Effect of valence state and incorporation site of cobalt dopants on the microstructure and electrical properties of 0.2PZN–0.8PZT ceramics

A CoCO3-added Pb((Zn1/3Nb2/3)0.20(Zr0.50Ti0.50)0.80)O3 (0.2PZN–0.8PZT) system was prepared and investigated. The results reveal that Co ions are present in the mixed valence form of +2 and +3, and their relative mole ratio depends on doping content. Co doping induces a phase transformation from the MPB to the tetragonal phase side, and the corresponding domain size increases accordingly. The solubility limit of Co ions in the perovskite matrix is near 0.2 wt.% in CoCO3 form. Below the solubility limit, Co ions enter the B sites of the oxygenic octahedral center, and the charge-compensating oxygen vacancies accelerate mass transport, which assists in the densification of the specimens. Above the solubility limit, excess Co ions gather in the grain boundaries and triple junctions, which facilitate the formation of a liquid phase with excess PbO and lead to remarkable grain growth. The resulting size effect plays a dominant role in improving the piezoelectric properties, which compensates for the hardening effect of acceptor doping. The optimum piezoelectric parameters are obtained at 0.8 wt.% CoCO3-doped 0.2PZN–0.8PZT system: d33 = 310 pC N−1, kp = 0.66, and εmax = 14,600.

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