Understanding the large strain behavior in the lead-free doped Bi1/2(Na0.78K0.22)1/2TiO3–BiMg1/2Ti1/2O3 (BNKT-BMT) piezoelectric system

The ternary solid solution Bi1/2(Na0.78K0.22)1/2TiO3-BiMg1/2Ti1/2O3 (BNKT-BMT) was investigated for high-strain actuator applications. This study of the BNKT-BMT system looked at the macroscopic properties as a function of BMT doping, temperature, and virgin vs poled states in order to gain new insights into the impact of quenched compositional disorder on non-ergodic to ergodic behavior in relaxor ferroelectrics. A gradual transition toward ergodic relaxor ferroelectric behavior was observed in the electric field induced polarization and strain of the BNKT-BMT system with an increase in concentration of BMT and an increase in temperature. BNKT-0%BMT exhibits a remarkable room temperature large-signal piezoelectric strain equivalent to an effective piezoelectric coefficient (d33*) >1000 pm/V at 5.5 kV/mm during the first virgin drive cycle due to the significant strain arising from an irreversible field induced phase transition characteristic of a non-ergodic relaxor ferroelectric. The maximum room temperature large-signal effective piezoelectric coefficient d33* observed for reversible bipolar cycling drive conditions was 685 pm/V at 5.5 kV/mm for BNKT-4%BMT. At elevated temperatures, compositions with reduced BMT exhibited superior d33* performance until a relative maximum d33* was reached and strain performance subsequently declined. This is interpreted in terms of competing strain mechanisms in relaxor ferroelectrics that vary with an increase in ergodicity of the system, as governed by factors such as composition and temperature. The work has implications for industrial actuator systems, such as those in inkjet printers.