Size Dependent Structures and Properties of Na0.5Bi0.5TiO3-Based Ceramics for Piezoelectric Sensors

Generally, film dielectric materials often exhibit size-dependent structure and electric properties. In this work, we demonstrate a similar behavior in bulk Na_0.5Bi_0.5TiO₃ (NBT)-based polycrystalline ceramics. According to the results from X-ray diffraction, the (Na_0.5Bi_0.5)_0.92Ba_0.08Ti_0.99Mg_0.01O_2.99 (NBT8M1.0) ceramic showed a complex structure that consists of rhombohedral, tetragonal and cubic symmetries. We found, when decreasing the thickness of a ϕ 10 mm NBT8M1.0 ceramic from 1230 μm to 230 μm, the ceramic showed increased content of cubic symmetry (CC) from 28% to 56%. Meanwhile, the piezoelectric response (d₃₃₎ increased from 107 pC/N to 134 pC/N and the depolarization temperature (T_d) decreased from 170 ℃ to 142 ℃. Similarly, when decreasing the diameter of a 1 mm thick NBT8M1.0 ceramic from ϕ 22 mm to ϕ 6 mm, the ceramic showed increased CC from 18% to 44%. Meanwhile, the d33 increased from 125 pC/N to 135 pC/N and the T_d decreased from 165 ℃ to 156 ℃. The origin of size-dependent structure and properties in macroscopic size range of NBT8M1.0 ceramics were further studied by investigating the difference of the size-dependence between the ceramics sintered from green bodies prepared by uniaxial pressing and isostatic pressing. The results demonstrated that the size-dependent structure and properties of NBT-based polycrystalline ceramics are closely related to the residual stress from the inhomogeneous densification during sintering process, as a result of the inhomogeneous density distribution in the pressed green body.