EARLY-STAGE OF MICROSTRUCTURE FORMATION IN THICK SUPERCONDUCTING YBA2CU3O7-DELTA FILMS - DEPENDENCE ON THE SINTERING CONDITIONS AND THE SUBSTRATE TYPE

A detailed examination of the microstructure of thick YBa2Cu3O7-delta films, prepared by painting onto four different types of substrates (S i3N4/Si, Y2O3, Fe and Ag/Si3N4/Si), shows the presence of fine needles locally appearing in each initial grain. This early growth stage is e videnced when a dwell is introduced in the sintering process, before t he maximal sintering temperature is reached. The dimensions of these n eedles are quite comparable, regardless of the substrate used, but the ir shape depends on the substrate type. At a later stage, grain coales cence occurs as the needles increase in size, owing to the existence o f partial liquid phases in several zones of the grains. We show that t his needle nucleation process is related to the existence of dwells du ring the sintering process; as for the doping with the substrate eleme nt, it governs the needle growth conditions and can affect the final m icrostructure shapes of the YBCO superconducting films.