Method for producing superconducting material

Abstract: This invention provides a method for producing an oxide superconducting material of a layered perovskite structure comprising Cu and O as basic constituting elements which has a uniform texture and is markedly high in sintered density and current density. This method comprises previously reacting CuO as one raw material with a carbonate, an oxide or a mixture thereof as another raw material to produce an inter­ mediate product or intermediate products containing Cu and O and then finally reacting these intermediate products with each other (in case two or more inter­ mediate products are produced) or the intermediate product with other raw material (in case only one intermediate product is produced) to produce an oxide of layered perovskite structure.

Description: This invention relates to a method for producing superconducting materials of oxide type ceramics and, more particularly, to a method for producing a superconducting material which is improved so that raw materials can readily be uniformly mixed, localization of composition can be prevented and thus uniform compositions can be obtained. As superconducting materials, there have been known metals (Pb, Hg, La, Ta, Sn, Ti, Zn, In, Nb, Al, V, Tc and the like), alloys (Nb-Tc, Nb-Zr and the like), compounds (Nb₃Sn, V₃Si, V₃Ga, Nb₃Ge 0.3 Al 0.7 , Nb₃Sn, CeCu₂Si₂, Mo₆Se₈, PbMo 6.4 S₈, SnMo₆S₈, SrTiO₃, BaPb 0.9 , Bi 0.1 O₃, LiTi₂O₄ and the like) and organic materials (TMTSF, BEDT-TTF and the like). Important characteristics of superconducting materials are (1) zero in electric resistance, (2) perfect diamagnetism (Meissner effect) and (3) Josephson effect. When these materials are utilized for power transmission, power distribution and power generation according to the characteristic that electric resistance is 0, the advantages are obtained that large current can be obtained with a small loss thereof. Recently, among the above superconducting materials, oxide type ceramics have been especially noticed in that they have a high critical temperature. The ceramic superconducting materials are generally synthesized by application of powder metallurgy. That is, for example, Y-Ba-Cu-O ceramics (YBCO) are obtained by mixing powders of raw materials BaCO₃, Y₂O₃ and CuO, calcining the mixture at 900°C to carry out a solid phase reaction, then grinding the mixture and press molding the resulting powder into pellets and then sintering the molded product in an oxygen atmosphere. This process is described in Journal of the American Ceramic Society, Vol. 70, No. 7, July, 1987, pages C-165 to C-167. This document mentions an additional firing of the calcined and grinded mixture at 950°C. In this case, uniformly mixing the above three raw material powders is relatively difficult and besides, mixing requires a long time due to the difference in specific gravity, shape and particle size. Furthermore, there is a problem that the raw materials which constitute the superconducting materials or BaO and CuO in the course of the reaction become liquid phase, which remains as non-superconducting phase mainly composed of CuO even after cooling, resulting in reduction of current density. Further problem is that a sintered density decreases owing to the local presence of CuO. OBJECT AND SUMMARY OF THE INVENTION The object of this invention is to provide a method for producing superconducting materials having a uniform texture and a high sintered density and current density with no problems mentioned above. The solution of this object is given in the claim. BRIEF DESCRIPTION OF DRAWINGS Fig. 1 and Fig. 2 are photographs which show crystal structure of superconducting material of one example according to this invention and a conventional superconducting material, respectively. Fig. 3 is a schematic view of an apparatus used in another example of this invention. DETAILED DESCRIPTION OF THE INVENTION According to the invention, there can be expected an effect to prevent ununiformity and localization of specific components caused by differences in specific gravity, shape of particles and particle size of a number of raw materials and to obtain uniform texture. Further effect is to inhibit the presence of a non-superconducting phase such as a phase mainly composed of CuO which gives a great influence on the characteristics of superconducting materials. Example 1 First, as raw materials, powdered Y₂O₃ and CuO were weighed so that Y : Cu was 1 : 1 and then were homogeneously mixed. The mixture was calcined at 900°C for 3 hours in an oxidizing atmosphere to obtain an oxide having the composition of Y₂Cu₂O₅. Then, powdered BaCO₃ and CuO were weighed so that Ba : Cu was 1 : 1 and homogeneously mixed and calcined in the same manner as above to obtain an oxide having the composition BaCuO₂. These two oxides were disintegrated and ground and were weighed so that Y : Ba : Cu was 1 : 2 : 3 and then molded into pellets of 25 mm in diameter x 5 mm. The pellets were sintered at 1000°C for 4 hours in an oxygen atmosphere to produce a superconducting material of a triple-layer perovskite structure deficient in oxygen. The average particle size of powders in the above steps is preferably 0.2 - 0.5 µm. The characteristics of the resulting superconducting material were measured to obtain a critical temperature of 91 K and a current density of 5000 A/cm². Figs. 1 and 2 are photographs which show the crystal structure of the superconducting material obtained in this example and a conventional superconducting material, respectively. In these photographs, black portions are cavities or vacancies which are discontinuous portions in crystal structure. As is clear from these figures, the material of this example of Fig. 1 had very few vacancy portion and had a high sintered density. On the other hand, in Fig. 2 which shows a conventional superconducting material indicates a great number of vacancy portions and this means a low sintered density. The sintered density of the former was 95% and that of the latter was 60 - 70%. This invention has the effects based on the above-mentioned construction and such an action that superconducting materials having a uniform texture and markedly high sintered density and current density can very easily be produced. Example 2 First, as raw materials, powdered Y₂O₃ and CuO were weighed so that Y : Cu was 1.89 : 1.11 and then were homogeneously mixed. The mixture was calcined at 890°C for 3 hours in an oxidizing atmosphere to obtain an oxide having the composition of Y 1.89 Cu 1.11 O 3.395 . Then, powdered BaCO₃ and CuO were weighed so that Ba : Cu was 0.9 : 1.1 and homogeneously mixed and calcined in the same manner as above to obtain an oxide having the composition of Ba 0.9 Cu 1.1 O₂. These two oxides were disintegrated and ground and were weighed so that Y : Ba : Cu was : 2 : 3 and then molded into pellets of 25 mm in diameter x 5 mm. The pellets were sintered at 1000°C for 4 hours in an oxygen atmosphere to produce a superconducting material of a triple-layer perovskite structure deficient in oxygen. The average particle size of powders in the steps above is preferably 0.2 - 0.5 µm. The characteristics of the resulting superconducting material were measured to obtain a critical temperature of 90 K and a current density of 4800 A/cm². Example 3 First, as raw materials, powdered Y₂O₃ and CuO were weighed so that Y : Cu was 1.26 : 1.74 and then were homogeneously mixed. The mixture was calcined at 890°C for 3 hours in an oxidizing atmosphere to obtain an oxide having the composition of Y 1.26 CU 1.74 O 2.76 . Then, powdered BaCO₃ and CuO were weighed so that Ba : Cu was 1.1 : 0.9 and homogeneously mixed and calcined in the same manner as above to obtain an oxide having the composition of Ba 1.1 Cu 0.9 O₂. These two oxides were disintegrated and ground and were weighed so that Y : Ba : Cu was 1 : 2 : 3 and then molded into pellets of 2 mm in diameter x 5 mm. The pellets were sintered at 1000°C for 4 hours in an oxygen atmosphere to produce a superconducting material of a triple-layer perovskite structure deficient in oxygen. The average particle size of powders in the steps above is preferably 0.2 - 0.5 µm. The characteristics of the resulting superconducting material were measured to obtain a critical temperature of 91 K and a current density of 5000 A/cm². Example 4 First, as raw materials, powdered Y₂O₃ and CuO were weighed so that Y : Cu was 1 : 1 and then were homogeneously mixed. The mixture was calcined at 900°C for 3 hours in an oxygen atmosphere to obtain an oxide having the composition of Y₂Cu₂O₅. Then, powdered BaCO₃ and CuO were weighed so that Ba : Cu was 1 : 1 and homogeneously mixed and calcined in the same manner as above to obtain an oxide having the composition of BaCuO₂. Then, these two oxides were disintegrated and ground and were weighed so that Y : Ba : Cu was 1 : 2 : 3. Then, the product was filled in a pipe of 5 mm in outer diameter, 4 mm in inner diameter and 300 mm in length and was sintered to produce a superconducting material having a triple-layer perovskite structure deficient in oxygen and having the composition of YBa₂Cu₃O 7-δ . Fig. 3 schematically shows the sintering means in the above example. In Fig. 3, 1 indicates a silica glass tube having an outer diameter of 180 mm, an inner diameter of 174 mm and a length of 1400 mm which supports pipe 3 of Al₂O₃ by means of supporting member 2 of SiC. 4 indicates a raw material powder and was filled in pipe 3 as mentioned above and the pipe was closed. 5 indicates a high frequency coil which is provided outside the silica glass tube 1 in such a manner that it can freely move in axial direction. Current was allowed to pass through the high frequency coil 5 to heat pipe 3 and raw material powder 4 to 1000 - 1100°C. In this case, the high frequency coil 5 was formed to have a length of, for example, 20 mm to provide a high temperature zone w of 10 - 30 mm where pipe 3 and raw material powder 4 were heated. Then, the high frequency coil 5 was moved at a speed of 30 mm/Hr from the right end portion of pipe 3 in the direction indicated by arrow and accordingly said high temperature zone also moved gradually to the left. By repeating this operation three times, it was possible to perform solid phase sintering and/or liquid phase sintering due to partial dissolution of raw material powder filled in pipe 3 . In this case, since the high temperature zone gradually moved from the right end of pipe 3 to the left, temperature gradient was imparted to the raw material powder 4 in the pipe 3 and this causes alignment of crystal grains and crystal axis in the axial direction of pipe 3 . According to the conventional method, raw material powders were calcined, then disintegrated and ground, press molded into pellets of 5 mm in diameter x 2 mm and then sintered at 950°C for 4 hours in an oxygen atmosphere. Crystal grains of YBa₂Cu₃O 7-δ in conventional examples were distributed in the form of amorphous particles or masses and the direction of crystal axis was not alinged at all. On the other hand, it was recognized that the crystal grains of the product of this example were completely aligned in the axial direction of pipe 3 .

Claims: A method for producing an oxide superconducting material of layered perovskite structure having a compositional formula: YBa₂Cu₃O 7-δ wherein δ is more than zero but less than 0.5, said process comprising the steps of: providing a first mixture comprised of Y₂O₃ and CuO; providing a second mixture comprised of BaCO₃ and CuO: reacting Y₂O₃ and CuO in said first mixture to form a first intermediate product, said first intermediate product consisting essentially of Y₂Cu₂O₅; reacting BaCO₃ and CuO in said second mixture to form a second intermediate product, said second intermediate product consisting essentially of BaCuO₂; mixing said first intermediate product and said second intermediate product to form an intermediate product mixture; and sintering said intermediate product mixture to form said oxide superconducting material.

Inventors: IIMURA TSUTOMU, IYORI YUSUKE, KASAKOSHI TOSHIYUKI, MARUTA KENZI, OGATA YASUNOBU, IIMURA, TSUTOMU, IYORI, YUSUKE, KASAKOSHI, TOSHIYUKI, MARUTA, KENZI, OGATA, YASUNOBU
Applicants: HITACHI METALS LTD, HITACHI METALS, LTD.
Application year: 1988
Main classification: H01L39
IPC codes: H01L39, C04B35