High temperature ultrasonic characterization of intrinsic and microstructural changes in ceramic YBa2Cu3O7−δ

Abstract

Young's modulus of ceramic YBa2Cu3O7−δ was measured between room temperature and 1000 °C, E(T), by an ultrasonic pulse-echo technique. Experimental results are presented for nonaligned ceramics with average grain sizes from 2 to 10 μm and densities from 80% to 95% of the theoretical value. Young's modulus is shown to be strongly sensitive to oxygen content with the orthorhombic phase being significantly stiffer than the tetragonal phase. In addition, the phase transition is denoted by a pronounced minimum in E(T) relating to softening of certain bonds in the unit cell. At high temperature (>900 °C) melting of the second phase gives a steep drop in E(T) while subsequent densification increases E. Finally, cooling of large-grained ceramic below 450 °C opens microcracks due to anisotropic volume changes. This results in hysteresis between the heating and cooling curves for E(T). Hence the technique is used to show that ceramic, with an average grain size of 2 μm and 85% of theoretical density, is well oxygenated and undamaged by its thermal treatment.