Stress-induced phase transition in lead-free relaxor ferroelectric composites

Piezoelectric materials are considered an enabling technology generating an annual turnover of about 20 billion $. At present, lead-based materials dominate the market with the known risk to health and environment. One of the three key competitors for their replacement is the class of sodium bismuth titanate (NBT)-based relaxor ferroelectrics, the use of which is limited by thermal depolarization. An increased thermal stability has recently been experimentally demonstrated for composites of Na1/2Bi1/2TiO3-6BaTiO3 with ZnO inclusions (NBT-6BT:xZnO). However, the exact mechanism for this enhancement still remains to be clarified. In this study, piezoresponse force microscopy and 23Na NMR spectroscopy were used to demonstrate that the incorporation of ZnO leads to a stabilization of the induced ferroelectric state at room temperature. Temperature-dependent measurements of the relative dielectric permittivity ε0(T), the piezoelectric coefficient d33 and the strain response revealed an increase of the working temperature by 37 °C. A simple mechanics model suggests that thermal deviatoric stresses stabilize the ferroelectric phase and increase, as well as broaden, the temperature range of depolarization.Our results reveal a generally applicable mechanism of enhancing phase stability in relaxor ferroelectric materials, which is also valid for phase diagrams of other ceramic matrix composites