Degree Candidacy: Master of Science in Mechanical Engineering
Date: Tuesday, November 26, 2013
Time: 9:30 AM
Location: 215 EIB
Advisor: Dr. Gang Li
Committee Members: Dr. Mohammed Daqaq and Dr. Xiangchun Xuan
Title: Phonon Transport Analysis of Semiconductor Nanocomposites using Monte Carlo Simulations
Nanocomposites are composite materials which incorporate nanosized particles, platelets or fibers. The addition of nanosized phases into the bulk matrix can lead to significantly different material properties compared to their macrocomposite counterparts. For nanocompoties, thermal conductivity is one of the most important physical properties. Manipulation and control of thermal conductivity in nanocomposites have impacted a variety of applications. In particular, it has been shown that the phonon thermal conductivity can be reduced significantly in nanocomposites due to the increase in phonon interface scattering while the electrical conductivity can be maintained . This extraordinary property of nanocomposites has been used to enhance the energy conversion efficiency of the thermoelectric devices which is proportional to the ratio of electrical to thermal conductivity.
This thesis investigates phonon transport and thermal conductivity in Si/Ge semiconductor nanocomposites through numerical analysis. The Boltzmann transport equation (BTE) is adopted for description of phonon thermal transport in the nanocomposites. The BTE employs the particle-like nature of phonons to model heat transfer which accounts for both ballistic and diffusive transport phenomenon. Due to the implementation complexity and computational cost involved, the phonon BTE is difficult to solve in its most generic form. Gray media (frequency independent phonons) is often assumed in the numerical solution of BTE using conventional methods such as finite volume and discrete ordinates methods. This thesis solves the BTE using Monte Carlo (MC) simulation technique which is more convenient and efficient when non-gray media (frequency dependent phonons) is considered. In the MC simulation, phonons are displaced inside the computational domain under the various boundary conditions and scattering effects. In this work, under the relaxation time approximation, thermal transport in the nanocomposites are computed by using both gray media and non-gray media approaches. The non-gray media simulations take into consideration the dispersion and polarization effects of phonon transport. The effects of volume fraction, size, shape and distribution of the nanoparticle fillers on heat flow and hence thermal conductivity are studied. In addition, the computational performance of the gray and non-gray media approaches are compared.