Mixed Topics in Computational Thermal Transport

Doctor of Philosophy, Graduate Program in Mechanical Engineering
University of California, Riverside, April 2017
Dr. Alex P. Greaney, Chairperson

Heat transfer is ubiquitous in both naturally occurring and engineered materials. As technology progresses, the length-and time-scales of thermal transport decreases, becoming comparable with the mean free paths and relaxation times of the vibrations that drive it. Increasingly, an atomistic-level understanding of thermal transport is pivotal in predicting and controlling heat transport in materials and devices. Modeling approaches  that permit an atomistic understanding of heat transport, and the implementation of complex approximations of the phonon Boltzmann transport formalism include classical molecular dynamics and density  functional  theory  (DFT).  Results  are  presented  for  equilibrium  molecular  dynamics (EMD) simulations  of the thermal conductivity of a series  of clustering and non-clustering point defects in graphite using the Green–Kubo method, aimed to advance our knowledge of the evolution of the microstructure of graphite while in service in a graphite-moderated nuclear reactor. The Green-Kubo method—commonly used for predicting transport properties by scientists and engineers across fields—relates the property of interest to the lifetime of fluctuations in its thermodynamic driving potential. The integral of the autocorrelation fluctuations requires a long averaging time to reduce remnant noise and is a principal source of error. A new approach is proposed to quantify ¬—on-the-fly—the uncertainty on transport properties computed using the Green-Kubo formulation, based on recognizing that the integrated noise is a random walk. EMD is also used to explore thermal transport in breathing
metal¬–organic  frameworks  (MOFs),  coveted  for  numerous  applications  due  to  their  large surface area and modularity. A simple geometric model of thermal conductivity is proposed as  a heuristic for the quick evaluation  of transport in  flexible MOFs,  and a quantum-based approach  is  undertaken  to  explore  deviations  from  the  heuristic,  such  as  rattler  modes  and phonon-focusing.  Phonon  properties  calculated  with  DFT  for  (1)  uranium  dioxide  and  (2) silicon, for fuel and spintronics applications respectively, are also briefly discussed.