JP7509524B2 - Substituted ε-iron oxide magnetic particles, method for producing substituted ε-iron oxide magnetic particles, compact, method for producing compact, and radio wave absorber - Google Patents

Description

The present invention relates to substituted ε iron oxide magnetic particles suitable for high density magnetic recording media, radio wave absorbers, etc., in particular to substituted ε iron oxide magnetic particles having a reduced content of non-magnetic α-type iron oxide which is a heterophase to the substituted ε iron oxide _, and a method for producing the same. Note that in this specification, an oxide in which part of the Fe site of ε- _Fe2O3 is substituted with another metal element may be referred to as an ε-type iron oxide, and substituted α-iron oxide particles having the same crystal system as that of α _{- Fe2O3} may be referred to as an α-type iron oxide.

Although ε-Fe ₂ O ₃ is an extremely rare phase among iron oxides, nanometer-sized particles at room temperature exhibit a huge coercive force (Hc) of about 20 kOe (1.59×10 ⁶ A/m), and therefore a manufacturing method for synthesizing ε-Fe ₂ O ₃ in a single phase has been studied (Patent Document 1). However, when ε-Fe ₂ O ₃ is used in a magnetic recording medium, there is currently no corresponding material for a magnetic head with a high level of saturation magnetic flux density, so in practice it is necessary to replace a part of the Fe site of ε-Fe ₂ O ₃ with a trivalent metal such as Al, Ga, or In to adjust the coercive force, and when used as a radio wave absorbing material, it is necessary to change the amount of substitution of the Fe site according to the required absorption wavelength (Patent Document 2).
On the other hand, since the magnetic particles of ε-type iron-based oxides are extremely fine, in order to improve the environmental resistance and thermal stability, it has been considered to replace part of the Fe sites of ε- _{Fe2O3 with} other metals having excellent heat resistance, and various partially substituted ε- _Fe2O3 products having excellent environmental resistance and thermal stability, represented by the general formula ε- _{AxByFe2 - xyO3 or} ε _{- AxByCzFe2 - xyzO3} (where A is a divalent metal element such as Co, Ni, Mn, Zn, etc., B is a tetravalent metal element such as Ti, etc., and C is a trivalent metal element such as In, _Ga , Al, etc.), have been proposed (Patent Document ₃ ).
Since ε-Fe ₂ O ₃ and ε-type iron oxides are not thermodynamically stable phases, a special method is required for their production. The above-mentioned Patent Documents 1 to 3 disclose a method for producing ε-Fe ₂ O ₃ or ε-type iron oxides by using fine crystals of iron oxyhydroxide or iron oxyhydroxide containing a substitution element produced by a liquid phase method as a precursor, covering the precursor with silicon oxide by a sol-gel method, and then heat treating the precursor. As the liquid phase method, the reverse micelle method using an organic solvent as a reaction medium and a method using only an aqueous solution as a reaction medium are disclosed.
Furthermore, it has been shown, for example in Patent Documents 4 and 5, that the above-mentioned ε-Fe ₂ O ₃ and ε-type iron oxides have a radio wave absorption peak in the high frequency range exceeding 100 GHz, and they are also expected to be used as radio wave absorbers.
However, the magnetic particles obtained by the manufacturing methods disclosed in Patent Documents 1 to 3 contain, in addition to ε-Fe ₂ O ₃ and ε-type iron oxides, a considerable amount of non-magnetic α-type iron oxides as impurities.
Patent Document 6 discloses a technique for reducing the amount of α-type iron oxides contained as impurities in substituted ε iron oxide magnetic particles.
Patent Document 7 discloses a method for producing an ε-type iron-based oxide by coating a silicon oxide with a sol-gel method in a wide pH range.

JP 2008-174405 A International Publication No. 2008/029861 WO 2008/149785 JP 2008-277726 A JP 2009-224414 A JP 2016-130208 A JP 2018-092691 A

The ε-type iron-based oxide in which a portion of the Fe sites is substituted, produced by the production method disclosed in the above-mentioned Patent Document 4, has a reduced content of α-type iron-based oxide, which is an impurity, compared to ε-type iron-based oxide produced by a conventional method. However, since ε-type iron-based oxide is a metastable phase, when the amount of Fe substituted by other metal elements is small, it is difficult to obtain the same space group as ε- _Fe2O3 even when the production method disclosed in Patent Document ₄ is used, and the reduction in the content of α-type iron-based oxide may be insufficient.
Since α-type iron oxides are non-magnetic, when the substituted ε-iron oxide magnetic particles are used as an electromagnetic wave absorbing material, they do not contribute to the electromagnetic wave absorbing characteristics, and when they are used in a magnetic recording medium, they do not contribute to increasing the recording density, so it is necessary to reduce the content of the α-type iron oxides.
That is, the technical problem to be solved by the present invention is to provide substituted ε-iron oxide magnetic particles having a reduced content of non-magnetic α-type iron oxides and a method for producing the substituted ε-iron oxide magnetic particles.

The present inventors have conducted intensive research focusing on the fact that in order to obtain substituted ε iron oxide magnetic particles, it is necessary to heat the precursor of the magnetic particles in a state in which it is coated with silicon oxide. As a result, it has been found that the content of α-type iron oxide can be reduced by adding a silicon compound having a hydrolyzable group used for coating to an aqueous solution containing the precursor when the pH is between 2.0 and 7.0.
Based on the above findings, the present inventors have completed the present invention described below.

In order to solve the above problems, the present invention provides
Provided is a substituted epsilon iron oxide magnetic particle mainly containing an epsilon type iron oxide in which part of the Fe sites of _epsilon - _Fe2O3 is substituted with another metal element, in which, when the number of moles of Fe contained in the substituted epsilon iron oxide magnetic particle is Fe and the number of moles of all metal elements substituting the Fe sites is Me, the amount of Fe substituted with the other metal element defined as Me/(Fe+Me) is 0.08 or more and 0.17 or less, and the content of α-type iron oxide measured by X-ray diffraction is 3% or less.
The other metal element partially substituting the Fe site is preferably one or more selected from Co, Ti, Ga, and Al. For example, an ε-type iron-based oxide containing Co and Ti as the other metal element partially substituting the Fe site and containing one or more selected from Ga and Al is a suitable target.
The present invention also provides a green compact made of the above-mentioned substituted ε-iron oxide magnetic particle powder.
The present invention also provides a radio wave absorber comprising the above-mentioned substituted ε iron oxide magnetic particles dispersed in a resin or rubber.
The present invention also provides a method for producing an ε-Fe 2 O dispersion comprising the steps of: using an acidic aqueous solution containing trivalent iron ions and ions of a metal which partially substitutes for the Fe sites as a raw material solution; adding an alkali to the raw material solution to neutralize the solution to a pH range of 8.0 to 10.0 to obtain a dispersion containing iron oxyhydroxide containing a substituting metal element or a mixture of iron oxyhydroxide and a hydroxide of the substituting metal element; adding a silicon compound having a hydrolyzable group to the dispersion containing the iron oxyhydroxide containing the substituting metal element or a mixture of iron oxyhydroxide and a hydroxide of the substituting metal element; and maintaining the dispersion containing the iron oxyhydroxide containing the substituting metal element or the mixture of iron oxyhydroxide and a hydroxide of the substituting metal element and the silicon compound at a pH of 8.0 to 10.0 to cover the iron oxyhydroxide containing the substituting metal element or the mixture of iron oxyhydroxide and a hydroxide of the substituting metal element with _a chemical reaction product of the silicon compound. a method for producing a substituted epsilon iron oxide magnetic particle powder mainly containing an epsilon type iron oxide in which a part of the Fe site of formula ₃ is substituted with another metal element, the method comprising: starting the addition of the silicon compound having a hydrolyzable group at a time when the pH of the dispersion is in the range of 2.0 to 7.0 in the neutralization step; and, when the number of moles of the silicon compound added to the dispersion having a pH of 2.0 to 7.0 is S1, the number of moles of Fe ions contained in the raw material solution is F, and the total number of moles of the substituting metal element ions is M, S1/(F+M) is 0.01 to 10.0, and when the total number of moles of the silicon compound added is S2, S2/(F+M) is 0.50 to 10.0.
In the above-mentioned manufacturing method, the addition of the alkali in the neutralization step and the addition of the silicon compound in the silicon compound addition step may both be carried out continuously, or the addition of the alkali may be carried out continuously and the addition of the silicon compound intermittently, or both the addition of the alkali and the silicon compound may be carried out intermittently, or the addition of the alkali may be carried out intermittently and the addition of the silicon compound may be carried out continuously.
In the above-mentioned manufacturing method, the other metal element partially substituting the Fe site is preferably one or more selected from Co, Ti, Ga, and Al. For example, the other metal element partially substituting the Fe site preferably includes Co and Ti, and one or more selected from Ga and Al.
The present invention also provides a method for producing a green compact, which comprises compression molding the substituted ε-iron oxide magnetic particles to obtain a green compact.

As described above, by using the manufacturing method of the present invention, it is possible to obtain substituted ε iron oxide magnetic particles with a reduced content of α-type iron oxide, as well as a compact and a radio wave absorber using the same.

FIG. 1 is a schematic diagram showing an example of an embodiment of the present invention. FIG. 1 is a schematic diagram showing an example of an embodiment of the present invention. FIG. 1 is a schematic diagram showing an example of an embodiment of the present invention. FIG. 1 is a schematic diagram showing an example of an embodiment of the present invention. FIG. 1 is a schematic diagram showing an example of an embodiment of the present invention. FIG. 1 is a schematic diagram showing an example of an embodiment of the present invention.

[Iron oxide magnetic particles]
The manufacturing method of the present invention is for producing substituted ε-iron oxide magnetic particles mainly containing ε-type iron oxides in which part of the Fe sites of ε _{- Fe2O3} are substituted with other metal elements, and the magnetic particles contain a heterogeneous phase which is an unavoidable impurity in the manufacturing process. The heterogeneous phase is mainly α-type iron oxide, and the iron oxide magnetic particles obtained by the present invention are substantially composed of ε-type iron oxide magnetic particles and α-type iron oxide. An object of the present invention is to reduce the content of the heterogeneous phase, α-type iron oxide.
Whether or not a partial substitution product in which part of the Fe sites _of ε- _Fe2O3 is substituted with other metal elements has an ε structure can be confirmed by X-ray diffraction (XRD), high energy electron diffraction (HEED), etc. In the present invention, the identification of ε-type and α-type iron-based oxides is performed by XRD.
Examples of partially substituted compounds that can be produced by the production method of the present invention include the following.
Those represented by the general formula ε-C _z Fe _2-z O ₃ (wherein C is one or more trivalent metallic elements selected from In, Ga and Al).
Those represented by the general formula ε-A _x B _y Fe _2-xy O ₃ (wherein A is one or more divalent metal elements selected from Co, Ni, Mn, and Zn, and B is one or more tetravalent metal elements selected from Ti and Sn).
Those represented by the general formula ε-A _x C _z Fe _2-xz O ₃ (wherein A is one or more divalent metal elements selected from Co, Ni, Mn, and Zn, and C is one or more trivalent metal elements selected from In, Ga, and Al).
Those represented by the general formula ε-B _y C _z Fe _2-yz O ₃ (wherein B is one or more tetravalent metal elements selected from Ti and Sn, and C is one or more trivalent metal elements selected from In, Ga and Al).
Represented by the general formula ε-A _x B _y C _z Fe _2-xyz O ₃ (wherein A is one or more divalent metal elements selected from Co, Ni, Mn, and Zn, B is one or more tetravalent metal elements selected from Ti and Sn, and C is one or more trivalent metal elements selected from In, Ga, and Al).

Here, the type substituted with only C element has the advantage that the coercive force of the magnetic particles can be controlled as desired and that the same space group as ε _{- Fe2O3} can be easily obtained, but it may have poor thermal stability. In particular, when Ga and Al are used as C, the thermal stability of the obtained substituted ε iron oxide magnetic particles is somewhat poor, so it is preferable to further substitute with A and/or B elements at the same time. The three-element substitution type of A, B and C has the best balance of the above-mentioned properties and is excellent in heat resistance, ease of obtaining a single phase, and controllability of coercive force, and when Ga and Al are used as C, it is preferable to also substitute with Co and Ti at the same time.
The manufacturing method of the present invention is applicable to any of the substituted iron oxide magnetic particles described above.
The manufacturing method of the present invention described later can be applied regardless of the amount of the metal element substituting the Fe site, but it is effective to apply a substitution amount that easily produces α-type iron oxide. Specifically, when the molar number of Fe contained in the substituted ε iron oxide magnetic particles is Fe and the molar number of all metal elements substituting the Fe site is Me, if the amount of Fe substituted by other metal elements defined as Me/(Fe+Me) is 0.08 to 0.17, it is possible to obtain substituted ε iron oxide magnetic particles having an α-type iron oxide content of 3% or less as measured by XRD, which was not obtained by the conventional method.

[Powder compact and radio wave absorber]
The substituted ε-iron oxide magnetic particles obtained by the present invention function as a radio wave absorber having excellent radio wave absorbing ability by forming a packed structure of the powder particles. The packed structure here means that the particles are in contact with each other or in close proximity to each other to form a three-dimensional structure. In order to use the particles as a radio wave absorber, it is necessary to maintain the packed structure. Examples of the method include a method of compressing the substituted ε-iron oxide magnetic particles to form a green compact, and a method of forming a packed structure by fixing the substituted ε-iron oxide magnetic particles using a nonmagnetic polymeric compound as a binder.
In the method using a binder, the substituted ε-iron oxide magnetic particles are mixed with a non-magnetic polymer base material to obtain a kneaded product. The amount of the radio wave absorbing material powder in the kneaded product is preferably 60% by mass or more. The greater the amount of the radio wave absorbing material powder, the more advantageous it is for improving the radio wave absorbing characteristics, but care must be taken because if the amount is too large, it becomes difficult to knead it with the polymer base material. For example, the amount of the radio wave absorbing material powder can be 80 to 95% by mass or 85 to 95% by mass.
As the polymer substrate, various materials that satisfy the heat resistance, flame retardancy, durability, mechanical strength, and electrical properties depending on the usage environment can be used. For example, an appropriate material may be selected from resins (such as nylon), gels (such as silicone gel), thermoplastic elastomers, rubbers, etc. Two or more polymer compounds may be blended to form the substrate.

[Average particle size]
In the present invention, the average particle size of the magnetic iron oxide particles obtained by the production method of the present invention is not particularly limited, but it is preferable that each particle is fine enough to have a single magnetic domain structure. Usually, the average particle size measured by a transmission electron microscope is 10 nm to 40 nm.

Starting Materials and Precursors
In the manufacturing method of the present invention, an acidic aqueous solution (hereinafter referred to as raw solution) containing trivalent iron ions and metal ions of the metal element that will eventually replace the Fe site is used as the starting material for the iron-based oxide magnetic particles. If divalent Fe ions are used instead of trivalent Fe ions as the starting material, a mixture containing divalent iron hydrate oxides and magnetite as well as trivalent iron hydrate oxides is generated as a precipitate, and the shape of the iron-based oxide particles finally obtained varies, making it impossible to obtain a substituted ε iron oxide magnetic particle powder with a reduced content of α-type iron oxide as in the present invention. Here, acidic refers to a solution pH of less than 7.0. As a supply source for these iron ions or metal ions of the replacing elements, it is preferable to use water-soluble inorganic acid salts such as nitrates, sulfates, and chlorides in terms of ease of availability and cost. When these metal salts are dissolved in water, the metal ions dissociate and the aqueous solution becomes acidic. When an alkali is added to this acidic aqueous solution containing metal ions to neutralize it, a mixture of iron oxyhydroxide and the hydroxide of the substituting element, or a precipitate of iron oxyhydroxide in which some of the Fe sites have been substituted with other metal elements (hereinafter these are collectively referred to as iron oxyhydroxide containing a substituting element). In the production method of the present invention, these iron oxyhydroxides containing a substituting element are used as precursors of the substituted ε iron oxide magnetic particles.
The total metal ion concentration in the raw material solution is not particularly specified in the present invention, but is preferably 0.01 mol/L or more and 0.5 mol/L or less. If it is less than 0.01 mol/L, the amount of substituted ε iron oxide magnetic particle powder obtained in one reaction is small, which is not economically preferable. If the total metal ion concentration exceeds 0.5 mol/L, the reaction solution is likely to gel due to rapid precipitation of hydroxides, which is not preferable.

[Neutralization process]
In the production method of the present invention, an alkali is added to the raw material solution, and the solution is neutralized until the pH reaches 8.0 to 10.0, to obtain a dispersion containing a precipitate of iron oxyhydroxide containing a substitution element. The hydroxide of the trivalent iron ion is mainly composed of oxyhydroxide. The pH of the dispersion is set to 8.0 or higher in order to complete the precipitation of the hydroxide of the substitution metal element, e.g. Co, and to promote the condensation reaction of the silanol derivative, which is the hydrolysis product. In the production method of the present invention, the upper limit of the pH reached in the neutralization step is not particularly specified, but it is preferably set to 10.0, since the neutralization effect is saturated and the effect of promoting the condensation reaction of the silanol derivative, which will be described later, decreases.
The alkali used for neutralization may be any of hydroxides of alkali metals or alkaline earth metals, aqueous ammonia, and ammonium salts such as ammonium hydrogen carbonate, but it is preferable to use aqueous ammonia or ammonium hydrogen carbonate, which are less likely to leave impurities when finally heat-treated to form an ε-type iron-based oxide. These alkalis may be added in the form of a solid to the aqueous solution of the starting material, but from the viewpoint of ensuring the uniformity of the reaction, it is preferable to add them in the form of an aqueous solution.
As described above, when an alkali is added to the raw material solution to carry out the neutralization treatment, a precipitate of iron oxyhydroxide containing a substitution element is formed, and therefore, during the neutralization treatment, the dispersion containing the precipitate is stirred by a known mechanical means.
The addition of the alkali to the raw material solution may be carried out continuously from the start to the end of the addition. Alternatively, the addition of the alkali may be interrupted before the pH of the dispersion reaches 8.0, and a predetermined pH holding time may be set. In this case, the pH holding time may be set multiple times, and the addition of the alkali may be carried out intermittently. Note that the number of times the pH holding time is set, i.e., the number of times the addition of the alkali is interrupted, is preferably set to 3 times or less in order to avoid complicating the production process.
In the production method of the present invention, the reaction temperature during the neutralization treatment is 5° C. or higher and 60° C. or lower. A reaction temperature lower than 5° C. is undesirable because the cost of cooling increases. A reaction temperature higher than 60° C. is undesirable because it ultimately tends to produce a heterogeneous phase, α-type oxide. A more preferable temperature is 10° C. or higher and 40° C. or lower. In the production method described in the above-mentioned Patent Document 4, the neutralization treatment needs to be carried out at a temperature of 5° C. or higher and 25° C. or lower, and a refrigerator needs to be used during the reaction, but in the production method of the present invention, it is also possible to carry out the neutralization treatment at a reaction temperature higher than room temperature.
The pH values described in this specification were measured using a glass electrode in accordance with JIS Z8802. The pH values are measured using a pH meter calibrated using an appropriate buffer solution according to the pH range to be measured as the pH standard solution. The pH values described in this specification are values obtained by directly reading the measured value indicated by a pH meter compensated by a temperature compensation electrode under reaction temperature conditions.

[Silicon compound addition process]
In the manufacturing method of the present invention, the iron oxyhydroxide containing a substitution element, which is a precursor of the substituted ε iron oxide magnetic particles produced in the above-mentioned step, is unlikely to undergo a phase change to an ε-type iron oxide even if it is subjected to heat treatment in its original state, so it is necessary to coat the iron oxyhydroxide containing a substitution element with a chemical reaction product obtained by hydrolysis and condensation reactions of a silicon compound prior to the heat treatment. Note that the chemical reaction product of a silicon compound is used here as a general term not only for silicon oxides of stoichiometric composition, but also for those of non-stoichiometric composition such as silanol derivatives and polysiloxane structures described below, and those that are converted to silicon oxides by heat treatment.
In the manufacturing methods described in Patent Documents 1 to 4, a sol-gel method is used as a coating method for the chemical reaction product of the silicon compound, and a silicon compound having a hydrolyzable group is added to the reaction solution after the neutralization process of the raw material solution is completed and the pH of the reaction solution becomes alkaline. On the other hand, the manufacturing method of the present invention also uses the sol-gel method as a coating method for silicon oxide, but is characterized in that the addition of the silicon compound having a hydrolyzable group is started at a point in time when the pH of the reaction solution is on the acidic side, in the range of pH 2.0 to 7.0, before the neutralization of the raw material solution is completed. The timing of the end of the addition of the silicon compound will be described later.
In the sol-gel method, a silicon compound having a hydrolyzable group, for example, alkoxysilanes such as tetraethoxysilane (TEOS) and tetramethoxysilane (TMOS) or a silane compound such as a variety of silane coupling agents, is added to a dispersion containing iron oxyhydroxide containing a substitution element, and a hydrolysis reaction is caused to occur under stirring. The resulting silanol derivative is condensed to form a polysiloxane bond, thereby coating the surface of the iron oxyhydroxide containing a substitution element.

The present inventors have found that when the addition of the silicon compound having a hydrolyzable group is started at a pH of 2.0 or more and 7.0 or less, it is possible to reduce the content of α-type iron oxides contained in the finally obtained substituted ε-iron oxide magnetic particles, and the reason for this is believed to be as follows.
The rates of the hydrolysis reaction of the silicon compound having the hydrolyzable group and the condensation reaction of the hydrolysis product, the silanol derivative, vary depending on the pH of the reaction system. The hydrolysis reaction rate is generally high in the low pH region on the acidic side, decreases with increasing pH, and increases again in the high pH region on the alkaline side. In contrast, the condensation reaction rate is low in the low pH region on the acidic side, increases with increasing pH, and increases in the neutral to alkaline pH region.
When a silicon compound having a hydrolyzable group is added to a dispersion containing a precipitate of iron oxyhydroxide containing a substitution element in a low pH region on the acidic side, hydrolysis of the silicon compound proceeds rapidly, and a silanol derivative containing a small amount of organic components is produced, but the condensation reaction of the produced silanol derivative does not proceed. Here, since the silanol derivative has an OH group, which is a hydrophilic group, and is uniformly distributed in an aqueous solution, it is considered that the precipitate of iron oxyhydroxide containing a substitution element and the silanol derivative coexist in a uniformly dispersed state in the dispersion.
Then, when the pH of the dispersion is further increased, the condensation reaction of the silanol derivative becomes dominant, so that the precipitate of the iron oxyhydroxide containing the substitution element is uniformly covered with the silanol derivative or its condensation reaction product, which is thought to result in a reduction in the content of α-type iron oxide contained in the substituted ε iron oxide magnetic particles finally obtained by heat treatment.
Incidentally, the above-mentioned Patent Document 5 discloses coating of silicon oxide by a sol-gel method over a wide range of pH regions. However, in this case, the silicon compound is added at a constant pH after the neutralization treatment is completed, and the technical idea of taking into consideration both the hydrolysis reaction rate and the condensation reaction rate of the silicon compound, as in the present invention, is not disclosed.

The pH at the time of starting the addition of the silicon compound having a hydrolyzable group is preferably 2.0 or more. If the pH is less than 2.0, the formation of precipitates of hydroxides of trivalent iron ions contained in the raw material solution, which is the main component of the substituted ε-iron oxide magnetic particles, may be insufficient. The pH at the time of starting the addition is more preferably 3.0 or more. The pH at the time of starting the addition of the silicon compound having a hydrolyzable group is preferably 7.0 or less. If the pH exceeds 7.0, the hydrolysis reaction slows down and the generation of silanol derivatives becomes insufficient, so that the precipitates of iron oxyhydroxide containing a substitution element and the silanol derivatives cannot be uniformly dispersed and coexist in the dispersion, and it becomes difficult for the precipitates of iron oxyhydroxide containing a substitution element to be uniformly covered by the silanol derivatives or their condensation reaction products. The pH at the time of starting the addition is preferably 6.0 or less, more preferably 4.0 or less.
The addition of the silicon compound having a hydrolyzable group is started when the pH of the raw material solution reaches a desired value in the neutralization step. The addition of the silicon compound may be carried out continuously from the start of the addition to the end of the addition. Here, the term "continuously" includes adding the entire amount of the silicon compound to be added to the dispersion liquid at one time. The addition of the silicon compound may also be carried out intermittently in multiple portions.

In the production method of the present invention, the amount of silicon compound added to the dispersion must simultaneously satisfy the following two conditions.
The first condition is the amount of silicon compound added to the dispersion having a pH of 2.0 to 7.0. When the number of moles of the silicon compound added to the dispersion having a pH of 2.0 to 7.0 is S1, the number of moles of Fe ions contained in the raw solution is F, and the total number of moles of the substituted metal element ions is M, S1/(F+M) is 0.01 to 10.0. If S1/(F+M) is less than 0.01, the amount of silanol derivative coexisting with the precipitate of iron oxyhydroxide containing a substitution element is small, and the effect of uniformly covering the precipitate of iron oxyhydroxide containing a substitution element with the silanol derivative or its condensation reaction product is reduced, which is not preferable. If the amount of silicon compound added is increased, the amount of processing in the heating step and the silicon oxide removal step described below increases, and the production cost increases, so S1/(F+M) is preferably 10.0 or less.
The second condition is the amount of silicon compounds added throughout the entire manufacturing process. When the total number of moles of silicon compounds added is S2, the number of moles of Fe ions contained in the raw material solution is F, and the total number of moles of substitution metal element ions is M, S2/(F+M) is set to 0.50 or more and 10.0 or less. If S2 is less than 0.50, the amount of the chemical reaction product of the silicon compounds coated on the surface of the precipitate of iron oxyhydroxide containing a substitution element is reduced, which is undesirable because it makes it easier for α-type iron oxides to be generated. Also, if S2/(F+M) exceeds 10.0, the amount of processing in the heating step and silicon oxide removal step described below increases, which increases the manufacturing cost, which is undesirable.
When the total amount of silicon compound is added within the pH range of 2.0 to 7.0, S1=S2.

[Embodiments of the invention]
As described above, in the manufacturing method of the present invention, the addition of the alkali in the neutralization step and the addition of the silicon compound in the silicon compound addition step can be carried out continuously or intermittently, and therefore, in the manufacturing method of the present invention, various embodiments can be taken depending on the combination of the method of adding the alkali and the method of adding the silicon compound. Some embodiments of the present invention are exemplified below, but the manufacturing method of the present invention is not limited to the embodiments described below.
FIG. 1 shows a schematic diagram of the time course of an embodiment in which both the addition of alkali and the addition of silicon compound are performed continuously. In this case, after the addition of alkali is started, the addition of silicon compound is started when the pH of the raw material solution reaches a predetermined pH in the range of 2.0 to 7.0. In this embodiment, the neutralization step and the silicon compound addition step are not consecutive steps in time, but are parallel steps. As shown in this figure, the addition of silicon compound may be continued after the addition of alkali is completed, may be completed when the addition of alkali is completed, or may be completed when the pH is 7.0 or less. When the addition of silicon compound is continued after the addition of alkali is completed, it is preferable to complete the addition of silicon compound within 120 min after the completion of the addition of alkali, taking into account the time of the entire manufacturing process. After the addition of silicon compound is completed, the aging step described below is provided.
Although not shown, in the embodiment of FIG. 1, the silicon compound may be added intermittently in several portions.

2 is a schematic diagram illustrating the time course of an example of an embodiment in which the addition of alkali is interrupted during the neutralization step. In this case, the addition of alkali is interrupted once, and the entire amount of silicon compound is added continuously within the pH holding time during which the addition of alkali is interrupted, so that S1 = S2.
3 is a schematic diagram illustrating the time course of another example of an embodiment in which the addition of the alkali is interrupted once. In this case, the addition of the silicon compound is started when the pH of the raw material solution reaches a predetermined pH in the range of 2.0 to 7.0, but the addition is terminated after the neutralization step is completed, and the addition of the silicon compound is continuous.
4 and 5 are schematic illustrations of the time course of an embodiment in which the addition of the alkali is interrupted once and the silicon compound is added intermittently. The silicon compound is added in two separate portions in FIG. 4 and in three separate portions in FIG. 5.
FIG. 6 illustrates a schematic diagram of the time course of an example embodiment in which the addition of the alkali is interrupted twice and the addition of the silicon compound is intermittently performed three times.
As described above, the manufacturing method of the present invention is not limited to the embodiments illustrated in FIGS. 1 to 6, and the addition of the alkali in the neutralization step and the addition of the silicon compound in the silicon compound addition step can be combined in any manner.

[Aging process]
Even if the pH is 8.0 or more, the condensation reaction of the silanol derivative proceeds slowly, so the dispersion containing the chemical reaction product of the iron oxyhydroxide containing a substitution element and the silicon compound obtained through the neutralization step and the silicon compound addition step is kept at a pH of 8.0 or more and aged to allow the condensation reaction of the silanol derivative to proceed. As a result, a uniform coating layer of the condensation reaction product of the silanol derivative is formed on the surface of the precipitate of the iron oxyhydroxide containing a substitution element. It is considered that this coating layer covers almost the entire surface of the precipitate of the iron oxyhydroxide containing a substitution element, but the presence of uncoated parts on the surface of the precipitate of the iron oxyhydroxide containing a substitution element is acceptable within the range in which the effects of the present invention can be achieved. The aging time is preferably 1 hour or more and 24 hours or less. If the holding time is less than 1 hour, the coating of the precipitate of the iron oxyhydroxide containing a substitution element by condensation of the silanol derivative is not completed, and α-type iron-based oxide is easily generated, and if the holding time exceeds 24 hours, the effect of aging becomes saturated, which is not preferable. The maturation time is the time after the end of the addition of the silicon compound if the addition of the silicon compound is continued after the end of the neutralization step, or after the end of the addition of the alkali if the addition of the silicon compound is completed before the end of the neutralization step.

In the method for producing the substituted ε iron oxide magnetic particles of the present invention, the steps after the aging step can be the same as those in the conventional production methods described in, for example, Patent Documents 1 to 4. Specifically, the following steps can be mentioned.
[Heating process]
In the manufacturing method of the present invention, the iron oxyhydroxide containing the substitution element coated with the condensation reaction product of the silanol derivative is recovered by a known solid-liquid separation method, and then heat-treated to obtain an ε-type iron-based oxide. Before the heat-treatment, a washing and drying step may be performed. The heat-treatment is performed in an oxidizing atmosphere, which may be an air atmosphere. The heating can be performed at a temperature in the range of about 700°C to 1300°C, but since a thermodynamically stable phase of α-Fe ₂ O ₃ (an impurity relative to ε-Fe ₂ O ₃ ) is easily generated at a high heating temperature, the heat-treatment is preferably performed at a temperature in the range of 900°C to 1200°C, more preferably 950°C to 1150°C.
The heat treatment time can be adjusted in the range of about 0.5H to 10H, but good results are likely to be obtained in the range of 2H to 5H. The presence of silicon-containing material covering the particles is considered to be advantageous in inducing a phase change to ε-type iron oxide rather than α-type iron oxide. The silicon oxide coating also has the effect of preventing sintering of iron oxyhydroxide crystals containing substitution elements during heat treatment.
When the ε-type iron-based oxide magnetic particles do not require a coating with silicon oxide, the silicon oxide coating may be removed after the heat treatment.

[Composition analysis by inductively coupled plasma emission spectrometry (ICP)]
The composition of the obtained substituted ε iron oxide magnetic particles was analyzed by a dissolution method using an ICP-720ES manufactured by Agilent Technologies, with the measurement wavelengths (nm) being Fe: 259.940 nm, Ga: 294.363 nm, Co: 230.786 nm, Ti: 336.122 nm, and Si: 288.158 nm.

[Measurement of magnetic hysteresis curve (bulk BH curve)]
Using a vibrating sample magnetometer VSM (VSM-P7 manufactured by Toei Industry Co., Ltd.), the magnetic properties were measured with an applied magnetic field of 1193 kA/m (15 kOe), an M measurement range of 0.005 A·m2 ( ⁵ emu), a step bit of 140 bit, a time constant of 0.03 sec, and a wait time of 0.1 sec. The coercive force Hc and saturation magnetization σs were evaluated using the BH curve.

[Evaluation of crystallinity by X-ray diffraction (XRD)]
The obtained sample was subjected to powder X-ray diffraction (XRD: Rigaku Corporation horizontal sample type multipurpose X-ray diffractometer Ultima IV, radiation source CuKα radiation, voltage 40 kV, current 40 mA, 2θ = 10° to 70°). The obtained diffraction pattern was evaluated by Rietveld analysis using integrated powder X-ray analysis software (PDXL2: Rigaku Corporation) based on No. 173025: Iron (III) Oxide-Epsilon and No. 82134: Hematite of ICSD (Inorganic Crystal Structure Database), and the crystal structure and the content of α phase were confirmed.

[BET specific surface area]
The BET specific surface area was determined by a single-point BET method using a Macsorb model-1210 manufactured by Mountec Corporation.

TEM observation of the substituted ε-iron oxide magnetic particles obtained by the production method of the present invention was carried out under the following conditions. JEM-1011 manufactured by JEOL Ltd. was used for TEM observation. TEM photographs taken at magnifications of 10,000x and 100,000x were used for particle observation (the ones after removing the silicon oxide coating were used).
- Measurement of average particle size and particle size distribution evaluation (coefficient of variation (%)) -
Digitizing was used for TEM average particle size and particle size distribution evaluation (coefficient of variation (%)). Mac-View Ver. 4.0 was used as image processing software. When using this image software, the particle size of a particle is calculated as the length of the long side of the rectangle that circumscribes the particle and has the smallest area. More than 200 particles were measured.
The criteria for selecting particles to be measured from among those shown on the transmission electron microscope photograph were as follows:
[1] Particles with any part lying outside the field of view of the photograph will not be measured.
[2] Particles that are clearly defined and exist in isolation are measured.
[3] Even if a particle deviates from the average particle shape, it will be measured if it is independent and can be measured as a single particle.
[4] When particles overlap but the boundaries between them are clear and the overall shape of the particle can be determined, each particle is measured as a single particle.
[5] Particles that overlap each other, have unclear boundaries, and whose overall shape cannot be determined are not measured as their shape cannot be determined.
The number average value of the particle diameters of the particles selected based on the above criteria was calculated, and this was taken as the average particle diameter of the substituted ε iron oxide magnetic particles as determined by TEM observation.

[Measurement of radio wave absorption characteristics]
1.2 g of the substituted ε-iron oxide powder was pressed at 28 MPa (20 kN) to obtain a green compact having a diameter of 13 mm and a thickness of 3 mm. The obtained green compact was subjected to transmission attenuation measurement by terahertz wave time domain spectroscopy. Specifically, a terahertz spectroscopy system TAS7400SL manufactured by Advantest Corporation was used to perform measurements when the green compact was placed on a sample holder and when it was a blank. The measurement conditions were as follows:
Sample holder diameter: φ10mm
Measurement Mode: Transmission
Frequency Resolution: 1.9GHz
・Vertical Axis: Absorbance
・Horizontal Axis:Frequency [THz]
・ Accumulated Number (Sample): 2048
・ Accumulated Number (Background): 2048
The observed signal waveform of the sample and the blank reference waveform were expanded to 2112 ps and Fourier transformed. The ratio (Ssig/Sref) of the obtained Fourier spectra (referred to as Sref and Ssig, respectively) was obtained, and the transmission attenuation of the powder compact placed in the sample holder was calculated.

[Example 1]
In a 5L reaction tank, 283.26 g of 99% purity ferric nitrate nonahydrate (III), 56.36 g of Ga(III) nitrate solution with a Ga concentration of 11.55 mass%, 6.25 g of 97% purity cobalt(II) nitrate hexahydrate, and 6.61 g of titanium(IV) sulfate with a Ti concentration of 15.1 mass% were dissolved in 3813.21 g of pure water while mechanically stirring with a stirring blade in an air atmosphere to obtain a raw material solution (Procedure 1). The pH of this raw material solution was about 1. The molar ratio of metal ions in this raw material solution was Fe:Ga:Co:Ti=1.677:0.223:0.050:0.050. The Roman numerals in parentheses after the names of the reagents represent the valence of the metal elements.
This raw material solution was stirred mechanically with a stirring blade in an air atmosphere at 20° C., and 294.85 g of a 22.30 mass % aqueous ammonia solution was added thereto over a period of 90 minutes (Procedure 2).
After 60 minutes, when the pH of the reaction solution during the addition of the aqueous ammonia solution reached 4.0, 519.22 g of tetraethoxysilane (TEOS) with a purity of 95.0 mass% as a silicon compound having a hydrolyzable group was added in parallel and dropped over 30 minutes. After the aqueous ammonia solution was all added, stirring was continued for 20 hours, and the precipitate of iron oxyhydroxide containing a substitution element was covered with the chemical reaction product of the silicon compound (step 3). The pH of the reaction solution at the end of the addition of the aqueous ammonia solution and throughout the 20 hours of stirring were both 8.8. Under these conditions, the molar ratio S1/(F+M) of the amount of elemental Si contained in tetraethoxysilane added to the dispersion having a pH of 2.0 or more and 7.0 or less to the amounts of iron, gallium, cobalt, and titanium ions contained in the raw solution was 0.34, and the molar ratio S2/(F+M) of the total amount of elemental Si contained in tetraethoxysilane dropped into the dispersion to the amounts of iron, gallium, cobalt, and titanium ions contained in the raw solution was 2.84.
The slurry obtained in step 3 was filtered, and the water in the precipitate of iron oxyhydroxide containing a substitution element coated with the chemical reaction product of the silicon compound was removed as much as possible, and the precipitate was dispersed again in pure water and repulp washed. The washed slurry was filtered again, and the cake obtained was dried in air at 110°C (step 4).
The dried product obtained in step 4 was heated in air at 1090°C for 4 hours in a box-type sintering furnace to obtain iron-based oxide magnetic powder coated with silicon oxide (step 5). Note that the chemical reaction product of the silicon compound is dehydrated and converted to an oxide when heat-treated in air.
The production conditions of this example, such as the conditions for preparing the raw material solution, are shown in Table 1. Table 1 also shows the production conditions of other examples and comparative examples.

The heat-treated powder obtained by subjecting the precipitate of iron oxyhydroxide containing a substitution element coated with a chemical reaction product of a silicon compound obtained in step 5 to heat treatment was stirred in a 17.58 mass% NaOH aqueous solution at about 60°C for 24 hours to remove the silicon oxide coating from the particle surface (step 6). Next, the slurry was washed using a centrifuge until the electrical conductivity was 500 mS/m or less, filtered through a membrane filter, and then dried. The composition of the obtained iron oxide magnetic powder was subjected to chemical analysis, XRD measurement, and measurement of magnetic properties. The measurement results are shown in Table 2. Table 2 also shows the physical properties of the substituted ε iron oxide magnetic particles obtained in other examples and comparative examples.
The substituted ε iron oxide magnetic particles according to Example 1 were subjected to XRD measurement, and the content of the α phase was found to be 1.3%. This value is superior to that of the substituted ε iron oxide powder obtained in Comparative Example 1, which was subjected to an intermediate aging step after the pH reached 8.9 following the addition of an ammonia solution, and then TEOS was added at a pH of 8.9. Chemical analysis of the composition and evaluation of the magnetic properties were also performed. The measurement results are shown in Table 2.

[Example 2]
In Example 2, substituted ε iron oxide magnetic particles were obtained in the same manner as in Example 1, except that the temperature during the addition of the aqueous ammonia solution was 30° C., the duration of the addition of the aqueous ammonia solution was 30 min, the pH at the start of the addition of TEOS was 6.0, and the TEOS addition rate was 5.88 g/min during the addition of the aqueous ammonia solution and 22.47 g/min after the end of the addition of the aqueous ammonia solution. The pH at the end of the addition of the aqueous ammonia solution was 8.9, and the pH of the reaction solution at the end of the addition of TEOS and throughout the stirring for the 20 hours were all 8.8. The substituted ε iron oxide magnetic particles according to Example 2 were subjected to XRD measurement, and the content of the α phase was found to be 3.0%. This value is superior to that of the substituted ε iron oxide magnetic particles obtained in Comparative Example 3, which was described later, in which a maturing step was carried out for 30 min after the pH reached 8.9 after the addition of the aqueous ammonia solution, and then TEOS was added at a pH of 8.9. Chemical analysis of the composition and evaluation of the magnetic properties were also carried out. The measurement results are shown in Table 2.

[Comparative Example 1]
As Comparative Example 1, substituted ε iron oxide magnetic particles were obtained in the same manner as in Example 1, except that an aqueous ammonia solution was added and TEOS was added when the pH reached 8.9, followed by an aging step. The pH at the end of the ammonia addition was 8.9, and the pH of the reaction solution at the end of the TEOS addition and throughout the 20 hours of stirring were all 8.8. XRD measurement was performed on the substituted ε iron oxide magnetic particles according to Comparative Example 1, and the content of the α phase was found to be 3.6%, which was higher than those of Examples 1 to 6. Chemical analysis of the composition and evaluation of the magnetic properties were also performed. The measurement results are shown in Table 2.

[Comparative Example 2]
In Comparative Example 2, the temperature during the addition of the aqueous ammonia solution was 25° C., the time for adding the aqueous ammonia solution was 60 min, an intermediate aging step was performed for 30 min after the pH of the dispersion reached 8.9 after the addition of the aqueous ammonia solution, and then TEOS was added at a pH of 8.9. The pH of the reaction solution at the end of the addition of TEOS and during the stirring for 20 h were both 8.8. XRD measurement was performed on the substituted ε iron oxide magnetic particles according to Comparative Example 2, and the content of the α phase was found to be 7.4%, which was higher than those of Examples 1 to 6. Chemical analysis of the composition and evaluation of the magnetic properties were also performed. The measurement results are shown in Table 2.

[Comparative Example 3]
As Comparative Example 3, a substituted ε iron oxide magnetic particle powder was obtained in the same manner as in Example 2, except that an aging step was performed after the pH reached 8.9 by adding an aqueous ammonia solution, and then TEOS was added at a pH of 8.9. The pH of the reaction solution at the end of the addition of TEOS and during the 20 hours of stirring were both 8.8. XRD measurement was performed on the substituted ε iron oxide magnetic particle powder according to Comparative Example 3, and the content of the α phase was found to be 5.4%, which was a higher value than those of Examples 1 to 6. Chemical analysis of the composition and evaluation of the magnetic properties were also performed. The measurement results are shown in Table 2.
From the above results, when the neutralization treatment is carried out continuously, the effect of starting the addition of TEOS at a pH of 2.0 to 7.0 is clear. Furthermore, as shown by the results of Example 2, in the case of the production method of the present invention, even when the neutralization treatment is carried out at 30° C., which is higher than room temperature, a substituted ε iron oxide magnetic particle powder having a low content of α phase can be obtained.

[Example 3]
In Example 3, the substituted ε iron oxide magnetic particles were obtained in the same manner as in Example 1, except that the addition of the aqueous ammonia solution was stopped after the addition of the aqueous ammonia solution until the pH reached 4.0 (first neutralization step), the intermediate aging step was not performed, the addition of TEOS was immediately started, and the remaining aqueous ammonia solution was added after the addition of TEOS was completed (second neutralization step). The pH of the reaction solution at the end of the second neutralization step and the pH of the reaction solution during the stirring for 20 hours were both 8.8. The substituted ε iron oxide magnetic particles according to Example 3 were subjected to XRD measurement, and the content of the α phase was found to be 0%. This value is superior to the values of the substituted ε iron oxide magnetic particles obtained in Comparative Examples 4 and 5, which will be described later, in which the addition of the aqueous ammonia solution was stopped after the addition of the aqueous ammonia solution until the pH reached 4.0 or 6.0 (first neutralization step), the intermediate aging step of 30 min was performed, and then the addition of TEOS was started, but the second neutralization step was not performed after the addition of TEOS was completed. Chemical analysis of the composition and evaluation of the magnetic properties were also performed. The measurement results are shown in Table 2.

[Example 4]
In a 1L reaction tank, 104.81g of ferric sulfate (III) solution with a Fe concentration of 11.65 mass%, 14.32g of Ga (III) nitrate solution with a Ga concentration of 11.55 mass%, 1.91g of cobalt (II) nitrate hexahydrate with a purity of 97%, and 2.02g of titanium (IV) sulfate with a Ti concentration of 15.1 mass% were dissolved in 737.71g of pure water while mechanically stirring with a stirring blade in an air atmosphere (Procedure 1). The pH of this solution was about 1. The molar ratio of metal ions in this charged solution was Fe:Ga:Co:Ti=1.714:0.186:0.050:0.050.
In an air atmosphere, this solution was mechanically stirred with a stirring blade at 30° C., while 15.00 g of 22.30 mass% aqueous ammonia solution was added over 1.9 min (first neutralization step), and after the dropwise addition was completed, stirring was continued for 30 min to age the resulting precipitate (intermediate aging step). At that time, the pH of the slurry containing the precipitate was 2.0 (Procedure 2).
While stirring the slurry obtained in step 2, 158.88 g of tetraethoxysilane (TEOS) with a purity of 95.0 mass% was added dropwise over 10 min at 30° C. in the air. After the addition of TEOS was completed, 62.77 g of ammonia solution with a purity of 22.30 mass% was added over 8.1 min (second neutralization step). The pH after the second neutralization step was 8.8. Stirring was continued for 20 h, and the precipitate was covered with the chemical reaction product of the silicon compound (step 3). The pH of the reaction solution during the 20 h of stirring was 8.8. Under these conditions, the molar ratio S1/(F+M) of the amount of elemental Si contained in tetraethoxysilane added to the dispersion having a pH of 2.0 or more and 7.0 or less to the amounts of iron, gallium, cobalt, and titanium ions contained in the raw solution is 2.84, and the molar ratio S2/(F+M) of the total amount of elemental Si contained in tetraethoxysilane added dropwise to the dispersion to the amounts of iron, gallium, cobalt, and titanium ions contained in the raw solution is also 2.84.
Thereafter, substituted ε-iron oxide magnetic particles were obtained in the same manner as in Example 1. The substituted ε-iron oxide powder according to Example 4 was subjected to XRD measurement, and the content of the α-phase was found to be 0%. This value is superior to those of the substituted ε-iron oxide magnetic particles obtained in Comparative Examples 4 and 5 described below. In addition, chemical analysis of the composition and evaluation of the magnetic properties were performed. The measurement results are also shown in Table 2.

[Example 5]
As Example 5, a substituted ε iron oxide magnetic particle powder was obtained in the same manner as in Example 4, except that the amount and time of the aqueous ammonia solution added in the first neutralization step were 51.00 g and 6.5 min, the pH after the first neutralization step was 3.0, and the amount and time of the aqueous ammonia solution added in the second neutralization step were 27.24 g and 3.5 min. The pH of the reaction solution at the end of the second neutralization step and during the 20 hours of stirring were both 8.8. The substituted ε iron oxide magnetic particle powder according to Example 5 was subjected to XRD measurement, and the content of the α phase was found to be 0%, which is superior to those of the substituted ε iron oxide magnetic particles obtained in Comparative Examples 4 and 5 described later. Chemical analysis of the composition and evaluation of the magnetic properties were also performed. The measurement results are shown in Table 2.

[Example 6]
As Example 6, a substituted ε iron oxide magnetic particle powder was obtained in the same manner as in Example 4, except that the amount and time of the aqueous ammonia solution added in the first neutralization step were 53.00 g and 6.8 min, the pH after the first neutralization step was 4.0, and the amount and time of the aqueous ammonia solution added in the second neutralization step were 24.78 g and 3.2 min. The pH of the reaction solution after the second neutralization step and the pH of the reaction solution during the 20 hours of stirring were 8.8. The substituted ε iron oxide powder according to Example 5 was subjected to XRD measurement, and the content of the α phase was found to be 0%, which is superior to those of the substituted ε iron oxide magnetic particle powders obtained in Comparative Examples 4 and 5 described later. Chemical analysis of the composition and evaluation of the magnetic properties were also performed. The measurement results are shown in Table 1.

[ Reference Example 7]
In a 1L reaction tank, 102.73 g of ferric sulfate (III) solution with a Fe concentration of 11.65 mass%, 10.74 g of aluminum nitrate (III) 98% purity 98% hexahydrate, 1.91 g of cobalt nitrate (II) 6-hydrate with a purity of 97%, and 2.02 g of titanium sulfate (IV) with a Ti concentration of 15.1 mass% were dissolved in 746.26 g of pure water while mechanically stirring with a stirring blade in an air atmosphere (Procedure 1). The pH of this solution was about 1. The molar ratio of metal ions in this charged solution was Fe:Al:Co:Ti=1.680:0.220:0.050:0.050.
In an air atmosphere, this solution was mechanically stirred with a stirring blade at 30° C., while 51.59 g of 22.30 mass% aqueous ammonia solution was added over 60 min (first neutralization step), and after the dropwise addition was completed, stirring was continued for 10 min to age the resulting precipitate (intermediate aging step). At that time, the pH of the slurry containing the precipitate was 3.0 (step 2).
While stirring the slurry obtained in step 2, 158.88 g of tetraethoxysilane (TEOS) with a purity of 95.0 mass% was added dropwise over 5 min at 30° C. in the air. After the addition of TEOS was completed, 27.09 g of ammonia solution with a purity of 22.30 mass% was added over 14 min (second neutralization step). The pH after the second neutralization step was 8.8. Stirring was continued for 20 h, and the precipitate was covered with the chemical reaction product of the silicon compound (step 3). The pH of the reaction solution during the 20 h of stirring was 8.8. Under these conditions, the molar ratio S1/(F+M) of the amount of elemental Si contained in tetraethoxysilane added to the dispersion having a pH of 2.0 or more and 7.0 or less to the amounts of iron, aluminum, cobalt, and titanium ions contained in the raw solution is 2.84, and the molar ratio S2/(F+M) of the total amount of elemental Si contained in tetraethoxysilane dropped into the dispersion to the amounts of iron, aluminum, cobalt, and titanium ions contained in the raw solution is 2.84.
Thereafter, substituted ε-iron oxide magnetic particles were obtained in the same manner as in Example 1. The substituted ε-iron oxide powder according to Reference Example 7 was subjected to XRD measurement, and the content of the α-phase was found to be 0%. This value is superior to those of the substituted ε-iron oxide magnetic particles obtained in Comparative Examples 4 and 5 described below. In addition, chemical analysis of the composition and evaluation of the magnetic properties were performed. The measurement results are also shown in Table 2.
The radio wave absorption characteristics of the obtained substituted ε-iron oxide magnetic particles were measured by the above-mentioned method, and the results showed that the maximum absorption frequency of the powder compact in the frequency range of 50 GHz to 100 GHz was 80.1 GHz, and the transmission attenuation per unit thickness was 4.1 dB/mm.

[Comparative Example 4]
As Comparative Example 4, a substituted ε iron oxide powder was obtained in the same manner as in Example 6, except that the second neutralization step was not carried out. The pH of the reaction solution at the end of the addition of TEOS and during the 20 hours of stirring were both 4.0. XRD measurement was carried out on the substituted ε iron oxide magnetic particles according to Comparative Example 4, and the content of the α phase was found to be 87.5%, which was a higher value than those of Examples 1 to 6 and Reference Example 7. Chemical analysis of the composition and evaluation of the magnetic properties were also carried out. The measurement results are shown in Table 2.

[Comparative Example 5]
As Comparative Example 5, a substituted ε iron oxide magnetic particle powder was obtained in the same manner as in Comparative Example 4, except that the amount of the aqueous ammonia solution added in the first neutralization step was 57.00 g and the addition time was 7.3 min, and the pH at the end of the first neutralization step was 6.0. The pH of the reaction solution at the end of the addition of TEOS and during the 20 hours of stirring were both 6.0. The substituted ε iron oxide magnetic particle powder according to Comparative Example 5 was subjected to XRD measurement, and the content of the α phase was found to be 36.9%, which was a higher value than those of Examples 1 to 6 and Reference Example 7. Chemical analysis of the composition and evaluation of the magnetic properties were also performed. The measurement results are shown in Table 2.
From the above results, it is understood that unless the second neutralization step is performed, substituted ε iron oxide magnetic particles having a low α phase content cannot be obtained. Furthermore, almost the same results are obtained whether nitrate or sulfate is used as the starting iron raw material.

[Example 8]
Except for changing the stirring time in the intermediate maturation step to 10 min and adding tetraethoxysilane (TEOS) over 5 min, a substituted ε-iron oxide magnetic particle powder was obtained in the same manner as in Example 5. The pH of the reaction solution at the end of the addition of TEOS was 3.0, and the pH of the reaction solution during the 20 h of stirring was 8.6.
The content of the α-phase in the obtained substituted ε-iron oxide magnetic particles was 0%, and the radio wave absorption characteristics were measured by the above-mentioned method. As a result, the maximum absorption frequency of the compact in the frequency range of 50 GHz to 100 GHz was 67.2 GHz, and the transmission attenuation per unit thickness was 4.6 dB/mm.

[Comparative Example 6]
In a 5L reaction tank, 527.22g of ferric sulfate (III) solution with a Fe concentration of 11.58 mass%, 71.61g of Ga (III) nitrate solution with a Ga concentration of 11.55 mass%, 9.57g of cobalt (II) nitrate hexahydrate with a purity of 97%, and 10.11g of titanium (IV) sulfate with a Ti concentration of 15.1 mass% were dissolved in 3688.56g of pure water while mechanically stirring with a stirring blade in an air atmosphere (Procedure 1). The pH of this solution was about 1. The molar ratio of metal ions in this charged solution was Fe:Ga:Co:Ti=1.714:0.186:0.050:0.050.
In an air atmosphere, this solution was mechanically stirred with a stirring blade at 30° C., while 388.91 g of 22.30 mass% aqueous ammonia solution was added over 10 min (first neutralization step), and after the dropwise addition was completed, stirring was continued for 30 min to age the resulting precipitate (intermediate aging step). At that time, the pH of the slurry containing the precipitate was 8.6 (step 2).
While stirring the slurry obtained in step 2, 794.40 g of tetraethoxysilane (TEOS) with a purity of 95.0 mass% was dropped over 10 min at 30° C. in the air. Stirring was continued for 20 h, and the precipitate was covered with the chemical reaction product of the silicon compound (step 3). The pH of the reaction solution during the 20 h of stirring was 8.6. Under these conditions, the molar ratio S1/(F+M) between the amount of Si element contained in the tetraethoxysilane added to the dispersion with a pH of 2.0 to 7.0 and the amount of iron, gallium, cobalt, and titanium ions contained in the raw solution was 0, and the molar ratio S2/(F+M) between the total amount of Si element contained in the tetraethoxysilane dropped into the dispersion and the amount of iron, gallium, cobalt, and titanium ions contained in the raw solution was 2.84.
Thereafter, substituted ε iron oxide magnetic particles were obtained in the same manner as in Example 1. XRD measurement was performed on the substituted ε iron oxide powder according to Comparative Example 6, and the content of the α phase was found to be 4.9%, which was higher than that of Example 8. In addition, chemical analysis of the composition, and evaluation of the magnetic properties and radio wave absorption properties were performed. The measurement results are also shown in Table 2.

[ Reference Example 9]
Substituted ε-iron oxide powder was obtained by the same procedure as in Reference Example 7, except that 101.50 g of ferric sulfate (III) solution and 11.7 g of aluminum nitrate (III) nonahydrate were used. The pH of the reaction solution at the end of the addition of TEOS was 3.0, and the pH of the reaction solution during the 20 hours of stirring was 8.6. The molar ratio of metal ions in this charged solution was Fe:Al:Co:Ti=1.660:0.240:0.050:0.050. Chemical analysis of the composition, and evaluation of magnetic properties and radio wave absorption properties were performed. The measurement results are shown in Table 2.

[Comparative Example 7]
In a 1L reaction tank, 101.51 g of ferric sulfate (III) solution with a Fe concentration of 11.65 mass%, 11.72 g of Al (III) nitrate nonahydrate with a purity of 98%, 1.91 g of cobalt (II) nitrate hexahydrate with a purity of 97%, and 2.12 g of titanium (IV) sulfate with a Ti concentration of 15.1 mass% were dissolved in 746.36 g of pure water while mechanically stirring with a stirring blade in an air atmosphere (Procedure 1). The pH of this solution was about 1. The molar ratio of metal ions in this charged solution was Fe:Al:Co:Ti=1.660:0.240:0.050:0.050.
In an air atmosphere, this solution was mechanically stirred with a stirring blade at 30° C., and 78.68 g of 22.30 mass% aqueous ammonia solution was added over 10 min (first neutralization step). After the dropwise addition was completed, stirring was continued for 30 min to age the resulting precipitate (intermediate aging step). At that time, the pH of the slurry containing the precipitate was 8.6 (Procedure 2).
While stirring the slurry obtained in step 2, 158.88 g of tetraethoxysilane (TEOS) with a purity of 95.0 mass% was dropped over 10 min at 30° C. in the air. Stirring was continued for 20 h, and the precipitate was covered with the chemical reaction product of the silicon compound (step 3). The pH of the reaction solution during the 20 h of stirring was 8.6. Under these conditions, the molar ratio S1/(F+M) between the amount of Si element contained in the tetraethoxysilane added to the dispersion with a pH of 2.0 to 7.0 and the amount of iron, gallium, cobalt, and titanium ions contained in the raw solution was 0, and the molar ratio S2/(F+M) between the total amount of Si element contained in the tetraethoxysilane dropped into the dispersion and the amount of iron, gallium, cobalt, and titanium ions contained in the raw solution was 2.84.
Thereafter, substituted ε iron oxide magnetic particles were obtained in the same manner as in Example 1. The substituted ε iron oxide powder according to Comparative Example 7 was subjected to XRD measurement, and the content of the α phase was found to be 8.7%, which was a higher value than that of Additional Example 2. In addition, chemical analysis, and evaluation of magnetic properties and radio wave absorption properties were performed. The measurement results are also shown in Table 2.

[Comparative Example 8]
This comparative example is an experimental example corresponding to the composition ratio of Reference Example 7 produced by a conventional method.
Substituted ε iron oxide magnetic particles were obtained in the same manner as in Additional Comparative Example 2, except that the amounts of metal salts added were 102.73 g of a ferric sulfate (III) solution with an Fe concentration of 11.65 mass% and 10.74 g of Al (III) nitrate nonahydrate with a purity of 98%.
The pH of the reaction solution at the end of the addition of TEOS and during the 20 hours of stirring were both 8.6. XRD measurement was performed on the substituted ε-iron oxide magnetic particles according to this comparative example to determine the content of the α-phase, which was 8.1%, a higher value than that of Reference Example 7. Chemical analysis of the composition and evaluation of the magnetic properties were also performed. The measurement results are shown in Table 2.

Claims (10)

The substituted epsilon iron oxide magnetic particles mainly contain an epsilon type iron oxide in which part of the Fe sites of _epsilon - _Fe2O3 is substituted with other metal elements, the other metal elements partially substituting the Fe sites being Co, Ti and Ga, the amount of Fe substituted by other metal elements defined as Me/(Fe+Me) being 0.08 or more and 0.16 or less, where Fe is the number of moles of Fe contained in the substituted epsilon iron oxide magnetic particles and Me is the number of moles of all metal elements substituting the Fe sites, the substituted epsilon iron oxide magnetic particles have a saturation magnetization σs of 17.1 Am2 ^/ kg or more, and the content of α-type iron oxide measured by X-ray diffraction is 3% or less. A green compact comprising the substituted ε iron oxide magnetic particle powder according to claim 1 . 10. A radio wave absorber comprising the substituted ε iron oxide magnetic particles according to claim 1 dispersed in a resin or rubber. A method for producing a substituted ε-iron oxide magnetic particle powder mainly containing an ε-type iron oxide in which a part of the Fe site of ε-Fe ₂ O ₃ is substituted with another metal element, comprising the steps of:
a neutralization step of using an acidic aqueous solution containing trivalent iron ions and ions of a metal that partially substitutes for the Fe sites as a raw material solution, and adding an alkali to the raw material solution to neutralize it to a pH of 8.0 to 10.0 to obtain a dispersion containing iron oxyhydroxide containing a substitution metal element or a mixture of iron oxyhydroxide and a hydroxide of the substitution metal element;
a silicon compound adding step of adding a silicon compound having a hydrolyzable group to the dispersion liquid containing the iron oxyhydroxide containing the substitution metal element or a mixture of the iron oxyhydroxide and the hydroxide of the substitution metal element;
an aging step of maintaining a pH of the dispersion containing the silicon compound and the iron oxyhydroxide containing the substituting metal element or the mixture of the iron oxyhydroxide and the hydroxide of the substituting metal element at 8.0 to 10.0, and coating the iron oxyhydroxide containing the substituting metal element or the mixture of the iron oxyhydroxide and the hydroxide of the substituting metal element with a chemical reaction product of the silicon compound;
Including,
Addition of the silicon compound having a hydrolyzable group is started when the pH of the dispersion becomes 2.0 or more and 7.0 or less in the neutralization step,
where S1 is the number of moles of the silicon compound added to the dispersion liquid having a pH of 2.0 or more and 7.0 or less, F is the number of moles of Fe ions contained in the raw material solution, and M is the total number of moles of the substitution metal element ions, and S1/(F+M) is 0.01 or more and 10.0 or less,
When the total molar number of the silicon compounds added is S2, S2/(F+M) is 0.50 or more and 10.0 or less.
A method for producing substituted ε-iron oxide magnetic particles. 5. The method for producing the substituted ε iron oxide magnetic particles according to claim 4 , wherein the addition of the alkali in the neutralization step and the addition of the silicon compound in the silicon compound addition step are both carried out continuously. 5. The method for producing substituted ε iron oxide magnetic particles according to claim 4 , wherein the addition of the alkali in the neutralization step is carried out continuously, and the addition of the silicon compound in the silicon compound addition step is carried out intermittently. 5. The method for producing the substituted ε iron oxide magnetic particles according to claim 4 , wherein the addition of the alkali in the neutralization step and the addition of the silicon compound in the silicon compound addition step are both carried out intermittently. 5. The method for producing the substituted ε iron oxide magnetic particles according to claim 4 , wherein the addition of the alkali in the neutralization step is carried out intermittently, and the addition of the silicon compound in the silicon compound addition step is carried out continuously. 5. The method for producing substituted ε iron oxide magnetic particles according to claim 4 , wherein the other metal elements partially substituting the Fe sites are Co, Ti and Ga . A method for producing a green compact, comprising compression molding the substituted ε-iron oxide magnetic particle powder according to claim 1 to obtain a green compact.

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