The effect of inter-granular constraints on the response of polycrystalline piezoelectric ceramics at the surface and in the bulk

The electro-mechanical coupling mechanisms in polycrystalline ferroelectric materials, including a soft PbZr_xTi_1−xO₃ (PZT) and lead-free 0.9375(Bi_1/2Na_1/2)TiO₃-0.0625BaTiO₃ (BNT-6.25BT), have been studied using a surface sensitive low-energy (12.4 keV) and bulk sensitive high-energy (73 keV) synchrotron X-ray diffraction with in situ electric fields. The results show that for tetragonal PZT at a maximum electric field of 2.8 kV/mm, the electric-field-induced lattice strain (ε₁₁₁) is 20% higher at the surface than in the bulk, and non-180° ferroelectric domain texture (as indicated by the intensity ratio I₀₀₂/I₂₀₀) is 16% higher at the surface. In the case of BNT-6.25BT, which is pseudo-cubic up to fields of 2 kV/mm, lattice strains, ε₁₁₁ and ε₂₀₀, are 15% and 20% higher at the surface, while in the mixed tetragonal and rhombohedral phases at 5 kV/mm, the domain texture indicated by the intensity ratio, I₁₁₁/I₁₁₁¯ and I₀₀₂/I₂₀₀, are 12% and 10% higher at the surface than in the bulk, respectively. The observed difference in the strain contributions between the surface and bulk is suggested to result from the fact that surface grains are not constrained in three dimensions, and consequently, domain reorientation and lattice expansion in surface grains are promoted. It is suggested that the magnitude of property difference between the surface and bulk is higher for the PZT than for BNT-6.25BT due to the level of anisotropy in the strain mechanism. The comparison of the results from different methods demonstrates that the intergranular constraints have a significant influence on the electric-field-induced electro-mechanical responses in polycrystalline ferroelectrics. These results have implications for the design of higher performance polycrystalline piezoelectrics.