Method of manufacturing bismuth type oxide superconductor

Abstract: A method of manufacturing a bismuth type oxide superconductor, wherein a molded body of a bismuth type oxide superconducting substance comprising bismuth, an alkaline earth metal, copper, and oxygen or a precursor thereof is subjected to a heat treatment for producing a superconducting phase and then subjected to one step selected from (1) a step of cooling the heated body from 700°C to 200°C in an atmosphere having an oxygen partial pressure of not less than 0.1 atm at a cooling rate of not less than 10°C/min, (2) a step of cooling the heated body from 700°C in an atmosphere having an oxygen partial pressure of less than 0.1 atm at a cooling rate of less than 10°C/min, and (3) a step of cooling the heated body, and then performing a heat treatment for the cooled heated body in an atmosphere having an oxygen partial pressure of not more than 0.1 atm at a tempera­ ture from 700°C to 200°C.

Description: The present invention relates to a method of manufacturing a bismuth type oxide superconductor. In recent years, an oxide superconductor (to be abbreviated as a Y type oxide superconductor hereinafter) comprising a rare earth element, an alkaline earth metal, copper, and oxygen, for example, Y-Ba-Cu-O type oxide superconductor has been developed. However, since the yttrium (Y) type oxide superconductor absorbs moisture and is degraded within a short time period, it is very difficult to put the superconductor of this type into practical use. Recently, an oxide superconductor (to be abbreviated as a Bi type oxide superconductor hereinafter) comprising bismuth, an alkaline earth metal, copper, and oxygen has been found. This Bi type oxide superconductor does not absorb moisture nor is degraded unlike the Y type oxide superconductor. Therefore, extensive studies have been made in various fields in order to put the superconductor of this type into practical use. The oxide superconductor of a Bi-Sr-Ca-Cu-O type or the like comprising Bi, an alkaline earth metal, copper, and oxygen has a high critical temperature (T C ) and therefore is expected to be used in a wide range of applications. An oxide type superconductor molded body consisting of the above oxide superconductor has conventionally been manufactured as follows. That is, an oxide, a carbonate, and the like (e.g., a combination of Bi₂O₃, SrCO₃, CaCO₃, CuO and the like) are used as materials, and these primary materials are mixed to obtain a desired composition. The resultant mixture is preburned to prepare a composite oxide. The prepared composite oxide is milled to obtain a secondary material powder. The secondary material powder is molded into a desired shape, sintered normally at a temperature of 800 to 900°C, and slowly cooled in the same atmosphere at a rate of 1 to 2°C/min, thereby manufacturing the molded body. Although the oxide superconductor molded body manufactured by the above conventional method slightly contains a high-temperature phase having a critical temperature (T C ) close to 110 to 120 K, it also contains a low-temperature phase (85-K phase) having T C close to 85 K, a phase having lower T C (50 to 60 K), and non-superconducting phases of a Ca-Cu-O type, a Bi-Sr-O type, and the like. The most widely studied method is a method of obtaining a so-called high-temperature phase having T C of 110 to 120 K. In order to produce the high-temperature phase, however, a heating/sintering treatment must be performed for a very long time period. In addition, critical current density (to be abbreviated as J C hereinafter) of the high-temperature phase is low. A so-called low-temperature phase having T C of 80 to 90 K and high J C can be produced by performing a heating/sintering treatment for a comparatively short time period. However, variations in units of lots are very large, and it is difficult to stably obtain a high-performance phase. In Japanese J. Appl. Physics, Vol. 27, No. 5, May 1988, pp 777-778, an amorphous material is made by quenching material. Further heat treatment of the quenched material at 850 C creates a superconducting material. Further heat treatment at 700 C, however, produces a semi-conductive material. In Applied Physics, A, Solids and Surfaces, Vol. 45, March 1988, pp 261-263, discloses synthesis of certain bismuth-type oxide superconductors including preparation of superconductive SrBiCaCu O7 by annealing at 800 C in air and quenching. In consideration of the above situation, the present inventors have made extensive studies and found that a low-temperature phase having high performance cannot be stably obtained because oxygen is excessively absorbed in the phase and have made further extensive studies to reach the present invention, i.e., a method of manufacturing a Bi type oxide superconductor superior in superconducting properties such as T C and J C . The present invention is a method of manufacturing a bismuth type oxide superconductor, wherein a molded body of a bismuth type oxide superconducting substance comprising bismuth, strontium; calcium, copper and oxygen or a precursor thereof is subjected to a heat treatment for producing a superconducting phase and then subjected to one step selected from the group consisting of (1) a step of quenching the heated body from a temperature no higher than 700°C to 200°C in an atmosphere having an oxygen partial pressure of not less than 10 KPa (0.1 atm) at a cooling rate of not less than 10°C/min (to be abbreviated as step A hereinafter), (2) a step of cooling the heated body from 700°C in an atmosphere having an oxygen partial pressure of less than 10 KPa (0.1 atm) at a cooling rate of less than 10°C/min (to be abbreviated as step B hereinafter), and (3) a step of cooling the heated body, and then performing an annealing treatment for the cooled heated body in an atmosphere having an oxygen partial pressure of not more than 10 KPa (0.1 atm) at a temperature from 700°C to 200°C (to be abbreviated as step C hereinafter). The precursor of the bismuth type oxide superconductor comprising bismuth, strontium, calcium, copper, and oxygen is a mixture of bismuth or a compound containing bismuth, strontium and calcium or a compound containing strontium and a compound containing calcium, and copper or a compound containing copper, or a composite oxide containing elements such as bismuth, strontium, calcium, and copper. These substances will be called a "precursor" hereinafter. In the method of the present invention, a molded body of a bismuth type oxide superconducting substance or its precursor can be subjected to a heat treatment for producing a superconducting phase, and then subjected to the step A. In this case, a cooling rate is increased in an atmosphere of an oxygen partial pressure of 10 KPa (0.1 atm) or more in order to prevent the heated body from excessively absorbing oxygen. If the cooling rate is less than 10°C/min, a large amount of oxygen is absorbed because a cooling time is prolonged, and therefore desired superconducting properties cannot be obtained. If cooling is performed from a temperature exceeding 700°C, a thermal strain occurs since cooling is fast, thereby producing cracks in a heated body. When the step B is to be performed, an oxygen partial pressure in the atmosphere is reduced at a low cooling rate of less than 10°C/min from 700°C in order to prevent the heated body from excessively absorbing oxygen. If the oxygen partial pressure in the atmosphere is 10 KPa (0.1 atm) or more, an absorption amount of oxygen is increased, and therefore desired superconducting properties cannot be obtained. The step C is performed in order to remove oxygen excessively absorbed by the heated body. If the oxygen partial pressure exceeds 10.133 KPa (0.1 atm) and a heat treatment temperature is more than 700 °C or less than 200°C, the above oxygen amount cannot be an adequate amount, and therefore desired superconducting properties cannot be obtained. In the present invention, when the molded body of the superconductor substance comprising bismuth, an alkaline earth metal, copper, and oxygen or its precursor is sintered within a temperature range from a temperature of T M - 20°C (inclusive) where T M is the melting start temperature of the superconductor substance or its precursor to a temperature of T M + 40°C (exclusive), a single phase of an 85-K class superconducting phase (Bi₂-Sr₂-Ca-Cu₂-Ox) can be formed within a comparatively short time period. When the resultant substance is fired in an atmosphere having an oxygen partial pressure of 20.265 KPa (0.2 atm) or more within a temperature range of 700 to 890°C to absorb oxygen, an optimal composition for realizing superconducting properties can be obtained. If the sintering temperature is less than a temperature of T M - 20°C where T M is the melting start temperature (T M ) of the superconducting substance or its precursor, it is difficult to form the single phase of the 85-K class superconducting phase within a comparatively short time period. If the sintering temperature exceeds a temperature of T M + 40°C, segregation of the composition occurs upon solidification. As a result, it is difficult to obtain the single phase of the 85-K class superconducting phase and to maintain the shape of the superconducting molded body. Therefore, the sintering treatment must be performed within a temperature range from a temperature of T M - 20°C (inclusive) where T M is the melting start temperature of the superconducting substance or its precursor to a temperature of T M + 40°C (exclusive) (e.g., 890°C to 950°C in an oxygen flow and 880°C to 940°C in the air). When the sintered molded body is to be fired to absorb oxygen, if an oxygen partial pressure in an atmosphere is less than 20.265 KPa (0.2 atm) or a firing temperature is less than 700°C or more than 890°C, oxygen absorption becomes insufficient, and an optimal composition for realizing superconducting properties cannot be obtained. Therefore, firing must be performed in an atmosphere having an oxygen partial pressure of 20.265 KPa (0.2 atm) or more within a temperature range from 700 to 890°C, more preferably, 800 to 850°C. The above firing treatment is preferably performed, from a viewpoint of a heat efficiency, by cooling to a predetermined firing temperature after the sintering treatment is finished and successively performing the firing treatment. Alternatively, the firing treatment may be performed by cooling to a room temperature after the sintering treatment is finished and then heating to a predetermined firing temperature. In the former case, a cooling pattern for cooling to a predetermined firing temperature after the sintering treatment is finished is not limited but may be a cooling pattern in which a desired temperature is held for a desired period of time during cooling. When the Bi type superconducting molded body which obtains the single phase of the 85-K class superconducting phase by the above sintering and firing treatments is cooled to a room temperature, a reaction for absorbing oxygen and decreasing a critical temperature occurs more easily at a lower temperature. Therefore, in order to prevent this reaction, quenching must be performed at least within a temperature range from 500°C to 200°C. That is, if a cooling rate in this temperature range is less than 10°C/min, the critical current density (J C ) of the superconductor is reduced. Therefore, quenching must be performed at a cooling rate of 10°C/min or more at least within the temperature range of 500 to 200°C. If a temperature at which quenching is started exceeds 700°C, cracks or the like are produced due to thermal strain to decrease the critical current density (J C ). In this case, a large crack may be produced in a large molded body, and therefore a molded body of a desired shape cannot be obtained. For this reason, the molded body must be quenched at a cooling rate of 10°C/min or more, or must be cooled in an atmosphere of low oxygen pressure, from a desired temperature falling within the range from 700 to 500°C to a temperature of 200°C or less. In the method of the present invention, the heated/sintered body of the molded body of the superconducting substance comprising bismuth, an alkaline earth metal, copper, and oxygen or its precursor is fired in an atmosphere having an oxygen partial pressure of 20 KPa (0.2 atm) or more within a temperature range from 700 to 890°C and then cooled under proper cooling conditions. Therefore, oxygen is sufficiently absorbed, and an optimal composition for realizing superconducting properties can be obtained. In addition, upon cooling to a room temperature performed after the firing treatment is finished, when quenching is performed at a cooling rate of 10°C/min or more from 700°C ∼ 500°C to 200°C or less, excessive oxygen absorption can be prevented. In this case, cracks or the like are not produced due to thermal strain upon quenching, and therefore a superconducting molded body having high critical current density (J C ) can be obtained. The present invention will be described in more detail below by way of its examples. [Example 1] Oxides and carbonates (Bi₂O₃, CaCO₃, SrCO₃, and CuO) were mixed at an atomic ratio of Bi : Ca : Sr : Cu of 2 : 2 : 1 : 2.05 to form a material powder, and the material powder was heated and preburned in an atmosphere at 800°C for six hours to form a pre-fired body. The obtained pre-fired body was milled and classified to form a pre-fired powder, and the pre-fired powder was pressurized and molded into 2 × 3 × 20-mm rectangular blocks. The pressurized/molded bodies were sintered, at 910°C for 30 minutes and then fired at 850°C for six hours in an oxygen atmosphere. Thereafter, the sintered/fired bodies were cooled in the above oxygen atmosphere to 700°C at a rate of 5°C/min and then cooled from 700°C to a room temperature at different cooling rates and in different cooling atmospheres, thereby manufacturing Bi-Sr-Ca-Cu-O type oxide superconductors. T C and J C of each Bi type superconductor were measured. The measurement results are listed in Table 1. As is apparent from Table 1, both the T C and J C values of the present invention products (1 to 5) were higher than those of the comparative example products (6 and 7). That is, although Nos. 1 to 3 of the present invention products were treated in cooling atmospheres having O₂ partial pressures of 10 KPa (0.1 atm) or more, cooling rates were high, 10°C/min or more. Nos. 4 and 5 were treated at low cooling rates of less than 10°C/min, but cooling atmospheres had O₂ partial pressures less than 10.133 KPa (0.1 atm). In either case, therefore, oxygen absorption upon cooling was suppressed to increase the T C and J C values. On the contrary, both the cooling atmospheres and rates of the comparative example products fell outside the limited ranges of the present invention. Therefore, oxygen was absorbed in the heated/sintered bodies to decrease both the T C and J C values. [Example 2] The Bi type oxide superconductors No. 6, i.e., the heated/sintered bodies manufactured in Example 1 were subjected to a heat treatment (step C treatment) under different treatment conditions. T C and J C values of the Bi type oxide superconductors subjected to the above heat treatment were measured. The measurement results are listed in Table 2 together with the heat treatment conditions. As is apparent from Table 2, both the T C and J C values of the present invention products (8 to 11) were higher than the product No. 6 manufactured by a conventional method. This is because excessive oxygen absorbed in the superconductors No. 6 were removed by the heat treatment (step C treatment) defined by the present invention. On the contrary, the T C and J C values of the comparative example products (12 to 14) were at most the same as those of the product No. 6. This is because excessive oxygen was removed insufficiently or too much since the heat treatment conditions fell outside the limited ranges of the present invention. [Example 3] Bi₂O₃, SrCO₃, CaCO₃, and CuO powders as materials were mixed at a molar ratio of Bi : Sr : Ca : Cu = 2 : 2 : 1 : 2 and pre-fired in an oxygen flow under the conditions of 800°C × 6 hr. The pre-fired body was milled and classified into a powder having an average particle size of 2 µm or less and then molded into a 2-mm thick, 3-mm wide, 20-mm long block. The molded block was sintered in an oxygen flow under the conditions of 910°C × 0.5 hr, cooled to 850°C, fired in an oxygen flow under the conditions of 850°C × 6 hr, slowly cooled to 700°C or 500°C at a cooling rate of 1°C/min, and then quenched to a room temperature (cooling rate: 20°C/min), as listed in Table 3. [Example 4] A block prepared following the same procedures as in Example 3 was sintered in an oxygen flow under the conditions of 900°C × 1 hr and 915°C × 0.5 hr, fired under the conditions of 880°C × 6 hr, further fired under the conditions of 850°C × 6 hr, slowly cooled to 700°C at a cooling rate of 1°C/min, and then quenched to a room temperature (cooling rate: 20°C/min). [Example 5] A block prepared following the same procedures as in Example 3 was sintered in an atmosphere under the conditions of 910°C × 0.5 hr, fired under the conditions of 850°C × 6 hr, slowly cooled to 700°C at a cooling rate of 1°C/min, and then quenched to a room temperature (cooling rate: 20°C/min). [Comparative Example 1] A block prepared following the same procedures as in Example 3 was sintered under the same conditions as in Example 1, cooled to 850°C, and fired under the conditions of 850°C × 6 hr. Thereafter, the resultant block was quenched to a room temperature (cooling rate: 20°C/min) or slowly cooled at a cooling rate of 1°C/min, as listed in Table 3, thereby preparing a superconducting molded body. [Example 6] A block prepared following the same procedures as in Example 3 was sintered under the same conditions as in Example 3, fired under the conditions of 880°C × 6 hr, slowly cooled to 700°C at a cooling rate of 1°C/min, and then quenched to a room temperature (cooling rate: 20°C/min). [Comparative Example 2] A block prepared following the same procedures as in Example 3 was sintered in an oxygen flow under the conditions of 880°C × 6 hr, fired under the conditions of 850°C × 6 hr, slowly cooled to 700°C at a cooling rate of 1°C/min, and then quenched to a room temperature (cooling rate: 20°C/min). [Comparative Example 3] A block prepared following the same procedures as in Example 3 was sintered in an oxygen flow under the conditions of 850°C × 6 hr or 850°C × 30 hr, slowly cooled to 700°C at a cooling rate of 1°C/min, and then quenched to a room temperature (cooling rate: 20°C/min). Microstructures of the superconducting molded bodies prepared by Examples 3 to 6 and Comparative Examples 1 to 3 were observed, and their superconducting properties such as a critical temperature (T C ) and a critical current density (J C ) were measured. Both the observation and measurement results are listed in Table 3. As is apparent from Table 3, each of the products of Examples 3 to Example 6 had a uniform single phase having T C of 85 K and had a high J C value. On the contrary, each of the products of Comparative Examples 2 and 3 sintered under the conditions outside the ranges of the present invention had a mixed phase in which the above 85-K phase, a phase having a lower TC value (50 to 60 K), a non-superconducting phase of a Ca-Cu-O type or Bi-Sr-Cu-O type, and the like were mixed and had only a lower J C value. The product of Comparative Example 1 sintered and fired under the conditions falling within the ranges of the present invention but then cooled under the conditions outside the ranges of the present invention had a single phase having an optimal composition for realizing superconducting properties. In the cooling process, however, cracks were produced or a reaction for degrading superconducting properties occurred in the molded body. Therefore, the product of Comparative Example 1 had only a low J C value. [Example 7] A) A block was prepared by mixing Bi₂O₃, SrCO₃, CaCO₃, and CuO at a molar ratio of 2 : 2 : 1 : 2 and compression-molding the mixture. B) A block was prepared by melting a substance consisting of Bi₂O₃, SrCO₃, CaCO₃, and CuO (molar ratio = 2 : 2 : 1 : 2) at a temperature of 950°C or more and solidifying the melted substance. C) A block was prepared by performing a heat treatment for a mixture consisting of SrCO₃, CaCO₃, and CuO, adding Bi₂O₃ to the resultant mixture to form a primary fired powder, and then compression-molding the primary fired powder. D) A paste was prepared by mixing Bi₂O₃, Sr(NO₃)₂, Ca(NO₃)₂, and CuO at a molar ratio of 2 : 2 : 1 : 2, performing a heat treatment at 800°C for six hours and milling the resultant mixture, and then mixing a binder consisting of an organic substance to the milled substance. In this case, the paste was applied on a zirconia substrate and its size was 5 mm × 0.5 mm × 30 mm. Precursor molded bodies of the Bi type oxide superconductors prepared by the above methods A) to D) were uniformly subjected to the following heat treatment for producing a superconducting phase in an oxygen atmosphere. The heat treatment was performed at 900°C for 30 minutes, at 920°C for ten minutes, and at 880°C for six hours. Thereafter, each molded body was cooled to 600°C at a cooling rate of 2°C/min, and then the atmosphere was switched to a nitrogen atmosphere to perform slow cooling to a room temperature. J C and T C values of the prepared molded bodies were measured. The measurement results are listed in Table 4 below. J C (A/cm²) T C (K) A 700 93 B 1,300 93 C 1,500 94 D 2,000 94 The Bi-Sr-Ca-Cu-O type oxide superconductor has been described above. However, the method of the present invention can be applied to another Bi type oxide superconductor such as a Bi-Pb-Sr-Ca-Cu-O type. As has been described above, according to the method of the present invention, a low-temperature phase of the Bi type oxide superconductor can be formed without excessively absorbing oxygen. Therefore, since the Bi type oxide superconductor superior in superconducting properties such as T C and J C can be stably manufactured, significant industrial effects can be obtained.

Claims: A method of manufacturing a bismuth type oxide superconductor, wherein a molded body of a bismuth type oxide superconducting substance comprising bismuth, strontium, calcium, copper, oxygen and optionally lead, or a precursor thereof, is subjected to a heat treatment for producing a superconducting phase, which heat treatment includes the step of sintering within a temperature range from a temperature of T M - 20 degree C (inclusive) where T M is a melting start temperature of the oxide superconductor substance or the precursor thereof to a temperature of T M + 40 degree C (exclusive), and thereafter the molded body is subjected to one step selected from (1) a step of quenching the heated body from a temperature no higher than 700 degree C to 200 degree C in an atmosphere having an oxygen partial pressure of not less than 10 KPa (0.1 atm) at a cooling rate of not less than 10 degree C/min, (2) a step of cooling the heated body from 700 degree C in an atmosphere having an oxygen partial pressure of less than 10 KPa (0.1 atm) at a cooling rate of less than 10 degree C/min, and (3) a step of cooling the heated body, and then performing a heat treatment for the cooled heated body, to fire the heated body in an atmosphere having an oxygen partial pressure of not more than 10 KPa (0.1 atm) at a temperature from 700 degree C to 200 degree C, thereby to exclude excess oxygen from the heated body. A method according to claim 1, characterized in that the precursor of the bismuth type oxide superconductor comprising bismuth, alkaline earth metals, copper, and oxygen is a mixture of bismuth or a compound containing bismuth, alkaline earth metals, or a compound containing an alkaline earth metal, and copper or a compound containing copper, or a composite oxide containing an element of bismuth, alkaline earth metals, and copper. A method according to claim 1 or 2, characterized in that the heated and sintered body subjected to the heating and sintering treatment in accordance with the claim 3 is fired in an atmosphere having an oxygen partial pressure of not less than 20 KPa (0.2 atm) at a temperature from 700 degree C to 890 degree C. A method according to claim 3, characterized in that after the fired treatment is performed at 700 degree C to 890 degree C, cooling to a room temperature is performed such that quenching is executed from 700 degree C - 500 degree C to not more than 200 degree C at a cooling rate of not less than 10 degree C. A method according to any one of the preceding claims characterized in that the Bi type oxide superconductor is a Bi-Sr-Ca-Cu-O type oxide superconductor.

Inventors: IMAI KUMIKO, IMAI, KUMIKO
Applicants: FURUKAWA ELECTRIC CO LTD, THE FURUKAWA ELECTRIC CO., LTD.
Application year: 1989
Main classification: C04B35
IPC codes: H01L39, C04B35