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 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 (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 .

Modified top-seeded melt growth for large sized Y1−xCaxBa2Cu3Oycrystals

“…Meanwhile, the Ca substitution level of x in the Y 1−x Ca x Ba 2 Cu 3 O y matrix was detected by energy dispersive spectroscopy (EDS), as shown in Fig. 9, indicating that the Ca concentration has a tendency to decrease from the initial growth region to the edge of the crystal due to a distribution coefficient effect (i.e., C s /C l = k > 1, 29 where C s denotes the solid composition and C l denotes the liquid composition). However, the average doping level detected by EDS was x = 0.192, which is larger than the nominal composition (x = 0.17).…”