JP2015176978A - nanocomposite magnet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a NdFeB-based nanocomposite magnet having both high residual magnetization and a high coercive force.SOLUTION: A nanocomposite magnet includes at least three phases of a NdFeB phase, a NdFephase and a rare earth phase. The rare earth phase is favorably a Nd rich phase, and the crystal grain sizes of the NdFeB phase and the NdFephase are favorably 300 nm or less.

Description

  The present invention relates to a NdFeB-based nanocomposite magnet having a high coercive force.

  The application of permanent magnets extends to a wide range of fields such as electronics, information communication, medical care, machine tool fields, industrial and automotive motors, and the demand for suppression of carbon dioxide emissions is increasing. In recent years, there are increasing expectations for the development of permanent magnets with even higher characteristics due to energy savings and improved power generation efficiency in the industrial field.

  At present, Nd—Fe—B magnets, which are dominating the market as high-performance magnets, are also used in drive motor magnets for HV / EHV. In response to the recent demand for further miniaturization and higher output of motors (increase in the remanent magnetization of magnets), Nd-Fe-B magnets also have higher performance, especially higher coercive force. And there is an increasing demand for an increase in remanent magnetization.

Research on nanocomposite magnets is underway as one of the developments of materials with performance exceeding that of Nd-Fe-B magnets. The material design philosophy of nanocomposite magnets is that the entire structure consists of a hard phase with high coercivity (Nd ₂ Fe ₁₄ B phase) and a soft phase with high saturation magnetization (α-Fe phase), both of which are fine crystal grains on the order of nm. The high energy product is achieved by coexisting the two phases and simultaneously exhibiting the characteristics of both phases through exchange bonding. Nanocomposite magnets are considered promising as a concept that can achieve both high coercivity and high saturation magnetization.

Various nanocomposite magnets using Nd—Fe—B based materials have been proposed. For example, Patent Document 1 discloses a two-phase mixture of Nd ₂ Fe ₁₄ B phase and α-Fe phase, or Nd ₂ Fe ₁₄ B. Proposed is a method for producing a nanocomposite magnet that is a three-phase mixture of a phase, an α-Fe phase, and an Nd—Cu phase, wherein the Nd ₂ Fe ₁₄ B phase is a hard phase and the α-Fe phase is a soft phase .

JP 2012-234985 A

In the case of a conventional nanocomposite magnet composed of two phases of Nd ₂ Fe ₁₄ B phase and α-Fe phase, since there is no magnetic separation between the hard phases, the squareness is poor and the coercive force is low. In addition, by using a soft phase having no magnetic anisotropy, the crystal grain size required for the soft phase was about 10 nm, and it was difficult to prepare a sample.

In the case of a nanocomposite magnet composed of Nd ₂ Fe ₁₄ B phase, α-Fe phase and Nd-Cu phase, the coexistence of α-Fe and Nd-Cu phase (Nd rich phase) Coexistence was difficult, the desired phase structure was difficult to obtain, and good magnetic properties could not be obtained.

  An object of the present invention is to provide an Nd-Fe-B-based nanocomposite magnet having both high magnetization remanence and high coercive force that can solve the above-mentioned problems of the prior art.

In order to solve the above-described problems, according to the present invention, there is provided a nanocomposite magnet characterized by including at least three phases of an Nd ₂ Fe ₁₄ B phase, an Nd ₂ Fe ₁₇ phase, and a rare earth phase. In the nanocomposite magnet, the rare earth phase is preferably an Nd-rich phase. The crystal grain sizes of the Nd ₂ Fe ₁₄ B phase and the Nd ₂ Fe ₁₇ phase are preferably 300 nm or less.

In the NdFeB-based nanocomposite magnet of the present invention, the upper limit of the crystal grain size can be relaxed by using the Nd ₂ Fe ₁₇ phase that is a semi-hard phase instead of α-Fe in the conventional nanocomposite magnet, By introducing a rare earth phase in addition to the two magnetic phases of Nd ₂ Fe ₁₄ B phase and Nd ₂ Fe ₁₇ phase, magnetic separation between the hard phases becomes possible, and the squareness can be improved and the coercive force can be increased. It became.

It is a graph which shows the composition area | region of the sample produced in the Example. It is a graph which shows the volume fraction of each phase in each sample in an Example. It is a graph which shows the magnetic characteristic of a quenching ribbon. It is a graph at the time of calculating | requiring squareness. It is a graph which shows the ratio of the Nd rich phase which shows parts other than a main phase, and the squareness. FIG. 2 is a structure diagram of a sample D. FIG.

The nanocomposite magnet of the present invention is characterized by including at least three phases of an Nd ₂ Fe ₁₄ B phase, an Nd ₂ Fe ₁₇ phase, and a rare earth phase.

  A nanocomposite magnet is a magnet in which a fine hard phase and a soft phase on the order of nanometers coexist in a tissue. In a conventional nanocomposite magnet, high magnetization is brought about by including a soft phase (α-Fe phase) with high saturation magnetization. In addition, the α-Fe phase is inherently soft magnetic and has a high coercive force. Although there is no exchange composite with the hard phase (Nd-Fe-B phase), nanocomposite magnets with high coercivity are also provided.

In the conventional nanocomposite magnet, the crystal grain size required for the soft phase (α-Fe phase) is about 10 nm. In the present invention, Nd ₂ Fe ₁₇ phase is used instead of this α-Fe phase. Thus, a larger crystal grain size can be used, and the upper limit of the crystal grain size can be relaxed.

Therefore, in the nanocomposite magnet of the present invention, the Nd ₂ Fe ₁₄ B phase as the nano-sized hard phase, the Nd ₂ Fe ₁₇ phase as the semi-hard phase and the rare earth phase are mixed in the magnet composition, and the hard phase and the semi-hard phase are mixed. The exchange coupling acts between the two to increase the magnetization, and the rare earth phase achieves a high coercive force by suppressing the propagation of the generated reverse magnetic domain.

In the nanocomposite magnet of the present invention, the Nd ₂ Fe ₁₄ B _phase, and Nd 2 _{Fe 17} phase, the ratio of the three phases of the rare earth phase, Nd ₂ Fe ₁₄ B phase: _85~95vol%, Nd 2 _{Fe 17} phase: It is preferable that the volume fraction excluding the Nd ₂ Fe ₁₄ B phase is 1 to 90 vol% when the volume fraction is 100%, and the rare earth phase is 10 to 99 vol%.

Generally, in a nanocomposite magnet, the Nd ₂ Fe ₁₄ B phase is required to exist as nanometer order particles. In the nanocomposite magnet of the present invention, it is preferable that the particle size of the Nd ₂ Fe ₁₄ B phase is about 10 to 300 nm. When the particle diameter is within this range, the ratio of single domain particles increases. A single magnetic domain is a state in which only one magnetic domain having no domain wall exists. In a structure in which single domain particles of a magnetic material having uniaxial anisotropy are aggregated, a change in magnetization of each magnetic domain occurs only by a magnetization reversal mechanism. In contrast to a single magnetic domain, a multiple magnetic domain is a state in which a domain wall exists and a plurality of magnetic domains exist. In the structure in which the multi-domain particles are aggregated, the magnetization of each magnetic domain is changed by the movement of the domain wall. Therefore, in the case of a single magnetic domain as compared with the case of multiple magnetic domains, there is no movement of the domain wall, so that the change in magnetization is difficult to occur, that is, the coercive force is increased. When the particle size of the Nd ₂ Fe ₁₄ B phase is larger than 300 nm, there is a problem that the magnetic layer is not a single magnetic domain and the intrinsic coercive force is reduced. On the other hand, when the particle size is reduced to about 10 nm, the magnetic anisotropy of the Nd ₂ Fe ₁₄ B phase becomes isotropic due to the effect of thermal fluctuation. Therefore, it is usually preferable to regulate the particle size of the Nd ₂ Fe ₁₄ B phase to 10 to 300 nm.

In the conventional nanocomposite magnet including the α-Fe phase, the particle size of the α-Fe phase is about 10 to 50 nm. In the present invention, Nd ₂ Fe ₁₇ is used instead of the α-Fe phase. By using the phase, the particle size of the Nd ₂ Fe ₁₇ phase can be set to about 10 to 300 nm, which is equivalent to the particle size of the Nd ₂ Fe ₁₄ B phase.

The rare earth phase can be a grain boundary phase between the Nd ₂ Fe ₁₄ B phase and the Nd ₂ Fe ₁₇ phase in the finally obtained magnet. Grain boundary _phase, Nd 2 _{Fe 14} exists between the magnetic structure comprising a B-phase and _Nd 2 _{Fe 17 phase,} separating magnetic structure each other comprising Nd 2 _{Fe 14} B phase and the _Nd 2 _{Fe 17} phase It is something to be made. The coercive force of the magnet can change depending on the situation of the grain boundary phase. For example, when two magnetic structures including an Nd ₂ Fe ₁₄ B phase are present with a grain boundary phase in between, even if there is a change in magnetization in one magnetic structure, This magnetic structure is less affected by magnetization reversal, resulting in an increase in coercivity.

As described above, each of the Nd ₂ Fe ₁₄ B phase and the Nd ₂ Fe ₁₇ phase is divided by the rare earth phase. The effect of this division will be described. First, the Nd ₂ Fe ₁₄ B phase is divided to improve the coercive force. In addition, the Nd ₂ Fe ₁₄ B phase and the Nd ₂ Fe ₁₇ phase are also separated, but the exchange junction between the Nd ₂ Fe ₁₄ B phase and the Nd ₂ Fe ₁₇ phase can be maintained. That is, the effect of high magnetization due to the semi-hard phase (Nd ₂ Fe ₁₇ phase), which is a feature of the nanocomposite magnet, can be maintained. That is, the distance between the divided Nd ₂ Fe ₁₄ B phase and the Nd ₂ Fe ₁₇ phase, that is, the thickness of the grain boundary phase can be adjusted appropriately, and consequently the Nd ₂ Fe ₁₄ B phase and the Nd ₂ Fe ₁₇ phase. It can be adjusted within a range in which exchange bonding between the phases is established.

In the nanocomposite magnet of the present invention, the thickness of the grain boundary phase (rare earth phase) is preferably 10 nm or less. This is because, in the nanocomposite magnetic structure, the distance at which the exchange junction between the Nd ₂ Fe ₁₄ B phase / Nd ₂ Fe ₁₇ phase is established is about 0 nm to 10 nm. This is because if the thickness of the grain boundary phase exceeds 10 nm, the exchange junction between the Nd ₂ Fe ₁₄ B phase / Nd ₂ Fe ₁₇ phase is not established, and high magnetization cannot be expected. The thickness of the grain boundary phase is preferably 0.5 nm or more. This is because if the thickness of the grain boundary phase is less than 0.5 nm, it is not possible to cut the exchange junction between the Nd ₂ Fe ₁₄ B phase / Nd ₂ Fe ₁₇ phase and sufficiently improve the magnetic separation property.

  The rare earth phase is preferably an Nd-rich phase. Here, the Nd-rich phase means a phase containing Nd at 50 at% or more. In addition to Nd, the Nd-rich phase may contain an element having an effect of lowering the melting point of Nd, such as Cu, Ag, Al, Fe, and Co.

  There is no particular limitation on the size and shape of the nanocomposite magnet of the present invention. For example, when a quenched ribbon formed by a liquid ribbon method is heat-treated from a master alloy prepared to have a desired magnet element composition, the shape of the quenched ribbon (ribbon shape) is about several mm. A width, length and thickness of about several tens of μm can be obtained. After that, depending on the purpose of use, bulk magnets and bonded magnets can be appropriately formed and processed, and a sensor, actuator, In general, it is finely pulverized and handled in a powder state with a particle size suitable for each purpose so that it can be used widely as a magnet part such as a motor.

  The nanocomposite magnet of the present invention can be obtained in the form of a powder having a particle size according to the purpose as described above. Therefore, it can be used as a so-called bonded magnet obtained by a compression molding method or an injection molding method in which a nanocomposite magnet powder having a desired particle size and a raw material material containing a polymer material, for example, a resin material, are mixed. Is possible. The polymer material to be mixed may be appropriately selected depending on the application, and the resin material may be a thermosetting resin such as an epoxy resin or a thermoplastic resin such as a polyamide resin (nylon), or an elastomer or rubber. Polymer materials such as these can also be used, but an epoxy resin is preferably used. Moreover, as long as it is a range which does not influence the performance of the obtained bonded magnet, the said raw material material may add suitably various additives and auxiliary agents as needed.

  It is also possible to create a bulk magnet in which the nanocomposite magnet powder of the present invention having a desired particle size is densified to a true density by using a hot press method or a discharge plasma sintering method. In particular, the density of the resulting bulk magnet is densified to the same level as that of the nanocomposite magnet, which is a raw material ribbon, by heat treatment using the spark sintering method or hot pressing method, which is a pressure sintering method. As a result, high magnet characteristics equivalent to the raw magnet material can be obtained.

  Since these bonded magnets or bulk magnets have a low rare earth composition, it is possible to greatly contribute to the cost reduction of magnet products, and it goes without saying that the cost of motors, sensors, and actuators using them can be reduced. Absent. In addition, when considering application as an automobile component (for example, a drive motor for an electric vehicle or a hybrid electric vehicle), the high cost of rare earth magnets has been the biggest bottleneck in the adoption so far. By using the magnet of the present invention, it is possible to solve the problem of cost of automobile driving motors, sensors, actuators, etc. at once.

The method for producing the nanocomposite magnet of the present invention is not particularly limited, and can be produced by a general method. That is, a rapid cooling ribbon is produced using a liquid quenching method from an alloy ingot obtained by casting. Specifically, a molten metal of a mother alloy (alloy ingot obtained by casting) adjusted to a predetermined composition so as to have a desired magnet composition is created. The melting method for making the mother alloy into a molten metal is not particularly limited as long as it can be heated above the melting point of the mother alloy. For example, the melting method includes melting by an arc, melting by a heater, melting by high frequency induction heating, etc. is there. A quenched ribbon is produced from the molten alloy having the desired composition thus obtained by using a known liquid quenching method. In the liquid quenching method, as described above, the alloy ingot obtained by casting is made into a molten metal (molten liquid metal; usually melted at about 1400 ° C. using high frequency induction melting or arc melting), and the molten metal is rotated. It is a method of producing a ribbon-like product (quenching ribbon) by spraying onto a roll (usually a copper rotating roll) and quenching. This liquid quenching method is performed by oxidizing the quenching ribbon by performing it in an inert atmosphere such as argon (Ar) or under reduced pressure (usually, it is generally reduced to about 10 ⁰ Pa = 1 Pa using a rotary pump). It is necessary to suppress deterioration. This is because rare earth (Nd) does not form an Nd ₂ Fe ₁₄ B phase due to oxidative degradation and becomes an oxide, so that the hard phase decreases and the magnetic flux density and coercive force decrease. The quenched ribbon generally contains an amorphous portion (contributed by element B), and then a heat treatment is performed to form a nano-order size hard phase, semi-hard phase, and rare earth phase. The quenching speed of the liquid quenching method, that is, the roll peripheral speed of the rotating roll is not particularly limited, but is preferably 15 to 50 m / s. This is because the structure becomes coarse because the cooling rate is slow at less than 15 m / s, and the magnet characteristics tend to deteriorate due to the amorphization of the structure when it exceeds 50 m / s.

  In using the rapid cooling method, the type, the material of the roll, the size of the roll, etc. are not particularly limited. For example, as the roll, a copper roll plated with Cr can be used. The size of the roll is preferably determined according to the production scale.

  A step of sintering the ribbon obtained by this rapid cooling to obtain a sintered body may be added. In the sintering step, means used in a known method for producing a sintered magnet can be employed. As a magnet sintering / heat treatment facility, a batch-type vacuum / atmosphere sintering furnace or heat treatment furnace can be used for small-scale production. The batch furnace can be heated and cooled in the same chamber according to the temperature pattern. By increasing the density of the sintered body by sintering, effects such as (i) an increase in residual magnetic flux density Br, (ii) an increase in mechanical strength, and (iii) resistance to corrosion such as oxidation are produced. .

The composition was calculated so that the volume fraction of the Nd ₂ Fe ₁₄ B phase (T1 phase) was 90%, and the other parts were the Nd rich phase and the Nd ₂ Fe ₁₇ phase, and based on the calculated composition, Iron (product made from high purity chemical), FeB (product made from high purity chemical), and Nd (product made from high purity chemical) were weighed in units of 1/1000 g. The compositions of the A, B, C, and D regions in FIG. 1 were synthesized, and the atomic ratio of each sample is shown in Table 1.

In FIG. 1, the A, B, and C regions are surrounded by the Nd ₂ Fe ₁₄ B phase-Nd ₂ Fe ₁₇ phase-Nd rich phase, so that these layers are obtained. FIG. 2 shows how much Nd ₂ Fe ₁₇ phase and Nd-rich phase occupy the remaining portion, with the Nd ₂ Fe ₁₄ B phase ratio fixed at 90 vol%.

As shown in FIG. 2, the Nd ₂ Fe ₁₄ B phase, which is the main phase in the sample, is 90%, and the remaining 10% is 100% Nd ₂ Fe ₁₇ phase and 0% Nd rich phase in Sample A. Yes, sample B is 99% Nd ₂ Fe ₁₇ phase, 1% Nd rich phase, sample C is 50% Nd ₂ Fe ₁₇ phase, 50 Nd rich phase, sample D is 0% Nd ₂ Fe ₁₇ phase, 100 % Nd rich phase.

  After weighing the weights of Nd, Fe and B, arc melting was performed three times in an Ar-substituted vacuum atmosphere to prepare an ingot having a uniform composition distribution. Thereafter, a quenching ribbon was obtained by melting and quenching the ingot in a liquid quenching apparatus. In this liquid quenching, an ingot in a quartz nozzle is melted by high frequency in a vacuum atmosphere substituted with Ar and reaches 1400 ° C., and then the molten metal discharged from a φ0.6 mm hole at the tip of the nozzle is 20 cm in diameter and a peripheral speed is 25 m / s. It was obtained by bringing it into contact with a rotating copper roll.

  The recovered rapidly cooled ribbon was completely magnetized with a pulse magnetic field of 10 T, and then the magnetic properties were measured with a sample vibration magnetometer VSM (manufactured by Lake Shore). The measurement results are shown in FIG. Sample D without the Nd-rich phase that is a grain boundary phase has poor squareness and low magnetization when a negative magnetic field is applied. On the other hand, samples B and C have improved squareness, and there is little decrease in magnetization even when a negative magnetic field is applied. Along with this, a significant increase in the coercive force Hc was observed.

  The squareness is the degree to which the demagnetization curve becomes square, and is expressed by the ratio Hk / Hc between the magnetic field Hk and the coercive force Hc. In the present invention, as shown in FIG. 4, the decrease in magnetization in the first quadrant of the MH curve is linearly approximated, and the magnetic field Hk / coercive force Hc at which the deviation from the straight line is 10% is defined as squareness.

  FIG. 5 shows the relationship between the ratio of the Nd-rich phase in the portion other than the main phase and the squareness. A marked increase in squareness was observed when the volume fraction of the Nd-rich phase was 1 vol% or more.

6 and 7 show tissue images of Sample B and Sample D. FIG. Moreover, the result of the point analysis of EDX in each structure | tissue image is shown below.
(1)
Element wt% at%
NdL 39.5 20.2
FeK 60.5 79.8
Total 100.0 100.0
(2)
Element wt% at%
NdL 42.1 22.0
FeK 57.9 78.0
Total 100.0 100.0
(3)
Element wt% at%
NdL 54.9 32.0
FeK 45.1 68.0
Total 100.0 100.0
(4)
Element wt% at%
NdL 41.1 21.3
FeK 58.9 78.7
Total 100.0 100.0
(5)
Element wt% at%
NdL 42.5 22.2
FeK 57.5 77.8
Total 100.0 100.0

6 and 7, the obtained sample is composed of crystal grains of 30 nm or less, and from the results of the preparation sample composition and composition analysis, there are Nd ₂ Fe ₁₄ B phase and Nd ₂ Fe ₁₇ phase, and the crystal grain spacing is It was found that it was divided by the Nd rich phase. According to the EDX point analysis, in consideration of the overlap of crystal grains, in Sample B, (1) is the Nd ₂ Fe ₁₇ phase, (2) is the Nd ₂ Fe ₁₄ B phase, and (3) is the Nd rich phase. . Thereby, the crystal grains are divided by the Nd-rich phase. It was confirmed that the structure had two magnetic phases (1) and (2). This prevents the magnetization reversal caused by the external magnetic field from propagating to the adjacent particles due to the presence of the Nd-rich phase, leading to improved squareness and increased coercive force. On the other hand, in sample D, it can be seen that there is no Nd-rich phase separating the crystal grains. When there is no magnetic separation between crystal grains, it is considered that the characteristics deteriorate due to the magnetization reversal chain.

Claims (3)

A nanocomposite magnet comprising at least three phases of an Nd ₂ Fe ₁₄ B phase, an Nd ₂ Fe ₁₇ phase, and a rare earth phase.   The nanocomposite magnet according to claim 1, wherein the rare earth phase is an Nd-rich phase. The nanocomposite magnet according to claim 1 or 2, wherein the crystal grain sizes of the Nd ₂ Fe ₁₄ B phase and the Nd ₂ Fe ₁₇ phase are 300 nm or less.