PROCESS FOR FABRICATING SUPERCONDUCTING CERAMICS MATERIALS

Abstract: A R-A-Cu-O based superconducting ceramics is fabricated as follows. First, a starting powder is provided and molded to form a molded body. Subsequently, the molded body is sintered in an oxygen-poor atmosphere with the partial pressure of oxygen being below 0.2 atm and then heat-treated in an oxygen-rich atmosphere with the partial pressure of oxygen being above 1 atm to thereby obtain ceramics having an increased density and improved superconducting characteristics. The ceramics is optionally subjected to cooling under specified cooling conditions in order to increase characteristics of the ceramics.

Description: DESCRIPTION PROCESS FOR FABRICATING SUPERCONDUCTING CERAMICS MATERIALS Technical Field The present invention relates to a process for fabricat¬ ing superconducting ceramics materials comprising a yttrium- including rare earth element, an alkaline earth metal and copper oxide (hereinafter referred to as "R-A-Cu-0 based ceramics" ) which has a high theoretical density ratio and thus has a high critical current value. Background Art Conventional processes for fabricating worked materials •such as wire rods, plates, strings, coils, and ribbons made of an R-A-Cu-0 based superconducting ceramics include a process in which starting powders, i.e., an 2O3 powder, an alkaline earth metal carbonate powder, and a CuO powder, each having an average grain size of not greater than 10 um, are provided and compounded in a predetermined compounding ratio followed by mixing to obtain a mixt powder, which is then calcined, e.g., in an atmosphere of dry air or an oxygen atmosphere of 1 atm, at a temperature of from 930 °C to 950 °C for a retention time of 5 hours to form a ceramics material having a composition of YBa2Cu3θ and having a perovskite structure, for example, the ceramics is then ground to obtain powder of an average grain size of from 0.4 jum to 10 μm and applied molding force of from 1 to 2 ton/cm 2 to obtain a molded body, the molded body is sintered in an electric furnace in an atmosphere of 1 atm, at a temperature of from 900 °C to 950 °C for a retention time of 12 hours and then cooled in the furnace and heat-treated in an oxygen flow of 1 atm, at a temperature of from 500 °C to 600 °C for a retention time of from 2 to 5 hours to obtain a superconduct¬ ing ceramics having a composition substantially the same as that of the ceramics as above described. With the above-described conventional processes, the superconducting ceramics having high density and thus high mechanical strength is obtained when the molded body is sintered in an oxygen-poor atmosphere with partial pressure of oxygen being low. In other words, the lower the partial pressure of oxygen, the higher the density of the super¬ conducting ceramics. However, the resulting superconducting ceramics tends to lack oxygen because the atmosphere of sintering is poor in oxygen. For example, when fabrication of YBa2Cu3θy is contemplated, product has a composition of YBa2Cug0 _ (j where d indicates degree of deficiency of oxygen. This causes deterioration of the superconducting characteristics such as high critical current value and high critical temperature of worked superconducting ceramics materials. Disclosure of Invention An object of the present invention is to obviate defects of the conventional processes and provide a process for fabricating worked superconducting ceramics materials having g ens y an g me , n having high critical current value and high critical tempe¬ rature. As a result of extensive investigation, it has now been found that sintering of superconducting ceramics powder in two steps with different partial pressure of oxygen gives rise to superconducting ceramics material having a high density and an excellent superconducting characteristics. Therefore, the present invention provides a process for fabricating worked superconducting ceramics material composed essentially of a yttrium-including rare earth element, an alkaline earth metal, copper and balance oxygen and unavoidable impurities, comprising: (A) providing starting powder composed of: (i) a member selected from the group consisting of: (1) a mixt powder composed of powders of component oxides: (1-1) a yttrium-including rare earth element oxide, (1-2) an alkaline earth metal oxide, and (1-3) copper oxide, and having a composition substantially the same as that of a contemplated superconducting ceramics, (2) a mixt powder composed of powders of: (2-1) a partial compound between any two of the component oxides constituting a contemplated superconductive ceramics selected from (1-1), (1-2) and (1-3), and (2-2) the rest of the component oxides selected from (1-1), (1-2) and (1-3), and having a composition substantially the same as that of a contemplated superconducting ceramics, (3) powder of a superconducting ceramics composed essen tially of a yttrium-including rare earth element, an alkaline earth metal, copper and balance oxygen and unavoidable impurities, (4) a mixt powder composed of at least two members . selecte from the mixt powders (1), (2) and (3); and (B) molding the starting powder to form a molded body; (C) optionally calcining the starting powder when the starting powder is the mixt powder (i)-(l), (i)-(2) or (i)- (4) at a temperature- sufficient for calcining the starting powder prior to or after molding; (D) sintering the molded body in an oxygen-poor atmosphere with the partial pressure of oxygen being below 0.2 atm; and (E) heat-treating the sintered molded body in an oxygen-rich atmosphere with the partial pressure of oxygen being above 1 atm. Best Mode for Carrying Out the Invention Hereinafter, description will be made on a typical embodiment of the present invention, in which the starting powder for fabricating superconducting ceramics material is powder of a superconducting ceramics (R-A-Cu-0) composed essentially of a yttrium-including rare earth element (R), an alkaline earth metal (A), copper (Cu), and balance oxygen (0) and unavoidable impurities. Suitable examples of the yttrium-including rare earth element which can be used include yttrium (Y), erbium (Er), dysprosium (Dy), ytterbium (Yb), gadolinium (Gd), thulium (Tm), neodymium (Nd), (Lu), and samarium (Sm). Of these, yttrium is preferred. The yttrium-including rare earth elements can be used singly or in combination. Suitable examples of the alkaline earth metal which can be used in the present invention include barium, strontium and calcium, with barium being preferred. The alkaline earth metal can be used singly or in combination. Powder of superconducting ceramics which can be used as a starting powder in the process of the present invention can be prepared, e.g., by calcining a mixture of single component oxides with a composition substantially the same as that of a contemplated superconducting ceramics material in an oven, e.g., electric oven at a temperature suitable for preparing ceramics, e.g., 350 °C to 950 °C, and grinding the resulting superconducting ceramics material to have a desired mean grain size, e.g., not greater than 10 um. Generally, smaller mean grain size, e.g., not greater than about 6 μm, is preferred. Of course, it is possible to use superconducting ceramics material otherwise prepared as far as it has a composition of R-A-Cu-0, e.g., YBa 2 Cu3θ 7 , to be fabricated after grinding, if desired. The superconducting ceramics powder may preferably have perovskite structure. Examples of known ceramics include those described, e.g., in Tomoji Kawai and Masaki Kanai, Japanese Journal of Applied Physics, vol. 26, No. 5, May, 1987, pp.L736-L737; and Masanobu Kobayashi et al., ibid. pp.L754-L756. The starting powder is subjected to molding treatment such as pressing and a like to obtain a molded body in the form of pellets, films, sheets, etc. having a predetermined density. It is noted that "molding treatment" includes formation in pellets by pressing, film formation using a doctor's blade method, film formation by screen printing and the like. Then, the molded body is sintered in an oxygen-poor atmosphere with the partial pressure of oxygen being below 0.2 atm, at a temperature (Ts) of from 870 °C to 950 °C for a retention time ranging from 4 to 12 hours. The sintering serves to increase the density of the sintered molded body so that it can have high mechanical strength. Then, the molded body sintered in an oxygen-poor atmosphere are heat-treated in an oxygen-rich atmosphere with the partial pressure of oxygen being above 1 atm, at a temperature (Th) of from 350 °C to 950 °C for a retention time ranging from 2 to 12 hours, thereby obtaining a super¬ conducting ceramics material having an excellent super¬ conducting characteristics and high mechanical strength. The heat-treatment serves to increase the oxygen content of the superconducting ceramics material without reducing the mechanical strength thereof. This is also useful for relieving or relaxing stress generated during molding and/ working. After molding, the molded body may be subjected to co - or ot-wor ng, e.g., swag ng, ro ng w groove rolls, processing with a die, or a like, to produce a worked body having a desired shape, e.g., wire rods, plates, ribbons, strings, coils, pellets and the like. If appropriate or desired, the cold- or hot-working may be preformed after molding and optional calcination and prior to sintering. In another typical embodiment of the present invention, the superconducting ceramics material can be fabricated starting from a mixture of powder of a yttrium-including rare earth element oxide (R2O3) , powder of an alkaline earth metal oxide (AO) and powder of copper oxide (CuO). The composition of these single component oxides are blended so that the resulting mixture has a composition substantially the same as that of a final superconducting ceramics material. Examples of the yttrium-including rare earth element oxide which can be used include oxides of yttrium-including rare earth element. The same yttrium-including rare earth elements described above with respect to the preceding embodiment can also be used in the instant embodiment. Of the oxides, generally yttrium oxide is preferred. The yttrium-including rare earth elements can be used singly or in combination with each other. Examples of the alkaline earth metal oxide which can be used in the present invention include oxides of barium, strontium and calcium. Barium oxide is preferred. The alkaline earth metal oxides can be used singly or in combinatio . The single component oxides, i.e., powder of a rare earth element oxide, powder of an alkaline earth metal oxide and powder of copper oxide, each are commercially available. They can be used as they are or after being ground in order to adjust the mean grain size to a desired size. Usually, single component oxide powders to be used have a mean grain size of not greater than 10 um. Generally, smaller mean grain size, e.g., not greater than 6 um, is preferred. The calcination of the mixt powder composition, which may be prior to or after being subjected to molding treatment, can be performed at a predetermined temperature, preferably ranging from 850 °C to 950 °C, more preferably from 900 °C to 940 °C, in the air for a period sufficient to convert the mixt powder composition into a superconductive ceramics. The same mold treatment as described with respect the preceding embodiment can be used. After the calcination and molding in this order or in reverse order, the resulting superconducting ceramics is then subjected to cold- or hot-working, which can be performed in the same manner as in the case of fabricating the super¬ conducting ceramics material starting from the superconduct¬ ing ceramics powder. The molded bodies can be heat-treated and cooled in the same manner as in the case of one fabricated starting from . Further, according to a variation of the above-described embodiments, powder of partial compounds between any two of the component oxides for fabricating superconducting ceramics articles, i.e., a compound between the yttrium-including rare earth element and the alkaline earth metal oxide, a compound between the alkaline earth metal and copper oxide, and a compound between copper oxide and the yttrium-including rare earth element, can be used together with powder of the remainder of the component oxides, i.e., copper oxide, the yttrium-including rare earth element, and the alkaline earth metal, respectively, instead of using a mixture of powders of single component oxides alone as a starting powder. For example, Y-Ba-CuO based superconducting ceramics materials can be fabricated using Y 2 θ3 * BaO and CuO in a composition substantially the same as that of the final ceramics. Other combinations such as CuO'BaO, and Y2θ3 * CuO, together with Y2O3, and BaO, respectively, can also be used. Similarly, partial compounds can be used for ceramics compositions containing other yttrium-including rare earth elements. The partial compounds between any two of the single component oxides for fabricating superconducting ceramics material can be commercially available or can be prepared in conventional manner, e.g., by calcining a mixture of powders of any two of the single component oxides described above in an oven at temperatures suitable for preparation of ceramics. The ratio of powder of the partial compound to the remainder single component oxide which can be added is not critical as far as the resulting mixture has a composition substantially the same as that of a finally obtained super¬ conducting ceramics material. According to another variation, the mixt powder of the single component oxides, and/or the mixt powder of the partial compound and the remainder of the single component oxides, can be added to powder of the superconducting ceramics powder. In this case, the mixing ratio therebetween is not critical. In the above-described variations, a superconducting ceramics material can be obtained with substantially the same operations including calcination, molding, sintering, heat- treating and cooling under substantially the same conditions as in the case of superconducting ceramics obtained starting from a mixture of powders of single component oxides, and detailed explanation is omitted herein. The superconducting ceramics may be the same as those used in the above-described embodiments of this invention, and it may preferably have a perovskite structure. On the other, hand, it is noted that; (1) A superconducting ceramics having a rhombic crystalline structure is stable in a high temperature region in an oxygen-rich atmosphere, and a tetragonal crystalline structure of the superconducting ceramics can be converted to the rhombic crystalline structure thereof with ease by increasing the content of oxygen in an atmosphere. (2) The conversion of the superconducting ceramics from the tetragonal to the rhombic crystalline structure is easily accelerated in the high temperature region because an atomic diffusion rate in the ceramics become higher in the temperature region in comparison with the rate at a temperature of not higher than the lowest temperature of the high temperature region. Thereofre, in a further variation, the lowest temperature of the high temperature region may preferably be a temperature of not lower than 300 °C, and that ceramics heat-treated may preferably be cooled at a cooling rate satisfying a relation expressed by a equation: wherein T indicates temperature (°C), t indicates cooling time (hour), and P Q indicates partial pressure of oxygen (atm) in order to obtain a superconducting ceramics having an excellent superconducting characteristics. The cooling serves to convert the crystalline structure of the super¬ conducting ceramic from tetragonal to rhombic. The reason why the lowest temperature of the high temperature region is not lower than 300°C is that when the heat-treated ceramics is rapidly cooled from a temperature of not higher than 300°C, the resulting ceramics has a superconducting characteristics sufficient for practical use. Any of the starting powders described above may be used in the process according to this variation. EXAMPLES The present invention will be explained in greater detail with reference to the following examples, which should in no way be construed as limiting the present invention. In tables, "rem." stands for "remainder". Example 1 Various starting powders composed of R-A-Cu-0 based superconducting ceramics having mean grain sizes and compositions as shown in Tables la and lb below were subjected to molding. The molding was performed by pressing at a pressure of about 1 ton/cm to obtain a molded body. The molded body was calcined in a atmosphere of 1 atm, at a temperature of 910 °C for a retention time of 30 hours, and then ground to obtain powders having an average grain size of 1.2 μm,- and having a perovskite structure. Further, the thus-obtained powders were subjected to molding to form a plurality of molded bodies in the form of pellets, each having an outer diameter of 2.0 mm and a height of 5 mm. These molded bodies were sintered under conditions as shown in Tables lc and Id below and then heat-treated under conditions as shown in Tables le and If below to obtain superconducting ceramics. Some molded bodies were subjected to cooling after the heat-treatment. The cooling was performed at a cooling rate as shown in Table If. For comparison, the comparative superconducting ceramics were obtained by the same procedures as above except the conditions of the sintering and heat-treating. The density ( f s), critical temperature (Tc) and critical current (Jc) of the superconducting ceramics thus obtained were measured and the results obtained are shown in Tables le and If below. From the results shown in Tables lg and lh, it can be seen that the superconducting ceramics fabricated according to the process of the present invention not only had high density but also had Tc and Jc values much higher than those of the comparative superconducting ceramics. Also, it can be seen that the superconducting ceramics fabricated according to the process of the present invention including step of cooling performed at the above-described cooling rate after the heat-treatment had Jc values much higher than that of the superconducting ceramics according to the process of the present invention in which the heat-treatment under the above-described cooling conditions was not performed. Table la Starting Powder Table lb Starting Powder Mean Run Composition (wt.%) Grain No. Cu Size (um) 1-23 Same as 1-1 above 1-24 Same as 1-2 above 1-25 Same as 1-3 above 1-26 Same as 1-4 above -27 Same as 1-5 above -28 Same as 1-6 above -29 Same as 1-7 above -30 Same as 1-8 above -31 Same as 1-9 above -32 Same as 1-10 above -33 Same as 1-11 above -34 Same as 1-1 above -35 Same as .1-2 above -36 Same. as 1-3 above -37 Same as 1-4 above -38 Same as 1-5 above -39 Same as 1-6 above -40 Same as 1-7 above -41 Same as 1-8 above -42 Same as 1-9 above -43 Same as 1-10 above -44 Same as 1-11 above Table Id Table l Example 2 As starting powder, R2O3 powder, AO powder and CuO powder each having a predetermined mean grain size of from 0.5 to 10 um were blended at a blend ratio shown in Table 2a below. The superconducting ceramics were fabricated from the starting powder as above described in the same manner as the process in Example 1 above except that additional cooling was per orme . In steps of sintering and heat-treating, molded bodies in the form of pellets were sintered under conditions as shown in Table 2b below and then heat-treated under condi¬ tions as shown in Table 2c below to obtain superconducting ceramics. For comparison, the comparative. superconducting ceramics were obtained by the same procedures as above except different conditions of steps of the sintering and heat- treating were used. The density ( ?s), critical temperature (Tc) and critical current (Jc) of the superconducting ceramics thus obtained were measured and the results obtained are shown in Table 2d below. From the results shown in Table 2d, it can be seen that the superconducting ceramics fabricated according to the process of the present invention had Jc values much higher than that of the comparative superconducting ceramics. Table 2b Sintering Table 2c Heat-Treating Example 3 As starting powders are provided yttrium oxide (Y 2 0 3) powder having a mean grain size of 6 jum, barium carbonate (BaCθ ) powder having a mean grain size of 6 um, and copper oxide (CuO) powder having a mean grain size of 6 jum. These powders were blended at a blend ratio as follows: Y 2 0 3 : BaC0 3 : CuO - 1/2 : 2 : 3 and well mixed. The resulting mixt powder was calcined in the air at a temperature of 910 °C for 30 hours to produce a compound having a composition of YBa 2 Cu3θ and a perovskite structure. The compound was then pulverized to obtain superconducting ceramics powder having a mean grain size of 1.2 um. The superconducting ceramics were fabricated from the starting powder as above described in the same manner as the process in Example 2. In steps of sintering and heat-treating, molded bodies in the form of pellets were sintered under conditions as shown in Table 3a below and then heat-treated under conditions as shown in Table 3b below to obtain superconduct¬ ing ceramics. For comparison, the comparative superconducting ceramics were obtained by the same procedures as above except different conditions of steps of the sintering and heat- treating. The density ( f s), critical temperature (Tc) and critical current (Jc) of the superconducting ceramics thus obtained were measured and the results obtained are shown in Table 3c below. From the results shown in Table 3c, it can be seen that the superconducting ceramics fabricated according to the process of the present invention had density and Jc values much higher than that of the comparative superconducting ceramics. Obviously many modifications and variations of the present invention are possible in the light of the abcve teachings. It is therefore, to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described. Table 3a Sintering Table 3c Characteristics of Superconducting Ceramics Density Run No. (fs, g/cm 3 ) Industrial Applicability The present of this invention provides superconducting ceramics materials having not only improved superconducting characteristics such as high critical temperature and critical current.

Claims: CLAIMS 1. A process for fabricating worked superconducting ceramics material composed essentially of a yttrium-including rare earth element, an alkaline earth metal, copper and balance oxygen and unavoidable impurities, comprising: (A) providing starting powder composed of: (i) a member selected from the group consisting of: (1) a mixt powder composed of powders of component oxides: (1-1) a yttrium-including rare earth element oxide, (1-2) an alkaline earth metal oxide, and (1-3) copper oxide, and having a composition substantially the same as that of a contemplated superconducting ceramics, (2) a mixt powder composed of powders of: (2-1) a partial compound between any two of the component oxides constituting a contemplated superconductive ceramics selected from (1-1), (1-2) and (1-3), and (2-2) the rest of the component oxides selected from (1-1), (1-2) and (1-3), and having a composition substantially the same as that of a contemplated superconducting ceramics, (3) powder of a superconducting ceramics composed essen¬ tially of a yttrium-including rare earth element, an alkaline earth metal, copper and balance oxygen and unavoidable impurities, (4) a mixt powder composed of at least two members e om e m x , (B) molding said starting powder to form a molded body; (C) optionally calcining said starting powder when said starting powder is said mixt powder (i)-(l), (i)-(2) or (i)- (4) at a temperature sufficient for calcining the starting powder prior to or after molding; (D) sintering said molded body in an oxygen-poor atmosphere with partial pressure of oxygen being below 0.2 atm; and (E) heat-treating said sintered molded body in an oxygen-rich atmosphere with partial pressure of oxygen being above 1 atm. 2. The process as claimed in Claim 1, wherein said oxygen-poor atmosphere has a partial pressure of oxygen of 10 "2 to 10 "10 atm. 3. The process as claimed in Claim 1, wherein said oxygen-rich atmosphere has a partial pressure of oxygen of 2 to 10 atm. 4. The process as claimed in Claim 1, wherein, said starting powder is mixt powder (1) and/or (2) and/or (4) . 5. The process as claimed in Claim 4, wherein said starting powder is composed of the mixt powder (1) . 6. The process as claimed in Claim 5, wherein said yttrium-including rare earth element oxide (1-1) is yttrium oxide. 7. The process as claimed in Claim 6, wherein said alkaline earth metal oxide (1-2) is barium oxide. 8. The process as claimed in Claim 1, wherein said starting powder is composed of the powder (3). 9. The process as claimed in Claim 8, wherein said yttrium-including rare earth element is yttrium. 10. The process as claimed in Claim 9, wherein said alkaline earth metal is barium. 11. The process as claimed in Claim 1, wherein said sintering is performed at a temperature of from 870 °C to 950 °C. 12. The process as claimed in Claim 1, wherein said heat-treatment is performed at a temperature of from 350 °C to 950 ° C. 13. The process as claimed in Claim 1, further comprising cooling said sintered molded body after heat- treating, said cooling is performed at a cooling rate satisfying, a relation expressed by an equation: wherein T indicates temperature (°C) of the superconducting ceramics material, t indicates cooling time (hour), and P Q indicates the partial pressure of oxygen (atm) in a high temperature region at a temperature of not lower than 300 °C.

Inventors: HAGINO SADAAKI, TAKESHITA TAKUO, HAGINO, SADAAKI, TAKESHITA, TAKUO
Applicants: MITSUBISHI METAL CORP, MITSUBISHI KINZOKU KABUSHIKI KAISHA
Application year: 1988
Main classification: C04B35
IPC codes: C01G1, C04B35, H01B12, H01B13, H01L39