PERMANENT MAGNET

Abstract: A permanent magnet according to the present invention is characterized in that it is composed of an alloy comprising mainly of iron, and R (at least one element from rare earth element and yttrium), cobalt, and boron as the remainder, wherein the alloy is formed principally of ferromagnetic Fe-rich phase of tetragonal system and includes a nonmagnetic laves phase. Compared with the prior rare earth-Fe based magnet, it has higher Curie temperature and has excellent magnetic characteristics, especially the temperature characteristics.

Description: BACKGROUND OF THE INVENTION The present invention relates to a rare earth-iron-based permanent magnet which includes a rare earth element, boron, and iron as its principal constituents. In the past, a rare earth-Co-based magnet is known as a high performance manget. Since, however, the maximum energy product (BH) max of the rare earth-Co-based magnet is not large enough, being about 30 MGOe at the most, the strong demand in recent years for more compactification and higher performance in electronic apparatus, makes it desirable to develop a permanent magnet with higher performance. In response to such a demand, development has been ongoing for a permanent magnet that has iron as its principal constituent (European Patent Application 101552, USP 4402770, USP 4533408, USP 4541877, and others). The iron-based permanent magnet includes a rare-earth element (R) such as Nd, and boron (B) with the balance occupied essentially by iron (Fe). It makes use of Fe which is less expensive than Co as the principal ingredient, and is capable of producing (BH) max that can exceed 30 MGOe. Therefore, it represents an extremely promising material that can provide a high performance magnet at low cost. The drawback of the iron-based permanent magnet is that the Curie temperature (Tc) is low compared with the permanent magnet of rare earth-Co-based permanent magnet, and has an inferior temperature characteristics of the magnetic characteristics. This will become a serious problem when it is to be used for a DC brushless motor or the like that is operated under conditions such as high temperature environment, and hence an improvement on this aspect has been desired. As such an improvement, there has been proposed a composition such as R-B-Co-Al-Fe (EPA 106948). The present inventors have also an application 85US773547 under file. However, the addition of Co brings about a deterioration in the magnetic characteristics so that a demand is strong for improving the performance of the rare earth-Fe-based permanent magnet, with development efforts under way at various laboratories. EP-A-0 106 948 discloses a magnetic material which comprises Fe, B, R (rare earth elements) and Co having a major phase of Fe-Co-B-R intermetallic compound of tetragonal system. A non-magnetic intermetallic phase is also included. EP-A-197 712 discloses a permanent magnet comprising a sintered alloy composed of R (rare earth elements), B, and Fe. The magnet essentially consists of a two-phase system made up of a ferromagnetic Fe-rich phase and a non-magnetic R-rich phase. The following conversion factors are provided for reference: 1 MGOe = 7.96 kJm -3 , 1 G = 10 -4 T, and 1 Oe = 79.6 Am -1 SUMMARY OF THE INVENTION An object of the present invention is to provide a rare earth-Fe-based permanent magnet which has a high Curie temperature (Tc) and excellent magnetic characteristics. Another object of the present invention is to provide a rare earth-Fe-based permanent magnet which has small temperature dependence of magnetic characteristics. Still further object of the present invention is to provide a permanent magnet whose powdered alloy has an excellent oxidation-resistance. According to one aspect, the present invention provides a permanent magnet comprising iron, cobalt, boron and a material R which consists of yttrium and/or at least one rare earth element, wherein the weight of iron exceeds that of any other individual constituent, and the magnet is formed principally of a ferromagnetic Fe-rich phase of tetragonal system, characterised in that the magnet includes a nonmagnetic laves phase of the type RB₂, wherein B is selected from Fe, Co and Al, and that the magnet further comprises 0.1 to 5% by weight of Al. The rare earth-Fe-based permanent magnet has a ferromagnetic Fe-rich phase of tetragonal system of Nd₂ Fe 14 B type as the principle phase and nonmagnetic Laves phase of Nd(Fe, Co, Al)₂ type. Besides, it may include a nonmagnetic R-rich phase of a cubic system such as Nd 97 Fe or Nd 95 Fe₅ that has more than 90% by weight of the R component, a nonmagnetic B-rich phase of a tetragonal system such as Nd 1+ε Fe₄ B₄, and others as the constituent phases, in addition to including some oxides. The composition is similar when an R component other than Nd is used. The addition of Co is effective in raising the Curie temperature, but it has also a disadvantage of lowering the coercive force. This is due to creation of a magnetic Laves phase. The magnetic Laves phase is considered responsible for lowering the coercive force by providing the sites of nuclei for generating the reversed magnetic domains. In the present invention, the coercive force is improved as a result of converting substantially all of the Laves phase to a nonmagnetic state. Consequently, it becomes possible to obtain satisfactory magnetic characteristics while optimizing the effect of rise in Curie temperature due to addition of Co. Further, the rare earth-Fe-based permanent magnet obtained in this manner is found to also possess satisfactory temperature characteristics of magnetic characteristics. It is preferable that the nonmagnetic Laves phase account for about 2 to 10% of the alloy by volume. If the content is too high, the percentage of the principal phase which produce the magnetic properties is decreased, and the value of Br (residual magnetic flux density) is lowered. On the other hand, if too little Laves phase is provided, then the amount of added Co decreases, hampering the full effect due to the rise in Curie temperature from being realized. For the purpose of the present invention, all phase other the principle phase and Laves phase are not essential to magnetic properties. However, the R-rich phase has a lower melting point compared to the principal phase, and contributes to the enhancement of coercive force and other magnetic properties by removing defects, foreign substances, and the like from the boundaries of the principal phase during the sintering, and by reducing the site of nuclei for generating the reversed magnetic domains. However, if the content is too much, the percentage of the principal phase is decreased and the magnetic characteristics are deteriorated. For this reason, the content should be less than 5% by volume, preferably in the range of about 2.5 to 5% by volume. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows X-ray diffraction diagrams of permanent magnets, Fig. 2 is a characteristic diagram for showing the relationship between the amount of B and the magnetic characteristics, and Fig. 3 is a characteristic diagram for showing the relationship between the number of days from pulverization to sintering and the coercive force. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT In the following the present invention will be described in detail. Appearance of the nonmagnetic Laves phase can be realized by adding and including a specific amount of Al to a specific composition, for example, an R-B-Co-Fe system. Example of this are shown in Fig. 1. Figure 1 (a) is an X-ray diffraction diagram when no Co is added, Fig. 1 (b) is for the case when Co alone is added, and Fig. 1 (c) is for the case when Al is added in addition. For all of these cases, the principal phase is the Fe-rich phase. However, in the case of addition of Co, there may be observed peaks that indicate the presence of a different phase in the neighborhood of the diffraction angle 2ϑ of 34° and 40°. By identifying the peaks by EPMA, TEM it was found that they are (220) and (311) peaks from the Nd(Fe,Co)₂ cubic Laves phase of MgCu₂ type. Considering the fact that the ratio of Fe to Co in the Laves phase is about 1 : 1, it will be seen that the Curie temperature is around 100°C and that the product possesses magnetic properties at room temperature. Moreover, in view of the fact that the coercive force of the rare earth-Fe-based permanent magnet is determined by the magnitude of the magnetic field that generates reversed magnetic domains, the magnetic Laves phase believe to be acting as the sites for generating reversed magnetic domains. In contrast to this, it is clear from Fig. 1 (c) that in the present invention there exist peaks at 2ϑ of 34° and 40°, and there are slight shifts of the peaks toward lower angles compared to the case of Fig. 1 (b). This indicates that the lattice constant of the Nd(Fe, Co)₂ phase is being increased. More specifically, it is increased from 7.38 Å to 7.42 Å. That the atomic radius of Al is large compared with l.26 Å for Fe and 1.25 Å for Co indicates that Al atoms exist in the Laves phase. Since Al atom is of nonmagnetic, Nd(Fe, Co, Al)₂ will become to have a nonmagnetic phase, and since this nonmagnetic Laves phase will not become the sites of nuclei for generating the reversed magnetic domains, the coercive force will be improved as a result. It is to be noted that in place of Al such nonmagnetic elements as Re, Os, Ag, Ir, Pt, Au, Ti, V, Cu, Zn, Cr, Mn, Ga, Mo, Ru, Rh, Pd and Ta may also be added. The total amount should be less than 5% by weight. However, when the magnetic characteristics are taken into consideration, more effective element will be Al and Ga. Aluminum is an element which is most effective to cause the Curie temperature of the Laves phase to drop, make the system nonmagnetic at room temperature, and improve the coercive force, and its addition in the range of 0.1 to 5% by weight is effective. An amount of Al will be changed with an amount of Co. It's needed that the amount of Al satisfy a relation (W Co - 9)/W Al ≦ 30 (W Co ; the amount of Co by weight, W Al ; the amount of Al by weight). When, insted of Al, Ga is used, the same relation is needed. Here, up to 80% of Al may be replaced by Ga. When Al is included in the Fe-rich phase, it increases the coercive force significantly so that magnetic characteristics such as (BH) max and the temperature characteristics will be improved. Although a detailed mechanism for this is not yet clear, it is guessed that the grain boundaries of the Fe-rich phase are cleansed by the mixing of Al. It should be noted that even when the total amount of Al is maintained the same, if Al is concentrated in phases other than the Fe-rich phase, the above-mentioned phenomenon will not occur. It is desirable to have at least more than 70%, and more than 80%, of Al be included in the Fe-rich phase. Although the composition for the permanent magnet of the present invention may appropriately be set, it is desirable to employ a compositional system of R of 10 to 40% by weight, B of 0.1 to 8% by weight, Co of 1 to 30% by weight, and essentially Fe for the balance. When the R component is below 10% by weight, the coercive force is small, and when it exceeds 40% by weight, Br is reduced and (BH) max is diminished. A content in the range of 25 to 35% by weight is more desirable. Moreover, of the rare earth elements, Nd and Pr are effective for obtaining high values for (BH) max . To include at least one of the two elements, especially Nd, as the R component is desirable. The ratio of the two elements in the R component is preferred to be more than 70% by weight. Cobalt contributes to increase the Curie temperature, effective for improving the temperature characteristics of the magnetic characteristics, and its addition of 9 to 30% by weight is effective. Although it is necessary to add Co to certain extent in order to obtain the full effect of rise in the Curie temperature, it is not advisable to exceed 30% by weight in view of the magnetic characteristics that will result in decreases in the coercive force and (BH) max . An addition of 23% or less by weight is preferred. It is desired to add as much amount of Co that does not deteriorate the magnetic characteristics, so that an addition of more than 9% by weight, in particular more than 13% by weight, is preferred. When the amount of B is less than 0.1% by weight, iHc falls off, and when it exceeds 8% by weight, Br and (BH) max are decreased. The amount of B affects the magnetic characteristics, especially conspicuously the value of Br and (BH) max , so that its amount of 1.25% or less by weight, in particular 0.8 to 0.95% by weight, in more particular 0.8 to 0.9% by weight, is preferred. As the amount of B is increased, the amount of the nonmagnetic B-rich phase will be increased also, which will result in reducing the amount of the principal phase and deteriorating the magnetic characteristics. Further, although a part of B may be substituted by C, N, Si, P, Ge, and others to improve the sintering characteristics and the like, the substituted amount should be up to 80% by weight of the amount of B. If Co, B and Al(Ga) is falling in the above range, a magnet has an excellent oxidation-resistance. The content of oxygen in the permanent magnet alloy has an important significance. Since large amount of oxygen leads to a decrease in the coercive force, it becomes impossible to obtain a large value of (BH) max . Therefore, it is preferred to includeless than 0.03% by weight. Moreover, if the content is too small, pulverization of the raw material alloy becomes difficult, resulting in a sharp increase in the cost of manufacturing. Fine pulverization which is required to be done to a fine particle size of about 2 to 10 µ m, becomes difficult to be accomplished, and moreover, there will arise a nonuniformity in the particle diameter. Accordingly, it leads to a decrease in the value of Br that accompanies a reduction in the orientability during the formation in a magnetic field, which eventually leads also to a lowering in the value of (BH) max . For these reasons, it is desirable to choose the content of oxygen in the range of 0.005 to 0.03% by weight. Although the role of oxygen in the alloy is not elucidated yet, it may be considered that a high performance permanent magnet is obtained by the behavior that will be described below. Namely, a part of oxygen in molten alloy is combined with R and Fe atoms that represent the major constituents, to form oxides. These oxides are considered segregated and exist, along with the remaining oxygen, in the grain boundaries and are absorbed especially by the R-rich phase to hamper the magnetic characteristics. Taking into consideration that the rare earth-Fe-based permanent magnet consists of corpuscular magents and its coercive force is determined by the magnetic field that generates reversed magnetic domains, it is considered, when there exist many defects such as oxides and segregations, that the coercive force will be decreased by the action of these defects as the generating sources of the reversed magnetic domains. On the other hand, when there are too few defects, boundary breakdown or the like are not easy to take place and it is considered that the pulverization characteristics will be deteriorated. The oxygen content in the alloy for permanent magnet can be controlled by the use of highly pure raw materials and by a strict control of the oxygen content in the molten raw material alloy in the furnance. A permanent magnet in accordance with the present invention will be manufactured, for example, as follows. First, raw material alloy with prescribed composition is crushed by a crushing means such as a ball mill. In this case, in order to facilitate the formation and sintering in the subsequent processes and to provide the product with satisfactory magnetic characteristics, it is desirable to crush it finely to powders with mean particle diameter of 2 to 10 µm. If the particle diameter is too large, it leads to a reduction in the coercive force. On the other hand, if it is too small, crushing becomes difficult and will result in a deterioration of magnetic characteristics such as Br. Next, pulverized powder of permanent magnet alloy is formed by pressing it into a desired form. During the formation, an orientation processing is performed under application of a magnetic field of, for example, 10 kOe, similar to the case of manufacturing ordinary sintered magnet. Following that, it is sintered under the conditions of 1000 to 1200°C and 0.5 to 5 hours. The sintering is preferred to be carried out in an inest gas such as Ar gas or in vacuum in order not to increase the oxygen content in the alloy. Then, an aging treatment is given under the conditions of 500 to 1000°C and 0.5 to 5 hours. These conditions may be set appropriately depending upon the composition in order to induce the appearance of the nonmagnetic Laves phase. In what follows, embodiments of the present invention will be described. Example 1 Elements that are combined in the composition of 0.5 - 1.4 wt% of B, 0.8 wt% of Al, 14.4 wt% of Co, 32.4 wt% of Nd, and balance of Fe are melted by arc in a water-cooled copper boat in an Ar atmosphere. The magnet alloy obtained (oxygen content of 0.02% by weight) is coarsely crushed in an Ar atmosphere, and is further pulverized to a grain diameter of about 3.5 µm in a jet mill. The pulverized powder was filled into a predetermined mold, and was formed under a pressure of 2 ton/cm² while applying a magnetic field of 20 kOe. After sintering the formed body in Ar atmosphere for one hour at 1020 to 1080°C and rapidly cooling to the room temperature, an aging treatment was given for one hour at 900°C, and again another aging treatment was given for one hour at 600°C, and is then cooled rapidly to the room temper­ ature. In each of the magnets obtained, there was confirmed the presence of a nonmagnetic Laves phase by X-ray diffraction. Further, that more than 90% of Al are included in the Fe-rich phase was confirmed by the composition observation by means of XMA. The relationship between the amount of B and iHc or (BH) max obtained for the manufactured magnet is shown in Fig. 2. For comparison purposes, the corresponding result for a sample with a composition of 0.5 - 1.4 wt% of B, 14.4 wt% of Co, 32.4 wt% of Nd, and the balance of Fe (Comparative Example 1) is also shown in Fig. 2. In the comparative example, there was not observed the presence of a nonmagnetic Laves phase, although the presence of a magnetic Laves phase was confirmed. As is clear from Fig. 2, both of iHc and (BH) max are improved markedly by the presence of the nonmagnetic Laves phase, and that (BH) max is especially high when the amount of B is in the range of 0.85 to 0.95% by weight. Further, an excellent result of the Curie temperature of 500°C and a temperature coefficient of -0.071%/°C was obtained for the present example. The characteristics for the magnets with various compositions manufactured in a similar manner are presented in the following Table 1. In set Table 1, permanent magnets obtained by the present invention show excellent characteristics. Example 2 The constituent phases and the magnetic characteristics for magnets with various compositions are shown in Table 2. In set Table 2, it is seen that the embodiments of the present invention that include nonmagnetic Laves phase possess excellent magnetic characteristics. Example 3 A mixture of elements in the composition of 0.9 wt% of B, 0.52 wt% of Al, 14.3 wt% of Co, 30.6 wt% of Nd, and the balance of Fe was melted by arc in a watercooled copper boat in an Ar atmosphere. The magnet alloy obtained (oxygen concentration of 0.02 wt%) was coarsely crushed in an Ar atmosphere, and was pulverized further to the grain size of about 3 µm in a jet mill. The relationship between the coercive force and the number of days from the day of pulverization to the day of sintering (kept in Ar), obtained for the magnets manufactured by similar process as in Example 1 using the powder obtained in the above, is shown in Fig. 3. For comparison, corresponding relationship for a sample with composition of 0.98 wt% of B, 32.7 wt% of Nd, and the remainder of Fe (Comparative Example 2) is also shown in Fig. 3. As may be clear from Fig. 3, in the case of Comparative Example 2, coercive force deteriorates markedly in two days after pulverization, whereas for the example of the present invention there was not observed deterioration in the coercive force. Example 4 An alloy for magnet with a composition of 34.6 wt% of Nd, 0.9 wt% of B, 14.1 wt% of Co, 0.4 wt% of Al, 0.02 wt% of oxygen, and the remainder of Fe was prepared, and a permanent magnet was manufactured using the alloy. The magnetic characteristics and the amount of Al in the Fe-rich phase as determined by XMA are shown in Table 3. In addition, a permanent magnet was manufactured using alloy powder obtained by mixing 0.4 g of Al powder and 100 g of powder of magnet alloy with composition of 34.4 wt% of Nd, 1.0 wt% of B, 14.2 wt% of Co, 0.02 wt% of oxygen, and the remainder of Fe. With this as a comparative example, the magnetic characteristics and the amount of Al in the Fe-rich phase as determined by XMA are shown in set Table 3. As may be seen from Table 3, in the case of the embodiment of the present invention in which more than 70 wt% of Al is included in the Fe-rich phase, there were obtained a large value for iHc and excellent magnetic characteristics.

Claims: 1. A permanent magnet comprising iron, cobalt, boron and a material R which consists of yttrium and/or at least one rare earth element, wherein the weight of iron exceeds that of any other individual constituent, and the magnet is formed principally of a ferromagnetic Fe-rich phase of tetragonal system, characterised in that the magnet includes a nonmagnetic laves phase of the type RB₂, wherein B is selected from Fe, Co an Al, and that the magnet further comprises 0.1 to 5% by weight of Al. 2. A permanent magnet according to claim 1, wherein the laves phase of the magnet is substantially entirely nonmagnetic. 3. An permanent magnet according to claim 1 or claim 2, wherein the laves phase is made up from 2 to 10% by volume of the alloy. 4. A permanent magnet according to any of claims 1-3 which comprises 10 to 40% by weight of R, 0.1 to 8% by weight of B, 1 to 23% by weight of Co, and the remainder consisting essentially of Fe. 5. A permanent magnet according to claim 4, comprising 0.8 to 0.95% by weight of B. 6. A permanent magnet according to any preceding claim which further comprises 0.1 to 5% by weight of Ga. 7. A permanent magnet according to any preceding claim wherein at least more than 70% by weight of Al is included in the Fe-rich phase. 8. A permanent magnet according to claim 7, wherein at least more than 80% by weight of Al is included in said Fe-rich phase. 9. A permanent magnet according to any preceding claim, having a (BH) max of 239 kJm -3 (30 MGOe) or more.

Inventors: INOMATA, KOICHIRO, MIZOGUCHI, TETSUHIKO, NIU, HIROMI, SAKAI, ISAO
Applicants: KABUSHIKI KAISHA TOSHIBA
Application year: 1986
Main classification: H01F1
IPC codes: H01F1