Process for making high temperature Bi-Sr-Ca-Cu oxide superconductor

A process for preparing a superconducting substance having essentially the overall composition

• Bi.sub.a (Sr, Ca).sub.b Cu.sub.6 O.sub.x
• wherein
• a=3 to 12,
• b=6 to 12, and
• x varies with the conditions of the process, with an atomic ratio Sr/Ca of 1:9 to 9:1, an atomic ratio Bi:(Sr+Ca) of 0.3 to 1.3,
• said superconducting substance having a principal phase which crystallizes in the orthorhombic system and a transition temperature T.sub.c for the superconductivity of at least 70 K, said process comprising:
• preparing an initial mixture containing the oxides or oxide precursors of bismuth, calcium, strontium, and copper in the atomic ratios necessary to form said superconducting substance,
• heating the resulting mixture to a temperature in the range of between 700.degree. C. and 900.degree. C. so that the mixture is not melted or is only partially melted, and keeping the mixture in this temperature range for at least 100 hours, and cooling the mixture.