Manufacturing method of rare earth magnet based on heat treatment of fine powder

Abstract

A manufacturing method of rare earth magnet based on heat treatment of fine powder includes the following: an alloy for the rare earth magnet is firstly coarsely crushed and then finely crushed by jet milling to obtain a fine powder; the fine powder is heated in vacuum or in inert gas atmosphere at a temperature of 100 C.1000 C. for 6 minutes to 24 hours; then the fine powder is compacted under a magnet field and is sintered in vacuum or in inert gas atmosphere at a temperature of 950 C.1140 C. to obtain a sintered magnet; and machining the sintered magnet to obtain a magnet; then the magnet performs a RH grain boundary diffusion at a temperature of 700 C.1020 C. An oxidation film forms on the surface of all of the powder.

Claims

1. A manufacturing method of rare earth magnet based on heat treatment of fine powder, the rare earth magnet including R.sub.2T.sub.14B main phase, R being selected from at least one rare earth element, and T being at least one transition metal element including the element Fe, the method comprising the steps of: strip casting a molten alloy fluid for the rare earth magnet and cooling the molten alloy fluid at a cooling rate between 10.sup.2 C/s to 10.sup.4 C/s, to thereby obtain an alloy for the rare earth magnet; coarsely crushing the alloy for the rare earth magnet and subsequently finely crushing by jet milling to obtain the fine powder; heating the fine powder in vacuum, of which a pressure is in a range of 10.sup.2 Pa-500 Pa with an oxygen content of 0.5 ppm-2000 ppm and a dew point of 60 C.-20 C., or in an inert gas atmosphere, of which a pressure is in a range of 10.sup.1 Pa-1000 Pa with an oxygen content of 0.5 ppm-2000 ppm and a dew point of 60 C.-20 C., at a temperature of 100 C.-700 C. for 1 hour to 24 hours, to thereby create an oxidation layer evenly on particle surfaces of the fine powder; compacting the fine powder under a magnet field; sintering in vacuum or in an inert gas atmosphere at a temperature of 950 C.-1140 C. to obtain a sintered magnet; and machining the sintered magnet to obtain a magnet, and subsequently performing a RH grain boundary diffusion on the magnet at a temperature of 1000 C.-1020 C.

2. The manufacturing method according to claim 1, wherein the temperature during the heating is 300 C.-700 C.

3. The manufacturing method according to claim 2, wherein the fine powder is vibrated or shaken during the heating.

4. The manufacturing method according to claim 1, wherein the coarse crushing comprises treating the alloy for the rare earth magnet by hydrogen decrepitation under a hydrogen pressure between 0.01 MPa to 1 MPa for 0.5-6 hours and subsequently dehydrogenated in vacuum.

5. The manufacturing method according to claim 2, wherein the alloy for the rare earth magnet is expressed, in atomic percent, as: R.sub.eT.sub.fA.sub.gJ.sub.hG.sub.iD.sub.k, where R is Nd or comprises Nd and at least one of the elements La, Ce, Pr, Sm, Gd, Dy, Tb, Ho, Er, Eu, Tm, Lu or Y; where T is Fe or comprises Fe and at least one of the elements Ru, Co or Ni; where A is B or comprises B and at least one of the elements C or P; where J is selected from at least one of the elements Cu, Mn, Si or Cr; where G is selected from at least one of the elements Al, Ga, Ag, Bi or Sn; where D is selected from at least one of the elements Zr, Hf, V, Mo, W, Ti or Nb; and where subscripts e, f, g, h, i and k are configured as: 12e16, 5g9, 0.05h1, 0.2i2.0, k is 0k4, and f=100-e-g-h-i-k.

6. The manufacturing method according to claim 1, wherein the oxidation layer is evenly formed on the surface of all of the fine powder after the heating.

7. The manufacturing method according to claim 3, wherein the coarse crushing comprises treating the alloy for the rare earth magnet by hydrogen decrepitation under a hydrogen pressure between 0.01 MPa to 1 MPa for 0.5-6 hours and subsequently dehydrogenated in vacuum.

8. The manufacturing method according to claim 2, wherein the coarse crushing comprises treating the alloy for the rare earth magnet by hydrogen decrepitation under a hydrogen pressure between 0.01 MPa to 1 MPa for 0.5-6 hours and subsequently dehydrogenated in vacuum.

9. The manufacturing method according to claim 3, wherein the oxidation layer is evenly formed on the surface of all of the fine powder after the heating.

10. The manufacturing method according to claim 2, wherein the oxidation layers is evenly formed on the surface of all of the fine powder after the heating.

Description

DETAILED DESCRIPTION OF THE EMBODIMENTS

(1) The present invention will be further described with the embodiments.

(2) Embodiment 1:

(3) Raw material preparing process: Nd, Pr, Dy, Tb and Gd with 99.5% purity, industrial FeB, industrial pure Fe, Co with 99.9% purity and Cu, Mn, Al, Ag, Mo and C with 99.5% purity are prepared; counted in atomic percent, and prepared in R.sub.eT.sub.fA.sub.gJ.sub.hG.sub.iD.sub.k components.

(4) The contents of the elements are shown in TABLE 1:

(5) TABLE-US-00001 TABLE 1 proportioning of each element R T A J G D Nd Pr Dy Tb Gd Fe Co C B Cu Mn Al Ag Mo 7 3 1 1 1 remain- 1 0.05 7 0.2 0.2 0.2 0.1 1 der

(6) Preparing 500 Kg raw material by weighing in accordance with TABLE 1.

(7) Melting process: the 500 Kg raw material is put into an aluminum oxide made crucible, an intermediate frequency vacuum induction melting furnace is used to melt the raw material in 1 Pa vacuum below 1650 C.

(8) Casting process: After the process of vacuum melting, Ar gas is filled to the melting furnace so that the Ar pressure would reach 80000 Pa, then the material is casted as a strip with an average thickness of 0.3 mm by strip casting method.

(9) Hydrogen decrepitation process (coarse crushing process): the strip of 0.3 mm average thickness is put into a stainless steel container of a rotating hydrogen decrepitation furnace with an inner diameter of 1200 mm, the container is then pumped to be vacuum and the vacuum level is below 10 Pa, then hydrogen of 99.999% purity is filled into the container, the hydrogen pressure would reach 0.12 MPa, the container rotates for 2 hours at a rotating rate of 1 rpm to absorb hydrogen, after that, the container is pumped for 2 hours at 600 C. to dehydrogenate, then the container rotates and gets cooled at a rotating rate of 30 rpm simultaneously, the cooled coarse powder is then taken out.

(10) Fine crushing process: a jet milling device is used to finely crush the coarse powder to obtain a fine powder with an average particle size of 4.2 m.

(11) Fine powder heat treatment process: the fine powder is divided into 8 equal parts, each part is respectively put into a stainless steel container of a rotating hydrogen decrepitation furnace with an inner diameter of 1200 mm, the container is then pumped to be vacuum and obtain a vacuum level of 10.sup.1 Pa with an oxygen content of 11000 ppm, and a dew point of 010 C., then the stainless steel container is put to an externally heating oven for heat treatment.

(12) The heating temperature and heat treatment time of each part of fine powder are shown in TABLE 2, the stainless steel container rotates at a rotating rate of 10 rpm when heated.

(13) After the heat treatment of the fine powder, the container is taken out of the externally heating oven, the container is then externally water cooled at a rotating rate of 20 rpm for 3 hours.

(14) Compacting process under a magnetic field: no organic additive such as forming aid and lubricant is added into the fine powder after heat treatment, a transversed type magnetic field molder is used, the powder is compacted in once to form a cube with sides of 40 mm in an orientation field of 2.1 T and under a compacting pressure of 0.2 ton/cm.sup.2, then the once-forming cube is demagnetized in a 0.2 T magnetic field.

(15) The once-forming compact (green compact) is sealed so as not to expose to air, the compact is secondary compacted by a secondary compact machine (isostatic pressing compacting machine) under a pressure of 1.0 ton/cm.sup.2.

(16) Sintering process: each of the green compact is moved to the sintering furnace, firstly sintering in a vacuum of 10.sup.3 Pa and respectively maintained for 2 hours at 200 C. and for 2 hours at 600 C., then in Ar gas atmosphere of 0.01 MPa, sintering for 2 hours at 1080 C., after that filling Ar gas into the sintering furnace so that the Ar pressure would reach 0.1 MPa, then cooling it to room temperature.

(17) Heat treatment process: the sintered magnet is heated for 1 hour at 600 C. in the atmosphere of high purity Ar gas, then cooling it to room temperature and taking it out.

(18) Magnetic property evaluation process: the sintered magnet is tested by NIM-10000H type nondestructive testing system for BH large rare earth permanent magnet from China Jiliang University.

(19) Oxygen content of sintered magnet evaluation process: the oxygen content of the sintered magnet is measured by EMGA-620W type oxygen and nitrogen analyzer from HORIBA company of Japan.

(20) TABLE-US-00002 TABLE 2 The magnetic property and oxygen content evaluation of the embodiments and the comparing samples in different heating temperature and heating time. Oxygen content of the Heating sintered temperature Heating Br SQ (BH)max magnet No. ( C.) time (hr) (kGs) Hcj (k0e) (%) (MG0e) (ppm) 0 Comparing None heat treatment of 10.1 11.4 82 21.4 2580 sample the fine powder 1 Comparing 80 30 10.2 11.6 82.3 22.8 1589 sample 2 Embodiment 100 24 12 35.1 98.2 31.2 562 3 Embodiment 300 6 12.3 35.4 99.1 35.3 375 4 Embodiment 500 4 12.3 36.7 99.1 35.2 369 5 Embodiment 700 1 12.3 37.8 99.2 35.2 383 6 Embodiment 1000 0.3 11.8 34.5 98.5 33.2 582 7 Comparing 1020 0.5 10.6 27.6 84.2 23.2 1587 sample 8 Comparing 1050 12 10.2 24.3 78.6 16.5 2598 sample

(21) As can be seen from TABLE 2, with the heat treatment of the fine powder, a very thin oxidation film is formed on the surface of the overall powder evenly, so that the lubricity is well among the powder, and the orientation degree of the powder is improved, so that it can obtain higher values of Br and (BH)max; furthermore, the phenomenon of abnormal grain growth would not happen when sintering, so that it can obtain a finer organization, and the value of coercivity Hcj is increased drastically; in addition, by the heat treatment of the fine powder, the sharp portion on the surface of the powder is melted and becomes round, so the counter magnetic field coefficient at the partial portion is increased, it can also obtain a higher value of coercivity. Moreover, during the processes from compacting to sintering, the powder with even oxidation film on the surface is weakened in activity, so that during those processes, even the powder is contacted with the air, drastic oxidation would not happen; on the contrary, the fine powder without heat treatment has a strong activity and is easily oxidized, during the processes from compacting to sintering, even contacted with a little amount of air, drastic oxidation would happen, leading to a higher oxygen content of the sintered magnet.

(22) It has to be noted that, if the heating temperature of the fine powder exceeds 1000 C., the oxidation film on the surface of the fine powder particle may be easily diffused into the inner of the particle, consequently it would be like no oxidation film, therefore the adhesion power between the powder gets stronger, in this case, the values of Br and (BH)max would be extremely adverse, the phenomenon of abnormal grain growth (AGG) would easily happen when sintering, and the value of coercivity Hcj would be reduced.

(23) In the past, in the low oxygen content process, as the adhesive power among the magnet powder is strong, and the orientation degree of the magnet powder is not too high, so that it also has problems of low values of Br and (BH)max; moreover, as the surface activity among the magnet powder is strong, the grains are easily welded when sintering, therefore the phenomenon of abnormal grain growth happens, and the value of coercivity is reduced rapidly. The above mentioned problems are solved by adopting the proposal of the present invention.

(24) Embodiment 2

(25) Raw material preparing process: Nd, Y with 99.9% purity, industrial FeB, industrial pure FeP, industrial FeCr, industrial pure Fe, Ni, si with 99.9% purity, and Sn, W with 99.5% purity are prepared.

(26) Counted in atomic percent, and prepared in R.sub.eT.sub.fA.sub.gJ.sub.hG.sub.iD.sub.k components.

(27) The contents of the elements are shown in TABLE 3:

(28) TABLE-US-00003 TABLE 3 proportioning of each element R T A J G D Nd Y Fe Ni B P Cr Si Sn W 12.7 0.1 remainder 0.1 5.9 0.05 0.2 0.1 0.3 0.01

(29) Preparing 500 Kg raw material by weighing in accordance with TABLE 3.

(30) Melting process: the 500 Kg raw material is put into an aluminum oxide made crucible, an intermediate frequency vacuum induction melting furnace is used to melt the raw material in 10.sup.2 Pa vacuum below 1600 C.

(31) Casting process: After the process of vacuum melting, Ar gas is filled to the melting furnace so that the Ar pressure would reach 50000 Pa after vacuum melting, then the material is casted as a strip with an average thickness of 2 mm on a water-cooling casting disk.

(32) Hydrogen decrepitation process: the strip is put into the stainless steel container of a rotating hydrogen decrepitation furnace with an inner diameter of 1200 mm, the container is then pumped to be vacuum and the vacuum level is below 10 Pa, then hydrogen of 99.999% purity is filled into the container, the hydrogen pressure would reach 0.12 MPa, the container rotates for 2 hours at a rotating rate of 1 rpm to absorb hydrogen, after that, the container is pumped for 2 hours at 600 C. to dehydrogenate, then the container rotates and gets cooled at a rotating rate of 30 rpm, the cooled coarse powder is then taken out.

(33) Fine crushing process: a jet milling device is used to finely crush the coarse powder to obtain a fine powder with an average particle size of 6.8 nm, then the powder is divided into 6 equal parts.

(34) Fine powder treatment process: 4 parts of the fine powder are respectively put into the stainless steel container of a rotating hydrogen decrepitation furnace with an inner diameter of 1200 mm, the container is then pumped to be vacuum to obtain a vacuum level of 10.sup.2 Pa with an oxygen content of 0.550 ppm, and a dew point of 1020 C., then the stainless steel container is put to an externally heating oven for heat treatment; the heating temperature is 600 C., the heating time is 2 hours, and the container is heated at a rotating rate of 1 rpm.

(35) After the heat treatment of the fine powder, the container is taken out of the externally heating oven, the container is then externally water cooled at a rotating rate 20 rpm for 3 hours.

(36) Compacting process under a magnetic field: no organic additive is added into the 4 parts of fine powder with the process of fine powder heat treatment and the rest 2 parts of fine powder without the process of fine powder heat treatment, and the transversed type magnetic field molder is respectively used for the two types of the fine powder; the two types of powder are respectively compacted in once to form a cube with sides of 40 mm in an orientation field of 2 T and under a compacting pressure of 0.20 ton/cm.sup.2, then the once-forming cube is demagnetized in a 0.2 T magnetic field. The once-forming compact (green compact) is sealed so as not to expose to air, then the compact is secondary compacted by a secondary compacting machine (isostatic pressing compacting machine) under a pressure of 1.2 ton/cm.sup.2.

(37) Sintering process: each of the green compact is moved to the sintering furnace to sinter, firstly sintering in a vacuum of 10.sup.3 Pa and respectively maintained for 2 hours at 300 C. and for 2 hours at 500 C., then sintering for 6 hours at 1050 C., after that filling Ar gas into the sintering furnace so that the Ar pressure would reach 0.1 MPa, then cooling it to room temperature.

(38) Heat treatment process: the sintered magnet is heated for 1 hour at 550 C. in the atmosphere of high purity Ar gas, then cooling it to room temperature and taking it out.

(39) Machining process: the sintered magnet compacted by the 2 parts of fine powder without fine powder heat treatment is machined to be a magnet with 15 mm diameter and 5 mm thickness, the 5 mm direction (along the direction of thickness) is the orientation direction of the magnetic field; thereinto, one sintered magnet is served as no grain boundary diffusion treatment and is tested its magnetic property (comparing sample 1), the other magnet is treated by Method A in TABLE 4 for grain boundary diffusion treatment after washed and surface cleaning (comparing sample 2).

(40) The 4 parts of sintered magnet compacted by fine powder with fine powder heat treatment is machined to be a magnet with 15 mm and 5 mm thickness, the 5 mm direction (the direction along the thickness) is the orientation direction of the magnetic field; one magnet of which is served as no grain boundary diffusion treatment and is directly tested its magnetic property (comparing sample 3).

(41) Grain boundary diffusion process: the other 3 parts of sintered magnet compacted by fine powder with heat treatment are respectively treated by Methods A, B, and C in TABLE 4 for grain boundary diffusion treatment after washed and surface cleaning.

(42) TABLE-US-00004 TABLE 4 grain boundary diffusion method Grain boundary diffusion type Detailed process A Dy oxide powder, Tb Dy oxide and Tb fluoride are prepared in proportion of fluoride powder coating 3:1 to make raw material to fully spray and coat on the diffusion method magnet, the coated magnet is then dried, then in high purity of Ar gas atmosphere, the magnet is treated with heat and diffusion treatment at 850 C. for 12 hours. B (Dy, Tb)NiCoAl serial The Dy.sub.30Tb.sub.30Ni.sub.5Co.sub.25Al.sub.10 alloy is finely crushed as fine alloy fine powder coating powder with an average grain particle size 15 m to diffusion method fully spray and coat on the magnet, the coated magnet is then dried, then in high purity of Ar gas atmosphere, the magnet is treated with heat and diffusion treatment at 950 C. for 12 hours. C Dy metal vapor diffusion In Ar gas atmosphere, the Dy metal plate, Mo screen method and magnet are put into a vacuum heating furnace for vapor treatment at 1010 C. for 6 hours.

(43) Magnetic property evaluation process: the sintered magnet is tested by NIM-10000H type nondestructive testing system for BH large rare earth permanent magnet from China Jiliang University.

(44) Oxygen content of sintered magnet evaluation process: the oxygen content of the sintered magnet is measured by EMGA-620W type oxygen and nitrogen analyzer from HORIBA company of Japan.

(45) The magnetic property and oxygen content evaluation of the embodiments and the comparing samples with the fine powder heat treatment and the grain boundary diffusion treatment are shown in TABLE 5.

(46) TABLE-US-00005 TABLE 5 The magnetic property and oxygen content evaluation of the embodiments and the comparing samples Oxygen Heat content of treatment the of Grain sintered the fine boundary Br SQ (BH)max magnet No. powder diffusion (kGs) Hcj (k0e) (%) (MG0e) (ppm) 0 Comparing no no 13.1 6.5 76.5 23.1 2687 sample 1 1 Comparing no A 13.2 13.2 86.6 32.5 2785 sample 2 2 Comparing yes no 15.4 9.5 86.7 46.4 421 sample 3 3 Embodiment yes A 15.5 22.3 98.4 56.5 278 4 Embodiment yes B 15.6 22.4 99.2 56.8 276 5 Embodiment yes C 15.6 24.2 99.1 57.2 289

(47) As can be seen from TABLE 5, the sintered magnet sintered by the fine powder with fine powder heat treatment has an obvious change in the existence state of the oxygen in the grain boundary, the diffusion rate of the elements Dy, Tb is accelerated and the diffusion efficiency is promoted, so that the grain boundary diffusion can be finished in a short time, the effect of the grain boundary diffusion is obvious and the coercivity is improved significantly.

(48) Embodiment 3

(49) Raw material preparing process: La, Ge, Nd, Tb, and Ho with 99.5% purity, industrial FeB, industrial pure Fe, Ru with 99.99% purity and P, Si, Cr, Ga, Sn, Zr with 99.5% purity are prepared; counted in atomic percent, and prepared in R.sub.eT.sub.fA.sub.gJ.sub.hG.sub.iD.sub.k components. The contents of the elements are shown as follows: R component, La is 0.1, Ce is 0.1, Nd is 12, Tb is 0.2, and Ho is 0.2; T component, Fe is the remainder, and Ru is 1; A component, P is 0.05, and B is 7; J component, Si is 0.2, and Cr is 0.2; G component, Ga is 0.2, and Sn is 0.1; and D component, Zr is 0.5.

(50) Preparing 500 Kg raw material by weighing in accordance with above contents of elements.

(51) Melting process: the 500 Kg raw material is put into an aluminum oxide made crucible, an intermediate frequency vacuum induction melting furnace is used to melt the raw material in 1 Pa vacuum below 1650 C.

(52) Casting process: Ar gas is filled to the melting furnace so that the Ar pressure would reach 80000 Pa after vacuum melting, then the material is casted as a strip with an average thickness of 0.15 mm by strip casting method (SC).

(53) Hydrogen decrepitation process: the strip is put into a stainless steel container of a rotating hydrogen decrepitation furnace with an inner diameter of 1200 mm, the container is then pumped to be vacuum and the vacuum level is below 10 Pa, then hydrogen of 99.999% purity is filled into the container, the hydrogen pressure would reach 0.12 MPa, the container rotates for 2 hours at a rotating rate of 1 rpm to absorb hydrogen, after that, the container is pumped for 2 hours at 600 C. to dehydrogenate, then the container rotates and gets cooled at a rotating rate of 30 rpm simultaneously, the cooled coarse powder is then taken out.

(54) Fine crushing process: a jet milling device is used to finely crush the coarse powder to obtain a fine powder with an average particle size of 5 nm.

(55) Fine powder heat treatment process: the fine powder is divided into 6 equal parts, each part is respectively put into the stainless steel container of a rotating hydrogen decrepitation furnace with an inner diameter of 1200 mm, the container is then pumped to be vacuum and the vacuum level is below 10 Pa, then Ar gas with 99.9999% purity is filled into the container to obtain a pressure of 500 Pa, the oxygen content is controlled as 18002000 ppm, and the dew point is 6050 C., then the stainless steel container is put into an externally heating oven for heat treatment, the stainless steel container rotates at a rotating rate of 5 rpm when heated.

(56) The heating temperature and heat treatment time of each part of fine powder are shown in TABLE 6.

(57) After the process of fine powder heat treatment, the container is taken out of the externally heating oven, the container is then externally water cooled at a rotating rate of 20 rpm for 3 hours.

(58) Compacting process under a magnetic field: no organic additive is added into the fine powder with the process of fine powder heat treatment, a transversed type magnetic field molder is directly used, the powder is compacted in once to form a cube with sides of 40 mm in an orientation field of 1.8 T and under a compacting pressure of 1.2 ton/cm.sup.2, then the once-forming cube is demagnetized in a 0.2 T magnetic field. The once-forming compact (green compact) is sealed so as not to expose to air, and then the green compact is delivered to a sintering furnace.

(59) Sintering process: each of the green compact is moved to the sintering furnace to sinter, in a vacuum of 10.sup.3 Pa and respectively maintained for 2 hours at 200 C. and for 2 hours at 600 C., then in Ar gas atmosphere of 0.02 MPa, sintering for 2 hours at 1080 C., after that filling Ar gas into the sintering furnace so that the Ar pressure would reach 0.1 MPa, then cooling it to room temperature.

(60) Heat treatment process: the sintered magnet is heated for 1 hour at 600 C. in the atmosphere of high purity Ar gas, then cooling it to room temperature and taking it out.

(61) Magnetic property evaluation process: the sintered magnet is tested by NIM-10000H type nondestructive testing system for BH large rare earth permanent magnet from China Jiliang University, and an average value is calculated.

(62) Oxygen content of sintered magnet evaluation process: the oxygen content of the sintered magnet is measured by EMGA-620W type oxygen and nitrogen analyzer from HORIBA company of Japan.

(63) The magnetic property and oxygen content evaluation of the embodiments and the comparing samples in same heating temperature and different heating time with the process of fine powder heat treatment are shown in TABLE 6.

(64) TABLE-US-00006 TABLE 6 The magnetic property and oxygen content evaluation of the embodiments and the comparing samples Oxygen content of Heating the sintered temperature Heating Br Hcj (BH)max magnet No. ( C.) time (hr) (kGs) (k0e) SQ (%) (MG0e) (ppm) 0 Comparing 700 0.05 13.8 9.8 81.2 45.3 2980 sample 1 Embodiment 700 0.1 15.1 13.3 97.8 54.3 565 2 Embodiment 700 1 15.2 13.6 98.2 54.8 354 3 Embodiment 700 4 15.3 14.2 99.1 55.2 375 4 Embodiment 700 12 15.4 14.1 99.2 56 395 5 Embodiment 700 24 15.3 13.5 99.1 55.3 573 6 Comparing 700 48 14.9 11.7 94.8 52.7 980 sample

(65) As can be seen from TABLE 6, at a temperature of 700 C., if the time of the fine powder heat treatment is less than 0.1 hour, the effect of the heat treatment of the fine powder is not sufficient, resulting in that it would be like no oxidation film, the adhesive power among the powder gets stronger, in this case, the values of Br, (BH)max would be extremely adverse, the phenomenon of abnormal grain growth would easily happen when sintering, and the value of coercivity Hcj would be reduced.

(66) At the same time, at a temperature of 700 C., when the time of the fine powder heat treatment exceeds 24 hours, the oxidation film on the surface of the fine powder particle would be absorbed and diffused into the particle, it would be like no oxidation film, consequently the oxygen content increases, in this case, the values of Br and (BH)max would be reduced, the phenomenon of abnormal grain growth would easily happen when sintering, and the value of coercivity Hcj would be reduced.

(67) Embodiment 4

(68) Raw material preparing process: Lu, Er, Nd, Tm, and Y with 99.5% purity, industrial FeB, industrial pure Fe, Co with 99.99% purity and C, Cu, Mn, Ga, Bi, Ti with 99.5% purity are prepared, counted in atomic percent, and prepared in R.sub.eT.sub.fA.sub.gJ.sub.hG.sub.iD.sub.k components. The contents of the elements are shown as follows: R component, Lu is 0.2, Er is 0.2, Nd is 13.5, Tm is 0.1, and Y is 0.1; T component, Fe is the remainder, and Co is 1; A component, C is 0.05, and B is 7; J component, Cu is 0.2, and Mn is 0.2; G component, Ga is 0.2, and Bi is 0.1; and D component, Ti is 1. Preparing 500 Kg raw material by weighing in accordance with above contents of elements.

(69) Melting process: the 500 Kg raw material is put into an aluminum oxide made crucible, an intermediate frequency vacuum induction melting furnace is used to melt the raw material in 0.1 Pa vacuum below 1550 C.

(70) Casting process: Ar gas is filled to the melting furnace so that the Ar pressure would reach 40000 Pa after the process of vacuum melting, then the material is casted as a strip with an average thickness of 0.6 mm by strip casting method (SC).

(71) Hydrogen decrepitation process: the strip is put into a stainless steel container of a rotating hydrogen decrepitation furnace with an inner diameter of 1200 mm, the container is then pumped to be vacuum and the vacuum level is below 10 Pa, then hydrogen of 99.999% purity is filled into the container, the hydrogen pressure would reach 0.12 MPa, the container rotates for 6 hours at a rotating rate of 2 rpm to absorb hydrogen, after that, the container is pumped for 3 hours at 600 C. to dehydrogenate, then the container rotates and gets cooled at a rotating rate of 10 rpm simultaneously, the cooled coarse powder is then taken out.

(72) Fine crushing process: a jet milling device is used to finely crush the coarse powder to obtain a fine powder with an average particle size of 2 nm.

(73) The fine powder after jet milling is divided into 2 equal parts.

(74) Fine powder heat treatment process: one part of the fine powder is put into the stainless steel container with an inner diameter of 1200 mm, the container is then pumped to be vacuum below 1 Pa, then Ar gas with 99.9999% purity is filled into the container and the pressure reaches 1000 Pa, the oxygen content is controlled as 8001000 ppm, and the dew point is 5040 C., then the stainless steel container is put into an externally heating oven to heat, the heating temperature is 600 C., the heating time is 2 hours. The stainless steel container rotates at a rotating rate of 5 rpm when heated.

(75) After the heat treatment, the container is taken out of the externally heating oven, the container is then externally water cooled at a rotating rate of 5 rpm for 5 hours.

(76) Compacting process under a magnetic field: no organic additive is added into the fine powder with the process of fine powder heat treatment, a transversed type magnetic field molder is directly used, the powder is compacted in once to form a cube with sides of 40 mm in an orientation field of 1.8 T and under a compacting pressure of 1.2 ton/cm.sup.2, then the once-forming cube is demagnetized in a 0.2 T magnetic field. The once-forming compact (green compact) is sealed so as not to expose to air, and then the green compact is delivered to a sintering furnace.

(77) Sintering process: each of the green compact is moved to the sintering furnace to sinter, in a vacuum of 10.sup.3 Pa and respectively maintained for 2 hours at 200 C. and for 2 hours at 600 C., then in Ar gas atmosphere of 0.02 MPa, sintering at 925 C. 1150 C., after that filling Ar gas into the sintering furnace so that the Ar pressure would reach 0.1 MPa, then cooling it to room temperature.

(78) Heat treatment process: the sintered magnet is heated for 1 hour at 600 C. in the atmosphere of high purity Ar gas, then cooling it to room temperature and taking it out.

(79) The other part of the fine powder is not treated with the process of fine powder heat treatment, and served as a comparing sample, which is sequentially treated with the above mentioned compacting process, sintering process and heating process except the process of fine powder heat treatment under the same treatment condition.

(80) Magnetic property evaluation process: the sintered magnet is tested by NIM-10000H type nondestructive testing system for BH large rare earth permanent magnet from China Jiliang University, and an average value is calculated.

(81) Oxygen content of sintered magnet evaluation process: the oxygen content of the sintered magnet is measured by EMGA-620W type oxygen and nitrogen analyzer from HORIBA company of Japan.

(82) The magnetic property and oxygen content evaluation of the embodiments and the comparing samples with or without the process of fine powder heat treatment in different sintering temperature are shown in TABLE 7. No. 111 are the sintered magnet without the process of fine powder heat treatment, No. 1222 are the sintered magnet with the process of fine powder heat treatment.

(83) TABLE-US-00007 TABLE 7 The magnetic property and oxygen content evaluation of the embodiments and the comparing samples Oxygen Fine content of powder Sintering the sintered heat temperature Density Br Hcj SQ (BH)max magnet No. treatment ( C.) (g/cc) (kGs) (k0e) (%) (MG0e) (ppm) 1 Comparing no 925 6.98 12.8 12.8 76.5 25.6 2840 sample 2 Comparing no 950 7.21 13.4 12.3 93.2 39.8 2940 sample 3 Comparing no 975 7.32 13.6 12.1 95.6 43.2 2850 sample 4 Comparing no 1000 7.38 13.9 11.9 96.3 44.5 2840 sample 5 Comparing no 1025 7.53 14.1 11.5 96.4 44.7 2840 sample 6 Comparing no 1050 7.54 14.2 11.2 96.3 45.9 2870 sample 7 Comparing no 1075 7.56 14.2 10.9 96.4 47.1 2780 sample 8 Comparing no 1100 7.57 14.3 10.2 96.2 47.2 2790 sample 9 Comparing no 1125 7.55 14.1 9.2 92.3 46.7 2830 sample 10 Comparing no 1140 7.51 13.8 8.5 87.4 39.8 2840 sample 11 Comparing no 1150 7.48 13.6 7.6 82.3 37.6 2980 sample 12 Comparing yes 925 7.23 13.8 9.8 81.2 45.3 982 sample 13 Embodiment yes 950 7.47 14.4 13.8 97.8 50.1 354 14 Embodiment yes 975 7.49 14.4 13.6 98.2 50.2 341 15 Embodiment yes 1000 7.51 14.5 13.5 98.3 50.4 340 16 Embodiment yes 1025 7.54 14.5 13.4 98.4 50.4 342 17 Embodiment yes 1050 7.56 14.6 13.4 98.5 50.6 345 18 Embodiment yes 1075 7.59 14.6 13.4 98.6 50.8 343 19 Embodiment yes 1100 7.61 14.7 13.4 98.9 50.8 346 20 Embodiment yes 1125 7.64 14.7 13.4 99 51.1 347 21 Embodiment yes 1140 7.65 14.8 13.4 99.1 51.2 349 22 Comparing yes 1150 7.32 13.4 12.2 76.5 38.4 768 sample

(84) As can be seen from TABLE 7, with heat treatment of the fine powder, it can expand the sintering temperature range to obtain a magnet with an excellent property. The reason is that, it avoids oxidation, so that the compacts can be sintered at a low sintering temperature, on the other hand, when sintering at a high temperature, the phenomenon of abnormal grain growth would not happen, thus it can obtain a magnet with an excellent property whether at the low sintering temperature or at the high sintering temperature.

(85) Embodiment 5

(86) Raw material preparing process: Lu, Er, Nd, Tm, and Y with 99.5% purity, industrial FeB, industrial pure Fe, Co with 99.99% purity and C, Cu, Mn, Ga, Bi, Ti with 99.5% purity are prepared, counted in atomic percent, and prepared in R.sub.eT.sub.fA.sub.gJ.sub.hG.sub.iD.sub.k components. The contents of the elements are shown as follows: R component, Lu is 0.2, Nd is 13.5, Tm is 0.1, and Y is 0.1; T component, Fe is the remainder, and Co is 1; A component, C is 0.05, and B is 7; J component, Cu is 0.2, and Mn is 0.2; G component, Ga is 0.2, and Bi is 0.1; and D component, Ti is 1.

(87) Preparing 500 Kg raw material by weighing in accordance with above contents of elements.

(88) Melting process: the 500 Kg raw material is put into an aluminum oxide made crucible, an intermediate frequency vacuum induction melting furnace is used to melt the raw material in 0.1 Pa vacuum below 1550 C.

(89) Casting process: After the process of vacuum melting, Ar gas is filled to the melting furnace so that the Ar pressure would reach 40000 Pa after vacuum melting, then the material is casted as a strip with an average thickness of 0.6 mm by strip casting method (SC).

(90) Hydrogen decrepitation process: the alloy is put into the stainless steel container of a rotating hydrogen decrepitation furnace with an inner diameter of 1200 mm, the container is then pumped to be vacuum and the vacuum level is below 10 Pa, then hydrogen of 99.999% purity is filled into the container, the hydrogen pressure would reach 0.12 MPa, the container rotates for 6 hours at a rotating rate of 2 rpm to absorb hydrogen, after that, the container is pumped for 3 hours at 600 C. to dehydrogenate, then the container rotates and gets cooled at a rotating rate of 10 rpm simultaneously, the cooled coarse powder is then taken out.

(91) Fine crushing process: a jet milling device is used to finely crush the coarse powder to obtain a fine powder with an average particle size of 2 nm.

(92) Fine powder heat treatment process: the fine powder is put into a stainless steel container with an inner diameter of 1200 mm, the container is then pumped to be vacuum obtain a pressure of below 1 Pa, then Ar gas with 99.9999% purity is filled into the container to obtain a pressure of 900 Pa, the oxygen content is controlled as 8001000 ppm, and the dew point 5040 C., then the stainless steel container is put to an externally heating oven for heat treatment, the heating temperature is 600 C., the heating time is 2 hours. The stainless steel container rotates at a rotating rate of 5 rpm when heated.

(93) After the heat treatment of the fine powder, the container is taken out of the externally heating oven, the container is then externally water cooled at a rotating rate of 5 rpm for 5 hours.

(94) Compacting under a magnetic field process: no organic additive is added into the fine powder with the process of fine powder heat treatment, a transversed type magnetic field molder is directly used, the powder is compacted in once to form a cube with sides of 40 mm in an orientation field of 1.8 T and under a compacting pressure of 1.2 ton/cm.sup.2, then the once-forming cube is demagnetized in a 0.2 T magnetic field. The once-forming compact (green compact) is sealed so as not to expose to air, and then the green compact is delivered to a sintering furnace.

(95) Sintering process: each of the green compact is moved to the sintering furnace to sinter, firstly sintering in a vacuum of 10.sup.3 Pa and respectively maintained for 2 hours at 200 C. and for 2 hours at 600 C., then in Ar gas atmosphere of 0.02 MPa, sintering at 980 C., after that filling Ar gas into the sintering furnace so that the Ar pressure would reach 0.1 MPa, then cooling it to room temperature.

(96) Heat treatment process: the sintered magnet is heated for 1 hour at 600 C. in the atmosphere of high purity Ar gas, then cooling it to room temperature and taking it out.

(97) Machining and RH diffusion processes: After the heat treatment process, the sintered magnet is machined as a magnet with a diameter of 15 mm and a thickness of 5 mm, the 5 mm direction (along the direction of thickness) is the orientation direction of the magnetic field. The machined magnet is washed and surface cleaned. A raw material with the Dy oxide and Tb fluoride is prepared in proportion of 3:1, fully sprayed and coated on the magnet, then the coated magnet is dried. In high purity of Ar gas atmosphere, the heat and diffusion process is performed at 6801050 C. for 12 hours.

(98) Magnetic property evaluation process: the sintered magnet is tested by NIM-10000H type nondestructive testing system for BH large rare earth permanent magnet from China Jiliang University, and an average value is calculated.

(99) Oxygen content of sintered magnet evaluation process: the oxygen content of the sintered magnet is measured by EMGA-620W type oxygen and nitrogen analyzer from HORIBA company of Japan.

(100) The magnetic property and oxygen content evaluation of the embodiments and the comparing samples at different sintering temperatures after heat treatment are shown in TABLE 8.

(101) TABLE-US-00008 TABLE 8 The magnetic property and oxygen content evaluation of the embodiments and the comparing samples Oxygen content of Diffusion Diffusion the sintered temperature time Density Br Hcj SQ (BH)max magnet No. ( C.) (hr) (g/cc) (kGs) (k0e) (%) (MG0e) (ppm) 1 Comparing 680 8 7.49 13.5 11.3 81.1 43.2 972 sample 2 Embodiment 700 8 7.50 14.0 19.8 98.2 46.6 954 3 Embodiment 750 8 7.52 14.2 20.8 98.6 47.2 941 4 Embodiment 800 6 7.52 14.2 21.3 98.3 46.8 940 5 Embodiment 850 6 7.51 14.4 22.1 99.4 47.6 942 6 Embodiment 900 4 7.51 14.2 22.5 99.5 46.6 945 7 Embodiment 950 4 7.52 14.2 23.0 99.6 46.2 943 8 Embodiment 1000 2 7.51 14.2 24.4 99.7 46.2 946 9 Embodiment 1020 2 7.52 14.2 24.4 99.3 46.1 947 10 Comparing 1040 2 7.50 14.2 23.1 99.1 46.1 949 sample 11 Comparing 1050 2 7.49 13.4 18.7 79.8 42.8 968 sample

(102) As can be seen from TABLE 8, as an oxidation layer is formed on the surface of the overall powder, the existence status of the oxygen at the grain boundary of the magnet is changed obviously, the diffusion rate of the heavy rare earth element is accelerated and the diffusion efficient is promoted; therefore it is capable of subverting the common sense and accomplishing the grain boundary diffusion in a short time.

(103) With the heat treatment of the fine powder, the property of the powder is changed drastically, the magnet is machined with a desired size after being sintered, and then treated with grain boundary diffusion; in the present invention, the grain boundary diffusion experiments are conducted at temperature of 680 C.1050 C., the temperature of 700 C.1020 C. is set as the grain boundary diffusion temperature and the temperature range of 1000 C.1020 C. is the most appropriate for the Dy grain boundary diffusion temperature.

(104) Common sense says that it generally takes more than 10 hours for the grain boundary diffusion of a magnet with a thickness of 5 mm in a temperature range of 800 C.950 C. so as to obtain an improving effect of coercivity; raising the diffusion temperature is benefit to shorten the diffusion time, but it may leads to the problems of deformation, surface molten and AGG, and the diffusion is simultaneously performed in the grain boundary phase and the main phase, resulting in losing of magnet property. In contrast, the diffusion to the magnet of the present invention is performed in a temperature range of 1000 C.1200 C. and only needs 2 hours, which is capable of obtaining an improving coercivity effect and shortening the production cycle without arising the above mentioned problems.

(105) Although the present invention has been described with reference to the preferred embodiments thereof for carrying out the patent for invention, it is apparent to those skilled in the art that a variety of modifications and changes may be made without departing from the scope of the patent for invention which is intended to be defined by the appended claims.