Piezoelectric bismuth titanate ceramics for high temperature applications
Bismuth titanate (Bi4Ti3O12) shows promise in piezoelectric applications in a temperature range (300-600 °C) which is not well served by standard piezoelectric ceramics. The proposal to use bismuth titanate ceramics for these applications has a major flaw, namely that the high electrical conductivity precludes the efficient polarization of these materials in an electric field. The degree of polarization is critical since it is directly related to the piezoelectric response. In addition, once bismuth titanate ceramics are successfully polarized, they must have a sufficiently low conductivity for use at temperatures above 300 °C. The premise of this study was that control of the conductivity in bismuth titanate ceramics would allow useful piezoelectric properties to be realized. The approach taken was to investigate the origins of the electrical conductivity and examine the impact of microstructure, starting materials, and dopants on the conductivity. The effects of these parameters on the piezoelectric coefficient in the longitudinal mode (d33) were examined. Results presented in this thesis show that the conductivity in undoped Bi4Ti3O12 ceramics is p-type and electronic. The conductivity was decreased by adding niobium, a donor dopant. The conductivity as a function of Nb concentration reached a minimum which corresponds to the condition for complete compensation of the ionized holes. Further increases in Nb concentration produced n-type behavior. A study of the frequency and temperature dependence of the dielectric properties revealed behavior characteristic of ionic motion in undoped bismuth titanate. It appears that oxygen vacancy migration promotes an ion jump relaxation at relatively low temperatures and frequencies (e.g.. 200 °C, 20 Hz). Any contribution of this ionic motion to the conductivity was masked by the high p-type conduction. The addition of Nb suppressed the dielectric relaxation which suggests that donor dopants restrict the diffusion of oxygen in bismuth titanate. The initial premise was proven, in that by decreasing the p-type conductivity in bismuth titanate, the piezoelectric coefficient was quadrupled from 5 pC/N to 20 pC/N. An unforeseen result was that bismuth titanate ceramics with the lowest conductivity did not possess the highest piezoelectric coefficient. An analysis of the phase lag between the applied sinusoidal force and resulting charge provided deeper insight into the effect of Nb doping on the piezoelectric response. An undesirable hysteretic behavior was observed between the force and charge for undoped (ptype) and lightly Nb doped bismuth titanate (fully compensated), but was absent in the highly Nb doped (n-type) compositions. The origins of the hysteresis in the piezoelectric coefficient was discussed in terms of extrinsic contributions from domain wall motion. It has been established that decreasing the conductivity in bismuth titanate is a necessary but not sufficient condition for achieving a useful piezoelectric response. Donor doping into the n-type region provides a bismuth titanate composition with stable and reproducible piezoelectric properties. This material can be produced via standard ceramic processing methods and is a candidate for high temperature piezoelectric applications.