Acoustic emission study of microcracking in 123-type ceramic superconductors
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Acoustic emission from sintered ceramic YBa2Cu3O7.-;C (YBCO) superconductor pellets provides a direct measure of microcracking behavior during processing. By detection and statistical analysis of acoustic events, the effects of cooling rates, processing atmosphere, average grain size, additives, and grain alignment on microcracking in YBCO have been studied. The onset temperature and duration of acoustic emission during cooling correlate well with the oxygen partial pressure in the furnace. Rapid changes in oxygen partial pressure at constant temperature produce acoustic emission that is characteristic of microcracking. A reported critical grain size for microcracking in sintered polycrystalline YBCO of about 1 jam has been confirmed. Two superconducting compounds, YSrBaCu3O7_^ and LaBaCaCuaOv-* with the 123 structure but with smaller crystallographic anisotropy were also examined. Recommendations are made for minimizing microcracking during processing of superconducting ceramics.
I. INTRODUCTION
Large-scale practical applications of oxide superconductors require bulk materials with greatly improved current-carrying capacities. Critical current densities for sintered YBa2Cu3O7-^ (YBCO) ceramics appear to be limited by intergranular resistances and percolation path density.1"4 The anisotropic expansion and contraction of YBCO grains during thermal processing, oxygenation, and cryo-cooling result in intraand intergranular stresses that are largely relieved by microcracking.5"7 While considerable effort has been devoted to investigation of grain boundary phases, grain orientation alignment and densification processes, limited attention has been paid to the role of microcracking in limiting the critical current density in bulk YBCO samples. Acoustic emission impulses produced during initiation and growth of cracks are qualitative and quantitative measures of the degradation in connectivity developing in a sintered ceramic form.8-9 Acoustic events produced by sintered ceramic superconductor pellets during processing under a wide range of conditions have been detected and subjected to statistical analysis with the goal of improving the current-carrying capacity of the polycrystalline materials.
and tetragonal (with stoichiometry YBa2Cu3O!=6.5 and c/a = 3.041) above about 600 °C. The material is therefore in the tetragonal form during sintering, which takes place between 850 °C and the melting point (which also depends on oxygen partial pressure). The ceramic takes up oxygen, if available, as it cools, producing the superconducting, orthorhombic structure (YBa 2 Cu 3 O =s7 . 0 , a/b = 1.017, c/b = 3.048) due to preferential occupancy of oxygen sites in one crystallographic direction (i.e., along the b-axis). The thermodynamics and kinetics of this phenomenon have been the subject of a vast number of studies using a variety of techniques.10"15 The rate of oxygen uptake is affected not only by temperature and pressure but also by grain size, porosity, and the presence or absence of second phases (e.g., BaCO3,Ag). The res
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