Electric field effect on the microstructure and properties of Ba0.9Ca0.1Ti0.9Zr0.1O3 (BCTZ) lead-free ceramics

Abstract

The attention on piezoelectric devices has been moved in the past decade to lead-free ceramics, especially on (Ba,Ca)(Ti,Zr)O₃-based materials, due to growing environmental concerns. Here we report a systematic evaluation of the effect that electric field application has on the structure, microstructure and electrical properties of Ba_0.9Ca_0.1Ti_0.9Zr_0.1O₃ (BCTZ) lead-free ceramics. Powders were prepared by Pechini method. Dense ceramic samples were sintered at 1275 °C for 5 h. As observed by X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDS), the ceramics are single perovskite-type structure phases and they are homogeneous both at global and local scales. Rietveld analysis of the diffraction patterns for the non-poled sample is consistent with a coexistence of ferroelectric tetragonal P4mm and rhombohedral R3m symmetries. It is shown that the electric field induces changes in rhombohedral–tetragonal relative content that facilitates the polarization process of the system. Ceramics exhibit a significantly high piezoelectric response with a d₃₃ value as high as ∼390 pC N⁻¹, which is comparable to soft Pb(Zr_1−xTi_x)O₃ (PZTs). Scanning Electron Microscopy (SEM) analysis showed an average grain size of 20 μm and complex ferroelectric domains. A confocal Raman spectroscopy and imaging study carried out on both types of samples reveals that, upon application of an electric field, the E(LO₃) + A₁(LO₂) + E(TO₄) mode of the Raman spectra shifts to lower frequencies, indicating a reduction of the bonding force between the ions at the B-site of the perovskite (Ti⁴⁺, Zr⁴⁺) and the oxygen ions of the octahedron for the poled samples. Besides, the mentioned spatially resolved mode of the Raman spectra shows a narrower distribution in the poled sample, monitoring the polarization alignment effect in the direction of the field. These characteristics are related to the complex ferroelectric domain distribution. The polar structure changes are revealed after the application of the electric field, as well by the changes on the dielectric permittivity curves as a function of the temperature. Ceramics also display a high ferroelectric remnant polarization (P_r = 15.3 μC cm⁻²) and low coercive field (E_C = 1.5 kV cm⁻¹) at room temperature.