JP6344129B2 - Iron nitride magnetic powder and magnet using the same - Google Patents

Description

The present invention relates to an iron nitride-based magnetic powder having a Fe ₁₆ N ₂ compound phase as a main phase and maintaining a high saturation magnetization and a high coercive force. Furthermore, a magnet using the iron nitride magnetic powder is provided.

In recent years, Nd-Fe-B magnets have been widely used as motor magnets for electric vehicles and hybrid vehicles. However, rare earths represented by Nd are raw materials for high-value-added components that support the industrial field, and since demand is increasing in recent years, there are concerns that resource depletion and raw material prices are unstable. . Furthermore, there is a problem in securing a stable supply because the demand is growing significantly in developing countries and its dependence on specific producing countries is high due to its uneven distribution.

In order to avoid the above problems, it is required to develop a high-performance magnet from elements (iron, nitrogen) that do not use rare earths and are inexhaustible in nature.

An Fe—N-based compound, particularly Fe ₁₆ N _2, has attracted attention as one of materials exhibiting a larger saturation magnetization than Fe. However, as it is said to be a metastable compound, it is very difficult to chemically synthesize this compound as an isolated powder.

In Patent Document 1, iron oxide magnetic powder is synthesized by a method of synthesizing iron oxide by a coprecipitation method and reducing and nitriding. However, since the magnetization of the obtained iron nitride powder is low, it is difficult to use it as a magnetic material for motor applications that require high coercive force and high saturation magnetization.

JP 2009-84115 A

The present invention has been made in view of the above, and an object thereof is to provide an iron nitride magnetic powder having high magnetization and high coercive force, and a magnet using the magnetic powder.

The present invention is an iron nitride magnetic powder having an average particle diameter of 10 to 60 nm mainly composed of Fe ₁₆ N ₂ , and an alkali metal compound containing an alkali metal on the surface of the iron nitride magnetic powder. Adhesive iron nitride magnetic powder, wherein the amount of alkali metal contained in the alkali metal compound is 0.15 to 1.80 mass% with respect to the iron nitride magnetic powder, and the alkali metal attached iron nitride magnetic The present invention relates to an iron nitride-based magnetic powder mainly composed of Fe ₁₆ N ₂ , wherein the powder has a saturation magnetization of 100 to 200 emu / g and a coercive force of 1.5 to 3.5 kOe.

According to the present invention, the amount of alkali metal contained in the alkali metal compound present on the surface of the iron nitride magnetic powder having an average particle size of 10 to 60 nm is 0.15 to 1. By setting the content to 80% by mass, it is possible to obtain a magnetic powder and a magnet exhibiting a coercive force of 1.5 kOe or more while maintaining a saturation magnetization of 100 emu / g or more. This is because the alkali metal contained in the alkali metal compound existing on the surface of the iron nitride-based magnetic powder promotes nitriding, so that the iron nitride magnetic powder has a saturation magnetization of 100 emu / g or more and a coercive force of 1.5 kOe or more. Can be obtained.

Hereinafter, preferred embodiments of the present invention will be described. The present invention is not limited by the contents of the embodiments and examples described below. In addition, the constituent elements shown in the embodiments and examples described below may be appropriately combined or may be appropriately selected.

A suitable method for producing the magnetic powder according to this embodiment will be described. The iron nitride-based magnetic powder of the present invention can be obtained by synthesizing iron oxide particles, attaching an alkali metal compound to the surface of the iron oxide particles, and sequentially performing reduction treatment and nitriding treatment.

The iron oxide particles can be produced by mixing an aqueous iron salt solution containing ferrous salt and / or ferric salt with an alkaline aqueous solution, aging and washing.

Examples of the early iron salt include sulfate, chloride, nitrate, etc., and these may be used in appropriate combination. Moreover, those hydrates can be used.

As the alkaline aqueous solution, one or more of ammonia water, ammonia salt aqueous solution, and urea aqueous solution can be used, but not limited thereto.

By the precipitation reaction by mixing the iron salt solution and the alkali solution, iron oxide particles not containing an alkali metal can be synthesized. In addition, after the iron oxide is generated, an in-liquid aging reaction such as hydrothermal treatment with an autoclave may be performed to improve crystallinity, control particle size, and particle shape.

The iron oxide particles can be recovered by filtering the aqueous solution after synthesis of iron oxide and subjecting it to a washing treatment such as washing as necessary.

For the adhesion of the alkali metal compound to the recovered iron oxide particles, the iron oxide slurry is dispersed in ion-exchanged water, and an aqueous solution of the alkali metal compound is dropped. As the alkali metal compound, one or more of potassium hydroxide, sodium hydroxide, potassium hydrogen carbonate, sodium hydrogen carbonate and sodium oleate can be used, but not limited thereto.

After the iron oxide synthesis, if necessary, the surface of the iron oxide may be coated with a Si compound in order to suppress sintering of the particles by reduction treatment. As the Si compound, colloidal silica, a silane coupling agent, a silanol compound, or the like can be used.

The coating amount of the Si compound is preferably 0.1% by mass or more and 20% by mass or less in terms of Si with respect to iron oxide. When the amount is less than 0.1% by mass, it is difficult to say that the effect of suppressing the sintering between particles during heat treatment is sufficient. When it exceeds 20 mass%, a nonmagnetic component will increase and it is unpreferable. A more preferable surface coating amount is 0.15% by mass or more and 15% by mass or less, and further more preferably 0.2% by mass or more and 10% by mass or less.

The obtained iron oxide slurry can be dried at 85 ° C. for 20 hours to produce an iron oxide powder in which an alkali metal compound is present on the particle surface.

The iron oxide has a specific surface area of 50 m ² / g or more and 180 m ² / g or less.

The iron oxide is not particularly limited, but magnetite, γ-Fe ₂ O ₃ , α-Fe ₂ O ₃ , α-FeOOH, β-FeOOH, γ-FeOOH, FeO, etc. can be used. Absent.

The particle shape of the iron oxide as a raw material is not particularly limited, but may be any shape such as a needle shape, a granular shape, a spindle shape, and a rectangular parallelepiped shape.

Next, a reduction process is performed. The temperature for the reduction treatment is preferably 200 to 400 ° C. When the temperature of the reduction treatment is less than 200 ° C., iron oxide is not sufficiently reduced to metallic iron. When the temperature of the reduction treatment exceeds 400 ° C., the iron oxide is sufficiently reduced, but it is not preferable because sintering between particles proceeds. More preferably, it is 230-350 degreeC.

The time for the reduction treatment is not particularly limited, but is preferably 1 to 96 hours. If it exceeds 96 hours, depending on the reduction temperature, the sintering proceeds and the subsequent nitriding process becomes difficult to proceed. If it is less than 1 hour, the reduction does not proceed sufficiently. More preferably, it is 2 to 72 hours.

The atmosphere for the reduction treatment is preferably a hydrogen atmosphere.

After the reduction treatment, nitriding treatment is performed.

The temperature of the nitriding treatment is 100 to 200 ° C. When the nitriding temperature is less than 100 ° C., the nitriding does not proceed sufficiently. When the temperature of the nitriding process exceeds 200 ° C., nitriding proceeds excessively, so that an iron nitride magnetic powder exhibiting desired Hc cannot be obtained. More preferably, it is 120-180 degreeC.

The nitriding time is not particularly limited, but is preferably 1 to 48 hours. If it exceeds 48 hours, depending on the nitriding temperature, an iron nitride magnetic powder exhibiting desired Hc cannot be obtained. In many cases, sufficient reduction cannot be achieved in less than 1 hour. More preferably, it is 3 to 24 hours.

The atmosphere of the nitriding treatment is preferably an NH ₃ atmosphere, and N ₂ , H ₂ or the like may be mixed in addition to NH ₃ .

In the iron nitride magnetic powder according to the present embodiment, an alkali metal compound is present on the surface of the magnetic particles, and the amount of alkali metal contained in the alkali metal compound is 0.15 to 1 with respect to the iron nitride magnetic powder. 80% by mass. When the amount of alkali metal contained in the alkali metal compound is less than 0.15% by mass, the effect of promoting nitriding is reduced, so the coercive force is reduced. When the amount of alkali metal contained in the alkali metal compound exceeds 1.8% by mass, the alkali metal present on the surface of the iron nitride magnetic powder penetrates into the grains and the reduction becomes insufficient, so the saturation magnetization is small. turn into.

The saturation magnetization σs of the alkali metal-attached iron nitride magnetic powder according to the present embodiment is 100 to 200 emu / g, and when the saturation magnetization is less than the above range, it is difficult to say that the magnetic properties are sufficient as the magnetic powder. More preferably, the saturation magnetization is 110 emu / g or more.

The coercive force Hc of the alkali metal-attached iron nitride magnetic powder according to the present embodiment is 1.5 to 3.5 kOe, and when the coercive force is less than the above range, it is difficult to say that the magnetic properties are sufficient as the magnetic powder. . More preferably, the coercive force is 2.0 kOe or more.

The average particle diameter of the iron nitride magnetic powder according to this embodiment is preferably 10 nm or more and 60 nm or less. If the average particle size is less than 10, the ratio of the oxide film on the particle surface is increased, or the superparamagnetism is exhibited when the particle size is small, so that the coercive force tends to decrease. If the average particle size exceeds 60 nm, the particle size is large, so the proportion of particles having a single domain critical diameter or less is small, and the coercive force tends to decrease.

In the iron nitride-based magnetic powder according to this embodiment, the main phase is an Fe ₁₆ N ₂ compound phase and may include an Fe ₄ N compound phase.

A magnet such as a bulk magnet or an anisotropic bonded magnet can be obtained by using the iron nitride magnetic powder obtained by the present embodiment. The manufacturing method will be described below.

First, an example of a bulk magnet manufacturing method will be described. The iron nitride powder obtained by this embodiment can be made into a bulk magnet by compression molding. Here, the compression molding conditions are not particularly limited, and may be adjusted so as to be the required characteristic values of the bulk magnet to be manufactured. For example, the compression molding pressure can be 1 to 10 ton / cm ² . Further, magnetic field orientation may be performed during molding. Further, a lubricant or resin may be applied to the surface of the iron nitride magnetic powder.

Next, an example of a method for producing an anisotropic bonded magnet using the iron nitride-based magnetic powder obtained by the present embodiment will be described. A resin binder containing resin and magnetic powder are kneaded with a pressure kneader such as a pressure kneader to prepare a compound (composition) for a bond magnet. Resins include thermosetting resins such as epoxy resins and phenolic resins, styrene-based, olefin-based, urethane-based, polyester-based, polyamide-based elastomers, ionomers, ethylene-propylene copolymer (EPM), ethylene-ethyl acrylate copolymer There are thermoplastic resins such as coalescence. Among them, the resin used for compression molding is preferably a thermosetting resin, and more preferably an epoxy resin or a phenol resin. The resin used for injection molding is preferably a thermoplastic resin. Moreover, you may add a coupling agent and another additive to the compound for bonded magnets as needed.

Moreover, it is preferable that the content ratio of the magnetic powder and the resin in the bonded magnet includes, for example, 0.5% by mass or more and 20% by mass or less of the resin with respect to 100% by mass of the magnetic powder. If the resin content is less than 0.5% by mass with respect to 100% by mass of the magnetic powder, shape retention tends to be impaired, and if the resin exceeds 20% by mass, sufficiently excellent magnetic properties are obtained. It tends to be difficult to obtain.

After preparing the above-described bonded magnet compound, the bonded magnet compound containing magnetic powder and resin can be obtained by injection molding the bonded magnet compound. When producing a bonded magnet by injection molding, the bonded magnet compound is heated to the melting temperature of the binder (thermoplastic resin) as necessary to obtain a fluid state, and then the bonded magnet compound has a predetermined shape. Injection into the mold and molding. Then, it cools and takes out the molded product (bond magnet) which has a predetermined shape from a metal mold | die. In this way, a bonded magnet is obtained. The manufacturing method of the bonded magnet is not limited to the above-described method by injection molding. For example, a bonded magnet containing magnetic powder and resin may be obtained by compression molding a bonded magnet compound. When producing a bonded magnet by compression molding, after preparing the above-mentioned bonded magnet compound, the bonded magnet compound is filled into a mold having a predetermined shape, and pressure is applied to obtain the predetermined shape from the mold. Take out the molded product (bonded magnet). When forming and taking out a bonded magnet compound with a mold, a compression molding machine such as a mechanical press or a hydraulic press is used. After that, it is cured by applying heat in a furnace such as a heating furnace or a vacuum drying furnace to obtain a bonded magnet.

The shape of the bonded magnet obtained by molding is not particularly limited, and can be changed, for example, in a flat plate shape, a column shape, or a cross-sectional shape in a ring shape, depending on the shape of the mold to be used. In addition, the obtained bonded magnet may be plated or painted on the surface in order to prevent deterioration of the oxide layer, the resin layer, and the like.

When the bonded magnet compound is molded into a desired predetermined shape, a molded body obtained by molding by applying a magnetic field may be oriented in a certain direction. Thereby, since the bonded magnet is oriented in a specific direction, an anisotropic bonded magnet having stronger magnetism can be obtained.

Hereinafter, the present invention will be described in more detail with reference to examples and comparative examples, but the present invention is not limited to the embodiments shown in the examples.

(Explanation of measurement method)
First, measurement methods in the present example and comparative example will be described.
The composition of the obtained magnetic powder was determined by dissolving a heated sample with an acid and using an inductively coupled plasma emission spectrometer (ICP). Moreover, the place where an alkali metal element exists was specified using the energy dispersive X-ray-analysis apparatus (STEM-EDS, JEOL JEM2100F) by a scanning transmission electron microscope. For the average particle size, the obtained magnetic powder was observed with a transmission electron microscope (TEM, JEM-2000FX manufactured by JEOL). Next, the diameter equivalent to the circular area of 1000 particles randomly selected from the observed image was calculated as the particle size by image processing, and the number distribution of the particle size was obtained. In addition, when it has layers, such as an alkali metal compound, a sintering inhibitor, and an oxide film, on the surface, the said average particle diameter shall mean the size of the whole particle | grains containing an oxide film etc. The magnetic properties of the magnetic powder were measured in a magnetic field of 0 to 20000 Oe at 296 K using a vibrating sample magnetometer (VSM, VSM-5-20 manufactured by Toei Kogyo).

Example 1
<Manufacture of iron oxide powder>
167 g of iron sulfate heptahydrate (FeSO ₄ · 7H ₂ O) and 85 g of iron chloride hexahydrate (FeCl ₃ · 6H ₂ O) were dissolved in ion exchange water to prepare an iron salt aqueous solution. Aqueous solution of 2.5 mol of ammonia water is made into 600 g aqueous solution, kept at 30 ° C., and the previously prepared iron salt aqueous solution is added. The iron oxide slurry was prepared by washing 3 times with 2 L of ion exchange water in a centrifuge.

<Adhesion of alkali metal compounds>
The iron oxide slurry was put into 500 g of ion exchange water, stirred at 5000 rpm, and redispersed. Sodium hydroxide 0.03mol was made into 100g aqueous solution, this was dripped, and it stirred at 70 degreeC for 1 hour.

<Adhesion of sintering inhibitor>
To the iron oxide slurry prepared above, 2.5 g of tetraethoxysilane, 21 g of ethanol, and 78 g of diethylene glycol monobutyl ether were added to perform Si deposition treatment. This iron oxide slurry was dried at 85 ° C. for 24 hours to prepare an iron oxide powder.

<Reduction treatment and nitriding treatment of iron oxide powder>
2 g of the powder obtained above was placed in a firing boat and left in a heat treatment furnace. After filling the furnace with nitrogen gas, the temperature was raised to 250 ° C. at a rate of 5 ° C./min while flowing hydrogen gas at a flow rate of 1 L / min, and the reduction treatment was carried out for 48 hours. Thereafter, the supply of hydrogen gas was stopped, and the temperature was lowered to 140 ° C. while flowing nitrogen gas at a flow rate of 2 L / min. Subsequently, nitriding was performed at 140 ° C. for 24 hours while flowing ammonia gas at 0.2 L / min. Thereafter, the temperature was lowered to 50 ° C. while flowing nitrogen gas at a flow rate of 2 L / min, and air replacement was performed for 24 hours.

Example 2
It was produced in the same manner as in Example 1 except that sodium hydroxide was replaced with potassium hydroxide.

Example 3
It was prepared in the same manner as in Example 1 except that sodium hydroxide was changed to 0.04 mol.

Example 4
It was produced in the same manner as in Example 1 except that sodium hydroxide was changed to 0.003 mol.

Example 5
It was prepared in the same manner as in Example 1 except that 28.3 g of sodium oleate was added in addition to sodium hydroxide.

Example 6
It was produced in the same manner as in Example 1 except that the aging reaction temperature in liquid was 50 ° C. and the reduction temperature was 230 ° C.

Example 7
It was produced in the same manner as in Example 1 except that the aging reaction temperature in liquid was 90 ° C and the reduction temperature was 270 ° C.

Comparative Example 1
It was produced in the same manner as in Example 1 except that 10 g of 0.025 mol sodium hydroxide aqueous solution was added.

Comparative Example 2
It was prepared in the same manner as in Example 1 except that 0.05 mol of sodium hydroxide was used.

Comparative Example 3
This was prepared in the same manner as in Example 1 except that the aqueous ammonia was sodium hydroxide and the alkali metal compound adhesion step was not performed.

Comparative Example 4
It was produced in the same manner as in Example 1 except that sodium hydroxide was changed to calcium hydroxide.

Comparative Example 5
It was produced in the same manner as in Example 1 except that sodium hydroxide was changed to magnesium hydroxide.

Comparative Example 6
It was produced in the same manner as in Example 1 except that the aging reaction temperature in the liquid was 40 ° C. and the reduction temperature was 225 ° C.

Comparative Example 7
It was prepared in the same manner as in Example 1 except that the autoclave reactor was used to set the aging reaction temperature in the liquid to 100 ° C. and the reduction temperature to 290 ° C.

<Evaluation>
Table 1 shows the results of alkali metal adhesion amount, saturation magnetization, and coercive force of the samples obtained in Examples 1 to 7 and Comparative Examples 1 to 7.

In all examples, the amount of alkali metal contained in the alkali metal compound present on the surface of the iron nitride magnetic powder having an average particle size of 10 to 60 nm is 0.15 to 1.80 mass relative to the iron nitride magnetic powder. %, It was confirmed that the saturation magnetization of the iron nitride magnetic powder was 100 to 200 emu / g and the coercive force was 1.5 to 3.5 kOe. This is considered to be because the alkali metal contained in the alkali metal compound promoted nitriding of the iron nitride magnetic powder.

In Comparative Example 1, the coercive force was lowered without nitriding because the amount of alkali metal deposited was small.

In Comparative Example 2, since the amount of alkali metal attached was too large, the reduction was suppressed, and the saturation magnetization and the coercive force were reduced.

In Comparative Example 3, both the saturation magnetization and the coercive force were reduced because alkali metal was present in the particles.

In Comparative Examples 4 and 5, a calcium compound and a magnesium compound were deposited, but both saturation magnetization and coercive force were lowered because there was no effect of promoting nitriding.

In Comparative Example 6, since the average particle diameter was small, the ratio of the oxide film on the particle surface was increased, or superparamagnetism was developed when the particle size was small, so the coercive force was lowered.

In Comparative Example 7, since the average particle diameter was large, the ratio of particles having a single domain critical diameter or less was small, and the coercive force was reduced.

Claims (2)

An iron nitride magnetic powder having an average particle diameter of 10 to 60 nm mainly composed of Fe ₁₆ N ₂ and having an alkali metal compound on the surface of the iron nitride magnetic powder. The amount of alkali metal contained in the alkali metal compound is 0.15 to 1.80 mass% with respect to the iron nitride magnetic powder, and the saturation magnetization of the alkali metal-attached iron nitride magnetic powder is 100 to 200 emu / g and an iron nitride magnetic powder having a coercive force of 1.5 to 3.5 kOe. A magnet using the iron nitride magnetic powder according to claim 1.