Development of bismuth ferrite derived piezoelectric ceramics for high temperature applications

The foundations of this project lay in the authors research into the xBiFeO3-(1-x)(K0.5Bi0.5)TiO3 ceramic system. A broad pseudocubic region was identified where x = 0.4-0.3, these compositions produced large electric-field induced strains (>0.3%) due to the presence of polar nanoregions that are easily switched by an applied electric field, however, these return to a random order upon removal of the field. Positive-up negative-down (PUND) showed that ferroelectric switching was reduced in the pseudocubic region and synchrotron studies with an applied electric-field in-situ demonstrated predominantly reversible domain switching.

Solid-solutions based upon the (1-x-y)BiFeO3-x(K0.5Bi0.5)TiO3-yPbTiO3 ternary system have been fabricated by conventional mixed oxide processing, this system is the primary focus of this thesis. Structural analysis using XRD established that long-range crystallographic order was present above a critical PbTiO3 concentration, in addition a broad mixed symmetry region was identified comprised of tetragonal and rhombohedral symmetries. The ferroelectric Curie temperature was between 410-590 ºC. Conventional ferroelectric and piezoelectric ordering was observed with the use of strain and polarization-field measurements. Rayleigh analysis was used to quantify the intrinsic and extrinsic contributions, a critical c/a ratio was identified at 1.045 with domain switching limited above this. Synchrotron x-rays with ex-situ electric-fields allowed for a more detailed structural analysis, selected compositions demonstrated significant phase changing behaviour. The room temperature tetragonal c/a ratio across the mixed symmetry region, Curie temperature, and piezoelectric d33 were found to correlate strongly with the PbTiO3 concentration in a number of compositions.

This work demonstrates a number of important results that have wider implications. Ternary and pseudo-quaternary systems have recently been subject to large amounts of research as many of the possible binary systems have been exhausted. This has led to a raft of materials with reported high electric-field induced strains, these are often electrostrictive or have limited operating temperatures. The partial substitution of PbTiO3 promotes long-range non-centrosymmetric order, which leads to increased piezoelectric activity and TC.