This award supports research and educational activities with an aim to advance our fundamental understanding of thermal transport in solid materials. Thermal transport is a critical consideration in applications ranging from energy-efficient buildings to cooling electronics to converting waste heat into electricity.
Thermal conductivity is a property that describes how heat moves through a material. Standard approaches for predicting the thermal conductivity of a solid are based on an assumption that its atoms are either perfectly ordered or fully disordered. The atomic structure of many solids, however, has elements of both order and disorder. For example, a class of materials, known as metal halide perovskites which can convert sunlight into electricity, contain molecules at regularly ordered positions, but the molecules rotate, which creates disorder. Their thermal conductivities, which must be known to determine device operating temperatures, cannot be predicted using standard approaches. This project first seeks to build theoretical tools and a computational framework for performing such calculations. The framework will then be applied to understand how thermal conductivity is impacted by co-existing order and disorder in metal halide perovskites and sodium superoxide, which is an important component of sodium-air batteries. The tools and framework will be suitable for studying thermal transport in any solid that contains elements of order and disorder.
The participating graduate and undergraduate students will carry out cutting-edge, interdisciplinary research that spans materials science, physics, and mechanical engineering. A modular, open-access molecular simulation course will be developed and distributed to the scientific community. An outreach activity that explores the order-disorder spectrum will be developed and presented to middle-school and high-school students in the Pittsburgh area.
This award supports research and educational activities with an aim to advance our fundamental understanding of thermal transport in statically and dynamically disordered crystals. Such materials have a well-defined lattice and basis, with elements of disorder emerging at higher temperatures. In a crystal with static disorder, a multi-well potential energy surface leads to differences between unit cells (e.g., the oxygen dimer orientations in sodium superoxide). In a crystal with dynamic disorder, some atoms do not vibrate around an equilibrium position (e.g., the rotating methylammonium ion in a halide perovskite).
The central hypothesis is that thermal transport in statically and dynamically disordered crystals can be described through an integrated treatment of phonons, diffusons (delocalized, non-propagating vibrational modes), and defects. A computational framework will be built that integrates finite-temperature force constants, lattice dynamics, the Boltzmann transport equation, Allen-Feldman theory, and perturbation theory to predict mode-dependent properties and thermal conductivities. Novel contributions will include the first-ever implementation of rotational lattice dynamics for thermal conductivity prediction, the formulation of a virtual crystal approximation approach for modeling scattering by static disorder, and the use of unstable equilibrium structures to include scattering by dynamic disorder. The framework will then be applied to understand how thermal conductivity is impacted by static and dynamic disorder in sodium superoxide and halide perovskites.
The participating graduate and undergraduate students will carry out cutting-edge, interdisciplinary research that spans materials science, physics, and mechanical engineering. A modular, open-access molecular simulation course will be developed and distributed through nanoHUB. An outreach activity that explores the order-disorder spectrum will be developed and presented to middle-school and high-school students in the Pittsburgh area.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
