Date of Award
Ferroelectrics; Multiferroic Composites
Two kinds of modern composites are studied in this thesis. One is ferroelectric materials, and the other is the multiferroic composites. Based upon the essential properties of these two types materials, several issues need to be considered in order to study their effective properties. For example, the microstructures of the system including the shape, size and the distribution; the phase connectivity; and the loading conditions will impact the results. First, we focus on the investigation of ferroelectrics and its composites. Based on the experimental observations, the nonlinear electromechanical coupling responses of ferroelectrics are strongly depending on the frequency of the applied electric field. In order to interpret the influence of a frequency, an exponential function is introduced to express the remanent polarization and coercive field in terms of the frequency. We incorporate the frequency-dependent exponential functions into the micromechanics-based model to establish the nonlinear constitutive relations of a ferroelectric material. Then, for the two-phase ferroelectric composites, the Mori-Tanaka Method can be applied to analyze and solve the overall effective electro-mechanical coupling behavior of the system. We demonstrate how the overall nonlinear effective physical properties depend on the phase volume concentration and inclusions’ shape and the loading frequency. Our predictions are shown to be in a good agreement with experimental data. Second, for multiferroic composites, both linear and nonlinear effective physical properties are studied. For the linear case, a popular BaTiO3-CoFe2O4 system is studied with different connectivity and aspect ratio. The effective physical properties of overall composites analyzed by the Mori-Tanaka method. The results shown that the magnetoelectric coupling coefficients 𝛼11 and 𝛼33 are highly depending on the volume fraction and aspect ratio. For the case of nonlinear, a two-level micromechanics model is developed to study the nonlinear magnetoelectric (ME) effects of 2-2 multiferroic composites consisting both ferromagnetic and ferroelectric phases. At the first level, similar to a ferroelectric material, a ferromagnetic phase can also be studied using a model involving a thermodynamically based evolution of the product domain from the parent one. Once the physical properties of ferroelectric and/or ferromagnetic phase can be obtained at the first level, their multiferroic composites can be studied as a two-phase composite at the second level. Then the two-level model is applied to evaluate the nonlinear magnetoelectric effects of Terfenol-D/PZT/Terfenol-D laminated system under the applied magnetic field. To verify the model, the voltage coefficient 𝛼E33 of the system is calculated and compared with the experimental data. The comparison between the theory and experiment is in a good agreement.
Hu, Zhiming, "A Micromechanics Study on Ferroelectrics and Multiferroic Composites" (2020). CUNY Academic Works.