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Energy transport provides the fundamental basis for operation of devices from transistors to solar cells. Despite past theories that successfully illustrate the principles behind the energy transport based on solid state physics, the microscopic details of the energy transport are not always clear due to the lack of tool to quantify the contribution from different degrees of freedom. Recent progress in first principles computations and development in optical characterization has offered us new ways to understand the energy transport at the nanoscale in a quantitative way. In this thesis, by leveraging these techniques, we aim to providing a detailed understanding of thermal and thermoelectric energy transport in crystalline and disordered materials, especially about how the energy transport depends on atomistic level details such as chemical bondings. Specifically, we will discuss three examples. 1) Electron transport in semiconductors: how electrons propagate as they interact with lattice and impurities. 2) Interaction between charge and heat: how the free carriers have an impact on the heat dissipation in semiconductors 3) Heat conduction in polymers: how the heat transfer in an amorphous system depends on its molecular structures. In the case of electron transport, we developed and applied first principles simulation to show that a large electron mobility can benefit from symmetry-protected non-bonding orbitals. Such orbitals result in weak electron-lattice coupling that explains the unusually large power factors in half-Heusler materials - a good thermoelectric material system. By devising an optical experiment to probe the ultrafast thermal decay, we quantified the effect of electron-phonon interaction on the thermal transport. Our results show that the thermal conductivity can be significantly affected by the free carriers. Lastly, we built a theoretical model to understand the heat conduction in amorphous polymers, and used this knowledge to design materials that are heat-conducting yet soft. These understandings will potentially facilitate discovery of new material systems with beneficial charge and heat transport characteristic.
Thermoelectric materials, which enable direct conversion between thermal and electrical energy, provide an alternative for power generation and refrigeration. The key parameter that defines the efficiency of thermoelectric materials is the 'dimensionless figure of merit' ZT, which is composed of the Seebeck coefficient, electrical conductivity and total thermal conductivity respectively. Ideally, to achieve high ZT both the Seebeck coefficient and electrical conductivity should be large, while total thermal conductivity must be minimized. In this thesis, first-principles calculations of the Seebeck coefficient, lattice thermal conductivity and electrical conductivity are performed to study mechanisms and factors that gives rise to high ZT. One effective way to enhance ZT is through direct reduction of lattice thermal conductivity. We perform calculation and analysis of lattice thermal conductivity for thermoelectric materials by solving the Boltzmann transport equation iteratively in the framework of perturbation theory. The second- and third-order interatomic force constants are extracted using the recently developed CSLD (compressive sensing lattice dynamics) method. Afterwards, we evaluate opportunities to achieve further reduction of lattice thermal conductivity. Our first study of ternary zinc-blende-based mineral compounds famatinite (Cu3SbS4) and permingeatite (Cu3SbSe4) shows that optical modes in these two compounds contribute a sizable portion of the total lattice thermal conductivity and thus cannot be neglected. Due to the fact that phonon modes with mean free paths larger than 10 nm carry about 80% of the heat, nanostructuring, which reduces the mean free path, is a promising way to reduce the lattice thermal conductivity by reducing the characteristic length. In addition, our simple alloying model including mass disorder reproduces experimental findings that forming solid solutions rapidly decreases the lattice thermal conductivity. An alternative way to reduce lattice thermal conductivity is to introduce guest atoms in host cage structures. Our study of type-I Si clathrates containing guest atoms Na and Ba shows that Na tends to form incoherent localized phonon mode while Ba coherently couples with the host cages. The low lattice thermal conductivities of Na- and Ba-filled Si clathrates should be attributed to the dramatic reductions in both phonon lifetime and group velocity. Analysis of phonon scattering process reveals that localized modes can be effectively emitted and absorbed, thus dramatically enhancing overall scattering rates. Another widely adopted approach to achieve high ZT is through maintaining a high power factor. To accurately determine the Seebeck coefficient and electrical conductivity, we estimate carrier lifetime due to electron-phonon interaction under relaxation time approximation using the electron-phonon Wannier interpolation technique. Our study of noble metals Cu and Ag shows that their positive Seebeck coefficients can be mostly attributed to the negative energy dependence of carrier lifetime. In contrast to the previous study of positive Seebeck in Li, which is due to the deviation of electronic behavior from that in free electron model, it is the nontrivial energy dependence of electron-phonon interaction vertex that leads to the positive Seebeck coefficient. Intermetallic compound B20-type CoSi has drawn considerable attention due to its exceptionally high power factor and large Seebeck coefficient. Our study shows that the large negative Seebeck coefficient of the pristine CoSi is mostly due to the strong energy dependence of carrier lifetime, which together with the high electrical conductivity leads to the high power factor. For heat transport, both electron-phonon and phonon-phonon interactions contribute significantly to phonon scattering at temperatures lower than 200 K. While at temperatures higher than 300 K, phonon-phonon interaction dominates over electron-phonon interaction. Based on the optimized power factor with properly adjusted carrier concentration, we predict that the maximum ZTs at 300 and 600 K are about 0.11 and 0.25 respectively without further reducing the total thermal conductivity. Known good thermoelectric materials often are comprised of elements that are in low abundance, toxic and require careful doping and complex synthesis procedures. High performance thermoelectricity has been reported in earth-abundant compounds based on natural mineral tetrahedrite (Cu12Sb4S13). Our first-principles electronic structure calculations of Cu12Sb4S13 show that Cu atoms are all in the monovalent state, creating two free hole states per formula unit of the pristine compound. Optimal thermoelectric performance can be achieved via electron doping. Substituting transition metals on Cu 12d sites does the job. Detailed analysis shows that Zn and Fe substitutions tend to fill the empty hole states, while Ni substitution introduces an additional hole to the valence band by forming ferromagnetic configuration. Experimentally observed extremely low lattice thermal conductivity can be attributed to the out-of-plane vibrations of the three-fold Cu ions. This is further verified by the large Gruneisen parameter calculated.
This book introduces readers to state-of-the-art theoretical and simulation techniques for determining transport in complex band structure materials and nanostructured-geometry materials, linking the techniques developed by the electronic transport community to the materials science community. Starting from the semi-classical Boltzmann Transport Equation method for complex band structure materials, then moving on to Monte Carlo and fully quantum mechanical models for nanostructured materials, the book addresses the theory and computational complexities of each method, as well as their advantages and capabilities. Presented in language that is accessible to junior computational scientists, while including enough detail and depth with regards to numerical implementation to tackle modern research problems, it offers a valuable resource for computational scientists and postgraduate researchers whose work involves the theory and simulation of electro-thermal transport in advanced materials.
There have been few books devoted to the study of phonons, a major area of condensed matter physics. The Physics of Phonons is a comprehensive theoretical discussion of the most important topics, including some topics not previously presented in book form. Although primarily theoretical in approach, the author refers to experimental results wherever possible, ensuring an ideal book for both experimental and theoretical researchers. The author begins with an introduction to crystal symmetry and continues with a discussion of lattice dynamics in the harmonic approximation, including the traditional phenomenological approach and the more recent ab initio approach, detailed for the first time in this book. A discussion of anharmonicity is followed by the theory of lattice thermal conductivity, presented at a level far beyond that available in any other book. The chapter on phonon interactions is likewise more comprehensive than any similar discussion elsewhere. The sections on phonons in superlattices, impure and mixed crystals, quasicrystals, phonon spectroscopy, Kapitza resistance, and quantum evaporation also contain material appearing in book form for the first time. The book is complemented by numerous diagrams that aid understanding and is comprehensively referenced for further study. With its unprecedented wide coverage of the field, The Physics of Phonons will be indispensable to all postgraduates, advanced undergraduates, and researchers working on condensed matter physics.
Thermoelectricity and Advanced Thermoelectric Materials reviews emerging thermoelectric materials, including skutterudites, clathrates, and half-Heusler alloys. In addition, the book discusses a number of oxides and silicides that have promising thermoelectric properties. Because 2D materials with high figures of merit have emerged as promising candidates for thermoelectric applications, this book presents an updated introduction to the field of thermoelectric materials, including recent advances in materials synthesis, device modeling, and design. Finally, the book addresses the theoretical difficulties and methodologies of computing the thermoelectric properties of materials that can be used to understand and predict highly efficient thermoelectric materials. This book is a key reference for materials scientists, physicists, and engineers in energy. - Reviews the most relevant, emerging thermoelectric materials, including 2D materials, skutterudites, clathrates and half-Heusler alloys - Focuses on how electronic structure engineering can lead to improved materials performance for thermoelectric energy conversion applications - Includes the latest advances in the synthesis, modeling and design of advanced thermoelectric materials
This book introduces readers to state-of-the-art theoretical and simulation techniques for determining transport in complex band structure materials and nanostructured-geometry materials, linking the techniques developed by the electronic transport community to the materials science community. Starting from the semi-classical Boltzmann Transport Equation method for complex band structure materials, then moving on to Monte Carlo and fully quantum mechanical models for nanostructured materials, the book addresses the theory and computational complexities of each method, as well as their advantages and capabilities. Presented in language that is accessible to junior computational scientists, while including enough detail and depth with regards to numerical implementation to tackle modern research problems, it offers a valuable resource for computational scientists and postgraduate researchers whose work involves the theory and simulation of electro-thermal transport in advanced materials.
This book provides an overview on nanostructured thermoelectric materials and devices, covering fundamental concepts, synthesis techniques, device contacts and stability, and potential applications, especially in waste heat recovery and solar energy conversion. The contents focus on thermoelectric devices made from nanomaterials with high thermoelectric efficiency for use in large scale to generate megawatts electricity. Covers the latest discoveries, methods, technologies in materials, contacts, modules, and systems for thermoelectricity. Addresses practical details of how to improve the efficiency and power output of a generator by optimizing contacts and electrical conductivity. Gives tips on how to realize a realistic and usable device or module with attention to large scale industry synthesis and product development. Prof. Zhifeng Ren is M. D. Anderson Professor in the Department of Physics and the Texas Center for Superconductivity at the University of Houston. Prof. Yucheng Lan is an associate professor in Morgan State University. Prof. Qinyong Zhang is a professor in the Center for Advanced Materials and Energy at Xihua University of China.
Phonon properties of Mo3Sb7-xTex (x = 0, 1.5, 1.7), a potential high-temperature thermoelectric material, have been studied with inelastic neutron and x-ray scattering, and with first-principles simulations. The substitution of Te for Sb leads to pronounced changes in the electronic struc- ture, local bonding, phonon density of states (DOS), dispersions, and phonon lifetimes. Alloying with tellurium shifts the Fermi level upward, near the top of the valence band, resulting in a strong suppression of electron-phonon screening, and a large overall stiffening of interatomic force- constants. The suppression in electron-phonon coupling concomitantly increases group velocities and suppresses phonon scattering rates, surpassing the effects of alloy-disorder scattering, and re- sulting in a surprising increased lattice thermal conductivity in the alloy. We also identify that the local bonding environment changes non-uniformly around different atoms, leading to variable perturbation strengths for different optical phonon branches. The respective roles of changes in phonon group velocities and phonon lifetimes on the lattice thermal conductivity are quantified. Lastly, our results highlight the importance of the electron-phonon coupling on phonon mean-free-paths in this compound, and also estimates the contributions from boundary scattering, umklapp scattering, and point-defect scattering.
Thermoelectric Materials and Devices summarizes the latest research achievements over the past 20 years of thermoelectric material and devices, most notably including new theory and strategies of thermoelectric materials design and the new technology of device integration. The book's author has provided a bridge between the knowledge of basic physical/chemical principles and the fabrication technology of thermoelectric materials and devices, providing readers with research and development strategies for high performance thermoelectric materials and devices. It will be a vital resource for graduate students, researchers and technologists working in the field of energy conversion and the development of thermoelectric devices. - Discusses the new theory and methods of thermoelectric materials design - Combines scientific principles, along with synthesis and fabrication technologies in thermoelectric materials - Presents the design optimization and interface technology for thermoelectric devices - Introduces thermoelectric polymers and organic-inorganic thermoelectric composites
Phonon properties of Mo3Sb7-xTex (x = 0,1.5,1.7), a potential high-temperature thermoelectric material, have been studied with inelastic neutron and x-ray scattering, and with first-principles simulations. The substitution of Te for Sb leads to pronounced changes in the electronic structure, local bonding, phonon density of states, dispersions, and phonon lifetimes. Alloying with tellurium shifts the Fermi level upward, near the top of the valence band, resulting in a strong suppression of electron-phonon screening and a large overall stiffening of interatomic force constants. The suppression in electron-phonon coupling concomitantly increases group velocities and suppresses phonon scattering rates, surpassing the effects of alloy-disorder scattering and resulting in a surprising increased lattice thermal conductivity in the alloy. We also identify that the local bonding environment changes nonuniformly around different atoms, leading to variable perturbation strengths for different optical phonon branches. Here, changes in phonon group velocities and phonon scattering rates are quantified, highlighting the large effect of electron-phonon coupling in this compound.