Piezoelectric anisotropy and energy-harvesting characteristics of novel sandwich layer BaTiO₃ structures

This paper presents a detailed modelling and experimental study of the piezoelectric and dielectric properties of novel ferroelectric sandwich layer BaTiO₃ structures that consist of an inner porous layer and dense outer layers. The dependencies of the piezoelectric coefficients and dielectric permittivity of the sandwich structure on the bulk relative density α are analysed by taking into account an inner layer with a porosity volume fraction of 0.5-0.6. The observed changes in and are interpreted within the framework of a model of a laminar structure whereby the electromechanical interaction of the inner porous layer and outer dense layers have an important role in determining the effective properties of the system. The porous layer is represented as a piezocomposite with a 1-3-0 connectivity pattern, and the composite is considered as a system of long poled ceramic rods with 1-3 connectivity which are surrounded by an unpoled ceramic matrix that contains a system of oblate air pores (3-0 connectivity). The outer monolithic is considered as a dense poled ceramic, however its electromechanical properties differ from those of the ceramic rods in the porous layer due to different levels of mobility of 90° domain walls in ceramic grains. A large anisotropy of d_3j ^∗ at α = 0.64-0.86 is achieved due to the difference in the properties of the porous and monolithic layers and the presence of highly oblate air pores. As a consequence, high energy-harvesting figures of merit d_3j ^∗ g_3j ^∗ are achieved that obey the condition d₃₃ ^∗ g₃₃ ^∗/( d₃₁ ^∗ g₃₁^∗) ∼ 10² at d₃₃^∗ g₃₃^∗ ∼ 10^-12 Pa^-1 and values of the hydrostatic piezoelectric coefficients d_h^z.ast; ≈ 100 pC N and g_h^z.ast; ≈ 20 m V m N are achieved at α= 0.64-0.70. The studied BaTiO₃-based sandwich structures has advantages over highly anisotropic PbTiO₃-type ceramics as a result of the higher piezoelectric activity of ceramic BaTiO₃ and can be used in piezoelectric sensor, energy-harvesting and related applications.