JP7578260B2 - Ferrite particles, carrier core material for electrophotographic developer, carrier for electrophotographic developer, and electrophotographic developer - Google Patents

Description

The present invention relates to ferrite particles, carrier core materials for electrophotographic developers, carriers for electrophotographic developers, and electrophotographic developers.

The electrophotographic development method is a method of developing an electrostatic latent image formed on a photoreceptor by attaching the toner in the developer to the electrostatic latent image formed on the photoreceptor. The developers used in this method are divided into two-component developers consisting of toner and carrier, and one-component developers that use only toner. In the past, the cascade method was used as a development method using two-component developers, but currently the magnetic brush method using a magnet roll is the mainstream.

In the magnetic brush method, the carrier and toner are stirred and mixed in a developing box filled with developer, giving the toner an electric charge. The carrier is then transported to the surface of the photoconductor by a developing roll holding a magnet. At that time, the carrier transports the charged toner to the surface of the photoconductor. After a toner image is formed on the photoconductor by electrostatic action, the carrier remaining on the developing roll is collected back into the developing box, stirred and mixed with new toner, and reused for a certain period of time.

Unlike single-component developers, two-component developers allow the magnetic and electrical properties of the carrier itself to be designed separately from the toner, allowing for good control when designing the developer. For this reason, two-component developers are suitable for full-color development devices that require high image quality, and high-speed printing devices that require reliable image maintenance and durability.

Compared to low-speed printing, high-speed printing places a greater agitation load, which makes the carrier more likely to crack or chip, resulting in so-called carrier scattering. Carrier scattering not only causes image defects, but also means that if cracked or chipped carrier adheres to the photoconductor or fixing roller, the sharp edges can damage these.

To address these issues, for example, Patent Document 1 proposes using magnesium ferrite particles containing a predetermined amount of _SiO2 as a carrier core material. It is said that by adding a predetermined amount of _SiO2 , the sintering state can be adjusted and the BET specific surface area can be set within a predetermined range, resulting in a high-strength carrier core material with fewer cracks and chips.

JP 2011-28060 A

However, it is difficult to sufficiently suppress the occurrence of carrier scattering in a high-temperature and high-humidity environment with conventional carrier core materials, and the same is true for the carrier core material disclosed in the above Patent Document 1. In addition, in the carrier using the carrier core material disclosed in the above Patent Document 1, although the addition of SiO ₂ prevents abnormal grain growth and suppresses cracking and chipping during stirring, it is difficult to sufficiently control the internal sintering state. In addition, in the carrier core material disclosed in Patent Document 1, a predetermined degree of internal voids is generated to cause many grain boundaries to exist inside the particles, and the strength of the grain boundaries is increased to prevent a decrease in the mechanical strength of the carrier core material. In other words, in order to cause many grain boundaries to exist in the carrier core material disclosed in Patent Document 1, it is necessary to generate a predetermined degree of internal voids, but the size of the internal voids cannot be sufficiently controlled, and therefore it is considered that the strength of the grain boundaries varies.

The objective of the present invention is to provide ferrite particles, a carrier core material for electrophotographic developer, a carrier for electrophotographic developer, and an electrophotographic developer that are high in strength, have little cracking or chipping, and can suppress the occurrence of carrier scattering.

In order to solve the above problems, the ferrite particles according to the present invention contain 50% by mass or more of a spinel-type crystal phase component represented by the composition formula of (MnO)x(MgO)y(Fe2O3 ₎ z ₍ wherein 15≦x≦50, 2≦y≦35, 45≦z≦60, x+y+z=100 (mol%)), and a crystal phase component consisting of a perovskite-type crystal represented by the composition formula of _SrZrO3 . When the particle size of each particle is expressed as a circle equivalent diameter in a backscattered electron image of the particle cross section taken by a scanning electron microscope, the content of particles having internal voids whose maximum Feret diameter is greater than 15% of the particle size is 4% or less. When a phase composition analysis of the crystal phase constituting the ferrite particles is performed by Rietveld analysis of an X-ray diffraction pattern, the ferrite particles contain 0.05% by mass or more and 4.00% by mass or less of the crystal phase component consisting of the perovskite-type crystal .

The apparent density of the ferrite particles according to the present invention is preferably within the range represented by the following formula.
2.15≦Y≦2.45
In the above formula, Y is the apparent density (g/cm ³ ) of the ferrite particles.

The BET specific surface area of the ferrite particles according to the present invention is preferably within the range represented by the following formula.
0.08≦X≦0.300
In the above formula, X is the BET specific surface area (m ² /g) of the ferrite particles.

The carrier core material for electrophotographic developer according to the present invention is characterized by containing the above-mentioned ferrite particles.

The electrophotographic developer carrier according to the present invention is characterized by comprising the above-mentioned ferrite particles and a resin coating layer provided on the surface of the ferrite particles.

The electrophotographic developer according to the present invention is characterized by containing the above-mentioned electrophotographic developer carrier and toner.

The electrophotographic developer according to the present invention is also preferably used as a refill developer.

The present invention provides ferrite particles, a carrier core material for electrophotographic developer, a carrier for electrophotographic developer, and an electrophotographic developer that are high in strength and capable of suppressing carrier scattering even in high-temperature and high-humidity environments.

FIG. 2 is a diagram showing a cross section of a ferrite particle, and is a diagram for explaining a method for determining the degree of dispersion of Zr in the ferrite particle.

Hereinafter, the embodiments of the ferrite particles, the carrier core material for electrophotographic developer, the carrier for electrophotographic developer, and the electrophotographic developer according to the present invention will be described. In this specification, the ferrite particles, the carrier core material for electrophotographic developer, the carrier for electrophotographic developer, and the electrophotographic developer mean an aggregate of individual particles, that is, powder, unless otherwise specified. First, the embodiments of the ferrite particles will be described. In the following, the ferrite particles according to the present invention will be described as being mainly used as a carrier core material for electrophotographic developer. However, the ferrite particles according to the present invention can be used for various applications such as magnetic inks, magnetic fluids, magnetic fillers, various functional fillers such as fillers for bonded magnets and fillers for electromagnetic wave shielding materials, electronic component materials, etc., and the applications of the ferrite particles are not limited to carrier core materials for electrophotographic developers.

1. Ferrite particles and carrier core material for electrophotographic developer First, an embodiment of the ferrite particles according to the present invention will be described. The ferrite particles according to the present invention contain a crystalline phase component consisting of perovskite crystals represented by the composition formula _RZrO3 (wherein R is an alkaline earth metal element), and are characterized in that, when the particle size of each particle is expressed as a circle equivalent diameter in a cross section in a backscattered electron image of the particle cross section photographed by a scanning electron microscope, the content of particles having internal voids whose maximum Feret diameter is greater than 15% of the particle size is 4% or less.

1-1. Crystal Phase Component Made of Perovskite-Type Crystal First, the crystal phase component made of perovskite-type crystal represented by the composition formula _RZrO3 (wherein R is an alkaline earth metal element) will be described.

As a carrier for a two-component electrophotographic developer suitable for the magnetic brush method, a resin-coated carrier is used, in which a magnetic particle is used as a core material and the surface is coated with a resin. Ferrite particles, which are a magnetic oxide mainly composed of ferric oxide (Fe ₂ O ₃ ), are mainly used as the magnetic particle used as the core material. In order to suppress carrier scattering due to the reduction in the particle size of the carrier, it is required to use ferrite particles with high magnetization and high resistance as the core material.

In recent years, multi-element ferrite particles containing metal elements such as Mg, Mn, Sr, and Ca in addition to Fe are widely used as core materials. When manufacturing ferrite, metal oxides, metal hydroxides, etc. containing metal elements of the desired composition are used as raw materials. To obtain highly magnetized ferrite particles, it is necessary to sufficiently advance the ferritization reaction of the raw materials so that unreacted raw materials that do not exhibit magnetism (hereinafter referred to as "unreacted raw materials") do not remain in the ferrite particles. However, in the case of multi-element ferrite, the ferrite formation temperature and formation speed differ depending on the combination of elements, and the ferrite reaction only proceeds at the contact surface between the raw materials, so it is difficult to completely ferrite the raw materials under general firing conditions. Therefore, unreacted raw materials remain in the ferrite particles.

Furthermore, the raw materials for ferrite contain metals or metal compounds, such as Na, that are not involved in the ferrite reaction as unavoidable impurities. These unavoidable impurities are not magnetic. Therefore, in order to obtain highly magnetized ferrite particles, it is necessary to reduce the amount of unavoidable impurities. However, no matter how pure the raw materials used are, there will always be traces of unavoidable impurities in the raw materials, and it is not realistic to completely remove the unavoidable impurities.

Furthermore, if there are defects in the organizational structure of the ferrite particles, the magnetization of the ferrite particles will decrease. Structural defects that cause a decrease in magnetization include defects within the ferrite particles (e.g., lattice defects, etc.). In the case of multi-element ferrites, the ferrite reaction is more complex than in single-element ferrites, making structural defects more likely to occur. Furthermore, if internal voids occur in the particles, especially if the internal voids have a large maximum Ferret diameter compared to the particle size, the strength of the particles will decrease.

Therefore, to obtain ferrite particles with high magnetization, high resistance, and high strength, it is necessary to reduce the amount of unavoidable impurities in the ferrite particles. However, as mentioned above, it is not realistic to completely remove the unavoidable impurities. Note that the unreacted raw materials (excluding the unavoidable impurities) are metal oxides, so the presence of unreacted raw materials does not cause the ferrite particles to have low resistance.

However, ferrite particles are often polycrystalline bodies that are aggregates of single crystals. Even if the composition of ferrite particles is the same, the magnetic and electrical properties of the ferrite particles change depending on the organizational structure of the ferrite particles. Therefore, the present inventors have focused on the organizational structure of ferrite particles and found that in order to obtain ferrite with high magnetization, high resistance, and high strength, it is important to include a perovskite-type crystal phase component represented by the composition formula _RZrO3 (wherein R is an alkaline earth metal element), and have come up with the present invention.

The reason why the above problem can be solved by including a perovskite crystal phase component represented by the composition formula _RZrO3 (wherein R is an alkaline earth metal element) is not clear, but the inventors of the present invention speculate as follows.

At the grain boundaries of ferrite particles, there are components that are not dissolved in ferrite, such as unreacted raw materials and unavoidable impurities. In addition, these components are pushed out to the grain boundaries as the crystal grains grow, and are also present on the surface of the ferrite particles. When the crystal grains that make up the ferrite particles are large, the grain boundaries are continuously formed inside the ferrite particles like a flow path connecting one point to another on the surface of the ferrite particles. The unavoidable impurities present on the surface and grain boundaries of ferrite particles are easily ionized. Therefore, if unavoidable impurities such as Na are continuously distributed at the grain boundaries continuously formed within the ferrite particles, when moisture adheres to the surface of the ferrite particles, the grain boundaries become like a conductive path, and the resistance of the ferrite particles decreases. However, as mentioned above, it is difficult to completely remove the unreacted raw materials and unavoidable impurities in the ferrite particles.

On the other hand, since the perovskite-type crystal phase component represented by the composition formula _RZrO3 (where R is an alkaline earth metal element) is not dissolved in other crystal phases with different crystal structures such as spinel ferrite phase, the crystal phase component is dispersed in the grain boundaries of the ferrite particles. Therefore, the grain boundary volume of the ferrite particles containing the crystal phase component is relatively increased compared to the case where the crystal phase component is not contained. If the amount of inevitable impurities contained in the ferrite particles is the same, when the grain boundary volume of the ferrite particles increases relatively, the distribution density of the inevitable impurities in the grain boundaries is relatively decreased. In addition to the inevitable impurities, the crystal phase component, which is an insulating material, is present in the grain boundaries. In the ferrite particles according to the present invention, the grain boundaries are distributed in a complex manner within the particles, and the inevitable impurities and insulating materials such as the crystal phase component are distributed discontinuously within the grain boundaries, so that the grain boundaries can be suppressed from functioning as conductive paths. Therefore, it is considered that the ferrite particles according to the present invention can also reduce the environmental dependency of resistance.

In addition, by uniformly dispersing the perovskite crystal phase component represented by the composition formula _RZrO3 (wherein R is an alkaline earth metal element) in the ferrite particles and adopting the manufacturing method described later, it becomes easy to obtain ferrite particles with fewer internal voids as described above, or even if internal voids are present, the maximum Feret diameter is smaller than the particle diameter and the strength is higher. In addition, according to the present invention, since the grain boundary volume can be relatively increased as described above, particles with high grain boundary strength can be obtained even without internal voids. Furthermore, since the perovskite crystal phase component is not solid-dissolved with other crystal phase components with different crystal structures such as spinel ferrite phase, when these crystal phases segregate or grow abnormally, cracks and chips are likely to occur starting from the interface.

In addition, when the grain boundary volume of a ferrite particle is relatively small, the unavoidable impurities are pushed out from the grain boundaries to the surface, resulting in a relatively large amount of unavoidable impurities present on the ferrite particle surface. On the other hand, when the grain boundary volume of a ferrite particle is relatively large, the amount of unavoidable impurities present on the ferrite particle surface can be relatively reduced. This makes it possible to increase the resistance of the ferrite particle. Furthermore, the environmental dependency of the resistance caused by the unavoidable impurities on the ferrite particle surface can be reduced.

Furthermore, the perovskite-type crystal phase component represented by the composition formula _RZrO3 (where R is an alkaline earth metal element) has high insulation properties, and the presence of the crystal phase component at the grain boundary or particle surface can increase the resistance of the ferrite particles. Furthermore, to obtain ferrite particles containing the crystal phase component, a compound containing Zr (e.g., _ZrO2 ) is used as a raw material. For example, when a multi-element ferrite has a composition containing R, the raw material containing R and the compound containing Zr undergo a solid-phase reaction, so that the content of the unreacted raw material in the ferrite particles can be reduced. Therefore, the occurrence of structural defects can be suppressed, and ferrite particles with relatively high magnetization can be obtained, and ferrite particles with few internal voids and a small maximum Feret diameter can be obtained even if internal voids exist.

For these reasons, it is presumed that by using ferrite particles containing a perovskite-type crystal phase component represented by the composition formula _RZrO3 (wherein R is an alkaline earth metal element), it is possible to achieve high magnetization, high resistance, and high strength for the reasons described above.

In the ferrite particles, the perovskite-type crystal phase component represented by the composition formula _RzO3 (wherein R is an alkaline earth metal element) means that the crystal phase component is contained at least inside the ferrite particles, and it is preferable that the crystal phase component is well dispersed inside the ferrite particles, and it is more preferable that the crystal phase component is uniformly dispersed on the surface and inside of the ferrite particles. The ferrite particles according to the present invention will be described in more detail below.

(1) R (alkaline earth metal element)
In the present invention, R is at least one element selected from the group consisting of Ca, Sr, Ba, and Ra, that is, an alkaline earth metal element. The alkaline earth metal element has an ionic radius sufficiently larger than that of zirconium, and forms a perovskite-type crystal structure with zirconium. In the present invention, R is more preferably at least one element selected from the group consisting of Sr, Ca, and Ba. These elements undergo a solid-phase reaction with zirconium under a predetermined temperature condition to form a perovskite-type crystal structure. Therefore, by controlling the firing temperature within a predetermined temperature range in the manufacturing process of the ferrite particles, ferrite particles containing a perovskite-type crystal phase component can be obtained.

(2) Content of Crystal Phase Component When a phase composition analysis of the crystal phase constituting the ferrite particles is performed by Rietveld analysis of an X-ray diffraction pattern, the perovskite-type crystal phase component represented by the composition formula _RZrO3 is preferably contained in an amount of 0.05 mass% or more and 4.00 mass% or less.

When the content ratio of the perovskite type crystal phase component represented by the composition formula of _RZrO3 is within the above range, it becomes easy to uniformly disperse the crystal phase component inside the ferrite particles, suppressing the occurrence of structural defects and making it easier to obtain ferrite particles with high magnetization and high resistance. In addition, by setting it within the above range, it is possible to suppress the segregation of the crystal phase component and the occurrence of internal voids, and obtain ferrite particles with high strength that are less likely to crack or chip when subjected to mechanical stress. As a result, it is also possible to suppress carrier scattering caused by cracks and chips.

To obtain these effects, the ferrite particles preferably contain 0.10% by mass or more of the crystalline phase component, more preferably 0.15% by mass or more, and even more preferably 0.20% by mass or more. In addition, the ferrite particles preferably contain 2.00% by mass or less of the crystalline phase component.

(3) Zirconium content The ferrite particles preferably contain zirconium in an amount of 0.1 mol% or more and 3.0 mol% or less based on the main raw material. By containing zirconium within this range, the content of the perovskite-type crystal phase component represented by the composition formula _RZrO3 is generally within the above range, and ferrite particles with high magnetization, high resistance, and high strength can be obtained. The content of zirconium in the ferrite particles is more preferably 0.2 mol% or more. In addition, the content of zirconium in the ferrite particles is more preferably 2.0 mol% or less, and even more preferably 1.5 mol% or less.

(4) Alkaline earth metal element content The content of the alkaline earth metal element (R) is more preferably 0.1 mol% or more and 3.0 mol% or less with respect to the main raw material. By including the alkaline earth metal element (R) within this range, the content of the perovskite crystal phase component represented by the composition formula of _RZrO3 is generally within the above range, and ferrite particles having high magnetization, high resistance, and high strength can be obtained. The content of the alkaline earth metal element (R) in the ferrite particles is more preferably 0.2 mol% or more. In addition, the content of the alkaline earth metal element (R) in the ferrite particles is more preferably 2.0 mol% or less, and even more preferably 1.5 mol% or less.

1-2. Content of the specified particle As described above, the ferrite particles are characterized in that, when the particle diameter of each particle is expressed as a circle-equivalent diameter in a cross section in a backscattered electron image (cross-sectional SEM photograph) of the particle cross section taken by a scanning electron microscope, the content of particles (hereinafter referred to as "specified particles") having internal voids with a maximum Feret diameter greater than 15% of the particle diameter is 4% or less. Here, the circle-equivalent diameter refers to a value equivalent to the diameter of a perfect circle having the same cross-sectional area as the cross-sectional area of the particle to be measured in the cross-sectional SEM photograph. In addition, the maximum Feret diameter refers to the projected width of the internal void of the particle to be measured, and refers to the maximum side length of the horizontal side length (horizontal Feret diameter) and the vertical side length (vertical Feret diameter) of a rectangle circumscribing the internal void to be measured. Such particles having internal voids with a maximum Feret diameter greater than 15% of the particle diameter have weak particle strength, and are prone to cracking and chipping when mechanical stress is applied during stirring with toner, etc. In the ferrite particles, the content of the specific particles is 4% or less, so that when mechanical stress is applied, cracks or chips are prevented from occurring in the powder as a whole, and carrier scattering due to cracks or chips in the carrier can be suppressed. The content of the specific particles can be measured by the method described below.

The smaller the content of the specified particles, the higher the particle strength of the powder as a whole. Therefore, it is more preferable that the content of the specified particles is 3.8% or less, and even more preferable that it is 3.5% or less.

1-3. Composition The composition of the ferrite particles is not particularly limited as long as it contains a perovskite crystal phase component represented by the composition formula _RZrO3 (wherein R is an alkaline earth metal element). However, from the viewpoint of obtaining ferrite particles with high magnetization and high resistance, it is preferable that the ferrite particles mainly contain a crystal phase component (hereinafter, spinel crystal phase component) made of a spinel crystal represented by the composition formula (MO)a( _{Fe2O3 )} b (wherein M is at least one metal element selected from the group consisting of Fe, Mg, Mn, Cu, Zn and Ni, a+b=100 (mol%)). Ferrite has a crystal structure such as a spinel crystal structure, a magnetoplumbite crystal structure, or a garnet crystal structure, but ferrite with a spinel crystal structure exhibits soft magnetism and is easy to adjust electrical properties such as resistance, making it suitable for use as a carrier core material for electrophotographic developers. In addition, the main component means that when the ferrite contains a plurality of crystal phase components (including the perovskite crystal phase component represented by the composition formula of _RZrO3 (wherein R is an alkaline earth metal element)), the spinel crystal phase occupies the largest proportion among the crystal phase components, and in particular, the spinel crystal phase component is preferably 50 mass% or more, more preferably 60 mass% or more, even more preferably 70 mass% or more, even more preferably 80 mass% or more, and even more preferably 90 mass% or more, and the majority of it may be 95 mass% or more and 99 mass% or less. The content of the spinel phase crystal phase component can be determined by the ratio of metal elements at the time of raw material blending. Generally, spinel ferrite is manufactured by a solid-phase reaction. In the ferritization reaction carried out in the firing process at that time, a solid solution (spinel crystal phase) begins to be generated from about 850 ° C., and the ferritization reaction is completed at about 1250 ° C. By appropriately adjusting the firing temperature, firing time, firing atmosphere, and cooling time in the firing step, the spinel crystal phase can be selectively generated, and the content ratio of the spinel crystal phase in the ferrite particles can be determined.

Furthermore, it is more preferable that the ferrite particles mainly contain a spinel crystal phase component represented by the formula (MnO)x(MgO)y( _Fe2O3 )z (where 15≦x≦50, 2≦y _≦ 35, 45≦z≦60, and x+y+z=100 (mol %)). By combining the above composition with a conventional ferrite production method, it is possible to obtain ferrite with a spinel crystal phase component of 50 mass % or more.

By using a composition containing Mn, it is possible to increase the magnetization on the low magnetic field side. In addition, by using a composition containing Mn, it is possible to prevent reoxidation of ferrite when it is removed from the furnace after the main firing. In particular, by making the Mn content 15 mol% or more, it is possible to suppress the relative increase in the Fe content and the increase in the content ratio of the magnetite component in the ferrite particles. Therefore, it is possible to suppress the decrease in magnetization on the low magnetic field side and the occurrence of carrier adhesion. In addition, since it is easy to adjust the resistance value to a good value for electrophotographic printing, it is possible to suppress the occurrence of image defects such as fog, deterioration of gradation, and whiteout. Furthermore, it is possible to maintain the toner consumption at an appropriate level. By making the Mn content 50 mol% or less, it is possible to suppress the occurrence of image defects such as whiteout caused by the resistance becoming too high.

By making the composition contain Mg, it is possible to obtain ferrite particles with high resistance. In addition, by making the Mg content 2 mol% or more, the Mn content becomes appropriate relative to the Fe content, and it becomes easy to adjust the magnetization and resistance of the ferrite particles within a favorable range for electrophotographic printing. Therefore, it is possible to suppress the occurrence of image defects such as the occurrence of fog, deterioration of gradation, the occurrence of brush lines, and carrier scattering. Furthermore, when magnesium hydroxide is used as the Mg raw material, if the firing temperature during the production of the ferrite particles is low, hydroxyl groups may remain in the ferrite particles. By making the Mg content 35 mol% or less, it is possible to reduce the amount of residual hydroxyl groups present due to the raw material. Therefore, it is possible to suppress the electrical properties of the ferrite particles, such as the charge amount and resistance, from fluctuating due to the influence of the atmospheric humidity due to the residual hydroxyl groups, and to improve the environmental dependency of the electrical properties of the ferrite particles.

The ferrite particles are magnetic oxides whose main component is ferric oxide. By setting the Fe content to 45 mol% or more and 60 mol% or less, it becomes easy to adjust the magnetization and resistance of the ferrite particles to within a favorable range for electrophotographic printing.

1-4. Magnetic properties When the ferrite particles are used as a core material of a carrier for electrophotographic developer, it is preferable that the saturation magnetization measured by VSM when a magnetic field of 1K·1000/4π·A/m is applied is 55 emu/g or more and 65 emu/g or less. When the saturation magnetization is 55 emu/g or more, the magnetic force of the core material is high, and carrier scattering caused by low magnetization can be suppressed well. In addition, although there is a trade-off between saturation magnetization and electrical resistance, when the saturation magnetization of the ferrite particles is within the above range, the balance between the two is good, and an electrophotographic developer capable of performing high-quality electrophotographic printing well can be obtained. In addition, even if the magnetization is high, if the resistance is low, carrier scattering due to low resistance may occur. By setting the saturation magnetization to 65 emu/g or less, carrier scattering due to low resistance can also be suppressed well.

1-5. Electrical Characteristics The resistance value M in a normal temperature and normal humidity environment (23° C., relative humidity 55%) is desirably 1.0×10 ⁶ (Ω) or more and 1.0×10 ^9.5 (Ω) or less when measured with an inter-electrode distance of 6.5 mm and an applied voltage of 1000 V. If the resistance value of the ferrite particles is within this range, when the ferrite particles are used as a core material and a resin coating layer is provided on the surface to form a carrier, even when the resin coating layer peels off during mixing with the toner, exposing the core material, carrier scattering due to charge injection can be suppressed.

1-6. Physical Properties (1) Apparent Density The apparent density of the ferrite particles is preferably within the range represented by the following formula.
2.15 ≦ Y ≦ 2.45
In the above formula, Y represents the apparent density (g/cm ³ ) of the ferrite particles.

The apparent density referred to here is a value measured by the funnel method in accordance with JIS Z 2504:2012. When the apparent density of the ferrite particles is within the range represented by the above formula, there are few internal voids, and even if there are internal voids, the maximum Feret diameter is small and the strength is high. Therefore, by using a carrier with the ferrite particles as the core material, cracking and chipping can be prevented even if mechanical stress is applied when mixed and stirred with the toner.

When the apparent density of the ferrite particles is low, the number of internal voids tends to be large, or the maximum Feret diameter of the internal voids tends to be large. Therefore, when mechanical stress is applied to a carrier having the ferrite particles as a core material during mixing and stirring with a toner, it is prone to cracking and chipping. If the carrier cracks or chips, it may cause image defects due to carrier scattering, or it may adhere to the photoconductor drum or fixing roller and damage them. On the other hand, when the apparent density of the ferrite particles exceeds 2.50, the internal voids are reduced, but the crystal grains become coarse due to oversintering. In this case, if the perovskite crystal phase component is segregated within the particles, as described above, when mechanical stress is applied, cracks and chips are likely to occur starting from the interface of the crystal phase, which is undesirable because it may cause image defects due to carrier scattering and damage to the photoconductor drum or fixing roller.

To achieve these effects, the apparent density of the ferrite particles is preferably 2.18 or more, and more preferably 2.20 or more. The apparent density of the ferrite particles is more preferably 2.40 or less, and even more preferably 2.35 or less.

In order to obtain ferrite particles having an apparent density within the above range, it is possible to appropriately adjust the blending amount of the alkaline earth metal element-containing compound and zirconium oxide used as raw materials in producing the ferrite particles, the particle size and specific surface area of the zirconium oxide, the firing temperature, the firing time, the oxygen concentration of the atmosphere during firing, etc. By adjusting these, it is possible to suppress the segregation of the perovskite-type crystal phase components and to uniformly disperse the crystal phase components on the particle surface and inside. In addition, for the reasons mentioned at the beginning, it is possible to prevent the occurrence of structural defects and suppress the growth of other crystal phase components such as spinel ferrite phase. For these reasons, it is believed that it is possible to obtain ferrite particles having the desired apparent density by appropriately adjusting the above conditions.

(2) BET Specific Surface Area The BET specific surface area of the ferrite particles is preferably within the range represented by the following formula.
0.08≦X≦0.300
In the above formula, X is the BET specific surface area (m ² /g) of the ferrite particles.

The BET specific surface area here can be a value measured using, for example, a specific surface area measuring device (Macsorb HM model-1208 (manufactured by Mountec Co., Ltd.)). When the BET specific surface area of the ferrite particles is within the range expressed by the above formula, the surface irregularities of the ferrite particles are within an appropriate range for the particle size. Therefore, when the ferrite particles are used as a core material, the surface can be well coated with resin. In addition, since the difference in surface irregularities relative to the particle size is small, when mechanical stress is applied to the surface, the load is suppressed from concentrating on the protrusions, and cracks and chips can be prevented. Therefore, if the ferrite particles are used as a core material, peeling of the resin from the surface during mixing and stirring with the toner can be suppressed, and cracks and chips of the carrier can be prevented, and carrier scattering, etc. can be prevented.

On the other hand, when the BET specific surface area is less than 0.08, the unevenness of the ferrite particle surface relative to the particle size is small or too small, so when the ferrite particle is used as a core material and the surface is coated with a resin, the resin is likely to peel off when mixed with toner, etc. When the resin peels off from the surface of the carrier, the core material is exposed at that part. In other words, the surface of the ferrite particle is exposed, so image defects due to carrier scattering and reduced charging ability are likely to occur. On the other hand, as the BET specific surface area increases, the unevenness of the ferrite particle surface relative to the particle size increases or becomes larger, making it difficult to coat the convex parts of the ferrite particles with resin, and the convex parts may be exposed. Therefore, it may not be possible to obtain a carrier with sufficient charging ability for the toner. In addition, if the surface unevenness becomes large, mechanical stress is applied to the convex parts of the carrier surface when mixed with toner, etc., and the load is concentrated on the convex parts, making the carrier more likely to crack or chip. For these reasons, it is preferable to set the BET specific surface area to less than 0.300, as this allows the carrier strength to be well maintained when the developer is in use.

The ferrite particles having a BET specific surface area within the above range can be obtained by appropriately controlling the granulation conditions and sintering conditions when producing the ferrite particles. Examples of the granulation conditions include the slurry particle size during granulation, the atomizer disk rotation speed, the slurry discharge amount, and the drying temperature. Examples of the sintering conditions include the sintering temperature, the sintering time, and the atmospheric oxygen concentration during sintering. By adjusting these, the raw material of the perovskite-type crystal phase component represented by the composition formula _RZrO3 (where R is an alkaline earth metal element) can be well mixed and dispersed with other raw materials, and as a result, ferrite particles in which the crystal phase component is uniformly dispersed on the particle surface and inside can be obtained, and the apparent density of the ferrite particles can be adjusted to a desired range.

(3) Particle Strength Index When the particle strength index is determined by the method described in the examples below, the particle strength index of the ferrite particles is preferably 0.5 or less, more preferably 0.4 or less. The ferrite particles having a content of the above-mentioned specific particles of 4% or less have high strength, and even when mechanical stress is applied during stirring with toner, the occurrence of cracks and chips can be suppressed, and the occurrence of carrier scattering due to the generation of fine powder can be suppressed. Furthermore, when a carrier is used in which ferrite particles are used as a core material and the surface of the carrier has a resin coating layer, if cracks or chips occur, the ferrite particles as the core material are exposed, and the chargeability of the carrier changes, which may cause a decrease in image density or toner scattering. However, the ferrite particles have high particle strength, so these problems can be suppressed for a long period of time, and a carrier with a longer life and higher reliability can be realized.

(4) Volume average particle size ( _D50 )
When the ferrite particles are used as a core material of a carrier for electrophotographic developer, the volume average particle diameter ( _D50 ) is preferably 24 μm or more and 40 μm or less. However, the volume average particle diameter referred to here refers to a value measured in accordance with JIS Z 8825:2013 by a laser diffraction/scattering method. When the volume average particle diameter is within this range, the charge imparting property to the toner is high and the charge imparting property can be maintained for a long period of time. Therefore, the life of the electrophotographic developer can be extended.

On the other hand, when the volume average particle diameter ( _D50 ) of the ferrite particles is less than 24 μm, the particle diameter is small, so that carrier scattering is likely to occur. Also, when the volume average particle diameter ( _D50 ) of the ferrite particles is less than 24 μm, the particle diameter is small, so that the ferrite particles are likely to aggregate. When the ferrite particles are used as a core material and the surface is coated with a resin to form a carrier, if the ferrite particles are aggregated, the surface of each ferrite particle cannot be well coated with the resin. After that, when the aggregate of the ferrite particles is loosened during the production or use of the developer, the content of the carrier having a large area not coated with the resin becomes high in the developer. Therefore, when a developer is produced using a carrier having such a ferrite particle as a core material, it is not preferable because sufficient charge imparting ability to the toner may not be obtained.

On the other hand, when the volume average particle diameter (D ₅₀ ) of the ferrite particles exceeds 40 μm, the particle diameter of each particle constituting the powder becomes large. Therefore, compared with ferrite particles having a small volume average particle diameter (D ₅₀ ), the surface area of the carrier contributing to triboelectric charging with the toner becomes small when viewed as a whole powder. Therefore, sufficient charge imparting ability to the toner may not be obtained. In order to improve this, if the surface area of each ferrite particle is increased by giving unevenness to the surface of each ferrite particle, the surface area of the carrier contributing to triboelectric charging with the toner can be increased. In this case, the charge imparting ability to the toner is improved, but it is not preferable because the carrier is likely to crack or chip due to mechanical stress applied to the convex parts of the carrier surface when mixed with the toner. In other words, it may not be possible to maintain the strength of the carrier when the developer is used, which is not preferable.

2. Carrier for electrophotographic developer Next, the carrier for electrophotographic developer according to the present invention will be described. The carrier for electrophotographic developer according to the present invention is characterized by comprising the above-mentioned ferrite particles and a resin coating layer provided on the surface of the ferrite particles. That is, the above-mentioned ferrite particles are characterized by being used as a core material of the carrier for electrophotographic developer. The ferrite particles are as described above, so here, the resin coating layer will be mainly described.

(1) Types of Coating Resin The type of resin (coating resin) constituting the resin coating layer is not particularly limited. For example, fluororesin, acrylic resin, epoxy resin, polyamide resin, polyamideimide resin, polyester resin, unsaturated polyester resin, urea resin, melamine resin, alkyd resin, phenol resin, fluoroacrylic resin, acrylic-styrene resin, silicone resin, etc. can be used. Modified silicone resins obtained by modifying silicone resins with acrylic resins, polyester resins, epoxy resins, polyamide resins, polyamideimide resins, alkyd resins, urethane resins, fluororesins, etc. may also be used. For example, from the viewpoint of suppressing resin peeling due to mechanical stress received during stirring and mixing with the toner, it is preferable that the coating resin is a thermosetting resin. Examples of thermosetting resins suitable for the coating resin include epoxy resins, phenol resins, silicone resins, unsaturated polyester resins, urea resins, melamine resins, alkyd resins, and resins containing them. However, as described above, the type of coating resin is not particularly limited, and an appropriate one can be selected depending on the type of toner to be combined and the usage environment.

The resin coating layer may be formed using one type of resin, or two or more types of resin. When two or more types of resin are used, two or more types of resin may be mixed to form one resin coating layer, or multiple resin coating layers may be formed. For example, it is preferable to provide a first resin coating layer having good adhesion to the ferrite particles on the surface of the ferrite particles, and provide a second resin coating layer on the surface of the first resin coating layer to impart the desired charging performance to the carrier.

(2) Resin Coating Amount The amount of resin (resin coating amount) coating the surface of the ferrite particles is preferably 0.1% by mass or more and 10% by mass or less with respect to the ferrite particles used as the core material. If the resin coating amount is less than 0.1% by mass, it becomes difficult to sufficiently coat the surface of the ferrite particles with resin, and it becomes difficult to obtain the desired charging ability. In addition, if the resin coating amount exceeds 10% by mass, aggregation of carrier particles occurs during production, which leads to a decrease in productivity such as a decrease in yield, and the developer characteristics such as the fluidity of the developer in the actual machine or the charging ability to the toner fluctuate, which is not preferable.

(3) Additives The resin coating layer may contain additives such as conductive agents and charge control agents for controlling the electrical resistance, charge amount, and charging speed of the carrier. Examples of conductive agents include conductive carbon, oxides such as titanium oxide and tin oxide, and various organic conductive agents. However, since the electrical resistance of the conductive agent is low, if the amount of the conductive agent added is too large, charge leakage is likely to occur. Therefore, the content of the conductive agent is preferably 0.25% by mass or more and 20.0% by mass or less, more preferably 0.5% by mass or more and 15.0% by mass or less, and even more preferably 1.0% by mass or more and 10.0% by mass or less, based on the solid content of the coating resin.

Examples of charge control agents include various charge control agents commonly used for toners and silane coupling agents. The types of these charge control agents and coupling agents are not particularly limited, but charge control agents such as nigrosine dyes, quaternary ammonium salts, organometallic complexes, and metal-containing monoazo dyes, as well as aminosilane coupling agents and fluorine-based silane coupling agents, can be preferably used. The content of the charge control agent is preferably 0.25% by mass or more and 20.0% by mass or less, more preferably 0.5% by mass or more and 15.0% by mass or less, and even more preferably 1.0% by mass or more and 10.0% by mass or less, based on the solid content of the coating resin.

3. Electrophotographic Developer Next, an embodiment of the electrophotographic developer according to the present invention will be described. The electrophotographic developer contains the above-mentioned carrier for electrophotographic developer and a toner.

As the toner constituting the electrophotographic developer, for example, either a polymerized toner produced by a polymerization method or a pulverized toner produced by a pulverization method can be preferably used. These toners may contain various additives, and any toner may be used as long as it can be used as an electrophotographic developer in combination with the above carrier.

The toner preferably has a volume average particle diameter ( _D50 ) of 2 μm or more and 15 μm or less, and more preferably 3 μm or more and 10 μm or less. When the toner has a volume average particle diameter ( _D50 ) within this range, an electrophotographic developer capable of performing high-quality electrophotographic printing can be obtained.

The mixture ratio of the carrier and the toner, i.e., the toner concentration, is preferably 3% by mass or more and 15% by mass or less. An electrophotographic developer containing toner at this concentration makes it easier to obtain the desired image density and can better suppress fogging and toner scattering.

On the other hand, when the electrophotographic developer is used as a replenishment developer, it is preferable that the toner amount is 2 parts by weight or more and 50 parts by weight or less per 1 part by weight of the carrier.

The electrophotographic developer can be suitably used in various electrophotographic developing devices that apply a magnetic brush development method in which a carrier is attracted and attached to a magnetic drum or the like by magnetic force to form a brush-like toner, and a visible image is formed by attaching the toner to an electrostatic latent image formed on a photoconductor or the like while applying a bias electric field. The electrophotographic developer can be used not only in electrophotographic developing devices that use a DC bias electric field when applying a bias electric field, but also in electrophotographic developing devices that use an alternating bias electric field in which an AC bias electric field is superimposed on a DC bias electric field.

4. Manufacturing Method Hereinafter, a manufacturing method of the ferrite powder, the carrier core material for an electrophotographic developer, the carrier for an electrophotographic developer, and the electrophotographic developer according to the present invention will be described.

4-1 Ferrite Powder and Carrier Core Material for Electrophotographic Developer The ferrite powder and carrier core material for electrophotographic developer according to the present invention can be produced as follows.

First, the raw materials are weighed out in appropriate amounts to obtain the desired ferrite composition, then they are ground and mixed in a ball mill or vibrating mill for 0.5 hours or more, preferably 1 hour to 20 hours, and then pre-fired.

For example, to produce ferrite particles mainly composed of a spinel _- type crystal phase component represented by the composition formula (MnO)x(MgO)y( _Fe2O3 )z (where 15≦x≦50, 2≦y≦35, 45≦z≦60, x+y+z=100 (mol%)), each raw material is weighed and pulverized and mixed so that x, y, and z are the desired values. As _the raw materials, for example, it is preferable to use _Fe2O3 , Mg(OH) ₂ and _/ or _MgCO3 , and _{one or more manganese compounds selected from the group consisting of MnO2, Mn2O3 , Mn3O4 ,} and _MnCO3 .

The ferrite particles according to the present invention contain a perovskite-type crystalline phase component represented by the composition formula _RZrO3 (wherein R is an alkaline earth metal element). Therefore, for the alkaline earth metal element (R), an oxide of the alkaline earth metal element (R) is used as a raw material, weighed out to obtain a desired amount, and pulverized and mixed with other raw materials. For Zr, _ZrO2 can be used as a raw material.

Here, when manufacturing ferrite particles containing the perovskite crystal phase component in an amount of 0.05% by mass or more and 2.50% by mass or less (however, this is the mass fraction obtained when performing a phase composition analysis of the crystal phase constituting the ferrite particles by Rietveld analysis of the X-ray diffraction pattern), it is preferable to mix ZrO ₂ : 0.1 to 3.0 and more preferably to mix ZrO ₂ : 0.2 to 1.5 with respect to 100 of the main component raw material in a molar ratio. Also, it is preferable to mix an oxide of an alkaline earth metal element (R) : 0.1 to 3.0 and more preferably to mix ZrO ₂ : 0.2 to 1.5 with respect to 100 of the main component raw material in a molar ratio. The perovskite crystal phase component is generated by a solid-phase reaction between the oxide of an alkaline earth metal element (R) and ZrO ₂ . Therefore, by appropriately changing the amounts and ratios of the oxides of alkaline earth metal elements (R) and _ZrO2 added within the above-mentioned preferred blending ranges, the amount of the perovskite crystal phase component produced can be adjusted.

When producing the ferrite particles according to the present invention, _ZrO2 is weighed out to a desired amount and pulverized and mixed with other raw materials. In order to uniformly disperse zirconium dioxide inside the particles, it is desirable to add zirconium dioxide when pulverizing and mixing the raw materials. It is also preferable to pulverize and mix raw materials other than _ZrO2 , calcinate them in the atmosphere, add _ZrO2 , and further pulverize and mix them. In that case, the pulverized product obtained by pulverizing and mixing the raw materials other than _ZrO2 is pelletized using a pressure molding machine or the like, calcined in the atmosphere at a temperature of 700°C to 1200°C, and then _ZrO2 is added.

After all the raw materials including _ZrO2 are ground and mixed, or after other raw materials other than _ZrO2 are ground and mixed and calcined, a predetermined amount of _ZrO2 is added to the calcined product obtained, and further ground with a ball mill or a vibration mill, etc., and in either case, water is added to the ground mixture and finely ground with a bead mill or the like to obtain a slurry. The degree of grinding can be controlled by adjusting the diameter, composition, and grinding time of the beads used as media. In order to uniformly disperse the raw materials, it is preferable to use fine beads having a particle size of 1 mm or less as media. In addition, in order to obtain ferrite particles whose content of the above-mentioned predetermined particles is within the range of the present invention, it is preferable to grind the ground product so that the volume average particle size ( _D50 ) is 2.0 μm or less, and more preferably to grind it so that it is 1.5 μm or less. By adopting this method, it is possible to obtain ferrite particles consisting of particles with few internal voids, or even if internal voids exist, whose maximum Feret diameter is small. In addition, in order to suppress abnormal grain growth, it is preferable to grind the grains so that the grain size ( _D90 ) on the coarse side of the grain size distribution is 3.5 μm or less. By adjusting these, the perovskite crystal phase components can be dispersed more uniformly from the surface to the interior of the particles.

Furthermore, in order to obtain ferrite particles having a BET specific surface area and apparent density in the above-mentioned preferred ranges, it is desirable to use ZrO ₂ having a BET specific surface area of 20 to 150 m ² /g and a volume average particle size (D ₅₀ ) of 0.5 μm to 2.5 μm as a raw material. By using a raw material having such characteristics, it is possible to uniformly advance the growth of the perovskite-type crystal phase component while dispersing ZrO ₂ well within the particles. It is possible to suppress the segregation of the crystal phase component within the particles and the abnormal grain growth of each crystal phase component. Therefore, even when mechanical stress is applied, it is possible to suppress the occurrence of cracks and chips at the interface between different crystal phases that are not solid-soluble with each other, and to improve the strength of the ferrite particles.

Next, it is preferable to add a dispersant, binder, etc. to the slurry obtained in this manner as necessary to adjust the viscosity to 2 poise or more and 4 poise or less. In this case, polyvinyl alcohol or polyvinylpyrrolidone can be used as the binder.

The slurry prepared as above is sprayed and dried using a spray dryer to obtain a granulated product. In this case, the granulation conditions are preferably set to a discharge rate of 20 Hz or more and 50 Hz or less, an atomizer disk rotation speed of 11,000 rpm or more and 20,000 rpm or less, and a drying temperature of 100°C or more and 500°C or less. For example, to obtain ferrite particles having an apparent density within the above range, it is preferable to set the atomizer disk rotation speed to 11,000 rpm or more and 16,000 rpm or less, and a drying temperature to 150°C or more and 300°C or less.

Next, it is preferable to classify the granulated material before firing and remove fine particles contained in the granulated material in order to obtain ferrite powder with a uniform particle size. The classification of the granulated material can be performed using a known air classification or a sieve.

Next, the classified granules are fired. It is preferable to perform primary firing of the granules as necessary, followed by main firing. When primary firing is performed, the firing temperature is preferably 600°C or higher and 900°C or lower.

In addition, the firing is preferably carried out in an inert atmosphere or a weakly oxidizing atmosphere by maintaining the temperature at 850°C or higher for 4 hours to 24 hours. However, the firing temperature is not particularly limited as long as the ferrite particles according to the present invention can be obtained. Here, an inert atmosphere or a weakly oxidizing atmosphere refers to an atmosphere of a mixed gas of nitrogen and oxygen in which the oxygen concentration is 0.1% by volume (1000 ppm) or more and 5% by volume (50000 ppm) or less, and more preferably, the atmospheric oxygen concentration is 0.1% by volume (1000 ppm) or more and 3.5% by volume (35000 ppm) or less, even more preferably, 0.1% by volume (1000 ppm) or more and 2.5% by volume (25000 ppm) or less, even more preferably, 0.1% by volume (1000 ppm) or more and 1.5% by volume (25000 ppm) or less, and even more preferably, 0.1% by volume (1000 ppm) or more and 1.0% by volume (25000 ppm) or less.

Furthermore, in order to obtain ferrite particles having a content of the above-mentioned specified particles within the range of the present invention, the heating rate up to the firing temperature is preferably 30° C./hour or more and 100° C./hour or less, more preferably 40° C./hour or more and 90° C./hour or less, and even more preferably 50° C./hour or more and 85° C./hour or less.

For example, in order to produce ferrite particles mainly composed of a spinel _type crystal phase component represented by the composition formula of (MnO)x(MgO)y( _Fe2O3 )z (where 15≦x≦50, 2≦y≦35, 45≦z≦60, x+y+z=100 (mol %)), it is more preferable to hold the mixture at a temperature suitable for generating the ferrite component made of spinel type crystals (850°C or higher and 1150°C or lower) for 3 hours or more in order to sufficiently generate the ferrite component made of spinel type crystals while dispersing the zirconium component at the grain boundaries, and then hold the mixture at a temperature suitable for generating a perovskite type crystal phase component represented by the composition formula of RZrO3, such as strontium _zirconate , for 1 hour or more to perform the main firing. In addition, at that time, the firing temperature, firing time, and atmospheric oxygen concentration during firing can be appropriately controlled according to the type of alkaline earth metal element (R) and the amount of _ZrO2 blended, thereby making it possible to make the BET specific surface area fall within the range of the present invention.

For example, in the case of strontium zirconate (SrZrO ₃ ), in order to obtain an apparent density within the range of the present invention while sufficiently generating perovskite crystal phase components, it is preferable to maintain the firing temperature at a temperature of preferably 1170° C. to 1400° C., more preferably 1180° C. to 1350° C., and even more preferably 1200° C. to 1330° C. for one hour or more. In this case, the amount of zirconium oxide (ZrO ₂ ) blended is preferably 0.05 mol to 3.00 mol.

In addition, for example, in the case of calcium zirconate (CaZrO ₃ ) or barium zirconate (BaZrO ₃ ), it is preferable to pulverize the raw material, add a reaction accelerator, and then sinter at a predetermined temperature. In order to generate the perovskite crystal phase components of calcium zirconate (CaZrO ₃ ) or barium zirconate (BaZrO ₃ ), if no reaction accelerator is added, it is necessary to sinter at a high temperature of 2000° C. or higher. On the other hand, by pulverizing the raw material to a primary particle size of about several tens of nanometers and adding an aluminum compound (e.g., alumina (Al ₂ O ₃ ) or the like) as a reaction accelerator, these perovskite crystal phase components can be generated even at a temperature of 1500° C. or lower. In this way, the ferrite particles according to the present invention can be obtained by maintaining the temperature suitable for generating the perovskite crystal phase components according to the target composition and adjusting other conditions as necessary.

In addition, when the main firing is performed, it is preferable to use a firing furnace such as a tunnel kiln or elevator kiln in which the granulated material (material to be fired) is passed through the hot section while being moved, rather than a firing furnace such as a rotary kiln in which the granulated material is passed through the hot section while being moved. In a firing furnace such as a rotary kiln in which the granulated material is passed through the hot section while being moved, if the oxygen concentration in the firing atmosphere is low, the granulated material may adhere to the inner surface of the furnace when passing through the hot section, and the granulated material passing through the inside while flowing may not be able to be sufficiently heated. In that case, the granulated material passes through the hot section without being sufficiently sintered, so that the surface of the obtained fired material is sufficiently sintered, but the inside is often insufficiently sintered. Such fired material does not meet the strength required for a carrier core material for electrophotographic developer, and the ferrite reaction inside is insufficient, so that the magnetic properties and electrical properties required for a carrier core material for electrophotographic developer may not be met. Furthermore, if the sintering inside the fired product is insufficient, the perovskite crystal phase component represented by the composition formula _RZrO3 cannot be sufficiently generated in the firing process, which makes it difficult to obtain the ferrite particles according to the present invention.

On the other hand, if the granulated material is placed in a sintering furnace such as a kiln and passed through a hot section while remaining stationary, the inside of the material to be sintered can be sufficiently sintered, making it easy to obtain ferrite particles that have high magnetization and high resistance and in which the perovskite crystal phase component represented by the composition formula _RZrO3 is sufficiently generated. For these reasons, it is preferable to use a tunnel kiln, elevator kiln, or the like when carrying out the main sintering step.

The fired product is then crushed and classified to obtain ferrite particles. As classification methods, the particle size is adjusted to the desired particle size using existing methods such as air classification, mesh filtration, and sedimentation. When performing dry recovery, it is also possible to recover using a cyclone, etc. When adjusting the particle size, two or more of the above classification methods may be selected and carried out, or the conditions may be changed for one classification method to remove coarse and fine particles.

Thereafter, if necessary, the surface of the ferrite particles can be heated at a low temperature to perform a surface oxidation treatment, thereby adjusting the surface resistance of the ferrite particles. The surface oxidation treatment can be performed by subjecting the ferrite particles to a heat treatment at 400°C to 730°C, preferably 450°C to 650°C, in an oxygen-containing atmosphere such as air, using a rotary electric furnace, a batch electric furnace, or the like. If the heating temperature during the surface oxidation treatment is lower than 400°C, the surface of the ferrite particles cannot be sufficiently oxidized, and the desired surface resistance characteristics may not be obtained. On the other hand, if the heating temperature is higher than 730°C, in the case of manganese-containing ferrite, the oxidation of manganese proceeds too much, and the magnetization of the ferrite particles decreases, which is not preferable. To form a uniform oxide film on the surface of the ferrite particles, it is preferable to use a rotary electric furnace. However, the surface oxidation treatment is an optional process.

4-2. Carrier for electrophotographic developer The carrier for electrophotographic developer according to the present invention is a carrier in which the above-mentioned ferrite particles are used as a core material and a resin coating layer is provided on the surface of the ferrite particles. The resin constituting the resin coating layer is as described above. When forming a resin coating layer on the surface of the ferrite particles, a known method such as a brush coating method, a spray drying method using a fluidized bed, a rotary drying method, a liquid immersion drying method using a universal stirrer, etc. can be adopted. In order to improve the ratio of the resin coating area to the surface of the ferrite particles (resin coating rate), it is preferable to adopt a spray drying method using a fluidized bed. In any case, the ferrite particles can be subjected to a resin coating treatment once or multiple times. The resin coating liquid used in forming the resin coating layer may contain the above-mentioned additives. In addition, since the amount of resin coating on the surface of the ferrite particles is as described above, a description thereof will be omitted here.

After the resin coating liquid is applied to the surface of the ferrite particles, baking may be performed by external or internal heating as necessary. For external heating, a fixed or flow type electric furnace, a rotary type electric furnace, a burner furnace, etc. can be used. For internal heating, a microwave furnace can be used. If a UV-curable resin is used as the coating resin, a UV heater is used. Baking should be performed at a temperature equal to or higher than the melting point or glass transition point of the coating resin. If a thermosetting resin or a condensation cross-linking type resin is used as the coating resin, baking should be performed at a temperature at which the curing of these resins progresses sufficiently.

4-3. Electrophotographic Developer Next, a method for producing an electrophotographic developer according to the present invention will be described.
The electrophotographic developer according to the present invention contains the above-mentioned carrier for electrophotographic developer and a toner. As the toner, either a polymerized toner or a pulverized toner can be preferably used as described above.

Polymerized toner can be produced by known methods such as suspension polymerization, emulsion polymerization, emulsion aggregation, ester elongation polymerization, and phase inversion emulsification. For example, a colored dispersion liquid in which a colorant is dispersed in water using a surfactant is mixed and stirred with a polymerizable monomer, a surfactant, and a polymerization initiator in an aqueous medium to emulsify and disperse the polymerizable monomer in the aqueous medium, and polymerize while stirring and mixing. A salting-out agent is then added to salt out the polymer particles. The particles obtained by salting out are filtered, washed, and dried to obtain a polymerized toner. External additives may then be added to the dried toner particles as necessary.

When producing the polymerized toner particles, a toner composition containing a polymerizable monomer, a surfactant, a polymerization initiator, a colorant, etc. may be used. The toner composition may also contain a fixation improver and a charge control agent.

To prepare pulverized toner, for example, binder resin, colorant, charge control agent, etc. are thoroughly mixed in a mixer such as a Henschel mixer, then melt-kneaded in a twin-screw extruder or the like to disperse uniformly, cooled, finely pulverized in a jet mill or the like, and classified, for example, by a wind classifier, to obtain a toner of the desired particle size. If necessary, wax, magnetic powder, viscosity modifier, and other additives may be added. Furthermore, external additives can be added after classification.

Next, the present invention will be specifically described with examples and comparative examples. However, the present invention is not limited to the following examples.

(1) Ferrite particles In Example 1, ferrite particles containing a spinel type crystal phase component represented by the composition formula of (MnO)x(MgO)y( _Fe2O3 )z as the main component and a _perovskite type crystal phase component represented by the composition formula of _SrZrO3 (R = Sr) were manufactured as follows. First, as the main component raw materials, MnO raw material, MgO raw material, and _Fe2O3 raw material were weighed out so that the molar ratio was MnO equivalent: 38.3, _MgO equivalent: 10.4, and _Fe2O3 : _51.3 . In addition, SrO raw material was weighed out so that the molar ratio was SrO: 0.8 with respect to the main component raw material: 100. Here, trimanganese tetroxide was used as the MnO raw material, magnesium oxide was used as the MgO raw material, ferric oxide was used as _{the Fe2O3} raw material, and strontium carbonate was used as the SrO raw material.

The weighed raw materials were then pulverized for 5 hours in a dry media mill (vibration mill, 1/8-inch diameter stainless steel beads), and the resulting pulverized material was made into pellets of approximately 1 mm square using a roller compactor. The resulting pellets were passed through a vibrating sieve with 3 mm openings to remove coarse powder, and then through a vibrating sieve with 0.5 mm openings to remove fine powder. The pellets were then heated at 800°C for 3 hours in a continuous electric furnace for pre-firing. The raw materials were then pulverized using a dry media mill (vibration mill, 1/8-inch diameter stainless steel beads) until the average particle size was approximately 5 μm. The pulverization time was 6 hours.

Water and zirconium dioxide as a _ZrO2 raw material having a BET specific surface area of 30 ^m2 /g and an average particle size of 2 μm were added to the obtained pulverized material, and the mixture was pulverized for 6 hours using a wet media mill (horizontal bead mill, zirconia beads having a diameter of 1 mm). At this time, zirconium dioxide was added to the pulverized material so that the molar ratio of the main component raw material: 100 to _ZrO2 : 1.0 was obtained. The particle size (primary particle size of the pulverized material) of the obtained slurry was measured using a laser diffraction particle size distribution analyzer (LA-950, Horiba, Ltd.), and _D50 was 0.9 μm.

Furthermore, an appropriate amount of dispersant was added to the slurry prepared as described above, and 0.4 mass% of PVA (polyvinyl alcohol) was added as a binder relative to the solid content (amount of pre-calcined material in the slurry), followed by granulation and drying using a spray dryer. The particle size of the resulting granulated material was adjusted, and then the material was heated in an air atmosphere at 800°C for 2 hours using a rotary electric furnace to remove organic components such as the dispersant and binder.

The granulated material was then fired in a tunnel-type electric furnace at a firing temperature (holding temperature) of 1230°C in an atmosphere with an oxygen concentration of 0.3% by volume for 3 hours. The heating rate was 50°C/hour and the cooling rate was 110°C/hour. The fired material was crushed using a hammer crusher, and further classified using a gyro sifter (vibrating sieve) and a turbo classifier (air classifier) to adjust the particle size. Low magnetic particles were separated by magnetic separation to obtain ferrite particles.

The obtained ferrite particles were subjected to a surface oxidation treatment in a rotary electric furnace equipped with a hot section and a cooling section following the hot section, and then cooled to obtain surface-oxidized ferrite particles. In the surface oxidation treatment, an oxide film was formed on the surface of the ferrite particles at 450°C in the hot section under air atmosphere. The main production conditions for the ferrite particles of Example 1 are shown in Table 1.

(2) Carrier for Electrophotographic Developer The above ferrite particles were used as a core material, and the ferrite particles were coated with an acrylic resin as follows to obtain a carrier of Example 1.

First, an acrylic resin solution (resin solid content 10% by mass) was prepared by mixing acrylic resin (Dianal LR-269, manufactured by Mitsubishi Rayon Co., Ltd.) with toluene. This resin solution was mixed with the ferrite particles of Example 1 using a universal stirrer to coat the surfaces of the ferrite particles with the resin solution. At that time, an amount of resin solution was used such that the resin solid content was 1.5% by mass relative to the ferrite particles. Next, the ferrite particles with the resin solution attached were heated while stirring at 145°C for 3 hours using a heat exchange type stirring and heating device, and the volatile components contained in the resin solution were volatilized to dry the ferrite particles. This resulted in the electrophotographic developer carrier of Example 1 having a resin coating layer on the surfaces of the ferrite particles.

(3) Electrophotographic Developer The above-mentioned electrophotographic developer carrier and toner were mixed by stirring for 30 minutes using a Turbula mixer to obtain 1 kg of developer (toner concentration 7.5% by weight). The toner used here was a commercially available negative toner (cyan toner, for DocuPrint C3530 manufactured by Fuji Xerox Co., Ltd.; average particle size approx. 5.8 μm) used in full-color printers.

In this example, the ferrite particles of Example 2 were produced in the same manner as in Example 1, except that the particle size (primary particle size of pulverization) _D50 of the slurry was set to 1.4 μm. The main production conditions of Example 2 are shown in Table 1. In addition, a carrier for an electrophotographic developer was produced in the same manner as in Example 1, except that the ferrite particles were used as a core material, and an electrophotographic developer was produced using the carrier for an electrophotographic developer.

In this example, the ferrite particles of Example 3 were produced in the same manner as in Example 1, except that the particle size (primary particle size of pulverization) _D50 of the slurry was set to 1.0 μm, and the temperature rise rate during main baking was set to 80° C./hour. The main production conditions of Example 3 are shown in Table 1. In addition, a carrier for an electrophotographic developer was produced in the same manner as in Example 1, except that the ferrite particles were used as a core material, and an electrophotographic developer was produced using the carrier for an electrophotographic developer.

In this example, the ferrite particles of Example 4 were produced in the same manner as in Example 1, except that the particle size (primary particle size after pulverization) _D50 of the slurry was set to 1.3 μm, and the temperature rise rate during main baking was set to 75° C./hour. The main production conditions of Example 4 are shown in Table 1. In addition, a carrier for an electrophotographic developer was produced in the same manner as in Example 1, except that the ferrite particles were used as a core material, and an electrophotographic developer was produced using the carrier for an electrophotographic developer.

Comparative Example

[Comparative Example 1]
In this comparative example, the ferrite particles of Comparative Example 1 were produced in the same manner as in Example 1, except that the particle size (primary particle size of pulverization) _D50 of the slurry was set to 2.3 μm, the temperature rise rate during main baking was set to 150° C./hour, and the baking temperature (retention temperature) during main baking was set to 1250° C. The main production conditions of Comparative Example 1 are shown in Table 1. Moreover, a carrier for an electrophotographic developer was produced in the same manner as in Example 1, except that the ferrite particles were used as a core material, and an electrophotographic developer was produced using the carrier for an electrophotographic developer.

[Comparative Example 2]
In this comparative example, the ferrite particles of Comparative Example 2 were produced in the same manner as in Example 1, except that the particle size (primary particle size of pulverization) _D50 of the slurry was set to 2.2 μm, the temperature rise rate during main baking was set to 150° C./hour, and the baking temperature (retention temperature) during main baking was set to 1300° C. The main production conditions of Comparative Example 2 are shown in Table 1. In addition, a carrier for an electrophotographic developer was produced in the same manner as in Example 1, except that the ferrite particles were used as a core material, and an electrophotographic developer was produced using the carrier for an electrophotographic developer.

[Comparative Example 3]
In this comparative example, the ferrite particles of Comparative Example 3 were produced in the same manner as in Example 1, except that the particle size (primary particle size of pulverization) _D50 of the slurry was set to 2.3 μm, the temperature rise rate during main baking was set to 150° C./hour, and the baking temperature (retention temperature) during main baking was set to 1210° C. The main production conditions of Comparative Example 3 are shown in Table 1. Moreover, a carrier for an electrophotographic developer was produced in the same manner as in Example 1, except that the ferrite particles were used as a core material, and an electrophotographic developer was produced using the carrier for an electrophotographic developer.

<evaluation>
The ferrite particles of each Example and Comparative Example obtained as described above were evaluated for (1) the perovskite type crystal phase component content, (2) the spinel type crystal phase component content, (3) the degree of dispersion of Zr element, (4) the volume average particle size, (5) the content ratio of a specified particle, (6) the apparent density, (7) the BET specific surface area, (8) the particle strength index, (9) the saturation magnetization, and (10) the resistance.
The evaluation methods/measurement methods and evaluation results are described below.

1. Evaluation method/measurement method (1) Perovskite crystal phase content (mass%)
The ferrite particles produced in each Example and Comparative Example were used as samples, and the perovskite crystal phase content in each ferrite particle was determined by Rietveld analysis of the powder X-ray diffraction pattern. Although it may be difficult to identify and quantify each crystal phase by waveform separation of the powder X-ray diffraction pattern, Rietveld analysis based on a crystal structure model makes it possible to identify and quantify each phase.

The X-ray diffraction device used was "X'PertPRO MPD" manufactured by PANalytical. A Co tube (CoKα ray) was used as the X-ray source. A focused optical system and a high-speed detector "X'Celarator" were used as the optical system. The measurement conditions were as follows.
Scan speed: 0.08°/sec Divergence slit: 1.0°
Scattering slit: 1.0°
Receiving slit: 0.15 mm
Voltage and current of the sealed tube: 40 kV/40 mA
Measurement range: 2θ = 15° to 90°
Number of times: 5

Based on the obtained measurement results, the crystal structure was identified as follows from the structure disclosed in the National Institute for Materials Science, "AtomWork" (URL: http://crystdb.nims.go.jp/).
Phase A: A crystalline phase consisting of manganese ferrite (spinel type crystal) Crystal structure: Space group F d -3 m (No. 227)
Phase B: Perovskite-type crystals represented by the formula _RZrO3 . Crystal structure: Space group P nma (No. 62)
Phase C: Zirconium dioxide (zirconia)
Crystal structure: space group P -4 2 m (No. 111)

Next, the identified crystal structure was refined using the analytical software “RIETAN-FP v2.83 (http://fujioizumi.verse.jp/download/download.html)” to calculate the abundance ratio of each crystal phase in terms of mass as the phase composition ratio.
The profile function used was the pseudo-Voigt function of Thompson, Cox and Hastings, and asymmetrization was performed by the Howard method.
The Rwp value and S value, which indicate the accuracy of fitting, were 2% or less and 1.5 or less, respectively, and it was confirmed that the main peaks of B and C phases were fitted at 2θ = 35 to 37°, after which each parameter was optimized.

Based on the results of the Rietveld analysis of the X-ray diffraction patterns obtained as described above, the content (mass%) of the perovskite-type crystal phase component (phase B) was determined when a phase composition analysis of the crystal phases constituting the ferrite particles was performed.

(2) Spinel-type crystal phase content (mass%)
The results of the X-ray diffraction measurements of _phases A, B, and C obtained above were analyzed in addition to _{phases D} , E _, and F, to determine the spinel crystal phase component content (mass%) in each ferrite particle.

(3) Dispersion Degree of Zr Element For the ferrite particles produced in each of the Examples and Comparative Examples, the dispersion degree of Zr element defined by the following formula was measured.
Dispersion degree of Zr=Zr(s)/Zr(c)
however,
Zr(s): Zr content (mass%) in the surface portion of the particle cross section measured by energy dispersive X-ray analysis
Zr(c): Zr content (mass%) at the center of the particle cross section measured by energy dispersive X-ray analysis

Here, we will explain with reference to Figure 1. The center of a ferrite particle's cross section (particle cross section) is defined as follows. If the maximum diameter in the particle cross section (for example, an SEM image) is defined as a line segment Dx, the midpoint of the line segment Dx is defined as the center C of the particle cross section, and the end points of the line segment Dx are defined as points P. Then, the center C is defined as the center position, and a square with one side that is 35% of the length of the line segment Dx is defined as square S. The area enclosed by this square S is defined as the center.

The surface portion of the cross section of the particle is defined as follows. Point P' is a point on the line segment Dx, and is located at a distance of 15% of the length of the line segment Dx from point P toward the center C. Rectangle R1 is a rectangle having a long side that is a line segment having a length of 35% of the length of the line segment Dx, perpendicular to the line segment Dx, and having point P or point P' as a midpoint, and a short side that is a line segment having a length of 15% of the length of the line segment _Dx . In the present invention, the area surrounded by this rectangle _R1 in the cross section of the ferrite particle is defined as the surface portion. Note that FIG. 1 shows the cross-sectional shape of the ferrite particle simplified to a circular shape for the purpose of explanation, and does not show the actual cross-sectional shape of the ferrite particle according to the present invention.

Energy dispersive X-ray analysis (EDX analysis) is performed on the central and surface areas of the particle cross section defined in this way to measure the Zr element content in each area. The specific measurement method is as follows.

(a) Ferrite particles were embedded in resin and subjected to cross-sectional processing by ion milling to prepare cross-sectional samples for measurement. Ion milling was performed using IM4000PLUS manufactured by Hitachi High-Technogies Corporation, with the ion beam acceleration voltage set to 6.0 kV under an argon atmosphere. Here, the ferrite single particles to be analyzed were particles with a maximum diameter Dx in the range of _D50 x 0.8 ≦ Dx ≦ _D50 x 1.2, where _D50 is the volume average particle diameter of the ferrite particles (powder) containing the ferrite single particles.

(b) The cross-section of the ferrite particle to be analyzed is observed using a scanning electron microscope (SEM, Hitachi High-Technogies SU8020) with an acceleration voltage of 15 kV and a WD of 15 mm for the obtained cross-sectional sample. The magnification is set so that only one ferrite particle to be analyzed is present in the field of view, and the entire particle fits within the field of view.

(c) Then, EDX analysis was performed on the center and surface regions (regions defined above) of the ferrite particle cross section. In the EDX analysis, mapping collection was performed for Fe, Mn, Mg, Sr, and Zr using an energy dispersive X-ray analyzer (Horiba, Ltd. EMax X-Max50), and the amount of each element (mass%) was calculated from the obtained X-ray spectrum peaks. The amount of Zr in the center of the obtained particle cross section was designated "Zr(c)," and the amount of Zr in the surface region of the particle cross section was designated "Zr(s)."

Then, by substituting the amount of Zr in the center of the particle cross section (Zr(c)) and the amount of Zr in the surface of the particle cross section (Zr(s)) obtained by EDX analysis into the above-mentioned formula (1), the degree of dispersion of Zr in the ferrite particles being analyzed can be obtained.

In this case, the amount of Zr in the surface portion of the cross section of the particle was defined as four regions surrounded by a rectangle _R2 defined similarly to the rectangle _R1 , and rectangles R3 and R4 defined based on points Q and Q' defined similarly to the points P and P' with respect to a line Dy passing through the center C of the cross section of the _particle and perpendicular to the line _Dx , and the surface portion was defined as the average value of the amount of Zr in each region. The rectangles _R1 , _R2 , _R3 , and _R4 were arranged at approximately equal intervals along the contour of the cross section of the particle.

(4) Volume average particle size ( _D50 )
The volume average particle diameter (D ₅₀ ) was measured using a Microtrack particle size analyzer (Model 9320-X100) manufactured by Nikkiso Co., Ltd., as follows: Ferrite particles manufactured in each Example and Comparative Example were used as samples, 10 g of each sample and 80 ml of water were placed in a 100 ml beaker, 2 to 3 drops of a dispersant (sodium hexametaphosphate) were added, and a sample was prepared by dispersing for 20 seconds using an ultrasonic homogenizer (UH-150 type manufactured by SMT.Co.LTD.) at an output level of 4, removing bubbles formed on the surface of the beaker, and measuring the volume average particle diameter of the sample using the Microtrack particle size analyzer.

(5) Content of Predetermined Particles The content of the above-mentioned predetermined particles was determined as follows.
a) A cross-sectional sample for measurement was prepared by embedding a ferrite carrier core material in resin and subjecting the core material to cross-sectional processing by ion milling. The ion milling was performed using an IM4000PLUS manufactured by Hitachi High-Technogies Corporation, with an ion beam acceleration voltage of 6.0 kV, in an argon atmosphere.

b) The cross-section of the obtained cross-section sample was photographed with a scanning electron microscope (SEM, Hitachi High-Technogies SU8020) at an acceleration voltage of 1.0 kV, a WD of 8 mm, and a 450x field of view to observe the cross-section. If there are less than 100 particles in the field of view, the measurement location may be changed and multiple fields of view may be photographed.

c) The secondary electron image information was input via an interface into Media Cybernetics' image analysis software (Image-Pro PLUS) for analysis, and the content of the above-mentioned specified particles was determined.

The particles to be measured were 100 ferrite particles having a circle equivalent diameter that satisfies the following condition (1). In the examples and comparative examples, ferrite particles were obtained by firing granulated powder having an approximately granular shape. Therefore, the obtained ferrite particles maintain approximately the same shape as the granulated powder. Particles that do not satisfy the following condition (1) are considered to be particles whose particle diameter is too small or too large with respect to the volume average particle diameter D ₅₀ of the ferrite particles, whose cross-sectional position is not appropriate, or whose particle shape deviates significantly from a spherical shape due to contact between particles during sample preparation, etc., and if such particles are included in the particles to be measured, they will not match the representative value of the powder.

Condition (1):
0.8X ≦ Y ≦ 1.3X
however,
X: Volume average particle diameter D ₅₀ (μm)
Y: Equivalent circle diameter of the cross-sectional area of the particle to be measured

d) For each particle to be measured, one particle was selected using the selection tool in the image analysis software, and the circle equivalent diameter Y was calculated from the particle area. After confirming that the above condition (1) was satisfied, voids (internal voids) in the cross-sectional particle were selected to determine the maximum Feret diameter Z. Note that an internal void refers to a void that is not in contact with the outer periphery of the measured particle in the cross-sectional SEM photograph and is not continuous from the surface. Using the same method, evaluation was performed on 100 particles within the field of view that satisfied the above condition (1).

e) The particles that satisfy the following formula for the circle equivalent diameter Y of the cross-sectional area of the measurement target particle and the maximum Feret diameter Z of the internal void are defined as the above-mentioned specified particles, the number of the specified particles is counted, and the ratio of the specified particles to all measurement particles (100 particles) is calculated. Note that one measurement target particle may have multiple internal voids. In such a case, when there is at least one internal void that satisfies the following condition, the measurement target particle is counted as the above-mentioned specified particle.
0.15Y≦Z
however,

(6) Apparent density This apparent density was measured in accordance with JIS Z2504:2012 (Test method for apparent density of metal powder).

(7) BET specific surface area The ferrite particles produced in each Example and Comparative Example were used as samples, and the BET specific surface area was determined using a specific surface area measuring device (Macsorb HM model-1208, Mountec Co., Ltd.) according to the following procedure. First, about 20 g of sample was placed in a glass petri dish, and then degassed to -0.1 MPa using a vacuum dryer. After degassing and confirming that the degree of vacuum in the glass petri dish had reached -0.1 MPa or less, the sample was heated at 200 ° C. for 2 hours. About 5 to 7 g of the sample subjected to these pretreatments was placed in a standard sample cell dedicated to the specific surface area measuring device. The mass of the sample placed in the standard sample cell was accurately weighed using a precision balance. Then, the standard sample cell containing the sample was set in the measurement port, and the BET specific surface area was measured by the one-point method at a temperature of 10 ° C. to 30 ° C. and a relative humidity of 20% to 80%. When the measurement was completed, the mass of the sample was entered, and the calculated value was regarded as the measured value of the BET specific surface area.

(8) Particle Strength Index The particle strength index of the ferrite particles produced in each of the Examples and Comparative Examples was determined as follows.

The particle strength index can be calculated from the difference in the amount of ferrite particles passing through the mesh before and after the crushing process.

(Mesh passing amount X before crushing treatment)
The mesh passing amount X before the crushing treatment was determined as follows. When measuring the mesh passing amount X, a charge amount measuring device Blow-off (Kyocera Chemical TB-200) was used, and ferrite particles were weighed out to have a volume of 30 mL as a sample.

First, the weight of a special cell (Faraday gauge) fitted with 635M (mesh) (opening 20 μm) is measured, 0.5000±0.0005 g of ferrite particles are weighed and put into the special cell (this is the put-in weight A), and the weight B of the special cell containing the ferrite particles is measured. Next, the special cell containing the ferrite particles is set in the blow-off charge measuring device and blown for 30 seconds at a blow pressure of 0.5 kgf/ ^cm2 , after which the special cell is removed and the weight C of the special cell containing the carrier core material after blowing is measured. Then, the amount of carrier core material passing through the mesh is calculated based on the following formula (1).

Amount passing through the mesh (weight %) = (weight before blow B - weight after blow C) / weight input A x 100 (%) ... (1)

(Mesh passing amount Y after crushing treatment)
The mesh passing amount Y after crushing was determined in the same manner as above after crushing the ferrite particles as follows: The crushing was performed by placing the particles in a sample case (inner diameter φ78 mm × inner height 37 mm, stainless steel) of a sample mill (SK-M2, Kyoritsu Riko Co., Ltd.) as a small grinder, and stirring for 30 seconds at a rotation speed of 15,000 rpm (unloaded) using a motor of AC 100 V, 120 W, 2.7 A.
The crushing blade in the sample mill was M2-04 (Kyoritsu Riko Co., Ltd.). A new crushing blade was used each time the crushing process for one sample was completed.

(Particle Strength Index)
Based on the mesh passing amounts before and after the crushing treatment obtained as described above, the particle strength index was calculated according to the following formula (2).
Particle strength index (wt%)=mesh passing amount Y (wt%) after crushing treatment−mesh passing amount X (wt%) before crushing treatment (2)

(9) Saturation magnetization The saturation magnetization was measured using a vibrating sample magnetic measuring device (model: VSM-C7-10A (manufactured by Toei Kogyo Co., Ltd.)). The specific measurement procedure was as follows. First, the ferrite particles produced in each Example and Comparative Example were used as samples, and the samples were filled into a cell with an inner diameter of 5 mm and a height of 2 mm and set in the above device. Then, a magnetic field was applied and swept up to 1K·1000/4π·A/m (=1 kOe). Next, the applied magnetic field was reduced, and a hysteresis curve was created on a recording paper. From the data of this curve, the magnetization at an applied magnetic field of 1K·1000/4π·A/m was read and taken as the saturation magnetization.

(10) Resistance The ferrite particles produced in each Example and Comparative Example were used as samples, and the resistance value M (Ω) was measured under normal temperature and humidity environment (23°C, relative humidity 55%) according to the following procedure. First, the electrodes were spaced 6.5 mm apart, and non-magnetic parallel plate electrodes (10 mm x 40 mm) were placed facing each other, and 200 mg of sample was filled between them. The sample was held between the parallel plate electrodes by a magnet (surface magnetic flux density: 1500 Gauss, area of magnet in contact with electrode: 10 mm x 30 mm) attached to the parallel plate electrodes. Then, a voltage of 1000 V was applied between the parallel plate electrodes facing each other, and the resistance was measured using an electrometer (KEITHLEY, insulation resistance meter model 16517A). The above-mentioned resistance value M is a resistance value measured by the above-mentioned procedure in a constant temperature and humidity chamber in which the atmospheric temperature and humidity are adjusted to a normal temperature and humidity environment (23°C, relative humidity 55%) by the method, after exposing the sample for 12 hours or more in the constant temperature and humidity chamber in which the atmospheric temperature and humidity are adjusted to the normal temperature and humidity environment (23°C, relative humidity 55%) by the method.

2. Evaluation Results Table 2 shows the measurement results for each of the above evaluation items.
(1) Perovskite-type crystal phase component content, etc. As shown in Table 2, it was confirmed that the ferrite particles of Examples 1 to 4 contained 0.94% by mass to 0.97% by mass of a perovskite-type crystal phase component, and the ferrite particles of Comparative Examples 1 to 3 contained 0.84% by mass to 0.95% by mass of a perovskite-type crystal phase component.
It was also confirmed that the ferrite particles of Examples 1 to 4 contained 96.6% by mass to 97.4% by mass of a spinel-type crystal phase component, and the ferrite particles of Comparative Examples 1 to 3 contained 95.7% by mass to 96.7% by mass of a spinel-type crystal phase component.
Furthermore, the Zr dispersion degree of the ferrite particles in each of the Examples and Comparative Examples was 1.0, and it was confirmed that the ferrite particles were mainly composed of a spinel crystal phase component and had a perovskite crystal phase well dispersed within the particles.

(2) Volume average particle size ( _D50 )
As shown in Table 2, the volume average particle size (D ₅₀ ) of each ferrite particle was about 32 μm to 33 μm, and it was confirmed that the sizes were approximately the same.

(3) Content of the specified particles In the ferrite particles of Examples 1 to 4, the content of the specified particles was 1% to 3%, whereas in the ferrite particles of Comparative Examples 1 to 3, the content of the specified particles was 5% to 8%. The ferrite particles of each Example and the ferrite particles of each Comparative Example were manufactured using the same amount of the same raw material and were manufactured to the same size, and both contained a perovskite crystal phase, but it was confirmed that by reducing the slurry particle size and slowing the heating rate during firing, it was possible to prevent the generation of voids with a large maximum Feret diameter compared to the circle equivalent diameter, and to reduce the content of the specified particles. In addition, when a separate verification experiment was conducted, it was also confirmed that the effect on the content of the specified particles was greater when the slurry particle size was kept constant and the heating rate was changed in the range of 50 ° C. / hour to 150 ° C. / hour than when the slurry particle size was changed in the range of 0.9 μm to 2.3 μm and the heating rate was constant.

(4) Apparent density and BET specific surface area The apparent densities of the ferrite particles of Examples 1 to 4 and those of Comparative Examples 1 and 2 were similar (2.28 g/ ^cm3 to 2.32 g/ ^cm3 ), but the apparent density of the ferrite particles of Comparative Example 3 was 2.14 g/ ^cm3 , a lower value compared to the other ferrite particles. The content of the above-mentioned specified particles in the ferrite particles of Comparative Example 3 was 8%, which is higher than the other ferrite particles, and this is thought to have resulted in the lower apparent density.
The ferrite particles of Examples 1 to 4 had a larger BET specific surface area than the ferrite particles of Comparative Examples 1 and 2, but had a smaller BET specific surface area than the ferrite particles of Comparative Example 3. It is believed that this result was due to the difference in firing temperature.

(5) Particle Strength Index The particle strength index of the ferrite particles of Examples 1 to 4 is 0.3 or less, whereas the ferrite particles of Comparative Examples 1 to 3 are 0.6 to 1.0. It was confirmed that the higher the content of the predetermined particles, the lower the particle strength index, and the stronger the ferrite particles obtained. In particular, the particle strength index of the ferrite particles of Comparative Example 3, which has a higher content of the predetermined particles than the others, is 1.0, and it was confirmed that the strength is lower than the others. The content of the predetermined particles of the ferrite particles of Comparative Examples 1 and 2 is 5%, and the content of the predetermined particles of the ferrite particles of Example 3 is 3%. On the other hand, the BET specific surface area of the ferrite particles of Comparative Examples 1 and 2 is smaller than the BET specific surface area of the ferrite particles of Example 3, and the surface unevenness is small. However, the particle strength index of the ferrite particles of Comparative Examples 1 and 2 is lower than that of the ferrite particles of Example 3, and it was confirmed that ferrite particles with extremely high particle strength can be obtained by obtaining ferrite particles with a low content of the predetermined particles.

(6) Saturation Magnetization and Resistance The saturation magnetization and resistance of each ferrite particle were approximately the same, and it was confirmed that the magnetic properties and electrical properties were also approximately the same.

As described above, the ferrite particles of Examples 1 to 4 and the ferrite particles of Comparative Examples 1 to 3 have high saturation magnetization and resistance, and can effectively suppress carrier scattering due to low magnetization, etc. On the other hand, the ferrite particles of Examples 1 to 4 have a lower content of the above-mentioned specific particles and higher particle strength compared to the ferrite particles of Comparative Examples 1 to 3. Therefore, the ferrite particles of Examples 1 to 4 can better prevent cracking and chipping when subjected to mechanical stress, and can effectively suppress carrier scattering associated with the generation of fine powder, compared to the ferrite particles of Comparative Examples 1 to 3.

The ferrite particles according to the present invention have high magnetization and high resistance, and furthermore, the particle strength is high. Therefore, by using the ferrite particles as the carrier core material, cracks and chipping are reduced, and the occurrence of carrier scattering caused by cracks and chipping can be effectively suppressed. As a result, a developer with a longer life than conventional ones can be obtained, and damage to the photoconductor or fixing roller due to the carrier being adhered thereto can be suppressed, thereby extending the life of the electrophotographic developing device.

Claims (7)

The composition of the present invention is such that the composition of the present invention contains 50% by mass or more of a spinel-type crystal phase component represented by the composition formula (MnO)x(MgO)y(Fe2O3 ₎ z ₍ wherein 15≦x≦50, 2≦y≦35, 45≦z≦60, and x+y+z=100 (mol%)),
Contains a crystalline phase component consisting of a perovskite type crystal represented by the composition formula SrZrO3 _,
Ferrite particles, in which when the particle size of each particle is expressed as a circle equivalent diameter in a cross section in a backscattered electron image of the particle cross section photographed by a scanning electron microscope, the content of particles having internal voids whose maximum Feret diameter is larger than 15% of the particle size is 4% or less,
When a phase composition analysis of a crystal phase constituting the ferrite particles is performed by Rietveld analysis of an X-ray diffraction pattern, the ferrite particles contain 0.05 mass % or more and 4.00 mass % or less of a crystal phase component consisting of a perovskite type crystal .
The ferrite particles are characterized by: 2. The ferrite particles according to claim 1, characterized in that the apparent density of the ferrite particles is within the range represented by the following formula:
2.15≦Y≦2.45
In the above formula, Y is the apparent density (g/cm ³ ) of the ferrite particles. 3. The ferrite particles according to claim 1, wherein the BET specific surface area of the ferrite particles is within the range represented by the following formula:
0.08≦X≦0.300
In the above formula, X is the BET specific surface area (m ² /g) of the ferrite particles. A carrier core material for an electrophotographic developer, comprising the ferrite particles according to any one of claims 1 to 3 . 5. A carrier for electrophotographic developer comprising the ferrite particles according to claim 1 and a resin coating layer provided on the surface of the ferrite particles. 6. An electrophotographic developer comprising the carrier for electrophotographic developer according to claim 5 and a toner. The electrophotographic developer according to claim 6, which is used as a replenishment developer.

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