Temperature-dependence of electromechanical coupling and strain in barium titanate hafnate transducer

Abstract

The electromechanical coupling factor and ferroelectric hysteresis loops were investigated at different temperatures for BaTio.ssHfo.tsO3 transducer. Maxirhum polarization and strain were obtained near the transition of the crystal structure. This correlated with increases in the ease and extent of ferroelectric domain boundary motion. The change in the crystal structure varied the unit cell volume, leading to higher strain. BaTiO3 and Pb(Ti, Zr)O 3 and their compositions containing different concentrations of doping ions have strong piezoelectric effects. The earliest investigations of the piezoelectric properties in these compositions [1-7] showed the coupling factor to be highest near the tetragonal-rhombohedral phase boundary. Compositions rich in PbTiO3 were not successfully poled, but a significant piezoelectric effect existed throughout the rhombohedral range. It is believed that the proximity to such a phase boundary between ferroelectric phases favours strong piezoelectrics in a ceramic, because of the increased ease of reorientation during poling. Subsequent work has yielded samples with much stronger piezoelectric coupling [5]. This resulted from a better ceramic quality and an improved poling technique. Instead of a maximum planar coupling factor of 0.4, values over 0.6 at room temperature have been achieved, both in these unmodified solid solutions and in others with various degrees of additional compositional modification. These compositions, as a class, are more useful than BaTiO3 compositions. In the present work, the electromechanical coupling factor and the ferroelectric hysteresis loops were investigated, with particular attention to tme behaviour of the system BaTio.85Hfo.1503 near the rhombohedral-cubic phase boundary. Large values of the coupling kp and spontaneous polarization p and strain have been reported near this phase boundary from cheap and unvolatile materials during the sintering process.