Extended crystal defects in perovskite related materials: Characterisation and technological relevance

Abstract

In perovskite related materials grain boundary phenomena exist that differ significantly from bulk properties. In BaTiO3 ceramics these properties have been used to design and fabricate a whole family of devices based on the positive temperature coefficient (PTC) effect, whereas for high-Tc superconductors grain boundary properties have delayed so far applications with large critical currents in the magnet and energy technology. The role of extended defects, particularly of {111} twins, for the anomalous grain growth was investigated. It was found that twinning alone does not trigger anomalous grain growth in BaTiO3 ceramics. Superconducting material relevant for applications in the magnet and energy technology should have critical current densities of 105–106 A/cm2. In truly polycrystalline YBa2Cu3O7 material the critical current density is reduced by several orders of magnitude, i.e. 102–103 A/cm2 at 77 K, due to the presence of grain boundaries. Bi-epitaxially textured material is required and can be produced for power applications yielding critical current densities of 106 A/cm2 at 77 K. Among the high-T c superconducting materials (Bi, Pb) 2Sr2Ca2Cu3O10+δ is the most complicated but also the one with the largest potential for high-T c superconducting components and devices. High-resolution TEM combined with image simulation was used to image and analyse the structure of the crystal defects. The anisotropy and granular microstructure limits the critical current densities in silver sheathed (Bi, Pb)2Sr2Ca2-Cu3O10+δ tapes to about 4.104 A/cm2 at 77 K.