Patent Application
20240363272
This application claims priority to Japanese Patent Application No. 2023-074811 filed on Apr. 28, 2023, and Japanese Patent Application No. 2024-066871 filed on Apr. 17, 2024. The entire disclosures of these Japanese Patent Applications are incorporated by reference herein.
The present disclosure relates to a sintered R-T-B based magnet.
Sintered R-T-B based magnets (R is at least one of rare earth elements; T is Fe, or Fe and Co; and B is boron) are known as permanent magnets with the highest performance. For this reason, sintered R-T-B based magnets are in use in various motors in the automobile field including electric vehicles (EV, HV, PHV), the renewable energy field including wind power generation, the home electric appliance field, the industrial field, and so on. Sintered R-T-B based magnets are materials essential for miniaturization, weight reduction, increase in efficiency, and energy saving (improvement in energy efficiency) of these motors. In addition, sintered R-T-B based magnets are used in drive motors for electric vehicles, and through the replacement of internal combustion engine vehicles with electric vehicles, sintered R-T-B based magnets also contribute to prevention of global warming by reduction of greenhouse gases such as carbon dioxide (reduction of fuel and exhaust gas). As described above, sintered R-T-B based magnets greatly contribute to the realization of a clean energy society.
Sintered R-T-B based magnets are composed of crystal grains made of an R2T14B type compound and grain boundary phases located at grain boundary portions of the crystal grains (for example, Patent Document 1). Patent Document 1 discloses a sintered rare earth magnet comprising R2T14B main-phase crystal grains and intergranular grain boundary phases between adjacent two R2T14B main-phase crystal grains, wherein the thickness of the intergranular grain boundary phases is 5 nm or more and 500 nm or less and the intergranular grain boundary phases are composed of phases with a magnetism different from that of a ferromagnet.
The R2T14B-type compound constituting the crystal grains is a ferromagnetic material having high saturation magnetization and an anisotropic magnetic field, and affects the characteristics of sintered R-T-B based magnets.
Sintered R-T-B based magnets have a problem that irreversible thermal demagnetization occurs because the coercivity HcJ (hereinafter, simply referred to as “HcJ”) decreases at high temperatures. Therefore, in particular, a sintered R-T-B based magnet to be used for a motor for an electric vehicle is required to have high HcJ even at high temperatures, that is, to have higher HcJ at room temperature.
It is known that when a light rare earth element RL (mainly Nd and Pr) in an R2T14B-type compound is replaced by a heavy rare earth element RH (mainly Tb and Dy), HcJ is improved. However, while He is improved, there is a problem that the saturation magnetization of the R2T14B-type compound phase decreases, and thus the residual magnetic flux density Br (hereinafter, simply referred to as “Br”) decreases. In particular, Tb has problems such as unstable supply and price fluctuation due to, for example, its originally small resource amount and limited production sites. Therefore, it is required to obtain high HcJ while inhibiting a decrease in Br with using Tb as less as possible (with reducing the amount used as much as possible).
The sintered rare earth magnet disclosed in Patent Document 1 is said to be capable of inhibiting a decrease in HcJ at high temperatures while reducing the amount of heavy rare earth elements RH such as Tb used, but the magnetic characteristics (Br and HcJ) are inferior when compared to sintered magnets containing heavy rare earth element RH. In recent years, Br and HcJ are required to be further improved particularly in motors for electric vehicles and the like.
Therefore, an object of an embodiment of the present invention is to provide a sintered R-T-B based magnet capable of further improving Br and HcJ while reducing the amount of heavy rare earth elements RH such as Tb used.
An aspect 1 of the present invention is
An aspect 2 of the present invention is
An aspect 3 of the present invention is
An aspect 4 of the present invention is
An aspect 5 of the present invention is
An aspect 6 of the present invention is
An aspect 7 of the present invention is
According to embodiments of the present invention, it is possible to provide a sintered R-T-B based magnet in which Br and HcJ are further improved while reducing the amount of heavy rare earth elements RH such as Tb used.
The present inventor has extensively conducted studies for improving magnetic characteristics (especially, Br and HcJ) of a sintered R-T-B based magnet while reducing the amount of heavy rare earths used. It was found when a “low boron” composition in which the B content of the sintered R-T-B based magnet is lower than that of the stoichiometric R2T14B compound, the effect of improving magnetic properties due to grain boundary diffusion of R and M is enhanced. Furthermore, it was found that, in the R2T14B compound, the same effect can be obtained even if a part of B is substituted by carbon (C). Further investigation revealed that C, which had substituted for B in the main phase R2T14B compound, binds with rare earth oxides in the grain boundary during the sintering step, to form rare earth oxy-carbide compounds (R-O-C compounds) within the grain boundary. It was also found that the atomic ratio in this case is R: (C, O)=1:1. When such R-O-C compounds are generated at the grain boundary, the content of C constituting the main phase R2T14B compound decreases accordingly. As mentioned above, even when the part of B in the R2T14B compound is substituted by C, the effect obtained by “low boron” can still be achieved. Therefore, a decrease in the content of C in the main phase R2T14B compound results in an effective decrease in the total amount of B and C. Furthermore, the formation of the R-O-C compounds at the grain boundary means that a part of the rare earth element R contained in the raw material alloy is consumed in the generation of the R-O-C compounds. The R-O-C compounds (rare earth oxy-carbides) shall include both R-O compounds (rare earth oxides) and R-C compounds (rare earth carbides).
Upon examination based on these findings, the present inventor has accomplished an invention relating to the embodiments of the present disclosure by finding that it is possible to obtain a sintered R-T-B based magnet having superior magnetic characteristics because of being a sintered R-T-B based magnet that satisfies a specific component composition and has a configuration in which a grain boundary phase between crystal grains includes a first phase and a second phase being disposed between the crystal grains and the first phase and having a Cu and Pr concentrations higher than that of the first phase, wherein the second phase contains Pr of 20 mass % or more 65 mass % or less.
In the following, a sintered R-T-B based magnet (hereinafter, sometimes simply referred to as a “sintered magnet”) according to an embodiment is described in detail.
<Sintered R-T-B based magnet>
The sintered R-T-B based magnet comprises:
As illustrated in
The grain boundary phase 14 includes an intergranular grain boundary phase 14a existing between two adjacent crystal grains 12 and a grain boundary triple junction 14b at which two or more intergranular grain boundary phases 14a intersect.
As illustrated in
The Cu and Pr concentrations of the second phase 142 are higher than the Cu and Pr concentrations of the first phase 141. That is, in the present embodiment, the intergranular grain boundary phase 14a has the first phase and the second phase differing in Cu and Pr contents from each other.
The Pr concentration of the second phase 142 is 20 mass % or more and 65 mass % or less. The Pr concentration of the first phase 141 may be also 20 mass % or more and less than 65 mass %.
The present inventor has confirmed that the intergranular grain boundary phase 14a of the sintered magnet 10 has an internal structure including the first phase 141 and the second phase 142 as described above and, owing to the structure, can achieve high magnetic characteristics. The reason why high magnetic characteristics can be realized by including such an intergranular grain boundary phase 14a is not clear, but it is considered that the higher Cu and Pr concentrations in the second phase 142 than in the first phase 141 cause the melting point of the second phase 142 to be lower than that of the first phase 141. It is considered that the lattice matching among the main phase (crystal grains 12), the second phase 142, and the first phase 142 is improved by the final solidification of the second phase 142.
In the sintered magnet according to the embodiment, R is a rare earth element, and necessarily contains Pr and preferably further contains Nd. By containing both Pr and Nd as R, the sintered magnet can have higher magnetic characteristics.
Similar to Pr, the Nd concentration of the second phase 142 is higher than the Nd concentration of the first phase 141.
In the sintered magnet 10 according to the embodiment, the intergranular grain boundary phase 14a has a two-phase structure, whereby the sintered magnet 10 exhibits higher magnetic characteristics, even though the component composition contains a small amount or none of the heavy rare earth element RH such as Tb.
When the sintered magnet 10 contains a heavy rare earth element RH, the content thereof is desirably smaller than the conventional level. In particular, the content of the heavy rare earth element RH is preferably limited to at least one selected from the group consisting of:
Owing to having an internal structure like that described above, the sintered magnet 10 according to the embodiment can achieve superior magnetic characteristics even if the contents of Tb and Dy, which are heavy rare earth elements RH, are controlled to 0.10 mass % or less and 0.20 mass % or less, respectively.
The sintered magnet 10 preferably satisfies the following Formula (1).
When the sintered magnet does not contain any one or more of Nd, Pr, Ce, La, Dy, Tb, O, and C, the content of the element not contained is regarded as “0 mass %”, which is substituted into the Formula (1).
The contents of R (especially, Nd, Pr, Ce, La, Dy, and Tb), O, and C in the sintered magnet 10 are adjusted, such that the above Formula (1) is satisfied. The content of C can be adjusted by adjusting the amount of the lubricant to be added in pulverization or molding. The value of Formula (1) is preferably 26.0 mass % or more and 27.5 mass % or less, more preferably 26.3 mass % or more and 27.3 mass % or less, and particularly preferably 27.0 mass % or more and 27.3 mass % or less. It is possible to obtain high Br and HcJ while further reducing the amount of heavy rare earth elements RH such as Tb used.
Regarding the component composition of the respective phases contained in the intergranular grain boundary phase 14a,
Preferably, the first phase 141 and the second phase 142 contain F, and the F concentration of the second phase 142 is higher than that of the first phase 141. It is considered that the lattice matching among the main phase (crystal grains 12), the second phase 142, and the first phase 142 is improved by the higher F concentration of the second phase 142 than that of the first phase 141, thereby obtained higher magnetic characteristics.
Each of the first phase 141 and the second phase 142 having such a component composition typically has a crystal structure of R6T13M, and may also contain R6T12.5M1.5 or the like. At least Cu is contained as M, and the Cu concentration of the second phase 142 is set higher than the Cu concentration of the first phase 141. It is considered that when the first phase 141 and the second phase 142 contain such a crystal structure, higher HcJ can be obtained.
Element concentrations in the first phase 141 and the second phase 142 can be measured by SEM-EDX analysis or TEM-EDX analysis.
The thickness of the first phase 141 is preferably 10 nm or more and 500 nm or less, and more preferably 20 nm or more and 300 nm or less. Within such a range, it is possible to control a decrease in magnetic characteristics due to a decrease in the main phase ratio while obtaining an effect of reducing the magnetic interaction between the main phase grains (crystal grains 12).
The thickness of the second phase 142 is preferably 0.5 nm or more and 10 nm or less, and more preferably 1.0 nm or more and 5.0 nm or less. Within such a range, it is possible to control a decrease in magnetic characteristics due to a decrease in the main phase ratio while satisfying the role of improving the lattice matching.
The respective component compositions of the sintered magnet 10 will be described in detail.
(R: 26.8 mass % or more and 31.5 mass % or less)
R is a rare earth element, and necessarily contains Pr. The content of R is 26.8 mass % or more and 31.5 mass % or less. When the content of R is less than 26.8 mass %, there may arise difficulty in densification during sintering, and when the content of R is more than 31.5 mass %, the ratio of the main phase may decrease, leading to a decrease in Br. The content of R is preferably 26.8 mass % or more and 30.0 mass % or less. When the content of R is in such a range, higher Br can be obtained.
(M: 0.05 mass % or more and 2.00 mass % or less)
M is at least one selected from the group consisting of Ga, Cu, Zn, Al, and Si, and necessarily contains Cu. The content of M (the total content of Ga, Cu, Zn, Al, and Si) is 0.05 mass % or more and 2.00 mass % or less. When M is contained in this range, the temperature coefficient may be improved and high HcJ can be achieved at high temperatures.
The sintered magnet 10 necessarily contains Cu as M, and preferably further contains Ga, and HcJ can thereby be further improved.
Cu is contained in both the crystal grains 12 and the intergranular grain boundary phase 14a. In particular, the Cu concentration in the second phase 142 of the intergranular grain boundary phase 14a is high, and as described later, the Cu concentration can be in the range of 2 mass % or more and 20 mass % or less. That is, Cu is concentrated in the second phase 142 of the sintered magnet 10. The first phase 141 may also contain Cu, but has a Cu concentration lower than that of the second phase 142.
Cu may be derived from a raw material, or may be derived from Cu diffused from a surface of a sintered body in a diffusion step performed after a sintering step.
(B: 0.84 mass % or more and 0.94 mass % or less)
The content of B is 0.84 mass % or more and 0.94 mass % or less. When the sintered magnet 10 contains B within the range of the present disclosure, the temperature coefficient is improved and high He can be achieved even at high temperatures. The content of B is preferably 0.85 mass % or more and 0.94 mass % or less.
(T: 61.5 mass % or more)
T is Fe and Co, and 90% or more of T is Fe in mass ratio. When Co is contained, corrosion resistance can be improved, but when the replacement amount of Co exceeds 10 mass % of T, there is a possibility that high Br is not obtained. The content of T is 61.5 mass % or more. When the content of T is less than 61.5 mass %, there is a possibility that Br significantly decreases. The content of T may be 70.0 or more. Preferably, T is the balance.
The sintered magnet 10 may contain Cr, Mn, La, Ce, Sm, Ca, Mg, or the like as inevitable impurities usually contained in didymium alloy (Nd-Pr), electrolytic iron, ferroboron, and the like. Furthermore, examples of the inevitable impurities contained during a production process include O (oxygen), N (nitrogen), and C (carbon).
The sintered magnet 10 according to the embodiment may contain one or more other elements. For example, as such an element, a small amount (about 0.1 mass %) of Ag, Zn, In, Sn, Ti, Ge, Y, H, F, P, S, V, Ni, Mo, Hf, Ta, W, Nb, Zr, Pb, Bi or the like may be contained. Such elements may be contained in a total amount of, for example, about 1.0 mass %. With this degree of content, it is sufficiently possible to obtain a sintered R-T-B based magnet having high HcJ at high temperatures.
Among these elements contained in a small amount, F is an essential element necessarily contained in the first phase 141 of the intergranular grain boundary phase 14a of the sintered magnet 10, but is hardly in the crystal grains 12, and thus F is treated as an element contained in a small amount as a component composition of the sintered magnet 10. However, as described above, the fact that the prescribed amount of F is contained in the second phase 142 can be confirmed by subjecting the intergranular grain boundary phase 14a to SEM-EDX analysis or TEM-EDX analysis.
F element may be intentionally added to the raw material such that the second phase 142 and, if necessary, the first phase 141 contain F in a prescribed amount.
In the sintered magnet 10 in the embodiment, the Pr and Cu concentrations may gradually decrease in a depth direction from a surface in a range from the surface to a depth of 200 μm. A sintered magnet 10 having such a concentration distribution is obtained by conducting a step of performing, in the production of the sintered magnet 10, diffusion from a surface of the magnet toward the inside of the magnet using a diffusion source containing Pr and Cu. The performance of such diffusion step surely provides the sintered R-T-B based magnetic in which the second phase 142, having higher Cu and Pr concentrations than those of the first phase 141, is included between the first phase 141 and one or both of two crystal grains 12, with the second phase 142 containing Pr of 20 mass % or more and 65 mass % or less.
Furthermore, as a result of the studies of the present inventor, the inventor has newly found that, in comparison to a sintered magnet produced by diffusing Pr, Cu, and some or all of Ga in the diffusion step, a sintered magnet produced by preparing the sintered compact contains 0.3 mass % or more of Ga all of which is derived from a raw material (that is, contained in the alloy for the sintered magnet), and then diffusing the Pr and Cu, has a significantly higher HcJ. In other words, preferably, the sintered R-T-B based magnet according to the present disclosure necessarily contains Cu and Ga as M, and a concentration of Ga is 0.3 mass % or more and does not gradually decrease in a depth direction from a surface in a range from the surface to a depth of 200 μm. The Ga concentration that does not gradually decrease in a depth direction means that diffusion from the surface of the magnet toward the inside of the magnet using a diffusion source containing Ga is not performed. As a result, it is considered that the first phase may be formed more uniformly in the sintered magnet, thereby achieving a higher HcJ.
The concentrations of Pr, and Cu in the range from the surface to the depth of 200 μm can be confirmed by performing line analysis in the range from a surface of the magnet toward the vicinity of the center of the magnet to a depth of 200 μm in a cross section of the sintered magnet 10 by energy dispersive X-ray spectroscopy (EDX). It is preferable that the measurement is performed in a cross section perpendicular to the surface and 200 μm or more away from the end of the surface. In the line analysis, measurement is performed in a direction orthogonal to the surface (the outer periphery of the measurement cross section) in a region 200 μm or more away from the end of the measurement cross section in order not to measure a range within 200 μm from the outer periphery of the measurement cross section.
The concentration of Ga can be confirmed by the similar measurement.
<Method for producing sintered R-T-B based magnet>
In the following, an embodiment of the method for producing the sintered R-T-B based magnet of the present disclosure will be described.
The production method in the present embodiment can comprise a step of preparing a sintered R-T-B based body, a step of preparing an R1-M alloy, a step of performing a first heat treatment, and a step of performing a second heat treatment. The step of performing the first heat treatment is a step in which at least a part of the R-M alloy is brought into contact with at least a part of a surface of the sintered R-T-B based body and then R1 and M are diffused into the magnet (a diffusion step) by performing the first heat treatment at a temperature of 700° C. or more and 950° C. or less in a vacuum or an inert gas atmosphere. The step of performing the second heat treatment is a step in which the sintered R-T-B based magnet resulting from the first heat treatment is subjected to the second heat treatment at a temperature that is 400° C. or more and 750° C. or less and is lower than the temperature of the first heat treatment, in a vacuum or an inert gas atmosphere. In the following, each of these steps will be described in more detail.
(Step of preparing sintered R-T-B based body)
First, the composition of the sintered R-T-B based body is described.
One of a special features of the sintered R-T-B based magnet of the present embodiment is to adjust the contents of R, O, and C in the sintered R-T-B based body to finally obtain a sintered R-T-B based magnet satisfying Formula (1) mentioned above. Therefore, it is preferred to prepare a sintered R-T-B based body which satisfies relationships of: 0.84 mass %≤[B]≤0.94 mass %, 25.8 mass %≤([Nd]+[Pr]+[Ce]+[La]+[Dy]+[Tb])−12([O]+[C])≤27.5 mass %, and 0.05 mass %≤[O]≤0.30 mass %. In addition, it is preferred to prepare a sintered R-T-B based body satisfying a relationship of 0.05 mass %≤[C]≤0.18 mass %. By performing a diffusion step described below on such R-T-B sintered body, it is possible to prevent excessive diffusion of R, M, and the like inside the R-T-B sintered body and to significantly promote grain boundary diffusion.
The sintered R-T-B based body to be prepared in this step has, for example, the following composition:
The sintered R-T-B based body also satisfies the above Formula (1).
Next, a method for preparing the sintered R-T-B based body is described.
First, an alloy for a sintered R-T-B based magnet is prepared, and then this alloy is coarsely pulverized by, for example, hydrogen pulverization or the like.
Examples of the method for producing the alloy for a sintered R-T-B based magnet will be described. An alloy ingot can be obtained by an ingot casting method in which a metal or an alloy adjusted in advance to have the above-described composition is melted, put into a mold, and solidified. Alternatively, an alloy may also be produced by a strip casting method in which a molten metal or alloy adjusted in advance to have the composition described above is quenched by being brought into contact with a single roll, a twin roll, a rotating disk, a rotating cylindrical mold, or the like and thus a quenched solidified alloy is made. Still alternatively, a flaky alloy may also be produced by another quenching method such as a centrifugal casting method.
In the embodiment of the present invention, an alloy produced by either the ingot method or the quenching method can be used, and it is preferable to use an alloy produced by the quenching method such as the strip casting method. The alloy prepared by the quenching method usually has a thickness of 0.03 mm or more 1 mm or less, and is flake-like. The molten alloy starts to solidify from a surface thereof in contact with a cooling roll (roll contact surface), and a crystal grows in a columnar shape in a thickness direction from the roll contact surface. The quenched alloy has been cooled in a shorter time period than an alloy (alloy ingot) produced by the conventional ingot casting method (mold casting method), and as a result, has a finer structure and a shorter crystal grain size. In addition, the area of grain boundaries is wide. Since an R-rich phase largely expands in grain boundaries, the quenching method is highly effective in dispersing the R-rich phase. For this reason, the R-rich phase is easily broken at the grain boundaries by hydrogen pulverization. By subjecting the quenched alloy to hydrogen pulverization, the size of the hydrogen pulverized powder (coarsely pulverized powder) can be adjusted to, for example, 1.0 mm or less. The coarsely pulverized powder thus obtained is pulverized by, for example, a jet mill.
In the present embodiment, the conditions of pulverization is adjusted such that the oxygen content of the sintered R-T-B based magnet is preferably 0.05 mass %≤[O]≤0.30 mass %. The jet mill pulverization is performed in an atmosphere of inert gas such as nitrogen. The pulverization may be performed by, for example, a jet mill in a humidified atmosphere. Preferably, the powder particles are reduced in size (the average particle size d50 is preferably 2.0 μm or more and 10.0 μm or less, more preferably 2.0 μm or more and 8.0 μm or less, still more preferably 2.0 μm or more and 4.5 μm or less, and further preferably 2.0 μm or more and 3.5 μm or less). By reducing the size of the powder particles, high HcJ can be obtained.
The average particle size (d50) can be measured using the airflow dispersion laser diffraction method, conforming to JIS Z 8825:2013 revised edition. In this specification, an average particle size means the particle diameter (median diameter) at which the integrated particle size distribution (volume basis) from the small particle size side becomes 50%.
The fine powder to be used for the preparation of the sintered R-T-B based body may be prepared from one type of raw material alloy (single raw material alloy) or alternatively may be prepared by a method of mixing two or more types of raw material alloys (a blend method) as long as the above-described conditions are satisfied.
In a preferred embodiment, a powder compact is prepared from the above-mentioned fine powder by pressing in a magnetic field, and then the powder compact is sintered to obtain the sintered R-T-B based magnet.
As to the pressing in a magnetic field when preparing a powder compact, it is preferable to form a powder compact by pressing in an inert gas atmosphere or wet pressing from the viewpoint of inhibiting oxidation. Especially in wet pressing, the surfaces of particles constituting the powder compact are covered with a dispersant such as an oil, and are inhibited from coming into contact with oxygen and water vapor in the air. For this reason, it is possible to prevent or inhibit the particles from being oxidized by the air before and after the pressing step or during the pressing step. Therefore, the oxygen content in the sintered magnet can be easily controlled within a prescribed range. In the case of performing wet pressing in a magnetic field, a slurry in which a dispersion medium is mixed with a fine powder is prepared, fed into a cavity in a mold of a wet press device, and press-molded in the magnetic field. The sintered R-T-B based body may be prepared using a known method such as a press-less process (PLP) described in, for example, JP 2006-019521 A without performing molding or the like. Next, the powder compact is sintered to afford a sintered R-T-B based body.
The sintering of the compact is preferably performed at a temperature in the range of 950° C. or more and 1150° C. or less. In order to prevent oxidation due to the sintering, the residual gas in the atmosphere may be replaced by an inert gas such as helium or argon. The sintered body obtained may be subjected to a heat treatment. As heat treatment conditions such as heat treatment temperature and heat treatment time, known conditions may be adopted.
(Step of preparing R1-M alloy
In the present embodiment, R1, or R1 and M are diffused from a surface to the inside of the sintered R-T-B based body. For performing this, an alloy containing the elements to be diffused (referred to as “an R1-M alloy”) is prepared.
First, the composition of the R1-M alloy will be described. R1 in the R1-M alloy is at least one of rare earth elements, and R1 preferably accounts for 65 mass % or more and 100 mass % or less, and more preferably 70 mass % or more and 95 mass % or less of the entire R1-M alloy. M in the R1-M alloy is at least one selected from the group consisting of Ga, Cu, Zn, Al, and Si. M preferably accounts for 0 mass % or more and 35 mass % or less, and more preferably 5 mass % or more and 30 mass % or less of the entire R1-M alloy.
In the R1-M alloy, R1 preferably contains at least one of Nd and Pr, and more preferably, necessarily contains Pr. The content of Pr is preferably 65 mass % or more and 86 mass % or less with respect to the entire R1-M alloy. Preferably, the content of Pr is 50 mass % or more of the entire R1, and more preferably 65 mass % or more of the entire R1. When Pr is contained, diffusion in the grain boundary phase easily proceeds, so that grain boundary diffusion can be promoted, and higher HcJ can be obtained.
The shape and size of the R1-M alloy are not particularly limited, and may be any shape and size. The R1-M alloy may be in the form of film, foil, powder, blocks, particles or the like.
Next, a method for preparing the R1-M alloy will be described.
The R1-M alloy may be prepared by a method for producing a raw material alloy that is adopted in methods for producing common sintered R-T-B based magnets, e.g., a mold casting method, a strip casting method, a single roll rapid quenching method (melt spinning method), and an atomization method. The R1-M alloy may be one obtained by pulverizing the alloy obtained as described above by a known pulverizing means such as a pin mill.
(Step of performing first heat treatment (Diffusion step))
A the first heat treatment step is performed at a temperature of 700° C. or more and 950° C. or less (a first heat treatment temperature) in a vacuum or an inert gas atmosphere, in which at least a part of the R1-M alloy is brought into contact with at least a part of a surface of the sintered R-T-B based body prepared by the above-described method. The first heat treatment is for diffusing R1 and M into the sintered magnet (that is, a diffusion step). By the heat treatment, a liquid phase containing R1 and M is generated from the R1-M alloy, and the liquid phase is diffused and introduced from a surface of the sintered body to the inside thereof via the grain boundaries in the sintered R-T-B based body.
When the first heat treatment temperature is lower than 700° C., the amount of the liquid phase containing R1 and M is excessively small, so that high HcJ cannot be obtained. On the other hand, when the first heat treatment temperature exceeds 950° C., there is a possibility that HcJ decreases. The first heat treatment temperature is preferably 850° C. or more and 950° C. or less. Within such a range, a higher HcJ can be obtained. Preferably, the sintered R-T-B based body subjected to the first heat treatment (700° C. or more and 950° C. or less) is cooled to 300° C. at a cooling rate of 5° C./min or more from the temperature at which the first heat treatment is performed. Within such a range, a higher HcJ can be obtained. More preferably, the cooling rate to 300° C. is 15° C./min or more.
The first heat treatment can be performed by using a known heat treatment apparatus with an R1-M alloy having an arbitrary shape disposed on a surface of the sintered R-T-B based body. For example, the first heat treatment may be performed with the surface of the sintered R-T-B based body covered with a powder layer of the R1-M alloy. For example, a slurry in which the R1-M alloy is dispersed in a dispersion medium may be applied to the surface of the sintered R-T-B based body, and then the dispersion medium may be evaporated to bring a R1-M alloy layer into contact with the sintered R-T-B based body to perform the first heat treatment. Examples of the dispersion medium include alcohols (ethanol and the like), aldehydes, and ketones. In addition, a film of the R1-M alloy may be formed on a surface of the sintered R-T-B based body, for example, with a known sputtering device or the like, followed by the first heat treatment. When the heavy rare earth element RH is introduced into the sintered R-T-B based body, the first heat treatment can be performed not only by using the R1-M alloy containing RH but also by arranging a fluoride, an oxide, an acid fluoride, or the like of the heavy rare earth element RH together with the R1-M alloy on a surface of the sintered R-T-B based body. Examples of the fluoride, oxide, and acid fluoride of the heavy rare earth element RH include TbF3, DyF3, Tb2O3, Dy2O3, TbOF, and DyOF.
The R1-M alloy may be disposed at any position as long as at least a part of the R1-M alloy is in contact with at least a part of the sintered R-T-B based body.
(Step of performing second heat treatment)
The sintered R-T-B based body resulting from the first heat treatment is then subjected to a heat treatment in a vacuum or an inert gas atmosphere at a temperature that is 400° C. or more and 750° C. or less and is lower than the first heat treatment temperature. In the present disclosure, this heat treatment is referred to as a second heat treatment. By performing the second heat treatment, high HcJ can be obtained. When the temperature of the second heat treatment (a second heat treatment temperature) is higher than that of the first heat treatment temperature or when the second heat treatment temperature is lower than 400° C. or higher than 750° C., high HcJ may not be obtained.
Raw materials of each of the elements were weighed such that the sintered R-T-B based bodies would have the compositions shown in Nos. A through D in Table 1, and alloys were produced by a strip casting method. Each of the alloys obtained was coarsely pulverized by hydrogen pulverization, affording a coarsely pulverized powder. Next, to the coarsely pulverized powder obtained, zinc stearate as a lubricant was added in an amount of 0.04 mass % with respect to 100 mass % of the coarsely pulverized powder, followed by mixing. Then, the mixture was dry-pulverized in a nitrogen flow using an airflow crusher (jet mill machine), affording finely pulverized powder (alloy powder) having an average particle size d50 of 3 μm.
Zinc stearate as a lubricant was added to the finely pulverized powder in an amount of 0.05 mass % with respect to 100 mass % of the finely pulverized powder, followed by mixing, and then the mixture was molded in a magnetic field, affording a powder compact. As a pressing apparatus, a so-called orthogonal magnetic field pressing apparatus (transverse magnetic field pressing apparatus) was used, in which the direction of magnetic field application was orthogonal to the pressurizing direction. The powder compact obtained was sintered at a temperature of 1060° C. or more and 1090° C. or less (a temperature at which a sufficiently densification caused by sintering would occur was chosen for each sample) for 4 hours, affording a sintered R-T-B based body. The sintered R-T-B based body obtained had a density of 7.5 Mg/m3 or more. The results of the components of the sintered R-T-B based bodies obtained are shown in Table 1. Each of the composition in Table 1 was measured using high-frequency inductively coupled plasma optical emission spectroscopy (ICP-OES). The O (oxygen) content was measured using a gas analyzer by a gas fusion-infrared absorption method and the C (carbon) content was measured using a gas analyzer by a combustion-infrared absorption method. The composition of the R1-M alloy (Table 2) was measured by the same measurement methods.
Raw materials of the respective elements were weighed such that the R1-M alloy would have approximately the compositions shown in Nos. a and b in Table 2, and these raw materials were melted and alloys in a ribbon or flake form were obtained by a single roll rapid quenching method (melt spinning method). Each of the alloys obtained was pulverized in an argon gas atmosphere using a mortar, and then was passed through a sieve with a mesh size of 425 μm, and thus R1-M alloys were prepared. The compositions of the R1-M alloys obtained are shown in Table 2.
The sintered R-T-B based bodies of Nos. A through D in Table 1 were each cut and ground into a 7.4 mm×7.4 mm×7.4 mm cube. Next, the R1-M alloy (No. a or b) was spread on the entire surface of each of the sintered R-T-B based bodies of Nos. A through D at a ratio of 3 mass % with respect to 100 mass % of the each of the sintered R-T-B based bodies. A heat treatment (the first heat treatment) was performed at 900° C. for 4 hours in argon gas under reduced pressure controlled to 50 Pa, and then the mixture was cooled to room temperature. Furthermore, the sintered R-T-B based body resulting from the first heat treatment was subjected to a heat treatment (the second heat treatment) at 500° C. for 1 hour in argon gas under reduced pressure controlled to 50 Pa, and then cooled to room temperature, and thus sintered R-T-B based magnets (Nos. 1 to 4) were prepared.
In Table 3 are shown the compositions and the value of the middle part of Formula (1) (that is, ([Nd]+[Pr]+[Ce]+[La]+[Dy]+[Tb])−12([O]+[C])) in the sintered R-T-B based magnets obtained. It has been confirmed that all of Nos. 1 to 3 satisfy Formula (1) of the present disclosure. Each of the composition in Table 3 was measured using high-frequency inductively coupled plasma optical emission spectroscopy (ICP-OES). The O (oxygen) content was measured using a gas analyzer by a gas fusion-infrared absorption method and the C (carbon) content was measured using a gas analyzer by a combustion-infrared absorption method.
The sintered R-T-B based magnets obtained were mechanically processed into magnetic samples sized 7 mm×7 mm×7 mm, and the magnetic characteristics (Br and HcJ) thereof were measured with a BH tracer. The measurement results are shown in Table 4. Each of the magnet samples Nos. 1 through 4 was cut along a plane parallel to a surface and passing through the vicinity of the central portion of the magnet, and in the cross section, the part from the magnet surface to the vicinity of the central portion of the magnet was subjected to line analysis by EDX. In all the samples, it was confirmed that the concentrations of Pr and Cu gradually decreased in a depth direction from the surface of the magnet in a range from the surface to a depth of 200 μm (the concentrations gradually decreased).
In addition, in the magnet sample No. 3 prepared by using a diffusion source No. b that contained Ga, it was confirmed that the concentration of Ga was gradually decreased in the depth direction from the surface of the magnet in the range from the surface to a depth of 200 μm. In the magnet samples Nos. 1, 2, and 4 prepared by using a diffusion source No. a that did not contain Ga, it was confirmed that the concentration of Ga was not gradually decreased in the depth direction from the surface of the magnet in the range from the surface to a depth of 200 μm.
Each of the cross sections of the sintered R-T-B based magnets of Nos. 1 through 4 was analyzed by FE-TEM/WDX/EDX (field emission transmission electron microscope/wavelength dispersive X-ray analysis/energy dispersive X-ray analysis). At this time, in order to eliminate the influence of elements diffused from the surface, analysis was performed at a position 3000 μm or more away from the surface.
First, the cross section was observed with TEM, and a TEM image of the intergranular grain boundary phase 14a between two adjacent crystal grains 12 was taken (
Similarly, TEM observation of Nos. 2 and 3 confirmed that the intergranular grain boundary phase 14a contained the first phase 141 and the second phase 142. Regarding No. 4, the intergranular grain boundary phase 14a was composed of only the first phase 141, and the second phase 142 was not confirmed.
For Nos. 1 to 4, each of the first phase 141 and the second phase 142 specified in the TEM image was subjected to point analysis by EDX, and the amounts of the elements contained in each phase were specified. For No. 4, the first phase 141 was subjected to point analysis by EDX. The results of the point analysis are shown in Table 4. In all of Nos. 1 to 3, it was confirmed that the Cu and Pr concentrations of the second phase 142 were higher than the Cu and Pr concentrations of the first phase 141, and the second phase contained Pr of 20 mass % or more 65 mass % or less, as shown in Table 4. As shown in Table 4, in Nos. 1 to 3 satisfying the conditions of the present disclosure, there was no large decrease in Br and high HcJ was obtained, as compared with the comparative example (No. 4).
The sintered magnets of Nos. 1 and 2 prepared by diffusing Pr and Cu into the sintered body containing all amount of Ga had a higher Her in comparison with the sintered magnet No. 3 prepared by diffusing Pr, Cu and Ga into the sintered body in the diffusion step.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2023-074811 | Apr 2023 | JP | national |
| 2024-066871 | Apr 2024 | JP | national |
