Preparation of tape of silver covered Bi-Pb-Ca;Sr-Cu-O oriented polycrystal superconductor

A process for producing a final tape comprised of a continuous elongated solid sintered body with its longitudinal surface enveloped with elemental silver leaving no significant portion of said longitudinal surface exposed, said sintered body containing oriented superconductive crystalline Bi.sub.2-y Pb.sub.y Ca.sub.2 Sr.sub.2 Cu.sub.3 O.sub.10.+-.z phase where y ranges from 0.1 to 0.5 and z ranges from zero to less than 1 in an amount of at least 90% by weight of said body, which comprises the following steps:

• (a) forming a particulate mixture of reactants comprised of Bi.sub.2 CaSr.sub.2 Cu.sub.2 O.sub.8.+-.x where x ranges from 0 to 0.5, Ca.sub.2 CuO.sub.3, cupric oxide, and lead oxide, said mixture of reactants being formulated to produce said superconductive phase and to provide an excess amount of said Ca.sub.2 CuO.sub.3 and cupric oxide sufficient to drive the reaction to produce said superconductive phase in said sintered body, said Ca.sub.2 CuO.sub.3 and cupric oxide in said mixture being present in substantially mole equivalents to each other, said Bi.sub.2 CaSr.sub.2 Cu.sub.2 O.sub.8.+-.x being comprised of substantially monodispersed crystals, said particulate mixture being a sinterable mixture;
• (b) providing a hollow tube open at one end comprised of elemental silver, said tube having dimensions sufficient to produce said tape;
• (c) packing said particulate mixture into said silver tube to produce a packed particulate mixture therein having a density ranging from 40% to 60% of its theoretical density;
• (d) closing the open end of the resulting packed tube with elemental silver producing a closed tube comprised of said packed mixture and silver;
• (e) applying physical force to the resulting packed tube in the radial direction to increase the density of said packed mixture to at least 65% of its theoretical density producing a deformed tube,
• (f) drawing said deformed tube to produce a wire, said wire having dimensions sufficient to produce said tape;
• (g) applying uniaxial pressure to said wire in the radial direction to orient said Bi.sub.2 CaSr.sub.2 Cu.sub.2 O.sub.8.+-.x crystals with their c-axis substantially parallel to each other producing an initial tape with two major opposed surfaces to which said c-axis of said Bi.sub.2 CaSr.sub.2 Cu.sub.2 O.sub.8.+-.x crystals are substantially perpendicular;
• (h) firing said initial tape in an oxidizing atmosphere at a reaction temperature ranging from 820.degree. C. to 860.degree. C. converting a sufficient amount of said Bi.sub.2 CaSr.sub.2 Cu.sub.2 O.sub.8.+-.x crystals to said superconductive crystalline Bi.sub.2-y Pb.sub.y Ca.sub.2 Sr.sub.2 Cu.sub.3 O.sub.10.+-.z phase to produce a first intermediate tape containing a significantly dilated intermediate partly reacted sintered product wherein said Bi.sub.2-y Pb.sub.y Ca.sub.2 Sr.sub.2 Cu.sub.3 O.sub.1-.+-.z phase comprises less than 50% by weight of said intermediate product;
• (i) cooling the first intermediate tape in an oxidizing atmosphere producing a first intermediate tape containing intermediate solid sintered product;
• (j) applying uniaxial pressure to a major surface of the resulting first intermediate tape to at least remove the dilation produced in step (h);
• (k) firing the resulting first pressed intermediate tape in an oxidizing atmosphere at a reaction temperature ranging from 820.degree. C. to 860.degree. C. converting a sufficient amount of said Bi.sub.2 CaSr.sub.2 Cu.sub.2 O.sub.8.+-.x crystals to said superconductive crystalline Bi.sub.2-y Pb.sub.y Ca.sub.2 Sr.sub.2 Cu.sub.3 O.sub.10.+-.z phase to produce a second intermediate tape containing a significantly dilated intermediate partly reacted sintered product wherein said Bi.sub.2-y Pb.sub.y Ca.sub.2 Sr.sub.2 Cu.sub.3 O.sub.10.+-.z phase comprises less than 90% by weight of said intermediate product;
• (l) cooling the second intermediate tape in an oxidizing atmosphere producing a second intermediate tape containing intermediate solid sintered product;
• (m) applying uniaxial pressure to a major surface of the resulting second intermediate tape to at least remove the dilation produced in step (k);
• (n) firing the resulting pressed intermediate tape containing pressed intermediate product in an oxidizing atmosphere at a reaction temperature ranging from 820.degree. C. to 860.degree. C. to produce a tape containing finally sintered reaction product wherein said superconductive phase comprises at least 90% by weight of said reaction product, said reaction temperature being below the temperature at which sufficient liquid forms to prevent production of said sintered products, said pressed intermediate product undergoing no significant dilation in said final firing;
• and
• (o) cooling the resulting tape in an oxidizing atmosphere producing said final tape wherein said solid sintered body has an open porosity ranging from 5% to 20% by volume of the body, said steps (h) to (m) being carried out sufficiently to produce said tape containing said finally sintered reaction product in step (n).