Method for preparing soft magnetic material by using liquid nitrogen through high-speed ball milling

Abstract

The disclosure discloses a method for preparing a γ′-Fe.sub.4N soft magnetic material by using liquid nitrogen through high-speed ball milling, and belongs to the field of the soft magnetic material. According to the method of the disclosure, high energy in the liquid nitrogen is used for obtaining a nanometer material Fe.sub.xN with a nitrogen atom supersaturation degree through cryogrinding. At a low temperature, the material is very brittle, and a surface volume ratio is very high, so that a content of nitrogen atoms adsorbed on a surface of a sample is as high as 22%. Through 300° C. post-annealing, γ′-Fe.sub.4N is directly obtained from α-Fe through phase change, so that a nanometer crystal γ′-Fe.sub.4N soft magnetic material is prepared. The method of the disclosure has the advantages that an operation is simple and convenient, the cost is low, the large-scale industrialized production can be realized, and the method belongs to a novel alternative method for preparing a high-grade soft magnetic material with ideal magnetism. The γ′-Fe.sub.4N soft magnetic material prepared by the method of the disclosure has the advantages of high Ms, low coercivity and high surface resistivity, and can be used for a transformer and an inductor operated in a high-frequency semiconductor switch.

Claims

1. A method for preparing a γ′-Fe.sub.4N soft magnetic material, using liquid nitrogen as a nitrogen source for preparation by combining ball milling and annealing processes, and comprising the following steps of: (1) putting an iron raw material into a ball milling machine according to a weight ratio of balls to the iron raw material being 5:1 to 20:1, introducing the liquid nitrogen into a ball milling tank; and starting ball milling at a speed of 3000 to 10000 rpm; and (2) heating to 250° C. to 300° C. for annealing to obtain the γ′-Fe.sub.4N soft magnetic material, wherein a purity is of the iron raw material is not lower than 90% by weight, wherein ball milling is continued for 90 to 200 hours, wherein ball milling is paused every hour for five minutes and a rotating direction of the milling machine reversed, wherein the liquid nitrogen is continuously supplemented and circulates through the ball milling machine from before ball milling and during the ball milling, and wherein in step (2) the annealing is performed in a furnace filled with nitrogen gas.

2. The method according to claim 1, wherein the weight ratio of the balls to the iron raw material is 10:1.

3. The method according to claim 1, wherein the iron raw material in step (1) comprises iron powder, and wherein a particle diameter of the iron powder is 10 nm to 1000 μm.

4. The method of claim 1, wherein the γ′-Fe.sub.4N soft magnetic material produced by the method has a resistivity of 375 μΩ.Math.m.

5. A method for preparing a γ′-Fe.sub.4N soft magnetic material, using liquid nitrogen as a nitrogen source for preparation by combining ball milling and annealing processes, and comprising the steps of: (1) putting an iron raw material into a ball milling machine, then introducing the liquid nitrogen into a ball milling tank, and starting ball milling at a speed of 3000 to 10000 rpm, wherein a ball milling temperature in step (1) is −196° C. to 25° C.; and (2) heating to 300° C. for annealing to obtain the γ′-Fe.sub.4N soft magnetic material wherein a purity is of the iron raw material is not lower than 90% by weight, wherein ball milling is continued for 90 to 200 hours, wherein ball milling is paused every hour for five minutes and a rotating direction of the milling machine reversed, wherein the liquid nitrogen is continuously supplemented and circulates through the ball milling machine from before ball milling and during the ball milling, and wherein in step (2) the annealing is performed in a furnace filled with nitrogen gas.

6. The method according to claim 5, wherein a ball milling time in step (1) is 9 hours.

7. The method according to claim 5, wherein a weight ratio of the balls to the iron raw material is 5:1 to 20:1.

8. The method according to claim 5, wherein the iron raw material in step (1) comprises iron powder, and wherein a particle diameter of the iron powder is 10 nm to 1000 μm.

9. The method according to claim 5, wherein a ball milling temperature in step (1) maintains a liquid nitrogen temperature of −196° C.

10. The method of claim 5, wherein the γ′-Fe.sub.4N soft magnetic material produced by the method has a resistivity of 375 μΩ.Math.m.

Description

BRIEF DESCRIPTION OF FIGURES

(1) FIG. 1 is an Auger Electron Spectroscopy (AES) spectrum of the material obtained after high-speed ball milling in Example 1.

(2) FIG. 2 is an XRD spectrum map of a material at different post-annealing temperatures in Example 2.

(3) FIG. 3 is a magnetic hysteresis loop VSM diagram of the material sample prepared in Example 1.

(4) FIG. 4 is SEM and TEM characterizations on a prepared sample subjected to 300° C. post-annealing in Example 1: (a) an SEM image of the prepared sample; and (b) a diffraction pattern of the sample.

(5) FIG. 5 is a schematic diagram of a phase change mechanism from α-Fe to γ′-Fe.sub.4N in a preparation process: (a) pure iron with a bcc structure; (b) cryomilling; and (c) a phase change into the γ′-Fe.sub.4N through post-annealing.

DETAILED DESCRIPTION

Example 1

(6) A starting raw material is pure iron with a purity being 99% (Alfa Aesar). Liquid nitrogen is provided by PRAXAIR. A high-speed ball milling system CM5100 (Luomen company) operates in a planetary rotation mode.

(7) Wear-resistant stainless steel iron balls are used as a grinding medium. A mass ratio of the balls to a sample is 10:1. Before and during a grinding process, a liquid nitrogen continuous cooling tank from an integrated cooling system is used, so that the sample becomes brittle, and a volatile nitrogen element is preserved. The liquid nitrogen circulates in the system, and is continuously supplemented from an external filling system. The external filling system is precisely controlled, so that a temperature is always maintained at −196° C.

(8) An iron raw material and a ball milling product are treated in a nitrogen gas environment in a glovebox, so that particles are protected from being oxidized. A grinding time is 90 h, and a rotating speed is 3000 rpm. The ball milling stops for 5 min each 1 h of operation. After each interval, a rotating direction is reversed so as to maintain a reaction in a uniform mode. After the ball milling is completed, a ball milling tank is put into the glovebox fully filled with a nitrogen gas. The sample in the ball milling tank is collected by a magnet, an ultrasonic method is used in an assisted way in a collection process, so that the sample attached onto a tank wall and the balls can be peeled off, and a recovery goal is achieved. After the ball milling, amorphous Fe.sub.xN powder with a 40-80 nm nanometer granularity is obtained. The ground powder is put into an annealing furnace, which is fully filled with the nitrogen gas and is heated to 300° C., so that the material generates a phase change, and a γ′-Fe.sub.4N material is obtained.

(9) The obtained γ′-Fe.sub.4N material is subjected to characterization:

(10) a result of an element concentration in the sample after a high-speed cryogrinding step by AES, as shown in FIG. 1, shows that the sample includes about 22% of nitrogen;

(11) FIG. 2 is an XRD spectrum of a sample prepared through post-annealing, and more γ′-Fe.sub.4N peaks and sharper bcc Fe are obtained through annealing at 300° C.;

(12) FIG. 3 shows magnetic hysteresis loops of a prepared sample, the sample prepared through the cryogrinding step shows good soft magnetic performance including Ms being 208 emu/g and Hc being 3.2 Oe. After the post-annealing, Ms value is a little reduced to about 155 emu/g, which corresponds to the phase change from α-Fe to γ′-Fe.sub.4N; however, besides the change of Ms, coercivity decreases (to 0.7 Oe) along with an increase of an annealing temperature, the low coercivity comes from an ultrafine structure of the sample after the high-speed cryogenic process in the liquid nitrogen, and is caused when a grain size is between 40 nm and 80 nm; on the other hand, the prepared sample has three phases including α-Fe, amorphous Fe and γ′-Fe.sub.4N, magnetostriction balance among structure phases enables the magnetostriction in the prepared sample to be close to zero, and this is another important reason for the ideal low coercivity. In a word, magnetism of the prepared sample of the disclosure shows that the sample is an ideal soft magnetic material; additionally, through the nitrogen supersaturation in the sample of the disclosure, resistivity of the sample is as high as 375 μam through measurement, which shows that the prepared γ′-Fe.sub.4N material of the disclosure can be used for a novel transformer magnetic core material with high performance and low cost; and

(13) FIG. 4 is SEM and TEM characterization results of the prepared sample: (a) an SEM image of the prepared sample, wherein the SEM image shows a regular shape of the prepared sample; and (b) a TEM transmission diffraction pattern of the sample, wherein FFT of an experimental HRTEM image with a clear contrast ratio is shown, the pattern is characterized in a γ′-Fe.sub.4N phase, growth of nitrides after a fibrous form can be observed, orientation of the image corresponding to a position near an axis [001] with an FCC structure can be determined, and the FCC structure exists. Feasibility of a ball milling synthesis method of the disclosure in the liquid nitrogen is verified by combining similar discoveries of SEM and TEM characterizations.

Example 2

(14) With the reference to method conditions in Embodiment 1, an annealing temperature is changed into 200° C. or 250° C., other conditions are unchanged, and a γ′-Fe.sub.4N material is prepared.

(15) The obtained material is characterized by an XRD spectrum, as shown in FIG. 2. For a sample prepared after cryogrinding, wide bcc Fe peaks are shown, and consistency with a metastable supersaturation degree of Fe converted from N is realized. Through annealing for 10 min at 200° C., a slight change of powder and slight sharpening of γ′-Fe.sub.4N peaks are caused. Through the annealing for 10 min at 250° C., sharp bcc Fe and γ′-Fe.sub.4N peaks are caused. Through the annealing at 300° C., more γ′-Fe.sub.4N peaks and sharper bcc Fe are caused. Additionally, the annealing temperature is further raised to be a little higher than 300° C., and no obvious influence is caused on XRD peaks. A result shows that wide BCC iron with nitrogen supersaturation is generated in a high-speed cryogenic process, and short-period post-annealing may cause formation of sharp BCC and γ′-Fe.sub.4N.

(16) Driving power of a phase change from α-Fe to γ′-Fe.sub.4N includes two parts: 1, surface activation energy of grinding particles; and 2, annealing energy. When ideal low-temperature materials are used, the surface activation energy does not have differences, so that annealing energy can generate an influence on generated γ′-Fe.sub.4N. On the one hand, the high annealing energy causes a higher volume ratio of the γ′-Fe.sub.4N in the sample. As shown in FIG. 2, the annealing at 300° C. corresponds to a highest volume ratio of the γ′-Fe.sub.4N at 200° C. to 250° C. However, a further raise of the annealing temperature cannot further improve the phase change. Iron recrystallization is a major reason for preventing further improvement of the phase change. An iron crystallization temperature is about 350° C. The post-annealing at a temperature higher than 350° C. can favorably increase a grain size of the iron. The phase change from the α-Fe to the γ′-Fe.sub.4N may be prevented by growth of iron particles. Therefore, the post-annealing at a temperature below 300° C. corresponds to optimization conditions, the maximum annealing energy is realized for assisting the phase change from the α-Fe to the γ′-Fe.sub.4N, and meanwhile, the temperature is lower than the iron crystallization temperature.

(17) It can be seen from a VSM characterization result (FIG. 3) of the obtained material that Ms of the sample subjected to ball milling is 208 emu/g, and coercivity is 3.2 Oe. After the annealing at 200° C., Ms is 188 emu/g, and the coercivity is 2.3 Oe. At this moment, through calculation according to an XRD spectrum map, a γ′-Fe.sub.4N phase accounts for about 20% in a whole. After the annealing at 250° C., Ms is 179 emu/g, and the coercivity is 1.2 Oe. At this moment, through calculation according to the XRD spectrum map, the γ′-Fe.sub.4N phase accounts for about 35% in the whole. After the annealing at 300° C., Ms is 155 emu/g, and the coercivity is 0.7 Oe. Through calculation according to the XRD spectrum map, the γ′-Fe.sub.4N phase accounts for about 75% in the whole.

Example 3

(18) With the reference to Embodiment 1, a weight ratio of balls to an iron powder material is changed from 10:1 to 30:1, other conditions are unchanged, and a Fe.sub.xN material is prepared. Magnetic performance of the obtained Fe.sub.xN material is similar to that of the material obtained in Embodiment 1, and a yield is about 30% of that of the material in Embodiment 1.

Comparative Example 1

(19) With the reference to Embodiment 1, a nitrogen source is changed into an ammonia gas from liquid nitrogen, other conditions are unchanged, and a Fe.sub.xN material is prepared.

(20) A nitrogen content of the obtained Fe.sub.xN material is 6%, Ms is 185 emu/g, the coercivity is 10 Oe, the resistivity is 25 μam, and the obtained γ′-Fe.sub.4N phase accounts for about 10% in the whole. It can be seen that a proportion of the γ′-Fe.sub.4N phase is low, so that integral performance of the prepared material is similar to that of pure iron.