Effects of Annealing on the Microstructure and Phase Chemistry of Directionally Solidified Bi 2 Sr 2 CaCu 2 O 8

The effects of annealing on the microstructure and phase chemistry of directionally solidified Bi 2 Sr 2 CaCu 2 O 8 (2212) were studied. Boules grown at fast growth rates are multiphase, contain no 2212, and exhibit some preferred orientation; Sr‐deficient 2201 exhibits a (220) texture. These samples, however, do show a significant amount of 2212 after annealing, with (220) texture similar to the texture of the preexisting 2201 phase in the as‐grown boules. In contrast, 2212 grown directly from the melt at slow growth rates has (200) texture. Boules grown at moderate growth rates are multiphase and contain 2212 as one of the phases; these boules show an increase in 2212 volume fraction after annealing. Microstructural observations in both cases indicate solidstate formation of 2212 occurs topotactically on or within the preexisting 2201 phase. Boules grown at fast growth rates are oxygen‐deficient, since they show an increase in mass with annealing, whereas boules grown at moderate growth rates show no appreciable mass increase with annealing. An estimate of the Cu(I) concentration in the liquid zone during crystal growth at fast growth rates was obtained from gravimetric analysis.