Synthesis and characterization of lead-free perovskite solid solutions

Piezo-/ferroelectrics form an important class of functional materials that can transduce mechanical energy to electrical energy and vice versa. They have large impacts in medicine (ultrasound imaging), in naval exploration/defence (sonar) and in consumer products (random access memories, capacitors). Currently, there is high interest in the development of new lead-free materials due to health and environmental risk factors of high-performance lead-based materials, such as Pb(Zr,Ti)O3. One promising lead-free system is the (K,Na)NbO3 (KNN) solid solution, as it has a high Curie temperature, which allows for a wide operating temperature range for devices. In order to better understand the structure and property relations, single crystals are needed. In this work, single crystals of K0.1Na0.9NbO3 (KNN) and 0.98K0.8Na0.2NbO3 – 0.02LiNbO3 (KNN-LN) have been grown using a high-temperature solution growth method with K2CO3 and B2O3 as flux. Polarized light microscopy was used to study the Na-rich KNN crystals, and the phase diagram on the Na-rich end of the (1-x)KNbO3 - xNaNbO3 solid solution has been updated. With the intention of addressing the issue of composition segregation, a modified vapour transport equilibration technique has been developed and demonstrated to be a viable approach to increase the Li-content in the KNN-LN crystals. In addition, a new ternary solid solution of y(K0.5Na0.5)NbO3 – (1-y)[(1-x)Bi0.5K0.5TiO3 – xBaTiO3] has been synthesized in the form of ceramics with compositions of y = 0.96 to y = 0.98 and its partial phase diagram has been established. Aside from KNN-based materials, translucent ceramics of (1-x)(Na0.5Bi0.5)TiO3 – xAgNbO3 (NBT-AN) and (1-x)(Na0.5Bi0.5)TiO3 – xAgTaO3 (NBT-AT) have been successfully prepared via a solid state method under ambient pressure. The dielectric permittivity as a function of temperature is found to be constant for compositions of x > 0.12 (e.g. its variation is within ±10 % for both NBT-AN and NBT-AT (x = 0.16) between 0 °C and 350 °C), while the polarization versus electric field relation shows pure capacitive behaviour up to 250 °C. With these properties, NBT-AN and NBT-AT are promising candidates for electro-optics and/or high-temperature capacitors.