title: Na1/2Bi1/2TiO3-Based Piezoceramics for High-Power Applications
Creators: name: Bremecker, Daniel
Publisher: name: UNSPECIFIED
Year: 2023

description: In addition to the need for the efficient use of resources, the toxicity of lead for humans and the environment has led to a broad research field for lead-free ferroelectric materials and triggered the development of new concepts and technologies. The use of lead-based ceramics is regulated by the Restriction of Hazardous Substances (RoHS) in the European Union (EU). PZT has an exception from the RoHS directive until a suitable replacement is found. Despite the efforts made in the past few decades, PZT remains the most widely used material system, and it seems there is no single material system that can completely replace it. Individual solutions for specific applications continue to emerge, and some material systems even outperform PZT in certain applications. Therefore, there is a need for further research to develop and optimize alternative lead-free materials with desirable piezoelectric properties while addressing the environmental and health concerns associated with the use of lead-based ceramics. This work addresses the challenges associated with (Na1/2Bi1/2)TiO3-based (NBT) materials and establishes their potential as replacements for PZT in high-power applications (high-power refers to electrical excitation and operation in mechanical resonance at high vibration velocity). NBT-based materials, such as (Na1/2Bi1/2)TiO3-xBaTiO3 (NBT-xBT) and (Na1/2Bi1/2)TiO3-x(K1/2Bi1/1)TiO3 (NBT-xKBT), have been identified as promising candidates due to their stable mechanical losses with increasing output power, in contrast to PZT, which exhibits strongly increasing mechanical losses under the same conditions. Despite these advantages, there remain several challenges associated with NBT-based materials, including a lack of mechanistic understanding and unknown issues regarding the transfer from laboratory to real-world applications. Therefore, the following questions are addressed: • What properties are crucial for the use in high-power applications, and why? • How do the material properties change when measured under application-like conditions? Systematic chemical modifications of the NBT-xBT and NBT-xKBT systems are discussed regarding their electromechanical properties, such as the piezoelectric coefficient, coupling factor, and different losses, including dielectric and electromechanical losses. This discussion includes their evaluation regarding temperature stability. A general doping strategy is established for the NBT-xBT and NBT-xKBT systems, enabling for a mechanistic discussion and comparison with doping in systems such as PZT and BaTiO3 (BT). In addition to the optimization and mechanistic discussion, the NBT-based materials are evaluated and classified for use in high-power applications. This process involves fatigue measurements and comparison with a current PZT standard material. Finally, a theory is developed and proposed to explain the underlying mechanism of why NBT-based materials have low and stable extrinsic contributions with increasing output power. Therefore, the following questions have not been adequately answered up to now and are a major part of this work: • Is there a general guideline for designing the properties of NBT-based materials for high-power applications, and what are the underlying physical mechanisms it is based on? • How do the long-term performances of NBT-based alternatives compare with currently used standard materials? • What is the origin of the exceptional stable extrinsic contribution to the strain against the increasing vibration velocity of NBT-based materials?