J.P. Zhou

Stabilisation of 110 K superconducting phase in BiSrCaCuO Pb substitution

Abstract The effect of Pb substitution for Bi in the Bi-Sr-Ca-Cu-O systems, together with heat treatment, on the superconducting transition was investigated. Up to 40% Pb substitution, the crystal structure remains essentially unchanged, but the sub-unit cell parameters decrease with an increase of Pb substitution; some extra peaks from the X-ray diffraction patterns could be indexed assuming c=37 A . The Pb substitution tends to stabilise the high- T c phase transition at 110 K and 95 K even for a wide range of heat treatment conditions due to the increase of Ca and Cu concentration in the superconducting phase. Ag addition sharpens all transitions due to the improvement of the connectivity between grains. Higher temperature treatment promotes the 95 K phase transition and raises the zero resistance temperature. Quenching treatment results in the enhancement of the 95 K phase transition for specimens without Pb substitutions; it also causes semiconductor behaviour for Pb-substituted specimens depending upon the optimisation of the levels of Cu 3+ concentration and oxygen deficiency.

Labile Cu3+ ions in the Bi-Sr-Ca-Cu-O system and the effects of varying the composition and heat treatment

While there is some difficulty involved in preparing single-phase superconducting Bi-Sr-Ca-Cu-O, this multi-phase assemblage shows evidence of two transitions at temperatures of 110 and 84 K. The labile-ion concentration and transition temperature are found to be increased compared with those of stoichiometric Bi2Sr3CaCu2O8+y by the presence of second phases rich in Cu, and by heat treatment that includes sintering at a temperature close to the melting point. Whiskers of the primary phase, of dimensions 5*0.1*0.1 mm3, have been grown by melting the system at 910 degrees c in a thermal gradient.

Exaggerated grain growth and improved properties of Y1 Ba2 Cu3 O7-x by Pt addition

Abstract Superconducting and mechanical properties of Y1 Ba2Cu3 O7 can be improved by the use of sintering aids. 0·2 wt% of finely divided laser-ablated platinum powder has been mixed with the parent material and produced a dense product with exaggerated grain growth and improved critical current density and Vickers hardness. At higher concentrations of platinum, while the mechanical properties are further improved, the increased density appeared to inhibit access of oxygen for the tetragonal to orthorhombic phase transition and the current density was reduced.

Crystallite alignment of YBa2Cu3O7-x through texture growth

Critical current densities of superconducting YBa2Cu3O7-x can be increased by several orders of magnitude by sintering processes that encourage textured growth of microcrystals from the melt. Alignment parallel to the a-b planes maximises the conduction within grains, reduces grain boundary resistivity, facilitates oxygen diffusion through vacancies in the a-b planes, and eliminates microcracking resulting from anisotropic thermal expansion. Measured critical current densities in excess of 2400 A cm-2 at 77 K are limited not by the specimen but by ohmic contact resistance.

The Important Role of High Oxidation States in Cuprate Superconductors

In cuprate superconductors the presence and concentration of the high oxidation state, Cu3+ or (Cu-O)+, are crucial to and correlate with the superconducting properties, such as transition temperature and critical current density. Likewise the stability of the materials in corrosive environments depends on the oxidation state.