Simultaneous enhancement of piezoelectric coefficient and depolarization temperature in BF-BT-based ceramics via heterogeneous structure design

BiFeO 3 -BaTiO 3 (BF-BT) lead-free ceramics are promising candidates for high-temperature piezoelectric applications due to their high Curie temperature (T c ), but they typically suffer from a low depolarization temperature (T d ) and a trade off between piezoelectric coefficient (d 33 ) and thermal stability. In this work, a sandwich structured composite 0.67BiFeO 3 -0.33BaTiO 3 -0.1wt%MnO 2 /0.75BiFeO 3 -0.25BaTiO 3 -0.1wt%MnO 2 /0.67BiFeO 3 -0.33BaTiO 3 -0.1wt%MnO 2 (abbreviated as 33/25/33BFBT) ceramics (abbreviated as 33/25/33BFBT) consisting of two high-d 33 0.67BiFeO 3 -0.33BaTiO 3 -0.1 wt% MnO 2 layers (abbreviated as BF-33BT) and a high-T d 0.75BiFeO 3 -0.25BaTiO 3 -0.1 wt% MnO 2 layer (abbreviated as BFBT) was designed to overcome this limitation. During high-temperature sintering, Ba 2+ ions diffused from the outer BF-33BT layers into the middle BFBT layer. This diffusion generated a Ba 2+ concentration gradient and formed diffuse layers on both sides of the ceramic. As a result, local heterogeneous structures were established within the composite. X ray diffraction and Rietveld refinement revealed lattice expansion and an increased tetragonal phase fraction (from 31.5% to 44.9%) due to substitution of smaller Bi 3+ by larger Ba 2+ . The heterogeneous structure enhanced relaxor behavior (diffusivity γ increased from 1.297 to 1.507), leading to a higher dielectric constant at T d and broader dielectric peaks. As a result, the 33/25/33BFBT ceramics achieved a d 33 of 140 pC/N (compared to 104 pC/N for the homogeneous BFBT ceramic) and a significantly increased T d of 420 ℃ (from 300 ℃). Moreover, an in situ d 33 of 365 pC/N was retained at 323 ℃. The improved performance is attributed to the presence of the diffuse layer, which contains polar nanoregions, reduced domain sizes, and defect induced internal bias fields that suppress depolarization. This work demonstrates that constructing local heterogeneous structures via A site ion diffusion is an effective strategy to simultaneously achieve a large d 33 and high T d in BF-BT based ceramics, providing new guidance for designing high temperature lead free piezoelectrics.