WO2023060791A1

WO2023060791A1 - Soft magnetic composite and preparation method therefor

Info

Publication number: WO2023060791A1
Application number: PCT/CN2022/070650
Authority: WO
Inventors: 钱坤明; 王红杰; 陈刚; 吴瑜; 杨晓禹
Original assignee: 内蒙金属材料研究所
Priority date: 2021-10-12
Filing date: 2022-01-07
Publication date: 2023-04-20
Also published as: CN113996781A; CN113996781B

Abstract

A soft magnetic composite and a preparation method therefor. The soft magnetic composite is of a core-shell structure, and takes a composite coating layer with a pure iron powder as a core, a chelate, which is originally generated by means of pretreatment and has hydroxyl groups on the surface thereof, as an inner layer, and SiO2-Al2O3 as an outer layer. The preparation method comprises the steps of: preparing a pretreatment liquid; preparing a chelate coating inner layer having hydroxyl groups on the surface thereof; coating same with an aluminum-silicon composite layer; and performing grinding and calcining at high temperature.

Description

A kind of soft magnetic composite material and preparation method thereof

technical field

The invention belongs to the technical field of soft magnetic materials, and relates to a soft magnetic composite material and a preparation method thereof.

Background technique

Soft Magnetic Composite (Soft Magnetic Composite, SMC) is a new type of iron-based powder soft magnetic material that has been gradually developed in recent years. Iron-based soft magnetic composite material is a metal soft magnetic composite material with high cost performance, which has broad application space in the fields of power factor correction circuit, pulse flyback transformer, energy storage filter inductor and line filter. But with the increase of its application frequency, the eddy current loss will increase. The eddy current loss will cause a lot of heat in the magnetic core, thereby reducing the magnetic performance of the soft magnetic composite material. According to statistics, about 9% of the electric energy is directly lost in the form of Joule heat during the working process of magnetic material components. Therefore, how to reduce eddy current loss and develop energy-saving iron-based soft magnetic composite materials is a hot and difficult point in current research.

Studies have shown that insulating and coating the magnetic particles is an effective way to reduce the eddy current loss of SMC, and the coating method and technology play a decisive role in the magnetic properties of SMC. Usually the thickness of the insulating coating should be as small as possible to reduce the magnetic loss. In addition, during the subsequent high-temperature (500°C or above) treatment and high-pressure (about 1000MPa or above) compression molding process of the coating, the insulating coating needs to maintain structural integrity to maintain its insulating properties. However, in actual situations, there is a contradiction between the lowest possible damage rate of the insulating coating and the smallest possible magnetic loss to a certain extent, that is, the lowest possible damage rate of the insulating coating requires a high thickness of the insulating coating and low magnetic loss. It is also required that the thickness of the insulating covering layer be small. The above problems have a lot to do with the insulation coating method and process of soft magnetic composite materials.

Therefore, it is necessary to develop a new soft magnetic composite material from the idea of insulation coating process to improve the performance of soft magnetic composite material.

Contents of the invention

The first technical problem to be solved by the present invention is to provide a soft magnetic composite material with excellent properties of low loss and high resistivity.

The second technical problem to be solved by the present invention is to provide a method for preparing a soft magnetic composite material with a simple preparation process and easy operation and implementation. The prepared soft magnetic composite material has excellent properties such as low loss and high resistivity.

The technical solution adopted by the present invention to solve the above-mentioned first technical problem is: a soft magnetic composite material, which is characterized in that: the soft magnetic composite material is a double-layer clad core-shell structure, wherein the core is pure iron powder, wrapped The cladding layer is a double-layer composite layer with the surface hydroxyl-containing chelate formed by pretreatment as the inner layer and SiO ₂ -Al ₂ O ₃ as the outer layer.

Further, the hydroxyl-containing chelate on the surface refers to the hydroxyl-containing metal salt layer formed by the reaction of pure iron powder and a pretreatment solution, the pretreatment solution being n-pentyl gallate, polyphosphoric acid, polyethylene glycol or For the mixed solution of isopropanol and water, the mass ratio of n-pentyl gallate, polyphosphoric acid, polyethylene glycol or isopropanol to deionized water is 2:1-3:5-7:7-9.

The technical scheme adopted by the present invention to solve the above-mentioned second technical problem is: a preparation method of a soft magnetic composite material, which is characterized in that it comprises the following steps:

1) preparing a pretreatment solution;

2) react the pure iron powder with the pretreatment solution, obtain the powder coated with phenolic hydroxyl iron chelate after filtering, cleaning and drying, i.e. powder A;

3) Coating an aluminum-silicon composite layer on the surface of powder A by sol-gel method, drying to obtain powder B;

4) After grinding and sieving the powder B, calcining at a high temperature to obtain a double-layer coated powder, that is, a double-layer coated soft magnetic composite material;

5) Mixing powder B and lubricant to form a magnetic ring;

6) Perform annealing treatment on the magnetic core obtained in step 5).

Preferably, the pretreatment solution in step 1) is a mixed solution of n-pentyl gallate, polyphosphoric acid, polyethylene glycol or isopropanol and deionized water, and the mass ratio of each component is: 2:1-3 : 5~7: 7~9.

Preferably, the particle size of the pure iron powder in step 2) is 20-150 μm, the purity is greater than 99.89%, and the purity requirements are:

C≤0.008wt%

O≤0.05wt%

S≤0.005wt%

Mn≤0.05wt%

Preferably, in the step 2) the mass volume ratio of the pure iron powder to the pretreatment solution is 50g: 20-30ml, and the reaction conditions of the pure iron powder and the pretreatment solution are: 20-35°C constant temperature water bath, stirring, and reacting for 10 ~30min.

Further, the specific process of the step 3) sol-gel method coating the aluminum-silicon composite layer on the surface of powder A is as follows: first, powder A, absolute ethanol, deionized water, silane coupling agent, tetraethyl orthosilicate The ester and ammonia water are mixed in turn, fully stirred for reaction, and then dried to a paste; then the newly prepared aluminum nitrate solution is added, continued to stir, and evaporated to dryness to obtain a coating precursor;

The mass parts of powder A, absolute ethanol, deionized water, silane coupling agent, tetraethyl orthosilicate and ammonia water are respectively 20 parts, 50-150 parts, 30-60 parts, 1-5 parts, 10 parts ～30 and 20～50 parts, the reaction conditions are: room temperature, stirring reaction for 3～5 hours, stirring speed 400～900rap/min; drying to paste refers to stirring in a water bath at 55～65°C until the mixture becomes paste; The mass parts of aluminum nitrate and deionized water in the aluminum nitrate solution are: 2 parts and 100-300 parts, adding the aluminum nitrate solution and stirring for 60-90 minutes, the stirring speed is 100-300 rap/min, and the reaction temperature is 50-70 ° C.

Preferably, the sieving in step 4) is carried out using 100-180# and 240-400# iron sieves; the high-temperature calcination is carried out at 600-900°C for 60-90 minutes under inert gas protection or vacuum conditions.

Preferably, the lubricant in step 5) is one or a mixture of zinc stearate or graphite, the amount of lubricant added is 0.1-1% of the total mass of powder B, and the pressing pressure is 900- 1800MPa.

Finally, the annealing treatment in the step 6) is heat treatment at 500-700° C. for 0.5-2 hours under inert gas protection or vacuum condition.

Compared with the prior art, the present invention has the advantages of:

1. Starting from improving SMC insulation and reducing SMC magnetic loss, using pure iron powder as raw material, the iron-based core-shell structure powder coated with phenolic hydroxyl iron/aluminum-silicon double layer was prepared by pre-filming method combined with sol-gel method , and then the prepared powder is sintered at high temperature and pressed into a high-density iron-based composite soft magnetic composite material with high resistivity and low loss.

2. Pretreatment can effectively remove bulky oxides and impurities on the surface of the iron powder, and at the same time, it can grow in situ on the surface of the matrix iron powder to form a chelate insulating layer mainly composed of phenolic hydroxyl iron. The structure of the insulating layer is regulated It is simple and has good bonding performance with the outer layer. During the pressing process, the double-layer coating process can maintain the insulation performance of the coating layer to a great extent, and can effectively block eddy current and reduce eddy current loss.

The present invention adopts the in-situ growth method to pre-form the film. Compared with the surface of pure iron powder, the physical and chemical properties of the hydroxyl-containing inner layer surface and the matching and compatibility of the outer coating layer are better. The prepared composite coating has good physical properties and thermal stability, can withstand higher processing temperature and higher pressing pressure, can effectively alleviate the cracking of the insulating layer of single-coated magnetic powder under high pressure, and avoid the deterioration of SMC insulation performance. decrease and increase in eddy current losses. The soft magnetic composite material of the invention has a double-layer coated core-shell structure, has excellent properties such as low loss and high resistivity, and has scientific and reasonable preparation technology and is easy to operate and implement.

Detailed ways

The present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments.

Example 1

A method for preparing a soft magnetic composite material, comprising the following steps:

(1) Take by weighing 20g of n-pentyl gallate, 20g of polyphosphoric acid, 60g of polyethylene glycol and 80g of deionized water, mix them, and stir them evenly, and configure them as a pretreatment solution;

(2) take by weighing the Fe powder of 40g, put into beaker;

(3) Measure 20ml of the pretreatment solution, add it to a beaker, put it in a constant temperature water bath at 25°C, stir, and react for 10 minutes, then filter, wash, and dry to obtain a powder coated with chelate, that is, powder A;

(4) The sol-gel method coats the aluminum-silicon composite layer on the surface of the powder A, and dries to obtain the powder B;

The specific process is: weigh 40g of powder A, take 200g of absolute ethanol, 90g of deionized water, 6g of silane coupling agent, 40g of tetraethyl orthosilicate and 70g of ammonia water, put them in a beaker, and stir for 3 hours at room temperature. Then stir in a 60°C water bath until the mixture becomes a paste, and the stirring speed is 400rap/min; weigh 4g of aluminum nitrate and 400g of deionized water, prepare an aluminum nitrate solution, and mix it with the pasty mixture in the previous step. Stir in the pot for 60-90 minutes at a stirring speed of 200 rap/min until the liquid is completely evaporated and dried to obtain powder B;

(5) Grinding the powder B, sieving the grinding product with 100# and 240# iron sieves, and then sintering at 750°C for 70min in a vacuum environment to obtain a double-layer coated powder, that is, a double-layer coated soft magnetic composite material;

(6) Weigh 20g of the sieved product in (5) and 0.11g of lubricant (one or two mixtures of zinc stearate or graphite), mix them thoroughly, and press them into a magnetic core (Φ13 ×Φ8×4mm);

(7) Under the condition of inert gas protection, heat-treat the pressed magnetic core in (6), the temperature is 600° C., and the treatment time is 1 h.

Wind the wire on the pressure ring for performance test, the primary coil and the secondary coil have 20 turns each, test with a dynamic magnetic property measuring instrument, the coercive force H _c is 118A/m, and the total loss is measured under the condition of (1T, 200kHz) It is 10.5W/kg. The sample was made into a thin disc (thickness ≤ 4mm), and the resistance value of the sample was measured by the four-probe method. After conversion, the resistivity was 6213 μΩ·m.

Example 2

(1) Take by weighing 20g of n-pentyl gallate, 10g of polyphosphoric acid, and 50g of polyethylene glycol, mix with 70g of deionized water, and stir evenly, to be configured as a pretreatment solution;

(2) take by weighing the Fe powder of 40g, put into beaker;

(3) Measure 26ml of pretreatment solution, add it to a beaker, put it in a constant temperature water bath at 25°C, stir, and react for 30 minutes, then filter, wash, and dry to obtain a powder coated with chelate, that is, powder A;

The specific process is: weigh 40g of powder A, take 100g of absolute ethanol, 60g of deionized water, 2g of silane coupling agent, 20g of tetraethyl orthosilicate and 40g of ammonia water, put them in a beaker, and stir for 3h at room temperature. Then stir in a 60°C water bath until the mixture becomes a paste, and the stirring speed is 400rap/min; weigh 4g of aluminum nitrate and 200g of deionized water, prepare an aluminum nitrate solution, and mix it with the pasty mixture in the previous step. In the pot for 60-90 minutes, the stirring speed is 100 rap/min, until the liquid is completely evaporated, and dried to obtain powder B;

(6) Weigh 20g of the sieved product in (5) and 0.11g of a lubricant (one or a mixture of zinc stearate or graphite), mix them well, and press them into a magnetic core (Φ13 ×Φ8×4mm);

(7) Under the condition of inert gas protection, heat-treat the pressed magnetic core in (6), the temperature is 500° C., and the treatment time is 0.5 h.

Wound the wire on the pressure ring for performance test, the primary coil and the secondary coil have 20 turns each, tested with a dynamic magnetic property measuring instrument, the coercive force H _c is 115A/m, and the total loss is measured under the condition of (1T, 200kHz) It is 9.1W/kg. The sample was made into a thin disc (thickness ≤ 4mm), and the resistance value of the sample was measured by the four-probe method. After conversion, the resistivity was 5735 μΩ·m.

Example 3

(1) Take by weighing 20g of n-pentyl gallate, 30g of polyphosphoric acid, and 70g of polyethylene glycol, mix with 90g of deionized water, and stir evenly, to be configured as a pretreatment solution;

(2) take by weighing the Fe powder of 40g, put into beaker;

(3) Measure 24ml of pretreatment solution, add it to a beaker, put it into a constant temperature water bath at 25°C, stir, and react for 30 minutes, then filter, wash, and dry to obtain a powder coated with chelate, that is, powder A;

The specific process is: weigh 40g of powder A, take 300g of absolute ethanol, 120g of deionized water, 10g of silane coupling agent, 60g of tetraethyl orthosilicate and 100g of ammonia water, put them in a beaker, and stir for 3h at room temperature. Then stir in a 60°C water bath until the mixture becomes a paste, and the stirring speed is 400rap/min; weigh 4g of aluminum nitrate and 600g of deionized water, prepare an aluminum nitrate solution, and mix it with the pasty mixture in the previous step. Stir in the pot for 60-90 minutes at a stirring speed of 300 rap/min until the liquid is completely evaporated and dried to obtain powder B;

(5) Grinding the powder B, sieving the ground product with 180# and 400# iron sieves, and then sintering at 900°C for 90min in a vacuum environment;

(6) Weigh 20g of the sieved product in (5) and 0.2g of lubricant (one or two mixtures of zinc stearate or graphite), mix them well, and press them into a magnetic core (Φ13 ×Φ8×4mm);

(7) Under the condition of inert gas protection, heat-treat the pressed magnetic core in (6), the temperature is 700° C., and the treatment time is 2 hours.

Wind the wire on the pressure ring for performance test. The primary coil and the secondary coil have 20 turns each. Tested with a dynamic magnetic property measuring instrument, the coercive force H _c is 124A/m, and the total loss is measured under the condition of (1T, 400kHz) It is 14.1W/kg. The sample was made into a thin disc (thickness ≤ 4mm), and the resistance value of the sample was measured by the four-probe method. After conversion, the resistivity was 6813 μΩ·m.

Comparative example 1

(1) Weigh 40g of iron powder, take 200g of absolute ethanol, 90g of deionized water, 6g of silane coupling agent, 40g of tetraethyl orthosilicate and 70g of ammonia water, put them in a beaker, and stir for 3h at 35°C. Then stir in a 60°C water bath until the mixture becomes a paste, and the stirring speed is 400rap/min; weigh 4g of aluminum nitrate and 400g of deionized water, put them in a beaker, stir in a 65°C water bath until the liquid evaporates completely, and stir The speed is 200rap/min;

(2) Grinding the solid product in (1), sieving the ground product with 100# and 240# iron sieves, and then sintering at 750°C for 70min in a vacuum environment;

(3) Weigh 20g of the sieved product in (2) and 0.11g of a lubricant (one or a mixture of zinc stearate or graphite), mix them well, and press them into a ring (Φ13× Φ8×4mm);

(4) Under the condition of inert gas protection, heat-treat the pressed magnetic core in (3), the temperature is 600° C., and the treatment time is 1 h.

Wind the wire on the pressure ring for performance test, the primary coil and the secondary coil have 20 turns each, test with a dynamic magnetic property measuring instrument, the coercive force H _c is 195A/m, and the total loss is measured under the condition of (1T, 400kHz) It is 48.3W/kg. The sample was made into a thin disc (thickness ≤ 4mm), and the resistance value of the sample was measured by the four-probe method. After conversion, the resistivity was 2023 μΩ·m.

Comparative example 2

(1) Take by weighing 20g of n-pentyl gallate, 20g of polyphosphoric acid, and 60g of polyethylene glycol, mix with 60g of deionized water, and stir evenly to be configured as a pretreatment solution;

(2) take by weighing the Fe powder of 40g, put into beaker;

(3) Measure 20ml of the pretreatment solution, add it to a beaker, put it into a constant temperature water bath at 25°C, stir, and react for 60min to obtain powder A;

The preparation of the aluminum-silicon-coated soft magnetic composite material comprises the following steps:

(4) Weigh 40g of powder A, take 200g of absolute ethanol, 90g of deionized water, 6g of silane coupling agent, 40g of tetraethyl orthosilicate and 70g of ammonia water, put them in a beaker, and stir for 3h at 35°C. Then stir in a 60°C water bath until the mixture becomes a paste, and the stirring speed is 400rap/min; weigh 4g of aluminum nitrate and 400g of deionized water, prepare an aluminum nitrate solution, and mix it with the pasty mixture in the previous step. Stir in the pot until the liquid evaporates completely, and the stirring speed is 200rap/min;

(5) The solid product in (4) is ground, and the ground product is screened with 100# and 240# iron sieves, and then sintered at 750°C for 70min in a vacuum environment;

(6) Weigh 20g of the screened product in (5) and 0.11g of lubricant (one or a mixture of zinc stearate or graphite), mix them well, and press them into a ring (Φ13× Φ8×4mm);

Wind the wire on the pressure ring for performance test, the primary coil and the secondary coil have 20 turns each, test with a dynamic magnetic property measuring instrument, the coercive force H _c is 121A/m, and the total loss is measured under the condition of (1T, 400kHz) It is 11.5W/kg. The sample was made into a thin disc (thickness ≤ 4mm), and the resistance value of the sample was measured by the four-probe method. After conversion, the resistivity was 6311 μΩ·m.

Comparative example 3

(2) take by weighing the Fe powder of 40g, put into beaker;

(3) Measure 20ml of the pretreatment solution, add it to a beaker, put it into a constant temperature water bath at 25°C, stir, and react for 10 minutes to obtain powder A;

(4) Weigh 40g of powder A, take 200g of absolute ethanol, 90g of deionized water, 15g of silane coupling agent, 90g of tetraethyl orthosilicate and 70g of ammonia water, put them in a beaker, and stir for 3h at 35°C. Then stir in a 60°C water bath until the mixture becomes a paste, and the stirring speed is 400rap/min; weigh 8g of aluminum nitrate and 400g of deionized water, prepare an aluminum nitrate solution, and mix it with the pasty mixture in the previous step. Stir in the pot until the liquid evaporates completely, and the stirring speed is 200rap/min;

Wound the wire on the pressure ring for performance test, the primary coil and the secondary coil have 20 turns each, tested with a dynamic magnetic property measuring instrument, the coercive force H _c is 163A/m, and the total loss is measured under the condition of (1T, 400kHz) It is 32.5W/kg. The sample was made into a thin disc (thickness ≤ 4mm), and the resistance value of the sample was measured by the four-probe method. After conversion, the resistivity was 7813 μΩ·m.

It can be seen from the test data obtained in the above examples that the pretreatment and coating parameter adjustment have a significant impact on the magnetic properties, and the increase in the content of the coating agent increases the insulation and total loss of the sample. The increase of the concentration of coating agent and the extension of coating treatment time are beneficial to the increase of the thickness of the insulating layer on the surface of the iron powder and the improvement of the resistivity. Comparative example 1 shows that the pretreatment plays a positive role in improving the insulating performance of the magnetic material. This is because during the high-pressure compression molding process, even if the outer layer is broken, the inner layer of the chelate can maintain good insulation. Comparative example 2 shows that prolonging the pretreatment time has no obvious influence on the magnetic properties of iron powder. This is mainly because during the pretreatment reaction, the surface of the iron powder will rapidly form and wrap a layer of dense phenolic hydroxyl iron chelate, which prevents the further progress of the pretreatment reaction. Therefore, prolonging the pretreatment reaction time had no significant effect on the formation of chelates. Comparative example 3 shows that too high content of the coating agent will lead to more impurities in the magnetic powder, which is not conducive to the improvement of the performance of the magnetic powder.

The above is only a preferred embodiment of the present invention, it should be pointed out that for those of ordinary skill in the art, without departing from the technical principle of the present invention, some improvements and modifications can also be made. These improvements and modifications It should also be regarded as the protection scope of the present invention.

Claims

A soft magnetic composite material, characterized in that: the soft magnetic composite material is a double-layer clad core-shell structure, wherein the core is pure iron powder, and the cladding layer is a chelating layer containing hydroxyl groups on the surface originally generated by pretreatment. SiO 2 -Al 2 O 3 is a double-layer composite layer as the inner layer and SiO 2 -Al 2 O 3 as the outer layer.
The soft magnetic composite material according to claim 1, characterized in that: the surface hydroxyl-containing chelate refers to the hydroxyl-containing metal salt layer formed by the reaction of pure iron powder and a pretreatment solution, and the pretreatment solution is gall Mixed solution of n-pentyl gallate, polyphosphoric acid, polyethylene glycol or isopropanol and water, the mass ratio of n-pentyl gallate, polyphosphoric acid, polyethylene glycol or isopropanol to deionized water is 2:1 ~3:5~7:7~9.
A kind of preparation method of soft magnetic composite material, it is characterized in that comprising the following steps:

1) preparing a pretreatment solution;

2) react the pure iron powder with the pretreatment solution, obtain the powder coated with phenolic hydroxyl iron chelate after filtering, cleaning and drying, i.e. powder A;

3) Coating an aluminum-silicon composite layer on the surface of powder A by sol-gel method, drying to obtain powder B;

4) After grinding and sieving the powder B, calcining at a high temperature to obtain a double-layer coated powder, that is, a double-layer coated soft magnetic composite material;

5) Mixing powder B and lubricant to form a magnetic ring;

6) Perform annealing treatment on the magnetic core obtained in step 5).
The preparation method according to claim 3, characterized in that: the pretreatment solution of said step 1) is a mixed solution of n-pentyl gallate, polyphosphoric acid, polyethylene glycol or isopropanol and deionized water, each The mass ratio of components is: 2:1～3:5～7:7～9.
The preparation method according to claim 3, characterized in that: the particle size of the pure iron powder in the step 2) is 20-150 μm, the purity is greater than 99.89%, and the purity requirements are:

C≤0.008wt%

O≤0.05wt%

S≤0.005wt%

Mn≤0.05wt%
The preparation method according to claim 3, characterized in that: said step 2) the mass volume ratio of pure iron powder and pretreatment solution is 40-50g: 20-30ml, the reaction conditions of pure iron powder and pretreatment solution For: 20 ~ 35 ℃ constant temperature water bath, stirring, reaction 10 ~ 30min.
The preparation method according to claim 3, characterized in that: said step 3) the specific process of coating the aluminum-silicon composite layer on the surface of powder A by the sol-gel method is: firstly powder A, absolute ethanol, deionized Water, silane coupling agent, tetraethyl orthosilicate and ammonia water are mixed in sequence, fully stirred and reacted, and then dried to a paste; then the newly prepared aluminum nitrate solution is added, continued to stir, evaporated to dryness, and the coating precursor is obtained;

The mass parts of powder A, absolute ethanol, deionized water, silane coupling agent, tetraethyl orthosilicate and ammonia water are respectively 20 parts, 50-150 parts, 30-60 parts, 1-5 parts, 10 parts ～30 and 20～50 parts, the reaction conditions are: room temperature, stirring reaction for 3～5 hours, stirring speed 400～900rap/min; drying to paste refers to stirring in a water bath at 55～65°C until the mixture becomes paste; The mass parts of aluminum nitrate and deionized water in the aluminum nitrate solution are: 2 parts and 100-300 parts, adding the aluminum nitrate solution and stirring for 60-90 minutes, the stirring speed is 100-300 rap/min, and the reaction temperature is 50-70 ° C.
The preparation method according to claim 3, characterized in that: the sieving in step 4) is carried out by using 100-180# and 240-400# iron sieves; high-temperature calcination is carried out under inert gas protection or vacuum conditions , 600 ~ 900 ℃ heat preservation 60 ~ 90min.
The preparation method according to claim 3, characterized in that: the lubricant in step 5) is a mixture of one or two of zinc stearate or graphite, and the amount of lubricant added is 1/2 of the total mass of powder B 0.1-1%, the pressing pressure is 900-2000MPa.
The preparation method according to claim 3, characterized in that: the annealing treatment in the step 6) is heat treatment at 500-700° C. for 0.5-2 hours under inert gas protection or vacuum condition.