Hardening Effect on the Electromechanical Properties of KNN-Based Ceramics Using a Composite Approach

The high mechanical quality factor (Q_m) of KNN-based ceramics is usually achieved by acceptor doping. However, this hardening effect has serious limitations due to the increased mobility of oxygen vacancies under large electric fields and hence is difficult to use in high-power applications. In this work, the hardening mechanism is demonstrated by the development composites of the 0.957(K_0.48Na_0.52)Nb_0.94Ta_0.06O₃-0.04(Bi_0.5Na_0.5)ZrO₃-0.003BiFeO₃ (KNNT-BNZ-BFO) matrix with the K₄CuNb₈O₂₃ (KCN) phase using the two-step ball-milling method. A decrease in remnant polarization and dielectric constant and an increase in resistivity and Q_m are observed compared to that in the KNNT-BNZ-BFO sample. A high Q_m of 160, Curie temperature, T_C, of 310 °C, and piezoelectric coefficient, d₃₃, of 330 pC/N can be obtained simultaneously in the composite with a 0.008 mole ratio of KCN. This can be explained by the mechanical clamping effect of KCN due to strain incompatibility and the domain wall pegging that traps charges at the KNNT-BNZ-BFO/KCN interface. This composite approach is considered a general hardening concept and can be extended to other KNN-based ceramic systems.