Growth of Ca‐Doped Large Grain Gd–Ba–Cu–O Superconductors

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The effect of calcium doping on growing GdBa2Cu3Oz (GdBCO or Gd123) bulk superconductors using top‐seeded melt‐textured growth (TSMTG) method in air was studied. NdBCO thin films deposited on MgO substrate were effectively used as cold‐seeds because of their superheating natures, which allow them enduring a higher maximum processing temperature (Tmax) than that of a conventional TSMTG process. Several heavily Ca‐doped GdBCO (up to 3 wt% CaCO3) single domains were obtained without heterogeneous nucleation. Moreover, an interesting phenomenon of negative growth was observed during the cooling‐down process of Ca–GdBCO, which prevented the grain from further crystallization. The compositional analysis revealed that the Ca content has a declined tendency from the initial growth region toward the edge. It is deduced that the effective Ca distribution coefficient k > 1, which gives rise to a decreased peritectic temperature (Tp) with the reduction of the Ca content, is the main origin of the negative growth behavior. To overcome this obstacle, a two‐stage cooling procedure was developed, i.e., slow cooling followed by fast cooling, to compensate the loss of the growth driving force caused by the decrease of Tp. Consequently, a highly doped Ca–GdBCO bulk was grown with a size of 14 mm2.