High-Tc oxide superconductor and method for producing the same

Abstract: A high-Tc oxide superconductor including Bi, Sr, Ca, Cu, O and at least one of Pb and Al, with a high critical temperature than that of liquid nitrogen. A method for producing the high-Tc oxide superconductor is also disclosed, in which Bi₂O₃, SrCO₃, CaCO₃, CuO and PbO or Al₂O₃ powders are mixed calcined, ground, cold-pressed and sintered. The calcined mixture may be melted and annealed to obtain a tape-form superconductor.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a method for producing a high-Tc oxide superconductor of a Bi-Sr-Ca-Cu-O system. Description of the Background Art In a conventional oxide superconductor of a Y-Ba-Cu-O system, its critical temperature Tc, at which the resistance becomes completely zero, is relatively high such as up to 90 K. However, in order to obtain high and good characteristics, it is necessary to set a composition ratio of Y:Ba:Cu to exactly 1:2:3, and it is quite difficult to control this composition ratio. Further, this superconductor is unstable and readily reacts with water or carbon dioxide to deteriorate its characteristics, and thus changes with the passage of time in the air. That is, this unstable superconductor must be handled carefully, e.g., it must be stored apart from the air. An oxide superconductor of a Bi-Sr-Ca-Cu-O system having a relatively high critical temperature Tc has been proposed. However, in actual, it is difficult to determine its producing conditions, and the obtained superconductor is liable to separate into two phases. Its zero resistance temperature is 75 K at most, and hence none of such superconductors operates in stable at temperature 77 K of liquid nitrogen. Accordingly, the application fields of the conventional superconductors are largely restricted. SUMMARY OF THE INVENTION It is an object of the present invention to provide a method for producing a high-Tc oxide superconductor having a high critical temperature Tc which is higher than 77 K of liquid nitrogen and having stable characteristics such as high onset Tc and high offset Tc (zero resistance temperature). In accordance with one aspect of the invention, there is provided a method for preparing an oxide superconductor including Bi, Sr, Ca, Cu, Pb and O having a Tc of greater than 77 K and in which the molar ratio of Pb to Bi is in the range of 5:95-85:15, comprising the steps of: mixing appropriate amounts of Bi 2 O 3 , SrCO 3 , CaCO 3 , PbO and CuO to obtain a mixture; calcining the mixture at 750-900 °C; grinding and then cold-pressing the calcined mixture; and sintering the pressed mixture at 750-900°C. In accordance with still another aspect of the invention, there is provided a method for preparing an oxide superconductor including Bi, Sr, Ca, Cu, Pb and O having a Tc of greater than 77 K and in which the molar ratio of Pb to Bi is in the range of 5:95-85:15 comprising the steps of: mixing appropriate amounts of Bi 2 O 3 , SrCO 3 , CaCO 3 , PbO and CuO to obtain a mixture; calcining the mixture at 750-900°C; melting the calcined mixture at a temperature higher than its melting point to obtain a melted material; and annealing the melted material at a temperature below the melting point of the melted material. Preferably, the annealing is conducted at a temperature of 750-900°C. BRIEF DESCRIPTION OF THE DRAWINGS These and other objects, features and advantages of the present invention will more fully appear from the following description of the preferred embodiments thereof with reference to the accompanying drawings, in which: Figs. 1 and 2 are graphical representations showing the relation between resistivity and temperature of superconductors prepared according to the present invention; Fig. 3 is a graphical representation showing the relation between offset Tc and Pb amount of another superconductor prepared according to the present invention; Fig. 4 is a graphical representation showing the relation between resistivity and temperature of another superconductor prepared according to the present invention; Fig. 5 is a graphical representation showing an X-ray diffraction pattern of the superconductor shown in Fig. 4; Fig. 6 is a graphical representation showing X-ray diffraction patterns of other superconductors prepared according to the present invention, in which the calcination temperature is varied; Fig. 7 is a graphical representation showing the relation between magnetization and temperature of the sample (d) in Fig. 6; and Figs. 8 to 11 are graphical representations showing the relations between offset Tc and amounts of components of other superconductors prepared according to the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will now be described in connection with preferred embodiments thereof with reference to the accompanying drawings. High-Tc superconductors prepared according to the present invention are stable oxides of Bi-Sr-Ca-Cu-O system essentially including bismuth (Bi), strontium (Sr), calcium (Ca), copper (Cu) and oxygen (O), and also including lead (Pb), and the critical temperatures Tc of these high-Tc oxide superconductors are increased to approximately 83 to 95 K which is higher than 77 K of liquid nitrogen. The composition ratio of Pb to Bi is in a range of 5:95 to 85:15, and more preferably 10:90 to 60:40. If the ratio lies outside this range, then the critical temperature Tc is lowered. The superconductor prepared according to the present invention includes at least one composition of the following formulae: (BiPb) 2 Sr 2 Ca 1 Cu 2 O y , and (BiPb) 2 Sr 2 Ca 2 Cu 3 O y . In these two formulae, the upper one is formed in a low-Tc phase, and y of O y is 8 + δ (δ = 0 to 1.0), whereas the lower one is formed in a high-Tc phase, and y of O y is 10 + δ (δ = 0 to 1.0). The value δ varies depending on the heat treatment conditions. According to the present invention, the superconductor is produced from a powder mixture of appropriate amounts of Bi 2 O 3 , SrCO 3 , CaCO 3 , CuO and PbO, and the powder mixture includes at least one composition of the following formulae: (Bi 1-x Pb x )Sr a Ca b Cu c O y , and (Bi 1-x Pb x ) 2 Sr a Ca b Cu c O y , wherein a = 1 to 5.0; b = 1 to 3.5; c = 1 to 4.5 and x = 0.05 to 0.85. These two formulae show only starting mixture compositions obtained by mixing the oxides Bi 2 O 3 , SrCO 3 , CaCO 3 , CuO and PbO, and thus only the suffixes of the positive ions are determined. The superconductor is produced as follows. Appropriate amounts of Bi 2 O 3 , SrCO 3 , CaCO 3 , CuO and Pbo powders are mixed so as to satisfy the formulae described above to prepare powder mixtures in a first step. The mixtures are calcined at approximately 750 to 900°C in air in a second step. The so formed calcined mixtures are ground and then cold-pressed in a third step. The pressed mixtures are sintered at approximately 750 to 900°C in air to obtain pellets in a fourth step. A bar-shaped specimen of 1 x 3 x 20 mm 3 is cut out from each pellet, and the electrical resistivity of each specimen with respect to temperature variation is measured by the standard four-probe method to obtain critical temperature Tc, onset Tc and offset Tc (zero resistance temperature). The critical temperatures of the obtained specimens are higher than that of the liquid nitrogen. In one preferred embodiment, the calcining and sintering are carried out at approximately 820 to 870°C in air. In this case, as to the calcining and sintering temperature, when the temperature is lower than 750°C, the critical temperature of the specimen is lowered, and when the temperature is higher than 900°C, non-superconducting phases dominate the pellet and the superconductor having excellent characteristics can not be obtained. In this embodiment, the sintering or heat treatment is carried out in air at an oxygen partial pressure of 1/5 or 20%. However, when the heat treatment is conducted in a reduced oxygen partial pressure atmosphere, the high-Tc phase area is effectively extended. When the specimen is heat-treated in a gas stream of oxygen of 7.7% and argon of 92.3%, the high-Tc phase is remarkably formed. This method is also effective at the reduced partial oxygen pressure of 1/100 or 1%. Alternatively, the calcined mixture obtained in the second step described above may be placed on a platinum (Pt) plate and then melted using an oxygen burner at a temperature of at least 900°C which is detected by a W-Re thermocouple in a third step. The melted specimens are annealed at a temperature below the melting points of the melted specimens in the air in a fourth step. The annealed specimens exhibit excellent characteristics such as onset Tc and offset Tc. A tape-form superconductor material can be produced according to this method, that is, an elongate material for use in superconductor magnet can be readily prepared. In this case, instead of using a platinum plate, a silver (Ag), gold (Au) or palladium (Pd) plate can be used. In this embodiment, the calcined material may be ground and this ground material used in the third step. The ground material may be pressed to form a pellet and this pellet also used in the third step. In the present invention, a low-Tc phase of (BiPb) 2 Sr 2 Ca 1 Cu 2 O y is formed and its critical temperature Tc is improved by adding Pb. Further, a high-Tc phase of (BiPb) 2 Sr 2 Ca 2 Cu 3 O y is formed by the sintering to improve the critical temperature Tc. Examples of the present invention will now be described. Example 1: Appropriate amounts of Bi 2 O 3 , PbO, SrCO 3 , CaCO 3 , CuO powders were mixed so as to satisfy a formula (Bi 1-x Pb x )Sr 1 Ca 1 Cu 2 O y , x = 0, 0.05, 0.1, 0.2, 0.3, 0.5, 0.7, 0.85 and 0.9, to prepare 10 powder mixtures in a first step. The mixtures were calcined at 850°C for 12 hours in the air in a second step, and the calcined mixtures were ground and then cold-pressed in a third step. The pressed mixtures were sintered at about 880°C for 12 hours in the air to obtain 10 pellets in a fourth step. A bar-shaped specimen of 1 x 3 x 20 mm 3 was cut out from each pellet, and electrical resistivity of each specimen with respect to temperature variation was measured by the standard four-probe method to obtain results shown in Fig. 1. In this case, the specimens A (x = 0.5 and 0.7) had a highest critical temperature Tc, and their onset Tc and offset Tc (zero resistance temperature) were 110 K and 90 K, respectively. It was confirmed that the critical temperatures of other specimens were somewhat lower than that of the specimens A but higher than that of the liquid nitrogen. On the other hand, specimens including no Pb prepared in the same manner as those of Example 1 had low offset Tc such as 77 K. The Pb 1 Sr 1 Ca 1 Cu 1 O y (x = 1) was an insulator. Example 2: Appropriate amounts of Bi 2 O 3 , PbO, SrCO 3 , CaCO 3 , CuO powders were mixed so as to satisfy a formula (Bi 0.5 Pb 0.5 )Sr 1 Ca 1 Cu 2 O y to prepare a powder mixture in a first step. The mixture was calculated at 850 °C for 12 hours in the air in a second step. The calcined mixture was placed on a platinum (Pt) plate and melted using an oxygen burner at 1300°C which was detected by a W-Re thermocouple in a third step. The melted specimen was annealed at a temperature below the melting point of the melted specimen such as 880°C for 5 hours in the air in a fourth step to obtain results shown in Fig. 2. The annealed specimen exhibited excellent characteristics such as onset Tc of 110 to 120 K and offset Tc of 90 K. A tape-form superconductor material having a length of 100 cm, a width of 1 cm and a thickness of 20 µm could be produced according to this method, that is, an elongate material for use in superconductor magnet can be readily prepared. In this case, instead of the platinum plate, a silver (Ag), gold (Au) or palladium (Pd) plate could be used. Example 3: Powder mixtures were prepared at a starting formula (Bi 1-x Pb x )Sr 1 Ca 1 Cu 2 O y in the same manner as Example 1, except that the amount of Pb was varied in a range of x = 0 to 1 and that mixtures were calcined at 800°C for 12 hours in the air in a second step and were sintered at 880 °C for 12 hours in the air in a fourth step to obtain results shown in Fig. 3. When x = 0.05 to 0.85, the high offset Tc was obtained. As described above, it is readily understood that, according to the present invention; a high-Tc oxide superconductor of Bi-Sr-Ca-Cu-O system with an offset Tc such as 83 to 95 K higher than 77 K of the liquid nitrogen can be prepared by adding Pb as compared with a conventional oxide superconductor of Bi-Sr-Ca-Cu-O system having no Pb, and further a stable high-Tc oxide superconductor for use in various fields can be obtained. According to the present invention, a method for producing a stable high-Tc oxide superconductor of Bi-Sr-Ca-Cu-O system with an offset Tc higher than that of the liquid nitrogen can be obtained.

Claims: A method for preparing an oxide superconductor including Bi, Sr, Ca, Cu, Pb and O having a Tc of greater than 77 K and in which the molar ratio of Pb to Bi is in the range of 5:95-85:15 comprising the steps of: mixing appropriate amounts of powdered Bi 2 O 3 , SrCO 3 , CaCO 3 , PbO and CuO to obtain a mixture; calcining the mixture at 750-900°C; grinding and then cold-pressing the calcined mixture; and sintering the pressed mixture at 750-900°C. A method according to Claim 1, wherein the calcining step and the sintering step are carried out in air. A method for preparing an oxide superconductor including Bi, Sr, Ca, Cu, Pb and O having a Tc of greater than 77 K and in which the molar ratio of Pb to Bi is in the range of 5:95-85:15 comprising the steps of: mixing appropriate amounts of powdered Bi 2 O 3 , SrCO 3 , CaCO 3 , PbO and CuO to obtain a mixture; calcining the mixture at 750-900°C; melting the calcined mixture at a temperature higher than its melting point to obtain a melted material; and annealing the melted material at a temperature below the melting point of the melted material. A method according to Claim 3, wherein the calcining and annealing are conducted in air. A method according to Claim 3, wherein the annealing is conducted at a temperature of 750-900°C. A method according to Claim 3, wherein the melting is carried out at a temperature of at least 900°C. A method according to Claim 3, wherein the melting is carried out by placing the calcined mixture on a metallic plate formed of platinum, silver, gold or palladium, and heating to melt the calcined mixture. A method according to Claim 7, wherein the melting step is carried out by placing the mixture on the metallic plate formed of platinum and firing it with an oxygen burner.

Inventors: MURASE SATORU, YAMADA YUTAKA, MURASE, SATORU, YAMADA, YUTAKA
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1989
Main classification: C04B35
IPC codes: C04B35, C01G29, H01L39, H01B12, H01B13, C01G1, C01G3