Diffuse multiphase coexistence renders temperature-insensitive lead-free energy-harvesting piezoceramics

Abstract

Lead-free piezoceramics with a temperature-insensitive figure of merit (FOM = d²/ε) are urgently required to build a new generation of piezoelectric energy harvesters (PEHs) that can be used for environmental protection and new energy applications. Although (K,Na)NbO₃ (KNN)-based ceramics are among the most promising lead-free piezoelectric candidates, the piezoelectric charge constant (d₃₃) and dielectric constant (ε_r) of KNN with an individual orthorhombic (O) phase exhibit opposite change trends in a certain temperature range, resulting in FOM with poor temperature stability. To break this bottleneck, a diffuse multiphase coexistence (DMC) strategy is proposed to realize a temperature-insensitive FOM. By constructing a DMC-featured orthorhombic–tetragonal (O–T) phase boundary associated with a hierarchical domain configuration, the temperature-driven synergistic variation of d₃₃ and ε_r was realized in the 0.965(K_0.48Na_0.52)NbO₃–0.035(Bi_0.5Li_0.5)ZrO₃ (KNN–BLZ) system. Benefiting from an excellent temperature-insensitive FOM, the power density of the cantilever beam-type PEH produced by KNN–BLZ obtained a record value of 865 μW cm⁻³ at 80 °C, which was superior to the reported value of approximately 380 μW cm⁻³ for KNN-based PEHs measured at room temperature. These results pave the way for applications of KNN–BLZ piezoceramics, and also greatly impact our understanding of the strong correlations between the thermal stability of DMC structure and temperature-dependent energy-harvesting properties.