WO2025230008A1 - Zirconia composition, zirconia calcined body, and method for producing same - Google Patents

Abstract

The present invention provides: a zirconia composition from which it is possible to obtain, after being sintered for a short period of time in which the holding time at the maximum sintering temperature is 10 minutes or less, a zirconia sintered body that can maintain high translucency comparable to that achieved after being sintered for a long period of time, and to obtain a zirconia calcined body that has excellent machinability and that can reduce the probability of chipping being generated during machining; a zirconia calcined body; and a method for producing the same. The present invention pertains to a zirconia composition which comprises a cubic-crystal zirconia powder containing a stabilizer capable of suppressing phase transition of zirconia, and in which the zirconia powder includes two or more types of zirconia powders containing the stabilizer at different ratios.

Description

Zirconia composition, zirconia calcined body, and method for producing the same

The present invention relates to a zirconia composition, a zirconia calcined body, and a method for producing the same. More specifically, the present invention relates to a zirconia composition, a zirconia calcined body, and a method for producing the same, which enable the production of a zirconia (zirconium (IV) oxide; ZrO ₂ ) sintered body having high translucency even with short sintering time.

Oxide ceramics are widely used industrially, and in particular, zirconia sintered bodies containing yttria have recently been used in dental materials such as dental prostheses due to their high strength and aesthetic properties. The raw materials for such zirconia sintered bodies are mainly zirconium oxide powder and yttrium oxide powder, either individually or as a solid solution. These raw materials are ground, mixed, dried, molded, and heat-treated to obtain a calcined body (mill blank), which is then machined into a shape similar to the desired dental prosthesis and further sintered to produce dental materials such as dental prostheses.

When sintering dental prostheses, it is important that the shorter the treatment time, the shorter the time it can be completed, and that the aesthetics of the prosthesis remain consistent even when sintered in various kilns with different outputs, i.e., that the translucency remains consistent regardless of the sintering time.

As a method for producing a zirconia sintered body, for example, a method is proposed in Patent Document 1. Patent Document 1 discloses a method for producing a zirconia sintered body that mixes a tetragonal crystal in which zirconia and yttria are solid-solved with a cubic crystal in which zirconia and yttria are solid-solved.

On the other hand, sintering of zirconia progresses while energy stabilization occurs, accompanied by changes in shape and crystalline phase due to mass transfer. Therefore, with the method of Patent Document 1, for example, if sintering is stopped after a short time, problems arise, such as insufficient densification resulting in low transparency, or insufficient crystalline phase transition and/or crystallization resulting in low strength and/or transparency.

Patent Document 2 has been proposed to solve the problem of sintering in such a short time. Patent Document 2 discloses that there is little difference in translucency between a zirconia sintered body obtained by short-term retention at the maximum sintering temperature of 30 minutes and a zirconia sintered body obtained by long-term retention at the same temperature of about 2 hours, and further discloses that excellent translucency can be achieved with sintering for 15 minutes.

Japanese Patent Application Laid-Open No. 2018-52806 International Publication No. 2018/056330

On the other hand, the shorter the sintering time, the more advantageous it is in terms of manufacturing costs. The inventors' investigations revealed that in order to further shorten the sintering time, the holding time at the maximum sintering temperature was further shortened from approximately 30 minutes to, for example, 10 minutes. In the invention of Patent Document 1, it was confirmed that, in a short-term retention period of, for example, 10 minutes, there was a problem in that the translucency was lower than with a long-term retention period of, for example, around 2 hours, because there was not a sufficient gradient in the concentrations of zirconium and yttrium elements to be able to utilize mass transfer between the tetragonal and cubic crystals to promote sintering.

Furthermore, in Patent Document 2, when the holding time at the maximum sintering temperature was shortened to 10 minutes, the proportion of monoclinic crystals remaining in the calcined body without undergoing phase transition was high, and the reaction rate for the phase transition from monoclinic to tetragonal or cubic crystals was insufficient. Comparing a short-term holding time at the maximum sintering temperature of, for example, 10 minutes with a long-term holding time of, for example, 2 hours, translucency was low, leaving room for further improvement.

The present invention aims to provide a zirconia composition, a zirconia calcined body, and a method for producing the same, which can obtain a zirconia sintered body that maintains high translucency equivalent to that obtained with long-term sintering after a short sintering time of 10 minutes or less at the maximum sintering temperature, and which further provides a zirconia calcined body with excellent machinability, reducing the likelihood of chipping during machining.

As a result of extensive research to solve the above-mentioned problems, the inventors discovered that using a zirconia calcined body produced using a zirconia composition containing cubic zirconia powder containing a stabilizer capable of suppressing the phase transition of zirconia, where the zirconia powder contains two or more types of zirconia powder with different stabilizer contents, is useful for promoting material diffusion and obtaining a zirconia sintered body that can maintain high translucency even after short-term sintering, at the same level as that obtained after long-term sintering. Based on this finding, the inventors conducted further research and completed the present invention.

The present invention includes the following inventions.
[1] A cubic zirconia powder containing a stabilizer capable of suppressing the phase transition of zirconia,
The zirconia composition, wherein the zirconia powder comprises two or more types of zirconia powder having different contents of the stabilizer.
[2] A zirconia powder having a first zirconia powder with a minimum content of the stabilizer and a second zirconia powder having a maximum content of the stabilizer,
The zirconia composition according to [1], wherein the difference between the content of the stabilizer in the first zirconia powder and the content of the stabilizer in the second zirconia powder is 1 to 10 mol%.
[3] The zirconia composition according to [2], wherein the content of the stabilizer in the first zirconia powder is 5.2 to 15 mol%, and the content of the stabilizer in the second zirconia powder is 6.2 to 17 mol%.
[4] The zirconia composition according to any one of [1] to [3], wherein the total content of the stabilizer in the entire zirconia composition is 5.5 to 12 mol% based on the total moles of zirconia and the stabilizer.
[5] The zirconia composition according to any one of [1] to [4], wherein the stabilizer is yttria (Y ₂ O ₃ ).
[6] The zirconia composition according to any one of [1] to [5], wherein the zirconia powder has an average primary particle size of 40 to 110 nm.
[7] The zirconia composition according to any one of [1] to [6], wherein, when ΔL ₁ *(W−B) of a first sintered body produced by firing at 1550°C for 120 minutes is compared with ΔL ₂ *(W−B) of a second sintered body produced by firing at 1550°C for 10 minutes, the following formula (1) is satisfied:
ΔL ₂ *(WB)/ΔL ₁ *(WB)≧0.85 (1)
[8] The zirconia composition according to [7], wherein the ΔL ₂ *(W−B) is 13 or more.
[9] The zirconia composition according to [7] or [8], wherein the second sintered body has a linear light transmittance of 0.7% or more.
[10] A zirconia calcined body in which zirconia powder is solidified to a degree that does not result in sintering,
Contains a stabilizer capable of suppressing the phase transition of zirconia,
The standard deviation of the stabilizer element distribution is 2 mol% or more and less than 21 mol%,
The cubic zirconia contains 55 to 100% zirconia.
Zirconia calcined body.
[11] The zirconia calcined body according to [10], wherein the stabilizer is yttria (Y ₂ O ₃ ).
[12] The zirconia calcined body according to [10] or [11], wherein the content of the stabilizer is 5.5 to 12 mol% based on the total moles of zirconia and the stabilizer.
[13] The zirconia calcined body according to any one of [10] to [12], having a density of 2.5 to 3.6 g/cm ³ .
[14] The zirconia calcined body according to any one of [10] to [13], having an average primary particle size of 40 to 110 nm.
[15] A zirconia calcined body according to any one of [10] to [14], wherein when ΔL ₁ *(W−B) of a first sintered body produced by firing at 1550°C for 120 minutes is compared with ΔL ₂ *(W−B) of a second sintered body produced by firing at 1550°C for 10 minutes, the following formula (1) is satisfied:
ΔL ₂ *(WB)/ΔL ₁ *(WB)≧0.85 (1)
[16] The zirconia calcined body according to [15], wherein the ΔL ₂ *(W−B) is 13 or more.
[17] The zirconia calcined body according to [15] or [16], wherein the second sintered body has a linear light transmittance of 0.7% or more.
[18] A method for producing a zirconia calcined body, comprising firing the zirconia composition according to any one of [1] to [9] to a degree that does not result in sintering.
[19] A method for producing a zirconia sintered body, comprising sintering the zirconia calcined body according to any one of [10] to [17].

By using the zirconia composition, the zirconia calcined body, and the method for producing the same of the present invention, it is possible to obtain a zirconia sintered body that can maintain high translucency equivalent to that of sintered for a long time after sintering in a short time in which the holding time at the maximum sintering temperature is 10 minutes or less (hereinafter, sintering in which the holding time at the maximum sintering temperature is 10 minutes will also be referred to as "10-minute sintering" or "short-time sintering").
Furthermore, by using the zirconia composition and zirconia calcined body of the present invention, the zirconia calcined body has excellent machinability (cuttability and grindability), and the probability of chipping during machining (hereinafter referred to as the "chipping rate") can be reduced.
Furthermore, by using the zirconia composition and zirconia calcined body of the present invention, it is possible to obtain a zirconia sintered body that can maintain high translucency equivalent to that obtained by long-term sintering after sintering for a short period of time, i.e., a maximum sintering temperature of less than 1,600°C (particularly preferably 1,560°C or less) and a holding time at the maximum sintering temperature of 10 minutes or less. Therefore, the production efficiency is excellent and the zirconia sintered body is industrially advantageous.
As described above, by using the zirconia composition of the present invention, it is possible to simultaneously solve the problems associated with zirconia calcined bodies and the problems associated with zirconia sintered bodies.

In this specification, the term "zirconia composition" refers to a raw material composition containing zirconia powder.
The zirconia calcined body can be a precursor (intermediate product) of a zirconia sintered body. In this specification, the term "zirconia calcined body" refers to a body in which zirconia particles are solidified to a degree that does not result in sintering.
"Not quite sintered" means a state in which the zirconia sintered body is not completely sintered (semi-sintered state). In a completely sintered sintered body, the relative density increases with sintering and densification progresses, so the relative density of the zirconia sintered body is 95% or more. The relative density can be calculated as the ratio of the actual density measured by the Archimedes method to the theoretical density.
In this specification, the term "zirconia sintered body" refers to a completely sintered state in which zirconia particles are solidified by sintering, the relative density increases as the sintering proceeds, and densification progresses to a relative density of 95% or more.
In this specification, "zirconia" refers to zirconium (IV) oxide ( _ZrO2 ), and _ZrO2 particles may contain a trace amount (0.5% by mass to 3% by mass) of _HfO2 relative to the amount of _ZrO2 . Because _HfO2 is difficult to separate, terms such as "zirconia,""zirconiaparticles," and "zirconia powder" refer to substances containing _ZrO2 and _HfO2 .
In this specification, the "maximum calcination temperature" refers to the maximum heating temperature that is finally reached during the calcination step (firing step for obtaining a calcined body) and maintained for a predetermined period of time.
In this specification, the "maximum sintering temperature" refers to the maximum heating temperature that is finally reached and maintained for a predetermined period of time during the sintering process (firing process for obtaining a sintered body).
In this specification, the content (mol %) of the stabilizer capable of suppressing the phase transition of zirconia means the content of zirconia and the stabilizer capable of suppressing the phase transition of zirconia calculated in terms of oxide.
In this specification, "normal pressure" means atmospheric pressure (0.1 MPa).
In this specification, the upper and lower limits of the numerical ranges (contents of each component, values calculated from each component, and each physical property, etc.) can be combined as appropriate.
In this specification, the range may be a range excluding the upper and/or lower limits of the numerical range. For example, the present invention includes the cases where the content of the stabilizer capable of suppressing the phase transition of zirconia is within the range of 5.2 to 15 mol%, 5.2 mol% to 15 mol%, 5.2 mol% to less than 15 mol%, more than 5.2 mol% to 15 mol%, and more than 5.2 mol% to less than 15 mol%.
In addition, in this specification, the total of the tetragonal crystal fraction f _t , cubic crystal fraction f _c , monoclinic crystal fraction f _m , and undissolved yttria content f _y calculated by the following formulas (2-1), (2-2), (2-3), and (2-4) does not exceed 100%.
Tetragonal crystal ratio f _t (%) = I _t / (I _m + I _t + I _c + I _y ) × 100 (2-1)
Cubic crystal ratio f _c (%) = I _c / (I _m + I _t + I _c + I _y ) × 100 (2-2)
Monoclinic rate f _m (%) = I _m / (I _m + I _t + I _c + I _y ) × 100 (2-3)
(wherein _fm represents the monoclinic fraction (%), _ft represents the tetragonal fraction (%), and _fc represents the cubic fraction (%); in XRD measurement, _Im represents the integrated intensity of the peak near 2θ=28.2° where the peak top of the main monoclinic peak appears; _It represents the integrated intensity of the peak near 2θ=30.2° where the peak top of the main tetragonal peak appears; _Ic represents the integrated intensity of the peak near 2θ=30.1° where the peak top of the main cubic peak appears; and _Iy represents the integrated intensity of the peak near 2θ=29.2° where the peak top of the main peak of undissolved yttria appears.)
f _y (%) = I _y / (I _m + I _t + I _c + I _y ) × 100 (2-4)
(In the formula, _fy represents the content (%) of undissolved yttria, and in XRD measurement, _Im represents the integrated intensity of the peak near 2θ=28.2° where the peak top of the main monoclinic peak appears, It represents the integrated intensity of the peak near 2θ=30.2° where the peak top of the main tetragonal peak appears, _Ic represents the integrated intensity of the peak near 2θ=30.1° where the peak top of the main cubic peak appears, _{and Iy} represents the integrated intensity of the peak near 2θ=29.2° where the peak top of the main peak of undissolved yttria appears.)

[Zirconia composition]
The zirconia composition of the present invention comprises a cubic zirconia powder containing a stabilizer capable of suppressing the phase transition of zirconia (hereinafter also simply referred to as "stabilizer");
The zirconia powder contains two or more types of zirconia powders having different contents of the stabilizer.

The reasons why, when a zirconia sintered body is produced from the zirconia composition of the present invention, after short-time sintering, it can maintain high translucency at the same level as that obtained with long-time sintering, and why the zirconia calcined body has excellent machinability and a reduced chipping rate during machining are not clear, but are thought to be as follows.
When two or more cubic zirconia crystal systems with different stabilizer contents are selected, the stabilizer content in the zirconia powder is closer to the target stabilizer content in the resulting zirconia sintered body than when a monoclinic zirconia with a 0 mol% dissolved stabilizer content is combined with a stabilizer that is not dissolved in zirconia. This suppresses the diffusion of stabilizer-derived components during sintering. In particular, when yttria is included as a stabilizer, the migration of yttrium element or yttrium ions is reduced, making it easier to maintain the concentration gradient of yttrium element or yttrium ions within a predetermined range. As a result, mass transfer occurs while particles are solidifying during sintering, allowing sufficient migration of yttrium element or yttrium ions even during short-term sintering of 10 minutes or less. This results in a more uniform distribution of yttrium element in the resulting zirconia sintered body, and it is believed that the translucency of the zirconia sintered body is so excellent that it can maintain the same level of high translucency after short-term sintering as after long-term sintering.
Furthermore, by selecting the content of the stabilizer contained in the cubic zirconia powder in the zirconia composition so that the content of the stabilizer in the zirconia calcined body falls within a predetermined range, it is believed that the stabilizer is integrated with the cubic crystal system, resulting in a zirconia calcined body that has excellent machinability.

Examples of stabilizers capable of suppressing the phase transition of zirconia contained in the cubic zirconia powder include calcium oxide (CaO), magnesium oxide (MgO), yttrium oxide ( _Y2O3 , hereinafter also referred to as "yttria"), cerium oxide ( _CeO2 ), scandium oxide _{( Sc2O3} ), _niobium oxide ( _Nb2O5 ), lanthanum oxide _{( La2O3} ), _{erbium oxide (Er2O3), praseodymium oxide (Pr2O3, Pr6O11 ) , samarium oxide} (Sm2O3), _europium oxide ( _Eu2O3 ), thulium oxide _{( Tm2O3 )} , gallium oxide ( _Ga2O3 ) _, indium oxide ( _{In2O3 )} , _and ytterbium oxide ( _{Yb2O3 ) . 3} ), and yttria is preferred because it provides excellent light transmittance to the zirconia sintered body.
The stabilizers may be used alone or in combination of two or more.

The content of the stabilizer in the zirconia powder of the present invention can be measured, for example, by inductively coupled plasma (ICP) emission spectroscopy, X-ray fluorescence analysis (XRF), etc.

The two or more types of cubic zirconia powders having different stabilizer contents may contain at least two types, and may contain three or more types of cubic zirconia powders having different stabilizer contents.
In one preferred embodiment, the zirconia composition includes two types of cubic zirconia powders having different contents of the stabilizer.

In this specification, of the two or more types of cubic zirconia powders, the cubic zirconia powder having the smallest stabilizer content is referred to as the "first zirconia powder," and the cubic zirconia powder having the largest stabilizer content is referred to as the "second zirconia powder."
In the zirconia composition of the present invention, the difference between the stabilizer content in the first zirconia powder and the stabilizer content in the second zirconia powder is preferably 1 to 10 mol %, more preferably 1.5 to 9.8 mol %, and even more preferably 2 to 9.5 mol % or more, from the viewpoint of excellent translucency of a zirconia sintered body obtained by short-time sintering.
In one preferred embodiment, the difference between the stabilizer content in the first zirconia powder and the stabilizer content in the second zirconia powder is 4.5 to 10 mol % (e.g., 4.5 mol % or more and 10 mol % or less).
Another preferred embodiment is a zirconia composition in which the difference between the stabilizer content in the first zirconia powder and the stabilizer content in the second zirconia powder is 1 to 4.5 mol% (e.g., 1 mol% or more and less than 4.5 mol%).
The stabilizer content in the first zirconia powder and the stabilizer content in the second zirconia powder refer to the stabilizer content relative to the total moles of zirconia and stabilizer in each zirconia powder.
The content of the stabilizer in the first zirconia powder and the content of the stabilizer in the second zirconia powder refer to the amount of the stabilizer dissolved in the powder.

The content of the stabilizer in the first zirconia powder is preferably 5.2 mol% or more, more preferably 5.4 mol% or more, and even more preferably 5.5 mol% or more, from the viewpoints that, when combined with the second zirconia powder, the zirconia sintered body obtained by short-time sintering has excellent translucency, the zirconia calcined body has excellent machinability, and the chipping rate can be reduced.
Furthermore, the content of the stabilizer in the first zirconia powder is preferably 15 mol% or less, more preferably 14 mol% or less, and even more preferably 12 mol% or less, from the viewpoints that, when combined with the second zirconia powder, the zirconia sintered body obtained by short-time sintering has excellent translucency, the zirconia calcined body has excellent machinability, and the chipping rate can be reduced.
In other words, the content of the stabilizer in the first zirconia powder is preferably 5.2 mol% or more and 15 mol% or less, more preferably 5.4 mol% or more and 14 mol% or less, and even more preferably 5.5 mol% or more and 12 mol% or less.

The content of the stabilizer in the second zirconia powder is preferably 6.2 mol% or more, more preferably 7.0 mol% or more, and even more preferably 7.5 mol% or more, from the viewpoints that, when combined with the first zirconia powder, the zirconia sintered body obtained by short-time sintering has excellent translucency, the zirconia calcined body has excellent machinability, and the chipping rate can be reduced.
Furthermore, the content of the stabilizer in the second zirconia powder is preferably 17 mol% or less, more preferably 16 mol% or less, and even more preferably 15 mol% or less, from the viewpoints that, when combined with the first zirconia powder, the zirconia sintered body obtained by short-time sintering has excellent translucency, the zirconia calcined body has excellent machinability, and the chipping rate can be reduced.
In other words, the content of the stabilizer in the second zirconia powder is preferably 6.2 mol% or more and 17 mol% or less, more preferably 7.0 mol% or more and 16 mol% or less, and even more preferably 7.5 mol% or more and 15 mol% or less.

The total content of the stabilizer in the entire zirconia composition is preferably 5.5 mol % or more, more preferably 5.8 mol % or more, and even more preferably 6.0 mol % or more, relative to the total moles of zirconia and the stabilizer, in terms of excellent translucency of the zirconia sintered body.
Furthermore, the total content of the stabilizer in the entire zirconia composition is preferably 12 mol % or less, more preferably 11 mol % or less, and even more preferably 10 mol % or less, relative to the total moles of zirconia and the stabilizer, from the viewpoints of being able to suppress a decrease in the strength of the zirconia sintered body, being able to provide a zirconia calcined body with excellent machinability, and being able to reduce the chipping rate.
In other words, the total content of the stabilizer in the entire zirconia composition is preferably 5.5 mol % or more and 12 mol % or less, more preferably 5.8 mol % or more and 11 mol % or less, and even more preferably 6.0 mol % or more and 10 mol % or less, based on the total moles of zirconia and stabilizer.

In this specification, the total stabilizer content in the entire zirconia composition and the zirconia calcined body means, in the case where stabilizers that are not dissolved in zirconia are present, the total content of stabilizers that are dissolved in zirconia and stabilizers that are not dissolved in zirconia.

It is preferable that the zirconia composition of the present invention satisfies the following formula (1) when comparing ΔL ₁ *(W−B) of a first sintered body produced by firing at 1550°C for 120 minutes with ΔL ₂ *(W−B) of a second sintered body produced by firing at 1550°C for 10 minutes.
ΔL ₂ *(WB)/ΔL ₁ *(WB)≧0.85 (1)
In the zirconia composition of the present invention, ΔL ₂ *(WB)/ΔL ₁ *(WB) is more preferably 0.86 or more, and even more preferably 0.88 or more.

ΔL ₁ *(W-B) for the first sintered body and ΔL ₂ *(W-B) for the second sintered body are both values calculated using the L* value of lightness (color space) in the L*a*b* color system (JIS Z 8781-4:2013 Colorimetry - Part 4: CIE 1976 L*a*b* color space). The lightness L* value can be measured under a D65 light source using, for example, a spectrophotometer (product name "Crystal Eye") manufactured by Olympus Corporation or a spectrophotometer CM-3610A or CM-36dGV manufactured by Konica Minolta, Inc.

ΔL ₁ *(W-B) for the first sintered body and ΔL ₂ *(W-B) for the second sintered body are both measured using the lightness (LW*) of the sample (zirconia sintered body) against a white background, and the lightness (LB*) of the same sample for which LW* was measured when the chromaticity was measured against a black background using the same measuring device, measuring mode, and light source.The difference between these values (ΔL* = (LW*) - (LB*)) represents an index of translucency.
In the evaluation of ΔL ₁ *(WB) for the first sintered body and ΔL ₂ *(WB) for the second sintered body, the maximum sintering temperature was 1550° C., and the heating rate and cooling rate were the same.
In the evaluation of ΔL ₁ *(W−B) for the first sintered body, the holding time (holding time) at the maximum sintering temperature was 120 minutes (hereinafter, sintering with a holding time at the maximum sintering temperature of 120 minutes will also be referred to as “120-minute sintering” or “long-term sintering”).
In evaluating ΔL ₂ *(W−B) for the second sintered body, the holding time (holding time) at the maximum sintering temperature was 10 minutes (hereinafter, sintering with a holding time of 10 minutes at the maximum sintering temperature is also referred to as “10-minute sintering” or “short-time sintering”).

The ΔL ₂ *(W−B) is preferably 13 or more, more preferably 14 or more, and even more preferably 15 or more.

The in-line light transmittance of the second sintered body is preferably 0.7% or more, and more preferably 0.8% or more.

[Method of producing zirconia composition]
Examples of methods for producing a zirconia composition include a method including a step of producing a cubic zirconia powder, and a step of mixing two or more types of cubic zirconia powders having different stabilizer contents to produce a powder containing zirconia-based particles.
In the zirconia composition and the method for producing a zirconia composition of the present invention, it is preferable not to include a powder consisting of only a stabilizer (stabilizer powder), since the target zirconia calcined body and zirconia sintered body having desired properties can be easily obtained.
Furthermore, the zirconia composition and the method for producing a zirconia composition of the present invention preferably do not contain monoclinic zirconia powder, since the target zirconia calcined body and zirconia sintered body having desired properties (particularly, the machinability of the zirconia calcined body and the effect of reducing the chipping rate during machining) can be easily obtained.
Furthermore, the zirconia composition and the method for producing a zirconia composition of the present invention preferably do not contain tetragonal zirconia powder, since the target zirconia calcined body and zirconia sintered body having desired properties (particularly, the machinability of the zirconia calcined body and the effect of reducing the chipping rate during machining) can be easily obtained.
In this specification, two or more types of cubic zirconia particles having different stabilizer contents are collectively referred to as "zirconia-based particles."

- Method for Producing Zirconia Powder There are no particular limitations on the method for preparing the cubic zirconia powder, which is the raw material powder. For example, a breakdown process in which coarse particles are pulverized or crushed to produce fine powder, or a building-up process in which atoms or ions are synthesized through a nucleation and growth process can be employed.

The average primary particle diameter of the cubic zirconia powder (hereinafter also referred to as "average particle diameter of zirconia powder" or "average particle diameter of cubic zirconia powder") is preferably 40 nm or more, more preferably 45 nm or more, and even more preferably 50 nm or more, in terms of excellent translucency of a zirconia sintered body obtained by short-time sintering, for any of the first zirconia powder, the second zirconia powder, and any zirconia powder other than the first zirconia powder and the second zirconia powder in a case where three or more cubic zirconia powders having different stabilizer contents are contained.
The average particle size of the cubic zirconia powder is preferably 110 nm or less, more preferably 105 nm or less, and even more preferably 100 nm or less, from the viewpoints of excellent light transmissivity of a zirconia sintered body obtained by short-time sintering and excellent machinability of a zirconia calcined body obtained.
Furthermore, the average particle size of the first zirconia powder and the average particle size of the second zirconia powder may be the same or different within the above range.
The average particle size of the cubic zirconia powder may be within any of the above-mentioned combinations. For example, the average particle size of the cubic zirconia powders (the first zirconia powder and the second zirconia powder) is preferably 40 to 110 nm, more preferably 45 to 105 nm, and even more preferably 50 to 100 nm, from the viewpoints of excellent light transmissivity of the zirconia sintered body obtained by short-time sintering and excellent machinability of the resulting zirconia calcined body.
When the average particle size of the cubic zirconia powder is within the above-mentioned average particle size range, the migration of the metal elements or ions thereof constituting the stabilizer can be sufficiently promoted even during short-time sintering of 10 minutes or less, and the resulting zirconia sintered body has excellent translucency.
Furthermore, in view of the fact that the zirconia composition of the present invention provides a zirconia sintered body with excellent translucency even in the above-mentioned short-time sintering, and that the zirconia calcined body has excellent machinability and can reduce the probability of chipping during machining, when the zirconia composition contains three or more types of cubic zirconia powders with different stabilizer contents, it is preferable that the average particle size of the cubic zirconia powder as a whole contained in the zirconia composition be within the suitable range of the average particle size of the zirconia powder.

The average particle size of the zirconia powder is the average primary particle size, and can be measured on a volumetric basis using, for example, a laser diffraction/scattering particle size distribution analyzer (product name "Partica LA-950") manufactured by Horiba, Ltd., by irradiating a slurry diluted with water with ultrasound for 30 minutes, and then applying ultrasound.

Below, an example of how to prepare zirconia powder is explained.

Cubic zirconia can be used as the zirconia raw material for producing the zirconia powder. These zirconia raw materials can be produced by, for example, the production method described in Japanese Patent No. 6543926 (produced by hydrolysis reaction). Commercially available zirconia raw materials can also be used.
Examples of commercially available products include 6 mol% yttria solid solution zirconia powder "TZ-6Y" (cubic 6Y), 8 mol% yttria solid solution zirconia powder "TZ-8Y" (cubic 8Y), and 10 mol% yttria solid solution zirconia powder "TZ-10Y" (cubic 10Y), all of which are manufactured by Tosoh Corporation.

In the breakdown process, coarse particles of zirconia raw material are pulverized separately for each crystal system to produce cubic zirconia powders adjusted to fall within the above-mentioned average particle size range.
In addition, it is preferable to prepare zirconia powder by pulverizing a cubic zirconia raw material for each stabilizer content, since it is easy to control the average particle size to a desired value.
The average particle size of the raw material compound before pulverization is not particularly limited as long as it can be adjusted to fall within the above average particle size range by pulverization.

The average particle size of the zirconia powder having the above average particle size can be adjusted by a known method such as pulverizing the raw material powder.
In order to facilitate adjustment to the above-mentioned predetermined average particle size, it is preferable to use fine-sized grinding media for grinding, for example, grinding media of 100 μm or less.
After the coarse particles are pulverized, the resulting zirconia powder is preferably classified.
Known methods and devices can be used for classification. Known methods include, for example, elutriation, which utilizes the difference in sedimentation velocity due to particle size-dependent dispersibility, and elutriation can be accelerated using a centrifuge. Known devices include, for example, porous membranes (membrane filters having pores of 100 nm, etc.), classification devices (wet classification devices, dry classification devices), etc.
By performing operations such as pulverization and classification, including changing the pulverization time as necessary, a raw material powder having a desired average particle size can be obtained.
By preparing a raw material powder having a desired average particle size, when zirconia having a predetermined cubic crystal system is selected as the zirconia crystal system, it becomes easier to keep the concentration gradient of elemental yttrium or yttrium ions within a predetermined range. As a result, mass transfer occurs while particles are solidifying together during sintering, and the migration of elemental yttrium or yttrium ions progresses sufficiently even in a short sintering time of 10 minutes or less, resulting in a more uniform distribution of elemental yttrium in the resulting zirconia sintered body, and it is believed that the translucency of the zirconia sintered body satisfies the above formula (1).

Furthermore, the following description will be given using an example in which the stabilizer is yttria, but the present invention is not limited to cases in which the stabilizer is yttria.

The amount of yttria dissolved in cubic zirconia as a zirconia raw material is the same as the stabilizer content in the first zirconia powder and the stabilizer content in the second zirconia powder described above.

The zirconia composition of the present invention may contain additives other than zirconia and stabilizers, as long as the effects of the present invention are achieved. Examples of such additives include colorants (including pigments, composite pigments, and fluorescent agents), binders, dispersants, emulsifiers, antifoaming agents, pH adjusters, lubricants, _alumina ( _Al2O3 ), titanium oxide ( _TiO2 ), and silica ( _SiO2 ). One type of additive may be used alone, or two or more types may be used in combination. Examples of additives include the same additives as those exemplified for the zirconia calcined body.
The additives may be added when the raw materials are mixed or pulverized, or may be added to the powder after pulverization.

In producing the zirconia composition of the present invention, the mixing of two or more types of zirconia powders having different stabilizer contents may be performed by dry mixing or wet mixing.

The mixing ratio of the first zirconia powder and the second zirconia powder can be adjusted appropriately so that the total content of the stabilizer in the zirconia powder falls within the desired range.

For example, in the zirconia composition, the mass ratio of the first zirconia powder to the mass of the second zirconia powder is preferably 15.0 mass %:85.0 mass % to 99.8 mass %:0.2 mass %.
In the zirconia composition, the respective contents of the first zirconia powder and the second zirconia powder can be adjusted within the above range so as to obtain the desired total content of the stabilizer in the entire zirconia composition.

The solvent used in the wet mixing is not particularly limited as long as it contains water, and an organic solvent may be used, a mixed solvent of water and an organic solvent may be used, or water alone may be used.
Examples of organic solvents include alcohols such as methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, 2-methyl-2-propanol, 2-methoxyethanol, 2-ethoxyethanol, 2-(2-ethoxyethoxy)ethanol, diethylene glycol monobutyl ether, and glycerin; ketones such as acetone and methyl ethyl ketone; ethers such as tetrahydrofuran, diethyl ether, diisopropyl ether, 1,4-dioxane, and dimethoxyethane (including modified ethers such as propylene glycol monomethyl ether acetate (commonly known as "PGMEA") (preferably ether-modified ethers and/or ester-modified ethers, more preferably ether-modified alkylene glycols and/or ester-modified alkylene glycols)); esters such as ethyl acetate and butyl acetate; hydrocarbons such as hexane and toluene; and halogenated hydrocarbons such as chloroform and carbon tetrachloride. One type of organic solvent may be used alone, or two or more types may be used in combination.

The zirconia composition of the present invention may be in a dry state, or may contain or be contained in a liquid. For example, the zirconia composition may be in the form of a powder, paste, slurry, etc.

The method for wet-mixing the raw materials in a solvent containing water is not particularly limited. For example, the raw materials may be wet-milled and mixed in a known milling and mixing device (such as a ball mill) to form a slurry containing zirconia-based particles, and then the slurry containing the zirconia-based particles may be dried and granulated to produce a granular zirconia composition.

In one embodiment, taking into consideration factors such as ease of adjusting the stabilizer content in the overall zirconia composition to the desired level, the powder containing zirconia-based particles may be a powder consisting solely of zirconia-based particles, or may contain other components (organic solvent, gelling agent, polymerizable monomer, polymerization initiator, resin (e.g., binder), etc.). The other components can be changed as appropriate, taking into consideration the form of the slurry, etc.

When the slurry is dried and granulated, there are no particular restrictions on the drying method, and methods such as spray drying, supercritical drying, freeze drying, hot air drying, and vacuum drying can be used. Of these, spray drying, supercritical drying, and freeze drying are preferred, with spray drying and supercritical drying being more preferred, and spray drying being even more preferred, as they can suppress particle aggregation during drying and result in a denser zirconia sintered body.

The slurry containing zirconia-based particles to be dried may be a slurry in which the dispersing medium is water, but it is preferable to use a slurry in which the dispersing medium is other than water, such as an organic solvent, as this can prevent particles from agglomerating during drying, resulting in a denser zirconia sintered body.

The water content in the slurry containing zirconia-based particles to be dried is preferably 3% by mass or less, more preferably 1% by mass or less, and even more preferably 0.1% by mass or less, because this prevents particles from agglomerating during drying and allows for the production of a denser zirconia sintered body. The water content can be measured using a Karl Fischer moisture meter.

There are no particular restrictions on the drying conditions for each drying method, and known drying conditions can be used as appropriate. When using an organic solvent as the dispersion medium, it is preferable to carry out the drying in the presence of a non-flammable gas, and it is even more preferable to carry out the drying in the presence of nitrogen, in order to reduce the risk of explosion during drying.

There are no particular restrictions on the supercritical fluid used in supercritical drying; for example, water or carbon dioxide can be used, but it is preferable that the supercritical fluid be carbon dioxide, as this can suppress particle aggregation and result in a denser zirconia sintered body.

Furthermore, particularly when spray drying is employed, it is preferable for the dispersion medium of the slurry containing zirconia-based particles to be dried to contain a liquid with a surface tension of 50 mN/m or less at 25°C, as this can prevent the zirconia particles from agglomerating during drying and allows for the production of a denser zirconia sintered body. From this perspective, the surface tension of the liquid is preferably 40 mN/m or less, and more preferably 30 mN/m or less.

For surface tension at 25°C, values listed in the Handbook of Chemistry and Physics can be used, for example. For liquids not listed there, values listed in WO 2014/126034 can be used. For liquids not listed in either of these publications, they can be determined using known measurement methods, such as the hanging ring method or the Wilhelmy method. Surface tension at 25°C is preferably measured using the automatic surface tensiometer "CBVP-Z" manufactured by Kyowa Interface Science Co., Ltd., or the "SIGMA 702" manufactured by KSV INSTRUMENTS LTD. (now Biolin Scientific AB, Sweden).

The liquid can be an organic solvent having the above-mentioned surface tension. Any of the organic solvents listed above that have the above-mentioned surface tension can be used as the liquid. However, at least one selected from the group consisting of methanol, ethanol, 2-methoxyethanol, 1,4-dioxane, 2-ethoxyethanol, and 2-(2-ethoxyethoxy)ethanol is preferred, as this can suppress particle aggregation during drying and result in a denser zirconia sintered body. At least one selected from the group consisting of methanol, ethanol, 2-ethoxyethanol, and 2-(2-ethoxyethoxy)ethanol is more preferred.

The content of the above liquid in the dispersion medium is preferably 50% by mass or more, more preferably 80% by mass or more, even more preferably 95% by mass or more, and particularly preferably 99% by mass or more, as this can suppress aggregation of particles during drying and result in a denser zirconia sintered body.

A slurry containing a dispersant other than water can be obtained by replacing the dispersant in a slurry containing water. There are no particular restrictions on the method for replacing the dispersant, and one method can be used, for example, by adding a dispersant other than water (such as an organic solvent) to a slurry containing water, and then evaporating the water. When evaporating the water, some or all of the dispersant other than water may be removed. The addition of the dispersant other than water and the distillation of the water may be repeated multiple times. Another method can be to add a dispersant other than water to a slurry containing water, and then precipitate the dispersoid. Furthermore, the dispersant in a slurry containing water may be replaced with a specific organic solvent, and then further replaced with another organic solvent.

The fluorescent agent may be added after replacing the dispersion medium, but it is preferable to add it before replacing the dispersion medium, as this will result in a more uniform zirconia sintered body with excellent physical properties. Similarly, if the slurry contains a colorant and/or a translucency adjuster, it may be added after replacing the dispersion medium, but it is preferable to add it before replacing the dispersion medium, as this will result in a more uniform zirconia sintered body with excellent physical properties.

On the other hand, examples of building-up processes include gas-phase pyrolysis, in which an oxide is precipitated by thermal decomposition while vaporizing an oxyacid salt of a metal ion or an organometallic compound; gas-phase reaction, in which synthesis is carried out through a gas-phase chemical reaction between a gaseous metal compound with a high vapor pressure and a reactant gas; evaporation concentration, in which the raw material is heated and vaporized, and then rapidly cooled in an inert gas at a specified pressure to condense the vapor into fine particles; melt method, in which a molten liquid is cooled into small droplets and solidified into a powder; solvent evaporation, in which a solvent is evaporated to increase the concentration in the liquid and cause precipitation to a supersaturated state; and precipitation, in which the solute concentration is supersaturated by reaction with a precipitant or hydrolysis, and then a nucleation-growth process is carried out to precipitate sparingly soluble compounds such as oxides and hydroxides.

Precipitation methods are further classified into homogeneous precipitation methods, in which a precipitant is produced in a solution by a chemical reaction, eliminating local non-uniformities in the precipitant concentration; coprecipitation methods, in which multiple metal ions coexisting in a solution are simultaneously precipitated by adding a precipitant; hydrolysis methods, in which an oxide or hydroxide is obtained by hydrolysis from a metal salt solution or an alcohol solution of a metal alkoxide or the like; and solvothermal synthesis methods, in which an oxide or hydroxide is obtained from a high-temperature, high-pressure fluid. Solvothermal synthesis methods are further classified into hydrothermal synthesis methods, in which water is used as a solvent, and supercritical synthesis methods, in which a supercritical fluid such as water or carbon dioxide is used as a solvent.
In the present invention, it is preferable not to use a coprecipitation method using zirconia and a stabilizer for the zirconia powder, in order to ensure that each particle has a desired average particle size.

The zirconium source used in the build-up process can be, for example, a nitrate, acetate, chloride, or alkoxide. Specific examples of the zirconium source include zirconium oxychloride, zirconium acetate, and zirconyl nitrate.

If the zirconia powder is produced by a method such as a building-up process, and the average particle size of the zirconia particles (such as the average particle size of the first zirconia powder and the average particle size of the second zirconia powder) is within the desired range, the cubic zirconia powders may be slurried without drying, and the resulting slurries may be used in a process of mixing two or more types of zirconia particles having different stabilizer contents to produce a slurry containing zirconia-based particles. There are no particular restrictions on the method for preparing each of the raw material slurries before obtaining the slurry containing zirconia-based particles; for example, they may be obtained via the breakdown process or building-up process described above.

When the zirconia composition of the present invention is in a dry state, the zirconia composition can be obtained by drying a slurry containing zirconia-based particles.

The zirconia composition of the present invention may be subjected to a molding process to form a molded body.
The term "molded body" refers to a body that has not yet reached either a semi-sintered state (calcined state) or a sintered state. That is, a molded body is distinguished from a calcined body and a sintered body in that the molded body is a body that has not yet been fired after being formed into a molded body by molding.

The type of the molding step is not particularly limited, but the molding step is preferably performed in the following manner, since it allows the zirconia molded body of the present invention, and further the zirconia calcined body and zirconia sintered body of the present invention to be easily obtained:
(i) a method comprising slip casting a slurry comprising zirconia-based particles;
(ii) a method comprising gel-casting a slurry comprising zirconia-based particles;
(iii) a method comprising a step of press-molding a powder containing zirconia-based particles;
(iv) a method comprising molding a composition comprising zirconia-based particles and a resin;
(v) a method comprising a step of polymerizing a composition containing zirconia-based particles and a polymerizable monomer or oligomer;
(vi) a method comprising additive manufacturing of granules comprising zirconia-based particles;
It is preferable that the method is at least one of the above, and more preferable that the method has a molding step of molding zirconia particles, a polyol, and a binder to obtain a zirconia molded body.

(i) Slip Casting When a zirconia molded body is produced by a method including a step of slip-casting a slurry containing zirconia-based particles, the specific method of slip-casting is not particularly limited. For example, a method can be employed in which a slurry containing zirconia-based particles is poured into a mold and then dried.

The content of the dispersion medium in the slurry containing zirconia-based particles used is preferably 80% by mass or less, more preferably 50% by mass or less, and even more preferably 20% by mass or less, as this makes it easy to pour the slurry into a mold, prevents excessive drying time, and allows the mold to be used more frequently.

The slurry can be poured into the mold under normal pressure, but from the perspective of production efficiency, it is preferable to do so under pressurized conditions. There are no particular restrictions on the type of mold used for slip casting; for example, porous molds made from plaster, resin, ceramics, etc. can be used. Porous molds made from resin or ceramics are superior in terms of durability.

The slurry containing the zirconia-based particles used in slip casting may further contain one or more of the other components described above, such as binders, plasticizers, dispersants, emulsifiers, antifoaming agents, pH adjusters, and lubricants.

(ii) Gel Casting When a zirconia molded body is produced by a method including a step of gel-casting a slurry containing zirconia-based particles, the specific method of gel-casting is not particularly limited. For example, a method can be employed in which a shaped wet body is obtained by gelling a slurry containing zirconia-based particles in a mold, and then the obtained wet body is dried.

The content of the dispersion medium in the slurry containing zirconia-based particles used is preferably 80% by mass or less, more preferably 50% by mass or less, and even more preferably 20% by mass or less, as this prevents drying from taking a long time and also suppresses the occurrence of cracks during drying.

The gelling may be achieved, for example, by adding a gelling agent, or by adding a polymerizable monomer and then polymerizing it. There are no particular restrictions on the type of mold used; for example, porous molds made of plaster, resin, ceramics, etc., or non-porous molds made of metal, resin, etc., can be used.

There are no restrictions on the type of gelling agent; for example, a water-soluble gelling agent can be used; specifically, agarose, gelatin, etc. are preferably used. A single gelling agent may be used alone, or two or more may be used in combination. The amount of gelling agent used is not particularly limited as long as it does not cause problems such as cracking during sintering, but it can be 10% by mass or less, 5% by mass or less, or 1% by mass or less, based on the mass of the slurry after the gelling agent has been blended.

The type of polymerizable monomer is not particularly limited, and examples thereof include (meth)acrylate monomers such as 2-hydroxyethyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 6-hydroxyhexyl (meth)acrylate, 10-hydroxydecyl (meth)acrylate, propylene glycol mono(meth)acrylate, glycerol mono(meth)acrylate, and erythritol mono(meth)acrylate; and (meth)acrylamide monomers such as N-methylol (meth)acrylamide, N-hydroxyethyl (meth)acrylamide, and N,N-bis(2-hydroxyethyl) (meth)acrylamide.
The polymerizable monomers may be used alone or in combination of two or more.

The amount of polymerizable monomer used is not particularly limited as long as it does not cause problems such as cracks during sintering, but it can be 10% by mass or less, 5% by mass or less, or 1% by mass or less, based on the mass of the slurry after the polymerizable monomer has been mixed.

When gelation is achieved by polymerization of a polymerizable monomer, it is preferable to carry out the polymerization using a polymerization initiator. There are no particular restrictions on the type of polymerization initiator, but photopolymerization initiators are particularly preferred. The photopolymerization initiator can be appropriately selected from photopolymerization initiators used in general industry, and photopolymerization initiators used for dental purposes are particularly preferred.

Specific photopolymerization initiators include, for example, (bis)acylphosphine oxides (including salts), thioxanthones (including salts such as quaternary ammonium salts), ketals, α-diketones, coumarins, anthraquinones, benzoin alkyl ether compounds, and α-aminoketone compounds. One photopolymerization initiator may be used alone, or two or more may be used in combination. Among these photopolymerization initiators, it is preferable to use at least one selected from the group consisting of (bis)acylphosphine oxides and α-diketones. This allows polymerization (gelation) to occur in both the ultraviolet (including near-ultraviolet) and visible light regions, and in particular, polymerization (gelation) can be sufficiently achieved using any light source, including lasers such as Ar lasers and He-Cd lasers; halogen lamps, xenon lamps, metal halide lamps, light-emitting diodes (LEDs), mercury lamps, and fluorescent lamps.

Among the above (bis)acylphosphine oxides, examples of acylphosphine oxides include 2,4,6-trimethylbenzoyldiphenylphosphine oxide (commonly known as "TPO"), 2,6-dimethoxybenzoyldiphenylphosphine oxide, 2,6-dichlorobenzoyldiphenylphosphine oxide, 2,4,6-trimethylbenzoylmethoxyphenylphosphine oxide, 2,4,6-trimethylbenzoylethoxyphenylphosphine oxide, 2,3,5,6-tetramethylbenzoyldiphenylphosphine oxide, benzoyldi(2,6-dimethylphenyl)phosphonate, sodium salt of 2,4,6-trimethylbenzoylphenylphosphine oxide, potassium salt of 2,4,6-trimethylbenzoyldiphenylphosphine oxide, and ammonium salt of 2,4,6-trimethylbenzoyldiphenylphosphine oxide.

Among the above (bis)acylphosphine oxides, examples of bisacylphosphine oxides include bis(2,6-dichlorobenzoyl)phenylphosphine oxide, bis(2,6-dichlorobenzoyl)-2,5-dimethylphenylphosphine oxide, bis(2,6-dichlorobenzoyl)-4-propylphenylphosphine oxide, bis(2,6-dichlorobenzoyl)-1-naphthylphosphine oxide, bis(2,6-dimethoxybenzoyl)phenylphosphine oxide, bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide, bis(2,6-dimethoxybenzoyl)-2,5-dimethylphenylphosphine oxide, bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide, and bis(2,3,6-trimethylbenzoyl)-2,4,4-trimethylpentylphosphine oxide. Additionally, compounds such as those described in JP 2000-159621 A can also be used.

Among these (bis)acylphosphine oxides, 2,4,6-trimethylbenzoyldiphenylphosphine oxide, 2,4,6-trimethylbenzoylmethoxyphenylphosphine oxide, bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide, and the sodium salt of 2,4,6-trimethylbenzoylphenylphosphine oxide are preferred.

Examples of α-diketones include diacetyl, benzyl, camphorquinone, 2,3-pentadione, 2,3-octadione, 9,10-phenanthrenequinone, 4,4'-oxybenzyl, and acenaphthenequinone. Of these, camphorquinone is preferred, especially when using a light source in the visible light range.

Slurries containing the above-mentioned zirconia-based particles used in gel casting, like slurries used in slip casting, may also contain one or more of the other components described above, such as binders, plasticizers, dispersants, emulsifiers, antifoaming agents, pH adjusters, and lubricants.

There are no particular restrictions on the drying method used to dry the shaped wet body, and examples include natural drying, hot air drying, vacuum drying, dielectric heating drying, induction heating drying, and constant temperature and humidity drying. These methods may be used alone or in combination with one or more other methods. Among these, natural drying, dielectric heating drying, induction heating drying, and constant temperature and humidity drying are preferred, as they can prevent cracks from occurring during drying.

(iii) Press Molding When a zirconia molded body is produced by a method including a step of press molding a powder containing zirconia-based particles, the specific method of press molding is not particularly limited, and the press molding can be carried out using a known press molding machine.
A specific example of the press molding method is uniaxial pressing.
The pressing pressure in the press molding is set to an optimum value depending on the size, open porosity, biaxial bending strength, and particle size of the raw material powder of the target molded body, and is usually 5 MPa to 1000 MPa. By increasing the pressing pressure during molding in the above-mentioned production method, the pores of the obtained molded body are more fully filled, the open porosity can be set lower, and the density of the molded body can be increased. Furthermore, in order to increase the density of the obtained zirconia molded body, a cold isostatic pressing (CIP) treatment may be further performed after uniaxial pressing.

The powder containing the zirconia-based particles used in press molding may further contain one or more of the other components described above, such as binders, plasticizers, dispersants, emulsifiers, antifoaming agents, pH adjusters, lubricants, and translucency adjusters. These components may be blended when the powder is prepared.

(iv) Molding of a Resin-Containing Composition When a zirconia molded body is produced by a method including a step of molding a composition containing zirconia-based particles and a resin, the specific method for molding the composition is not particularly limited, and for example, injection molding, cast molding, extrusion molding, etc. may be used. Alternatively, a method of molding the composition by fusion dynamic molding (FDM), an inkjet method, a powder/binder lamination method, or other additive manufacturing methods (e.g., 3D printing) may also be used. Among these molding methods, injection molding and cast molding are preferred, and injection molding is more preferred.

There are no particular restrictions on the type of resin used, and those that function as binders are preferably used. Specific examples of such resins include paraffin wax, polyvinyl alcohol, polyethylene, polypropylene, ethylene-vinyl acetate copolymer, polystyrene, atactic polypropylene, methacrylic resin, and fatty acids such as stearic acid. These resins may be used alone or in combination of two or more.

The composition containing the zirconia-based particles and resin may further contain one or more of the other components described above, such as plasticizers, dispersants, emulsifiers, antifoaming agents, pH adjusters, and lubricants.

(v) Polymerization of a composition containing a polymerizable monomer or oligomer By polymerizing a composition containing zirconia-based particles and a polymerizable monomer or oligomer, the polymerizable monomer in the composition can be polymerized to harden the composition.
When a zirconia molded body is produced by a method including the polymerization step, the specific method is not particularly limited, and examples that can be used include (a) a method of polymerizing a composition containing zirconia-based particles and a polymerizable monomer or oligomer in a mold, and (b) a stereolithography (SLA) method using a composition containing zirconia-based particles and a polymerizable monomer or oligomer. Among these, the stereolithography method (b) is preferred.
According to the stereolithography method, a shape corresponding to the desired shape of the zirconia sintered body to be finally obtained can be imparted to the zirconia molded body at the time of production, and therefore, the stereolithography method may be particularly suitable when the zirconia sintered body of the present invention is used as a dental material for dental prostheses and the like.

The type of polymerizable monomer in the composition containing the zirconia-based particles and the polymerizable monomer or oligomer is not particularly limited, and may be any of monofunctional polymerizable monomers such as monofunctional (meth)acrylates and monofunctional (meth)acrylamides, and polyfunctional polymerizable monomers such as bifunctional aromatic compounds, bifunctional aliphatic compounds, trifunctional or higher functional compounds, etc. One type of polymerizable monomer may be used alone, or two or more types may be used.
The oligomer is not particularly limited as long as it is a compound in which two or more of the polymerizable monomers are bonded together and has polymerizability.
Among these, it is preferable to use a polyfunctional polymerizable monomer, particularly when a stereolithography method is employed.

Examples of monofunctional (meth)acrylates include (meth)acrylates having a hydroxyl group such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 6-hydroxyhexyl (meth)acrylate, 10-hydroxydecyl (meth)acrylate, propylene glycol mono(meth)acrylate, glycerol mono(meth)acrylate, and erythritol mono(meth)acrylate; methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, and sec-butyl (meth)acrylate. Examples of suitable acrylates include alkyl (meth)acrylates such as t-butyl (meth)acrylate, isobutyl (meth)acrylate, n-hexyl (meth)acrylate, lauryl (meth)acrylate, cetyl (meth)acrylate, and stearyl (meth)acrylate; alicyclic (meth)acrylates such as cyclohexyl (meth)acrylate and isobornyl (meth)acrylate; aromatic group-containing (meth)acrylates such as benzyl (meth)acrylate and phenyl (meth)acrylate; and (meth)acrylates with functional groups such as 2,3-dibromopropyl (meth)acrylate, 3-(meth)acryloyloxypropyltrimethoxysilane, and 11-(meth)acryloyloxyundecyltrimethoxysilane.

Examples of monofunctional (meth)acrylamides include (meth)acrylamide, N-(meth)acryloylmorpholine, N,N-dimethyl(meth)acrylamide, N,N-diethyl(meth)acrylamide, N,N-di-n-propyl(meth)acrylamide, N,N-di-n-butyl(meth)acrylamide, N,N-di-n-hexyl(meth)acrylamide, N,N-di-n-octyl(meth)acrylamide, N,N-di-2-ethylhexyl(meth)acrylamide, N-hydroxyethyl(meth)acrylamide, and N,N-bis(2-hydroxyethyl)(meth)acrylamide.

Among these monofunctional polymerizable monomers, (meth)acrylamide is preferred due to its excellent polymerizability, with N-(meth)acryloylmorpholine, N,N-dimethyl(meth)acrylamide, and N,N-diethyl(meth)acrylamide being more preferred.

Examples of bifunctional aromatic compounds include 2,2-bis((meth)acryloyloxyphenyl)propane, 2,2-bis[4-(2-hydroxy-3-acryloyloxypropoxy)phenyl]propane, 2,2-bis[4-(2-hydroxy-3-methacryloyloxypropoxy)phenyl]propane (commonly known as "Bis-GMA"), 2,2-bis(4-(meth)acryloyloxyethoxyphenyl)propane, 2,2-bis(4-(meth)acryloyloxypolyethoxyphenyl)propane, 2,2-bis(4-(meth)acryloyloxydiethoxyphenyl)propane, 2,2-bis(4-(meth)acryloyloxytetraethoxyphenyl)propane, 2,2-bis(4-(meth)acryloyloxypentaethoxyphenyl)propane, 2,2-bis(4-(meth)acryloyloxypentaeth ... and (meth)acrylates such as bis(4-(meth)acryloyloxydipropoxyphenyl)propane, 2-(4-(meth)acryloyloxydiethoxyphenyl)-2-(4-(meth)acryloyloxyethoxyphenyl)propane, 2-(4-(meth)acryloyloxydiethoxyphenyl)-2-(4-(meth)acryloyloxytriethoxyphenyl)propane, 2-(4-(meth)acryloyloxydipropoxyphenyl)-2-(4-(meth)acryloyloxytriethoxyphenyl)propane, 2,2-bis(4-(meth)acryloyloxypropoxyphenyl)propane, 2,2-bis(4-(meth)acryloyloxyisopropoxyphenyl)propane, and 1,4-bis(2-(meth)acryloyloxyethyl)pyromellitate. Among these, Bis-GMA and 2,2-bis(4-(meth)acryloyloxypolyethoxyphenyl)propane are preferred due to their excellent polymerizability and the mechanical strength of the resulting zirconia molded body. Of the 2,2-bis(4-(meth)acryloyloxypolyethoxyphenyl)propanes, 2,2-bis(4-methacryloyloxypolyethoxyphenyl)propane (average number of moles of ethoxy groups added: 2.6, commonly known as "D-2.6E") is preferred.

Examples of bifunctional aliphatic compounds include glycerol di(meth)acrylate, ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, butylene glycol di(meth)acrylate, neopentyl glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, 1,3-butanediol di(meth)acrylate, and 1,4-butanediol di(meth)acrylate. Examples of (meth)acrylates include triethylene glycol dimethacrylate (TEGDMA) and 2,2,4-trimethylhexamethylenebis(2-carbamoyloxyethyl)dimethacrylate (UDMA). Among these, triethylene glycol dimethacrylate (TEGDMA) and 2,2,4-trimethylhexamethylenebis(2-carbamoyloxyethyl)dimethacrylate are preferred due to their excellent polymerizability and the mechanical strength of the resulting zirconia molded body.

Examples of trifunctional or higher functional compounds include (meth)acrylates such as trimethylolpropane tri(meth)acrylate, trimethylolethane tri(meth)acrylate, trimethylolmethane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol penta(meth)acrylate, N,N-(2,2,4-trimethylhexamethylene)bis[2-(aminocarboxy)propane-1,3-diol]tetra(meth)acrylate, and 1,7-diacryloyloxy-2,2,6,6-tetra(meth)acryloyloxymethyl-4-oxaheptane. Among these, N,N-(2,2,4-trimethylhexamethylene)bis[2-(aminocarboxy)propane-1,3-diol]tetramethacrylate and 1,7-diacryloyloxy-2,2,6,6-tetraacryloyloxymethyl-4-oxaheptane are preferred due to their excellent polymerizability and the mechanical strength of the resulting zirconia molded body.

In either method (a) or (b) above, polymerization of the composition is preferably carried out using a polymerization initiator, and the composition preferably further contains a polymerization initiator. There are no particular restrictions on the type of polymerization initiator, but photopolymerization initiators are particularly preferred. The photopolymerization initiator can be appropriately selected from photopolymerization initiators used in general industry, and photopolymerization initiators used in dental applications are particularly preferred. Specific examples of photopolymerization initiators are the same as those described above in the explanation of gel casting, so a duplicate explanation will be omitted here.

The composition containing the zirconia-based particles and polymerizable monomer may further contain one or more of the other components described above, such as plasticizers, dispersants, emulsifiers, antifoaming agents, pH adjusters, and lubricants.

When manufacturing a zirconia molded body by stereolithography using a composition containing zirconia-based particles and a polymerizable monomer, there are no particular limitations on the specific method of stereolithography, and any known method can be appropriately adopted for stereolithography. For example, a method can be used in which a stereolithography device is used to photopolymerize a liquid composition with ultraviolet light, a laser, or the like, thereby sequentially forming layers having the desired shape to obtain the desired zirconia molded body.

When obtaining a zirconia molded body by stereolithography, it is preferable that the content of zirconia-based particles in a composition containing zirconia-based particles and a polymerizable monomer be as high as possible, from the perspective of subsequent sintering properties, etc. Specifically, the content of zirconia-based particles in the composition is preferably 20% by mass or more, more preferably 30% by mass or more, even more preferably 40% by mass or more, and particularly preferably 50% by mass or more. On the other hand, in stereolithography, due to the principles of layered molding, it is preferable that the viscosity of the composition be within a certain range. Therefore, the content of zirconia-based particles in the above composition is preferably 90% by mass or less, more preferably 80% by mass or less, even more preferably 70% by mass or less, and particularly preferably 60% by mass or less. Adjusting the viscosity of the composition can be particularly important when using the controlled liquid level method, in which zirconia molded bodies are formed one layer at a time by curing layers by irradiating light from below the container through the bottom surface of the container, in order to raise the cured layer by one layer and allow the composition to form the next layer to smoothly flow between the underside of the cured layer and the bottom surface of the container.

Specific viscosity of the composition at 25°C is preferably 20,000 mPa·s or less, more preferably 10,000 mPa·s or less, and even more preferably 5,000 mPa·s or less. Furthermore, the viscosity is preferably 100 mPa·s or more. Since the viscosity of the composition tends to increase as the zirconia-based particle content increases, it is preferable to appropriately adjust the balance between the zirconia-based particle content and viscosity of the composition in accordance with the performance of the stereolithography device used, taking into account factors such as the balance between the stereolithography speed and the precision of the resulting zirconia molded body. The viscosity can be measured using an E-type viscometer.

In the method for producing a zirconia molded body of the present invention, in order to further improve the density of the zirconia molded body, the zirconia molded body may be subjected to a humidification treatment and then a CIP treatment.
When press molding is performed, the powder containing zirconia-based particles may be subjected to a humidification treatment before press molding, followed by press molding. The humidification treatment may be performed by any known method without any limitation, such as spraying water with a spray bottle or using a hygrostat or thermo-hygrostat. The amount of moisture increase due to the humidification treatment depends on the average particle size of the zirconia-based particles contained (the average particle size of the first zirconia powder, the average particle size of the second zirconia powder, etc.), but is preferably greater than 2% by mass, more preferably greater than 3% by mass, even more preferably greater than 4% by mass, and particularly preferably greater than 5% by mass, relative to the mass of the pre-moistened powder (powder before the humidification treatment) and the molded body. It is also preferably 15% by mass or less, more preferably 13% by mass or less, and even more preferably 11% by mass or less. The amount of moisture increase due to the humidification treatment can be calculated as a percentage by subtracting the masses of the pre-moistened powder and the molded body from the mass of the moistened powder (powder after the humidification treatment) and the molded body, and dividing the resultant value by the mass of the pre-moistened powder and the molded body.
The pressure of the CIP treatment is the same as that described above in the description of press molding.

(vi) additive manufacturing of granules containing zirconia-based particles
There are no particular limitations on the specific method for producing granules containing zirconia-based particles. For example, a method can be used in which a slurry is obtained and then dried in a spray dryer to form granules, and the resulting granules can be used in powder additive manufacturing.
The powder additive manufacturing method is not particularly limited, and examples include the powder bed method, SLS method (selective laser sintering method), SLM method (selective laser melting method), electron beam method, arc discharge method, binder jet method, etc. For methods in which it is better not to use organic substances during additive manufacturing, it is also preferable not to use organic substances in the granule manufacturing stage.

[Zirconia calcined body]
The zirconia calcined body of the present invention contains zirconia and a stabilizer capable of suppressing a phase transition of zirconia,
The standard deviation of the stabilizer element distribution is 2 mol% or more and less than 21 mol%,
The zirconia contains cubic zirconia in an amount of 55 to 100%.

A preferred embodiment will be described below taking as an example a case where the stabilizer is yttria.
The present invention is not limited to the case where the stabilizer is yttria. Therefore, when another stabilizer is used, the "yttria content" can be read as the content of the other stabilizer, and the "yttrium element distribution" can be read as the element distribution of the other metal element.

The reason why the zirconia calcined body of the present invention can provide a zirconia sintered body that can maintain high translucency equivalent to that of a zirconia sintered body sintered for a long period of time, even after sintering for a short period of time in which the holding time at the maximum sintering temperature is 10 minutes or less, is considered to be as follows.
Hereinafter, an example in which the stabilizer is yttria will be described, but the present invention is not limited to the case in which the stabilizer is yttria.
Generally, if the holding time at the maximum sintering temperature is shortened to 10 minutes, when the concentration gradient of the stabilizer metal element (preferably yttrium element) in the zirconia crystal system is small, the transfer of yttrium element or yttrium ions between substances hardly occurs. Therefore, the stabilization of energy accompanying the substance transfer is insufficient, sintering becomes insufficient, and the translucency decreases, which is a problem with 10-minute sintering.
In contrast, when yttria is used as a stabilizer, as in the present invention, two or more types of cubic zirconia powders having different stabilizer contents are selected and calcined according to the amount of yttria dissolved, etc., so that a source for supplying elemental yttrium or yttrium ions and a destination for the elemental yttrium or yttrium ions exist within the system of the zirconia calcined body, and an appropriate concentration gradient of elemental yttrium or yttrium ions is achieved between the two, and elemental yttrium or yttrium ions migrate between crystals in the zirconia crystal system during sintering.
In this way, by having a zirconia crystal system as a supplier that supplies elemental yttrium or yttrium ions, and a zirconia crystal system as a destination to which elemental yttrium or yttrium ions can migrate, present within the system of the zirconia calcined body, the difference in concentration of elemental yttrium or yttrium ions between the substances present in the system during sintering is increased, and material transfer occurs as particles solidify during sintering. As a result, even during a short sintering time of 10 minutes or less, the migration of elemental yttrium or yttrium ions progresses sufficiently, and when combined with a raw material powder having a predetermined average particle size, they act together, resulting in a more uniform distribution of elemental yttrium in the resulting zirconia sintered body. It is therefore believed that the translucency of the zirconia sintered body satisfies the above formula (1).

As described above, in this invention, by increasing the difference in the concentrations of yttrium element or yttrium ions between substances present in the system during sintering and promoting the mass transfer that occurs during sintering, excellent translucency is achieved even in short sintering times of 10 minutes or less, even at maximum sintering temperatures below 1600°C. Therefore, because the maximum sintering temperature is lower and the sintering time is short, both a lower maximum sintering temperature and a shorter sintering time are achieved, which is industrially advantageous.

In the zirconia calcined body of the present invention, the standard deviation of the yttrium element distribution is preferably 2 mol% or more, since migration of yttrium element or yttrium ions is likely to occur during short-time sintering and the resulting zirconia sintered body has excellent translucency. In order to achieve even better translucency, the standard deviation is more preferably 2.1 mol% or more, even more preferably 2.5 mol% or more, and particularly preferably 3 mol% or more.
Furthermore, in the case of short-time sintering, the standard deviation of the yttrium element distribution is preferably less than 21 mol%, more preferably 20 mol% or less, even more preferably 18 mol% or less, and particularly preferably 15 mol% or less, because migration of yttrium element or yttrium ions between substances present in the system during sintering is likely to occur and the resulting zirconia sintered body has superior translucency.
The standard deviation of the yttrium element distribution may be any of the above-mentioned combinations. For example, the standard deviation of the yttrium element distribution is preferably 2 mol% or more and less than 21 mol%, more preferably 2.1 mol% or more and 20 mol% or less, even more preferably 2.5 mol% or more and 18 mol% or less, and particularly preferably 3 mol% or more and 15 mol% or less.
In one preferred embodiment, the zirconia calcined body has a standard deviation of the yttrium element distribution of 2.1 mol % or more and 15 mol % or less.
The standard deviation of the yttrium element distribution can be adjusted by the content f _y of undissolved yttria, the predetermined average particle size of the raw material powder used as the yttrium source, the content and blending ratio of the predetermined raw material powder in the raw material composition, etc., and can be more easily adjusted by the content f _y of undissolved yttria and the predetermined average particle size of the raw material powder used as the yttrium source.

The method for calculating the standard deviation of the yttrium element distribution is as described in the Examples section below.

The zirconia calcined body of the present invention uses two or more types of cubic zirconia having different stabilizer contents in the zirconia composition, which facilitates migration of elemental yttrium or yttrium ions during short-time sintering, and the resulting zirconia sintered body has excellent translucency, excellent machinability, and a reduced chipping rate.
The zirconia calcined body of the present invention preferably has a content of cubic zirconia of 55 to 100%, more preferably 80 to 100%, further preferably 85 to 100%, and particularly preferably 90 to 100%. The content of cubic zirconia means the cubic fraction (f _c ).
The content of cubic zirconia (cubic fraction f _c ) is measured as described in the Examples below.
Furthermore, the zirconia calcined body of the present invention preferably does not contain monoclinic zirconia.
Furthermore, the zirconia calcined body of the present invention preferably has a content of tetragonal zirconia (tetragonal fraction f _t ) of 20% or less, more preferably 15% or less, even more preferably 10% or less, and particularly preferably 5% or less.
In addition, the zirconia calcined body of the present invention preferably does not contain yttria that is not solid-dissolved in zirconia, in order to obtain the effects of the present invention more effectively.

The content of the stabilizer in the zirconia calcined body of the present invention is preferably 5.5 mol % or more, more preferably 5.8 mol % or more, and even more preferably 6.0 mol % or more, relative to the total moles of zirconia (zirconium (IV) oxide; _{ZrO 2} ) and stabilizer, from the viewpoints of improving the translucency of the zirconia sintered body, providing excellent machinability of the zirconia calcined body, and reducing the chipping rate.
When the content is 5.5 mol % or more, the crystal form contained in the zirconia sintered body contains many cubic crystals, which is preferable in terms of improving the translucency.
Furthermore, the content of the stabilizer is preferably 12 mol % or less, more preferably 11 mol % or less, and even more preferably 10 mol % or less, from the viewpoints of being able to suppress a decrease in the strength of the sintered body, providing excellent machinability for the zirconia calcined body, and reducing the chipping rate.
The content of the stabilizer may be within any of the above-mentioned ranges, for example, the content of the stabilizer is preferably 5.5 to 12 mol%, more preferably 5.8 mol% to 11 mol%, and even more preferably 6.0 to 10 mol%.
The content of the stabilizer in the zirconia calcined body of the present invention can be adjusted by changing the compounding ratio in consideration of the content of the stabilizer dissolved in the crystalline system, such as by adjusting the content of cubic zirconia in the zirconia composition, which has a high content of the stabilizer dissolved in zirconia.

The stabilizer content in the zirconia calcined body of the present invention can be measured using the same method as for measuring the stabilizer content in zirconia powder.

The density of the zirconia calcined body of the present invention is preferably 3.6 g/cm ³ or less, more preferably 3.5 g/cm ³ or less, and even more preferably 3.4 g/cm ³ or less.
The density of the zirconia calcined body of the present invention is preferably 2.5 g/cm ³ or more, more preferably 2.7 g/cm ³ or more, and even more preferably 2.9 g/cm ³ or more.
In other words, the density of the zirconia calcined body of the present invention is preferably 2.5 g/cm or ^more and 3.6 g/cm ^or less, more preferably 2.7 g/cm ^{or more} and 3.5 g/cm ^or less, and even more preferably 2.9 g/cm ^{or more} and 3.4 g/cm or ^less .

The density of the zirconia calcined body can be calculated by dividing the mass of the zirconia calcined body by the volume of the zirconia calcined body. The density of the zirconia calcined body can be determined, for example, by cutting 10 mm square specimens (n=3) from any desired locations on the zirconia calcined body while varying the cutting position, measuring the mass and volume of the resulting specimens, calculating the arithmetic mean of the measured values, and then using the arithmetic mean value to calculate the density using the above formula.

The average primary particle size of the particles in the zirconia calcined body of the present invention is preferably 40 nm or more, more preferably 45 nm or more, and even more preferably 50 nm or more.
The average primary particle size of the particles in the zirconia calcined body of the present invention is preferably 110 nm or less, more preferably 105 nm or less, and even more preferably 100 nm or less, from the viewpoints of excellent light transmissivity and excellent machinability of a zirconia sintered body obtainable by short-time sintering.
The average primary particle size of the particles in the zirconia calcined body may be within any of the above-mentioned ranges, and is preferably 40 to 110 nm, more preferably 45 to 105 nm, and even more preferably 50 to 100 nm.

The method for measuring the average primary particle size of particles in the zirconia calcined body is as described in the Examples section below.

The zirconia calcined body of the present invention satisfies the following formula (1) when comparing ΔL ₁ *(W-B) for a first sintered body produced by sintering at 1550°C for 120 minutes with ΔL ₂ *(W-B) for a second sintered body produced by sintering at 1550°C for 10 minutes.
ΔL ₂ *(WB)/ΔL ₁ *(WB)≧0.85 (1)
In the zirconia calcined body of the present invention, ΔL ₂ *(WB)/ΔL ₁ *(WB) is more preferably 0.86 or more, and even more preferably 0.88 or more.

The ΔL ₂ *(W−B) is preferably 13 or more, more preferably 14 or more, and even more preferably 15 or more.

The in-line light transmittance of the second sintered body is preferably 0.7% or more, and more preferably 0.8% or more.

In one embodiment, the zirconia calcined body may not include a zirconia calcined body having a porosity of 15 to 30%. In other words, the zirconia calcined body according to the present invention may exclude a zirconia calcined body having a porosity of 15 to 30%. The porosity is a value calculated from the following formula:
Porosity (%) = pore volume / (pore volume + skeleton volume) × 100
The pore volume is a value determined by measuring the number of interconnected pores, excluding closed pores, having a diameter of about 5 nm to 250 μm by mercury intrusion porosimetry. The skeletal volume is a value calculated from the true density measured by gas-phase displacement spectroscopy. The skeletal volume is a value calculated by the formula: skeletal volume (cm ³ /g) = 1 / true density (g/cm ³ ).
The pore volume and skeletal volume can be measured using a fully automatic multifunction mercury porosimeter (POREMASTER, manufactured by Anton Paar Japan K.K.) with a zirconia calcined body cut into a rectangular column (5 mm x 5 mm x 5 mm) as a sample (measurement conditions: mercury surface tension: 480 erg/cm ² , contact angle: 140°, discharge contact angle: 140°, pressure: 0 to 50,000 psia), and the skeletal volume can be calculated by measuring the true density using a dry automatic density meter (AccuPyc II 1340, manufactured by Shimadzu Corporation).

The zirconia calcined body of the present invention may contain additives other than zirconia and stabilizers, as long as the effects of the present invention are achieved. Examples of such additives include colorants (including pigments, composite pigments, and fluorescent agents), binders, dispersants, emulsifiers, antifoaming agents, pH adjusters, lubricants, and translucency adjusters. One type of additive may be used alone, or two or more types may be used in combination.

The pigment may be, for example, an oxide of at least one element selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Y, Zr, Sn, Sb, Bi, Ce, Sm, Eu, Gd, and Er.

Examples of the composite pigment include (Zr, V) _O2 , Fe(Fe _{, Cr)2O4, (Ni, Co, Fe)(Fe, Cr)2O4.ZrSiO4 , and ( Co} , _Zn ) _Al2O4 .

The zirconia calcined body of the present invention may contain a fluorescent agent. When the zirconia calcined body contains a fluorescent agent, the zirconia sintered body has fluorescence. There are no particular restrictions on the type of fluorescent agent, and one or more fluorescent agents that can emit fluorescence when exposed to light of any wavelength can be used.
The fluorescent agent may contain a metal element. Examples of the metal element include Ga, Bi, Ce, Nd, Sm, Eu, Gd, Tb, Dy, and Tm. The fluorescent agent may contain one of these metal elements alone or two or more of them. Among these metal elements, Ga, Bi, Eu, Gd, and Tm are preferred, and Bi and Eu are more preferred.
Examples of the fluorescent agent include oxides, hydroxides, _acetates , and nitrates of the above-mentioned metal elements. The fluorescent agent may also be _Y2SiO5 : _Ce , _Y2SiO5 :Tb _, (Y, _Gd , Eu) _BO3 , _Y2O3 :Eu, YAG:Ce, _ZnGa2O4 : _Zn , _BaMgAl10O17 :Eu, etc.

The content of the fluorescent agent in the zirconia calcined body is not particularly limited and can be adjusted as appropriate depending on the type of fluorescent agent or the application of the zirconia sintered body. However, from the viewpoint of favorable use as a dental prosthesis, the content of the fluorescent agent is preferably 0.001% by mass or more, more preferably 0.005% by mass or more, and even more preferably 0.01% by mass or more, calculated as the oxide of the metal element contained in the fluorescent agent, relative to 100% by mass of zirconia contained in the zirconia calcined body.
Furthermore, the content of the fluorescent agent is preferably 1 mass % or less, more preferably 0.5 mass % or less, and even more preferably 0.1 mass % or less, calculated as the oxide of the metal element contained in the fluorescent agent. When the content is equal to or greater than the lower limit, the fluorescence is not inferior to that of human natural teeth, and when the content is equal to or less than the upper limit, the deterioration of the translucency and mechanical strength of the zirconia sintered body can be suppressed.

Examples of binders include polyvinyl alcohol, methyl cellulose, carboxymethyl cellulose, acrylic binders, wax binders, polyvinyl butyral, polymethyl methacrylate, and ethyl cellulose.
In order to improve translucency, the content of the binder in the zirconia composition of the present invention is preferably 10% by mass or less, more preferably 5% by mass or less, and even more preferably 3% by mass or less, relative to 100% by mass of zirconia.

Plasticizers include, for example, polyethylene glycol, glycerin, propylene glycol, and dibutyl phthalate.

Dispersants include, for example, ammonium polycarboxylate (such as triammonium citrate), ammonium polyacrylate, acrylic copolymer resin, acrylic ester copolymer, polyacrylic acid, bentonite, carboxymethyl cellulose, anionic surfactants (such as polyoxyethylene alkyl ether phosphate esters, such as polyoxyethylene lauryl ether phosphate esters), nonionic surfactants, oleic glycerides, amine salt surfactants, oligosaccharide alcohols, and stearic acid.

Emulsifiers include, for example, alkyl ethers, phenyl ethers, sorbitan derivatives, and ammonium salts.

Examples of antifoaming agents include alcohol, polyether, polyethylene glycol, silicone, and wax.

Examples of pH adjusters include ammonia and ammonium salts (including ammonium hydroxides such as tetramethylammonium hydroxide).

Lubricants include, for example, polyoxyethylene alkylate ether and wax.

Examples of the light transmittance adjusting agent include aluminum oxide (Al ₂ O ₃ ), titanium oxide (TiO ₂ ), silicon dioxide (SiO ₂ ), zircon, lithium silicate, and lithium disilicate.

[Method of manufacturing zirconia calcined body]
The zirconia calcined body of the present invention can be produced by firing (calcining) the above-described zirconia composition (for example, a molded body) to a degree that does not result in sintering of the zirconia particles together.
In this specification, the zirconia composition is a composition containing zirconia powder and a powder of a stabilizer capable of suppressing the phase transition of zirconia.
The zirconia composition may be, for example, a molded compact.
The molded body is formed by applying an external force to a powder containing zirconia-based particles. The zirconia composition and zirconia molded body are not necked (adhered) because they are not yet fired.

In one embodiment, there is mentioned a method for producing a calcined zirconia body, which comprises firing a zirconia composition containing a cubic zirconia powder containing a stabilizer capable of suppressing a phase transition of zirconia, the zirconia powder including two or more types of zirconia powder having different contents of the stabilizer, at a temperature that does not reach a sintering temperature.
The zirconia composition to be fired may be any of the zirconia compositions of the embodiments.

By calcining the zirconia composition, organic matter is removed, the desired amount of stabilizer is dissolved in the zirconia, and a calcined zirconia body with necked primary particles is obtained.

The temperature at which the zirconia composition is fired (maximum calcination temperature) is preferably 200°C or higher, more preferably 300°C or higher, and even more preferably 400°C or higher, from the viewpoints of obtaining excellent translucency in a short time in the subsequent sintering step, being able to remove organic substances, and not adversely affecting the subsequent sintering step.
The maximum calcination temperature is preferably 1200°C or lower, more preferably 1100°C or lower, further preferably 1000°C or lower, and particularly preferably 900°C or lower.
The maximum calcination temperature may be within any of the above-mentioned ranges, and is preferably 200 to 1200°C, more preferably 300 to 1100°C, and even more preferably 400 to 900°C.
In some embodiments, the calcination temperature may be 900 to 1200° C. in order to increase the hardness of the calcined body.
The pressure during calcination is not particularly limited, and may be atmospheric pressure.

The time for treatment at the maximum calcination temperature (calcination time) is preferably 30 minutes or more, more preferably 120 minutes or more, which makes it possible to remove organic substances and easily avoid adverse effects in the subsequent sintering step.
The calcination time is preferably 360 minutes or less, more preferably 240 minutes or less. By setting the calcination time to 240 minutes or less, the diffusion distance of the stabilizer can be reduced and the concentration gradient of the stabilizer can be maintained, which is preferable because excellent translucency can be obtained in a short time in the subsequent sintering step.
The calcination time may be within any of the above-mentioned ranges, and is preferably, for example, 30 to 360 minutes, and more preferably 120 to 240 minutes.

[Zirconia sintered body]
Next, the zirconia sintered body of the present invention will be described.
The zirconia sintered body of the present invention can be produced using a zirconia calcined body. Specifically, the zirconia sintered body of the present invention can be obtained, for example, by sintering the above-mentioned zirconia calcined body.

The stabilizer content in the zirconia sintered body of the present invention is the same as the stabilizer content in the zirconia calcined body.

The zirconia sintered body of the present invention satisfies the above formula (1) when the first translucency ΔL ₁ *(WB) of a zirconia sintered body produced by sintering at 1550°C for 120 minutes is compared with the second translucency ΔL ₂ *(WB) of a zirconia sintered body produced by sintering at 1550°C for 10 minutes.
Therefore, the zirconia sintered body of the present invention can maintain high translucency at the same level as that achieved by long-term sintering after sintering for a short period of time, i.e., a holding time at the maximum sintering temperature of 10 minutes or less.

The zirconia sintered body of the present invention may contain a fluorescent agent. The fluorescent agent is the same as the fluorescent agent in the zirconia calcined body. The fluorescent agent may be contained alone or in combination of two or more types.
In this specification, with regard to the fluorescent agent, "based on 100% by mass of zirconia contained in the zirconia calcined body" can be read as "based on 100% by mass of zirconia contained in the zirconia sintered body."
The same applies to the colorant and the light transmittance adjusting agent.

The zirconia sintered body of the present invention may contain a colorant. Examples of colorants include those similar to those used in the zirconia calcined body.

There are no particular restrictions on the colorant content in the zirconia sintered body, and it can be adjusted as appropriate depending on the type of colorant and the application of the zirconia sintered body. However, from the perspective of favorable use as a dental prosthesis, the content, calculated as the oxide of the metal element contained in the colorant, relative to 100% by mass of zirconia contained in the zirconia sintered body is preferably 0.001% by mass or more, more preferably 0.005% by mass or more, and even more preferably 0.01% by mass or more. Furthermore, the content of the colorant, calculated as the oxide of the metal element contained in the colorant, is preferably 5% by mass or less, more preferably 1% by mass or less, even more preferably 0.5% by mass or less, and may be 0.1% by mass or less, or even 0.05% by mass or less.

In order to adjust the translucency of the zirconia sintered body of the present invention, the zirconia sintered body of the present invention may contain a translucency adjuster. Examples of the translucency adjuster include the same ones as those used in the zirconia calcined body.

There are no particular restrictions on the content of the translucency adjuster in the zirconia sintered body, and it can be adjusted as appropriate depending on the type of translucency adjuster and the application of the zirconia sintered body. However, from the perspective of favorable use as a dental prosthesis, it is preferable that the content be 0.1% by mass or less relative to 100% by mass of zirconia contained in the zirconia sintered body.

[Method of manufacturing zirconia sintered body]
The method for producing the zirconia sintered body of the present invention includes a method for producing a zirconia sintered body by sintering the above-mentioned zirconia calcined body.
The method for producing the zirconia sintered body preferably includes a step of sintering the above-mentioned zirconia calcined body under normal pressure at a temperature higher than 900°C and not higher than 1700°C (hereinafter also referred to as the "sintering step"). By using such a production method, it is possible to easily produce the zirconia sintered body of the present invention, which can maintain high translucency equivalent to that achieved by long-term sintering after sintering for a short period of time, i.e., a holding time at the maximum sintering temperature of 10 minutes or less.

The zirconia sintered body of the present invention can also be produced by sintering the zirconia calcined body of the present invention under atmospheric pressure.

When a sintered body is produced by sintering the zirconia calcined body of the present invention, the sinterable temperature (for example, the maximum sintering temperature) is preferably set at a condition that maximizes the translucency of the zirconia sintered body.
From the viewpoint of easily obtaining the desired zirconia sintered body under normal pressure, the sinterable temperature is preferably higher than 900°C, more preferably 1000°C or higher, and even more preferably 1050°C or higher, and is preferably 1700°C or lower, more preferably 1650°C or lower, and even more preferably lower than 1600°C.
In a preferred embodiment, a method for producing a zirconia sintered body includes sintering a zirconia calcined body under atmospheric pressure at a temperature higher than 900°C and not higher than 1560°C, because the zirconia sintered body has excellent translucency even at a lower sintering temperature in a short time. Compared to conventional techniques, the method has industrial advantages because the zirconia sintered body has excellent translucency even at a lower sintering temperature in a short time.
By setting the sinterable temperature to the above lower limit or higher and the above upper limit or lower, sintering can be sufficiently carried out, and a dense sintered body can be easily obtained. In addition, by setting the sinterable temperature to the above upper limit or lower, deactivation of the fluorescent agent can be suppressed.

When producing a sintered body, there is no particular limit to the sintering time as long as the holding time at the maximum sintering temperature is 10 minutes or less, but in order to be able to produce the desired zirconia sintered body efficiently and stably, the holding time at the sinterable temperature (e.g., the maximum sintering temperature) is preferably 10 minutes or less, more preferably 9 minutes or less, even more preferably 8 minutes or less, even more preferably 7 minutes or less, particularly preferably 6 minutes or less, and most preferably 5 minutes or less. The holding time is preferably 1 minute or more, and more preferably 2 minutes or more.

When producing a sintered body, the sintering time required to produce the sintered body can be shortened without reducing the translucency of the zirconia sintered body produced. In particular, the holding time at the maximum sintering temperature required to produce the sintered body can be shortened to 10 minutes or less. This improves production efficiency, and when the zirconia calcined body of the present invention is applied to dental products, it is possible to shorten the time required from determining the dimensions of the dental product to be used in treatment and cutting it to making the dental product ready for treatment, thereby reducing the time burden on patients. It also reduces energy costs.

The heating rate and temperature drop rate in the sintering process are preferably set so as to shorten the time required for the sintering process. For example, the heating rate can be set so as to reach the maximum sintering temperature in the shortest time possible, depending on the performance of the firing furnace. The heating rate up to the maximum sintering temperature can be, for example, 10°C/min or more, 50°C/min or more, 100°C/min or more, 120°C/min or more, 150°C/min or more, or 200°C/min or more. The temperature drop rate is preferably set so as to prevent defects such as cracks from occurring in the sintered body. For example, after heating is complete, the sintered body can be allowed to cool at room temperature.

In the present invention, sintering can be carried out using a sintering furnace. There are no particular restrictions on the type of sintering furnace, and for example, electric furnaces and degreasing furnaces used in general industry can be used. In particular, when used for dental material applications, in addition to conventional sintering furnaces for dental zirconia, dental porcelain furnaces with relatively low sintering temperatures (e.g., maximum sintering temperatures) can also be used.

The zirconia sintered body of the present invention can be easily produced without HIP treatment, but by performing HIP treatment after sintering under atmospheric pressure as described above, it is possible to further improve translucency and mechanical strength.

[Uses of zirconia sintered body]
There are no particular limitations on the uses of the zirconia sintered body of the present invention.
The zirconia sintered body of the present invention has excellent translucency and excellent in-line light transmittance, and is therefore particularly suitable as a dental material for dental prostheses and the like. In particular, it is extremely useful not only as a dental prosthesis to be used in the cervical region of teeth, but also as a dental prosthesis to be used on the occlusal surfaces of molars and the incisal edges of front teeth.
The zirconia sintered body of the present invention is extremely useful as a dental prosthesis, particularly for use on the incisal ends of front teeth.

The present invention includes embodiments that combine the above configurations in various ways within the scope of the technical concept of the present invention, as long as the effects of the present invention are achieved.

Next, the present invention will be explained in more detail using examples, but the present invention is not limited to these examples in any way, and many modifications within the technical scope of the present invention are possible for those skilled in the art.

[Raw material powder]
Monoclinic 0Y: zirconia powder ("TZ-0" manufactured by Tosoh Corporation)
Monoclinic 2.5Y: Produced by the method described in Comparative Example 7 below.
Tetragonal 3Y: zirconia powder with 3 mol% yttria solid solution ("TZ-3Y-E" manufactured by Tosoh Corporation)
Cubic 5.5Y: A zirconia powder containing 5.5 mol % yttria as a solid solution was produced in Production Example 1 below.
Cubic 6Y: zirconia powder with 6 mol% yttria solid solution ("TZ-6Y" manufactured by Tosoh Corporation)
Cubic 10Y: A hydrated zirconia sol obtained by hydrolyzing an aqueous zirconium oxychloride solution and yttria were mixed and dried in such a proportion that the yttria content relative to the total moles of zirconia and yttria in the resulting yttria-containing zirconia powder was 10 mol %. The resulting powder was then heat-treated in air at 1,160°C for 2 hours to obtain an yttria-containing zirconia powder having an yttria content of 10 mol % relative to the total moles of zirconia and yttria.
Cubic 15Y: A hydrated zirconia sol obtained by hydrolyzing an aqueous zirconium oxychloride solution and yttria were mixed and dried in an adjusted ratio so that the yttria content relative to the total moles of zirconia and yttria in the resulting yttria-containing zirconia powder was 15 mol %. The resulting powder was then heat-treated in air at 1,160°C for 2 hours to obtain an yttria-containing zirconia powder having an yttria content of 15 mol % relative to the total moles of zirconia and yttria.
Yttria: Undissolved yttria powder ( _Y2O3 manufactured by Treibacher _Industrie AG)

<Production Example 1>
A zirconia powder containing 5.5 mol % yttria as a solid solution was produced by the following method.
A hydrated zirconia sol obtained by hydrolyzing an aqueous zirconium oxychloride solution and yttria were mixed and dried in such a proportion that the yttria content relative to the total moles of zirconia and yttria in the resulting yttria-containing zirconia powder was 5.5 mol %. The resulting powder was then heat-treated in air at 1,160°C for 2 hours to obtain an yttria-containing zirconia powder having an yttria content of 5.5 mol % relative to the total moles of zirconia and yttria.

<Examples 1 to 4 and Comparative Examples 1 to 3 and 6>
[Preparation of zirconia compositions of Examples 1 to 4 and Comparative Examples 1 to 3 and 6]
To prepare the granular raw material compositions of each of the Examples and Comparative Examples, the above raw material powders were mixed to obtain the compositions shown in Table 1, water was added, and the mixture was wet-pulverized and mixed in a ball mill for 20 hours. A binder was added to the pulverized slurry, and the mixture was dried in a spray dryer to obtain granular zirconia compositions.
In Tables 1 and 2 below, "total yttria content" represents the content of yttria relative to the total moles of zirconia and yttria.

<Comparative Examples 4 and 5>
[Preparation of zirconia compositions of Comparative Examples 4 and 5]
The above raw material powders were mixed to obtain the composition shown in Table 1, water was added, and the mixture was wet-pulverized and mixed in a ball mill for 20 hours. A binder was added to the pulverized slurry, and the mixture was dried in a spray dryer to obtain granules. The granules were heated to 1000°C at a rate of 10°C/min and held for 2 hours. Water was then added, and the mixture was wet-pulverized and mixed in a ball mill for 20 hours. A binder was added to the pulverized slurry, and the mixture was dried in a spray dryer to obtain a granular zirconia composition.

Comparative Example 7
[Preparation of zirconia composition of Comparative Example 7]
A zirconia composition was prepared in the same manner as in Example 2 of WO2023/042893.
Specifically, the method was as follows.
A hydrated zirconia sol obtained by hydrolyzing an aqueous solution of zirconium oxychloride was mixed with yttria ( _Y2O3 ) so that the yttria content was 2.5 mol%, dried, and then heat-treated in an air atmosphere at 1160°C for 2 hours to obtain a calcined powder (monoclinic _2.5Y : yttria-containing zirconia powder with an yttria content of 2.5 mol%) having a matrix of zirconia containing yttrium with an yttria content of 2.5 mol% (yttrium-stabilized zirconia).
The obtained calcined powder and pure water were mixed and pulverized for 18 hours in a ball mill using 2 mm diameter beads as the pulverizing medium to obtain a slurry containing a powder (yttrium-stabilized zirconia powder) having a matrix of zirconia with an yttria content of 2.5 mol% and a BET specific surface area of 11.9 ^m2 /g, which was designated as Slurry A.
A slurry containing powder with a BET specific surface area of 9.7 m/g and a zirconia matrix containing 5.5 mol% yttria (yttrium-stabilized zirconia) was obtained by the same method as for slurry A, except that yttrium was mixed so that the yttria content was 5.5 mol% and the mixture was milled for 10 hours. This was designated slurry B. The BET specific surface area of the powder contained in slurry A was 2.2 ^m /g larger than that of the powder contained in slurry ^B.
Slurry A was added to and mixed with stirred slurry B so that the total amount of yttria in the entire powder composition obtained by mixing slurry A and slurry B was 5.2 mol %.
The powder composition was then dried at 110°C under air flow to obtain a powder composition having a matrix of zirconia containing 5.2 mol% yttrium (yttrium-stabilized zirconia) and a BET specific surface area of 9.9 ^m /g. The difference in the amount of stabilizing elements between the two stabilized zirconias (stabilized zirconia powders contained in the two slurries) was 3.0 mol%.

[Preparation of zirconia calcined body]
For each of the Examples and Comparative Examples, a pellet-shaped calcined body was prepared as follows so as to obtain a sample of a zirconia sintered body for evaluating translucency.
First, a cylindrical mold having a diameter of 19 mm was used, and the raw material zirconia composition was placed in the mold so that the thickness of the zirconia sintered body after sintering would be 1.2 mm. Next, the raw material composition was press-molded using a uniaxial press molding machine at a surface pressure of 200 MPa to produce a pellet-shaped molded body. The obtained pellet-shaped molded body was heated to 1000°C at a rate of 10°C/min using a sintering furnace "Noritake Katana (registered trademark) F-1" manufactured by SK Medical Electronics Co., Ltd., and then held for 2 hours. After cooling, a zirconia calcined body was obtained.

[Preparation of zirconia sintered body]
The pellet-shaped calcined bodies obtained were sintered at 1550°C for 120 minutes using a Noritake Katana (registered trademark) F-1 furnace manufactured by SK Medical Electronics Co., Ltd. to obtain a sample for measuring ΔL ₁ *(W-B) of the first sintered body, and a sample for measuring ΔL ₂ *(W-B) of the second sintered body was sintered at 1550°C for 10 minutes.

The properties of the zirconia sintered bodies produced in each example and comparative example were measured using the following methods.

<Method for measuring average primary particle size of particles in zirconia composition>
The average primary particle diameter of each particle (zirconia particles, yttria particles) in the zirconia composition was measured by the following method for measuring the average primary particle diameter of the primary particles constituting the granules obtained by the above-mentioned method for producing the zirconia composition using each raw material powder alone.
The surface of the obtained granular powder was imaged (SEM image) using a scanning electron microscope (product name "VE-9800", manufactured by Keyence Corporation). After the grain boundaries of each particle were noted in the obtained image, the average primary particle size was calculated by image analysis.
The particle diameter was measured using image analysis software (product name "Image-Pro Plus ver. 7.0.1", manufactured by Hakuto Co., Ltd.), and the captured SEM image was binarized, the brightness range was adjusted so that the grain boundaries were clearly visible, and the particles were recognized from the field of view (area).
The particle diameter obtained by Image-Pro Plus is the diameter passing through the center of gravity of the particle, and the diameter passing through the center of gravity of the particle is obtained by measuring the length of a line segment connecting the outlines passing through the center of gravity, which is determined from the outline of the particle, at intervals of 2 degrees around the center of gravity, and averaging the measured values (180 pieces).
In measuring particle size, particles that were not on the edge of the image were measured. "Particles that were not on the edge of the image" refers to particles excluding particles whose outlines did not fit completely within the screen of the SEM photograph (particles whose outlines are interrupted at the top, bottom, left, and right boundary lines). The particle sizes of all particles that were not on the edge of the image were determined by selecting the option to exclude particles on all boundary lines in Image-Pro Plus.
For one sample of each example and comparative example, the particle diameter of each granule in three fields of view was obtained, and the average primary particle diameter was calculated.

<Content (mol %) of stabilizer in zirconia composition and zirconia calcined body>
The stabilizer content (mol %) in the zirconia composition and the zirconia calcined body was measured as the stabilizer content relative to the total moles of zirconia and stabilizer using an X-ray fluorescence analysis (XRF) device (RX3000, manufactured by Matsusada Precision Co., Ltd.).

<Evaluation of Crystalline Phase Ratio of Zirconia Calcined Body>
The tetragonal fraction f _t , cubic fraction f _c , and monoclinic fraction f _m of the calcined zirconia body were determined by analyzing the crystalline phase in the calcined zirconia body.
Specifically, X-ray diffraction measurements were performed using a fully automated horizontal multipurpose X-ray diffractometer (SmartLab, manufactured by Rigaku Corporation) and integrated X-ray analysis software (SmartLab Studio II, manufactured by Rigaku Corporation) under the following conditions, and the area intensity of each peak (area intensity I of peak) was determined.
X-ray source: Cu Kα (λ=1.54186 Å)
Goniometer length: 300 mm
Optical system: focusing method Detector: high-speed one-dimensional X-ray detector (D/teX Ultra250)
Monochromatization: Kβ filter Tube voltage: 40 kV
Tube current: 30mA
Scan axis: 2θ/θ
Measurement range (2θ): 5 to 90°
Scan speed: 0.2°/min Sampling step: 0.01°

The crystalline phase ratio was calculated from the following formulas (2-1), (2-2), and (2-3), respectively, by assigning each peak to a crystalline phase.
Tetragonal crystal ratio f _t (%) = I _t / (I _m + I _t + I _c + I _y ) × 100 (2-1)
Cubic crystal ratio f _c (%) = I _c / (I _m + I _t + I _c + I _y ) × 100 (2-2)
Monoclinic rate f _m (%) = I _m / (I _m + I _t + I _c + I _y ) × 100 (2-3)
(wherein _fm represents the monoclinic fraction (%), _ft represents the tetragonal fraction (%), and _fc represents the cubic fraction (%); in XRD measurement, _Im represents the integrated intensity of the peak near 2θ = 28.2° where the peak top of the main monoclinic peak appears; _It represents the integrated intensity of the peak near 2θ = 30.2° where the peak top of the main tetragonal peak appears; _Ic represents the integrated intensity of the peak near 2θ = 30.1° where the peak top of the main cubic peak appears; and _Iy represents the integrated intensity of the peak near 2θ = 29.2° where the peak top of the main peak of undissolved yttria appears.)
For the measurement, disk-shaped zirconia calcined bodies of each of the examples and comparative examples were used as samples.

<Measurement of the content of yttria not dissolved in zirconia in the zirconia calcined body>
The content f _y of undissolved yttria in the calcined zirconia body was determined by X-ray diffraction (XRD) measurement using CuKα radiation and from the following formula (2-4).
f _y (%) = I _y / (I _m + I _t + I _c + I _y ) × 100 (2-4)
(In the formula, _fy represents the content (%) of undissolved yttria, and in XRD measurement, _Im represents the integrated intensity of the peak near 2θ=28.2° where the peak top of the main monoclinic peak appears, It represents the integrated intensity of the peak near 2θ=30.2° where the peak top of the main tetragonal peak appears, _Ic represents the integrated intensity of the peak near 2θ=30.1° where the peak top of the main cubic peak appears, _{and Iy} represents the integrated intensity of the peak near 2θ=29.2° where the peak top of the main peak of undissolved yttria appears.)

<Measurement of standard deviation of yttrium element distribution in zirconia calcined body>
The yttrium element distribution in the zirconia calcined body was measured using a field emission scanning electron microscope (FE-SEM Reglus 8220, manufactured by Hitachi High-Tech Corporation) and an energy dispersive X-ray analyzer (Aztec Energy X-Max 50, manufactured by Oxford Instruments) under the following conditions, and the yttrium element was observed at 1.923 keV.
The standard deviation (mol %) of the yttrium element of 10 particles derived from the stabilizer was determined.
Measurement magnification: 20,000 times Analysis mode: Point analysis Acceleration voltage: 5 kV
Working distance: 15mm ±1mm
X-ray take-off angle: 30 degrees Dead time: 7%
Measurement time: 100 seconds

<Method for measuring the average primary particle size of particles in zirconia calcined body>
For the zirconia calcined bodies obtained in the Examples and Comparative Examples, images (SEM images) of the surfaces were taken using a scanning electron microscope (product name "VE-9800", manufactured by Keyence Corporation). After the grain boundaries of each crystal particle were noted in the obtained images, the primary particle diameter of each crystal particle was measured by image analysis.
The particle diameter was measured using image analysis software (product name "Image-Pro Plus ver. 7.0.1", manufactured by Hakuto Co., Ltd.), and the captured SEM image was binarized, the brightness range was adjusted so that the grain boundaries were clearly visible, and the particles were recognized from the field of view (area).
The particle size obtained by Image-Pro Plus is the diameter passing through the center of gravity of the particle, and the diameter passing through the center of gravity of the particle is the length of the line segment connecting the outlines passing through the center of gravity determined from the outline of the particle, measured at intervals of 2 degrees around the center of gravity, and the measured values (180 pieces) are averaged.
In measuring particle size, particles that were not on the edge of the image were measured. "Particles that were not on the edge of the image" refers to particles excluding particles whose outlines did not fit completely within the screen of the SEM photograph (particles whose outlines are interrupted at the top, bottom, left, and right boundary lines). The particle sizes of all particles that were not on the edge of the image were determined by selecting the option to exclude particles on all boundary lines in Image-Pro Plus.
For one sample of each example and comparative example, the particle diameters of the crystal particles in three fields of view were obtained, and the average primary particle diameter was calculated.

<Evaluation of machinability of zirconia calcined body (measurement of tool wear amount)>
A disk-shaped calcined body having a thickness of 14 mm and a diameter of 98.5 mm was prepared by the method described in the Examples and Comparative Examples, except that the size of the mold into which the raw material composition was placed was changed.
Based on the three-dimensional NC data, this disk-shaped calcined body was cut using a milling machine "DWX-52DC" manufactured by Kuraray Noritake Dental Co., Ltd. with an unused Katana (registered trademark) drill (manufactured by Kuraray Noritake Dental Co., Ltd., Φ2 mm, not diamond coated) so as to leave a disk with a thickness of 1 mm. After machining, the drill was removed, and the amount of wear on the cutting edge at the tip of the drill was measured using an optical microscope.
The machining pattern was contour machining, from the center of the disk-shaped calcined body to the outside, with a spindle rotation speed of 30,000 rpm, a feed rate of 2000 mm/min, and a machining pitch of Z=0.5 mm, XY=1 mm.

<Measurement of Chipping Rate in Zirconia Calcined Body>
The side of a 1 mm thick disk cut out for measuring the amount of tool wear was photographed using an optical microscope, and the chipped area was painted black, with the other areas being white (binarized).
The chipping rate was expressed as a percentage of the black area relative to the total of the black and white areas.
The area was measured using image analysis software (trade name "Image-Pro Plus", manufactured by Hakuto Co., Ltd.).

The evaluation criteria were as follows (n=4): The arithmetic mean value of n=4 was calculated, and "A,""B," and "C" were classified as pass, and "D" was classified as fail. In the following, "3% or less" refers to the percentage value (chipping rate) of the black area relative to the total area of black and white in the captured image, and means that the arithmetic mean value of the chipping rate calculated for each of the four samples was "3% or less."
<Judgment criteria>
A: 3% or less B: 3% or more but 7% or less C: 7% or more but 10% or less D: More than 10%

<Evaluation of Translucency of Zirconia Sintered Body (Measurement of ΔL*(W−B))>
The calcined bodies obtained in the examples and comparative examples were fired at a maximum sintering temperature of 1550°C for a holding time at the maximum sintering temperature of 120 minutes (120-minute sintering) to produce zirconia sintered bodies (first sintered bodies).
Next, the calcined body prepared in the same manner was fired at a maximum sintering temperature of 1550°C for a holding time at the maximum sintering temperature of 10 minutes (10-minute sintering) to prepare a zirconia sintered body (second sintered body).
The temperature increase and decrease rates were set to be the same for 10-minute sintering and 120-minute sintering.
The two types of zirconia sintered bodies thus obtained were each polished into a flat plate sample having a thickness of 1.20 mm, which was used as a sample for measuring translucency. The translucency of the sample was measured using a spectrophotometer (product name "Crystal Eye") manufactured by Olympus Corporation in measurement mode with a 7-band LED light source.
Specifically, the lightness (LW*) of a zirconia sintered body, which is a flat sample, measured against a white background, and the lightness (LB*) of the same sample measured against a black background using the same measuring device, measuring mode, and light source were measured to determine the light transmittance (ΔL*(W-B)) as the difference between the two (ΔL*=(LW*)-(LB*)) (arithmetic mean of n=3). The L* value is the L* value of the chromaticity (color space) in the L*a*b* color system (JIS Z 8781-4:2013).
The first translucency ΔL ₁ *(W-B) of the first sintered body produced by sintering at 1550°C for 120 minutes and the second translucency ΔL ₂ *(W-B) of the second sintered body produced by sintering at 1550°C for 10 minutes were determined, and the ratio of ΔL 2 *(W-B) to ΔL ₁ *(W-B) (ΔL ₂ *(W-B) _{/ΔL 1 *} (W-B)) was calculated as the rate of change in translucency.
A rate of change in light transmittance of 0.85 or more (85% or more) was considered acceptable.

<Measurement of in-line light transmittance of zirconia sintered body>
The linear light transmittance of the zirconia sintered body at a thickness of 1.0 mm was measured using a turbidity meter (manufactured by Nippon Denshoku Industries Co., Ltd., "Haze Meter NDH 4000"), in which light generated from a light source was transmitted through and scattered by the sample, using an integrating sphere. In this measurement, the linear light transmittance was measured in accordance with ISO 13468-1:1996 and JIS K 7361-1:1997, and the haze was measured in accordance with ISO 14782-1:1999 and JIS K 7136:2000, and the linear light transmittance was measured. For the measurement, the second zirconia sintered body, which was disc-shaped with a diameter of 16 mm and a thickness of 1.0 mm and had both sides mirror-polished, was used as a sample.

The evaluation results of the zirconia calcined body and the zirconia sintered body are shown in Table 2.

As shown in Table 2, in Comparative Examples 1 to 7, the translucency was lowered and did not meet the pass criteria when the holding time at the maximum sintering temperature was 10 minutes, compared to when the holding time at the maximum sintering temperature was 2 hours.
In contrast to this, in Examples 1 to 4, even with a short sintering time of 10 minutes at the maximum sintering temperature, the same level of translucency was obtained as with sintering with a sintering time of 2 hours at the maximum sintering temperature.
Furthermore, in Examples 1 to 4, the zirconia calcined bodies were excellent in machinability and were able to reduce the chipping rate.
Furthermore, Examples 1 to 4 also exhibited excellent linear light transmittance.
As described above, by using the zirconia composition of the present invention, not only can excellent effects be obtained in the zirconia sintered body, but also in the zirconia calcined body, excellent machinability can be obtained and the chipping rate can be reduced, and it has been confirmed that excellent effects can be obtained in both the zirconia sintered body and the zirconia calcined body.

The zirconia composition and zirconia calcined body of the present invention, as well as their manufacturing method, are useful for use as dental materials such as dental prostheses.

Claims (19)

The present invention includes a cubic zirconia powder containing a stabilizer capable of suppressing the phase transition of zirconia,
The zirconia composition, wherein the zirconia powder comprises two or more types of zirconia powder having different contents of the stabilizer. The zirconia powder includes a first zirconia powder having a minimum content of the stabilizer, and a second zirconia powder having a maximum content of the stabilizer,
2. The zirconia composition according to claim 1, wherein the difference between the content of the stabilizer in the first zirconia powder and the content of the stabilizer in the second zirconia powder is 1 to 10 mol %. The zirconia composition according to claim 2, wherein the stabilizer content in the first zirconia powder is 5.2 to 15 mol%, and the stabilizer content in the second zirconia powder is 6.2 to 17 mol%. The zirconia composition according to claim 3, wherein the total content of the stabilizer in the entire zirconia composition is 5.5 to 12 mol% based on the total moles of zirconia and stabilizer. 3. The zirconia composition of claim 1, wherein the stabilizer is yttria ( _{Y2O3 )} . The zirconia composition according to claim 1 or 2, wherein the average primary particle size of the zirconia powder is 40 to 110 nm. The zirconia composition according to claim 1 or 2, wherein when ΔL ₁ *(W-B) of a first sintered body produced by firing at 1550°C for 120 minutes is compared with ΔL ₂ *(W-B) of a second sintered body produced by firing at 1550°C for 10 minutes, the following formula (1) is satisfied:
ΔL ₂ *(WB)/ΔL ₁ *(WB)≧0.85 (1) 8. The zirconia composition according to claim 7, wherein ΔL ₂ *(W−B) is 13 or more. The zirconia composition according to claim 7, wherein the second sintered body has an in-line light transmittance of 0.7% or more. A zirconia calcined body in which zirconia powder is solidified to a degree that does not result in sintering,
Contains a stabilizer capable of suppressing the phase transition of zirconia,
The standard deviation of the stabilizer element distribution is 2 mol% or more and less than 21 mol%,
The cubic zirconia contains 55 to 100% zirconia.
Zirconia calcined body. The zirconia calcined body according to claim 10, wherein the stabilizer is yttria ( _{Y2O3 )} . The zirconia calcined body according to claim 10 or 11, wherein the stabilizer content is 5.5 to 12 mol% based on the total moles of zirconia and stabilizer. 12. The zirconia calcined body according to claim 10, having a density of 2.5 to 3.6 g/cm ³ . The zirconia calcined body according to claim 10 or 11, having an average primary particle size of 40 to 110 nm. The zirconia calcined body according to claim 10 or 11, wherein when ΔL ₁ *(W-B) of a first sintered body produced by firing at 1550°C for 120 minutes is compared with ΔL ₂ *(W-B) of a second sintered body produced by firing at 1550°C for 10 minutes, the following formula (1) is satisfied:
ΔL ₂ *(WB)/ΔL ₁ *(WB)≧0.85 (1) 16. The zirconia calcined body according to claim 15, wherein ΔL ₂ *(W−B) is 13 or more. The zirconia calcined body according to claim 15, wherein the second sintered body has an in-line light transmittance of 0.7% or more. A method for producing a zirconia calcined body, comprising firing the zirconia composition according to claim 1 or 2 to a degree that does not result in sintering. A method for producing a zirconia sintered body, comprising sintering the zirconia calcined body according to claim 10 or 11.