UCLA

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.