Title of Invention

RARE EARTH ALLOY - MELTING CRUCIBLE AND RARE EARTH ALLOY

Abstract

A crucible for the melting of a rare earth alloy, comprising AI2O3., Al2TiO5 and at least one selected from rare earth oxides inclusive of Y2O3. as main components, characterized in that the crucible is obtained by firing at 1,000 to 1,700°C, the rare earth oxide is distributed at a higher proportion in a portion of fine particles having a particle size of up to 0.5 mm than in a portion of coarse particles having a particle size in excess of 0.5 mm, and the crucible is substantially free of the reaction products of the rare earth oxide with AI2O3. and Al2TiO5.

Full Text

FORM 2 THE PATENTS ACT, 197 0 (39 of 1970) COMPLETE SPECIFICATION [See Section 10, rule 13) RARE EARTH ALLOY-MELTING CRUCIBLE. AND RARE EARTH.ALLOY. SHIN-ETSU CHEMICAL CO., LTD. of 6-1, OTEMACHI 2-CHOME, CHIYODA-KU, TOKYO, 100-0004, JAPAN JAPANESE Company The following specification particularly describes the nature of the invention and the manner in which it is to be performed : - 276/MUMNP/2004 12/5/2004 22-3-2006 This invention relates to a crucible for use in the high-frequency melting of rare earth alloys and rare earth alloys obtained using the crucible. 10 BACKGROUND ART. Rare earth alloys are recently used in a variety of fields as magnet material, battery electrode material and the like. It is important to manufacture rare earth alloys 15 of consistent' quality at a low cost. In general, the rare earth alloys are produced by weighing amounts of raw materials so as to give a desired composition, placing them in a crucible, high-frequency melting, and feeding the melt into a mold or rotating rolls 20 for casting. The crucible used for high-frequency melting can be manufactured by a conventional process, as used in the ceramic preparation, involving mixing predetermined raw materials with water or a binder and water to of orm a slurry, 25 and shaping the slurry, followed by removal from the mold and firing (or drying). Since the molten rare earth alloy is extraordinarily reactive, the crucible used for melting of the crucible material must meet the following requirements 30 Namely, the crucible material must have, (1) least reactivity with water and (2) a certain degree of stability. On the other hand, the crucible is given requirements Including (1) resistance to cracking by thermal- expansion upon high-frequency heating (resistance to thermal shocks), 35 (2) high mechanical strength, (3) easy removal of the slag which deposits within the crucible at the end of melting, and (4) low cost To meet, these requirements, the crucible.s used in the melting of rare earth alloys have been constructed of AI2O3, or A1203 with additives. Al203 used herein has a stability which is relativoly 5 high among oxides, but inferior to rare earth metals, allowing reaction to proceed little by little.. The reacted portion becomes a slag. The slag, which is strongly bound to the crucible and difficult to remove, gives rise to some' problems that the slag removal operation takes a long time 10 to detract from productivity and that the crucible can be damaged during the slag removal operation. These problems may be solved by using rare earth oxides and calcium oxide which are highly stable and least reactive with rare earth metals. Because of. their 15 likelihood to form hydroxides,, they can be used as unshaped refractory or added only in small amounts, but are difficult to use as the primary material of shaped refractory to construct the crucible. 20 DISCLOSURE OF THE INVENTION Therefore, an object of the invention ;is to provide a rare earth alloy melting crucible which allows any slag generating thereon to be readily removed, can. be used repeatedly, and is inexpensive; and a rare earth alloy 25 obtained using the crucible. Analyzing the reaction of a conventional crucible made of A1203 with an Nd-Fe-B base alloy typical of rare earth alloys and a slag thereof, the inventors found the following problems. 30 More particularly, since the particle size formulation of the crucible is important in order that the. crucible be unsusceptible to cracking by thermal expansipn upon high-frequency heating (or be improved in strength to thermal shocks), the particle size distribution is adjusted 35 in accordance with the purpose or the size of .the crucible. Al2OJ particles having a particle size of up to., 2 mm. to. 100% by volume of the coarse particle portion is Al2O3 and (3) a rare earth alloy obtained using a crucible according . to (1) or (2). 5 Also, the present invention provides: . (4) a crucible for the melting of a rare earth alloy, comprising Al2Ti05 and at least one selected from rare earth oxides inclusive of Y203 as main components, characterized in that the crucible is obtained by firing at 1,000 to 1,700" C, 10 the rare earth oxide is distributed at a higher proportion in a portion of fine particles having a particle size of up to 0. !3 nun than in a portion of coarse particlose having particle size in excess of 0.5 nun, and the crucible in substantially free of the reaction product of the rare eartli 15 oxide with Al2Ti05; (5) a crucible for the melting of a rare earth alloy, comprising A1203, Al2Ti05 and at least one selected from rare earth; oxides inclusive of Y,03 as main components, characterized in that the crucible is obtained by firing at 20 1,000 to 1,700°C, the rare earth oxide is distributed at a higher proportion in a portion of fine particles having a particle size of up to 0.5 mm than in a portion of coarse particles having a particle size in excess of; 0.5 nun, and the crucible is substantially free of the reaction products 25 of the rare earth oxide with A1203 and Al2Ti05 (6) a crucible for the melting of a rare earth alloy according to (.4) or (5), characterized, in that 2 to 100% by volume of the fine particle portion is the rare,earth oxide; (7) a crucible for the melting of a rare earth alloy 30 according to (4), (5) or (6), characterized in that the at least one rare earth oxide is Y203; and (8) a rare earth alloy obtained using a crucible according to any one of (4) to (7). BRIF DESCRIPTION OF THE DRAWING FIG. 1 is a frogmental. cross-sectional view of a crucible according to the invention in which a slay has generated. 5 FIG. 2 is a fragmental cross-sectional view of a conventional crucible in which a slag has generated. FIG. 3 is a fragmental cross-sectional view of another conventional crucible in which a slag has generated. 10 BEST MODE FOR CARRYING OUT THE INVENTION A rare earth alloy melting crucible according to a first embodiment of the invention is one comprising A1203 and one or more oxides selected from rare earth oxides inclusive of Y203 as main components, characterized in that the 15 crucible is obtained by firing at 500 to 1,800°C, the rare earth oxide is distributed at a higher proportion in a portion of fine particles having a. particle size of up to 0.5s mm than in a portion of coarse particles having a particle size in excess of 0.5 mm, and the crucible is 20 substantially free of the reaction product of the rare earth oxide with A1203. The rare earth oxide which may be used is one or more oxides selected from oxides of rare earth elements including yttrium (Y) and ranging from La to Lu. Of these, Y203, Ce02, 25 Dy203, Tb407 and Sm203 having a lower degree of hydroxide formation are preferred for use. The rare earth oxide is distributed in ,the fine particle portion with a particle size of up to 0.5 mm in a high proportion in order to restrain reaction of the 30 cifucible with a rare earth alloy. Specifically, it is preferred.that at least 50% by volume, especially at least 60% by volume of the rare earth oxides in the entire crucible be present in the fine particle portion. Also preferably, the content of rare earth oxide in 35 the fine particle portion is 2 to 100% by volume, and especially 10 to 100% by volume. In this portion, the balance may be one of ceramics such as A1203, Si02, T102. Zr02, MgO, CaO, Sl^N,,. DN and T103 or a combination thereof. On the other hand, since the coarse particle portion 5 having a particle size in excess of .0.5 mm is relatively less reactive, it is possible to use A1203 having a good balance of mechanical strength, stability and cost. The content of A1203 is 20 to 100% by volume, and especially 50 to 100% by volume of the coarse particle portion. 10 In this portion, the balance may be one of rare earth oxides such as Y203, Ce02, Dy203, Tb407 and Srn203 and ceramics such as Si02, Ti02, ZrOz, MgO, CaO, Si3N„, BN and TiB2, or a combination thereof. The addition amount thereof is preferably 50% by volume or less. When the rare earth oxide 15 is used in the coarse particle portion, it must be included such that it is richer in the fine particle portion than in the coarse particle portion. i It is noted that the size of coarse particles is preferably up to 10 mm, and especially up to:5 mm. 20 A rare earth alloy melting crucible according to a second embodiment of the invention is one qomprising Al2T:LOs and at least one selected from rare earth oxides inclusive of Y203 as main components, characterized in, .that the crucible is obtained by firing at 1,000 to 1,700°C, the rare 25 earth oxide is distributed at a higher proportion in a portion of fine particles having a particle.size of up to 0.5 mm than in a portion of coarse particles having a particle size in. excess of 0.5 mm, and the crucible is substantially free of the reaction product of the rare earth 30 oxide with Al2Ti05; and a rare, earth alloy melting crucible ..according to a third embodiment of the invention is one comprising A1203, Al2Tid5 and at least one selected from rare,-earth oxides inclusive of.'Y203 as main components, characterized in that 35 the crucible, is obtained by firing at l,00q,,,to 1,700°C, the rare earth oxide is distributed at a higher,proportion in a portion of fine particles having a parhH cle,size of up to 0.5 mm than in a portion of coarse particles-having a particle size in excess of 0.5 nun, and the crucible is substantially.free of the reaction products of the rare earth oxide with A1203 and Al2Ti05. 5 The rare earth oxide which may be used is one or more oxides selected from oxides of rare earth elements including yttrium (Y) and ranging from La to Lu, as in the first embodiment. Of these, Y203, Ce02, Dy203, Tb407 and Srn203 having a lower degree of hydroxide formation are preferred for use. 10 Inter alia, Y203 having excellent stability is most preferred for use. , The rare earth oxide is distributed in the fine particle portion with a particle size of up to 0.5 nun in a high proportion in order to restrain reaction of the 15 crucible with a rare earth alloy. Specifically, it is preferred that at least 50% by volume, especially at least 60% by volume of the rare earth oxides in the entire crucible be present in the fine particle por.tl.on. Also preferably, the content of rare earth oxide in 20 the fine particle portion is 2 to 100% by volume, and especially 10 to 100% by volume. In this portion, the balance is preferably Al2Ti05, although it is. acceptable from the standpoints, of mechanical strength and the like to add one of ceramics such as AlzOJ32, 25 Si02, Ti02, Zr02, MgO, CaO, Si3N„, BN and TiB2 or a combination thereof in an amount of up to 50% by volume. On the other hand, Al2TiOs becomes the main component of the coarse particle portion having a particle size in excess of 0.5 mm, and preferably up to 5 mm. ;!. 0, Herein, part of Al2Ti05 may be replacedi by, A1203 which is commonly used as the raw material of the crucible, although a mixture with a higher proportion of Al2Ti05 is preferred as long as mechanical strength is ensured for the crucible. 5 When part of Al2Ti05 is replaced by A1?03^ the replacement proportion is preferably up to 8,0,%, by volume and especially up to 30% by volume. It is noted that to the coarse particle portion, one or a mixture of the above-mentioned tare earth oxide::; and ceramics such as Si02, Ti02, Zr02., MgO, CaO, Si3N, BN and TiB2, may be added in a proportion of up to 50% by volume: 5 When the rare earth oxide is used in the coarse particle portion, it, must be included such that it is- richer in the fine particle portion than in the coarse particle portion, as previously described. In the practice of the invention, the mixing 10 proportion of fine particles and coarse particles va:ries with the size of the crucible or the like, and is preferably such that the fine particle portion is 10 to 60% by volume, and especially 20 to 40% by volume. If the fine particle portion is less than 10% by volume, strength lowers with a 15 high probability. If the fine particle portion is more than 60% by volume, a possibility of failure by thermal shocks-increases . The crucible of the invention is produced, for example, by the following procedure. 20 Rare earth oxide and A1?03 and/or Al2Ti05 are placed on a sieve having openings of a predetermined diameter (e.g., 5 mm or 0.5 mm), and classified into a fine-particle portion having a particle size of up to 0.5 mm and a coarse particle portion (e.g., with a particle size of 0.5, ,to 5 mm). The 25 rare earth oxide is used for the fine particle portion, and Al2O3 ond/or Al2O3 is used for the coarse particle problem The fine particle portion and the coarse particle portion are,mixed so that the fine particle portion accounts for 10' to 60% by volume. A slurry of the mixture is filled 30 in a predetermined mold where it is fired in an air atmosphere, vacuum atmosphere or inert gas atmosphere of Ar or the like, at 500 to 1,800°C, preferably 1,000 to 1,7 00°C, obtaining a crucible. If firing is effected at temperatures higher than 35 1,800°C, reaction can occur between the rare earth oxide and A1203 and/or-. Al2Ti05, and some areas are densified at the same time, with an increased possibility of becoming brittle to placing the blend in the inventive crucible, effecting high- frequency inciting In an Inort gas atmosphere such as A1 at 500 to 1,800°C, preferably 1.000 to l,700C, and pouring the resulting melt into a mold, followed by cooling. 5 It .is noted that the temperature used in high-frequency melting is preferably controlled within the above range in order to prevent any reaction with the rare earth oxide, Al2Ti05 and other components of the crucible. 10 EXAMPLE Examples and Comparative Examples are given below by way of illustration although the invention is not limited to these Examples. Ce02 and A1203 were passed through two sieves of 5 mm and 0.5 mm and classified into a fine particle portion having a particle size of up to 0.5 rnm and a coarse particle portion having a particle size of 0.5 to 5 mm CeO2, was 20 assigned to the fine particle portion., and QAl2O3 was assigned to the coarse particle portion. They w.ere mixed in a proportion of 3,0;% by volume the fine particle portion and 70% by volume the coarse particle portion. A flurry of the mixture was: prepared and poured in 25 a gypsum mold where it was allowed to stand over 2 days. By subsequent removal from the mold, holding for a further 2 days and firing 1,550°C, a crucible was obtained. The crucible had an outer diameter of 540 mm, a height of 840 mm, a thickness of 40 mm and a. weight of 229 kg. 3p In this crucible was placed 500 kg of a raw material for Nd-Fe-B base magnet which had been weighed so as to give the compositional formula: 30 . 5Nd-l. 2Dy-l. 0B'-2'. OCo-O . 2A1-65.1Fe (in %.by weight). After melting in 'an Ar atmosphere by high-frequency heating at 1,500°C for 70 minutes, the 35 melt was cast into a mold. The crpcible was allowed to stand for 8Q minutes for cooling and then opened to air, followed by.^slag removal operation. Theareafter, the raw material was placed there in and melted again. In this way, the process- was repeated until the crucible became unusable. The crucible failed during the slag removal operation at the end of 50th melting 5 cycle. The products of these 58 cycles were in an average yield of 98.7%, and the average time taken- for slag removal was 13 minutes. .! Example 2 10 A crucible was prepared as in Example 1 except that a mixture consisting of 50% by volume of Ce02, 30% by volume of A1203 and 20% by volume of Si02 was prepared and used for the fine particle portion with a particle size of up to 0.5 mm, and a mixture consisting of 10% by volume of Ce02 and 15 90% by volume of A120;, was prepared and used for the coarse particle portion with a particle size in excess of.0.5 mm. Using this crucible, a rare earth alloy was melted as ln'\ Example 1. The crucible failed during/.- the slag removal operation at the end of 54th melting cycle;. The products of 20 these 54 cycles were In an ivnrogo yield of- Oft..1",'?,, and I lie j , . ■ ■ average time taken foa: slag aremoval was 15 minutes. i Example 3 A crucible was prepared as in Example 1 except that a 25 mixture consisting of 80% by volume of Ce02 and 20% by volume of Y203 was prepared and used for the fine particle portion with a particle size of up to 0.5,mm, and a mixture consisting, of 70% by volume of A1203 and 30% by volume of Si02 was prepared and used foa: the coarse particle portion 30 with a particle size in excess of 0.5 mm. Using this crucible, a rare earth alloy was melted as in Examplei 1. The crucible failed during the slag removal operation ,at the end of 68th melting cycle. The products of these 68 cycles were in an average,-yield;,of 098 . 9%, and the 35 average time taken for slag removal, was .10.minutes. Example 4 A crucible was prepared as in Example 1 except that Y203 was used for the fine particle portion with a particle size of up to 0.5 mm. 5 Using this crucible, a rare earth alloy was melted as in Example 1. The crucible failed during the slag removal operation at.the end of 75th melting cycle. The products of these 75 cycles were in an average yield, of ,.99 . 0%, and the average time taken for slag removal was 9 minutes. 10 A crucible was prepared as in Example 1 except that a mixture consisting of 50% by volume of Y203, 30% by volume of. A1203 and 20% by volume of Si02 was prepared and used for the 15 fine particle portion with a particle size of up to 0.5 mm, and a mixture consisting of 10% by volume of Y203 and 90% by volume of A1203 was prepared and used for the coarse particle portion with a particle size in excess of 0.5 mm. Using this crucible, a rare earth alloy was melted as 20 in Example,1. The crucible failed during the slag removal operation at the end of 65th melting cycle. .The products of these 65 cycles were in an average yield of 98.8%, and the average time taken for slag removal was 11 minutes. 25 Example 6 , A crucible was prepared as in Example.. 1 except that a mixture consisting of 90% by volume of Y203 and 10% by volume of Dy203 was prepared and used for the fine particle portion with a particle size of up to 0.5 mm, and,a mixture 30 consisting of 70% by volume of A12Q3 and 3,0% by volume of Si02 was prepared and used for the coarse article portion with a particle size in excess of 0.5 min.. Using this crucible, a rare earth alloy was melted as in Example1; 1. The crucible failed during the slag removal 35 operation at the end of 72nd melting cycle. The products of these 72 cycles were in an average yield ,of. ,98.8%, and the average time taken for slag removal was 10 minutes. Example 8 A crucible was properd as in Example 7 exept that a mixture consisting Of 50% by volume of Y2Q3 30% by volume 01 Al2TiOs and: 20% by volume of Si02 was prepared and used for 5 the fine particle portion with a particle size of up to 0.5 mm, and a mixture consisting of 10% by volume of Y203 and 90 2 by volume of Al2Ti05 was prepared and used for the coarse particle portion with a particle size in excess of 0.5 mm. Using this crucible, a rare earth alloy was melted as 10 in Example 7. The crucible failed during the slag removal operation at the end of 188th melting cycle. The products of these 188 cycles were in an average yield of 98.9%, and the average time taKen for slag removal was 9 minutes. 15 Example 9 A crucible was prepared as in Example 7 except that a mixture consisting of 30% by volume of Y20:,, 30% by volume of Al2Ti05, 10% by volume of A1203 and 30% by volume of Si02 was prepared and used for the fine particle portion with a 20 particle size of up to 0.5 mm, and a mixture consisting of 70% by volume of Al2Ti05, 20% by volume of A1203 and 10% by volume of Si,02 was prepared and used for the coarse particle portion with a particle size in excess of , 0 . 5 mm. Using this crucible, a rare earth alloy was melted as 25 in Example 7. The crucible failed during the slag removal operation at the end of 196th melting cycle. The products of these 196 cycles were in an average yiel,d of 98.8%, and the average time taJcen for slag removal was II minutes. 30 Comprative Example 5 A crucible was prepared as in Example 7 except that A1203 was used for both the fine particle portion with a particle size of up to 0.5 mm and the coarse particle portion with a particle size in excess of 0.5 mm. 35 Using this crucible, a rare earth alloy was melted as in Example 7 The crucible failed during the slag removal operation at the end of 36th melting cycle- The products of these 36 cycles were in an average yield of 98.0%, and the average time taken for slag removal was 33 minutes. A crucible was prepared as in Example 7 except that A1203 was used for the fine particle portion with a particle size of up to 0.5 mm and Al2TiO5 was used for the coarse particle portion with a particle size in excess of 0.5 mm. Using "this crucible, a rare earth alloy was melted as in Example 7. The crucible failed during the slag removal operation at the end of 51st melting cycle. The products of these 51 cycles were in an average yield of 98.2%, and the average time taken for slag removal' was 24 minutes. The results of the foregoing Examples and Comparative Examples are shown in Table 2. Table 2 \ Fine particle portion (vol%) Coarse particle portion (vol%) Use cycles Yield (%) Slag removal time (min) Y»03 M,TiO., A 1,0, Sio, Y03, M,Ti05 Al,03 SIO, Example 7 100 -' - - "!■ 100 - - 17 2 99. 1 7 Example 8 50' 30 - 20 10 90 - i.:/ ' 10U 9 0 . 9 9 Example 9 30\ 30 10 30 - 70 20 10 V "196 90.8 11 Comparative Example 5 • - - 100 . - - - 100 36 90.0 33 Comparative Example 6 - - 100 - - 100 - ~l f ' 51 98.2 24 As shown in Table 2, a comparison of(Examples 7-9 with Comparative Examples 5-6 reveals that Examples, containing rare earth oxide at a higher proportion in -the, portion having a particle size of up to 0.5 mm succeed in significantly increasing the use cycles of the crucible and are easy to remove slag and improved in product yield. A comparison of Example 7 with Example 4 reveals that the life of the crucible is significantly prolonged using , Al2Ti05. When A1203 or Si02 is added in order to increase the strength of the crucible as in Examples 8 and 9, the use cycles are further increased despite some drops in product yield and slag removal time as compared with Example 7. As described above, the present invention is successful in extending the lifetime of the crucible, reducing the slag removal time, increasing the throughput of products, reducing the labor cost, and achieving an increase in product yield. We Claim: 1. A crucible for the melting of a rare earth alloy, comprising AI2O3., Al2TiO5 and at least one selected from rare earth oxides inclusive of Y2O3. as main components, characterized in that the crucible is obtained by firing at 1,000 to 1,700°C, the rare earth oxide is distributed at a higher proportion in a portion of fine particles having a particle size of up to 0.5 mm than in a portion of coarse particles having a particle size in excess of 0.5 mm, and the crucible is substantially free of the reaction products of the rare earth oxide with AI2O3. and Al2TiO5. 2. A crucible for the melting of a rare earth alloy as claimed in claim 1, wherein 2 to 100% by volume of the fine particle portion is the rare earth oxide and 20 to 100% by volume of the coarse particle portion is AI2O3. A crucible for the melting of a rare earth alloy, comprising Al2TiO5 and at least one selected from rare earth oxides inclusive of Y2O3 as main components, wherein the crucible is obtained by firing at 1,000 to 1.700°C, the rare earth oxide is distributed at a higher proportion in a portion of fine particles having a particle size of up to 0.5 mm than in a portion of coarse particles having a particle size in excess of 0.5 mm. and the crucible is substantially free of the reaction product of the rare earth oxide with Al2TiO5. A crucible for the melting of a rare earth alloy, comprising Al2O3 and at least one selected from rare earth oxides inclusive of Y2O3 as main components, wherein the crucible is obtained by firing at 500 to 1,800°C, the rare earth oxide is distributed at a higher proportion in a portion of fine particles having a particle size of up to 0.5 mm than in a portion of coarse particles having a particle size in excess of 0.5 mm, and the crucible is substantially free of the reaction product of the rare earth oxide with AI2O3. A crucible for the melting of a rare earth alloy as claimed in claim 4 or 5, wherein 2 to 100% by volume of the fine particle portion is the rare earth oxide. A crucible for the melting of a rare earth alloy as claimed in claim 4, 5 or 6, wherein the at least one rare earth oxide is Y2O3.. Dated this 11th day of May, 2004. HIRAL CHANDRAKANT JOSHI AGENT FOR SHIN-ETSU CHEMICAL CO., LTD.

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276-mumnp-2004-correspondence(17-07-2007).pdf

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