BEYOND THE PHONON GAS MODEL (PGM): UNRAVELING THE UNIQUE MECHANISTIC PROCESSES DICTATING THERMAL TRANSPORT IN HIGHLY ANHARMONIC AND DISORDERED MATERIALS

Author

Sandip Thakur, University of Rhode IslandFollow

Date of Award

2025

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Mechanical Engineering and Applied Mechanics

Department

Mechanical, Industrial and Systems Engineering

First Advisor

Ashutosh Giri

Abstract

Thermal transport in nonmetallic solids has traditionally been described by the phonon gas model (PGM), in which heat is carried by weakly interacting phonon quasiparticles in an ordered crystalline lattice. However, this model breaks down in materials characterized by strong anharmonicity, dynamic disorder, or structural complexity, features prevalent in many next-generation materials used in energy conversion, optoelectronics, and thermal management. This dissertation investigates thermal transport in such complex materials, including metal halide perovskites (MHPs), covalent organic frameworks (COFs), metal organic frameworks (MOFs), and 2D-3D heterostructures, through large-scale molecular dynamics (MD) simulations and frequency-resolved spectral analyses.

In MHPs, the work reveals that thermal conductivity is governed by two-dimensional octahedral tilt correlations, which are further reinforced by the dynamics of A-site cations, contrary to conventional picture of “rattling” suppressing heat flow. In COFs, supramolecular interactions in interpenetrated frameworks significantly enhance thermal conductivity by stiffening lattice vibrations. A high-throughput screening of 10,750 COFs uncovers key structure-property relationships and provides design guidelines linking framework topology, bonding chemistry, and structural motifs to thermal properties. In MOFs, CO₂ gas adsorption is shown to modulate thermal conductivity in a non-monotonic manner, while vibrational scattering by adsorbed gas molecules to the framework reduces thermal conductivity at low temperatures, enhanced gas diffusivity at higher temperatures introduces additional pathways for heat transport. For 2D-3D interfaces, spectral analyses reveal that thermal boundary conductance is highly dependent to the intrinsic anharmonicity of the 2D material and to the the spectral coupling of vibrational modes across the interface.

Together, these studies establish a new mechanistic framework for understanding and engineering thermal transport in structurally complex and dynamically disordered systems, well beyond the limits of the traditional PGM. The insights presented herein not only advance the fundamental understanding of heat conduction but also provide rational design strategies for tailoring thermal properties in emerging materials.

Recommended Citation

Thakur, Sandip, "BEYOND THE PHONON GAS MODEL (PGM): UNRAVELING THE UNIQUE MECHANISTIC PROCESSES DICTATING THERMAL TRANSPORT IN HIGHLY ANHARMONIC AND DISORDERED MATERIALS" (2025). Open Access Dissertations. Paper 4486.
https://digitalcommons.uri.edu/oa_diss/4486