New investigation for Tc depression by Ca in Y1−xCax:123 superconducting systems

Two similar sets of the series Y1−xCaxBa2Cu3OY (0.00 ⩽ x ⩽ 0.50) are prepared by two different heat treatments at the final process of calcinations. The first set is quenched in air down to room temperature (ssq); while the second is left in the furnace and slowly cooled to room temperature (sss). After that, the samples are separately sintered in two different oxygen purity (99.99% Hp and 93% Lp, respectively) to obtain four different sets called ssqHp, sssHp, ssqLp and sssLp. Then, the considered samples are tested by XRD, SEM, resistivity and microhardness (VHN) techniques. It is found that c-parameter for ssqHP and sssLP samples increases slightly with Ca doping; while orthorhombic distortion remains nearly invariant. Interestingly, unlike for ssqHp and sssLp samples, the OD decreases continuously with Ca doping in sssHp and ssqLp samples; while c-parameter increases. SEM micrographs indicate that Ca generally improve the link between superconducting grains for all set of samples. The oxygen content is gradually decreased by Ca for all set of samples and generally follow the relation, y = 6.93 − 0.5x. The effective Cu valence slightly decreases, as compared to pure samples, but stays nearly independent of Ca content for all the series. The relative decrease in critical temperature Tc by Ca is more in ssqHp and sssLp samples, when compared with those of sssHp and ssqLp samples. Although, an approximately linear increase in VHN with various Ca is obtained, the relative increase is more in ssqHp and sssLp samples with respect to sssHp and ssqLp samples. These results indicate that substitution of Ca at Y site leads towards oxygen vacancies predominantly created in CuO2 planes (CuO chains) of ssqHp and sssLp (sssHp and ssqLp) based samples, and far from overdoping.