JP2011207656A - Ferrite particle, and carrier for electrophotographic development containing the same, developer for electrophotography, and method for manufacturing ferrite particle - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide ferrite particles having desired saturation magnetization and high electric resistance.SOLUTION: The ferrite particles have a composition represented by MgFeO(0≤x≤0.95), wherein the excessive oxygen amount δ in a spinel lattice in the ferrite phase is in the range of 0.01≤δ≤0.18. A method for manufacturing the ferrite particles includes a step of mixing a raw material of Fe and a raw material of Mg in a liquid medium to obtain a slurry, a step of subjecting the slurry to spray drying to obtain particles, and a step of firing the particles to obtain a fired product. Preferably, the firing is carried out under a condition to meet expression (1):-5,000≤T×logP≤0, wherein T is temperature, and Pis oxygen pressure/total pressure.

Description

  The present invention relates to ferrite particles, an electrophotographic developer carrier using the same, an electrophotographic developer, and a method for producing ferrite particles.

  In image forming apparatuses such as facsimiles, printers, and copiers using an electrophotographic system, powder toner as a developer is attached to an electrostatic latent image on a photosensitive member, and the attached toner image is applied to a predetermined sheet or the like. After being transferred to the sheet, it is heated and pressurized to melt and fix it on a sheet or the like. Here, the developer is roughly classified into a one-component development method using a one-component developer containing only toner and a two-component development method using a two-component developer containing toner and carrier. . In recent years, the two-component development method has been used in most cases because toner charge control is easy, stable image quality can be obtained, and high-speed development is possible.

  In the two-component development method, the carrier forms a magnetic brush conjugated with toner on the developing sleeve, and functions to move the toner to the photoreceptor via the magnetic brush. Here, when the carrier forms a magnetic brush on the developing sleeve, the magnetic force of the carrier is important. When the magnetic force of the carrier is too low, the coupling force between the carrier and the developing sleeve and between the carrier and the carrier is weakened. When the coupling force is weak, the carrier forming the magnetic brush cannot withstand the centrifugal force of the rotating developing sleeve, and the carrier is scattered on the photosensitive member together with the toner, so that the carrier adheres to the photosensitive member. (Carrier adhesion) occurs, resulting in image abnormality.

  On the other hand, if the magnetic force of the carrier is too high, the coupling force between the carriers becomes strong and the magnetic brush becomes too hard. Then, when the toner moves from the magnetic brush to the photoconductor, the toner moves in a concentrated manner, resulting in a phenomenon that the variation increases and the image quality deteriorates. Further, since the development torque is high, the developer is rapidly deteriorated and the durability is deteriorated.

  Therefore, for example, in Patent Document 1, in a carrier in which the surface of magnesium (hereinafter sometimes referred to as “Mg”) ferrite particles is resin-coated, the Mg content is set within a predetermined range, and in a nitrogen atmosphere or in the presence of nitrogen + oxygen. An electrophotographic carrier having predetermined magnetic characteristics by firing is proposed.

JP 2001-154416 A

  In recent years, in order to meet the market demand for higher image formation speed and higher image quality in image forming apparatuses, it is required to increase the electrical resistance and charge of ferrite particles used as carriers.

  However, in the proposed electrophotographic carrier, the saturation magnetization is considered to be maintained within a predetermined range. However, since the electrical resistance has not been studied, the electrical resistance is low, charge leakage occurs, and the image quality deteriorates. May cause.

  The present invention has been made in view of such a conventional problem, and an object thereof is to provide a ferrite particle having a desired saturation magnetization and a high electric resistance, and a method for producing the same.

  Another object of the present invention is to provide an electrophotographic developer carrier and an electrophotographic developer that satisfy high speed and high image quality.

As a result of intensive investigations to obtain ferrite particles having desired saturation magnetization and high electric resistance, the present inventors have suitable ferrite particles represented by Mg _x Fe _3-x O ₄ as the composition. In addition, when the ferrite particles take in oxygen excessively, the valence of iron is changed, and the magnetic force and electric resistance including saturation magnetization are changed, and the present invention has been made. That is, the ferrite particles according to the present invention are represented by the composition formula: Mg _x Fe _3-x O _{4 + δ} (where 0 ≦ x ≦ 0.95), and the oxygen excess amount δ in the spinel lattice in the ferrite phase is It is characterized in that 0.01 ≦ δ ≦ 0.18.

In addition, the present inventors can control the oxygen uptake amount in the ferrite particles by the oxygen chemical potential μ _O2 (hereinafter sometimes referred to as “μ _O2 ”) in the firing process, and adjust μ _O2 in the cooling process. By doing so, it has been found that oxygen can be uniformly excessively provided inside the ferrite particles. Here, μ _O2 can be expressed by an equation of μ _O2 = RTlnP _O2 and can be changed according to temperature and oxygen concentration. Therefore, the method for producing ferrite particles according to the present invention includes an Fe raw material whose components are adjusted so that ferrite particles having a composition represented by Mg _x Fe _3-x O ₄ (where 0 ≦ x ≦ 0.95) are generated. And a step of mixing a Mg raw material in a medium solution to obtain a slurry, a step of spray drying the slurry to obtain a granulated product, and a step of firing the granulated product to obtain a fired product, The firing is performed under conditions satisfying the following formula (1) to generate ferrite particles represented by Mg _x Fe _3-x O _{4 + δ} (where 0.01 ≦ δ ≦ 0.18).
−5000 ≦ T × log P _O2 ≦ 0 (1)
(Where T: temperature (° C.), P _O2 = oxygen pressure / overall pressure)

  Here, the average particle diameter of the ferrite particles is preferably in the range of 10 μm to 100 μm.

  In addition, according to the present invention, there is provided an electrophotographic developing carrier characterized in that the surface of the ferrite particles described above is coated with a resin.

  Furthermore, according to the present invention, there is provided an electrophotographic developer comprising the carrier described above and a toner.

  Since the ferrite particles of the present invention have a predetermined amount of excess oxygen in the spinel lattice in the ferrite phase, they have a desired saturation magnetization and at the same time a high electrical resistance.

  In addition, since the electrophotographic developer carrier and the electrophotographic developer of the present invention use the ferrite particles, it is possible to cope with high-speed image formation and high image quality.

  Moreover, in the production method of the present invention, the firing is performed under the condition satisfying the formula (1), so that the amount of excess oxygen taken into the ferrite particles can be controlled.

The X-ray-diffraction pattern about the ferrite particle of Examples 1-6. The figure which shows the relationship between oxygen chemical potential (mu) _O2 and a lattice constant.

First, the ferrite particles according to the present invention will be described. The major feature of the ferrite particles according to the present invention is represented by the composition formula: Mg _x Fe _3-x O _{4 + δ} (where 0 ≦ x ≦ 0.95), and the oxygen excess δ in the spinel structure in the ferrite is 0. It is in the range of .01 ≦ δ ≦ 0.18. Thereby, for example, the electric resistance can be maintained high while maintaining the predetermined saturation magnetization σ _s . As x, the range of 0.2-0.7 is preferable, More preferably, it is the range of 0.4-0.6. This is because when x is in the above preferred range, the ferrite particles are of a type that takes in oxygen excessively. Also, if the excess oxygen amount δ is less than 0.01, the effects of the present invention cannot be obtained, and if the excess oxygen amount δ exceeds 0.18, too much oxygen is taken in to obtain single-phase ferrite. The effect of the present invention cannot be obtained. A more preferable range of the oxygen excess amount δ is 0.05 to 0.12.

  The average particle diameter of the ferrite particles according to the present invention is preferably in the range of 10 μm to 100 μm. When the average particle diameter is 10 μm or more, the necessary magnetic force is reliably imparted to each of the particles. For example, when ferrite particles are used as a carrier for electrophotographic development, carrier adhesion to the photoreceptor is suppressed. become. On the other hand, when the average particle diameter is 100 μm or less, the image characteristics can be kept good. In order to set the average particle diameter of the ferrite particles within the above range, classification may be performed using a sieve or the like during the manufacturing process of the ferrite particles or after the manufacturing process.

The preferred saturation magnetization σ _s of the ferrite particles according to the present invention is in the range of 20 to 90 emu / g. When the saturation magnetization σ _s is less than 20 emu / g, for example, when ferrite particles are used as a carrier for electrophotographic development, there is a possibility that carrier adhesion to the photoreceptor frequently occurs. On the other hand, when the saturation magnetization σ _s exceeds 90 emu / g, the ears of the magnetic brush become hard, and there is a possibility that the image quality is lowered in electrophotographic development. The more preferable saturation magnetization σ _s of the ferrite particles is in the range of 30 to 80 emu / g, and more preferably in the range of 40 to 70 emu / g.

The preferred electrical resistance of the ferrite particles according to the present invention is in the range of 1 × 10 ⁷ to 1 × 10 ¹² Ωcm at an applied voltage of 10V. If the electrical resistance is less than 1 × 10 ⁷ Ωcm, charge leakage may occur. On the other hand, if the electrical resistance exceeds 1 × 10 ¹² Ωcm, the edge effect may increase and the image density may decrease. More preferable electrical resistance of the ferrite particles is in the range of 1 × 10 ⁸ to 1 × 10 ¹¹ Ωcm, and more preferably in the range of 1 × 10 ⁹ to 1 × 10 ¹⁰ Ωcm.

  The ferrite particles of the present invention can be used in various applications, for example, electrophotographic developer carriers, electromagnetic wave absorbing materials, electromagnetic shielding material powders, rubber, fillers / reinforcing materials for plastics, paints, paints / adhesives It can be used as a matting material, filler, reinforcing material and the like. Among these, it is particularly preferably used as a carrier for electrophotographic development.

  Although the manufacturing method of the ferrite particle of the present invention is not particularly limited, the manufacturing method described below is preferable.

First, an Fe raw material and an Mg raw material are weighed, put into a dispersion medium, and mixed to prepare a slurry. Fe ₂ O ₃ powder, Fe oxide, Fe hydroxide, etc. can be used as the Fe raw material, and MgFe ₂ O ₄ calcined powder, Mg oxide, Mg hydroxide, etc. are suitably used as the Mg raw material it can. The solid content concentration of the slurry is desirably in the range of 50 to 90 wt%. If necessary, the raw materials Fe and Mg may be pulverized and mixed before being introduced into the dispersion medium.

  Water is preferred as the dispersion medium used in the present invention. In addition to the Fe raw material and Mg raw material, a binder, a dispersing agent, and the like may be blended in the dispersion medium as necessary. For example, polyvinyl alcohol can be suitably used as the binder. The blending amount of the binder is preferably about 0.5 to 2 wt% in the slurry. Moreover, as a dispersing agent, polycarboxylate ammonium etc. can be used conveniently, for example. As the blending amount of the dispersant, the concentration in the slurry is preferably about 0.5 to 2 wt%. In addition, you may mix | blend a lubricant, a sintering accelerator, etc.

  Next, the slurry prepared as described above is wet-pulverized as necessary. For example, wet grinding is performed for a predetermined time using a ball mill or a vibration mill. The average particle diameter of the raw material after pulverization is preferably 50 μm or less, more preferably 10 μm or less. The vibration mill or ball mill preferably contains a medium having a predetermined particle diameter. Examples of the material of the media include iron-based chromium steel and oxide-based zirconia, titania, and alumina. As a form of a grinding | pulverization process, any of a continuous type and a batch type may be sufficient. The particle size of the pulverized product is adjusted depending on the pulverization time and rotation speed, the material and particle size of the media used, and the like.

  Then, the pulverized slurry is spray-dried and granulated. Specifically, the slurry is introduced into a spray dryer such as a spray dryer, and granulated into a spherical shape by spraying into the atmosphere. The atmospheric temperature during spray drying is preferably in the range of 100 to 300 ° C. Thereby, a spherical granulated product having a particle size of 10 to 200 μm is obtained. In addition, it is desirable that the obtained granulated product has a sharp particle size distribution by removing coarse particles and fine powder using a vibration sieve or the like.

  Next, the obtained granulated product is put into a heated furnace and fired to obtain a fired product having a magnetic phase. The firing temperature may be set to a temperature range in which the target magnetic phase is generated. However, when producing the ferrite particles according to the present invention, firing is preferably performed in a temperature range of 1000 to 1400 ° C. More preferably, it is the temperature range of 1100 degreeC-1350 degreeC.

What is important here is that the firing is performed under the condition satisfying the above-mentioned formula (1). As a result, the oxygen excess δ in the spinel structure in the ferrite particles can be set in the range of 0.01 ≦ δ ≦ 0.18, and as a result, ferrite particles having appropriate saturation magnetization and high electrical resistance can be obtained. . Specifically, for example, the oxygen concentration in the furnace in a range of 0.03% ~20.6%, μ _O2 is -100KJ / mоl or more at 1200 ℃, -19KJ / mоl less, -74KJ at 800 ° C. / A cooling process of not less than −14 KJ / mol and not more than −45 KJ / mol and not more than −45 KJ / mol and not more than −8 KJ / mol at 400 ° C. is performed. By controlling the temperature T and oxygen pressure P _O2 in the furnace so as to have such a range, oxygen-excess ferrite particles are obtained to the inside particles. Further, in the cooling stage in the firing process, the fired product in the furnace is immersed in a cooling solvent such as liquid nitrogen or water at the μ _O2 controlled to have a desired saturation magnetization and electrical resistance. You may make it obtain. As a result, the reaction during the temperature drop is suppressed, and the intended saturation magnetization and electrical resistance at μ _O2 can be obtained. In Examples described later, ferrite particles having a predetermined oxygen excess were produced by this method of rapid cooling in the cooling stage. The oxygen pressure in the furnace may be controlled by causing the atmosphere or a mixed gas of air and nitrogen to flow into the furnace.

  Next, the obtained fired product is crushed. Specifically, for example, the fired product is crushed by a hammer mill or the like. As a form of a crushing process, any of a continuous type and a batch type may be sufficient. And if necessary, classification may be performed in order to make the particle size in a predetermined range. As a classification method, a conventionally known method such as air classification or sieve classification can be used. In addition, after primary classification with an air classifier, the particle size may be aligned within a predetermined range with a vibration sieve or an ultrasonic sieve. Furthermore, you may make it remove a nonmagnetic particle with a magnetic field separator after a classification process.

  When the ferrite particles of the present invention produced as described above are used as a carrier for electrophotographic development, the ferrite particles can be used as they are as a carrier for electrophotographic development. However, from the viewpoint of chargeability and the like, It is preferable to coat the surface with a resin.

  As the resin for covering the surface of the ferrite particles, conventionally known resins can be used, for example, polyethylene, polypropylene, polyvinyl chloride, poly-4-methylpentene-1, polyvinylidene chloride, ABS (acrylonitrile-butadiene-styrene). Examples thereof include resins, polystyrene, (meth) acrylic resins, polyvinyl alcohol resins, polyvinyl chloride-based, polyurethane-based, polyester-based, polyamide-based, polybutadiene-based thermoplastic elastomers, fluorine silicone-based resins, and the like.

  In order to coat the surface of the ferrite particles with a resin, a resin solution or dispersion may be applied to the ferrite particles. Solvents for the coating solution include aromatic hydrocarbon solvents such as toluene and xylene; ketone solvents such as acetone, methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone; cyclic ether solvents such as tetrahydrofuran and dioxane; ethanol, propanol, and butanol Alcohol solvents such as ethyl cellosolve, cellosolve solvents such as butyl cellosolve; ester solvents such as ethyl acetate and butyl acetate; amide solvents such as dimethylformamide and dimethylacetamide, etc. . The concentration of the resin component in the coating solution is generally in the range of 0.001 to 30 wt%, particularly 0.001 to 2 wt%.

  As a method for coating the resin on the ferrite particles, for example, a spray drying method, a fluidized bed method, a spray drying method using a fluidized bed, an immersion method, or the like can be used. Among these, the fluidized bed method is particularly preferable in that it can be efficiently applied with a small amount of resin. For example, in the case of the fluidized bed method, the resin coating amount can be adjusted by the amount of resin solution sprayed and the spraying time.

  The electrophotographic developer according to the present invention is obtained by mixing the carrier prepared as described above and a toner. The mixing ratio of the carrier and the toner is not particularly limited, and may be determined as appropriate based on the developing conditions of the developing device to be used. In general, the toner concentration in the developer is preferably in the range of 1 to 20 wt%. When the toner density is less than 1 wt%, the image density becomes too low, and when the toner density exceeds 20 wt%, the toner scatters in the developing device, and the toner adheres to the background portion such as internal dirt or transfer paper. This is because there is a risk of occurrence. A more preferable toner concentration is in the range of 3 to 15 wt%.

  A known mixing device can be used for mixing the carrier and the toner. For example, a Henschel mixer, a V-type mixer, a tumbler mixer, a hybridizer, or the like can be used.

(Example 1)
Fe ₂ O ₃ and MgFe ₂ O ₄ calcined powders were prepared and weighed so that Fe ₂ O ₃ : MgFe ₂ O ₄ = 3: 8 (molar ratio). The mixing ratio of the raw materials corresponds to x = 0.8 in the composition formula Mg _x Fe _3−x O _{4 + δ} . The weighed Fe ₂ O ₃ and MgFe ₂ O ₄ calcined powders were dispersed in pure water to which ammonium polycarboxylate as a dispersant was added so that the concentration in the medium liquid was 1% to obtain a mixture. This mixture was pulverized by a wet ball mill (media diameter: 2 mm) to obtain a slurry.

The obtained slurry was sprayed into hot air at about 150 ° C. with a spray dryer to obtain a dry granulated product having a particle size of 10 to 100 μm. And the granulated material with a particle size exceeding 100 micrometers was removed using the sieve. The obtained dried granulated product was put into an electric furnace and fired at 1200 ° C. for 3 hours in an air flow atmosphere, and then immediately cooled to obtain a fired product (temperature T: 1200 ° C., oxygen content). Pressure P _O2 (oxygen pressure / overall pressure): 2.1 × 10 ⁻¹ , T × log P _O2 = −813). After the obtained fired product was pulverized, coarse particles and fine particles were removed using a sieve to obtain Mg ferrite particles having an average particle diameter of 45 μm. The average particle size of the ferrite particles is measured using a laser diffraction particle size distribution measuring device (Microtrack, Model 9320-X100 manufactured by Nikkiso Co., Ltd.).

  The obtained ferrite particles were measured for oxygen excess δ, saturation magnetization σs, and electrical resistance by the following methods. Further, ferrite particles were coated with a resin and used as a carrier, which was then put into an evaluation machine for image evaluation. The results are summarized in Table 1.

(Measurement method of excess oxygen δ)
In Mg _x Fe _3-x O _{4 + δ} , Mg is divalent (+2) and O is divalent (−2), so there is no excess oxygen in the Mg _x Fe _3-x O _{4 + δ} lattice (δ = 0). In this case, the theoretical valence of Fe is calculated from the following formula.
Theoretical valence of Fe = {(8-2x) / (3-x)}
On the other hand, when oxygen excess is present in the lattice, the average valence of Fe is calculated from the following formula.
Average valence of Fe = {(4 + δ) × 2-2x} / (3-x)
= (8-2x + 2δ) / (3-x)
= (8-2x) / (3-x) + 2δ / (3-x)
= Theoretical valence of Fe + 2δ / (3-x)
Therefore, the excess oxygen amount δ in the Mg _x Fe _3-x O _{4 + δ} lattice can be obtained from the following equation.
δ = (3-x) / 2 (average valence of Fe−theoretical valence of Fe)

The average valence of Fe is obtained as follows. First, ferrite particles are dissolved in hydrochloric acid (HCl) water, which is a reducing acid, in a bubble of carbon dioxide gas to obtain a solution. Thereafter, the amount of Fe ²⁺ ions in the solution is quantitatively analyzed by potentiometric titration with a potassium permanganate solution, and the quantitative value (α) of Fe ²⁺ is obtained from the titration amount.
Next, the ferrite particles are weighed in the same amount as above, and dissolved in a mixed acid water of hydrochloric acid and nitric acid to obtain a solution. Next, after evaporating the solution to dryness, sulfuric acid water is added to the dried product and redissolved to obtain a solution, and excess hydrochloric acid and nitric acid are volatilized. Solid Al is added to the solution to reduce Fe ³⁺ in the solution to Fe ²⁺ . Subsequently, the amount of Fe ²⁺ ions in the reduced solution is quantitatively analyzed by potentiometric titration with a potassium permanganate solution, and the quantitative value (β) of total Fe is obtained from the titration amount.
Subsequently, the quantitative value (α) of Fe ²⁺ is subtracted from the quantitative amount (β) of total Fe to obtain the Fe ³⁺ amount of the ferrite particles. And the average valence of Fe is calculated from the following formula.
Fe average valence = {3 × (β−α) + 2 × α} / β

(Saturation magnetization measurement)
The magnetic properties of the ferrite were measured for magnetic susceptibility using VSM (manufactured by Toei Kogyo Co., Ltd., VSM-P7), and the saturation magnetization σ _s (emu / g) in an applied magnetic field of 10 kOe was measured.

(Electrical resistance measurement)
Two brass plates as electrodes having a thickness of 2 mm whose surfaces were electropolished were arranged to face each other with a distance of 2 mm. After inserting 200 mg of ferrite particles between the electrodes, a magnet having a cross-sectional area of 240 mm ² (ferrite magnet having a surface magnetic flux density of 1500 gauss) is placed behind each electrode to form a bridge of ferrite particles between the electrodes. I let you. A DC voltage of 10 V to 1000 V was applied between the electrodes, the value of the current flowing through the ferrite particles was measured, and the electrical resistance of the ferrite particles was calculated.

(Image evaluation)
A silicone resin (manufactured by Shin-Etsu Chemical, KR251) was dissolved in toluene to prepare a coating resin solution. Then, the ferrite particles and the coating resin solution were put into a stirrer at a ratio of 9: 1 by weight, and stirred with heating at 150 to 250 ° C. for 3 hours while immersing the ferrite particles in the resin solution. The resin-coated ferrite particles were heated at 250 ° C. for 5 hours with a hot air circulation type heating device to cure the resin coating layer to obtain a carrier.
92% by weight of the obtained carrier and 8% by weight of toner (cyan) of a full color copying machine were mixed with a V-type mixer to obtain an electrophotographic developer. This electrophotographic developer is put into an evaluation machine based on an image forming apparatus of 20 sheets / minute of digital reversal development method, in which an AC bias voltage is applied as a developing bias voltage, and initially, 50K sheets, 100K sheets, A gray image was output at 150K sheets. Then, image evaluation was performed according to the following criteria, paying attention to the presence or absence of white spots due to carrier adhesion.
“◎” is very good “○” is good “△” is usable level “×” is not usable

(Example 2)
In the firing step, the granulated product is fired at 1200 ° C. for 3 hours, and in the cooling stage, the temperature T is 1000 ° C., the oxygen partial pressure P _O2 (oxygen pressure / overall pressure) is 2.1 × 10 ⁻¹ , T × log P _A fired product was obtained in the same manner as in Example 1 except that quenching was performed at _O2 = −678. After the obtained fired product was pulverized, coarse particles and fine particles were removed using a sieve to obtain Mg ferrite particles having an average particle diameter of 45 μm.
The obtained Mg ferrite particles were measured for oxygen excess δ, saturation magnetization σs, and electrical resistance in the same manner as in Example 1, and the Mg ferrite particles were coated with a resin and used as a carrier for evaluation into images. It was. The results are summarized in Table 1.

(Example 3)
In the firing step, the granulated product is fired at 1200 ° C. for 3 hours, and in the cooling stage, the temperature T is 800 ° C., the oxygen partial pressure P _O2 (oxygen pressure / total pressure) is 2.1 × 10 ⁻¹ , T × log P _A fired product was obtained in the same manner as in Example 1 except that quenching was performed at _O2 = -542. After the obtained fired product was pulverized, coarse particles and fine particles were removed using a sieve to obtain Mg ferrite particles having an average particle diameter of 45 μm.
The obtained Mg ferrite particles were measured for oxygen excess δ, saturation magnetization σs, and electrical resistance in the same manner as in Example 1, and the Mg ferrite particles were coated with a resin and used as a carrier for evaluation into images. It was. The results are summarized in Table 1.

Example 4
In the firing step, the granulated product is fired at 1200 ° C. for 3 hours, and in the cooling stage, the temperature T is 600 ° C., the oxygen partial pressure P _O2 (oxygen pressure / total pressure) is 2.1 × 10 ⁻¹ , T × log P _A fired product was obtained in the same manner as in Example 1 except that quenching was performed at _O2 = −407. After the obtained fired product was pulverized, coarse particles and fine particles were removed using a sieve to obtain Mg ferrite particles having an average particle diameter of 45 μm.
The obtained Mg ferrite particles were measured for oxygen excess δ, saturation magnetization σs, and electrical resistance in the same manner as in Example 1, and the Mg ferrite particles were coated with a resin and used as a carrier for evaluation into images. It was. The results are summarized in Table 1.

(Example 5)
In the firing step, the granulated product is fired at 1200 ° C. for 3 hours, and in the cooling stage, the temperature T is 400 ° C., the oxygen partial pressure P _O2 (oxygen pressure / overall pressure) is 2.1 × 10 ⁻¹ , T × log P _A fired product was obtained in the same manner as in Example 1 except that quenching was performed at _O2 = -271. After the obtained fired product was pulverized, coarse particles and fine particles were removed using a sieve to obtain Mg ferrite particles having an average particle diameter of 45 μm.
The obtained Mg ferrite particles were measured for oxygen excess δ, saturation magnetization σs, and electrical resistance in the same manner as in Example 1, and the Mg ferrite particles were coated with a resin and used as a carrier for evaluation into images. It was. The results are summarized in Table 1.

(Example 6)
In the firing step, the granulated product is fired at 1200 ° C. for 3 hours, and in the cooling stage, the temperature T is 25 ° C., the oxygen partial pressure P _O2 (oxygen pressure / total pressure) is 2.1 × 10 ⁻¹ , T × log P _A fired product was obtained in the same manner as in Example 1 except that a fired product was obtained at _O2 = -17. After the obtained fired product was pulverized, coarse particles and fine particles were removed using a sieve to obtain Mg ferrite particles having an average particle diameter of 45 μm.
The obtained Mg ferrite particles were measured for oxygen excess δ, saturation magnetization σs, and electrical resistance in the same manner as in Example 1, and the Mg ferrite particles were coated with a resin and used as a carrier for evaluation into images. It was. The results are summarized in Table 1.

(Example 7)
A granulated product was obtained in the same manner as in Example 1 except that Fe ₂ O ₃ and MgFe ₂ O ₄ calcined powder were mixed at 1: 1 (molar ratio) so that x = 0.6. . In the firing step, the granulated product was fired at 1200 ° C. for 3 hours in an atmosphere having an oxygen concentration of 1%, and then immediately cooled to obtain a fired product (temperature T: 1200 ° C., oxygen partial pressure P _O2 (oxygen). Pressure / overall pressure): 1.0 × 10 ⁻² , T × log P _O2 = −2400). After the obtained fired product was pulverized, coarse particles and fine particles were removed using a sieve to obtain Mg ferrite particles having an average particle diameter of 45 μm.
The obtained Mg ferrite particles were measured for oxygen excess δ, saturation magnetization σs, and electrical resistance in the same manner as in Example 1, and the Mg ferrite particles were coated with a resin and used as a carrier for evaluation into images. It was. The results are summarized in Table 1.

(Example 8)
In the firing step, the granulated product is fired at 1200 ° C. for 3 hours, and in the cooling stage, the temperature T is 1000 ° C., the oxygen partial pressure P _O2 (oxygen pressure / total pressure) is 1.0 × 10 ⁻² , T × log P _A fired product was obtained in the same manner as in Example 7 except that quenching was performed at _O2 = -2000. After the obtained fired product was pulverized, coarse particles and fine particles were removed using a sieve to obtain Mg ferrite particles having an average particle diameter of 45 μm.
The obtained Mg ferrite particles were measured for oxygen excess δ, saturation magnetization σs, and electrical resistance in the same manner as in Example 1, and the Mg ferrite particles were coated with a resin and used as a carrier for evaluation into images. It was. The results are summarized in Table 1.

Example 9
In the firing step, the granulated product is fired at 1200 ° C. for 3 hours, and in the cooling stage, the temperature T is 800 ° C., the oxygen partial pressure P _O2 (oxygen pressure / total pressure) is 1.0 × 10 ⁻² , T × log P _A fired product was obtained in the same manner as in Example 7 except that quenching was performed at _O2 = -1600. After the obtained fired product was pulverized, coarse particles and fine particles were removed using a sieve to obtain Mg ferrite particles having an average particle diameter of 45 μm.
The obtained Mg ferrite particles were measured for oxygen excess δ, saturation magnetization σs, and electrical resistance in the same manner as in Example 1, and the Mg ferrite particles were coated with a resin and used as a carrier for evaluation into images. It was. The results are summarized in Table 1.

(Example 10)
In the firing step, the granulated product is fired at 1200 ° C. for 3 hours, and in the cooling stage, the temperature T is 600 ° C., the oxygen partial pressure P _O2 (oxygen pressure / total pressure) is 1.0 × 10 ⁻² , T × log P _A fired product was obtained in the same manner as in Example 7 except that quenching was performed at _O2 = -1200. After the obtained fired product was pulverized, coarse particles and fine particles were removed using a sieve to obtain Mg ferrite particles having an average particle diameter of 45 μm.
The obtained Mg ferrite particles were measured for oxygen excess δ, saturation magnetization σs, and electrical resistance in the same manner as in Example 1, and the Mg ferrite particles were coated with a resin and used as a carrier for evaluation into images. It was. The results are summarized in Table 1.

(Example 11)
In the firing step, the granulated product is fired at 1200 ° C. for 3 hours, and in the cooling stage, the temperature T is 400 ° C., the oxygen partial pressure P _O2 (oxygen pressure / total pressure) is 1.0 × 10 ⁻² , T × logP. _A fired product was obtained in the same manner as in Example 7 except that quenching was performed at _O2 = -800. After the obtained fired product was pulverized, coarse particles and fine particles were removed using a sieve to obtain Mg ferrite particles having an average particle diameter of 45 μm.
The obtained Mg ferrite particles were measured for oxygen excess δ, saturation magnetization σs, and electrical resistance in the same manner as in Example 1, and the Mg ferrite particles were coated with a resin and used as a carrier for evaluation into images. It was. The results are summarized in Table 1.

(Example 12)
In the firing step, the granulated product is fired at 1200 ° C. for 3 hours, and in the cooling stage, the temperature T is 25 ° C., the oxygen partial pressure P _O2 (oxygen pressure / total pressure) is 1.0 × 10 ⁻² , T × log P _A fired product was obtained in the same manner as in Example 7 except that a fired product was obtained at _O2 = -50. After the obtained fired product was pulverized, coarse particles and fine particles were removed using a sieve to obtain Mg ferrite particles having an average particle diameter of 45 μm.
The obtained Mg ferrite particles were measured for oxygen excess δ, saturation magnetization σs, and electrical resistance in the same manner as in Example 1, and the Mg ferrite particles were coated with a resin and used as a carrier for evaluation into images. It was. The results are summarized in Table 1.

(Example 13)
A granulated product was obtained in the same manner as in Example 1 except that Fe ₂ O ₃ and MgFe ₂ O ₄ calcined powder were blended at 9: 4 (molar ratio) so that x = 0.4. . In the firing step, the granulated product was fired at 1200 ° C. for 3 hours in an atmosphere having an oxygen concentration of 0.1%, and then immediately cooled to obtain a fired product (temperature T: 1200 ° C., oxygen partial pressure P _O2 (Oxygen pressure / overall pressure): 1.0 × 10 ⁻³ , T × logP _O2 = −3600). After the obtained fired product was pulverized, coarse particles and fine particles were removed using a sieve to obtain Mg ferrite particles having an average particle diameter of 45 μm.
The obtained Mg ferrite particles were measured for oxygen excess δ, saturation magnetization σs, and electrical resistance in the same manner as in Example 1, and the Mg ferrite particles were coated with a resin and used as a carrier for evaluation into images. It was. The results are summarized in Table 1.

(Example 14)
In the firing step, the granulated product is fired at 1200 ° C. for 3 hours, and in the cooling stage, the temperature T is 1000 ° C., the oxygen partial pressure P _O2 (oxygen pressure / total pressure) is 1.0 × 10 ⁻³ , T × log P _A fired product was obtained in the same manner as in Example 13 except that quenching was performed at _O2 = -3000. After the obtained fired product was pulverized, coarse particles and fine particles were removed using a sieve to obtain Mg ferrite particles having an average particle diameter of 45 μm.
The obtained Mg ferrite particles were measured for oxygen excess δ, saturation magnetization σs, and electrical resistance in the same manner as in Example 1, and the Mg ferrite particles were coated with a resin and used as a carrier for evaluation into images. It was. The results are summarized in Table 1.

(Example 15)
In the firing step, the granulated product is fired at 1200 ° C. for 3 hours, and in the cooling stage, the temperature T is 800 ° C., the oxygen partial pressure P _O2 (oxygen pressure / total pressure) is 1.0 × 10 ⁻³ , T × log P _A fired product was obtained in the same manner as in Example 13 except that quenching was performed at _O2 = -2400. After the obtained fired product was pulverized, coarse particles and fine particles were removed using a sieve to obtain Mg ferrite particles having an average particle diameter of 45 μm.
The obtained Mg ferrite particles were measured for oxygen excess δ, saturation magnetization σs, and electrical resistance in the same manner as in Example 1, and the Mg ferrite particles were coated with a resin and used as a carrier for evaluation into images. It was. The results are summarized in Table 1.

(Example 16)
In the firing step, the granulated product is fired at 1200 ° C. for 3 hours, and in the cooling stage, the temperature T is 600 ° C., the oxygen partial pressure P _O2 (oxygen pressure / total pressure) is 1.0 × 10 ⁻³ , T × log P _A fired product was obtained in the same manner as in Example 13 except that quenching was performed at _O2 = -1800. After the obtained fired product was pulverized, coarse particles and fine particles were removed using a sieve to obtain Mg ferrite particles having an average particle diameter of 45 μm.
The obtained Mg ferrite particles were measured for oxygen excess δ, saturation magnetization σs, and electrical resistance in the same manner as in Example 1, and the Mg ferrite particles were coated with a resin and used as a carrier for evaluation into images. It was. The results are summarized in Table 1.

(Example 17)
In the firing step, the granulated product is fired at 1200 ° C. for 3 hours, and in the cooling stage, the temperature T is 400 ° C., the oxygen partial pressure P _O2 (oxygen pressure / total pressure) is 1.0 × 10 ⁻³ , T × log P _A fired product was obtained in the same manner as in Example 13 except that quenching was performed at _O2 = -1200. After the obtained fired product was pulverized, coarse particles and fine particles were removed using a sieve to obtain Mg ferrite particles having an average particle diameter of 45 μm.
The obtained Mg ferrite particles were measured for oxygen excess δ, saturation magnetization σs, and electrical resistance in the same manner as in Example 1, and the Mg ferrite particles were coated with a resin and used as a carrier for evaluation into images. It was. The results are summarized in Table 1.

(Example 18)
In the firing step, the granulated product is fired at 1200 ° C. for 3 hours, and in the cooling stage, the temperature T is 25 ° C., the oxygen partial pressure P _O2 (oxygen pressure / total pressure) is 1.0 × 10 ⁻³ , T × log P _A fired product was obtained in the same manner as in Example 13 except that a fired product was obtained at _O2 = -75. After the obtained fired product was pulverized, coarse particles and fine particles were removed using a sieve to obtain Mg ferrite particles having an average particle diameter of 45 μm.
The obtained Mg ferrite particles were measured for oxygen excess δ, saturation magnetization σs, and electrical resistance in the same manner as in Example 1, and the Mg ferrite particles were coated with a resin and used as a carrier for evaluation into images. It was. The results are summarized in Table 1.

(Example 19)
The Fe ₂ O ₃ and the MgFe ₂ O ₄ calcined powder are blended at a ratio of 6: 1 (molar ratio) so that x = 0.2, and carbon black as a reducing agent is added to the Fe ₂ O ₃ and MgFe ₂ O ₄ calcined powder. A granulated product was obtained in the same manner as in Example 1 except that 0.75 wt% was blended with respect to the total amount. In the firing step, the granulated product was fired at 1200 ° C. for 3 hours in an atmosphere having an oxygen concentration of 0.03%, and then immediately cooled to obtain a fired product (temperature T: 1200 ° C., oxygen partial pressure P _O2 (Oxygen pressure / overall pressure): 2.0 × 10 ⁻⁴ , T × logP _O2 = −4439). After the obtained fired product was pulverized, coarse particles and fine particles were removed using a sieve to obtain Mg ferrite particles having an average particle diameter of 45 μm.
The obtained Mg ferrite particles were measured for oxygen excess δ, saturation magnetization σs, and electrical resistance in the same manner as in Example 1, and the Mg ferrite particles were coated with a resin and used as a carrier for evaluation into images. It was. The results are summarized in Table 1.

(Example 20)
In the firing step, the granulated product is fired at 1200 ° C. for 3 hours, and in the cooling stage, the temperature T is 1000 ° C., the oxygen partial pressure P _O2 (oxygen pressure / total pressure) is 2.0 × 10 ⁻⁴ , T × log P _A fired product was obtained in the same manner as in Example 19 except that quenching was performed at _O2 = -3699. After the obtained fired product was pulverized, coarse particles and fine particles were removed using a sieve to obtain Mg ferrite particles having an average particle diameter of 45 μm.
The obtained Mg ferrite particles were measured for oxygen excess δ, saturation magnetization σs, and electrical resistance in the same manner as in Example 1, and the Mg ferrite particles were coated with a resin and used as a carrier for evaluation into images. It was. The results are summarized in Table 1.

(Example 21)
In the firing step, the granulated product is fired at 1200 ° C. for 3 hours, and in the cooling stage, the temperature T is 800 ° C., the oxygen partial pressure P _O2 (oxygen pressure / total pressure) is 2.0 × 10 ⁻⁴ , T × log P _A fired product was obtained in the same manner as in Example 19 except that quenching was performed at _O2 = -2959. After the obtained fired product was pulverized, coarse particles and fine particles were removed using a sieve to obtain Mg ferrite particles having an average particle diameter of 45 μm.
The obtained Mg ferrite particles were measured for oxygen excess δ, saturation magnetization σs, and electrical resistance in the same manner as in Example 1, and the Mg ferrite particles were coated with a resin and used as a carrier for evaluation into images. It was. The results are summarized in Table 1.

(Example 22)
In the firing step, the granulated product is fired at 1200 ° C. for 3 hours, and in the cooling stage, the temperature T is 600 ° C., the oxygen partial pressure P _O2 (oxygen pressure / total pressure) is 2.0 × 10 ⁻⁴ , T × log P _A fired product was obtained in the same manner as in Example 19 except that quenching was performed at _O2 = 22-219. After the obtained fired product was pulverized, coarse particles and fine particles were removed using a sieve to obtain Mg ferrite particles having an average particle diameter of 45 μm.
The obtained Mg ferrite particles were measured for oxygen excess δ, saturation magnetization σs, and electrical resistance in the same manner as in Example 1, and the Mg ferrite particles were coated with a resin and used as a carrier for evaluation into images. It was. The results are summarized in Table 1.

(Example 23)
In the firing step, the granulated product is fired at 1200 ° C. for 3 hours, and in the cooling stage, the temperature T is 400 ° C., the oxygen partial pressure P _O2 (oxygen pressure / total pressure) is 2.0 × 10 ⁻⁴ , T × log P _A fired product was obtained in the same manner as in Example 19 except that quenching was performed at _O2 = -1480. After the obtained fired product was pulverized, coarse particles and fine particles were removed using a sieve to obtain Mg ferrite particles having an average particle diameter of 45 μm.
The obtained Mg ferrite particles were measured for oxygen excess δ, saturation magnetization σs, and electrical resistance in the same manner as in Example 1, and the Mg ferrite particles were coated with a resin and used as a carrier for evaluation into images. It was. The results are summarized in Table 1.

(Example 24)
In the firing step, the granulated product is fired at 1200 ° C. for 3 hours, and in the cooling stage, the temperature T is 25 ° C., the oxygen partial pressure P _O2 (oxygen pressure / total pressure) is 2.0 × 10 ⁻⁴ , T × log P _A fired product was obtained in the same manner as in Example 19 except that a fired product was obtained at _O2 = -92. After the obtained fired product was pulverized, coarse particles and fine particles were removed using a sieve to obtain Mg ferrite particles having an average particle diameter of 45 μm.
The obtained Mg ferrite particles were measured for oxygen excess δ, saturation magnetization σs, and electrical resistance in the same manner as in Example 1, and the Mg ferrite particles were coated with a resin and used as a carrier for evaluation into images. It was. The results are summarized in Table 1.

(Example 25)
A granulated product was obtained in the same manner as in Example 1 except that only Fe ₂ O ₃ was prepared and weighed, and that carbon black as a reducing agent was blended in an amount of 1 wt% with respect to the total amount of Fe ₂ O ₃ . . In the firing step, the granulated product was fired at 1200 ° C. for 3 hours in an atmosphere having an oxygen concentration of 0.03%, and then immediately cooled to obtain a fired product (temperature T: 1200 ° C., oxygen partial pressure P _O2 (Oxygen pressure / overall pressure): 2.0 × 10 ⁻⁴ , T × logP _O2 = −4439). After the obtained fired product was pulverized, coarse particles and fine particles were removed using a sieve to obtain ferrite particles having an average particle diameter of 45 μm.
The oxygen excess amount δ, saturation magnetization σs, and electric resistance of the obtained ferrite particles were measured in the same manner as in Example 1, and the ferrite particles were coated with a resin as a carrier and put into an evaluator for image evaluation. The results are summarized in Table 1.

(Example 26)
In the firing step, the granulated product is fired at 1200 ° C. for 3 hours, and in the cooling stage, the temperature T is 1000 ° C., the oxygen partial pressure P _O2 (oxygen pressure / total pressure) is 2.0 × 10 ⁻⁴ , T × log P _A fired product was obtained in the same manner as in Example 25 except that quenching was performed at _O2 = -3699. After the obtained fired product was pulverized, coarse particles and fine particles were removed using a sieve to obtain ferrite particles having an average particle diameter of 45 μm.
The oxygen excess amount δ, saturation magnetization σs, and electric resistance of the obtained ferrite particles were measured in the same manner as in Example 1, and the ferrite particles were coated with a resin as a carrier and put into an evaluator for image evaluation. The results are summarized in Table 1.

(Example 27)
In the firing step, the granulated product is fired at 1200 ° C. for 3 hours, and in the cooling stage, the temperature T is 800 ° C., the oxygen partial pressure P _O2 (oxygen pressure / total pressure) is 2.0 × 10 ⁻⁴ , T × log P _A fired product was obtained in the same manner as in Example 25 except that quenching was performed at _O2 = -2959. After the obtained fired product was pulverized, coarse particles and fine particles were removed using a sieve to obtain ferrite particles having an average particle diameter of 45 μm.
The oxygen excess amount δ, saturation magnetization σs, and electric resistance of the obtained ferrite particles were measured in the same manner as in Example 1, and the ferrite particles were coated with a resin as a carrier and put into an evaluator for image evaluation. The results are summarized in Table 1.

(Example 28)
In the firing step, the granulated product is fired at 1200 ° C. for 3 hours, and in the cooling stage, the temperature T is 600 ° C., the oxygen partial pressure P _O2 (oxygen pressure / total pressure) is 2.0 × 10 ⁻⁴ , T × log P _A fired product was obtained in the same manner as in Example 25 except that quenching was performed at _O2 = -2219. After the obtained fired product was pulverized, coarse particles and fine particles were removed using a sieve to obtain ferrite particles having an average particle diameter of 45 μm.
The oxygen excess amount δ, saturation magnetization σs, and electric resistance of the obtained ferrite particles were measured in the same manner as in Example 1, and the ferrite particles were coated with a resin as a carrier and put into an evaluator for image evaluation. The results are summarized in Table 1.

(Example 29)
In the firing step, the granulated product is fired at 1200 ° C. for 3 hours, and in the cooling stage, the temperature T is 400 ° C., the oxygen partial pressure P _O2 (oxygen pressure / total pressure) is 2.0 × 10 ⁻⁴ , T × log P _A fired product was obtained in the same manner as in Example 25 except that quenching was performed at _O2 = -1480. After the obtained fired product was pulverized, coarse particles and fine particles were removed using a sieve to obtain ferrite particles having an average particle diameter of 45 μm.
The oxygen excess amount δ, saturation magnetization σs, and electric resistance of the obtained ferrite particles were measured in the same manner as in Example 1, and the ferrite particles were coated with a resin as a carrier and put into an evaluator for image evaluation. The results are summarized in Table 1.

(Example 30)
In the firing step, the granulated product is fired at 1200 ° C. for 3 hours, and in the cooling stage, the temperature T is 25 ° C., the oxygen partial pressure P _O2 (oxygen pressure / total pressure) is 2.0 × 10 ⁻⁴ , T × log P _A fired product was obtained in the same manner as in Example 25 except that a fired product was obtained at _O2 = -92. After the obtained fired product was pulverized, coarse particles and fine particles were removed using a sieve to obtain ferrite particles having an average particle diameter of 45 μm.
The oxygen excess amount δ, saturation magnetization σs, and electric resistance of the obtained ferrite particles were measured in the same manner as in Example 1, and the ferrite particles were coated with a resin as a carrier and put into an evaluator for image evaluation. The results are summarized in Table 1.

(Example 31)
A granulated product was obtained in the same manner as in Example 1 except that Fe ₂ O ₃ and MgFe ₂ O ₄ calcined powder were blended at 3:38 (molar ratio) so that x = 0.95. . In the firing step, the granulated product is fired at 1200 ° C. for 3 hours in an air flow atmosphere. Then, in the cooling stage, the temperature T is 25 ° C. and the oxygen partial pressure P _O2 (oxygen pressure / total pressure) is 2.1. A fired product was obtained at × 10 ⁻¹ and T × log P _O2 = −17. After the obtained fired product was pulverized, coarse particles and fine particles were removed using a sieve to obtain Mg ferrite particles having an average particle diameter of 45 μm.
The obtained Mg ferrite particles were measured for oxygen excess δ, saturation magnetization σs, and electrical resistance in the same manner as in Example 1, and the Mg ferrite particles were coated with a resin and used as a carrier for evaluation into images. It was. The results are summarized in Table 1.

(Example 32)
Fe ₂ O ₃ and MgFe ₂ O ₄ calcined powder are blended at 57: 2 (molar ratio) so that x = 0.05, and carbon black as a reducing agent is further added to Fe ₂ O ₃ and MgFe _2. A granulated product was obtained in the same manner as in Example 1 except that 0.95 wt% was added to the total amount of the O ₄ calcined powder. In the firing step, the granulated product is fired at 1200 ° C. for 3 hours in an air flow atmosphere, and then in the cooling stage, the temperature T is 25 ° C. and the oxygen partial pressure P _O2 (oxygen pressure / total pressure) is 2.0. A fired product was obtained at × 10 ⁻⁴ , T × logP _O2 = −92. After the obtained fired product was pulverized, coarse particles and fine particles were removed using a sieve to obtain Mg ferrite particles having an average particle diameter of 45 μm.
The obtained Mg ferrite particles were measured for oxygen excess δ, saturation magnetization σs, and electrical resistance in the same manner as in Example 1, and the Mg ferrite particles were coated with a resin and used as a carrier for evaluation into an evaluation machine. . The results are summarized in Table 1.

(Comparative Example 1)
A granulated product was obtained in the same manner as in Example 1 except that MgO and Fe ₂ O ₃ were mixed at a molar ratio of 7: 3 so that x = 0.7. In the firing step, the granulated product was fired at 1200 ° C. for 3 hours in a nitrogen atmosphere, and then immediately cooled to obtain a fired product (temperature T: 1200 ° C., oxygen partial pressure P _O2 (oxygen pressure / total pressure). ): 2.0 × 10 ⁻⁴ , T × logP _O2 = −92). After the obtained fired product was pulverized, coarse particles and fine particles were removed using a sieve to obtain Mg ferrite particles having an average particle diameter of 45 μm.
The obtained Mg ferrite particles were measured for oxygen excess δ, saturation magnetization σs, and electrical resistance in the same manner as in Example 1, and the Mg ferrite particles were coated with a resin and used as a carrier for evaluation into images. It was. The results are summarized in Table 1.

(Comparative Example 2)
A granulated product was obtained in the same manner as in Example 1 except that MgO and Fe ₂ O ₃ were mixed at 8:23 (molar ratio) so that x = 0.45. In the firing step, the granulated product was fired at 1200 ° C. for 3 hours in a nitrogen atmosphere, and then immediately cooled to obtain a fired product (temperature T: 1200 ° C., oxygen partial pressure P _O2 (oxygen pressure / total pressure). ): 2.0 × 10 ⁻⁴ , T × logP _O2 = −92). After the obtained fired product was pulverized, coarse particles and fine particles were removed using a sieve to obtain Mg ferrite particles having an average particle diameter of 45 μm.
The obtained Mg ferrite particles were measured for oxygen excess δ, saturation magnetization σs, and electrical resistance in the same manner as in Example 1, and the Mg ferrite particles were coated with a resin and used as a carrier for evaluation into images. It was. The results are summarized in Table 1.

  The ferrite particles of Examples 1 to 32 had an oxygen excess δ of 0.013 to 0.173, and had a desired saturation magnetization and high electrical resistance. Further, in the carrier using these ferrite particles, a white spot that would hinder actual use was not generated even after a 150 K printing test. On the other hand, since the Mg ferrite particles of Comparative Example 1 and Comparative Example 2 were fired in a reducing atmosphere, the excess oxygen amount δ was zero, and desired saturation magnetization and electrical resistance were not obtained. For this reason, in the carrier using the Mg ferrite particles of Comparative Example 1 and Comparative Example 2, a white spot that would hinder actual use was generated.

In order to confirm the crystal structure of each of the ferrite particles produced in the examples, X-ray diffraction (“XRD” X-ray diffraction) analysis was performed on the ferrite particles of Examples 1 to 6 as a representative example. FIG. 1 shows the results of XRD analysis. As is clear from this figure, it can be seen that the ferrite particles of Examples 1 to 6 are substantially composed of single-phase MgFe ₂ O ₄ although they contain a little Fe ₂ O ₃ phase.

Moreover, in order to confirm that each particle produced in the Example was a continuous solid solution of ferrite and oxygen, the particles of Examples 1 to 6 were obtained from the oxygen chemical potential _μO2 and the XRD pattern as representative examples. Lattice constants were obtained and their relationship was investigated. FIG. 2 shows the relationship between the oxygen chemical potential μ _O2 and the lattice constant. As is apparent from this figure, together with oxygen chemical potential mu _O2 becomes large and increasing number lattice constant. Therefore, it turns out that each particle produced in Examples 1-6 is a continuous solid solution of ferrite and oxygen.

  Since the ferrite particles of the present invention have a predetermined amount of excess oxygen in the spinel lattice in the ferrite phase, they have a desired saturation magnetization and at the same time a high electrical resistance. Accordingly, for example, when the ferrite particles of the present invention are used as a carrier for electrophotographic development, it becomes possible to cope with high speed image formation and high image quality.

Claims (5)

A ferrite particle represented by a composition formula: Mg _x Fe _3-x O _{4 + δ} (where 0 ≦ x ≦ 0.95),
A ferrite particle, wherein an excess amount of oxygen δ in a spinel lattice in a ferrite phase is 0.01 ≦ δ ≦ 0.18.   The ferrite particles according to claim 1, wherein the average particle diameter is in the range of 10 µm to 100 µm.   3. A carrier for electrophotographic development, wherein the surface of the ferrite particles according to claim 1 or 2 is coated with a resin.   An electrophotographic developer comprising the carrier according to claim 3 and a toner. The Fe raw material and the Mg raw material whose components were adjusted so as to produce ferrite particles having a composition represented by Mg _x Fe _3-x O ₄ (where 0 ≦ x ≦ 0.95) were mixed in a medium solution to form a slurry. A step of obtaining the granulated product by spray drying the slurry, and a step of obtaining a calcined product by firing the granulated product,
Performing the firing under conditions satisfying the following formula (1),
A method for producing ferrite particles, characterized by producing ferrite particles represented by Mg _x Fe _3-x O _{4 + δ} (where 0.01 ≦ δ ≦ 0.18).
−5000 ≦ T × log P _O2 ≦ 0 (1)
(Where T: temperature (° C.), P _O2 = oxygen pressure / overall pressure)