Effects of Ge⁴⁺ acceptor dopant on sintering and electrical properties of (K_0.5Na_0.5)NbO₃ lead-free piezoceramics

Abstract

Lead-free (K_0.5Na_0.5)(Nb_1−xGe_x)O₃ (KNN-xGe, where x = 0–0.01) piezoelectric ceramics were prepared by conventional ceramic processing. The effects of Ge⁴⁺ cation doping on the phase compositions, microstructure and electrical properties of KNN ceramics were studied. SEM images show that Ge⁴⁺ cation doping improved the sintering and promoted the grain growth of the KNN ceramics. Dielectric and ferroelectric measurements proved that Ge⁴⁺ cations substituted Nb⁵⁺ ions as acceptors, and the Curie temperature (T_C) shows an almost linear decrease with increasing the Ge⁴⁺ content. Combining this result with microstructure observations and electrical measurements, it is concluded that the optimal sintering temperature for KNN-xGe ceramics was 1020°C. Ge⁴⁺ doping less than 0.4 mol.% can improve the compositional homogeneity and piezoelectric properties of KNN ceramics. The KNN-xGe ceramics with x = 0.2% exhibited the best piezoelectric properties: piezoelectric constant d₃₃ = 120 pC/N, planar electromechanical coupling coefficient k_p = 34.7%, mechanical quality factor Q_m = 130, and tanδ = 3.6%.

Keywords

lead-free piezoelectric ceramics / potassium sodium niobate / doping / sintering / dielectric relaxation / piezoelectric properties

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Acknowledgements

The authors thank Jianqiang Zhou for his technical help with SEM measurements. This work was supported by the National Natural Science Foundation of China (Grant No. 21371056) and the Fundamental Research Funds for the Central Universities (Grant No. 2015ZZD04).

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2017 Higher Education Press and Springer-Verlag Berlin Heidelberg