Investigation of the processing conditions for the growth of large cross section meet-powder-meet-growth YBCO samples in a bidirectional temperature gradient

A bidirectional gradient furnace with sample displacement was used for the growth of large meet-powder-meet-growth (MPMG) YBCO samples. The optimization of the macrogranular structure with consequent extension of the central part of the sample, characterized by large aligned domains, was studied. Considering the large cross section of our samples, we studied the combined effects of different ratios between the longitudinal and transversal temperature gradients, different pulling rates (R = 0, 0.5, l, and 2 mm/h), and one or more stages at the beginning of the sample growth on grain nucleation, dimension, and orientation. Different mechanical pretreatments of sample surfaces were also tested in order to control domain nucleation. We obtained 20 × 25 × 45 mm³ samples with low misoriented extended domains. Moreover, we were able to vary their c-axis directions from perpendicular to parallel with respect to the sample length.