DE3750661T2 - Permanent magnet with good thermal stability. - Google Patents

Description

BACKGROUND OF THE INVENTION

The present invention relates to rare earth permanent magnet materials, in particular to R-Fe-B permanent magnet materials with good thermal stability.

R-Fe-B permanent magnet materials have been developed as new compositions with higher magnetic properties than R-Co permanent magnet materials (Japanese Patent Publication Nos. 59-46008, 59-64733 and 59-89401, and M.Sagawa et al, "New Material for Permanent Magnets on a Basis of Nd and Fe", J.of Appl. Phys. 55 (6) 2083 (1984)). According to these documents, an alloy of Nd15Fe77B8 [Nd(Fe0.091B0.09)5, for example, has magnetic properties such as (BH)max of nearly 280 kJ/m3 and iHc of nearly 800 kA/m. However, the R-Fe-B magnets have lower Curie temperatures so that they have weak thermal stability. To solve these problems, attempts have been made to increase the Curie temperature by adding Co (Japanese Patent Publication No. 59-64733). Specifically, the R-Fe-B permanent magnet has a Curie temperature of about 300ºC and at most 370ºC (Japanese Patent Publication No. 59-46008), while substitution of Co for a part of Fe in the R-Fe-B magnet serves to increase the Curie temperature to 400-800ºC (Japanese Patent Publication No. 59-64733). And the addition of Co also reduces the coercive force iHc of the R-Fe-B magnet.

Attempts have also been made to improve the coercivity by adding Al, Ti, V, Cr, Mn, Zn, Hf, Nb, Ta, Mo, Ge, Sb, Sn, Bi, Ni, etc. It was found that that Al is particularly effective in improving the coercive force (Japanese Patent Publication No. 59-89401). However, since these elements other than Ni are non-magnetic, the addition of larger amounts of such elements would result in a decrease in the residual magnetic flux density Br, which in turn leads to the decrease in (BH) max.

Furthermore, substitution of heavy rare earth elements such as Tb, Dy and Ho for a portion of Nd has been proposed to improve the coercivity while maintaining a high (BH)max (Japanese Patent Publication Nos. 60-32306 and 60-34005). By substituting the heavy rare earth elements for a portion of Nd, the coercivity is increased from about 720 kA/m to 960 to 1440 kA/m at (BH)max of about 240 kJ/m3. However, since heavy rare earth elements are very expensive, substitution of such heavy rare earth elements for a portion of neodymium in large amounts undesirably increases the cost of R-Fe-B magnets.

The addition of both Co and Al has been further proposed to improve the thermal stability of the R-Fe-B magnet (T. Mizoguchi et al., Appl. Phys. Lett. 48, 1309 (1986)). The substitution of Co for a portion of Fe increases the Curie temperature Tc but causes a decrease in iHc, presumably because ferromagnetic precipitation phases of Nd(Fe,Co)2 appear at the grain boundaries, which form nucleation sites of reversed regions. The addition of Al in combination with Co serves to form non-magnetic Nd(Fe,Co,Al)2 phases, which suppress the generation of nucleation sites of reversed magnetic regions. However, since the addition of Al greatly lowers the Curie temperature Tc, R-Fe-B magnets containing Co and Al inevitably have poor thermal stability at temperatures up to 100ºC or more. Moreover, the coercive force iHc of such magnets is only about 9 kOe.

OBJECT AND SUMMARY OF THE INVENTION

An object of the present invention is therefore to provide an R-Fe-B permanent magnet with an increased Curie temperature and sufficient coercive force and thus improved thermal stability.

As a result of intensive research in view of the above task, the inventors have found that by adding Ga or Co and Ga in combination, R-Fe-B magnets with higher Curie temperature, sufficient coercivity and thus higher thermal stability can be obtained at cost advantages.

That is, the permanent magnet having good thermal stability according to the present invention consists essentially of a composition set forth in claim 1.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a graph showing the variations of irreversible flux loss of Nd-Fe-B, Nd-Dy-Fe-B and Nd-Fe-B-Ga magnets versus heating temperatures;

Fig. 2 is a graph showing the variations of irreversible flux losses of Nd-Fe-Co-B, Nd-Dy-Fe-Co-B and Nd-Fe-Co-B-Ga magnets versus heating temperatures;

Fig. 3 is a graph showing the variations of irreversible flux losses of Nd-Fe-Co-B, Nd-Fe-Co-B-Ga and Nd-Fe-Co-B-Ga-W magnets versus heating temperatures;

Fig. 4 is a graph showing the variations of the irreversible flux losses of Nd(Fe0.85-xCo0.06B0.08GaxW0.01)5.4 versus heating temperatures;

Fig. 5 is a graph showing the variations of irreversible flux losses versus heating temperatures of magnets prepared by (a) rapid quenching → heat treatment → resin bonding, (b) rapid quenching → heat treatment → hot pressing, and (c) rapid quenching → compression forming;

Fig. 6 is a graph showing the comparison of magnetic properties of Nd-Dy-Fe-Co-B, Nd-Fe-Co-B-Al and Nd-Fe-Co-B-Ga magnets;

Fig. 7 is a graph showing the variations of irreversible flux losses of Nd(Fe0.72Co0.2B0.08)5.6, Nd0.8Dy0.2(Fe0.72Co0.2B0.08)5.6, Nd(Fe0.67Co0.2B0.08Al0.05)5.6 and Nd(Fe0.67Co0.2B0.08Ga0.05)5.6 magnets versus heating temperatures;

Fig. 8(a)-(d) are graphs showing the variations of the open fluxes of Nd(Fe0.72Co0.2B0.08)5.6, Nd0.8Dy0.2(Fe0.72Co0.2B0.08)5.6, Nd(Fe0.67Co0.2B0.08Al0.05)5.6 and Nd(Fe0.67Co0.2B0.08Ga0.05)5.6 magnets versus heating temperatures; and

Fig. 9(a)-(d) are graphs showing the demagnetization curves of Nd (Fe0.67-z-uCo0.25B0.08GazWu)5.6, Nd (Fe0.67Co0.25B0.08)5.6, Nd (Fe0.65Co0.25B0.08Ga0.02)5.6 and Nd (Fe0.635Co0.25B0.08Ga0.02W0.015)5.6 magnets prepared at different sintering temperatures.

DETAILED DESCRIPTION OF THE INVENTION

The reasons for limiting the composition ranges of components in the magnet alloy of the present invention are described below.

Adding Co to the R-Fe-B magnet raises its Curie temperature but lowers its magnetic crystal anisotropy constant, resulting in the reduction in coercivity. However, adding Co and Ga in combination gives the magnet a higher Curie temperature and thus a higher coercivity. Although adding elements such as Al and Si to an R-Fe-Co-B magnet can result in improved coercivity, the maximum improvement in coercivity can be achieved by adding Ga. And although heavy rare earth elements such as Tb, Dy and Ho are commonly added to improve coercivity, the use of Ga can minimize the use of expensive, heavy rare earth elements, if any are needed. Thus, the disadvantage of the R-Fe-B magnet that it has a low Curie temperature, which leads to poor thermal stability, can be overcome by adding Ga or Co and Ga in combination, thereby giving the magnet a higher coercive force and a higher Curie temperature, thus providing better thermal stability and cost advantages.

The amount of Co represented by "x" is 0 to 0.7. If it exceeds 0.7, the remaining magnetic flux density Br of the resulting magnet becomes too low. In order to sufficiently improve the Curie temperature Tc, the lower limit of Co is preferably 0.01, and in order to obtain a well-balanced combination of magnetic properties such as iHc and Br and Tc, the upper limit of Co is preferably 0.4. The most preferred amount of Co is 0.05 to 0.25.

The addition of Ga leads to a remarkable improvement in the coercivity. This improvement appears to be due to an increase in the Curie temperature of a BCC phase in the magnet. The BCC phase is a polycrystalline phase with a body-centered cubic crystal structure that surrounds a main phase of the R-Fe-B magnet (Nd2Fe14B) in a width of 10 to 500 nm. This BCC phase is in turn surrounded by a phase rich in Nd (Nd: 70-95 at.% and the balance Fe). The Curie temperature of this BCC phase corresponds to a temperature at which the coercivity of the magnet becomes lower than 50 Oe, which severely affects the temperature characteristics of the magnet. The addition of Ga serves to raise the Curie temperature of the BCC phase, thereby improving the temperature characteristics.

The amount of Ga represented by "z" is 0.001 to 0.15. If it is less than 0.001, substantially no effect is obtained in improving the Curie temperature of the magnet. On the other hand, if "z" exceeds 0.5, it leads to an extreme decrease in saturation magnetization and Curie temperature, resulting in undesirable permanent magnet materials. The preferred amount of Ga is 0.002 to 0.10, and the most preferred amount of Ga is 0.005 to 0.05.

If the amount of boron represented by "y" is less than 0.02, the Curie temperature is low and a high coercive force cannot be obtained. On the other hand, if the amount "y" of B is higher than 0.3, the saturation magnetization decreases, thereby forming phases undesirable for magnetic properties. Accordingly, the amount of B should be 0.02 to 0.3. The preferred range of "y" is 0.03 to 0.20. The most preferred amount of B is 0.04 to 0.15.

If "A" is less than 4, the saturation magnetization is low, and if it exceeds 7.5, phases which are rich in Fe and Co, resulting in an extreme decrease in coercive force. Accordingly, "A" should be 4.0 to 7.5. The preferred range of "A" is 4.5 to 7.0. The most preferred range of "A" is 5.0 to 6.8.

The permanent magnet of the present invention further contains an additional element represented by the formula

R(Fe1-x-y-z-uCOxByGazMu)A

generally designated "M", where R is Nd alone or one or more rare earth elements composed mainly of Nd, Pr or Ce, a part of which may be substituted by Dy, Tb or Ho, M is one or more elements selected from the group consisting of Nb, W, V, Ta and Mo, 0≤x≤0.7;0.02≤y≤0.3;0.001≤z≤0.15;0.001≤u≤0.1 and 4.0≤A ≤7.5.

Nb, W, V, Ta or Mo is added to prevent grain growth. The amount of these elements represented by "u" is 0.001 to 0.1. If it is less than 0.001, sufficient effects cannot be obtained, and if it exceeds 0.1, the saturation magnetization is extremely reduced, resulting in undesirable permanent magnets.

The addition of Nb does not decrease Br as much as the addition of Ga, but it slightly increases iHc. Nb is effective for increasing corrosion resistance, and since it is likely to be exposed to relatively high temperatures in the case of highly heat-resistant alloys, it is a highly effective additive. If the amount of Nb represented by "u" is less than 0.001, sufficient effects for increasing iHc cannot be obtained, and the magnet alloy does not have sufficiently high corrosion resistance. On the other hand, if the amount of Nb exceeds 0.1, a undesirably large reduction of Br and Curie temperature. The preferred range of Nb is 0.002≤z≤0.04.

The addition of tungsten (W) serves to greatly improve the temperature characteristics. If the amount of W("u") exceeds 0.1, the saturation magnetization and the coercive force are extremely reduced. If "u" is less than 0.001, sufficient effects cannot be achieved. The preferred amount of W is 0.002 to 0.04.

As for the rare earth element "R", it can be Nd alone or a combination of Nd and a light rare earth element such as Pr or Ce, or Pr plus Ce. When Pr and/or Ce are included, the ratio of Pr to Nd can be 0:1 to 1:0, and that of Ce to Nd can be 0:1 to 0.3:0.7.

Nd can also be substituted by Dy, which acts to slightly raise the Curie temperature and enhance the coercive force iHc. Thus, the addition of Dy is effective to improve the thermal stability of the permanent magnet of the present invention. However, an excessive amount of Dy leads to the reduction of the remaining magnetic flux density Br. Accordingly, the ratio of Dy to Nd should be 0.03:0.07 to 0.4:0.6 in atomic ratios. The preferred atomic ratio of Dy is 0.05 to 0.25.

The permanent magnet of the present invention can be manufactured by a powder metallurgy method, a rapid quenching method or by resin bonding.

(1) Powder metallurgy processes

A magnet alloy is obtained by arc melting or high frequency melting. The purity of raw materials can be 90% or more for R, 95% or more for Fe, 95% or more for Co, 90% or more for B, 95% or more for Ga and 95% or more for M (Nb, W, V, Ta, Mo). A starting material for B may be ferroboron and a starting material for Ga may be ferrogallium. Furthermore, a starting material for M (Nb, W, V, Ta, Mo) may be ferroniobium, ferrotungsten, ferrovanadium, ferrotantal or ferromolybdenum. Since the ferroboron and ferrogallium contain inevitable impurities such as Al and Si, a high coercive force can be achieved by synergistic effect of such elements as Ga, Al and Si.

Pulverization may consist of the steps of pulverization and grinding. Pulverization may be carried out by a stamp mill, jaw crusher, brown mill, disc mill, etc., and grinding may be carried out by a jet mill, vibratory mill, ball mill, etc. In any case, pulverization is preferably carried out in a non-oxidizing atmosphere to avoid oxidation of the alloy. The final particle size is desirably 2-5 µm (FSSS).

The resulting fine powders are pressed through a matrix in a magnetic field. This is essential to impart anisotropy to the alloy, namely that the magnetic powders to be pressed have their C-axes aligned in the same direction. Sintering is carried out in an inert gas such as Ar, He, etc., or in vacuum or in hydrogen at 1050ºC to 1150ºC. Heat treatment is carried out on the sintered magnetic alloy at 400ºC to 1100ºC.

(2) Rapid deterrence

A magnet alloy is prepared in the same way as in the powder metallurgy process (1). A melt of the resulting alloy is rapidly quenched by a single-roll or double-roll quenching apparatus. That is, the alloy melted by, for example, high frequency through a nozzle onto a roller rotating at high speed, thereby rapidly quenching it. The resulting flaky products are heat treated at 500 to 800ºC. Materials provided by this rapid quenching process can be used for three types of permanent magnets.

(a) The resulting flaky products are pulverized to a particle size of 10-500 µm by a disk mill, etc. The powders are mixed with, for example, an epoxy resin for matrix molding or a nylon resin for injection molding. In order to improve the adhesion of the alloy powders to the resins, suitable coupling agents may be used for the alloy powders before mixing. The resulting magnets are isotropic.

(b) The flaked products are pressed through a hot press or a hot isostatic press (HIP) to provide bulky isotropic magnets. The magnets thus prepared are isotropic.

(c) The bulky isotropic magnets obtained in (b) above are made flat by compression deformation. This plastic deformation gives the magnets anisotropy, namely that their C-axes are aligned in the same direction. The magnets thus prepared are anisotropic.

(3) Resin bonding

The starting material may be an R-Fe-Co-B-Ga alloy obtained in (1), sintered bodies obtained by pulverizing and sintering the above alloy, rapidly quenched flakes obtained in (2), or bulk products obtained by hot pressing or compression molding the flakes. These bulk products are pulverized to a particle size of 30-500 µm by a jaw crusher, a brown mill, a disk mill, etc. The resulting fine powders are mixed with resins and Matrix molding or injection molding. Applying a magnetic field during the molding operation produces anisotropic magnets in which the C-axes are aligned in the same direction.

The present invention is described in more detail in the following examples.

In the examples, the starting materials used were 99.9% pure Nd, 99.9% pure Fe, 99.9% pure Co, 99.5% pure B, 99.9999% pure Ga, 99.9% pure Nb and 99.9% pure W, and all other elements used had a purity of 99.9% or more.

example 1

Various alloys represented by the composition Nd(Fe0.70Co0.2B0.07M0.03)6.5 (M=B, Al, Si, P, Ti, V, Cr, Mn, Ni, Cu, Ga, Ge, Zr, Nb, Mo, Ag, In, Sb, W) were prepared by arc melting. The resulting ingots were coarsely pulverized by a stamp mill and a disk mill, and after sieving to a size of less than 32 meshes, ground by a jet mill. The pulverizing medium was N2 gas, and fine powders of a particle size of 3.5 µm (FSSS) were obtained. The resulting powders were pressed in a magnetic field of 1200 kA/m, the direction of which was perpendicular to the pressing direction. The pressing pressure was 2 kbar. The resulting green bodies were sintered in a vacuum at 1090 °C for two hours. Heat treatment was carried out at 500 to 900ºC for one hour, followed by quenching. The results are shown in Table 1. Table 1 Magnetic properties of a Nd(Fe0.7Co0.2B0.07M0.23)6.5 magnet Tc: Curie temperature *: almost 0

Among 19 elements "M" studied, only Ga had an iHc exceeding 800 kA/m. This shows that Ga is extremely effective for improving the coercive force. Incidentally, although the coercive force is also increased by the addition of Al, in this case the coercive force is only 680 kA/m.

Example 2

Pulverization, grinding, sintering and heat treatment were carried out in the same manner as in Example 1 on alloys having the following compositions:

Nd(Fe0.9-xCoxB0.07Ga0.03)5.8 (x=0; 0.05; 0.1; 0.15; 0.2; 0.25);

Nd(Fe0.93-xCoxB0.07)5.8 (x=0; 0.05; 0.1; 0.15; 0.2; 0.25); and

Nd0.9Dy0.1(Fe0.93-xCOxB0.07)5.8 (x=0; 0.05; 0.1; 0.15; 0.2; 0.25).

The obtained magnets were measured for their magnetic properties. The results are shown in Tables 2, 3 and 4. Table 2 Magnetic properties of Nd(Fe0.9-xCoxB0.07Ga0.03)5.8 magnets Magnetic properties Table 3 Magnetic properties of Nd(Fe0.93-xCoxB0.07)6.5 magnets Magnetic properties Table 4 Magnetic properties of Nd0.9Dy0.1(Fe0.93-xCoxB0.07)5.8 magnets Magnetic properties

And the samples in which the amount of Co was 0 and 0.2, respectively, were heated at different temperatures for 30 minutes and then measured for the change in open fluxes (irreversible flux loss) to gain insight into their thermal stability. The samples tested were those that were processed to have a permeance coefficient (Pc) of -2. The samples were magnetized at a magnetic field strength of 2000 kA/m and their magnetic fluxes were measured for the first time at 25ºC. The samples were heated to 80ºC and then cooled to 25ºC to measure the magnetic fluxes again. Thus, the irreversible flux loss at 80ºC was determined. By raising the heating temperature to 200ºC in steps of 20ºC, the irreversible flux loss at each temperature was obtained in the same way. The results are shown in Fig. 1 and 2. It can be clearly seen that the addition of Ga increases the coercive force of the magnets, thereby extremely improving their thermal stability.

Example 3

Pulverization, grinding, sintering and heat treatment were carried out in the same manner as in Example 1 on magnet alloys having the following compositions:

Nd(Fe0.7Co0.2B0.08Ga0.02)A (A=5.6; 5.8; 6.0; 6.2; 6.4; 6.6), and

Nd(Fe0.92B0.08)A (A=5.6; 5.8; 6.0; 6.2; 6.4; 6.6).

The thus prepared magnets were measured for their magnetic properties. The results are shown in Tables 5 and 6. Table 5 Magnetic properties of Nd(Fe0.7Co0.2B0.08Ga0.02)A magnets Magnetic properties Table 6 Magnetic properties of Nd(Fe0.92B0.08)A magnets Magnetic properties

For the ternary Nd-Fe-B alloy, iHc, (BH) max was almost 0 when A=6.2 or more. However, the addition of both Co and Ga resulted in a high coercive force when A=6.6, thereby providing high magnetic properties. This could be theoretically explained by the fact that in the ternary Nd-Fe-B alloy, when A=6.2 or more, a Nd-rich phase serving as a liquid phase in the process of sintering is reduced by the oxidation of Nd, so that a high coercive force cannot be obtained. On the other hand, when both Co and Ga are added, Ga acts as a liquid phase instead of Nd which would otherwise be oxidized, thus providing a high coercive force.

Example 4

Alloys of compositions Nd(Fe0.82Co0.1B0.07Ga0.01)6.5 and Nd(Fe0.93B0.07)6.5 were prepared by arc melting. The resulting alloys were flash quenched from their melts by a single roll process. The resulting flaked materials were heat treated at 700°C for one hour. The samples thus prepared were pulverized to approximately 100 µm by a disk mill. The obtained coarse powders of each composition were separated into two groups; (a) one was mixed with an epoxy resin and molded by die casting, and (b) the other was hot pressed. The magnetic properties of each of the resulting magnets are shown in Table 7. Table 7 Magnetic properties of magnets prepared by the rapid quenching process Magnetic properties Irreversible flux loss¹

Note: ¹Irreversible flux loss after heating at 100ºC for 0.5 hours

(a) glued magnet

(b) hot-pressed magnet

As can be seen from the above data, when both Co and Ga were added, iHc took values up to 1600 kA/m or more, thus providing magnets with good thermal stability.

Example 5

An alloy with the composition Nd(Fe0.82Co0.1B0.07Ga0.01)5.4 was prepared by arc melting. The resulting alloy was flash quenched from its melt by a single roll process. The sample was compressed by HIP and flattened by compression deformation. The resulting magnet had the following magnetic properties: 4ΠIr=1.18 T, iHc=1035 kA/m and (BH)max=257 kJ/m³

Example 6

Alloys having the compositions Nd(Fe0.82Co0.1B0.07Ga0.01)5.6 and Nd(Fe0.92B0.08)5.6 were prepared by arc melting. The resulting alloys were further processed in two ways: (a) one was pulverized to 50 µm or less, and (b) the other was rapidly quenched from its melt by a single roll process, and the resulting flaky product was subjected to a hot isotropic pressing (HIP) process and flattened by compression molding and then pulverized to 50 µm or less. These powders were mixed with an epoxy resin and formed into magnets in a magnetic field. The resulting magnets had the magnetic properties shown in Table 8. It should be noted that the ternary Nd-Fe-B alloy had an extremely low coercivity, while the magnet containing both Co and Ga had a sufficient coercivity. Table 8 Magnetic properties of bonded magnets Magnetic properties

Remark:

(a) Ingot → Pulverization → Mixing with resin

(b) Ingot → Rapid quenching → HIP → Compression forming → Pulverization → Mixing with resin

Example 7 (of the invention)

An alloy having the composition (Nd0.8Dy0.2)(Fe0.835Co0.06B0.08Nb0.015Ga0.01)5.5 was formed into an ingot by high frequency melting. The resulting alloy ingot was roughly pulverized by a stamp mill and a disk mill and then finely pulverized in a nitrogen gas as a pulverizing medium to provide fine powders with a particle size of 3.5 µm (FSSS). The fine powders were pressed in a magnetic field of 1200 kA/m perpendicular to the compression direction. The compression pressure was 2000 kbar. The resulting green bodies were sintered at 1100 ºC for 2 hours in vacuum and then cooled in a furnace to room temperature. A number of the resulting sintered alloys were heated at 900ºC for 2 hours and then slowly cooled to room temperature at 1.5ºC/min.

After cooling, annealing was carried out at various temperatures between 540ºC and 640ºC. Magnetic properties were measured on the heat-treated magnets. The results are shown in Table 9. Table 9 Healing temp.

After demagnetization of these magnets, they were processed to obtain a permeance coefficient Pc=-2, and magnetized again at 2000 kA/m. They were further heated between 180ºC and 280ºC every 20ºC for one hour. The irreversible flux loss at each heating temperature was measured. The results are shown in Table 10. Table 10 Irreversible flux loss (%, Pc=-2) Healing temp.

In Table 10, it is shown that the irreversible flux loss is 5% or less even when heated at 260ºC, which means that the magnets have good thermal stability.

For comparison purposes, an alloy of (Nd0.8Dy0.86)(Fe0.86Co0.06B0.08)5.5 was prepared in the same manner as above. The annealing temperature was 600ºC. The magnetic properties of the resulting magnets were as follows: Br of nearly 1.12 T, bHc of nearly 852 kA/m, iHc of nearly 1910 kA/m and (BH)max of nearly 273 kJ/m3. The irreversible flux loss by heating was 1.0% for 180ºC heating, 1.8% for 200ºC heating, 5.7% for 220ºC heating and 23.0% for 240ºC heating when Pc=-2.

Thus, it is clear that the addition of both Nb and Ga increases the heat resistance by about 40ºC.

Example 8

Three kinds of alloys represented by the formulas: (Nd0.8Dy0.2)(Fe0.92-xCoxB0.08)5.5, where x=0.6-0.12, (Nd0.8Dy0.2)(Fe0.905-xCoxB0.08Nb0.015)5.5, where x=0.06-0.12, and according to the invention (Nd0.8Dy0.22t)(Fe0.895-xCoxB0.08Nb0.015Ga0.01)5.5, where x=0.06-0.12, were melted, pulverized and molded in the same manner as in Example 7.

Each of the resulting green bodies was sintered in vacuum at 1090ºC for 1 hour and then heat treated at 900ºC for 2 hours and then cooled to room temperature at a rate of 1ºC per minute. It was reheated for annealing in an Ar gas stream at 600ºC for 1 hour and flash cooled in water. Magnetic properties were measured on each sample. The results are shown in Tables 11(a)-(c). Table 11(a) (Nd0.8Dy0.2) (Fe0.92-xCoxB0.08)5.5 Table 11(b) (Nd0.8Dy0.2) (Fe0.905-xCoxB0.08Nb0.015)5.5 Table 11(c) (Nd0.8Dy0.2) (Fe0.895-xCoxB0.08Nb0.015Ga0.01)5.5

The irreversible flux loss due to heating is also shown in Tables 12(a)-(c). In each of these three types of alloys, the increase in Co content leads to the decrease in iHc without significantly changing (BH) max. The irreversible flux loss becomes larger with the increase in Co content. When the Co content is 0.06, the highest heat resistance can be obtained. The comparison of these three types of alloys shows that those containing both Ga and Nb have the highest heat resistance. Table 12(a) (Nd0.8Dy0.2) (Fe0.92-xCoxB0.08)5.5 Irreversible flux loss (%, Pc=-2) Table 12(b) (Nd0.8Dy0.2) (Fe0.905-xCoxB0.08Nb0.015)5.5 Irreversible flux loss (%, Pc=-2) Table 12(c) (according to the invention) (Nd0.8Dy0.2) (Fe0.895-xCoxB0.08Nb0.015Ga0.01)5.5 Irreversible flux loss (%, Pc=-2)

Example 9

Various alloys represented by the formula: (Nd0.8Dy0.2)(Fe0.86-uCo0.06B0.08Nbu)5.5, where u=0-0.05, were melted, pulverized and molded in the same manner as in Example 7. The resulting green bodies were sintered at 1080°C for 2 hours in vacuum. The resulting sintered bodies were again heated at 900°C for 2 hours and cooled to room temperature at a cooling rate of 2°C/min. They were further heated for annealing in an Ar stream at 600°C for 0.5 hour and flash cooled in water. Magnetic properties were measured on each sample. The results are shown in Table 13. Table 13 (Nd0.8Dy0.2) (Fe0.86-uCo0.06B0.08Nbu)5.5

It is seen that the addition of Nb decreases Br and (BH)max while increasing iHc. As shown in Table 14, the irreversible flux loss by heating at 220ºC decreases with the increase of iHc.

Table 14 (Nd0.8Dy0.2) (Fe0.86-uCo0.06B0.08Nbu)5.5 u Irreversible loss of flow due to heating at 200ºC

0 10.1

0.003 8.7

0.006 6.3

0.009 5.0

0.012 4.6

0.015 3.1

0.020 2.5

0.030 2.0

0.040 1.8

0.050 1.5

Example 10

Alloys having the formula: (Nd0.8Dy0.2)(Fe0.86-zCo0.06B0.08Gazu)5.5 where z=0-0.15 were melted, pulverized and molded in the same manner as in Example 7. After sintering, each of them was heated at 900ºC for 2 hours and cooled to room temperature at 1.5ºC/min. It was annealed at 580ºC for 1 hour in an Ar gas stream and flash quenched in water. The magnetic properties of the resulting magnets are shown in Table 15, and their irreversible flux losses by heating at 220ºC are shown in Table 16. Table 15 (Nd0.8Dy0.2) (Fe0.86-zCo0.06B0.08Gau)5.5 Table 16 (Nd0.8Dy0.2) (Fe0.86-zCo0.06B0.08Gazt)5.5

z Irreversible flux loss due to heating at 220ºC (%, Pc=-2)

0 10.1

0.002 7.5

0.01 2.7

0.03 0.7

0.07 0.5

0.10 0.3

0.12 0.1

0.15 0.1

It is found that the addition of Ga Br and (BH) max greatly decreases while it greatly increases iHc, thereby improving the heat resistance (thermal stability) of the magnets.

Example 11 (according to the invention)

Alloys having the formula (Nd0.9Dy0.1)(Fe0.845-zCo0.06B0.08Nb0.015Gaz)5.5, where z=0-0.06, were melted, pulverized and molded in the same manner as in Example 10. The measured magnetic properties are shown in Table 17, and the irreversible flux losses measured by heating at 220°C are shown in Table 18. Table 17 (Nd0.9Dy0.1) (Fe0.845-zCo0.06B0.08Nb0.015Gaz)5.5

Table 18 (Nd0.9Dy0.1) (Fe0.845-zCo0.06B0.08Nb0.015Gaz)5.5 z Irreversible loss of flow due to heating at 220ºC

0 38.1

0.01 20.3

0.02 4.5

0.03 1.8

0.04 1.2

0.05 0.7

It is shown that even with a small amount of Dy substituted for Nd, the addition of Ga serves to improve the thermal stability of the magnets.

Example 12

Alloys represented by the compositions Nd(Fe0.86Co0.06B0.08)5.5, Nd(Fe0.84Co0.06B0.08Ga0.02)5.5, and Nd(Fe0.825Co0.06B0.08Ga0.02W0.015)5.6 were prepared by arc melting according to the present invention. The resulting ingots were coarsely pulverized by a stamp mill and a disk mill, and after sieving to a size of less than 32 meshes, they were ground by a jet mill. N2 gas was used as a pulverizing medium, and fine powders with a particle size of 3.5 µm (FSSS) were obtained. The resulting powders were molded in a magnetic field of 1200 kA/m, the direction of which was perpendicular to the pressing direction. The pressing pressure was 2000 kbar. The resulting green bodies were sintered in vacuum at 1080ºC for 2 hours. Heat treatment was carried out at 500-900ºC for one hour, followed by quenching. The results are shown in Table 19. Table 19 Magnetic properties of Nd-Fe-Co-B-Ga-W magnets Composition

Each sample was heated at different temperatures for 30 minutes and then measured for the change in open flows to find out their thermal stability. The samples tested were those that processed to have a permeance coefficient (Pc) of -2. The results are shown in Fig. 3. It can be seen from Fig. 3 that the addition of Co, Ga and W in combination gives the magnets high thermal stability.

Example 13 (according to the invention)

Pulverization, grinding, sintering and heat treatment were carried out in the same manner as in Example 12 on alloys with the composition:

Nd(Fe0.85-zCo0.06B0.08GazW0.01)5.4 (z=0; 0.01; 0.02; 0.03; 0.04; 0.05) was performed.

The magnetic properties of the resulting magnets are shown in Table 20. Table 20 Magnetic properties of Nd (Fe0.85-zCo0.06B0.08-zGazW0.01)5.4

Magnets

The thermal stabilities of the samples of Nd(Fe0.85-zCo0.06B0.08GazW0.01)5.4 (z=0; 0.02; 0.04) were measured in the same manner as in Example 12. The results are shown in Fig. 4.

Example 14 (according to the invention)

An alloy of composition Nd(Fe0.825Co0.06B0.08Ga0.02W0.015)6.0 was prepared by arc welding. The resulting alloy was rapidly quenched from its melt by a single roll process. The resulting flaky products were converted to bulk ones by the following three processes:

(a) Heat treatment at 500-700ºC, mixing with an epoxy resin and pressure casting molds,

(b) heat treatment at 500-700ºC and hot pressing, (c) hot isostatic pressing and flattening by compression molding.

The magnetic properties of the resulting magnets are shown in Table 21. Table 21 Magnetic properties of Nd (Fe0.825Co0.06B0.08Ga0.02W0.015)6.0 magnets

Each sample was measured for thermal stability in the same manner as in Example 12. The results are shown in Fig. 5.

Example 15 (according to the invention)

An alloy with the composition Nd (Fe0.85Co0.04B0.08Ga0.02W0.01)6.1 was prepared by arc melting. The resulting alloy was rapidly quenched from its melt by a single roll process. The resulting prepared sample was compressed by HIP and flattened by compression molding. This bulky sample was pulverized to less than 80 µm, mixed with an epoxy resin and molded in a magnetic field. The resulting magnet had the following magnetic properties: 4ΠIr=0.86 T, iHc=1051 kA/m and (BH)max=127 kJ/m³.

Example 16

Alloys with compositions represented by the formulas Nd1-αDyα(Fe0.72Co0.2B0.08)5.6 (α=0; 0.04; 0.08; 0.12; 0.16; 0.2), Nd(Fe0.72-zCo0.2B0.08Alz)5.6 (z=0; 0.01; 0.02; 0.03; 0.04; 0.05) and Nd(Fe0.72-zCo0.2B0.08Gaz)5.6 (z=0; 0.01; 0.02; 0.03; 0.04; 0.05) were prepared by arc melting. The resulting ingots were roughly pulverized by a stamp mill and a disk mill and, after screening to a size of less than 32 meshes, ground by a jet mill. N2 gas was used as a pulverizing medium and fine powders of a particle size of 3.5 µm (FSSS) were obtained. The resulting powders were molded in a magnetic field of 1200 kA/m whose direction was perpendicular to the pressing direction. The pressing pressure was 1500 kbar. The resulting green bodies were sintered in vacuum at 1040°C for 2 hours. A heat treatment was carried out at 600 to 700°C for 1 hour, followed by quenching. The results are shown in Fig. 6. The magnets containing Ga had a higher coercivity and a smaller decrease in 4�Pi;Ir and (BH) max than those containing Dy or Al.

The magnets with the compositions Nd(Fe0,72Co0,2B0,08)5,8, Nd0,8Dy0,2(Fe0,72Co0,2B0,08)5,6, Nd(Fe0,67Co0,2B0,08Al0,05)5,6 and Nd(Fe0,67Co0,2B0,08Ga0,05)5,6 were processed to obtain a shape with a permeance coefficient Pc=-2, magnetized and different temperatures for 30 minutes and then measured for the change in open fluxes to find their thermal stabilities. The results are shown in Fig. 7. It is shown that the variation of irreversible flux loss with temperature depends on the coercivity and that the addition of Ga gives the magnets a good thermal stability of about 5% or less irreversible flux loss at 160ºC.

Example 17

From the magnets prepared in Example 16

(a) Nd(Fe0.72Co0.02B0.08)5.6

(b) Nd0.8Dy0.2(Fe0.72Co0.2B0.08)5.6

(c) Nd(Fe0.67Co0.2B0.08Al0.05)5.6 and

(d) Nd(Fe0.67Co0.2B0.08Ga0.05)5.6 were taken as small pieces of a few millimeters on a side, magnetized and measured for the variations of their magnetic fluxes as a function of temperatures by a vibration magnetometer. The measurement was carried out without a magnetic field. The results are shown in Fig. 8. The variation of the magnetic flux as a function of temperature has two inflection points; one on the low temperature side corresponding to the Curie temperature of the BCC phase and the other on the higher temperature side corresponding to the Curie temperature of the main phase. The magnets with Ga have lower Curie temperatures in their main phases than those containing no additive. On the other hand, the former has a higher Curie temperature of the BCC phase than the latter. However, the addition of Al greatly reduces the Curie temperatures of the main phase and the BCC phase, resulting in undesirable thermal stability.

Example 18

Pulverization, grinding, sintering and heat treatment were carried out in the same manner as in Example 16 on alloys having the following compositions: Nd(Fe0.67Co0.25B0.08)5.6, Nd(Fe0.65Co0.25B0.08Ga0.02)5.6 and Nd(Fe0.635Co0.25B0.08Ga0.02W0.015)5.6 (according to the invention)

The sintering temperatures were 1020ºC, 1040ºC, 1060ºC and 1080ºC, respectively, and the magnetic properties were measured. The results are shown in Fig. 9(b)-(c). Fig. 9(a) shows the comparison of the demagnetization curve of the above magnets, which are summarily expressed by the following formula:

Nd(Fe0.67-zCo025B0.08GazWu)5.6, where z=0 or 0.02 and u=0 or 0.015. As shown in Fig. 9(b) and (c), where W is not included, the higher the sintering temperature, the worse the squareness of the resulting magnet, resulting in the growth of coarse crystal grains with low coercivity. On the other hand, as shown in Fig. 9(d), when W is added, the higher sintering temperature does not result in the growth of coarse crystal grains, resulting in good squareness. Fig. 9(a) shows that the inclusion of Ga and W increases the coercivity of the magnet.

Example 19 (according to the invention)

Alloys with the compositions Nd(Fe0.69Co0.2B0.08Ga0.02M0.01)5.6, where M can be V, Nb, Ta, Mo or W, were pulverized, ground, sintered and heat treated in the same manner as in Example 16. The magnetic properties of the resulting magnets are shown in Table 22. Table 22 Magnetic properties of Nd (Fe0.69Co0.2B0.08Ga0.02M0.01)5.6 (M: V, Nb, Ta, Mo, W) Composition

Example 20

Alloys having the composition (Nd0.8Dy0.2)(Fe0.85-uCo0.06B0.08Ga0.01Mou)5.5, where u=0-0.03, were pulverized, ground, sintered and heat treated in the same manner as in Example 16. The resulting magnets were measured for magnetic properties and irreversible flux loss by heating at 260°C (Pc=-2). The results are shown in Table 23. Table 23 (Nd0.8Dy0.2) (Fe0.85-uCo0.06B0.08Ga0.01Mou)5.5

Note: ¹) Irreversible flow loss

Example 21 (according to the invention)

Alloys of the composition Nd(Fe0.855-uCo0.06B0.075Ga0.01Vu)5.5 where u=0-0.02 were pulverized, ground, sintered and heat treated. The resulting magnets were measured for magnetic properties and irreversible flux loss by heating at 160ºC (Pc=-2). The results are shown in Table 24. Table 24 Nd (Fe0.855-uCo0.06B0.075Ga0.01Vu)5.5

Note:¹) Irreversible flow loss

Example 22 (according to the invention)

Alloys of the composition (Nd0.9Dy0.1)(Fe0.85-uCo0.06B0.08Ga0.01Tau)5.5, where u=0-0.03, were pulverized, ground, sintered and heat treated in the same manner as in Example 16. The resulting magnets were measured for magnetic properties and irreversible flux loss by heating at 160°C (Pc=-2). The results are shown in Table 25. Table 25 (Nd0.9Dy0.1) (Fe0.85-uCo0.06B0.08Ga0.01Tau)5.5

Note:¹) Irreversible flow loss

As described in the above examples, the addition of Ga or Co and Ga together in Nd-Fe-B magnets increases the Curie temperature and coercivity of the magnets, thereby providing magnets with better thermal stability. The addition of M (one or more of Nb, W, V, Ta, Mo) together with Co and Ga in Nd-Fe-B magnets further increases the Curie temperature and coercivity.

Claims (5)

1. Permanent magnet with good thermal stability, which essentially consists of the general formula R (Fe1-x-y-z-uCoxByGazMu)A where R is only Nd or one or more of the rare earths, mainly Nd, Pr or Ce, which may be partially substituted by Dy, Tb or Ho; M is one or more of the elements Nb, W, V, Ta and Mo; 0 ≤ x ≤ 0.7; 0.02 ≤ y ≤ 0.3; 0.001 ≤ z ≤ 0.15; 0.001 ≤ u ≤ 0.1 and 4.0 ≤ A ≤ 7.5. 2. Permanent magnet with good thermal stability according to claim 1, wherein 0.01 ≤ x ≤ 0.4; 0.03 ≤ y ≤ 0.2; 0.002 ≤ z ≤ 0.1; 0.002 ≤ u ≤ 0.04 and 4.5 ≤ A ≤ 7.0. 3. Permanent magnet with good thermal stability according to claim 1, wherein R is mainly composed of Nd and Dy and the atomic ratio of Nd to Dy is between 0.97:0.03 and 0.6:0.4. 4. Permanent magnet with good thermal stability according to claim 3, wherein 0.01 ≤ x ≤ 0.4; 0.03 ≤ y ≤ 0.2; 0.002 ≤ z ≤ 0.1; 0.002 ≤ u ≤ 0.04 and 4.5 ≤ A ≤ 7.0. 5. Permanent magnet with good thermal stability according to any of claims 1 to 4, wherein M is Nb.