Date of Award
Doctor of Philosophy
The immense amount of research on graphene in the last decade has led to advancement in techniques to control the thermo-physical properties of graphene by making use of engineered graphene structures. Carefully designed defective graphene, van der Waals heterostructures, functionalized graphene and engineered graphene surfaces have potential applications in photoelectronics and thermoelectrics devices, as electrode materials, phase-change materials for thermal energy storage, sensors, coatings etc. In this study, we focus on the interfacial and thermal transport phenomena within these engineered graphene nano-structures.
The main objective of this dissertation is to understand the mechanism of heat transfer across the nano-structures and analyze the molecular interactions at the interface which governs the surface properties like interfacial thermal resistance, viscosity and wettability. We use atomistic modelling to evaluate these materials and compare the results with experiments. The degradation of thermal conductivity with concentration of isotope and vacancy defects in graphene was analyzed using molecular dynamics (MD). The results qualitatively match with collaborative experimental and theoretical efforts based on Boltzmann transport equation (BTE). We next investigate contrasting behavior of thermal conductivity and sheet conductance in graphene/MoS2 van der Waals heterostructure. The phonon relaxation times and thermal conductivity in graphene are suppressed due to the weak van der Waals interaction with the adjacent layers. We also experimentally and computationally characterize thermo-physical properties of a mixture of n-eicosane phase change material and graphite particles (GP). Our results show a large enhancement in k (450%) and µ (1200%) for a 3.5% vol. concentration of GP fillers. The surprising reduction in interfacial thermal resistance (Reff) with increasing filler concentration, together with the high thermal conductivity of GP contributes to the large enhancement in k. While the viscosity of the n-eicosane around GP increases, we explore graphene’s potential as a solid lubricant when used with nanodiamond. The frictional force at the atomic scale in nanodiamond wrapped with graphene and graphitized nanodiamond is studied using molecular dynamics. Our results show that the rotational degree of freedom of nanodiamond leads to a lower friction in graphene wrapped nanodiamond. Finally, we show that hydrophobicity of graphene surfaces can be tuned by changing the orientation of graphene flakes. Our results show that the hydrophobicity due to the graphene flake orientation determines the Kapitza resistance and evaporation rates of water over the surface. The faster heated evaporation over hydrophilic surfaces is attributed to the efficient heat transfer from the substrate to water. Our results matches with collaborative experimental observations.
Srinivasan, Srilok, "Atomistic modeling of interfacial and thermal transport properties of engineered graphene structures" (2018). Graduate Theses and Dissertations. 17327.