JP2012253248A - Iron nitride material and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an iron nitride material which has a high content of iron nitride particles containing α''FeNas a major ingredient, and to provide a method for manufacturing the same.SOLUTION: Granulated powders having an average particle diameter of 1 μm or more are formed by mixing raw powders made of iron nitride particles which contain α''FeNas a major ingredient and have a minor axis average length of 100 nm or less, and a binder. After a molding die is filled with the granulated powders, a molded body (iron nitride material) is formed by pressure molding. The pressure molding is carried out while performing exhaust of the molding die to be 0.9 atmospheres or less, in a heated state at a temperature of ±20°C of a decomposition temperature of the binder and an applied state of a magnetic field equal to or larger than 2T. By applying a strong magnetic field in the presence of the binder which has melted by heating, transfer and rotation of iron nitride particles become easy and crystal orientation can be oriented in a specific direction. When removing the binder by heating and exhausting, a filling rate of the iron nitride particles can be increased. According to the manufacturing method, iron nitride material having a high content of the iron nitride particles and an oriented texture can be obtained, and the iron nitride material excels in magnetic properties.

Description

The present invention relates to an iron nitride material suitable for a material of a magnetic member such as a permanent magnet, and a manufacturing method thereof. In particular, the present invention relates to an iron nitride material having a large content of powder mainly composed of α ″ Fe ₁₆ N ₂ .

Conventionally, as a raw material for a magnetic member such as a magnetic recording medium, nano-powder made of spherical iron nitride having a particle size of nano order or columnar iron nitride having a short axis of nano order has been used. As this iron nitride, α ”type Fe ₁₆ N ₂ with very high saturation magnetization and excellent magnetic properties (saturation magnetization: about 2.8 T in tetragonal calculations, thin film experiments, tetragonal, a = 5.72Å, c = 6.29 mm, crystal symbol: I4 / mmm) is used (Patent Document 1, etc.).

  On the other hand, rare earth magnets containing rare earth elements such as Nd (neodymium) and Sm (samarium) are widely used as permanent magnets used in motors and generators.

JP 2007-335592 A

Although rare earth magnets are excellent in magnetic properties, since rare earth elements are rare elements, it is desired to reduce the amount used. On the other hand, iron elements and nitrogen elements are more abundant than rare earth elements. Therefore, it is expected that a permanent magnet having a magnetic property superior to that of a rare earth magnet can be obtained by using an iron nitride material containing α ″ Fe ₁₆ N ₂ as a main component for a magnet material.

However, the conventional alpha "Fe ₁₆ N ₂ nitriding iron containing is, alpha" Fe ₁₆ N less content of _2, inferior magnetic properties, it is difficult to apply to a magnetic member such as a magnet material.

For example, an iron nitride material used for a magnetic recording medium is typically a tape-like material in which a mixture of an α "Fe ₁₆ N ₂ powder and a binder such as a resin or an organic material is applied to a support film made of a resin or the like. (Refer to Patent Document 1.) Due to the presence of the binder and the support film, the iron nitride material has a low filling rate of the powder, and as a result, the content of α ”Fe ₁₆ N ₂ is low.

  Moreover, when using the nanopowder described above as a raw material powder, the magnetic properties can be further improved by forming the nanoparticle with the crystal orientation in a specific direction. However, nanopowder generally tends to aggregate and has high surface energy. For this reason, it is difficult to shape the crystal orientation of the nanoparticles in a specific direction. Further, when coarsening is caused by aggregation, the magnetic properties are deteriorated.

Therefore, it is desired to develop a bulk material having a high filling rate of nano-powder containing α "Fe ₁₆ N ₂ and excellent magnetic properties.

Accordingly, one of the objects of the present invention is to provide an iron nitride material having a high content of powder mainly composed of α ″ Fe ₁₆ N _2. Another object of the present invention is to provide the iron nitride material described above. It is in providing the manufacturing method of.

  The inventors filled a granulated powder prepared by mixing a raw material powder with a specific binder into a mold, heated to a specific temperature, deaerated while removing the binder, and applied a specific strong magnetic field. It has been found that application can improve the orientation of the crystal and increase the filling rate. The present invention is based on the above findings.

The method for producing an iron nitride material of the present invention relates to a method for producing an iron nitride material used as a material for a magnetic member such as a magnet, and includes the following preparation step, granulation step, and molding step. .
Preparation step: A step of preparing a raw material powder composed of columnar iron nitride particles containing α "Fe ₁₆ N ₂ as a main component and having an average minor axis length of 100 nm or less.
Granulation step: a step of mixing the raw material powder and a binder having a decomposition temperature lower than that of α ″ Fe ₁₆ N ₂ to form a granulated powder having an average particle size of 1 μm or more.
Molding step: A step of forming a molded body by filling the granulated powder into a mold and then press-molding it.
In the molding step, while the inside of the mold is evacuated to 0.9 atm or less, it is heated to a temperature of {(decomposition temperature of the binder) −20} ° C. or more {(decomposition temperature of the binder) +20} ° C. or less. In addition, pressure molding is performed with a magnetic field of 2 T or more applied.

The decomposition temperature in the binder is the temperature at which the binder begins to lose weight (TG / DTA) after the differential thermal / thermogravimetric curve (TG / DTA) of the binder is obtained at atmospheric pressure (1 atm ≒ 0.1 MPa). Temperature), and here, offset temperature is used. Offset temperature: T _o is determined as follows. First, in the TG curve obtained as described above, as the temperature at which the reduction of the weight of the binder starts, as shown in FIG. 2, it takes the straight line L ₁ parallel to the X axis. The weight ratio of the amount of the binder to the weight of the iron nitride particles used in the raw material powder X (wt%), when the initial weight of the binder M1, which is parallel to the straight line L _1, the initial weight from the straight line L ₁ : Take a straight line L ₂ which takes (5 × X) wt% smaller weight of M1. Further, a tangent line L ₃ at the intersection of the TG curve and the straight line L ₂ is taken. Then, taking the intersection of the straight line L ₁ and the tangent line L _3, the temperature of the intersection point and the offset temperature T _O.

The iron nitride material of the present invention having a high content of the iron nitride particles can be obtained by the method for producing the iron nitride material of the present invention. Specifically, the iron nitride material of the present invention is an iron nitride material obtained by the above-described production method of the present invention, and from a molded body composed of a plurality of iron nitride particles mainly containing α ″ Fe ₁₆ N _2. In the iron nitride material of the present invention, the content of the iron nitride particles in the molded body is 85% by volume or more.

Alternatively, the iron nitride material of the present invention comprises a molded body composed of a plurality of iron nitride particles containing α ″ Fe ₁₆ N ₂ as a main component, and the iron nitride particles are columnar and have an average length of a short axis. In which the content of the iron nitride particles in the molded body is 85% by volume or more.

  In the production method of the present invention, nano powder is used as the raw material powder, and agglomerated powder can be effectively suppressed by the inclusion of the binder by using the granulated powder obtained by mixing the nano powder and the binder. Moreover, since the said binder can be easily removed by heating and exhausting to a specific temperature at the time of shaping | molding, this invention manufacturing method can raise the filling rate of the nanopowder in the molded object obtained. In other words, the production method of the present invention can produce a molded body that does not substantially contain the binder and has a high content of nanopowder.

  Moreover, in the production method of the present invention, the nanopowder can be moved and rotated to some extent by the presence of a binder that is in a molten state by heating during molding. Therefore, when the nanopowder is substantially made of a single crystal magnetic material, the easy magnetization axis of the crystal can be oriented in the direction of application of the magnetic field by applying a specific strong magnetic field. That is, the production method of the present invention can efficiently produce a molded article having high orientation by using a binder and applying a specific strong magnetic field.

  The iron nitride material obtained by the production method of the present invention (typically, the iron nitride material of the present invention) has excellent magnetic properties due to the high content of iron nitride particles. Moreover, since the molded object which comprises the said iron nitride material has the orientation structure | tissue where the crystal orientation aligned in one direction, this invention iron nitride material is excellent in a magnetic characteristic. Furthermore, since the iron nitride material of the present invention is excellent in magnetic properties even if it does not contain rare earth elements, it can be suitably used for a magnet material such as a permanent magnet, and is expected to contribute to a rare earth-free magnet. .

As one mode of the present invention production process, the content of the iron nitride α in the particle "Fe ₁₆ N ₂ can be mentioned embodiment is 85 vol% or more.

The content of α "Fe ₁₆ N _{2 in} the iron nitride particles constituting the raw material powder is high (high purity), so the content of α" Fe ₁₆ N ₂ in the molded product obtained after molding also increases. Therefore, the above embodiment can produce an iron nitride material that is more excellent in magnetic properties.

  One aspect of the production method of the present invention is an aspect in which the aspect ratio is 2 or more when the ratio of the length of the major axis to the length of the minor axis in the iron nitride particles is defined as the aspect ratio.

By using the raw material powder having a large aspect ratio, the aspect ratio of the iron nitride particles constituting the molded body obtained after the molding also increases. Therefore, the above form has a large shape magnetic anisotropy. In addition, by applying a specific strong magnetic field, the c-axis of the α "Fe ₁₆ N ₂ crystal is oriented in the major axis direction of the iron nitride particles. The easy magnetization direction and the easy magnetization axis of crystal magnetic anisotropy of iron nitride are aligned by the application of the above-described strong magnetic field, so that an iron nitride material having further excellent magnetic properties can be manufactured.

  As one form of this invention manufacturing method, the form whose decomposition temperature of the said binder is 240 degrees C or less is mentioned.

Since the decomposition temperature of α ”Fe ₁₆ N ₂ is about 260 ° C., the above form does not decompose α” Fe ₁₆ N ₂ even when the heating temperature during molding is (binder decomposition temperature) + 20 ° C. Therefore, it is possible to produce an iron nitride material that can be satisfactorily molded and sufficiently contains α ″ Fe ₁₆ N ₂ .

As one form of the production method of the present invention, in the granulation step, a low oxygen atmosphere with an oxygen concentration of 3000 ppm by mass or less is used, and the product is cooled from {(melting point of the binder) +5} ° C. to room temperature. The form which performs a grain is mentioned. In addition, melting | fusing point in a binder is calculated | required as follows. The differential scanning calorimetry curve (DSC) of the binder is obtained in the state of atmospheric pressure (1 atm.apprxeq.0.1 MPa), and a tangent L _p1 at the peak of heat amount change is taken as shown in FIG. When the heat quantity change peak value is Y, a straight line L _p2 parallel to the tangent L _p1 and separated by Y is taken. The straight line L _p2 in parallel, taking the direction of the peak point from the straight line L _p2 the (Y / 2) apart by a straight line L _p3. The intersection of the DSC and the straight line L _p3 is taken, and the tangent L _p4 at the intersection on the low temperature side (the intersection on the left side in FIG. 3) is taken. Then, an intersection point between the straight line L _p2 and the tangent line L _p4 is taken, and the temperature of this intersection point is set as an offset temperature T _Op .

Since the oxidation of α "Fe ₁₆ N ₂ which is easily oxidized can be effectively prevented by using a low oxygen atmosphere, and the deterioration of the magnetic properties due to the presence of the oxide can be suppressed, the above form is a nitriding with excellent magnetic properties. Iron material can be produced, and the raw material powder and the binder can be easily mixed by setting the temperature higher than the melting point of the binder, and by cooling from the above temperature to room temperature, Since it is easy to form granule, the said form is excellent in the productivity of granulated powder.

  As one form of this invention manufacturing method, the form which performs the application of the said magnetic field in the said formation process using a high temperature superconducting magnet is mentioned.

  In the above embodiment, a strong magnetic field of 2 T or more can be stably applied to a large space. In addition, since the above-mentioned form can change the magnetic field at high speed, the process time can be shortened, or it is easy to set an appropriate magnetic field strength in accordance with the change in the orientation state of the iron nitride particles in the molding process. Productivity of iron nitride materials can be increased.

  As one form of the iron nitride material of the present invention, a form satisfying at least one of a coercive force of the molded body of 2.0 kOe (160 kA / m) or more and a saturation magnetization of the molded body of 2.0 T or more can be mentioned.

  Since the above form has a high coercive force and saturation magnetization, it can be suitably used for a magnet material such as a permanent magnet.

As an embodiment of the iron nitride material of the present invention, in the molded body, the integrated intensity of the X-ray diffraction peak of the (202) plane is I ₂₀₂ , the integrated intensity of the X-ray diffraction peak of the (004) plane is I ₀₀₄ , the integrated intensity: _{Assuming that} the ratio of integrated intensity to I ₂₀₂ : I ₀₀₄ is I ₀₀₄ / I _{202, there} is a form that satisfies I ₀₀₄ / I ₂₀₂ > 0.2.

  The above-mentioned form has an oriented structure in which the (004) plane is oriented, i.e., the c-axis, which is the easy axis of crystalline magnetic anisotropy of iron nitride, has a texture oriented in a specific direction. Excellent.

The iron nitride material of the present invention has a large content of iron nitride particles containing α ″ Fe ₁₆ N ₂ as a main component. The method of producing the iron nitride material of the present invention can produce the iron nitride material of the present invention with high productivity.

FIG. 1 is a process explanatory view showing a method for producing an iron nitride material of the present invention. FIG. 2 is an explanatory diagram for explaining the decomposition temperature of the binder. FIG. 3 is an explanatory diagram for explaining the melting point of the binder.

Hereinafter, the present invention will be described in more detail.
[Manufacturing method of iron nitride]
(Preparation process)
As the raw material powder, a powder made of iron nitride particles mainly containing α ″ Fe ₁₆ N ₂ is prepared. The iron nitride particles used for the raw material powder are particles constituting the finally obtained iron nitride material. In other words, the iron nitride particles constituting the iron nitride material obtained by the production method of the present invention substantially maintain the components, shapes and sizes of the iron nitride particles constituting the raw material powder. In order to improve the magnetic properties due to shape magnetic anisotropy (particularly, coercive force), as the iron nitride particles, columnar particles having an average minor axis of 100 nm or less are used.

Iron nitride particles with a short axis length of nano-order are typically obtained by nitriding columnar iron powder (hereinafter referred to as nano-iron powder) with a short axis length of nano-order. And a known production method can be used. For the production of the nano iron powder, for example, a known method such as a coprecipitation method, a reverse micelle method or a sol-gel method can be used. When the coprecipitation method is used, nano iron powder can be obtained by reducing nano-order iron oxide (hematite: Fe ₂ O ₃ ), and when reverse micelle method is used, iron carbonyl: Fe (CO) ₅ By synthesizing, nano iron powder can be obtained. Nano iron powder is substantially composed of α-Fe. It can be made columnar by adjusting the manufacturing conditions, or the length of the short axis of the nano iron powder and the length of the long axis of the nano iron powder can be changed. When an initial crystal of nano-order iron oxide in the precursor state (hereinafter referred to as nano-iron oxide) or nano-order iron (hereinafter referred to as nano-iron) is generated or the particle size is grown, an external magnetic field or When an electric field is applied, the growth direction of nano iron oxide or nano iron which is a precursor state can be controlled. In addition, the particle size of nano iron oxide or nano iron in the precursor state tends to decrease when the temperature during the reaction is lowered or when the reaction time is shortened. Therefore, by adjusting the reaction temperature and reaction time with the growth direction controlled by a magnetic field or electric field, the columnar shape, the length of the short axis can be shortened, or the length of the long axis can be lengthened. Can do. When a crystal grows to a desired size, a coupling agent having a hydrophilic group is added to modify the end of the oxygen-iron bond existing in the iron oxide or iron surface layer to be grown. You can stop. The external magnetic field and electric field may be applied with direct current or applied with alternating current. Examples of the coupling agent having a hydrophilic group include silane coupling agents and unsaturated fatty acids (oleic acid, linoleic acid, etc.).

“Having α ″ Fe ₁₆ N ₂ as a main component” means that the content (purity) of α ”Fe ₁₆ N ₂ is 80% by volume or more. As the content of α "Fe ₁₆ N _{2 in} the iron nitride particles increases (the purity is higher), the content of α" Fe ₁₆ N _{2 in} the iron nitride material increases. 90 volume% or more is preferable. The iron nitride particles allow the inclusion of impurities. Examples of the impurity include α-Fe. Increasing the content of α ”Fe ₁₆ N ₂ (in order to increase the purity), for example, when performing nitriding treatment on the nano-iron powder that is a precursor state obtained as described above, low temperature (150 ° C ~ Reacts in a state (for example, ammonia (NH ₃ ) or plasma nitrogen state) where the nitrogen atom is more reactive than the nitrogen molecule (N ₂ ) state (for example, about 300 ° C) for a long time (about 10 to 50 hours). A method for measuring the content of α ″ Fe ₁₆ N _{2 in} the iron nitride material will be described later.

  The average length of the minor axis of the iron nitride particles is preferably 80 nm or less, more preferably 50 nm or less, and particularly preferably 20 nm or less because the shorter the average aspect ratio becomes, the easier the aspect ratio becomes and the iron nitride particles as a whole tend to become small. Further, if the average length of the short axis is 10 nm or more, the aspect ratio does not become too small, so that it becomes difficult to be in a so-called superparamagnetic state, and loss of magnet characteristics can be suppressed. Nano iron powder is prepared by adjusting the manufacturing conditions as described above so that iron nitride particles having a desired minor axis length and aspect ratio can be obtained. A method for measuring the average length of the minor axis will be described later.

  The iron nitride particles have better magnetic properties due to shape magnetic anisotropy as the minor axis is shorter and the major axis is longer, that is, the aspect ratio is larger. Accordingly, the aspect ratio is preferably 2 or more, and more preferably 2.2 or more. The aspect ratio is adjusted by applying an external magnetic field or electric field at the time of initial crystal generation of nano iron oxide or nano iron powder in the precursor state or growing the particle size as described above, and adjusting the magnitude of this external magnetic field or electric field. It can be controlled by doing. Increasing the external magnetic field or electric field makes it easier to increase the aspect ratio. When the magnitude of the external magnetic field or electric field is constant, the particle size varies depending on the reaction temperature and reaction time, but the aspect ratio is generally constant.

(Granulation process)
One feature of the production method of the present invention is that the raw material powder and the binder are mixed to form a granulated powder.

The binder is temporarily present from granulation to the middle of molding and is removed by heating and exhaust during molding. This binder ensures the fluidity of the granulated powder until it is put into the mold, and moves the iron nitride particles by an external magnetic field by melting and liquefying in the process of increasing the temperature due to external heating. And has a function to facilitate rotation. Therefore, the binder, the decomposition temperature alpha "Fe ₁₆ N less than _two of the decomposition temperature, alpha" does not react with the Fe ₁₆ N _2, and capable granulated. In particular, the production method of the present invention utilizes a binder that can be vaporized and removed by volatilization or decomposition into gas at a decomposition temperature of + 20 ° C. or lower. Since the binder can be removed at such a relatively low temperature, the molding temperature can be lowered, so that thermal decomposition of the binder itself and α "Fe ₁₆ N ₂ can be effectively prevented. Α" Fe _Since the decomposition temperature of ₁₆ N ₂ is about 260 ° C., the binder preferably has a decomposition temperature of 240 ° C. or lower, more preferably 220 ° C. or lower. Examples of the binder having a decomposition temperature of 240 ° C. or lower include organic substances such as oleic acid amide, erucic acid amide, and ricinoleic acid amide. Commercially available wax that satisfies the above specifications (decomposition temperature etc.) may be used.

  In granulation, when the binder is in a molten or softened state, the iron nitride particles constituting the raw material powder and the binder are easily mixed uniformly, and the binder exists so as to cover the entire circumference of the iron nitride particles. Granulated powder can be formed. Therefore, it is preferable to mix the raw material powder and the binder at a temperature equal to or higher than the melting point of the binder, preferably higher than the melting point + 5 ° C. If the temperature at the time of mixing is too low, (1) the raw material powder and the binder are difficult to mix and granulate, (2) the binder does not adhere uniformly to the raw material powder, and the raw material powder does not slide well. There is a possibility that a granulated powder having inferior formability is formed, and (3) that the slippage is poor, leading to a decrease in the filling rate of the raw material powder into the mold. On the other hand, if the temperature at the time of mixing is too high, the binder is reduced due to adhesion or deposition in the mixing equipment during granulation. Therefore, the melting point is preferably about (melting point + 5) ° C. to (melting point + 15) ° C. The melting point of the binder is preferably 90 ° C. or less. Since the melting point is low, the binder can be easily melted or softened. Therefore, the binder and the raw material powder can be mixed uniformly at a relatively low temperature, and the workability of the granulation process is excellent, and a granulated powder in which the binder exists uniformly can be formed. This granulated powder can prevent the oxidation of the raw material powder, can improve the sliding of the raw material powder, and is excellent in moldability.

  When sufficiently mixed, the mixture of the raw material powder and the binder is cooled to room temperature, and the binder is solidified to form granulated powder. The granulated powder is formed so that the average particle size is 1 μm or more. If the average particle size is less than 1 μm, the fluidity is remarkably low, and since it has scattering properties, it is inferior in the filling property to the mold. The larger the average particle size, the easier it is to produce the granulated powder and the better the moldability, so the average particle size is preferably 5 μm or more, more preferably 10 μm or more. However, if the granulated powder is too large, the binder becomes excessive or the filling rate is lowered due to the remaining binder component, so the average particle size is preferably 100 μm or less. As described above, it is easy to produce granulated powder having an average particle diameter of 1 μm or more by melting the binder and then cooling it. A method for measuring the average particle diameter will be described later.

Iron nitride particles containing α "Fe ₁₆ N ₂ as a main component are easy to oxidize, and when oxidized, the orientation deteriorates and the magnetic properties of the iron nitride material deteriorate due to the presence of the generated oxide. The granulation step is preferably performed in a low oxygen atmosphere, specifically, the oxygen concentration is preferably 3000 ppm by mass or less, more preferably 2500 ppm by mass or less, and particularly preferably 2000 ppm by mass or less. Examples thereof include a rare gas atmosphere such as Ar (argon) and He (helium) and an inert atmosphere such as N ₂ (nitrogen).

  When the granulated powder covers the entire circumference of the iron nitride particles with the binder as described above, the binder can also function as an oxidation-preventing layer for the iron nitride particles.

  The binder content can be selected as appropriate. However, if the content is too large, the removal time becomes longer, or the residual content causes a decrease in the filling rate of the iron nitride particles. Therefore, the content of the binder is preferably 0.5% by mass to 5% by mass with respect to the entire mixture of the raw material powder and the binder.

(Molding process)
One feature of the production method of the present invention is that when the granulated powder is pressure-formed, it is heated and exhausted and a strong magnetic field is applied.

{heating}
Heating at the time of molding is performed mainly for melting and vaporizing (volatilizing) the binder in the granulated powder. Therefore, the heating temperature (final temperature reached) in this step is preferably in the vicinity of the binder decomposition temperature, and is set to the decomposition temperature ± 20 ° C. If this heating temperature is less than (decomposition temperature −20) ° C., the binder cannot be sufficiently melted, the movement and rotation of the iron nitride particles are hindered to make it difficult to be oriented, and the binder cannot be sufficiently vaporized, and the binder remains. As a result, the filling rate of iron nitride particles is reduced. The higher the heating temperature, the easier it is to melt and vaporize the binder, and it can be removed reliably. For example, C (carbon) or the like remains in the molded body as a residue, or α "Fe ₁₆ N ₂ decomposes, resulting in a decrease in the magnetic properties of the iron nitride material. In addition, it can be realized by heating the molding die to a predetermined temperature, and if the granulated powder is preheated at a temperature lower than the melting point of the binder, the temperature rise time of the granulated powder itself can be shortened and production can be performed. It is possible to improve the performance.

{exhaust}
Exhaust during molding is performed mainly to remove the binder vaporized by the heating to the outside. Specifically, exhaust is performed so that the pressure becomes 0.9 atm (91.2 kPa) or less. That is, the molding is performed in a reduced pressure atmosphere. When the atmospheric pressure exceeds 0.9 atm, sufficient degassing cannot be performed, and the binder remains, resulting in a decrease in the filling rate of the iron nitride particles. The lower the pressure of the atmosphere, the more the exhaust can be performed, the binder can be easily discharged, and the oxygen concentration in the atmosphere can be reduced. Therefore, 0.8 atm (81.1 kPa) or less is more preferable. At the time of molding, when the oxygen concentration in the atmosphere is low, even if the binder is removed and the iron nitride particles are exposed, the oxidation of the particles can be suppressed.

{Magnetic field applied}
The application of the magnetic field at the time of molding is mainly performed in order to orient the crystal orientation of the iron nitride particles in a certain direction. Specifically, a strong magnetic field of 2T or more is applied. Such a strong magnetic field can be stably formed by using a high-temperature superconducting magnet. In addition, the high-temperature superconducting magnet can change the magnetic field at high speed, for example, the arrival time from the pre-applied state to the maximum magnetic field is short. When a low-temperature superconducting magnet is used, the magnetic field fluctuation speed is generally about 5 to 10 minutes per 1T, whereas with a high-temperature superconducting magnet, it can be performed in a very short time, for example, within 10 seconds per 1T. When the high-temperature superconducting magnet is used in this way, a desired strong magnetic field can be obtained in a short time, and therefore the time for the molding process can be shortened. By shortening the process time, it is possible to reduce the coarsening by suppressing the growth of crystal grains in the particles constituting the molded body, and thus it is easy to obtain an iron nitride material having a large coercive force. In addition, because the magnetic field fluctuation speed is fast, when applying the granulated powder to the mold or taking out the compact, stop applying the magnetic field (OFF), start applying the magnetic field after filling (ON), It is possible to quickly control the application of the magnetic field. When the high-temperature superconducting magnet is used in this way, the productivity of the iron nitride material is excellent. A high-temperature superconducting magnet is typically used by cooling a superconducting coil composed of an oxide superconductor, for example, by conducting cooling with a refrigerator (operating temperature is about −260 ° C. or more). If the magnitude of the magnetic field is less than 2T, it is difficult to orient the crystal orientation of the iron nitride particles in one direction, leading to a decrease in orientation. As the magnitude of the magnetic field is increased, the orientation is improved and an iron nitride material having excellent magnetic properties is finally obtained. Therefore, it is preferably 2.2 T or more, and more preferably 3 T or more. The application direction of the magnetic field is preferably the same as the forming direction (compression direction) when forming the granulated powder.

  When filling the granulated powder into the mold, it is preferable that the above-described specific strong magnetic field is not applied to easily prevent the granulated powder from being charged unevenly in the mold. On the other hand, after filling the granulated powder into a mold (preferably in a state heated to the above heating temperature), until the temperature of the granulated powder rises and the binder is in a molten state, for example, It is desired to make the above-mentioned specific strong magnetic field within a very short time of about 10 seconds to 15 seconds. A high-temperature superconducting magnet is an electromagnet having a high-temperature superconducting coil that can rapidly excite a strong magnetic field, and can sufficiently meet this requirement. Therefore, after filling granulated powder into a mold, the above-mentioned high-temperature superconducting magnet is used. It is preferable to apply a specific magnetic field.

{Pressurization}
The pressurization at the time of molding is mainly performed to increase the density of the iron nitride particles. The molding pressure is preferably 1 ton / cm ² or more, and about ¹ ton / cm ² to ³ ton / cm ² is easy to use. By pressurization, the binder existing between the iron nitride particles is easily discharged. After sufficiently removing the binder, the molding pressure may be increased to about 10 ton / cm ² to further increase the density.

[Iron nitride]
The iron nitride material of the present invention consists of a molded body composed of a plurality of iron nitride particles mainly composed of α "Fe ₁₆ N _2. Therefore, the iron nitride material of the present invention is a powder particle of iron nitride particles. The world can be confirmed.

  The iron nitride particles constituting the iron nitride material of the present invention are columnar, and the average length of the minor axis is 100 nm or less, like the raw material powder described above. The average length of the short axis is preferably 80 nm or less, more preferably 50 nm or less, and particularly preferably 20 nm or less. Here, when the crystal grain size is 10 nm or more, the coercive force increases as the crystal grain boundary increases, that is, as the crystal grain size decreases. Since the shorter the length of the minor axis relative to the major axis, that is, the larger the aspect ratio, the easier it is to crystallize the inside of the iron nitride particles, resulting in the same meaning as the smaller crystal size. The coercive force can be increased. The aspect ratio of the iron nitride particles is preferably 2 or more, and more preferably 2.2 or more.

The content (purity) of α ”Fe ₁₆ N _{2 in} the iron nitride particles constituting the iron nitride material of the present invention is 80% by volume or more, 85% by volume or more, and further 90% by volume, like the raw material powder described above. The above is preferable. Since the content of α ″ Fe ₁₆ N ₂ is large, the orientation can be improved, and as a result, improvement of magnetic properties such as improvement of coercive force and improvement of saturation magnetization can be expected.

  The iron nitride material of the present invention is characterized in that the content of the iron nitride particles is 85% by volume or more. The iron nitride material of the present invention does not include a binder or a support film unlike conventional magnetic recording media, and has a high content of iron nitride particles by removing the binder. By adjusting the heating temperature at the time of molding and the pressure of the atmosphere described above, an iron nitride material in which the content of iron nitride particles is 90% by volume or more can be obtained.

  The iron nitride material of the present invention has a high content of iron nitride particles, preferably high orientation, and thus has excellent magnetic properties. Specifically, the coercive force satisfies 2 kOe (160 kA / m) or higher, the saturation magnetization satisfies 2.0 T or higher, the coercive force: 2 kOe (160 kA / m) or higher, and the saturation magnetization: 2.0 T or higher. A form is mentioned. Thus, since it is excellent in a magnetic characteristic, this invention iron nitride material can be utilized suitably for the raw material of a permanent magnet.

Since the iron nitride material of the present invention is manufactured by applying a specific strong magnetic field as described above, it typically has an oriented structure. Specifically, when the present invention iron nitride was subjected to X-ray diffraction integrated intensity of (202) plane: integrated intensity of for I ₂₀₂ (004) plane ratio of I _004: I ₀₀₄ / I ₂₀₂ is more than 0.2 (I ₀₀₄ / I ₂₀₂ > 0.2). Depending on the manufacturing conditions, it is possible to have a configuration satisfying I ₀₀₄ / I ₂₀₂ ≧ 0.4 and further I ₀₀₄ / I ₂₀₂ ≧ 0.6. The (004) plane is oriented, that is, the iron nitride material of the present invention is excellent in magnetic properties because the c-axis which is the axis of easy magnetization of α ″ Fe ₁₆ N ₂ is oriented. In the molded body constituting the iron nitride material, the surface (which may be a surface or a cross section) whose normal is the direction of application of the magnetic field is performed.If the surface of the molded body is oxidized, the surface oxide layer, etc. It is preferable to perform X-ray diffraction after removing.

Hereinafter, more specific embodiments of the present invention will be described with reference to test examples.
[Test Example 1]
A raw material powder composed of iron nitride particles containing α "Fe ₁₆ N ₂ as a main component and a binder are mixed to produce a granulated powder, and this granulated powder is molded by pressing to produce an iron nitride material. In this test, in particular, the influence of the size of the iron nitride particles, the size of the granulated powder, and the molding conditions (temperature, atmospheric pressure, applied magnetic field) were investigated.

  As shown in FIG. 1, the iron nitride material was prepared in the order of preparation step: production of raw material powder → granulation step: production of granulated powder → molding step: molding of iron nitride material.

(Preparation process)
According to the coprecipitation method, iron (II) chloride and sodium hydroxide were charged and controlled so that the pH was about 8 to 9, and nano-Fe ₂ O ₃ powders of various sizes were prepared, and hydrogen Reduction was performed to produce columnar nano iron powder made of α-Fe. The nano iron powder was subjected to nitriding treatment in an ammonia atmosphere (in an ammonia stream, 200 ° C. × 24 Hr (low temperature, long time)) to obtain a raw material powder. The nano Fe ₂ O ₃ powder used for each sample was prepared by placing an electromagnet outside the reaction vessel and applying a constant external magnetic field (0.1 T). In this synthesis, the particle size of the nano Fe ₂ O ₃ powder was appropriately changed by adjusting the reaction time (30 minutes to 180 minutes) and the reaction temperature (60 ° C. to 90 ° C.). In addition, when the sizes of the powder before and after the hydrogen reduction and the powder before and after the nitriding treatment were examined, an increase in the particle size was not substantially observed. That is, the nano Fe ₂ O ₃ powder, the nano iron powder, and the raw material powder obtained after the nitriding treatment all have substantially the same particle size.

When the obtained raw material powders were examined, the content of α "Fe ₁₆ N _{2 in} the iron nitride particles was 90% by volume, and α" Fe ₁₆ N ₂ was the main component (80% by volume or more). It was confirmed to be composed of a plurality of iron nitride particles. The content of α "Fe ₁₆ N ₂ is determined by performing electron beam diffraction by focusing iron nitride particles on a transmission electron microscope: TEM. Electron diffraction intensity and other components in α" Fe ₁₆ N ₂ crystals (α -Fe, Fe ₃ N, Fe ₄ N, etc.) and the ratio to the electron diffraction intensity are measured. The volume ratio of α "Fe ₁₆ N ₂ in iron nitride particles can be measured from this ratio of electron diffraction intensity. The components of iron nitride particles can be measured by Mossbauer spectrum analysis or when the size is large. It can also be measured by X-ray diffraction.

When the obtained iron nitride particles were observed with a transmission electron microscope: TEM, all were columnar. The average length of the short axis was measured using this observation image. Here, the observation image is image-processed, and for each iron nitride particle present in the field of view, the length of the minor axis perpendicular to the major axis is measured at the center position in the major axis direction. Is the short axis length of the particles, and the average of the short axis lengths of 50 or more iron nitride particles is the average short axis length. The results are shown in Table 1. Depending on the size of the Fe ₂ O ₃ powder, the average length of the short axis is different, and sample Nos. 101 to 103 were obtained by using a large Fe ₂ O ₃ powder. Became larger. Also, the length of the major axis was measured to determine the aspect ratio (length of major axis / length of minor axis), and all samples were 2.2.

(Granulation process)
A commercially available wax made of oleic amide (melting point: 75 ° C., decomposition temperature: 220 ° C.) was prepared as the binder. The amount of binder added was adjusted so that the binder content was 1.0 mass% with respect to the mixture of the raw material powder and the binder. Then, under a nitrogen atmosphere with an oxygen concentration of 2000 mass ppm, the raw material powder and the binder are mixed while being heated to the temperature shown in Table 1, and after thoroughly mixing, a cooling rate of about 1 ° C / min to room temperature The mixture was cooled and kneaded to form granulated powder. Sample Nos. 1-11, 1-12, 111 to 113 were made different in average particle size of the granulated powder by making the cooling rate faster than other samples. The average particle size of the granulated powder was measured as follows. A sufficient amount of granulated powder is dispersed on the glass plate, and a projected image of the granulated powder on the glass plate is taken with an optical microscope. About 50 or more granulated powders present in the obtained image, The ferret diameter of the granulated powder (here, the average value of the vertical ferret diameter and the parallel ferret diameter) was determined, and the average of the ferret diameters of 50 or more granulated powders was defined as the average particle diameter. The results are shown in Table 1. The granulated powder can be classified using sieve classification, air classification, or the like.

(Molding process)
Each of the obtained granulated powders was subjected to pressure molding under the conditions shown in Table 1 with a molding pressure of 1 ton / cm ² . Specifically, after filling the mold with the granulated powder, the deaeration pump exhausts so that the pressure in the mold becomes a reduced pressure atmosphere of the size shown in Table 1 (atmosphere atmosphere of the pressure shown in Table 1). While maintaining the state heated to the temperature shown in Table 1, and applying the magnetic field shown in Table 1, pressure molding was performed. Sample No. 132 was at atmospheric pressure (1 atm≈101.3 kPa) and was not evacuated. The application of the magnetic field was performed using a high-temperature superconducting magnet, and the application direction of the magnetic field was the same as the pressing direction.

After sufficiently pressurizing, the molded body (a cylindrical body having a diameter of 10 mm × height of 10 mm) was taken out from the mold. Sample No. 111 could not be molded and a molded body could not be obtained. The obtained molded body was examined by X-ray diffraction, and it was confirmed that the molded body was composed of a plurality of iron nitride particles mainly composed of α ”Fe ₁₆ N ₂ . It can be measured by using an energy dispersive X-ray spectroscopy apparatus, etc. Further, the content (volume%) of iron nitride particles in each compact was examined, and the results are shown in Table 1. The content of iron nitride particles was measured as follows: About the obtained molded body, a plane perpendicular to the pressing direction during molding (here, a circular plane (plane) of a cylindrical molded body) The surface parallel to the pressing direction (here, the circumferential surface (curved surface) of the cylindrical molded body) is polished so that the particles do not fall off or deform, and then polished with an optical microscope or the like. For each observation image, the ratio of the voids in the observation image (%): (the area of the void / the area of the observation image) × 100 When P1 is the porosity on the flat surface and P2 is the porosity on the parallel plane, (P1) x (P2) x (P2) is the porosity of the molded body (%) and the volume of the molded body (100% ) From which the porosity of the compact was removed was taken as the content of iron nitride particles.

  Saturation magnetization (T) and coercivity (kOe) of the obtained compact (iron nitride material) were investigated. The results are shown in Table 1. Here, the saturation magnetization (T) and coercive force (kOe) in the axial direction of the cylinder of the compact (= the direction of application of the magnetic field = the pressurizing direction during molding) are measured using a BH tracer (DCBH tracer manufactured by Riken Denshi Co., Ltd.). I investigated.

Also, take a cross section of the compact (iron nitride material), perform X-ray diffraction on this cross section, examine the (202) plane peak integrated intensity: I ₂₀₂ , (004) plane peak integrated intensity: I ₀₀₄ , integrated intensity: integrated intensity for I _202: the ratio of I _004: was obtained I ₀₀₄ / I _202. The results are shown in Table 1. Here, the cross section was a surface in a direction perpendicular to the pressing direction during molding.

As shown in Table 1, as a raw material powder, α ”Fe ₁₆ N ₂ as a main component (80% by volume or more), a powder composed of columnar iron nitride particles having an average short axis length of 100 nm or less, The raw material powder and binder are mixed to form granulated powder of 1 μm or more, and this granulated powder is subjected to specific conditions (heating temperature: binder decomposition temperature ± 20 ° C, atmospheric pressure: 0.9 atmospheres or less, applied magnetic field: 2T From the above, it can be seen that an iron nitride material having an iron nitride particle content of 85% by volume or more can be obtained by press molding.The obtained iron nitride material has a saturation magnetization of 2 T or more and a coercive force. In particular, it is found that an iron nitride material satisfying saturation magnetization: 2T or more and coercive force: 2 kOe or more can be obtained by adjusting the manufacturing conditions and the like. The obtained iron nitride material tends to have a structure oriented along the c-axis direction and satisfies I ₀₀₄ / I ₂₀₂ > 0.2. It was confirmed that the aspect ratio and purity of the iron nitride particles constituting the obtained iron nitride material substantially maintained the value of the raw material powder.

  On the other hand, it can be seen that when a raw material powder having an average minor axis length of more than 100 nm is used, the coercive force is low and the magnetic properties are poor. Alternatively, it can be seen that when granulated powder having an average particle size of less than 1 μm is used, it cannot be molded, or the content of iron nitride particles is small and the filling property is poor. Or when the heating temperature at the time of shaping | molding is too low, or exhaust_gas | exhaustion is inadequate and the pressure of atmosphere is too large, it turns out that content of an iron nitride particle is small and it is inferior to a filling property. Or when the heating temperature at the time of shaping | molding is too high, or a magnetic field is too small, it turns out that orientation falls, coercive force is low, and it is inferior to a magnetic characteristic.

[Test Example 2]
An iron nitride material was produced in the same process as in Test Example 1, and the magnetic properties were examined. In this test, in particular, the influence of the size of the iron nitride particles, the purity of the iron nitride particles, the binder material, and the conditions of the granulation process (temperature, oxygen concentration) were investigated.

(Preparation process)
A columnar nano iron powder made of α-Fe produced in the same manner as in Test Example 1 was subjected to nitriding treatment in an ammonia atmosphere under the same conditions as in Test Example 1 to obtain a raw material powder. In Sample Nos. 2-1 to 2-3 and 2-5, the aspect ratio was changed by changing the applied magnetic field during the synthesis of the nano-Fe ₂ O ₃ powder. In Sample Nos. 2-11 to 2-14, the treatment time during the nitriding treatment was shorter than that of the other samples.

When the obtained raw material powders were examined by X-ray diffraction, they were confirmed to be composed of a plurality of iron nitride particles containing α ″ Fe ₁₆ N ₂ as a main component (80% by volume or more). Table 2 shows the content (volume%) of α ″ Fe ₁₆ N ₂ therein. Samples Nos. 2-11 to 2-14 were used with a short nitriding treatment time, so that the content of α "Fe ₁₆ N ₂ was small.

  When the obtained iron nitride particles were observed with a transmission electron microscope: TEM, all were columnar, and the average length of the minor axis was measured in the same manner as in Test Example 1. As a result, it was 50 nm. Table 2 shows the aspect ratio of each sample. Sample Nos. 2-1 to 2-3 have a smaller aspect ratio by making the applied magnetic field smaller than that of sample No. 2-4 (0.1T). The aspect ratio was large by making it larger than -4 (0.1T).

(Granulation process)
The binder shown in Table 1 was used. The sample described as “oleic amide” in Table 1 utilized the same binder as in Test Example 1. Erucamide (melting point: 85 ° C., decomposition temperature: 240 ° C.) and ethylenebisstearic acid amide (melting point: 115 ° C., decomposition temperature: 250 ° C.) are all commercially available waxes. The content of each binder was 1% by mass as in Test Example 1. Then, in a nitrogen atmosphere with the oxygen concentration shown in Table 2, the raw material powder and the binder are mixed in a state heated to the temperature shown in Table 2, and after thoroughly mixing, a cooling rate of about 1 ° C / min to room temperature The mixture was cooled and kneaded to form granulated powder having an average particle size of 10 μm. The average particle size was measured in the same manner as in Test Example 1.

(Molding process)
Each obtained granulated powder was subjected to pressure molding in the same manner as in Test Example 1 to produce a molded body having the same shape and size as in Test Example 1. In this test, the molding conditions were as follows: heating temperature: temperature shown in Table 1, atmospheric pressure: exhausted to 0.8 atm, applied magnetic field: 3 T, molding pressure: 1 ton / cm ² . The application of the magnetic field was performed using a high-temperature superconducting magnet, and the application direction of the magnetic field was the same as the pressing direction.

After sufficiently pressurizing, the molded body was taken out from the mold, and the obtained molded body was examined by X-ray diffraction. As a result, it was found that the molded body was composed of a plurality of iron nitride particles mainly containing α "Fe ₁₆ N _2. The content (volume%) of iron nitride particles in each molded body was examined in the same manner as in Test Example 1. The results are shown in Table 2.

With respect to the obtained molded body (iron nitride material), saturation magnetization (T), coercive force (kOe), and I ₀₀₄ / I ₂₀₂ were examined in the same manner as in Test Example 1. The results are shown in Table 2.

As shown in Table 2, raw material powder with an aspect ratio of 2 or higher, raw material powder with high α "Fe ₁₆ N ₂ content (purity), or binder with decomposition temperature of 240 ° C or lower It can be seen that an iron nitride material superior in magnetic properties can be obtained, and that an iron nitride material superior in magnetic properties can be obtained if the oxygen concentration is 3000 mass ppm or less, particularly 2000 mass ppm or less in the granulation step. In addition, it was confirmed that the average length, aspect ratio, and purity of the short axis of the iron nitride particles constituting the obtained iron nitride material substantially maintained the value of the raw material powder.

  Note that the present invention is not limited to the above-described embodiment, and can be appropriately changed without departing from the gist of the present invention. For example, the mixing amount of the binder can be appropriately changed.

  The iron nitride material of the present invention can be suitably used as a permanent magnet, for example, a material of a permanent magnet used in various motors, in particular, a high-speed motor provided in a hybrid vehicle (HEV) or a hard disk drive (HDD). . In addition, the iron nitride material of the present invention is expected to be usable for electromagnetic wave interference / absorption materials up to a frequency region (terahertz region) in which the skin depth of the magnetic phase is close to the width of the magnetic phase. The manufacturing method of the iron nitride material of the present invention can be suitably used for manufacturing the iron nitride material of the present invention.

Claims (10)

a preparation step of preparing a raw material powder composed of columnar iron nitride particles having α ”Fe ₁₆ N ₂ as a main component and having an average minor axis length of 100 nm or less;
Mixing the raw material powder and a binder having a decomposition temperature lower than the decomposition temperature of α "Fe ₁₆ N ₂ to form a granulated powder having an average particle size of 1 μm or more,
After filling the granulated powder into a molding die, comprising a molding step of forming a molded body by pressure molding,
In the molding step, while evacuating the inside of the mold to 0.9 atm or less, while being heated to a temperature of {(decomposition temperature of the binder) −20} ° C. or more {(decomposition temperature of the binder) +20} ° C. or less. And a method of producing an iron nitride material, wherein pressure forming is performed in a state where a magnetic field of 2 T or more is applied. _2. The method for producing an iron nitride material according to claim 1, wherein the content of α ″ Fe ₁₆ N ₂ in the iron nitride particles is 85% by volume or more.   The iron nitride material according to claim 1 or 2, wherein the aspect ratio is 2 or more when the ratio of the length of the major axis to the length of the minor axis in the iron nitride particles is an aspect ratio. Production method.   The method for producing an iron nitride material according to any one of claims 1 to 3, wherein a decomposition temperature of the binder is 240 ° C or lower.   The granulation step is performed in a low oxygen atmosphere with an oxygen concentration of 3000 ppm by mass or less, and granulated by cooling from a temperature of {(melting point of the binder) +5} ° C. to room temperature. Item 5. The method for producing an iron nitride material according to any one of Items 1 to 4.   6. The method of manufacturing an iron nitride material according to claim 1, wherein the magnetic field is applied in the forming step using a high-temperature superconducting magnet. An iron nitride material obtained by the iron nitride material manufacturing method according to any one of claims 1 to 6,
It consists of a molded body composed of a plurality of iron nitride particles containing α "Fe ₁₆ N ₂ as the main component,
The iron nitride material, wherein the content of the iron nitride particles in the molded body is 85% by volume or more. It consists of a molded body composed of a plurality of iron nitride particles mainly composed of α "Fe ₁₆ N ₂ ,
The iron nitride particles are columnar, the average length of the minor axis is 100 nm or less,
The iron nitride material, wherein the content of the iron nitride particles in the molded body is 85% by volume or more.   9. The iron nitride material according to claim 7, wherein a coercive force of the molded body satisfies at least one of 2.0 kOe or more and a saturation magnetization of the molded body of 2.0 T or more. The integrated intensity of the peaks of the X-ray diffraction of the molded article (202) plane I _202, the integrated intensity of the peaks of the X-ray diffraction of the (004) plane I _004, integrated intensity: integrated intensity for I _202: the I ₀₀₄ 10. The iron nitride material according to claim 7, wherein when the ratio is I ₀₀₄ / I ₂₀₂ , I ₀₀₄ / I ₂₀₂ > 0.2 is satisfied.

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