Lead-free piezoelectric K0.5Bi0.5TiO3–Bi(Mg0.5Ti0.5)O3 ceramics with depolarisation temperatures up to ~220 C

The properties of K0.5Bi0.5TiO3-rich ceramic solid solutions in the system (1 - x)K0.5Bi0.5TiO3– xBi(Mg0.5Ti0.5)O3 are reported. The highest values of piezoelectric charge coefficient, d33, and field-induced strains are found in compositions located close to a compositional boundary between single-phase tetragonal and mixed tetragonal ? cubic perovskite phases. Maximum d33 values were *150 pC/N for x = 0.03, with positive strains of *0.25 %; the x = 0.04 composition had a d33 * 133 pC/N and strain of 0.35 % (bipolar electric field, 50 kV/ cm, 1 Hz). Depolarisation temperature Td is an important selection criterion for any lead-free piezoelectric for actuator or sensor applications. A Td of *220 C for x = 0.03 is *100 C higher than for the widely reported Na0.5Bi0.5TiO3–BaTiO3 system, yet d33 values and strains are similar, suggesting the new material is worthy of further attention as a lead-free piezoceramic for elevated temperature applications

High temperature dielectric ceramics: a review of temperature-stable high-permittivity perovskites

Recent developments are reviewed in the search for dielectric ceramics which can operate at temperatures >200 °C, well above the limit of existing high volumetric efficiency capacitor materials. Compositional systems based on lead-free relaxor dielectrics with mixed cation site occupancy on the perovskite lattice are summarised, and properties compared. As a consequence of increased dielectric peak broadening and shifts to peak temperatures, properties can be engineered such that a plateau in relative permittivity–temperature response (εr–T) is obtained, giving a ±15 %, or better, consistency in εr over a wide temperature range. Materials with extended upper temperature limits of 300, 400 and indeed 500 °C are grouped in this article according to the parent component of the solid solution, for example BaTiO3 and Na0.5Bi0.5TiO3. Challenges are highlighted in achieving a lower working temperature of −55 °C, whilst also extending the upper temperature limit of stable εr to ≥300 °C, and achieving high-permittivity and low values of dielectric loss tangent, tan δ. Summary tables and diagrams are used to help compare values of εr, tan δ, and temperature ranges of stability for different material