Rare earth based magnet

Abstract

The present invention provides a rare earth based magnet that inhibits the high temperature demagnetization rate even when less or no heavy rare earth elements such as Dy, Tb and the like are used. The rare earth based magnet according to the present invention includes R.sub.2T.sub.14B main phase crystal grains and grain boundary phases between adjacent main phase crystal grains. In any cross-section of the rare earth based magnet, when evaluating the circular degree of the main phase crystal grains with Wadell's Roundness A, the shape of the main phase crystal grains is controlled such that the Roundness A becomes 0.24 or more.

Claims

1. A rare earth based sintered magnet, comprising: R.sub.2T.sub.14B main phase crystal grains and grain boundary phases between adjacent main phase crystal grains, wherein the grain boundary phases comprise an antiferromagnetic compound Nd.sub.6Fe.sub.13Ga phase, in any cross-section of said rare earth based sintered magnet observed by an electron microscope, circular degree of the main phase crystal grains is denoted by Roundness A and the Roundness A of five grains with different sizes is measured, and the average measured value thereby obtained is 0.24 or more and 0.36 or less, said rare earth based sintered magnet does not contain heavy rare earth elements, said rare earth based sintered magnet contains C, O and N, and the relation of [O]/([C]+[N])<0.60 is satisfied, wherein [C] is the molar amount of C, [O] is the molar amount of O, and [N] is the molar amount of N, said Roundness A is defined by the following formula (1),
A=r.sub.i/(Rn)(1) wherein, r.sub.i represents a radius of a circle inscribed in a corner of a selected crystal grain, R represents a radius of the biggest circle inscribed in the selected crystal grain, n represents the number of the corners of the selected crystal grain, represents the sum of the corners of the selected crystal grain, and said rare earth based sintered magnet has an absolute value of a demagnetization rate D at a high temperature is 0.77% or less, in which
D=100(B1B0)/B0(%), B0: residual magnetic flux at room temperature (25 C.), and B1: residual magnetic flux brought back to room temperature after being exposed to a temperature of 140 C. for 2 hours.

2. The rare earth based magnet according to claim 1, wherein said grain boundary phases further comprise a non-magnetic R-rich phase.

3. The rare earth based magnet according to claim 1, wherein [O]/([C]+[N]) is 0.45 or less.

4. The rare earth based magnet according to claim 1, wherein [O]/([C]+[N])0.51.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a drawing showing the relationship between the Roundness and the high temperature demagnetization rate according to the present embodiment.

(2) FIG. 2 is a drawing illustrating the evaluation method of the Roundness in the present embodiment.

(3) FIG. 3 is a cross-section photograph exhibiting the microstructure of the rare earth based magnet of the example.

(4) FIG. 4 is a cross-section photograph exhibiting the microstructure of the rare earth based magnet of the comparative example.

DETAILED DESCRIPTION OF EMBODIMENTS

(5) Hereinafter, the preferred embodiments of the present invention are illustrated while making a reference to the drawings. Moreover, the rare earth based magnet according to the present invention is a sintered magnet comprising R.sub.2T.sub.14B main phase crystal grains and grain boundary phases, and contains B and additional components that are to add various well known elements, and R contains one or more rare earth elements. T contains one or more elements of iron group including Fe as an essential element.

(6) FIG. 2 is a drawing schematically illustrating the cross-section structure of the rare earth based magnet of the embodiments according to the present invention. The rare earth based magnet according to the present embodiment is a magnet with the cross-section of the R.sub.2T.sub.14B main phase crystal grains having a roundish shape.

(7) Firstly. Wadell's Roundness, as the evaluation index of the circular degree of the main phase crystal grains in the present invention, is described on the basis of FIG. 2. In the present specification, Roundness A is defined by formula (1) below.
A=r.sub.i/(Rn)(1)
Here, r.sub.i represents the radius of a circle inscribed in a corner of a selected crystal grain,
R represents the radius of the biggest circle inscribed in the selected crystal grain,
n represents the number of the corners of the selected crystal grain, and
represents the sum of the corners of the selected crystal grain.

(8) Based on this definition, in the case that the selected grain is an ideal circle (sphere), Roundness A is 1. Therefore, the Roundness described in the present specification does not exceed 1. In view of the schematic drawing of FIG. 2, the crystal grain has 5 corners (n=5), and the radiuses of circles inscribed in these corners are r.sub.1, r.sub.2, r.sub.3, r.sub.4 and r.sub.5, respectively, and the radius of the biggest circle inscribed is R.

(9) Next, the evaluation for the high temperature demagnetization rate of the rare earth based magnet according to the present embodiment is described. The shape of the sample used for evaluation is not particularly limited, and for example, it is a shape, that is generally used, with a Permeance Coefficient of 2. Firstly, residual flux of the sample at room temperature (25 C.) is measured and taken as B0. The residual flux may be measured by for example a magnetic flux meter. Next, the sample is exposed to a high temperature of 140 C. for 2 hours, and then returns to the room temperature. Once the temperature of the sample returns to the room temperature, the residual flux is measured again and taken as B1. As such, the high temperature demagnetization rate D is evaluated by the formula (2) below.
D=(B1B0)B0*100(%)(2)

(10) Since the above Roundness is allowed to be 0.24 or more, the rare earth based magnet according to the present embodiment is formed such that the gap between the main phase crystal grains is larger than that of a current structure filled with rectangular grains. Thus, magnetic isolation of the adjacent main phase crystal grains is enhanced, and the high temperature demagnetization rate is inhibited.

(11) Hereinafter, an example of the method for producing the rare earth based magnet according to the present embodiment is described.

(12) The composition of the rare earth based magnet according to the present embodiment is formed such that in comparison to the element T, the element R is excessive to the stoichiometric ratio of R.sub.2T.sub.14B. Specifically, atomic percentage of R may be around 14.4%. Moreover, the rare earth based magnet according to the present embodiment may be produced by a usual powder metallurgic method comprising a preparation step of producing the raw alloys, a pulverization step of pulverizing the raw alloys to obtain raw fine powders, a molding step of molding the raw fine powders to obtain a molded body, a sintering process of firing the molded body to obtain a sintered body, and an heat treating step of subjecting the sintered body to an aging treatment.

(13) The preparation step is the step for producing the raw alloys having the elements contained in the rare earth based magnet according to the present embodiment. Firstly, the raw metals having the specified elements are prepared, and subjected to a strip casting method and the like. The raw alloys are thus produced. As the raw metals, for examples, rare earth based metals or rare earth based alloys, pure iron, pure cobalt, ferroboron or alloys thereof are exemplified. These raw metals are used to produce the raw alloys of the rare earth based magnet having the desired composition.

(14) The pulverization step is the step for pulverizing the raw alloys obtained in the preparation step into raw fine powders. This step is preferably performed in two stages comprising a coarse pulverization step and a fine pulverization step, and may also be performed as one stage. The coarse pulverization may be performed by using, for example, a stamp mill, a jaw crusher, a braun mill and the like under an inert gas atmosphere. A hydrogen adsorption pulverization in which pulverization is performed after adsorbing hydrogen may also be performed. In the coarse pulverization step, the raw alloys are pulverized until the particle size is around several hundred micrometers to several millimeters.

(15) The fine pulverization step is the step in which the coarse powders obtained in the coarse pulverization step is finely pulverized to prepare the raw fine powders with the average particle size of several micrometers. The average particle size of the raw fine powders may be set under the consideration of the growth of the crystal grains after sintering. For example, the fine pulverization may be performed by a jet mill.

(16) The molding step is a step for molding the raw fine powders in the magnetic field to produce a molded body. Specifically, after the raw fine powders are filled into a mold equipped in an electromagnet, the molding is performed by orientating the crystallographic axis of the raw fine powders by applying a magnetic field via the electromagnet, while pressurizing the raw fine powders. The molding may be performed in a magnetic field of 10001600 kA/m under a pressure of about 30300 MPa.

(17) The sintering step is a step for firing the molded body to obtain a sintered body. After being molded in the magnetic field, the molded body may be fired in a vacuum or an inert gas atmosphere to obtain a sintered body. Preferably, the firing conditions are suitably set depending on the factors such as composition of the molded body, the pulverization method of the raw fine powders, grain size and the like. For example, the sintering may be performed at 1000 C.1100 C. for 110 hours.

(18) The heat treating step is a step for subjecting the sintered body to an aging treatment. After this step, the Roundness A of the R.sub.2T.sub.14B main phase crystal grains and the width of the grain boundary phases are determined. However, these microstructures are not only controlled in this step, but are determined in view of the conditions of the above sintering step and the situation of the raw fine powders. Hence, the relationship between the conditions of the heat treatment and the microstructure of the sintered body may be considered to set the temperature and time period of the heat treatment. The heat treatment may be performed at a temperature ranging from 400 C. to the sintering temperature, and may also be performed in two stages comprising a heat treatment at 800 C. nearby followed by a heat treatment at 550 C. nearby. The cooling rate during the cooling process of the heat treatment may also alter the microstructure. The cooling rate is preferably 100 C./min or more, particularly preferably 300 C./min or more. By the above aging treatment of the present invention in which the cooling rate is faster than before, the segregation of the ferromagnetic phase in the grain boundary phase can be effectively inhibited. Thus, the causes for reducing coercivity and further deteriorating the high temperature demagnetization rate can be eliminated. By respectively setting the composition of the raw alloys, the above sintering conditions and the conditions of heat treatment at 800 C. nearby, the shape of the main phase crystal grains can be controlled. Here, an example of the heat treatment step is described as the method for controlling the shape of the main phase crystal grains. The shape of the main phase crystal grains may also be controlled by the following main factors of the composition.

(19) According to the information based on the experiments of the inventors, it is considered that by suitably selecting the additive elements besides the elements R, T and B of the main phase crystal grains, the corners of the main phase are melt in the heat treatment at a high temperature of 800 C. nearby, the corners of the main phase crystal grains are made to be rounded, and the elements R and Fe precipitate in the grain boundary phases. Based on such a mechanism, the raw alloys are taken as R-rich composition, and the non-magnetic R-rich phase precipitates in the grain boundary phases. Further, as the additive elements, the elements that, together with R and T, form compounds with a magnetic structure having antiferromagnetism, ferrimagnetism or the like different from ferromagnetism are preferred. Specifically, Al, Ge, Si, Sn, Ga and the like may be added, and other elements may also be feasible, as long as compounds with a magnetic structure different from a ferromagnetic structure can be formed. As such, if the non-magnetic R-rich phase precipitates in the grain boundary phase or the grain boundary phase become an antiferromagnetic or ferrimagnetic compound, magnetic isolation of the main phase crystal grains is easily resulted, and the high temperature demagnetization rate and the like are inhibited. Thus, a magnet having excellent magnetic properties is formed.

(20) In addition, O contained in the resultant rare earth based magnet may be measured by an inert gas fusion-nondispersive infrared absorption method, C may be measured by a combustion in oxygen flow-infrared absorption method. N may be measured by an inert gas fusion-thermal conductivity method. The composition of the rare earth based magnet according to the present embodiment is formed such that, in comparison to the element T, the element R is excessive to the stoichiometric ratio of R.sub.2T.sub.14B. Further, when the atom numbers of the contained C, O and N are denoted as [C], [O], and [N] respectively, the relationship of [O]/([C]+[N])<0.60 is preferably satisfied. With such a composition, the absolute value of the high temperature demagnetization rate can be inhibited to be small.

(21) Next, the present invention is further described in detail with reference to the specific examples, but the present invention is not limited thereto.

EXAMPLES

(22) Nd was used as the element R, and Fe was used as the element T. In addition, Cu and Ga were used as the additive elements for forming the grain boundary phases. The raw metals of the rare earth based magnet were prepared. The composition having various elements consisting of

(23) Nd: 31.08 mass %,

(24) B: 0.95 mass %,

(25) Ga: 0.72 mass %,

(26) Cu: 0.10 mass %,

(27) Fe (the residual part except the inevitable impurities and the like is Fe): balance, and other inevitable impurities and the like: 1 mass % or less, were dissolved, and raw alloys were prepared by a strip casting method. Further, in order to form thicker grain boundary phases, a composition that was richer in Nd and Ga than the above composition also could be prepared. Next, after adsorption of hydrogen onto the resultant raw alloys, hydrogen pulverization by desorbing hydrogen was performed in Ar atmosphere at 600 C. for 1 hour. Then, the resultant pulverized materials were cooled to room temperature in Ar atmosphere.

(28) After adding oleic amide as a grinding aid to the resultant pulverized materials and mixing therewith, a fine pulverization was performed by using a jet mill to obtain raw powders with an average particle size of 34 m.

(29) The resultant raw powders were molded in a low-oxygen atmosphere under the condition of an alignment magnetic field of 1200 kA/m and a molding pressure of 120 MPa to obtain a molded body.

(30) Then, the molded body was fired in a vacuum at 1060 C. for 3 hours, and quenched to obtain a sintered body.

(31) For the resultant sintered body, various samples with the main phase crystal grains different in Roundness were prepared by varying the temperature, time period, cooling rate of the cooling process in the heat treatment as illustrated in Tables 1 and 3. Moreover, the Roundness of the main phase crystal grains may also vary depending on the composition of the raw alloys and the sintering conditions.

(32) TABLE-US-00001 TABLE 1 Temperature Time period High of heat of heat temperature treatment treatment Round- demagnetization Sample No. ( C.) (hr) ness rate (%) Comparative 850 0.2 0.15 2.3 Example 1 Comparative 850 0.5 0.16 2.2 Example 2 Comparative 850 1 0.19 2.1 Example 3 Comparative 850 5 0.21 2.1 Example 4 Example 1 700 0.2 0.24 0.77 Example 2 700 0.5 0.26 0.51 Example 3 700 1 0.31 0.27 Example 4 700 5 0.35 0.2 Example 5 700 20 0.36 0.18

(33) For the samples obtained as above, firstly, the high temperature demagnetization rate was measured, and then the cross-section was observed by an electron microscope, followed by measurement of the Roundness and observation of the grain boundary phases. For the Roundness, five grains with different sizes were measured respectively, and the average of these measured values was taken as the Roundness of the sample. Table 2 illustrated a specific example of Roundness measurement for one main phase crystal grain from the example and the comparative example, respectively. The main phase crystal grain of the evaluated example as Example 1 had 10 corners, and the radiuses of circles inscribed in respective corners were indicated as the values shown in Table 2. Moreover, the main phase crystal grain of the evaluated example as Comparative Example 1 had 7 corners, and the radiuses of circles inscribed in respective corners were indicated as the values shown in Table 2. For the respective examples and comparative examples, the residual 4 crystal grains were subjected to the same measurements, and the average was taken as the Roundness.

(34) TABLE-US-00002 TABLE 2 Example 1 Comparative Example 1 R(m) 7.98 4.23 r1(m) 3.04 2.2 r2(m) 1.48 1.15 r3(m) 1.4 0.79 r4(m) 2.95 0.43 r5(m) 2.03 0.15 r6(m) 3.11 0.15 r7(m) 4.83 0.04 r8(m) 0.39 r9(m) 0.39 r10(m) 1.05 n 10 7 Roundness A 0.26 0.17

(35) Additionally, when the atom numbers of the elements N, C and O contained in the resultant rare earth based magnet were denoted as [N], [C] and [O] respectively, the values of [O]/([C]+[N]) of respective samples were calculated and shown in Table 3. The amounts of oxygen and nitrogen contained in the rare earth based magnet were adjusted to the ranges shown in Table 3 by controlling the atmospheres from the pulverization step to the heat treating step, especially adjusting the amounts of oxygen and nitrogen contained in the atmosphere in the pulverization step. Moreover, the amount of carbon contained in the raw materials of the rare earth based magnet was adjusted to the range shown in Table 3 by adjusting the amount of the grinding aid added in the pulverization step.

(36) TABLE-US-00003 TABLE 3 Amounts of N, C, O contained in the rare Ratio Cooling rate earth based magnet of Atom of the N C O numbers heat treatment Sample No. mass % mass % mass % [O]/[C] + [N]) C./min Comparative 0.05 0.09 0.11 0.62 60 Example 1 Comparative 0.05 0.09 0.11 0.62 60 Example 2 Comparative 0.05 0.09 0.10 0.57 100 Example 3 Comparative 0.05 0.09 0.10 0.57 100 Example 4 Example 1 0.05 0.09 0.08 0.45 100 Example 2 0.05 0.09 0.08 0.45 100 Example 3 0.05 0.09 0.08 0.45 100 Example 4 0.05 0.09 0.09 0.51 300 Example 5 0.05 0.09 0.09 0.51 300

(37) FIG. 1 is a drawing showing the relationship between the Roundness A of the examples and the comparative examples evaluated by the above method and the high temperature demagnetization rate D. It can be seen from FIG. 1 that the effect on inhibiting the high temperature demagnetization rate D was especially improved when the Roundness A was 0.24 or more.

(38) FIG. 3 is a cross-section photograph exhibiting the microstructure of the rare earth based magnet according to the present embodiment. The Roundness A of the rare earth based magnet was 0.31, and the high temperature demagnetization rate was inhibited to a low value of 0.27%. It can be seen from FIG. 3 that an extremely broad grain boundary phase was formed between the roundish main phase crystal grains. By the above heat treatment at a high temperature, the corners of the main phase crystal grains were melted, and based on it, thick grain boundary phases were formed, allowing the main phase crystal grains to have a smooth surface and to be magnetically isolated. If analyzing the composition of the grain boundary phase, it can be confirmed that Nd-rich phase with 90 at % of Nd was formed, or the antiferromagnetic compound Nd.sub.6Fe.sub.13Ga phase and the like was formed even containing the element Fe. By allowing the composition of the raw alloys to be rich in R and further adding Ga and the like, the grain boundary phase might be formed into a magnetic structure that is not ferromagnetic, resulting in magnetic isolation of the main phase crystal grains.

(39) FIG. 4 is a cross-section photograph exhibiting the microstructure of the rare earth based magnet of the comparative example. Although the comparative example had an approximately the same composition as the above example, the main phase crystal grains were formed into an angular shape due to insufficient heat treatment at a high temperature. The Roundness A of the rare earth based magnet of the comparative example was 0.15, and the high temperature demagnetization rate was up to 2.3%.

(40) Additionally, as shown in Table 3, in the samples of Examples 1-5 that meet the requirements of the present invention, the above microstructure was formed in the sintered magnet, and the atom numbers of O, C and N contained in the sintered magnet satisfied the following specific relationship. That is, when the atom numbers of O, C and N were denoted as [O], [C], and [N] respectively, the relationship of [O]/([C]+[N])<0.60 was satisfied. As such, when [O]([C]+[N])<0.60, the high temperature demagnetization rate D can be effectively inhibited.

(41) As demonstrated by the above examples, by forming the R.sub.2T.sub.14B main phase crystal grains into a roundish shape with the Roundness A being 0.24 or more, the rare earth based magnet according to the present invention can allow the width of the grain boundary phase formed adjacent to the main phase crystal grains to become thick, and the high temperature demagnetization rate is inhibited to be low.

(42) The present invention was described with reference to the embodiments above. The embodiments were exemplified, and various modification and changes may be included within the claims of the present invention. In addition, one skilled in the art will understand that the modified examples and changes are within the claims of the present invention. Thus, the description and the drawings in the present specification should be considered as illustrative but not limited.

(43) According to the present invention, a rare earth based magnet that is applicable even at a high temperature environment may be provided.

DESCRIPTION OF REFERENCE NUMERALS

(44) 1 Main phase crystal grains 2 Grain boundary phase