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.