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Starting from a broad overview of heat transport based on the Boltzmann Transport Equation, this book presents a comprehensive analysis of heat transport in bulk and nanomaterials based on a kinetic-collective model (KCM). This has become key to understanding the field of thermal transport in semiconductors, and represents an important stride. The book describes how heat transport becomes hydrodynamic at the nanoscale, propagating very much like a viscous fluid and manifesting vorticity and friction-like behavior. It introduces a generalization of Fourier’s law including a hydrodynamic term based on collective behavior in the phonon ensemble. This approach makes it possible to describe in a unifying way recent experiments that had to resort to unphysical assumptions in order to uphold the validity of Fourier’s law, demonstrating that hydrodynamic heat transport is a pervasive type of behavior in semiconductors at reduced scales.
Advanced materials with extreme thermal conductivity are critically important for various technological applications including energy conversion, storage, and thermal management. High thermal conductivity is desirable for efficient heat spreading in electronics, and low thermal conductivity is needed for thermal insulation and thermoelectric energy harvesting. However, practical application deployments are usually limited by the materials availability and understanding the fundamental origins for extreme thermal conductivity remains challenging. My PhD research focuses on applying and developing first-principles computations to understand the microscopic thermal transport mechanisms of the emerging materials and to discover new materials with ultrahigh and ultralow thermal conductivity. My dissertation is composed of three themes. The first theme is focused on understanding the fundamental origins and transport mechanisms for a group of high thermal conductivity semiconductors that were discovered recently by our group. In particular, boron phosphide (BP) and boron arsenide (BAs) crystals have been synthesized and measured with thermal conductivities of 460 and 1300 W/mK respectively, representing the best thermal conductor among common bulk metals and semiconductors. I have conducted ab initio calculations based on density functional theory to investigate phonon anharmonicity, size-dependent transport from diffusive to ballistic regime, as well as the effect from defect scattering. Our study shows that, unlike the commonly accepted rule for most materials near room temperature, high-order anharmonicity through the four-phonon process is significant in BA because of its unique band structure. In addition, I have performed multiscale Monte Carlo simulations to solve phonon Boltzmann transport equations to compute heat dissipation in three-dimensional practical measurement samples and electronic devices, which quantitively determines temperature distributed resulted by non-equilibrium phonon transport and underscores the promise of our developed BP and BAs for the next generation of thermal management technologies. The second theme of my thesis is to theoretical search for new ultra-high thermal conductivity materials, with the aim to push the limit of existing materials database. We have calculated the thermal conductivity of several B-C-X ternary compounds and found the R3m-BNC2 has ultrahigh thermal conductivity at ~2200 W/mK, which is comparable with the existing highest thermal conductivity materials, diamond. We also calculate the thermal conductivity of single-layer boron compounds in III-V group, and find high thermal conductivity of single-layer h-BAs at around 400 W/K. My computational studies enable atomistic understanding through their phonon band structures, scattering spaces, lifetimes, etc. The third theme of my thesis is to investigate phonon transport in ultralow thermal conductivity materials with a focus on tin selenide (SnSe). SnSe is a recently discovered high performance thermoelectric material, but its intrinsic low thermal conductivity remains debating in recent literature. In collaboration with my labmates, we combine phonon theory and experiments to investigate phonon softening physics. In particular, my calculated phonon frequencies of SnSe under varying temperatures indicate strong phonon renormalization due to higher-order anharmonicity. The comparison of my theory results with experiments indicates that the widely used harmonic model fails to descript the phonon renormalization and thus thermal conductivity of SnSe. Instead, I have developed self-consistent phonon theory to capture the higher order interactions and provided very good agreement with the experimentally measured ultralow thermal conductivity and thermophysical properties of SnSe.
This is a classic text of its time in condensed matter physics.
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.
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.
It took us a long time to write this book. In 1959, two of us (Lifshits and Kaganov) pub lished a review of the mechanics of electrons with a complex dispersion law. About that time, geometrical terms such as extremal sections, curvatures, diameters, limiting points began to appear in papers on the electron theory of metals. They were followed by terms quite unusual in the scientific literature: monsters, pockets, arms, sheets, and so on. With their excitingly shaped figures, papers on the electron theory of metals began to resemble catalogs of exhibitions of abstract or ultramodern sculpture. The modern theory of metals was passing through its romantic period. Each newly interpreted Fermi surface and each discovery of a new structure sensitive phenomenon was an emotional experience for the authors and readers alike. The atti tude of the theoreticians was epitomized by phrases such as "This method or this phenomenon can be used to reconstruct the Fermi surface . . . ," which were found at the end of almost every paper on the electron theory of metals. The experimentalists selected convenient meth ods, being guided not so much by the elegance of a particular method as by its experimental capabilities. Gradually, the romantic approach gave way to a systematic activity, which re sulted in the interpretation of the energy spectra of the majority of metals. There were some unavoidable disappointments.
The present status of the theory of thermal conductivity in solids is such that although the processes leading to thermal resistance are understood in a qualititative manner, there is, as yet no reliable quantitative theory. To illustrate the successes and failures of the present theory we have calculated the heat conduction in a perfect linear lattice. It is possible to perform the necessary calculations for this model without the dange of becoming lost in a forest of indices. In a perfect crystal of reasonable size phonon-phonon interactions are the only scattering events which determine thermal resistance and the transition probability vanishes for phonon-phonon scattering in a one-dimensional lattice. We have demonstrated that to simultaneously satisfy the selection rules on the wave vectors and the energy at least one of the wave vectors must be to a reciprocal lattice vector (more precisely zero in the one-dimensional case). This causes the transition probability as calculated by first order perturbation theory to vanish for cubic anharmonic terms in the potential energy of the lattice. The thermal conductivity of simple alloys has been calculated by assuming that the addtional scattering in the alloy is due to mass difference of the elements comprising the alloy. This "isotope effect" has been used to calculate the thermal conductivity of Si-Ge alloys.