Quenching-Induced Changes in the Structural and Electrical Properties of Lead-Free Ferroelectric Ceramics

Ferroelectrics are an important class of functional materials utilized in various electronic devices, often in the form of polycrystalline ceramics. The most commonly used ferroelectric ceramics are lead-containing Pb(Zr,Ti)O3-based materials. However, due to the toxicity of lead and the resulting environmental concerns, the EU legislation on the Restriction of Hazardous Substances has emphasized the need to identify lead-free alternatives. Na1/2Bi1/2TiO3-BaTiO3 (NBT-BT) and BiFeO3-BaTiO3 (BF-BT) solid solutions are among the promising lead-free candidates for high power and high temperature applications, respectively. Both compositions exhibit certain drawbacks, with NBT-BT having a low temperature stability and BF-BT rather low piezoelectric coefficient (d33) and resistivity. These disadvantages can be mitigated to some extent by quenching the material from high temperatures. The present study aims to investigate the quenching-induced changes in the structural and electrical characteristics of lead-free Bi-based ceramics to advance the understanding of the underlying mechanisms. The investigated materials are NBT-BT with 3, 6, 9 and 12 mol% BT, and BF-BT with 30 and 33 mol% BT with and without the addition of 5 mol% NBT. High-resolution X-ray powder diffraction and synchrotron diffraction experiments on bulk samples confirmed that quenching increases the lattice distortion in NBT-BT. This effect is particularly pronounced for 6 mol% BT, located at the morphotropic phase boundary (MPB). For this composition, which is a non-ergodic relaxor at room temperature, quenching alters the phase fractions, leading to a notable decrease in the volume of the cubic phase. In the poled state, the lattice distortion for the quenched samples is more pronounced in comparison to the furnace cooled counterparts, presumably being the reason for the increased depolarization temperature, Td. In situ electric field-dependent synchrotron diffraction revealed a shift in the onset of electric field-induced structural changes to higher electric field amplitudes. Further, quenched NBT-BT exhibits strongly reduced volumetric strain compared to the furnace cooled sample. This can be attributed to the stabilized ferroelectric domain state, which limits the possibility for quenching-induced phase transformation. The increase in lattice distortion and spontaneous strain in NBT-BT upon quenching is also reflected in increased polarization, coercive field and total strain as established from ferroelectric hysteresis measurements. However, quenching decreases the d33 in all the investigated NBT-BT compositions. Dielectric characterization revealed an increase in the ferroelectric to relaxor transition temperature, and thus, Td. To elucidate the influence of VI quenching on the nanoscale structure of NBT-BT at elevated temperatures, temperature-dependent Young’s modulus and a composite model were used to calculate the volume fraction of polar nanoregions (PNRs). Above the temperature of maximum permittivity, quenching causes an increase in PNR volume fraction at and close to the MPB, likely due to the enhanced non-cubic distortions, which is consistent with the change in phase composition at room temperature. The addition of 5 mol% NBT into BF-BT with 30 and 33 mol% BT promotes a more cubic structure with a smaller rhombohedral distortion and lower rhombohedral phase fraction. Furthermore, the ternary composition exhibits significantly reduced dielectric losses. BF-BT-NBT demonstrates characteristics of a partially ergodic relaxor material in its ferroelectric response, displaying low remanent polarization and strain, which can be correlated to a decrease in d33 values. Quenching BF-BT-NBT leads to vanishing of the ergodicity and enhanced negative strain and remanent polarization. The d33 value is increased by up to 107 %, and the rhombohedral lattice distortion is enhanced upon quenching. In both investigated material systems, quenching induces changes in structural and electrical characteristics, yielding beneficial effects on application-relevant properties. The enhanced lattice distortion in quenched NBT-BT results in an increased Td, improving the temperature-range for applications. Moreover, the Young’s modulus is only marginally lower compared to the furnace cooled material, indicating insignificant deterioration of the mechanical properties. In quenched BF-BT-NBT, the d33 value is greatly improved, and the increase in resistivity upon introducing NBT into BF-BT is maintained. These findings emphasize that quenching Bi-based piezoceramics can help bringing these lead-free materials into application