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

Abstract

The effect of calcium doping on growing GdBa 2 Cu 3 O z ( GdBCO or Gd 123) 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 ( T max ) than that of a conventional TSMTG process. Several heavily Ca ‐doped GdBCO (up to 3 wt% CaCO 3 ) 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 ( T p ) 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 T p . Consequently, a highly doped Ca – GdBCO bulk was grown with a size of 14 mm 2 .