Control of successive phase transition in YBCO superconductive ceramics

The ceramic YBa 2 Cu 3 O 7– δ samples composed of sub-micron size grains are considered as random Josephson-coupled networks of 0 and π junctions and show successive phase transition. With decreasing temperature, a transition occurs at first inside each grain at T c1 (just the same with the bulk superconducting transition temperature T c ) and successively another transition occurs among the grains at T c2 ( T c1 ). The various ceramic samples were prepared with not only changing the calcination and the sintering temperature but also changing the pressure in pelleterizing process between calcination and sintering, using sub-micron size powder prepared by the coprecipitation method. With increasing sintering temperature (equal to calcination temperature), T c2 increases and both the absolute values of linear resistivity and the divergence of nonlinear resistivity (just above T c2 ) decrease. With increasing the density of sample by increasing the pressure in the pelleterizing process, the T c2 and the absolute value of conductivity increase monotonously. These effects may be considered as the changes in Josephson-couple strength between superconducting grains in the ceramic specimens.