Influence of processing parameters on the ferroelectric-relaxor crossover in BNT-based piezoelectric ceramics

The effects of sintering time on the ferroelectric to relaxor crossover were systematically investigated for Sr(Hf0.5Zr0.5)O-3-modified Bi-0.5(Na0.8K0.2)TiO3 ceramics, prepared using the conventional solid-state mixed-oxide route. Scanning electron microscopy indicated a modest increase in grain size from 1.0 & PLUSMN; 0.2 to 2.0 & PLUSMN; 0.5 mu m when the sintering time increased from 2 to 24 h. Furthermore, it was observed that the sintering time does not affect the long-range average crystal structure, as x-ray diffraction data suggest the presence of a single pseudocubic phase for all the samples, irrespective of the sintering time. Interestingly, ferroelectric and piezoelectric characterization showed evidence of a ferroelectric to relaxor transition when the sintering time increased from 2 to 6 h. This transition was marked by a sudden decrease in remanent polarization, a loss in negative strain along with a drastic increase in the maximum electromechanical strain. This was further exemplified in the unipolar strain data, which showed a transition from linear to non-linear dependence with electric field when the sintering time increased from 2 to 6 h. The piezoelectric properties were enhanced with further increase in sintering time up to 12 h, with the corresponding normalized strain value (S-max/E-max) d 33 * = 647 pm / V. However, the d 33 * decreased with further increase in sintering time to 24 h. As the sintering time increased, temperature-dependent dielectric data show a decrease in the maximum permittivity along with the slight shift of the T-max (temperature of maximum permittivity) to a higher temperature. In addition, results from impedance spectroscopy indicate that the DC resistivity increased by approximately two orders of magnitude when the sintering time increased from 2 to 12 h. These results suggest that while sintering time has a minimal impact on either the microstructure or the long-range average structure, it has a strong influence on the ferroelectric to relaxor crossover, which is often associated with enhanced electromechanical properties. This work presents further evidence that the crossover phenomenon is closely tied to the local structure, where disruption of the long-range dipole order results in stabilization of the relaxor state.