Speaker: David Cann, Oregon State University
Title: BISMUTH PEROVSKITE RELAXOR DIELECTRICS: THE ROLE OF NON-STOICHIOMETRY
Abstract: New dielectric materials designed for SiC and GaN-based power electronics technologies are needed because existing ceramic capacitor technologies are not compatible with extreme environments, especially for temperatures higher than 200 ºC. In recent years, several research groups have developed new Pb-free relaxor materials that exhibit promising dielectric properties. This presentation will focus on the solid solution BaTiO3–Bi(Zn1/2Ti1/2)O3 (BT-BZT) that has been shown to have a temperature independent relative permittivity, high electrical resistivity, and a relative permittivity above 1000 at fields in excess of 100 kV/cm. Structural data on these compositions show a smooth transition between tetragonal symmetry to cubic symmetry over this same range in composition. The relaxor characteristics seen in this solid solution and many similar compounds are likely due to the chemical disorder introduced through the addition of Bi3+ onto the A-sublattice and Zn2+ onto the B-sublattice of the perovskite structure. The temperature dependence can be manipulated through the addition of other compounds such as BiScO3, BiInO3 and NaNbO3, for example. Furthermore, the addition of BiMO3 to p-type BT is accompanied by significant improvement in insulation properties, often leading to an n-type behavior in BT-BiMO3 ceramics. It will be shown that donor doping can originate primarily due to two factors—the presence of secondary phases which modifies the stoichiometry of the perovskite phase to become rich in high valence cations, and an increased oxygen vacancy concentration. It was proposed that these defects had an effect of shifting the conductivity minima to higher pO2 values in the Kröger-Vink diagram, as a means to explain both the higher resistivity values and shift to n-type behavior while still showing a near-intrinsic activation energy. In conclusion, with an understanding of the underlying defect equilibria, BT-BZT relaxor dielectrics show promising dielectric properties with a high permittivity over broad temperature ranges which is of interest for high performance capacitor applications.
Bio:
Seminar Host: Xiaoli Tan