Invited Lecture: Dr. David Hall

Bulk ferroelectric heterostructures for high performance lead-free piezoelectrics

Piezoelectric ceramics based on lead zirconate titanate (PZT) ceramics are used widely in electro-mechanical sensors, actuators and ultrasonic transducers. In the search for more environmentally benign piezoceramic materials, bismuth ferrite-barium titanate (BF-BT) solid solutions have emerged as the leading candidate for high-temperature and high-power applications. It is now well-established that the ferroelectric and piezoelectric properties of such BF-BT ceramics are strongly dependent on the rate of cooling after high temperature sintering, being enhanced by quenching or degraded by slow cooling. However, there is currently no consensus on the mechanism responsible for the ‘quenching effect’ in BF-BT, despite intensive research efforts during the past 10 years.

Systematic annealing of quenched BF-BT ceramics at intermediate temperatures, in the region of 500 °C, was conducted to exaggerate the structural modifications induced by slow-cooling procedures and hence facilitate identification of the controlling mechanism. It is shown that long-term annealing (over a period up to 200 hours) induces nanoscale phase separation in a 0.7BF-03BT solid solution, driven by thermodynamic immiscibility. TEM investigations revealed the presence of periodic compositional fluctuations (varying from 60% to 80% BF content) with a wavelength of approximately 16 nm, formed by anisotropic spinodal decomposition. Crystallographic alignment of the inter-phase boundaries in such bulk ferroelectric heterostructures (BFH) stabilises the pre-existing ferroelectric domain configuration, resulting in a novel ferroelectric hardening effect with unprecedented strength. For example, the macroscopic internal bias field in an exemplar BFH material can be tuned up to 8 kV/mm, which is an order of magnitude greater than that of a typical ‘hard’ PZT ceramic. The presentation will illustrate the structural features of BFH BF-BT ceramics and demonstrate the optimisation of their functional properties by a combination of domain engineering and modification of heat treatment parameters.