Studies On The Properties Of Thermoelectric Materials And Thermal-electrical-mechanical Behaviors Of Thermoelectric Devices

Thermoelectric (TE) energy is a new renewable clean source of energy, which can be obtained through TE devices. Based upon the TE effects of TE materials, the TE device can realize the conversion between thermal energy and electric energy in the most direct way, i.e. electron transport. Currently, the TE conversion efficiency is relatively low, such that the practical value of TE devices has been greatly restricted. How to improve the TE conversion efficiency has become a key scientific issue in TE technology. In this dissertation, the TE properties of polycrystalline thin films and the coupled thermal-electrical-mechanical performance of TE devices have been studied comprehensively, in order to provide a theoretical reference for improving the performance of TE materials and TE devices. The content of the dissertation can be summarized as:Firstly, from the point of view of the microstructure in films and the Boltzmann transport theory, a theoretical model is presented to predict the Seebeck coefficient and the electrical conductivity for a polycrystalline TE film by considering the combined effects of the film-surface scattering and grain-boundary scattering. The model takes the scattering effects as the boundary condition for the electron distribution function. The transmission coefficient and reflection coefficient are introduced to describe the levels of the film-surface scattering and grain-boundary scattering, respectively. The relations between the Seebeck coefficient, the electrical conductivity and the film thickness, grain number and scattering coefficient are then discussed. It can be found that in contrast to the film-surface effect, the grain-boundary effect plays a key role in the TE properties of TE films. To study the dependence of TE properties on magnetic field, the grain-boundary scattering is considered as the periodic boundary conditions of the electron distribution function. By analyzing the distribution characteristics of electron within a single grain, a theoretical model of the magnetic effect on the Seebeck coefficient and the electrical conductivity is established. It can be noticed from the theoretical calculations that the Seebeck coefficient, and the electrical conductivity also shows the grain size effect; the applied magnetic field increases the electron density of states, while TE properties of TE film has been significantly improved.Secondly, using the theory of heat transfer theory and finite element method, the predictive models on the TE performance of linear-shaped TE generators under two operating temperature conditions are built, individually. For the small temperature difference condition, a simple ID analytical model is developed to predict the TE performance. For the large temperature difference condition, a 2D finite element model is developed to simulate the TE behaviors. And the proposed models are verified with each other. The effect of the geometry dimension of the thermoelements on the output power and the conversion efficiency is investigated. In addition, taking into account the ever-changing practical application environments, a transient numerical model is developed to study the dynamic response characteristics of linear-shaped TE generators. The temperature of the heat source is treated as the time-varying thermal boundary condition of the TE generator. For heating and cooling processes, the effect of the output power and the conversion efficiency on variation factors (i.e. loading way of thermal load, time to reach a stable thermal load, geometry dimension of the thermoelements) are investigated. The results indicate that, the linear-shaped TE generator exhibits better design flexibility and TE performance than a traditional π-shaped TE generator; some useful criteria for the optimal design are given; under dynamic heat supply, an internal heat source exists in the generator, caused by the delay of thermal diffusion from the hot end of the thermoelements to cold end. This heat source renders a heat release from the generator to the ambient. A process relying on its own heat to generate electricity is thus formed. The duration of this process can be controlled by changing the operating conditions.Finally, we construct the functionally graded TE legs by compositing two different materials in segmentation fashion. And a 3D finite element model is established to estimate the TE and mechanical performance of the segmented TE generator. Based upon thermal-electric coupling calculation, the TE conversion efficiency under a given operating temperature is optimized by selecting the appropriate segmented material length. And for different operating temperatures, the theoretical maximum conversion efficiency is also discussed. Then, the thermal-mechanical coupling calculation is performed and the maximum stress levels of TE legs are obtained under different operating conditions. By the mechanical strength evaluation and mechanical reliability analysis, the TE behaviors of the segmented generator are verified, and the actual optimal operating conditions which can achieve satisfied mechanical reliability and conversion efficiency are determined. The numerical results show that, in contrast to TE devices with homogenous material, the segmented TE generator has a better TE performance; for a given operating temperature, some segmented cases do not satisfy the strength requirements of TE materials; for different operating temperatures, the segmented generator cannot always achieve the maximum design value of conversion efficiency. In practice, the design for TE devices should balance the maximization of efficiency and the mechanical reliability.In summary, this dissertation focuses on the theoretical investigations of the TE properties in polycrystalline TE films. These extend the theory of existing scale effects to low-dimensional/nanostructured TE materials, and provides a way to understanding of the electron transport characteristics of polycrystalline TE materials and the electron scattering mechanism, and also offers a theoretical basis for predicting the effect of applied magnetic field on TE properties of TE materials. In addition, the studies on the coupled thermal-electrical-mechanical performance of linear-shaped and segmented generators, not only show the design and performance advantages of TE devices with novel structure, and further strengthen the understanding of the behavior of TE devices in service, but also provide new ideas and methods for optimal design of the TE devices.