Superconducting material

Abstract: A superconductive material is disclosed which has the following composition: (R 1-x Ca x )(Ba 1-y La y )₂Cu₄O₈ wherein R is at least one element selected from Y, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu, x is a number in the range of 0-­ 0.3 and y is a number in the range of 0.001-0.3 with the proviso that y is not greater than 0.2 when x is not 0.

Description: This invention relates to a novel superconducting material having an RBa₂Cu₄O₈ (R = a rare earth element) crystallographic phase. A superconducting material having a composition RBa₂Cu₃O₇ (R = a rare earth element) is known to have a superconducting transition temperature Tc higher than the boiling point (77K) of liquid nitrogen (Appl. Phys. Lett. 51 , 57(1987)). The oxygen content of this material varies according to conditions such as temperature and oxygen partial pressure under which the material has been sintered, so that the crystal phase thereof shifts between the tetragonal and orthorhombic systems. As a consequence of this phase change, the Tc of the superconducting material considerably varies in the range of from 90 to 0 K (insulation). Thus, it would be possible to control the Tc of the superconducting material to a desired point by controlling the oxygen content thereof. However, such superconducting material with a controlled Tc cannot be suitably used for the preparation of superconductor articles. Namely, when such a superconducting material is used as a raw material for the preparation of, for example, a wiring rod by a silver-sheathed wiring method in which a silver pipe filled with powder of the superconducting material is subjected to cold drawing, followed by sintering at 800-900 °C, the superconductivity is deteriorated due to loss of its oxygen during the sintering stage. It is, therefore, extremely difficult to obtain a superconductor having a desired Tc using RBa₂Cu₃O₇. There is proposed a high Tc superconducting material composed of RBa₂Cu₄O₈ (R = Y, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm) and having a three-layered pervoskite crystal structure with double CuO one-dimensional chains (Phys. Rev. B. 39 , 7347(1989)). This material does not encounter increase or decrease in its oxygen content at temperatures up to about 850°C and thus is stable in such an environment. However, no means have been proposed for controlling the Tc of the material to a desired point. Another problem of this superconducting material arises in actual industrial applications and is concerned with the fact that it is necessary to use, as a raw material for the production thereof, Ba(NO₃)₂ which is expensive and poisonous. A variation of the superconducting transition temperature in the pseudo-quaternary Y(Ba 1-x La x )₂Cu₃O y compound is known from Appl. Phys. Lett. 52 (22), 1901-190 (1989). The present invention has been made in consideration of the above-mentioned problems of the state of the art. In accordance with the present invention there is provided a superconductive material having the following composition: (R 1-x Ca x )(Ba 1-y La y )₂Cu₄O₈ wherein R is at least one element selected from Y, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu, x is a number in the range of 0-0.3 and y is a number in the range of 0.001-0.3, with the proviso that y is not greater than 0.2 when x is not 0. The invention will be described in conjunction with the accompanying drawings, in which: Fig. 1 is a schematic illustration of the crystal structure of RBa₂Cu₄O₈ which is the main component of the superconducting material of the present invention; Fig. 2 is a schematic illustration of the crystal structure of RBa₂Cu₃O₇; Fig. 3 is a powder X-ray diffraction pattern of Y(Ba 0.9 La 0.1 )₂CU₄O₈ obtained in Example 1; Fig. 4 is a graph showing temperature-resistivity characteristics of Y(Ba 1-y La y )₂Cu₄O₈ obtained in Example 1; Fig. 5 is a graph showing the results of thermogravimetric analysis of Y(Ba 1-y La y )₂Cu₄O₈ obtained in Example 1 and of YBa₂Cu₃O₇; Fig. 6 is a powder X-ray diffraction pattern of Ho(Ba 0.9 La 0.1 )₂Cu₄O₈ obtained in Example 2; Fig. 7 is a graph showing temperature-resistivity characteristics of Ho(Ba 1-y La y )₂Cu₄O₈ obtained in Example 2; Fig. 8 is a graph showing the results of thermogravimetric analysis of Ho(Ba 1-y La y )₂Cu₄O₈ obtained in Example 2 and of YBa₂Cu₃O₇; Fig. 9 shows powder X-ray diffraction patterns of YBa₂Cu₄O₈ and (Y 0.9 Ca 0.1 )(Ba 0.9 La 0.1 )₂Cu₄O₈ obtained in Example 5; Fig. 10 is a graph showing temperature-resistivity characteristics of YBa₂Cu₄O₈ and (Y 0.9 Ca 0.1 )(Ba 0.9 La 0.1 )₂Cu₄O₈ obtained in Example 5; Fig. 11 is a graph showing the results of measurement of alternating-current magnetic susceptibility of YBa₂Cu₄O₈ and (Y 0.9 Ca 0.1 )(Ba 0.9 La 0.1 )₂Cu₄O₈ obtained in Example 5; Fig. 12 is a graph showing the results of thermogravimetric analysis of YBa₂Cu₄O₈ and (Y 0.9 Ca 0.1 )(Ba 0.1 La 0.1 )₂Cu₄O₈ obtained in Example 5; Fig. 13 shows powder X-ray diffraction patterns of YBa₂Cu₄O₈ and (Y 0.9 Ca 0.1 )(Ba 0.9 La 0.1 )₂Cu₄O₈ obtained in Example 6; Fig. 14 is a graph showing temperature-resistivity characteristics of YBa₂Cu₄O₈ and (Y 0.9 Ca 0.1 )(Ba 0.9 La 0.1 )₂Cu₄O₈ obtained in Example 6; Fig. 15 is a graph showing the results of measurement of alternating-current magnetic susceptibility of YBa₂Cu₄O₈ and (Y 0.9 Ca 0.1 )(Ba 0.9 La 0.1 )₂Cu₄O₈ obtained in Example 6; and Fig. 16 is a graph showing the results of thermogravimetric analysis of YBa₂Cu₄O₈ and (Y 0.9 Ca 0.1 )(Ba 0.9 La 0.1 )₂Cu₄O₈ obtained in Example 6; As stated above, the oxide superconductor according to the present invention has a composition expressed by the following formula (I): (R 1-x Ca x )(Ba 1-y La y )₂Cu₄O₈ (I) wherein R is at least one element selected from Y, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu, x is a number in the range of 0-0.3 and y is a number in the range of 0.001-0.3 with the proviso that y is not greater than 0.2 when x is not 0. When x is 0 in the above formula (I), the composition is expressed by the following formula (II): R(Ba 1-y La y )₂Cu₄O₈ (II) In this case, y is 0.001-0.3. This composition is that obtained by substituting La for a portion of Ba of RBa₂Cu₄O₈. Fig. 1 illustrates the basic structure of RBa₂Cu₄O₈ which is the main component of the superconducting material of the present invention. For the comparison purpose, the crystal structure of RBa₂Cu₃O₇ is shown in Fig. 2. In these Figures, the reference numeral 1 designates a rare earth element R, 2 designates Ba, 3 designates Cu and 4 designates O positioned in the intersections. The superconducting material of the formula (II) has a crystal structure in which the single CuO chain of the structure shown in Fig. 2 is substituted by a double CuO chain as shown in Fig. 1 and in which a portion of Ba in Fig. 1 is replaced by La. The superconducting transition temperature Tc of the material of the formula (II) varies with the amount of La Thus, by controlling the amount of La substituted for Ba, the Tc may be controlled at will. Further, this material is stable at temperatures up to about 850 °C. Moreover, the substitution of La for Ba has a merit that the amount of the poisonous Ba raw material compound may be decreased. The superconducting material of the formula (II) is, therefore, well suited for actual industrial applications such as in formation of highly condensed wiring rods by the previously described silver-sheathed wiring method. When x is not 0 in the composition of the formula (I), y is in the range of 0.001-0.2. In this case, x is preferably in the range of 0.001-0.3 and R is preferably selected from Y, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm and mixtures thereof. The superconducting material of the formula (I) in which x is not 0 has a crystal structure in which a portion of each of R and Ba in Fig. 1 is replaced by Ca and La, respectively. This superconducting material generally has Tc of higher than 80 K. Additionally, the material is stable at temperatures up to around 850 °C and advantageously contains a reduced amount of Ba. Further, the material allows the use of inexpensive BaCO₃ as a raw material for the preparation thereof. The thermal stability of the superconducting material also permits the use of a binder for forming a high density shaped article, because the binder may be completely removed during heat treatment at a high temperature of up to about 850 °C. This is advantageous because such an increase of density can improve the superconducting current density. Because of these advantages, the superconducting material according to the present invention is suitably used on an industrial scale for the production of various superconductors and other applications such as wirings for low temperature electrical instruments and magnetic shielding materials. The following examples will further illustrate the present invention. Example 1 Metal oxide compositions having the formula Y(Ba 1-y La y )₂Cu₄O₈ wherein y = 0, 0.01, 0.05, 0.1, 0.2, 0.3 and 0.4 were prepared. Thus, quantities of Y₂O₃ powder, Ba(NO₃)₂ powder, CuO powder and La₂O₃ were mixed and the mixture was calcined at 850 °C in an oxygen atmosphere for 24 hours. The calcined mass was ground and formed into a rectangular block, followed by heating at 800 °C in an oxygen atomosphere for 5 hours. The resulting pre-sintered sample was then heat-treated in an mixed gas atmosphere of 80 % argon and 20 % oxygen at a pressure of 1,000 kg/cm². The heat treatment was performed by first heating the sample to 960 °C with a heating rate of 200 °C/hour, then maintaining at 960 °C for 6 hours, increasing the temperature to 1,050 °C with a heating rate of 200 °C/hour and finally maintaining at 1,050 °C for 6 hours. Thereafter, the sample was cooled to 300 °C with a cooling rate of 200 °C/hour. The pressure was then released and the treated sample was allowed to stand in air, ground, shaped and sintered again at 800 °C in an oxygen atmosphere to obtain a sintered product. This product was then subjected to powder X-ray diffraction analysis and thermogravimetric analysis and was further tested for temperature dependence of the resistivity thereof. The results are summarized in Table 1, in which Tc onset refers to an onset Tc which is a temperature at which the material starts transition from the normal conductive state to superconductive state and Tc R=0 refers to perfect Tc which is a temperature at which the material has completely become superconductive state (the resistivity has become 0). The powder X-ray diffraction pattern of the product in the case of y = 0.1 is shown in Fig. 3 while temperature dependency of resistivity and change in weight in thermogravimetry of the products (y = 0 and y = 0.1) are shown in Figs. 4 and 5, respectively. From the results shown in Table 1 and Fig. 4, Tc reduces with the increase of the La content. Thus, it is possible to control Tc by control of La content. When y = 0.4, the sintered product becomes an insulator. As seen from the results shown in Fig. 5, the sintered product with y = 0.1 is stable up to about 850 °C, whereas conventional YBa₂Cu₃O₇ loses its oxygen at temperatures above about 400 °C. Example 2 Example 1 was repeated in the same manner as described except that Ho₂O₃ was used in place of Y₂O₃, thereby to produce Ho(Ba 1-y La y )₂Cu₄O₈ wherein y = 0, 0.01, 0.05, 0.1, 0.2, 0.3 and 0.4. The product was then tested in the same manner as that in Example 1. The results are summarized in Table 2. The powder X-ray diffraction pattern of the product in the case of y = 0.1 is shown in Fig. 6 while temperature dependency of resistivity and change in weight in thermogravimetry of the products (y = 0 and y = 0.1) are shown in Figs. 7 and 8, respectively. From the results shown in Table 2 and Fig. 7, Tc reduces with the increase of the La content. Thus, it is possible to control Tc by control of La content. When y = 0.4, the sintered product becomes an insulator. As seen from the results shown in Fig. 8, the sintered product with y = 0.1 is stable up to about 850 °C. Example 3 Metal oxide compositions having the formula R(Ba 0.9 La 0.1 )₂Cu₄O₈ (R = Nd, Sm, Eu, Gd, Dy, Er, Tm, Yb and Lu) were prepared in the same manner as that in Example 1. The resulting products were tested for temperature dependence of the resistivity thereof. The results are summarized in Table 3. The results shown in Table 3 indicate that the addition of La is effective in controlling Tc of various rare earth-containing superconductors. Example 4 Metal oxide compositions having the formula (Y 1-z Ho z )(Ba 0.9 La 0.1 )₂Cu₄O₈ (z = 0.2, 0.4, 0.6 and 0.8) were prepared in the same manner as that in Example 1. The resulting products were tested for temperature dependence of the resistivity thereof. The results are summarized in Table 4. The results shown in Table 4 indicate that the addition of La is effective in controlling Tc of superconducting material containing two rare earth elements with various mixing ratios. Example 5 Metal oxide compositions having the formula (R 1-x Ca x )(Ba 1-y La y )₂Cu₄O₈ wherein R, x and y are numbers as shown in Table 5a were prepared. Thus, quantities of R₂O₃ powder, CaO powder, Ba(NO₃)₂ powder, CuO powder and La₂O₃ were mixed in an inert gas atmosphere and the mixture was calcined, under an oxygen stream, at a temperature as indicated in Table 5b for 10 hours. The calcined mass was ground and formed into a rectangular block, followed by heating at a pre-sintering temperature as indicated in Table 5b under an oxygen stream for 5 hours. The resulting pre-sintered sample was then heat-treated in an mixed gas atmosphere of 80 % argon and 20 % oxygen at a pressure of 1,000 kg/cm². The heat treatment was performed by heating the sample to 1000 °C with a heating rate of 200 °C/hour and then maintaining at that temperature for 10 hours. Thereafter, the sample was cooled to 300 °C with a cooling rate of 200 °C/hour. The pressure was then released and the treated sample was allowed to stand in air, ground, shaped and sintered again at 800 °C for 20 hours under an oxygen stream to obtain a sintered product. This product was then subjected to powder X-ray diffraction analysis and thermogravimetric analysis and was further tested for the temperature dependence of resistivity and alternate current-magnetic susceptibility. The results are shown in Table 5b and in Figs. 9-12. The X-ray diffraction pattern shown in Fig. 9 (x = 0.1, y = 0.1) indicates the presence of the YBa₂Cu₄O₈ crystal phase. The sintered samples with x of 0-0.15 and y of 0-0.1 were found to have a single layer superconducting phase. From the results summarized in Table 5 and in Fig. 10, it is seen that the addition of Ca and La in specific amounts is effective in reducing the amount of Ba while maintaining Tc above about 80 K. The high Tc is also confirmed by the results shown in Fig. 11 indicating the occurrence of diamagnetism at a temperature of above about 80 K. As seen from the results shown in Fig. 12, the sintered product with x = 0.1 and y = 0.1 is stable up to about 850 °C, similar to YBa₂Cu₄O₈. Example 6 Metal oxide compositions having the formula: (R 1-x Ca x )(Ba 1-y La y )₂Cu₄O₈ wherein R, x and y are numbers as shown in Table 6a were prepared. Thus, quantities of R₂O₃ powder, CaO powder, BaCO₃ powder, CuO powder and La₂O₃ were mixed in an inert gas atmosphere and the mixture was calcined, under an oxygen stream, at a temperature as indicated in Table 6b for 10 hours. The calcined mass was ground and formed into a rectangular block, followed by heating at a pre-sintering temperature as indicated in Table 6b under an oxygen stream for 5 hours. The resulting pre-sintered sample was then heat-treated in an mixed gas atmosphere of 80 % argon and 20 % oxygen at a pressure of 1,000 kg/cm². The heat treatment was performed by heating the sample to 1000 °C with a heating rate of 200 °C/hour and then maintaining at that temperature for 10 hours. Thereafter, the sample was cooled to 300 °C with a cooling rate of 200 °C/hour. The pressure was then released and the treated sample was allowed to stand in air, ground, shaped and sintered again at 800 °C for 20 hours under an oxygen stream to obtain a sintered product. This product was then subjected to powder X-ray diffraction analysis and thermogravimetric analysis and was further tested for the temperature dependence of resistivity and alternate current-magnetic susceptibility. The results are shown in Table 6b and in Figs.13-16. The X-ray diffraction pattern shown in Fig. 13 (x = 0.1, y = 0.1) indicates the presence of the YBa₂Cu₄O₈ crystal phase. The sintered samples with x of 0-0.15 and y of 0-0.1 were found to have a single layer superconducting phase. From the results summarized in Table 6 and in Fig. 14, it is seen that the addition of Ca and La in specific amounts is effective in reducing the amount of Ba while maintaining Tc above about 80 K. The high Tc is also confirmed by the results shown in Fig. 15 indicating the occurrence of diamagnetism at a temperature of above about 80 K. As seen from the results shown in Fig. 16, the sintered product with x = 0.1 and y = 0.1 is stable up to about 850 °C, similar to YBa₂Cu₄O₈. This example also indicates that good results are obtainable even when BaCO₃ which is available at low costs is used as a source of Ba.

Claims: A superconductive material having the following composition: (R 1-x Ca x )(Ba 1-y La y )₂Cu₄O₈ wherein R is at least one element selected from Y, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu, x is a number in the range of 0-0.3 and y is a number in the range of 0.001-0.3 with the proviso that y is not greater than 0.2 when x is not 0. A superconductive material as claimed in claim 1, wherein x is a number in the range of 0.001-0.3 and R is at least one element selected from the group consisting of Y, Nd, Sm, Eu, Gd, Dy, Ho, Er and Tm. A superconducting material as claimed in claim 1 and having the following composition: R(Ba 1-y La y )₂Cu₄O₈.

Inventors: KORIYAMA SHINICHI, KOSHIZUKA NAOKI, MIYATAKE TAKAYUKI, SAKURAI TAKESHI, SUZUKI NOBUO, TANAKA SHOJI, WADA TAKAHIRO, YAMAUCHI HISAO, KORIYAMA, SHINICHI, KOSHIZUKA, NAOKI, MIYATAKE, TAKAYUKI, SAKURAI, TAKESHI, SUZUKI, NOBUO, TANAKA, SHOJI, WADA, TAKAHIRO, YAMAUCHI, HISAO
Applicants: INT SUPERCONDUCTIVITY TECH, KYOCERA CORP, MITSUBISHI METAL CORP, TOKYO ELECTRIC POWER CO, INTERNATIONAL SUPERCONDUCTIVITY TECHNOLOGY CENTER, KYOCERA CORPORATION, MITSUBISHI METAL CORPORATION, THE TOKYO ELECTRIC POWER CO., INC.
Application year: 1990
Main classification: H01L39
IPC codes: H01L39