Influence of defect chemistry on ferroelectric properties of KNN
Synthesis and Properties of Functional Materials
Final Report Abstract
For about 20 years, lead-free ferroelectrics have been researched as environmentally friendly and high-performance substitutes for lead-containing ceramics, particularly focusing on piezoelectric properties for actuator applications. (K,Na)NbO3 (KNN), improved by Saito et al. in 2004, is considered a promising material system. Despite optimized piezoelectric and ferroelectric properties, KNN is rarely used in technical applications (with few exceptions) because the material typically exhibits high leakage currents and the ceramic density is often too low. Leakage currents are caused by high grain boundary densities, internal surfaces in insufficiently sintered materials, and intrinsic defects or impurities that increase electrical conductivity. Furthermore, doping with foreign elements does not seem to have a similar large impact on KNN as in the case of lead-based ceramics. Therefore, a precise knowledge of defect chemistry is a prerequisite for improving the material with regard to reduced leakage currents, improved sintering behavior, degradation, and aging. The project dealt with the defect chemistry of KNN-based lead-free ferroelectric ceramics. The research aimed to understand the influence of doping on the electrical and ferroelectric properties of the material and to identify the defect-induced mechanisms that lead to leakage currents, degradation, and aging. Using a combination of experimental investigations and quantum mechanical modeling, a complete picture of defect interactions and their effects on the performance of the material was obtained. It could be determined that intrinsically formed defects (A-site nonstoichiometry) dominate the behavior of KNN, and therefore doping has little effect on the properties. Sintering methods such as hot pressing and cold sintering can help suppress the formation of intrinsic defects. It has also been shown that the variation of the K/Na ratio influences the dominant charge carriers and thus the electronic structure. The findings allow for the development of new strategies for controlling KNN properties.
Publications
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Identifying the Interfacial Polarization in Non‐stoichiometric Lead‐Free Perovskites by Defect Engineering. Angewandte Chemie International Edition, 62(9).
Xu, Ze; Liu, Yi‐Xuan; Azadeh, Maryam; Thong, Hao‐Cheng; Jiang, Yuqi; Yao, Fang‐Zhou; Yue, Zhen‐Xing; Zhang, Zhong‐Tai; Tang, Zi‐Long; Li, Jing‐Feng; Wang, Heng; Frömling, Till & Wang, Ke
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Effect of iron acceptor doping and calcium donor doping in potassium sodium niobate‐based lead‐free piezoceramics. Journal of the American Ceramic Society, 107(7), 4949-4961.
Azadeh, Maryam; Zhao, Changhao; Pawadi, Apoorva; Gao, Shuang & Frömling, Till
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Water-mediated production of K0.5Na0.5NbO3 piezoelectric ceramics with cold-sintering assisted route: Enhanced dielectric properties. Journal of the European Ceramic Society, 45(1), 116815.
Azadeh, Maryam; Zhuo, Fangping; Zahler, Marc Pascal; Sayyadi-Shahraki, Ahmad; Ni, Fan; Rheinheimer, Wolfgang & Frömling, Till
