Lead-free piezoelectric Materials based on Silver Niobate
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Studies done on base material AgNbO3 reveals that a large polarization, which is comparable to values reported for PZT, occurs under application of a high electric field. Density functional calculations show that solid solutions of end component AgNbO3 with suitable materials can unlock and enhance the piezoelectric response. In this study, the effect of chemical substitutions of Li and Ta for Ag and Nb in the base material AgNbO3 was examined with respect to structure, phase transitions, dielectric and piezoelectric properties. The effect of chemical substitution of Ba and Ti for Ag and Nb was also examined, but resulted in very limited solid solubility. Consequently, no further investigations were done. Solid state synthesis of different material compositions of ALNT was performed. Structural refinements revealed that the material compositions ALN5 and ALN5T10 were close to a morphotropic phase boundary between an orthorhomic and rhombohedral phase. Sintered samples of these compositions yielded densities of 96-97% of theoretical density. A converse piezoelectric coefficient of 180 pm/V was measured for ALN5T10, and 50 pm/V for ALN5. A direct piezoelectric coefficient of 58 pC/N was measured for ALN5T10, and 20 pC/N for ALN5, which demonstrates that ALN5T10 performs better than ALN5, due to increased structural disorder with introduction of Ta into the structure.Increasing amounts of Li enhance the rhombohedral distortion and phase transition temperatures, whereas a decrease in phase transition temperatures is observed with introduction of Ta. A general problem for the examined samples is that maximum densification takes place close to the melting temperatures, which in turn are low. Introduction of sintering aids into the material system may result in lower sintering temperature. A tuning of the composition of the promising ALNT solid solutions may result in materials that show further enhanced piezoelectric properties, which can also be achieved by examination of single crystals and texturing of the ceramics.