Designing with single crystal piezoelectrics and ferroelectrics

In the last 15 years, the field of functional materials with applications in sensors, actuators and smart materials, has expanded rapidly. Central to this development are piezoelectric materials which offer solid state actuation and sensing under direct electrical control. Around 1997 Shrout and co-workers pioneered the development of complex perovskite-relaxor single crystals with giant piezoelectric strain; this led to intense interest in the processing and characterisation of these new materials. The dominant processing issues are now overcome, allowing good quality bulk crystals to be made, though there remain significant challenges in reducing the cost of provision of high quality single crystals for industry. However, the fundamental issues of designing with and modelling of bulk single crystals remain largely unaddressed. Although many of the potential applications are piezoelectric in nature, the greatest strains are achieved at high field levels, which can induce both ferroelectric switching and phase transformations. At present, applications are severely limited by the issue of robustness: internal stresses give rise to cracks that grow in low-cycle fatigue. Yet there is no reliable model for the internal stress state of large single crystals. Similarly, a predictive understanding of the behaviour of ferroelectric single crystals under combined electrical, mechanical and thermal loads is needed. Such an understanding would enable the engineering design process for existing single crystal piezoelectrics, and would be equally applicable in the future to Lead-free piezoelectric single crystals that have the same underlying mechanisms of piezoelectricity and ferroelectric switching.