1. Field of the Invention
This invention relates to Bi oxide superconductors.
2. Description of the Related Art
Oxide superconductors have recently been considered a potential as a high transition temperature (Tc) superconductor. Among them, a so-called Bi oxide superconductor consisting essentially of Bi--(Sr,Ca)--Cu--O exhibits an extremely high Tc with comparatively good stability to external environment such as moisture. In addition, its nontoxic property incontradistinction with Tl oxide superconductors is an advantage that makes its industrial applicability still higher.
It is known that the Bi oxide superconductor comes in three types depending on the length of the c-axis: 24 .ANG. phase (Tc=about 8K); 30 .ANG. phase (Tc=about 80K); and 36 .ANG. phase (Tc=about 110K). These three types of Bi oxide superconductors are known to have one, two, and three planes, each plane consisting of copper and oxygen ions (Cu--O plane) per unit cell, respectively.
Tl oxide superconductors represented by a formula: Tl.sub.m Ba.sub.2 Ca.sub.n-1 Cu.sub.n O.sub.y (where m is 1 or 2; n is the number of Cu--O planes per unit cell which is 1, 2, or 3), has a crystal structure substantially identical to that of the above Bi oxide superconductors, and are reported to have a Tc of about 20K with n=1. Also, a Tc of about 90K with n=2 and a Tc of about 120K with n=3 have been reported.
As revealed by these examples, it has been said that the number of Cu--O planes per unit cell intimately correlates with the Tc in the oxide superconductors and that the larger the number is, the higher the Tc becomes.
However, the latest research reports that the above correlation is not always strictly applied and that higher Tcs could be obtained with smaller number of Cu--O planes by changing the carrier concentration in the superconductive material. For example, it is known that TlBa.sub.2 CuO.sub.6, one of the Tl oxide superconductors, which has only one Cu--O plane per unit cell, exhibits a variation in Tc from 0 to 90K depending on the oxygen content.
A similar phenomenon has been observed in the Bi oxide superconductors. It has been reported that Bi.sub.2 Sr.sub.2 CuO.sub.6 (24 .ANG. phase) exhibits an improvement of the Tc from 8 to about 20K by controlling the carrier concentration while substituting part of Sr for a rare earth element such as Nd. However, the maximum Tc thus obtained, being quite low compared to that of the TlBa.sub.2 CuO.sub.6 superconductor, must be improved. It has also been revealed that the Tc value can be improved on Bi oxide superconductors of 30 .ANG. phase represented by Bi.sub.2 (Sr,Ca)Cu.sub.2 O.sub.8 by controlling the carrier concentration through substitution of part of Sr and/or Ca for rare earth elements. However, since the rate of Tc improvement is small in this system, further efforts in increasing the Tc are strongly called for.
Generally, oxide superconductors of simpler structure can be formed more easily. To obtain a superconductor in the form of thin films by such method as a sputtering method, a simple crystal structure, particularly simple in the c-axis direction in case of Bi oxide superconductors, allows easier formation. From the viewpoint of reproducibility, the best Bi oxide superconductors is of Bi.sub.2 Sr.sub.2 CuO.sub.6 system whose lattice constant in the c-axis direction is smallest, followed by Bi oxide superconductors of Bi.sub.2 (Sr,Ca).sub.3 Cu.sub.2 O.sub.8 system. Thus, high quality, high Tc Bi oxide superconductive thin films will be prepared with satisfactory reproducibility only if the Tc of Bi.sub.2 Sr.sub.2 CuO.sub.6 and Bi.sub.2 (Sr,Ca).sub.3 Cu.sub.2 O.sub.8 are improved. From this standpoint, further improvements in Tc superconductivity of the Bi superconductors are strongly demanded.
By the way, to obtain a long conductor using the above described Bi oxide superconductors, attempts have been made to prepare a desired wire or tape by using a superconductive powder to fill, e.g., a metal coated tube, drawing it to a predetermined diameter and shape, and then subjecting it to a thermal treatment in an oxygen containing atmosphere. Since a material for the metal coated tube must have not only good conductivity but also high oxygen content during the preparation and thermal treatment of the Bi oxide superconductor, excellent oxygen permeability is also required. In addition, the thermal treatment at high temperatures require that the material be stabile with respect to the Bi oxide superconductor. From the above points, noble metals such as silver are often used as materials for the metal coated tube.
However, the noble metals are not only expensive but also of poor mechanical properties in forming, e.g., a superconductive magnet. Copper and copper alloys which are inexpensive and have satisfactory mechanical properties are, on the other hand, susceptible to oxidation during thermal treatment in an oxygen containing atmosphere. The insufficient oxygen supply to the oxide superconductor in addition to their oxidation prevents copper and copper alloys from serving as materials suitable for obtaining the desired superconductivity.
To this end, there is a strong demand for development of the Bi oxide superconductors that exhibit excellent superconducting properties even at a sufficiently low oxygen content in a thermal treatment to reduce deterioration of the metal coated tube materials such as copper and copper alloys. Such a demand also applies not only to the preparation of superconductive wires and tapes but also to the preparation of superconductive elements by laminating an oxide superconductive thin film and a metal thin film one upon the other.