A Mesoscopic Electromechanical Theory of Ferroelectric Films and Ceramics

We present a multi-scale modelling framework to predict the effective electromechanical behavior of ferroelectric ceramics and thin films. This paper specifically focuses on the mesoscopic scale and models the effects of domains and domain switching taking into account intergranular constraints. Starting from the properties of the single crystal and the pre-poling granular texture, the theory predicts the domain patterns, the post-poling texture, the saturation polarization, saturation strain and the electromechanical moduli. We demonstrate remarkable agreement with experimental data. The theory also explains the superior electromechanical property of PZT at the morphotropic phase boundary. The paper concludes with the application of the theory to predict the optimal texture for enhanced electromechanical coupling factors and high-strain actuation in selected materials

Imaging Ferroelectric Domains via Charge Gradient Microscopy Enhanced by Principal Component Analysis

Local domain structures of ferroelectrics have been studied extensively using various modes of scanning probes at the nanoscale, including piezoresponse force microscopy (PFM) and Kelvin probe force microscopy (KPFM), though none of these techniques measure the polarization directly, and the fast formation kinetics of domains and screening charges cannot be captured by these quasi-static measurements. In this study, we used charge gradient microscopy (CGM) to image ferroelectric domains of lithium niobate based on current measured during fast scanning, and applied principal component analysis (PCA) to enhance the signal-to-noise ratio of noisy raw data. We found that the CGM signal increases linearly with the scan speed while decreases with the temperature under power-law, consistent with proposed imaging mechanisms of scraping and refilling of surface charges within domains, and polarization change across domain wall. We then, based on CGM mappings, estimated the spontaneous polarization and the density of surface charges with order of magnitude agreement with literature data. The study demonstrates that PCA is a powerful method in imaging analysis of scanning probe microscopy (SPM), with which quantitative analysis of noisy raw data becomes possible

The effective behavior of thermoelectric composites

Thermoelectric materials are promising due to its capability of converting heat directly into electricity and vice versa, and can be used for both waste heat recovery and thermal management. In this study, we developed a homogenization method to study the effective behavior of thermoelectric composites with periodic microstructure. Unit cell problem is established first from asymptotic analysis, which is then solved numerically by finite element method. The effective thermoelectric properties are calculated, and the corresponding conversion efficiency is analyzed. It is discovered that the homogenized thermoelectric equations are significantly different from those of homogeneous materials. More importantly, higher conversion efficiency than those of the constituent phases is demonstrated, and the condition for improved conversion efficiency is identified. The analysis provides considerable insight into the effective behavior of thermoelectric composites, and it can be used to guide the design and optimization of high efficiency thermoelectric materials

Electrochemical strain microscopy: principle, analysis, and applications

In this discussion, electrochemical strain microscopy (ESM) as a power tool to probe electrochemistry at the nanoscale is introduced. The imaging mechanism is based on ionic Vegard strain perturbed by charged SPM probe, and the methods to distinguish electromechanical coupling arising from piezoelectric strain, dipolar electrostriction, and electrochemical Vegard strain are discussed first. A phase field based model is then developed to interpret strain response in electrochemical system probed by ESM, and a combination of scanning and spectroscopic experiments are proposed to measure local ionic diffusivity and concentration at the nanoscale. The method has been applied to a variety of lithium ion battery electrodes to correlate the local electrochemistry at the nanoscale to the macroscopic battery performance, and future developments as well as some practical limitations are also discussed

Proterozoic crustal evolution of NE Australia during the supercontinent Nuna Assembly: new insights from a coupled thermochronological and geophysical study

This thesis primary focus on the NE Australian Craton, with particular interests in the Protozoic rocks of the Mount Isa and Georgetown inliers, to resolve the crustal evolution record across this region that reflects on tectonic processes and crustal history related to the assembly of the supercontinent Nuna

Comments on the spontaneous strain and polarization of polycrystalline ferroelectric ceramics

A framework to calculate the spontaneous strain and polarization of a polycrystalline ferroelectric is presented, and various applications are discussed

Electroelastic moduli of piezoelectric polycrystals with bulk and film textures

The effect of crystallographic texture on the electroelastic moduli of piezoelectric polycrystals has been studied using micromechanical modeling that makes use of the uniform field concept. An orientational averaging scheme has been developed for textured piezoelectric polycrystals, which, when combined with the conventional self-consistent approach, provides an estimate of the effective electroelastic moduli in terms of texture. In the special situation where the polycrystal exhibits a fiber texture, a class of uniform fields exist under certain crystal symmetries, so that the effective electroelastic moduli can be determined exactly. This is confirmed by the coincidence of the corresponding upper and lower bounds. Numerical results are presented for both cases and compared to known theoretical predictions where possible