Modeling energy transport in nanostructures
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Heat transfer in nanostructures differ significantly from that in the bulk materials since the characteristic length scales associated with heat carriers, i.e., the mean free path and the wavelength, are comparable to the characteristic length of the nanostructures. Nanostructure materials hold the promise of novel phenomena, properties, and functions in the areas of thermal management and energy conversion. Example of thermal management in micro/nano electronic devices is the use of efficient nanostructured materials to alleviate 'hot spots' in integrated circuits. Examples in the manipulation of heat flow and energy conversion include nanostructures for thermoelectric energy conversion, thermophotovoltaic power generation, and data storage. One of the major challenges in Metal-Oxide Field Effect Transistor (MOSFET) devices is to study the 'hot spot' generation by accurately modeling the carrier-optical phonon-acoustic phonon interactions. Prediction of hotspot temperature and position in MOSFET devices is necessary for improving thermal design and reliability of micro/nano electronic devices. Thermoelectric properties are among the properties that may drastically change at nanoscale. The efficiency of thermoelectric energy conversion in a material is measured by a non-dimensional figure of merit (ZT) defined as, ZT = σS 2 T/k where σ is the electrical conductivity, S is the Seebeck coefficient, T is the temperature, and k is the thermal conductivity. During the last decade, advances have been made in increasing ZT using nanostructures. Three important topics are studied with respect to energy transport in nanostructure materials for micro/nano electronic and thermoelectric applications; (1) the role of nanocomposites in improving the thermal efficiency of thermoelectric devices, (2) the interfacial thermal resistance for the semiconductor/metal contacts in thermoelectric devices and for metallic interconnects in micro/nano electronic devices, (3) the interaction between the energy carriers namely electrons/carriers with phonons which lead to a significant non-equilibrium at the semiconductor-metal contacts. This dissertation aims to focus on these three important topics. The first topic is addressed by modeling the thermal transport in 2-D and 3-D nanocomposites. The Boltzmann transport equation (BTE) for the phonon intensity is solved in conjunction with suitable boundary and interface treatment. Unlike in bulk composites, the results show a strong dependence of thermal conductivity, temperature, and heat flux on the wire size, wire atomic ratio, and interface specularity parameter. The second topic is addressed through a computational study for modeling the interfacial thermal resistance in carbon nanotube (CNT) contacts. A detailed parametric study is conducted by varying the dimensions of the CNT. The results of this study are compared with experimental data and the theory developed for nanoscale contacts. The third topic is addressed by modeling the non-equilibrium between energy carriers in metals and semiconductors. The Boltzmann transport model (BTM) has been introduced to study the electron-phonon non-equilibrium due to short pulsed laser interaction with thin gold and silicon films. A three stage Runge Kutta (RK) time stepping and a higher order Discontinuous Galerkin (DG) scheme using two Legendre basis functions are implemented for temporal and spatial discretization of the BTM. A parametric study is conducted by varying the laser parameters and studying their effect on electron/carrier and phonon thermal characteristics.