TEM study of the formation of twins in PMP-processed YBCO bulks with submicron Y₂BaCuO₅ particles

YBCO bulks with submicron Y₂BaCuO₅ (211) particles have been prepared under oxygen partial pressures of 1 and 0.21 atm, respectively, by the powder melting process (PMP) technique. The microstructures of the as-grown samples and the oxygenated samples have been characterized by transmission electron microscopy and their oxygen contents have also been measured. No twins could be found in the as-grown sample prepared under P(O₂) = 0.21 atm. On the other hand, twins were observed to nucleate at the 211/123 interface and subgrain boundaries and extend into the interior of a YBCO matrix in the as-grown sample produced under P(O₂) = 1 atm. The oxygen contents were 6.63 and 6.76 for the two samples, respectively. High densities of twins with an average width of 60-80 nm were found in the oxygenated samples prepared both under P(O₂) = 1 atm and under P(O₂) = 0.21 atm. Moreover, a kind of twin domain boundary structure has been observed. The formation mechanisms of the high density of twins and the twin domain boundary structure have been discussed in this paper.