Text

theses

Energy materials play a significant role in modern material science. To understand the mechanism of functional materials, an energy functional formulation method can provide an efficient way to systematically describe the behavior of energy materials. Energy formulation method also has the advantage in dealing with the difficulties in the field formulation. In this thesis, we mainly have three parts of work based on energy formulation method.
First, an interesting problem on the equilibrium shape of a bubble/droplet in an electric field is investigated. This is important for electrowetting over dielectrics (EWOD), electrohydrodynamic enhancement for heat transfer, and electro-deformation of a single biological cell among others. In this part of work, we develop a general variational formulation on account of electro-mechanical couplings. In the context of electrohydrodynamics (EHD), we identify the free energy functional and the associated energy minimization problem that determines the equilibrium shape of a bubble in an electric field. Based on this variational formulation, we implement a fixed mesh level-set gradient method for computing the equilibrium shapes. This numerical scheme is efficient and validated by comparing with analytical solutions at the absence of electric field and experimental results at the presence of electric field. We also present simulation results for zero gravity which will be useful for space applications. The variational formulation and numerical scheme are anticipated to have broad applications in areas of EWOD, EHD, and electro-deformation in biomechanics.
Secondly, based on the continuum theory of thermoelectric materials developed by Liu[71], we predict that the power factor of thermoelectric (TE) composites can be significantly enhanced by simple laminate structures. This prediction is numerically verified by the Finite Element Model (FEM) that is implemented to compute the local fields in heterogeneous TE structures of general geometries and boundary conditions. Among many other applications, the FEM enables to investigate the effects of small electrical contact on power generation. For a cylindrical sandwich TE structure, we show that the power output of the TE sandwich structure, though lowered by a small contact area, is still significantly larger than that of the constituent TE semiconductor.
Thirdly, we study the type II superconducting materials. Many applications of high-temperature superconductors(HTS) need a high critical current density Jc, especially under a strong external magnetic field. An effective way to enhance Jc is to pin the vortex array to avoid flux flow. Therefore, fluxing pinning plays an important role in the properties of HTS. Here, based on Ginzburg- Landau theory and classic Landau theory of micromagnetics, we formulate the total free energy of the system associated with superconducting materials coupling with paramagnetic inhomogeneities. Consider thin film scenario, pinning force which is related to the size of inhomogeneity, paramagnetic permeability and distance of vortex to inhomogeneity interface is investigated with/without external transport current at dilute limit. We develop a self-consistent model, leading to an estimation of paramagnetic interface effect on pinning force in different structures of the thin film composite. The theoretical results fit well with existing experiments in the literature qualitatively.

ETD_9430

Ph.D.

Includes bibliographical references

by Hanxiong Wang

doi:10.7282/t3-eg6m-7283

ETD doctoral

The author owns the copyright to this work.