Method for producing NdFeB system sintered magnet

Abstract

A method for producing a NdFeB system sintered magnet. The method includes: a hydrogen pulverization process, in which coarse powder of a NdFeB system alloy is prepared by coarsely pulverizing a lump of NdFeB system alloy by making this lump occlude hydrogen; a fine pulverization process, in which fine powder is prepared by performing fine pulverization for further pulverizing the coarse powder; a filling process, in which the fine powder is put into a filling container; an orienting process, in which the fine powder in the filling container is oriented; and a sintering process, in which the fine powder after the orienting process is sintered as held in the filling container. The processes from hydrogen pulverization through orienting are performed with neither dehydrogenation heating nor evacuation each for desorbing hydrogen occluded in the hydrogen pulverization process. The processes from hydrogen pulverization through sintering are performed in an oxygen-free atmosphere.

Claims

1. A method for producing a NdFeB system sintered magnet, comprising: a) coarsely pulverizing a lump of NdFeB system alloy by making the lump occlude hydrogen via a hydrogen pulverization process, in which coarse powder of the NdFeB system alloy is prepared; b) finely pulverizing the coarse powder via a fine pulverization process, in which fine powder is prepared; c) filling a filling container with the fine powder via a filling process; d) orienting the fine powder held in the filling container via an orienting process; and e) sintering the fine powder in the filling container after the orienting process via a sintering process, and performing evacuation during the sintering process only after a predetermined temperature in a range of from 100 to 500° C. is reached such that evacuation is not performed during a beginning of the sintering process before the predetermined temperature is reached; wherein: the processes from the hydrogen pulverization process through the orienting process are performed with neither dehydrogenation heating nor evacuation each for desorbing hydrogen occluded in the hydrogen pulverization process; and the processes from the hydrogen pulverization process through the sintering process are performed in an oxygen-free atmosphere.

2. The method according to claim 1, wherein the predetermined temperature is within a range of from 100° C. to 400° C.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a flowchart showing one embodiment of the method for producing a NdFeB system sintered magnet according to the present invention.

(2) FIG. 2 is a flowchart showing a method for producing a NdFeB system sintered magnet as a comparative example.

(3) FIG. 3 is a graph showing a temperature history of the hydrogen pulverization process in the method for producing a NdFeB system sintered magnet of the present embodiment.

(4) FIG. 4 is (a) a graph showing a temperature history of the hydrogen pulverization process in the comparative example of the method for producing a NdFeB system sintered magnet, and (b) a resized version of the graph in FIG. 3 fitted to the scale of graph (a) in FIG. 4.

DESCRIPTION OF EMBODIMENTS

(5) One embodiment of the method for producing a NdFeB system sintered magnet according to the present invention is hereinafter described.

(6) As shown in FIG. 1, the method for producing a NdFeB system sintered magnet according to the present embodiment includes: a hydrogen pulverization process (Step S1), in which a piece or pieces of alloy sheets of a NdFeB system alloy prepared beforehand by strip casting is coarsely pulverized by making the alloy sheet occlude hydrogen; a fine pulverization process (Step S2), in which 0.05-0.1 wt % of methyl caprylate or similar lubricant is mixed in the coarse powder of the NdFeB system alloy prepared by hydrogen pulverization of the NdFeB system alloy sheet in the hydrogen pulverization process without the subsequent dehydrogenation heating, and in which the coarse powder is finely pulverized in a nitrogen gas stream using a jet mill so that the grain size of the alloy will be equal to or smaller than 3.2 μm in terms of the median (D.sub.50) of the grain size distribution measured by a laser diffraction method; a filling process (Step S3), in which 0.05-0.15 wt % of methyl laurate or similar lubricant is mixed in the finely pulverized powder (alloy powder) and the mixture is put in a mold (filling container) at a density of 3.0-3.5 g/cm.sup.3; an orienting process (Step S4), in which the alloy powder held in the mold is oriented in a magnetic field at room temperature; and a sintering process (Step S5), in which the oriented alloy powder in the mold is sintered.

(7) The processes of Steps S3 through S5 are performed as a press-less process. Step S1 is performed in hydrogen gas without evacuation, and Steps S2 through S4 are performed in an inert gas without evacuation. An evacuation may be performed before Step S1 in order to prevent oxidization of the alloy as well as to prevent a detonating reaction of hydrogen and oxygen and thereby ensure safety. However, this is a pre-process to be performed before the hydrogen pulverization process is initiated. Step S5 in the present embodiment is initially performed in argon gas until the temperature being increased reaches 500° C. which is a halfway to the sintering temperature and subsequently performed in vacuum. Examples of the inert gas used in those steps include argon gas, helium gas and other kinds of noble gas, nitrogen gas, as well as a mixture of two or more of those kinds of gas.

(8) For comparison, an example in which the dehydrogenation heating and/or evacuation is performed is described by means of FIG. 2. The production method in the present example is identical to the method shown by the flowchart in FIG. 1 except for the following two differences: The first difference is that the hydrogenation heating and/or evacuation for desorbing hydrogen is performed after the NdFeB system alloy is made to occlude hydrogen in the hydrogen pulverization process (Step S1A). More specifically, one of the three following operations is chosen in Step S1A: (i) the dehydrogenation heating is performed (without evacuation), (ii) the evacuation is performed (without dehydrogenation heating), and (iii) both the dehydrogenation heating and the evacuation are performed. The second difference is that, in the orienting process, the alloy powder may (optionally) be heated before or in the middle of the process of orienting the alloy powder in the magnetic field (Step S4A). Such an orientation process accompanied by heating is called the “temperature-programmed orientation.” The temperature-programmed orientation is a technique for temporarily lowering the coercive force of each individual grain of the alloy powder to suppress the mutual repulsion of the grains in the orienting process so as to improve the degree of orientation of the eventually obtained NdFeB system sintered magnet in the case where an alloy powder having a high coercive force is used as in the present embodiment. This technique lowers the production efficiency since it includes heating and cooling processes. Therefore, the temperature-programmed orientation is not performed in the present embodiment.

(9) The following description is focused on the dehydrogenation heating, leaving the evacuation out of consideration, to explain what difference occurs depending on whether or not the dehydrogenation heating is performed, using a temperature history of the hydrogen pulverization process. The graph in FIG. 3 is a temperature history of the hydrogen pulverization process in the method for producing a NdFeB system sintered magnet without dehydrogenation heating (Step S1, or case (ii) in Step S1A of the comparative example), while graph (a) in FIG. 4 is a temperature history of the hydrogen pulverization process in the method for producing a NdFeB system sintered magnet with dehydrogenation heating (case (i) or (iii) in Step S1A). Graph (b) in FIG. 4 is a resized version of the graph in FIG. 3 with the horizontal and vertical scales fitted to those of graph (a) in FIG. 4.

(10) In the hydrogen pulverization process, the NdFeB system alloy lump is made to occlude hydrogen. The hydrogen occlusion process is an exothermic reaction and causes the NdFeB system alloy lump to self-heat to temperatures of 200° C. to 300° C. During this process, the Nd rich phase in the alloy lump reacts with hydrogen and expands, creating a large number of cracks, to eventually pulverize the lump. A portion of the hydrogen is also occluded in the main phase. In general, after being naturally cooled, the obtained powder is heated to approximately 500° C. to desorb a portion of the hydrogen which has reacted with the Nd rich phase (dehydrogenation heating), in order to suppress oxidization of the alloy, after which the powder is naturally cooled to room temperature. In graph (a) of FIG. 4, which shows the example with dehydrogenation heating, the period of time required for the hydrogen pulverization process is approximately 1,400 minutes, including the period of time for desorbing hydrogen.

(11) In the case where the dehydrogenation heating is not performed, as shown in FIG. 3 and in graph (b) of FIG. 4, the hydrogen pulverization process can be completed within approximately 400 minutes after the temperature begins to rise due to the heat resulting from the hydrogen occlusion process, even if a somewhat long period of time is allotted for the cooling of the alloy powder to room temperature. As compared to the example (a) in FIG. 4, the production time can be reduced by approximately 1,000 minutes (16.7 hours). Thus, by omitting the dehydrogenation heating, the production process can be simplified and the production time can be significantly shortened.

(12) Hereinafter described is the result of an experiment in which NdFeB system sintered magnets were actually created using the method of the present embodiment and that of the comparative example. The inert gases used in the present embodiment were nitrogen gas in the fine pulverization process (Step S2) and argon gas in the other processes. In the comparative example, neither the dehydrogenation heating in the hydrogen pulverization process (Step S1A) nor the temperature-programmed orientation in the orientation process (Step S4A) was performed, but the evacuation in the hydrogen pulverization process was performed (i.e., the method of the aforementioned case (ii) was adopted). A NdFeB system alloy lump with the same composition was used as the material in both the present embodiment and the comparative example. Specifically, the composition (in percent by weight) was as follows: Nd: 26.95, Pr: 4.75, Dy: 0, Co: 0.94, B: 1.01, Al: 0.27, Cu: 0.1, and Fe: balance.

(13) The result of this experiment was such that the NdFeB system sintered magnet created in the comparative example had a coercive force of 17.6 kOe, while the NdFeB system sintered magnet created in the present embodiment had a higher coercive force, 18.1 kOe.

(14) Another experiment was also conducted in which a grain boundary diffusion process was performed as follows using the NdFeB system sintered magnets created in the present embodiment and the comparative example as the base material.

(15) Initially, a TbNiAl alloy powder composed of 92 wt % of Tb, 4.3 wt % of Ni and 3.7 wt % of Al was mixed with silicon grease by a weight ratio of 80:20. Then, 0.07 g of silicon oil was added to 10 g of the aforementioned mixture to obtain a paste, and 10 mg of this paste was applied to each of the two magnetic pole faces (7 mm×7 mm in size) of the base material.

(16) After the paste was applied, the rectangular base material was placed on a molybdenum tray provided with a plurality of pointed supports. The rectangular base material, being held by the supports, was heated in a vacuum of 10.sup.−4 Pa. The heating temperature was 880° C., and the heating time was 10 hours. Subsequently, the base material was quenched to room temperature, after which it was heated at 500° C. for two hours and then once more quenched to room temperature. Thus, the grain boundary diffusion process was completed.

(17) The result of this experiment of the grain boundary diffusion process was such that the NdFeB system sintered magnet created in the comparative example had a coercive force of 25.5 kOe, while the NdFeB system sintered magnet created in the present embodiment had a higher coercive force, 26.4 kOe.

(18) Thus, it has been confirmed that a NdFeB system sintered magnet with higher magnetic properties can be obtained by omitting the evacuation as in the present embodiment.

(19) Not only the magnetic properties but also the pulverization rate in the fine pulverization process was improved in the present embodiment. Specifically, in the case where coarse powder was pulverized to a mean grain size of 2 μm (in terms of the D.sub.50 value measured by the laser method), the pulverization rate was 12 g/min in the comparative example, while the rate in the present embodiment was 21 g/min, an approximately 70% improvement. This is most likely due to the fact that the fine pulverization in the present embodiment is performed under the condition that a larger amount of hydrogen is occluded in the coarse powder, and particularly, that a considerable amount of hydrogen is occluded in the main phase. As described thus far, by omitting the evacuation for dehydrogenation, it becomes possible to shorten the period of time for the fine pulverization process which constitutes a temporal bottleneck in the mass production of NdFeB system sintered magnets, and to thereby enhance the production efficiency.