Analysis of the Phase Transitions in BNT-BT Lead-Free Ceramics Around Morphotropic Phase Boundary by Mechanical and Dielectric Spectroscopies

In this work, the syntheses and characterization by mechanical and dielectric spectroscopies of (1-x) Bi_0.5Na_0.5TiO_3-xBaTiO₃ (BNT-100xBT), with x = 0.05, 0.06 and 0.07, lead-free piezoelectric ceramics is reported. Ceramic samples of BNT-BT have been prepared by mixed-oxide method and then conventionally sintered. X-ray diffraction patterns of sintered samples, indicated for BNT-7BT the presence of tetragonal (P4mm) complex perovskite structure, whereas for BNT-5BT and BNT-6BT the samples exhibit a mixture of tetragonal (P4mm) and rhombohedral (R3c) crystalline phases, which reveal the presence of a morphotropic phase boundary (MPB) in the BNT-BT system. Measurements of internal friction, Q-1, and the storage modulus, E’, as a function of temperature at various frequencies were carried out in a Dynamic Mechanical Analyzer (DMA), in the temperature range from 0ºC to 600ºC. Dielectric profiles are recorded in the frequency range from 1kHz to 100kHz and the temperature range from room temperature to 475ºC. Mechanical loss spectra obtained for investigated compositions of BNT-BT samples showed different frequency-independent anomalies. Two main anomalies for BNT-5BT and BNT-6BT, observed around 100ºC and 430ºC were associated with the ferroelectric-antiferroelectric and antiferroelectricparaelectric phase transitions, respectively. The results obtained from mechanical loss measurements were supported by dielectric relaxation spectra. For BNT-7BT, outside the MPB, four different frequency-independent processes were observed.