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 d3j ∗ 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 d3j ∗ g3j ∗ 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 dhz.ast; ≈ 100 pC N and ghz.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.</p

Finite element modelling of bilayer porous PZT structures with improved hydrostatic figures of merit

A finite element model is presented in which bilayer lead zirconate titanate (PZT) structures that are formed from a dense layer and a porous layer are investigated for their hydrostatic sensing properties. The model simulates the poling of the porous ferroelectric material to determine the distribution of poled material throughout the structure. The fraction of PZT successfully poled is found to be closely related to resulting piezoelectric and dielectric properties of the composite. Structures with high layer porosity (&gt;40 vol.%) and porous layer relative thickness (&gt;0.5) were found to have a significantly improved hydrostatic piezoelectric coefficient, dh, hydrostatic voltage coefficient, gh, and hydrostatic figure of merit, dh.gh. The highest dh.gh of 7.74 × 10-12 m2/N was observed in the structure with a porous layer relative thickness of 0.6 and porosity of 60 vol.%, which was more than 100 times higher than that for dense PZT (dh.gh = 0.067 × 10-12 m2/N) and over three times that of PZT with 60 vol.% porosity with 3-3 connectivity (dh.gh = 2.19 × 10-12 m2/N). The results demonstrate the potential for layered porous materials for use in hydrophones.</p

Electric field distribution in porous piezoelectric materials during polarization

High piezoelectric coupling coefficients enable the harvesting of more energy or increase the sensitivity of sensors which work using the principle of piezoelectricity. These coefficients depend on the material properties, but the manufacturing process can have a significant impact on the resulting overall coefficients. During the manufacturing process, one of the main steps is the process of polarization where a poling electric field aligns the ferroelectric domains in a similar direction in order to create a transversely isotropic material able to generate electric fields or deformations. The degree of polarization depends on multiple factors and it can strongly influence the final piezoelectric coefficients. In this paper, a study on the electric field distribution on the sensitivity of the main piezoelectric and dielectric coefficients to the polarization process is performed, focusing on porous piezoelectric materials. Different inclusion geometries are considered, namely spherical, ellipsoidal and spheres with cracks. The electric field distribution at the micro scale within a representative volume element is modelled to determine the material polarization level using the finite element method. The results show that the electric field distribution is highly dependent on the inclusion geometries and cracks and it has a noticeable impact on the equivalent piezoelectric coefficients. These results are compared with experimental measurements from published literature. Good agreement is found between the ellipsoidal model and the experimental data