WO2024202735A1 - Silicon nitride powder and resin composition using same - Google Patents

Abstract

This silicon nitride powder contains multiple silicon nitride particles and satisfies formula (1).　(1): 0%≤|X-Y|/Y×100<240.0% Wherein, X (nm) represents the thickness of a SiO₂ coating film as calculated from the total oxygen amount TO (mass%) in the silicon nitride powder, and Y (nm) represents the thickness of the SiO₂ coating film as calculated from the surface oxygen percentage SO (atom%) of the silicon nitride particles.

Description

Silicon nitride powder and resin composition using same

This disclosure relates to silicon nitride powder and a resin composition using the same.

Heat generated by passing current through an electronic component is dissipated via a heat sink. In order to improve the efficiency of heat dissipation, a technique is known in which a heat dissipation material is filled between the electronic component and the heat sink.
One type of heat dissipation material is a resin composition containing a resin and inorganic particles, and it is known that silicon nitride powder can be used as the inorganic particles (for example, Patent Document 1).

Moreover, Patent Document 2 discloses silicon nitride powder for other applications.
Patent Document 2 discloses silicon nitride powder having an internal oxygen content of 0.6 mass % or less as a silicon nitride powder from which a silicon nitride sintered body having high thermal conductivity can be obtained.

Special Publication No. 2022-522814 International Publication No. 2020/203695

When the resin composition comes into contact with moisture, ammonia may be generated by hydrolysis of the silicon nitride powder in the resin composition. Since ammonia may cause dielectric breakdown in the resin composition, the silicon nitride powder used as a filler in the resin composition is required to have low reactivity with water (i.e., to be water resistant).
However, Patent Documents 1 and 2 make no consideration at all about the water resistance of silicon nitride powder.

In view of this situation, one embodiment of the present invention aims to provide a silicon nitride powder that is used as a filler for a resin composition and that exhibits water resistance. Furthermore, another embodiment of the present invention aims to provide a resin composition that uses the silicon nitride powder according to one embodiment.

Aspect 1 of the present invention is
The silicon nitride powder contains a plurality of silicon nitride particles and satisfies the following formula (1):

0%≦｜X-Y｜/Y×100<240.0% (1)

Where:
X (nm) is the thickness of the _SiO2 coating converted from the total oxygen content TO (mass%) of the silicon nitride powder,
Y (nm) is the thickness of the _SiO2 coating converted from the surface oxygen ratio SO (atom%) of the silicon nitride particles.

Aspect 2 of the present invention is
The silicon nitride powder according to aspect 1, which satisfies the following formula (2):

0<X(nm)<50.0 (2)

Aspect 3 of the present invention is
The silicon nitride powder according to aspect 1 or 2, which satisfies the following formula (3):

SO(atom%)/SA( ^m2 /g)>10.1(atom% ^/(m2 /g)) (3)

Here, SA (m ² /g) is the specific surface area of the silicon nitride powder.

Aspect 4 of the present invention is
The silicon nitride powder according to any one of aspects 1 to 3, wherein the total oxygen content TO (mass%) of the silicon nitride powder satisfies the following formula (4):

TO (mass%) <1.21 (4)

Aspect 5 of the present invention is
The silicon nitride powder according to any one of Aspects 1 to 4, wherein the surface oxygen ratio SO (atom %) of the silicon nitride particles satisfies the following formula (5):

SO(atom%)>10.0 (5)

Aspect 6 of the present invention is
A silicon nitride powder according to any one of Aspects 1 to 5, having a specific surface area of less than 2.66 m ² /g.

Aspect 7 of the present invention is
The silicon nitride powder according to any one of Aspects 1 to 6, having a surface roughness Ra of less than 0.97 nm.

Aspect 8 of the present invention is
The silicon nitride powder according to any one of Aspects 1 to 7, wherein the β-formation rate is 65% or more.

Aspect 9 of the present invention is
The silicon nitride powder according to any one of aspects 1 to 8, wherein the ratio (L2/L1) of the total length L2 of the internal boundary lines to the length L1 of the outer edge is 1% or less, and the maximum particle size of the silicon nitride particles is 6.8 μm or more.

Aspect 10 of the present invention is
A resin composition comprising a resin and the silicon nitride powder according to any one of Aspects 1 to 9.

The silicon nitride powder according to one embodiment of the invention can exhibit sufficient water resistance by controlling the ratio of the amount of oxygen present on the particle surface to the amount of oxygen present in the entire powder. In addition, a resin composition according to another embodiment of the present invention can exhibit high water resistance because it uses the silicon nitride powder according to one embodiment.

[Silicon nitride powder]
The present inventors have conducted extensive research to obtain silicon nitride powder that can achieve excellent water resistance. They have found that silicon nitride powder with excellent water resistance can be obtained by significantly localizing oxygen on the surface of the silicon nitride particles that constitute the silicon nitride powder. In order to indirectly know the water resistance of the silicon nitride powder, the present inventors have newly introduced an index that indicates the degree of surface localization of oxygen, and have made it possible to obtain silicon nitride powder with high water resistance based on the index.

The characteristics of the silicon nitride powder according to the embodiment of the present invention are described below.

The silicon nitride powder according to the embodiment includes a plurality of silicon nitride particles and further satisfies the following formula (1):

0%≦｜X-Y｜/Y×100<240.0% (1)

Where:
X (nm) is the thickness of the _SiO2 coating converted from the total oxygen content TO (mass%) of the silicon nitride powder,
Y (nm) is the thickness of the _SiO2 coating converted from the surface oxygen ratio SO (atom%) of the silicon nitride particles.
The method of calculating the thicknesses X and Y will be described in detail later.

The "|X-Y|/Y×100" (%) written in the middle of formula (1) is a new index that indicates the degree to which oxygen is localized on the surface of silicon nitride particles. This is called the "oxygen localization index."
The thickness X is the thickness of the _SiO2 coating calculated from the total oxygen content TO (mass%) of the silicon nitride powder. If a large amount of oxygen is contained inside the silicon nitride particle, the thickness X becomes large.
The thickness Y is the thickness of the _SiO2 coating calculated from the oxygen content (surface oxidation rate SO) contained on the surface of the silicon nitride particle (up to a depth of about 10 nm). Even if a large amount of oxygen is contained inside the silicon nitride particle (parts other than the surface), the effect on the thickness Y is small.

The closer the values of X and Y are to each other (i.e., the less oxygen is contained inside the silicon nitride particles), the closer the oxygen localization index is to 0%. In other words, when most of the total amount of oxygen TO (mass%) contained in the silicon nitride powder is localized on the surface of the silicon nitride particles, the above index approaches 0%.
On the other hand, the greater the difference between the values of X and Y (i.e., the greater the amount of oxygen contained inside the silicon nitride particle), the greater the oxygen localization index. In other words, when the degree of localization of oxygen on the surface is low, the oxygen localization index is a value farther away from 0%.

The present inventors have found that when the oxygen localization index is equal to or greater than 0% and less than 240.0%, the effect of oxygen localizing on the surface is exerted, and the water resistance of the silicon nitride powder can be improved.
The oxygen localization index is preferably 0% or more and 230.0% or less, more preferably 0.01% or more and 220.0% or less, even more preferably 0.1% or more and 200.0% or less, still more preferably 0.5% or more and 180.0% or less, and particularly preferably 1.0% or more and 170.0% or less.

(Regarding total oxygen amount TO (mass%) and thickness X)
The total oxygen content TO (mass%) of the silicon nitride powder refers to the total amount of oxygen in the silicon nitride powder, and is measured by an inert gas fusion-infrared absorption method in accordance with JIS G 1239:2014.

The total oxygen content TO (mass %) of the silicon nitride powder can be set to, for example, less than 1.21 mass % (i.e., satisfying the following formula (4)).

TO (mass%) <1.21 (4)

The total oxygen content TO (mass%) of the silicon nitride powder may be 1.00 mass% or less, 0.70 mass% or less, 0.60 mass% or less, or even 0.50 mass% or less. The lower limit of the total oxygen content TO is not particularly limited, but in order to better exert the effect of water resistance, it is preferably 0.05 mass% or more, more preferably 0.10 mass% or more, even more preferably 0.20 mass% or more, and particularly preferably 0.25 mass% or more.

The method for converting the total oxygen content TO (mass%) of the silicon nitride powder into the thickness X (nm) of the _SiO2 coating is as follows.
First, it is assumed that the silicon nitride particles constituting the silicon nitride powder are composed of spheres made of _{Si3N4 and SiO2} unevenly distributed on the surface, and that the _volume density of _Si3N4 and _SiO2 are the same. In addition, the particle size _D50 of the cumulative 50% from the fine particle side of the cumulative particle size distribution on a volume basis is used as the silicon nitride particle size. The method for measuring D50 will be described later.

The silicon content l (mol%), oxygen content m (mol%), and nitrogen content n (mol%) relative to the total oxygen content TO (mass%) in silicon nitride can be calculated as follows:

(i) Silicon content l (mol %) If all of _the N in n (mol %) comes from _Si3N4 and all of the O in m (mol %) comes from _SiO2 , the silicon content l (mol %) can be calculated using formula (6).

l=3/4×n+1/2×m (6)

(ii) Silicon content (mass %) The silicon content (mass %) can be calculated from the atomic weight of silicon (=28) and the silicon content l (mol %) using formula (7).
Moreover, by substituting equation (6) into equation (7), equation (8) is obtained.

Silicon content (mass%) = 28 × l (7)

Silicon content (mass%) = 28 x 3/4 x n + 28 x 1/2 x m
=21×n+14×m (8)

(iii) Since the atomic weight of nitrogen is 14 and the atomic weight of oxygen is 16, the nitrogen content (mass %) and the oxygen content (mass %) can be calculated from equations (9) and (10), respectively.

Nitrogen content (mass%) = 14 × n (9)
Oxygen content (mass%) = 16 × m (10)

(iv) Assuming that silicon nitride is composed only of silicon, nitrogen, and oxygen, the total mass of silicon nitride can be expressed as shown in equation (11) from equations (8) to (10).
(21×n+14×m)+14×n+16×m (11)

(v) Total oxygen content TO (mass %) If we assume that silicon nitride is composed only of silicon, nitrogen, and oxygen, the total oxygen content TO (mass %) can be calculated by dividing the right side of formula (10) by the left side of formula (11). In other words, the following formula (12) holds for the total oxygen content TO (mass %).

TO (mass%) = 16 x m x 100/{(21 x n + 14 x m) + 14 x n + 16 x m} (12)

(vi) Next, if we assume that silicon nitride is composed only of silicon, nitrogen, and oxygen, equation (13) holds for the total amount of silicon nitride.

(3/4×n+1/2×m)+n+m=100 (13)

From the formulas (12) and (13), the oxygen content m (mol %) and the nitrogen content n (mol %) are calculated as shown in the following formulas (14) and (15), respectively.

m=5/4×TO (14)
n=(800-15×TО)/14 (15)

Next, the proportions (mol %) of the amounts of substance occupied by Si ₃ N ₄ and SiO ₂ in the silicon nitride can be calculated from the following formulas (16) and (17).

(Ratio of the amount of Si originating from Si ₃ N ₄ (mol%))/3=3n/4=(2400-45×TO)/56 (16)
(Ratio of the amount of Si derived from _SiO2 (mol%)) = m / 2 = 5 / 8 × TO (17)

Assuming that the volume densities of _Si3N4 and _SiO2 are equal and that _{a SiO2} coating is formed with a uniform thickness over the entire surface of _{the Si3N4} particle, the following equation (18) holds for the ratio k of the _Si3N4 film thickness to the silicon _nitride particle diameter.

k ³ = (3/4×n)/(3/4×n+1/2×m)
=3×n/(3×n+2×m) (18)

In addition, the film thickness (thickness X) of the _SiO2 coating is expressed by the following formula (19) using the particle diameter D50 (nm).

_SiO2 coating thickness (thickness X) (nm) = D50 × (1-k) / 2 (19)

By substituting equations (14) and (15) into equation (18) to find k, and then substituting that value of k and the value of D50 into equation (19), the thickness of the _SiO2 coating can be calculated.

The thickness X (nm) of the _SiO2 coating, calculated from the total oxygen content TO (mass%) of the silicon nitride powder, is preferably more than 0 nm and less than 50 nm. In other words, it is preferable to satisfy the following formula (2). Such silicon nitride powder can have improved water resistance.

0<X(nm)<50 (2)

The thickness X is more preferably greater than 0 nm and equal to or less than 30 nm, even more preferably 0.5 nm or more and 15.0 nm or less, even more preferably 1.0 nm or more and 11.0 nm or less, even more preferably 3.0 nm or more and 10.0 nm or less, even more preferably 6.1 nm or more and 10.0 nm or less, and particularly preferably 7.1 nm or more and 10.0 nm or less.

(Surface oxygen ratio SO (atom%) and thickness Y)
The surface oxygen ratio SO (atom%) of silicon nitride particles refers to the amount of oxygen localized on the surface of silicon nitride particles contained in silicon nitride powder. XPS measurement of silicon nitride powder (more precisely, silicon nitride particles) is performed to measure the content of each element of Si, N, C, and O present on the silicon nitride particle surface. The surface oxygen ratio SO (atom%) is the content (atom%) of O element when the total content of these four elements is 100 atom%.
That is, SO can be calculated by the following equation (20).

SO=[O]/([Si]+[N]+[C]+[O])×100 (20)

Here, [Si], [N], [C] and [O] are the contents (atom %) of Si, N, C and O present on the surface of the silicon nitride particles.

The measurement target of the XPS measurement (XPS measurement portion) is the portion from the surface of the silicon nitride particle to a depth of 10 nm, and it is considered that the elements contained in the XPS measurement portion are all detected with equal intensity regardless of the depth from the surface.
The XPS measurement of the silicon nitride particle surface is performed, for example, using a JFS-9010 model manufactured by JEOL Ltd., with excitation X-ray: AlKα, X-ray output of 110 W, photoelectron escape angle of 45 degrees, and pass energy of 50 EV to measure the contents of the elements Si, N, C, and O on the silicon nitride particle surface.

The surface oxygen ratio SO (atom %) of the silicon nitride particles is preferably, for example, more than 10.0 atom % (i.e., satisfying the following formula (5)). This increases the amount of localized oxygen present on the surface of the silicon nitride particles, and can improve the water resistance of the silicon nitride powder.

SO(atom%)>10.0 (5)

The surface oxygen ratio SO (atom%) of the silicon nitride particles may be 11.0 atom% or more, 13.0 atom% or more, 15.0 atom% or more, or 20.0 atom% or more. The upper limit of the surface oxygen ratio SO of the silicon nitride particles is not particularly limited, but is, for example, 60.0 atom% or less.

The method for converting the surface oxygen ratio SO (atom%) of the silicon nitride particles into the thickness Y (nm) of the _SiO2 coating is as follows.
First, among the elements measured by XPS, all N elements are derived from _Si3N4 , and all O elements are derived from _SiO2 . It is assumed that the film is composed of spheres made of _{Si3N4 and SiO2} unevenly distributed on the surface, and that _the volume density of _Si3N4 and _SiO2 are the same. It is also assumed that the film thickness of the _SiO2 film and _{the Si3N4 film} are proportional to the Si content contained in each film.
Based on these assumptions, equation (21) is obtained.

The measurement results of the N content [N] (atom%) and the O content [O] (atom%) on the surface of the silicon nitride particles are substituted into the following formula (21) to determine the thickness Y (nm) of the _SiO2 coating.

Y (nm) = 10 x ([O]/2)/(3[N]/4+[O]/2) (21)

In the right side of the formula (21), “10” is the thickness of the XPS measurement part (=10 nm),
"[O]/2" is the content (atom%) of silicon derived from _SiO2 , and "3[N]/ ₄ " is the content (atom%) of silicon derived from _Si3N4 ,
“([O]/2)/(3[N]/4+[O]/2)” is the ratio of the content of silicon derived from _SiO2 to the total content of silicon.

The thickness Y is preferably 1.0 nm or more, more preferably 1.5 nm or more, even more preferably 2.0 nm or more, even more preferably 2.6 nm or more, and particularly preferably 3.6 nm or more.

(SO/SA value)
The value (SO/SA) obtained by dividing the surface oxygen ratio SO (atom%) of the silicon nitride particles by the specific surface area SA ( ^m2 /g) of the silicon nitride powder is preferably more than 10.1 (atom%/( ^m2 /g)) (i.e., satisfying the following formula (3)). If the value of SO/SA is large, that is, if the amount of surface oxygen per unit specific surface area is large, the oxide film will become thicker, and it is expected that the water resistance will improve.

SO(atom%)/SA( ^m2 /g)>10.1(atom% ^/(m2 /g)) (3)

SO/SA is more preferably 11.0 (atom %/(m ² /g)) or more, even more preferably 15.0 (atom %/(m ² /g)) or more, still more preferably 20.0 (atom %/(m ² /g)) or more, and particularly preferably 22.0 (atom %/(m ² /g)) or more.
The method for measuring the specific surface area SA (m ² /g) of the silicon nitride powder will be described later.

(Specific surface area)
The specific surface area SA of the silicon nitride powder according to the embodiment is the BET specific surface area measured by the krypton adsorption method based on JIS Z 8830:2013. The specific surface area SA of the silicon nitride powder is preferably less than 2.66 m ² /g, and the surface of the silicon nitride particles constituting the silicon nitride powder can be covered with a small amount of SiO _2. In addition, for silicon nitride particles containing the same amount of SiO ₂ , the smaller the specific surface area, the thicker the SiO ₂ film formed on the surface can be. Therefore, it is expected that the water resistance of the silicon nitride powder will be improved.
The specific surface area of the silicon nitride powder is more preferably 2.00 ^m2 /g or less, even more preferably 1.50 ^m2 /g or less, and even more preferably less than 1.36 ^m2 /g. The lower limit of the specific surface area is not particularly limited, but may be, for example, 0.01 ^m2 /g or more, 0.10 ^m2 /g or more, or 0.50 ^m2 /g or more.

(Surface roughness Ra)
The surface roughness Ra (arithmetic mean roughness) of the silicon nitride powder is preferably less than 0.97 nm. If the surface roughness Ra is less than 0.97 nm, the surface area is small, so the _SiO2 film becomes thicker and water resistance can be improved. Furthermore, the small surface area reduces the contact area with water, so water resistance can be improved.
The surface roughness Ra is preferably less than 0.97 nm, more preferably 0.95 nm or less, even more preferably less than 0.93 nm, still more preferably 0.90 nm or less, even more preferably 0.80 nm or less, and particularly preferably 0.50 nm or less.

To measure surface roughness Ra, a measurement area of 100 nm x 100 nm is observed on the surface of any silicon nitride particle using a scanning probe microscope (SPM). The surface roughness Ra is calculated from the obtained shape image. The surface roughness Ra is measured for four silicon nitride particles, and the average value is taken as the surface roughness Ra of the silicon nitride powder.

(Beta rate)
The β-phase rate of the silicon nitride powder is preferably 65% or more. When the β-phase rate is 65% or more, the water resistance of the silicon nitride powder can be improved.
The β-formation rate is more preferably 70% or more, further preferably 80% or more, even more preferably 85% or more, and particularly preferably 90% or more.

In this specification, the "β-type rate" refers to the content (volume %) of β-type silicon nitride relative to the total silicon nitride contained in the silicon nitride powder.

In calculating the β-phase ratio, the silicon nitride powder is measured by powder X-ray diffraction, and the diffraction pattern is analyzed by the Gazzara & Messier method (G.P. Gazzara and D.P. Messier, "Determination of Phase Content of _Si3N4 by X _- ray Diffraction Analysis", Am. Ceram. Soc. Bull., 56[9]777-80 (1977)), to calculate the β-phase ratio.

(The maximum particle size of silicon nitride particles having a ratio (L2/L1) of the total length L2 of the internal boundary lines to the length L1 of the outer edge of 1% or less is 6.8 μm or more)
The fewer the grain boundaries and cavities inside the silicon nitride particle, the less moisture will penetrate inside, and the more water resistance can be improved. Therefore, the ratio of the total length L2 of the boundary lines to the length L1 of the outer edge (L2/L1) is introduced as an index of the content of grain boundaries inside the silicon nitride particle. L1 and L2 are obtained by observing the cross section of the silicon nitride particle. Here, the "grain boundary" and "boundary line" refer to a boundary (also called a high-angle grain boundary) whose crystal orientation difference (oblique angle) exceeds 15° as a result of EBSD analysis.

If the length of the outer edge of one silicon nitride particle is L1 and the total length of the boundary lines of that silicon nitride particle is L2, then silicon nitride particles with a small value of L2/L1 are silicon nitride particles with a low content of boundary lines. In this embodiment, silicon nitride particles with an L2/L1 of 1% or less are considered to be silicon nitride particles made of a single crystal (these are referred to as "single crystal particles").

In this embodiment, the maximum particle size of silicon nitride particles with L2/L1 of 1% or less (i.e., single crystal) is preferably 6.8 μm or more, more preferably 10.0 μm or more, and even more preferably 15.0 μm or more. There is no particular upper limit, but it may be, for example, 200.0 μm or less, 100.0 μm or less, or 80.0 μm or less.

(Average aspect ratio of silicon nitride particles)
The silicon nitride particles contained in the silicon nitride powder preferably have an average aspect ratio of the minor axis to the major axis of more than 0.50, more preferably more than 0.60, even more preferably 0.70 or more, and even more preferably 0.72 or more. On the other hand, the average aspect ratio of the silicon nitride particles is preferably less than 0.87, more preferably 0.85 or less, and even more preferably 0.80 or less.

The average aspect ratio of the silicon nitride particles is measured as follows.
SEM images of silicon nitride particles contained in the silicon nitride powder are analyzed using image processing software (e.g., Image J (manufactured by the National Institute of Health)). The maximum particle size of the silicon nitride particles (referred to as the "major axis") is identified, and the particle size in the direction perpendicular to the major axis is regarded as the "minor axis". The major and minor axes are measured for 20 random silicon nitride particles, and the ratio of the minor axis to the major axis (minor axis/major axis) is determined for each particle. The arithmetic mean value of these ratios is regarded as the average aspect ratio of the silicon nitride particles.

(D50: cumulative 50% particle size from the fine side of the cumulative particle size distribution on a volume basis)
The silicon nitride powder preferably has a particle size D50 (hereinafter sometimes simply referred to as "D50") of 1.5 μm or more, more preferably 2.0 μm or more, at the cumulative 50% particle size from the fine particle side of the cumulative particle size distribution based on volume. The upper limit is not particularly limited, but is, for example, preferably 500.0 μm or less, more preferably 300.0 μm or less, and even more preferably 200.0 μm or less.

The D50 of silicon nitride powder is measured by the laser diffraction method. Specifically, the powder dispersed in water is irradiated with a laser beam, and the diffraction is measured to determine each particle size. A measuring device such as the CILAS 1090L can be used.

[Method of producing silicon nitride powder]
A method for producing a silicon nitride powder according to an embodiment of the present invention will be described.
Silicon nitride powder is
(1) synthesizing a silicon nitride composite crystal by a combustion synthesis method under a nitrogen atmosphere using a raw material containing Si;
(2) crushing the silicon nitride synthetic crystals to obtain a coarsely pulverized silicon nitride powder; and (3) finely pulverizing the coarsely pulverized silicon nitride powder to obtain a finely pulverized silicon nitride powder.
In addition, optionally,
(4) A step of heat treating the finely pulverized silicon nitride powder may be included.

Step (1): Step of synthesizing silicon nitride synthetic crystal As the raw material containing Si, for example, Si powder is used.
The average particle diameter D50 of the raw material is, for example, within the range of 2 to 10 μm. This makes it possible to suppress the amount of oxygen impurities and increase the combustion speed to raise the synthesis temperature, thereby obtaining good crystal growth. As an example, the average particle diameter D50 of Si is 5 μm.

The diluent is used to adjust the amount of Si in the raw materials. Separately prepared silicon nitride powder is used as the diluent. The diluent may be either α-type silicon nitride powder or β-type silicon nitride powder, or a mixture of these may be used. The average particle diameter D50 of the diluent is preferably in the range of 0.5 to 2.0 μm. As an example, the average particle diameter D50 of the diluent is 1.0 μm. The amount of diluent added is less than 10 mass% of the entire raw materials (including the diluent). As an example, the diluent is added in an amount of 5 to 8 mass% of the entire raw materials. By adding an amount of diluent within the above range, a predetermined amount of silicon nitride particles with oxygen localized on the surface of the silicon nitride particles can be generated.

In this embodiment, a diluent is mixed into the raw materials and filled into an insulating heat-resistant container. The thermal conductivity of this insulating heat-resistant container is 1 W/mK or less, and the material can be alumina or zirconia, but carbon is preferred in consideration of the inclusion of impurities. After filling with the raw materials, the container is covered with a lid made of the same material as the insulating heat-resistant container. Furthermore, in order to increase the temperature inside the synthesis body during combustion, the thickness of the mixed raw materials is made to be more than 100 mm, preferably more than 100 mm and not more than 150 mm. Combustion synthesis is performed in a nitrogen atmosphere in the range of 0.5 to 1 MPa (for example, 0.9 MPa). By adjusting the pressure range within the above range, efficient synthesis can be achieved while suppressing increases in equipment costs.

When filling the crucible with the mixed raw materials, a layer of powder (silicon nitride) with a thickness of 1 mm to 80 mm is placed on the bottom and sides of the crucible, the mixed raw materials are then filled, and the top surface is covered with a layer of powder with a thickness of 1 mm to 80 mm. By covering the entire surface with powder, the mixed raw materials can be kept warm, and a specified amount of silicon nitride particles with oxygen localized on the surfaces of the silicon nitride particles can be produced.

To promote crystal growth more effectively, a catalyst may be used, for example, about 0.01 to 0.1 mass % of Y ₂ O ₃ , Fe ₂ O ₃ , CaO, Ni, Co, C, etc. is added. In addition, external auxiliary heating in the range of 500° C. to 1700° C. (for example, 1500° C.) is performed to increase the combustion temperature in the combustion synthesis method by self-ignition.

Step (2): Obtaining a coarsely pulverized silicon nitride powder The silicon nitride composite crystal is in the form of an aggregate of multiple silicon nitride particles. In step (2), the silicon nitride composite crystal is crushed to obtain a coarsely pulverized silicon nitride powder. For example, the composite is crushed using a general crushing device such as a hammer mill or a disk mill until it passes through a sieve with a predetermined mesh size (for example, a sieve with a mesh size in the range of 400 μm to 500 μm).

Step (3): Step of obtaining finely pulverized silicon nitride powder The coarsely pulverized silicon nitride powder is further pulverized to obtain finely pulverized silicon nitride powder. The pulverization is carried out using a pulverizing device such as a jet mill or a ball mill. If necessary, the obtained finely pulverized powder may be classified. The classification may be carried out by sieving, wet classification, or the like.
The resulting finely pulverized powder may be used as it is as silicon nitride powder, or the finely pulverized powder may be subjected to a heat treatment step (step (4) described below) before being used as silicon nitride powder.

Step (4): Step of heat treating the finely pulverized silicon nitride powder Optionally, the finely pulverized silicon nitride powder may be heat treated. By heat treating, oxygen can be further localized on the surface, and an oxide film is formed on the surface of the silicon nitride particles. This has the effect of chemically stabilizing the silicon nitride particles, and a silicon nitride powder with excellent water resistance is obtained. The heat treatment is performed in air at 500°C or higher and 1200°C or lower, preferably 550°C or higher and 1100°C or lower, and particularly preferably more than 800°C and 1000°C or lower. The heat treatment time can be appropriately adjusted according to the heat treatment temperature. The heat treatment time is, for example, 5 hours.

In the method for producing silicon nitride powder, the heat generated by the combustion synthesis method is used to synthesize silicon nitride composite crystals, which are then crushed, classified, and finely pulverized to produce the silicon nitride powder according to this embodiment.

[Resin composition]
The silicon nitride powder according to the embodiment of the present invention has excellent water resistance and is therefore suitable as a filler for a resin composition. The resin composition contains a resin and the silicon nitride powder according to the embodiment of the present invention.

The compounding ratio of the silicon nitride powder and the resin according to the embodiment of the present invention can be appropriately determined depending on the purpose and application. As an example, the ratio of the resin to the silicon nitride powder may be 5 to 75 volume % and 95 to 25 volume % relative to the resin composition (composite).
The filling rate of the silicon nitride powder in the resin composition refers to the content (volume %) of the silicon nitride powder when the volume of the resin composition (including the silicon nitride powder) is taken as 100 volume %.

A method for producing the resin composition will be described.
A resin composition can be obtained by mixing silicon nitride powder and a resin using a commonly used known method. For example, when the resin is liquid (such as liquid epoxy resin), the resin composition can be obtained by mixing the liquid resin, silicon nitride powder, and a curing agent, and then curing with heat or ultraviolet light. Known curing agents, mixing methods, and curing methods can be used. On the other hand, when the resin is solid, the silicon nitride powder and the resin are mixed, and then kneaded by a known method such as melt kneading to obtain the desired resin composition.

　The resin used in the resin composition may be a known resin such as an epoxy resin. The type of resin may be selected from thermoplastic resins, thermoplastic elastomers, and thermosetting resins. The resin may be used alone or in combination of two or more types.

Furthermore, these resin compositions may contain, as necessary, one or more of known additives such as plasticizers, curing accelerators, coupling agents, fillers, pigments, flame retardants, antioxidants, surfactants, compatibilizers, weather resistance agents, antiblocking agents, antistatic agents, leveling agents, and release agents, within the scope of the invention that does not impair the effects of the invention.

Below, the embodiments of the present invention will be described in detail with reference to examples carried out to clarify the effects of the embodiments of the present invention. Note that the embodiments of the present invention are not limited in any way by the following examples.

<Preparation of silicon nitride powder>
(Samples Nos. 1 to 3, 5, and 6)
Si powder (particle size = 5 μm) and silicon nitride powder (particle size = 1 μm) prepared separately as a diluent were mixed in a rolling ball mill. The amount of diluent added was 5 to 8 mass% of the total raw material (including diluent). The mixed powder was filled in a carbonaceous heat-insulating heat-resistant container with a layer of powder having a thickness of 1 mm to 80 mm on the bottom and sides so that the raw material layer thickness was more than 100 mm and 150 mm or less, and the raw material layer was further covered with a layer of powder having a thickness of 1 mm to 80 mm or less. Then, a lid made of a carbonaceous heat-insulating heat-resistant material was placed, and synthesis was performed under a nitrogen atmosphere of 0.9 MPa. After synthesis, coarse pulverization (disintegration) was performed in a mortar until the powder passed through a sieve with a predetermined opening. The openings of the sieve for each sample are shown in Table 1.

The obtained coarsely pulverized powder was further finely pulverized using a Nano Jet Mizer (manufactured by Aisin Nano Technologies Co., Ltd.). For the fine pulverization, a Nano Jet Mizer with a model number shown in Table 1 was used. The obtained finely pulverized powder was classified by the method shown in Table 1. Note that classification was not performed for sample No. 2.
The resulting classified powders (samples No. 1, 3, and 6) and finely pulverized powder (sample No. 2) were then placed in an alumina crucible and heat-treated in an air atmosphere using a small programmable electric powder furnace (MMF series manufactured by AS ONE) under the conditions (heat treatment temperature, heat treatment time) listed in Table 1 to obtain silicon nitride powders (samples No. 1 to 3 and 6). The classified powder (sample No. 5) was not heat-treated and was used as it was as silicon nitride powder (sample No. 5).

Various measurements were carried out on the obtained samples Nos. 1-3 and 5-6 (Examples) and, as a comparative example, a commercially available silicon nitride powder (manufactured by Aldrich, silicon nitride (predominantly β-phase, ≦10 micron primary particle size, product code 248622); hereafter referred to as "Sample No. 4").

(1) Measurement of the β-phase ratio of silicon nitride powder The diffraction pattern of the sample (silicon nitride powder) was obtained using a powder X-ray diffractometer (manufactured by Rigaku Denki Co., Ltd.) under the following measurement conditions.
・X-ray source: CuKα rays ・X-ray output: 45kV, 200mA
・Graphite monochromator ・Diffraction angle (2θ): Step scan in the range of 2 to 90° in 0.02° increments ・Scanning speed: 21.7 deg/min

When a sample contained components other than silicon nitride, the peaks of those components were compared with the corresponding peaks of standard samples of those components to determine the proportions of those components. For samples No. 1 to 6, the powder X-ray diffraction patterns obtained confirmed that the samples were composed only of alpha-type silicon nitride and beta-type silicon nitride. The proportion of beta-type silicon nitride in the sample (beta ratio) was then calculated using the Gazzara & Messier method.

(2) The maximum particle size of silicon nitride particles having an L2/L1 ratio of 1% or less (the maximum particle size of single crystal particles)
A sample for cross-sectional observation was prepared using the sample (silicon nitride particles). In preparing the sample for cross-sectional observation, the silicon nitride particles were embedded in resin, and then the resin and silicon nitride particles were cut with a diamond cutter. Thereafter, Pt was vapor-deposited on the cross-section as a protective film, the cross-section was prepared by Ar ion milling, and the sample was fixed to the SEM sample stage with Cu double-sided tape, and SEM-EBSD measurement was performed without vapor deposition. The observation position was determined so that two or more silicon nitride particles were completely contained within the observation area (i.e., two or more silicon nitride particles were not in contact with the frame of the observation area). The measurement was performed with β-type silicon nitride particles.

The following equipment was used for sample pretreatment and EBSD measurement.
・Equipment used: Ion milling device: E-3500 (manufactured by Hitachi High-Tech Corporation)
Ion sputtering device: E-1030 (manufactured by Hitachi, Ltd.)
Schottky scanning electron microscope: SU5000 (Hitachi High-Tech Corporation)
Backscattered electron diffraction device: Velocity (manufactured by METEK Corporation)

The conditions for the EBSD measurement were as follows:
・Measurement area: 500.0μm x 400.0μm
Acceleration voltage: 15.0 kV
Magnification: x500
Vacuum degree: 30 Pa

In the obtained EBSD image, two or more silicon nitride particles that were not in contact with the frame of the observation area were selected, and the outer edge length L1 of each silicon nitride particle was averaged using image processing software Image J (manufactured by the National Institute of Health). The total boundary length L2 was also calculated. The "total boundary length L2" is the sum of the boundary lines contained inside the silicon nitride particle, and does not include the outer edge of the silicon nitride particle. The total boundary length L2 was calculated by adding the total length of the grain boundaries inside the silicon nitride particle and the total length of the inner walls of the cavity (if there is a cavity inside the silicon nitride particle).

Such measurements were carried out once for 20 random silicon nitride particles, and silicon nitride particles with an L2/L1 ratio of 1% or less were regarded as single crystal particles, and the length of their major axis was measured and recorded as the "maximum particle size of the single crystal particle." If the 20 silicon nitride particles measured contained multiple single crystal particles, the arithmetic average of their maximum particle sizes was calculated.

(3) Measurement of Surface Roughness Ra The surface roughness Ra was measured using a scanning probe microscope SPA300HV manufactured by Seiko Instruments Inc. The measurement conditions were as follows:
Probe station/unit SPI4000/SPA300HV
Cantilever: SI-DF20
Scanner: 20 μm
Data type: Shape image Observation mode: DFM (Dynamic Force Mode Microscope)
Scanning area: 100 nm x 100 nm
Scanning frequency: 0.25Hz
Analysis software: (included with the measuring device)

Five random silicon nitride particles were selected from the image, and a measurement area of 100 nm x 100 nm on the surface of each particle was observed using a scanning probe microscope (SPM). The surface roughness Ra of each particle was calculated from the obtained shape image, and the average value was taken as the surface roughness Ra of the silicon nitride powder.

(4) Measurement of total oxygen content TO of silicon nitride powder The total oxygen content TO (mass%) of the silicon nitride powder was measured by an inert gas fusion-infrared absorption method in accordance with JIS G 1239:2014.

(5) Measurement of Surface Oxygen Ratio SO of Silicon Nitride Particles Using K-Alpha manufactured by Thermo Fisher Scientific, the contents (atom %) of each element of Si, N, C and O on the surface of silicon nitride particles were measured under the following conditions: excitation X-ray: AlKα, X-ray output: 72 W, photoelectron escape angle: 90 degrees, and pass energy: 50 EV.
The measurement results were substituted into the following formula (6) to determine the surface oxygen ratio SO (atom %) of the silicon nitride particles.

SO (atom%) = [O] / ([Si] + [N] + [C] + [O]) x 100 (6)

Here, [Si], [N], [C] and [O] are the contents (atom %) of Si, N, C and O present on the surface of the silicon nitride particles.

(6) Thickness X, Y of _SiO2 Coating, and Oxygen Localization Index Using the method described in the embodiment, the thickness X (nm) of the _SiO2 coating was obtained from the measurement results of the total oxygen content TO (mass%) of the silicon nitride powder, and the thickness Y (nm) of the SiO2 coating was obtained from the measurement results of the surface oxygen ratio SO (atom%) of the silicon _nitride particles.
The D50 of the silicon nitride powder, which is necessary for determining the thickness, was measured as follows.

(Measurement of D50 of silicon nitride powder)
The particle size distribution of the sample (silicon nitride powder) was measured, and the cumulative 50% particle size D50 was determined.
The particle size distribution of silicon nitride powder was measured by laser diffraction. A sample dispersed in water was irradiated with a laser beam, and the diffraction was measured to determine the particle size. The measuring device used was a CILAS 1090L model. The particle size was taken as the equivalent circle particle size. The equivalent circle particle size is the particle size of a perfect circle that has the same area as a projected particle image. The particle size was based on volume.

The oxygen localization index (|X-Y|/Y x 100(%)) was calculated from the calculated thicknesses X and Y.

(7) Ratio of surface oxygen content SO of silicon nitride particles to specific surface area SA of silicon nitride powder (SO/SA)
The surface oxygen ratio SO (atom %) of the silicon nitride particles was divided by the specific surface area SA (m ² /g) of the silicon nitride powder measured by the method described below to obtain SO/SA.

(8) Ratio of the total oxygen content TO of the silicon nitride powder to the specific surface area SA of the silicon nitride powder (TO/SA)
The total oxygen content TO (mass %) of the silicon nitride powder was divided by the specific surface area SA (m ² /g) of the silicon nitride powder measured by the method described below to obtain TO/SA.

(9) Specific surface area SA of silicon nitride powder
The specific surface area of the sample (silicon nitride powder) was measured.
The specific surface area of a powder (solid) was measured by gas adsorption in accordance with JIS Z 8830: 2013, using krypton as the adsorption gas. In the measurement, 1 g of silicon nitride powder was placed in a sample tube, an adsorption/desorption isotherm was obtained, and the specific surface area ( ^m2 /g) was calculated by the multipoint plot method.

(10) Water Resistance Test 2 g of silicon nitride was immersed in 18 g of pure water, heated to 80° C., and held for 72 hours. Thereafter, the concentration of ammonium ions (NH ₄ ⁺ ) contained in the immersion water was measured by ion chromatography in accordance with JIS K 0127:2013.
If the NH ₄ ⁺ concentration is 30 ppm or less, the water resistance of silicon nitride is judged to be excellent (◎), if it is more than 30 ppm but not more than 50 ppm, the water resistance is judged to be good (◯), and if it exceeds 50 ppm, the water resistance is judged to be poor (×).

The measurement results are summarized in Tables 2 and 3.

The measurement results are discussed below.
The silicon nitride powders of samples Nos. 1 to 3 and 5 to 6, which satisfy the requirements of this embodiment, were evaluated as having good (◯) or excellent (◎) water resistance. On the other hand, the silicon nitride powder of sample No. 4, which did not satisfy the requirements of this embodiment, had poor water resistance (×).

This application claims priority from Japanese Patent Application No. 2023-058618, filed on March 31, 2023. Japanese Patent Application No. 2023-058618 is incorporated herein by reference.

Claims (10)

A silicon nitride powder comprising a plurality of silicon nitride particles and satisfying the following formula (1):

0%≦｜X-Y｜/Y×100<240.0% (1)

Where:
X (nm) is the thickness of the _SiO2 coating converted from the total oxygen content TO (mass%) of the silicon nitride powder,
Y (nm) is the thickness of the _SiO2 coating converted from the surface oxygen ratio SO (atom%) of the silicon nitride particles. The silicon nitride powder according to claim 1, which satisfies the following formula (2):

0<X(nm)<50.0 (2)
The silicon nitride powder according to claim 1, which satisfies the following formula (3):

SO(atom%)/SA( ^m2 /g)>10.1(atom% ^/(m2 /g)) (3)

Here, SA (m ² /g) is the specific surface area of the silicon nitride powder. The silicon nitride powder according to claim 1, wherein a total oxygen content TO (mass%) of the silicon nitride powder satisfies the following formula (4):

TO (mass%) <1.21 (4)
The silicon nitride powder according to claim 1, wherein the surface oxygen ratio SO (atom%) of the silicon nitride particles satisfies the following formula (5):

SO(atom%)>10.0 (5)
2. The silicon nitride powder according to claim 1, having a specific surface area of less than 2.66 m ² /g. The silicon nitride powder according to claim 1, having a surface roughness Ra of less than 0.97 nm. The silicon nitride powder according to claim 1, in which the beta conversion rate is 65% or more. The silicon nitride powder according to claim 1, in which the ratio (L2/L1) of the total length L2 of the internal boundary lines to the length L1 of the outer edge is 1% or less, and the maximum particle size of the silicon nitride particles is 6.8 μm or more. A resin composition comprising a resin and the silicon nitride powder according to any one of claims 1 to 9.