KNN-Based Lead-Free Piezoelectric Ceramics with High Q_m and Enhanced d₃₃ via a Donor-Acceptor Codoping Strategy

The wide application of KNN-based lead-free piezoelectric ceramics is constrained by the contradictory relationship between its mechanical quality factor and piezoelectric constant. From an application point of view, searching for chemical composition with enhanced piezoelectric constant (d₃₃) and mechanical quality factor (Q_m) is one of the key points of KNN-based ceramics. In this work, KNN-based ceramics with enhanced d₃₃ and high Q_m values were obtained by the solid solution method via a donor-acceptor codoping strategy. The donor dopant Ho³⁺ enhanced d₃₃ values by refining the domain size, while the acceptor dopant (Cu_1/3Nb_2/3)⁴⁺ improved Q_m by the formation of defect dipoles. The composition (KNN-5Ho-4CN) exhibits optimal integrated performances, of which d₃₃, Q_m, and T_C values are 120 pC/N, 850, and 392 °C, respectively. Moreover, the temperature coefficient of resonant frequency (TCF = -429 ppm/K) indicates that KNN-5Ho-4CN ceramic has good temperature stability. This work provides a new insight for developing KNN-based ceramics with enhanced d₃₃ and high Q_m.