Phonon-Defect Interactions in Thermoelectric Materials

Gurunathan, Ramya

doi:https://doi.org/10.21985/n2-cgdp-zr83

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Phonon-Defect Interactions in Thermoelectric Materials

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Engineering heat transport in materials is essential for thermal management in a wide range of technologies, from batteries to thermoelectrics. Materials host a wide spectrum of heat-carrying phonons, which vary in their frequency, spatial extent, and degree of plane-wave character. This diversity in phonon properties leads to complex behavior, especially in materials with structural complexity, defects, and internal strain sources. The study of thermal conductivity has benefited from simple, physics-based models for over 70 years, as they are easily implemented, elucidate underlying mechanisms, and can even help point to exotic physics when they fail to describe a system. Their lasting relevance supports the argument for continued work on analytic, physical expressions in emerging fields of materials science even as new techniques in simulation and materials informatics become widespread. This thesis primarily focuses on the development of analytic theory to describe the phonon interactions with structural defects. Special focus is given to the phonon scattering effects of point defects and low energy interfaces, which are composed of an underlying array of interfacial dislocations. We start by reviewing previous descriptions of phonon—point-defect interactions and presenting a conceptually clear model of point defect scattering. We then apply this model to study the thermal properties in multicomponent alloy systems for thermoelectrics. We additionally show an extension of alloy scattering models for charge carriers to high dimensional alloy systems by drawing an analogy to the calculation of excess Gibbs free energy. Design rules are suggested based on this modeling for when a reduction in thermal conductivity can be expected from multicomponent alloying. We end with a discussion of phonon-interface scattering and introduce our model of the thermal boundary resistance RK of low-energy grain boundaries and interfaces. Our modeling of symmetric tilt and twist grain boundaries as well as semicoherent heterointerfaces helps to address fundamental questions about RK such as: How does the RK of a twist and tilt grain boundary compare? How does RK relate to grain boundary angle and energy? How does the degree of misfit at a heterointerface impact its RK?

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