Control of Y₂BaCuO₅size and morphology in melt-processed YBa₂Cu₃O_{7_£}superconductor

Important factors governing the size and the morphology of Y₂BaCuO₅(Y211) at 1100 °C in air were investigated for three different starting materials having the same nominal composition of Y: Ba: Cu = 1.8 :2.4 : 3.4, Y₂BaCuO₅-BaCuC₂-CuO-Pt, Y₂O₃-BaCu0₂-CuO-Pt, and melt-quenched materials from 1400 °C in a Pt crucible. With various amounts of Pt doping, the heating rate and the holding time were employed as the processing parameters. While, with the aid of Pt doping as the effective growth inhibitor, fine round Y211 grains could be obtained by simply employing a refined round Y211 precursor and a rapid healing, there were several important factors for obtaining fine acicular (or needle-like) Y211 grains as follows: (i) The Pt dopants dissolved in the liquid phase should act as the effective heterogeneous nucleation sites, (ii) Y211 grains should grow into the acicular shapes before the system reaches an equilibrium amount of Y211 in the liquid (i.e., reaction-controlled), (iii) A large amount of the liquid phase should be supplied instantly at the partial melt stage with a rapid heating.