Optimizing the grain size and grain boundary morphology of (K,Na) NbO3-based ceramics: Paving the way for ultrahigh energy storage capacitors

Relaxor dielectric ceramic capacitors are very attractive for high-power energy storage. However, the low breakdown strength severely restricts improvements to the energy storage density and practical application. Here, a strategy of designing small grain sizes and abundant amorphous grain boundaries is proposed to improve the energy storage properties under the guidance of phase field theory. 0.925(K0.5Na0.5)NbO3-0.075Bi(Zn-2/3(Ta0.5Nb0.5)(1/3))O-3 (KNN-BZTN) relaxor ferroelectric ceramic is taken as an example to verify our strategy. The grain sizes and grain boundaries of the KNN-BZTN ceramics are carefully controlled by the high-energy ball milling method and two-step sintering strategy. Impedance analysis and diffusion reflectance spectra demonstrate that KNN-BZTN ceramics with a small grain size and abundant amorphous grain boundary exhibit a lower charge carrier concentration and higher band gap. As a consequence, the breakdown electric field of KNN-BZTN ceramics increases from 222 kV/cm to 317 kV/cm when the grain size is decreased from 410 nm to 200 nm, accompanied by a slightly degraded maximum polarization. KNN-BZTN ceramics with an average grain size of similar to 250 nm and abundant amorphous grain boundaries exhibit optimum energy storage properties with a high recoverable energy density of 4.02 J/cm(3) and a high energy efficiency of 87.4%. This successful local structural design opens up a new paradigm to improve the energy storage performance of other dielectric ceramic capacitors for electrical energy storage. (C) 2020 The Chinese Ceramic Society. Production and hosting by Elsevier B.V.

Automated summary

A strategy of designing small grain sizes and abundant amorphous grain boundaries was proposed to improve the energy storage properties of Relaxor dielectric ceramic capacitors. Using KNN-BZTN ceramic as an example, it was demonstrated that ceramics with an average grain size of around 250 nm and abundant amorphous grain boundaries exhibited optimal energy storage properties.