Atomistic and mesoscopic simulations of heat transfer across heterogeneous material interfaces
The study of heat transfer and the associated thermal interface resistance at heterogeneous material interfaces is over 70 years old since the first measurements of thermal interface resistance by Kapitza in 1941. However, recent developments in experimental metrology techniques that enable spectrally-resolved phonon transport measurements at the nanoscale along with the development of high-fidelity simulation methods have provided a renewed interest in the fundamental physics of heat transfer across interfaces. Miniaturized electronic devices and nanostructured materials for energy applications are among technologies that would benefit from a fundamental understanding of interfacial thermal transport. This dissertation focuses on the study of problems in interfacial heat transfer that span the atomistic and mesoscopic length scales and have broad applications in electronic thermal management.^ The first part of this dissertation develops a mesoscale simulation framework to predict the mechanical and thermal performance of carbon nanotube (CNT) thermal interface materials (TIMs). CNT arrays have been widely studied for use as TIMs due to the high thermal conductivity and mechanical compliance of CNTs. However, modeling of CNT TIMs has been largely limited to semi-empirical methods that lack detailed consideration of CNT array microstructure. We develop a physics-based, microstructure-sensitive, thermo-mechanical simulation framework that can be used in the design and optimization of CNT TIMs. Coarse-grain mechanics simulations are used to predict the CNT array microstructure and the finite volume method is used to solve the Fourier conduction equations for CNTs embedded in a filler matrix. The simulations provide insights on the sensitivity of thermal resistance of the CNT array to microscopic CNT-CNT and CNT-substrate contact resistances. Microstructural parameters that are not readily accessible in experiments such as the contact areas and the fraction of CNTs in contact with the opposing substrate are reported to demonstrate the usefulness of the simulation approach.^ The latter part of this dissertation deals with the development of a first-principles atomistic simulation framework to study heat transfer across metal-semiconductor heterojunctions which form an important class of interfaces used in electronic devices. The silicides of transition metals such as titanium and cobalt (TiSi2, CoSi2) are commonly used as metal contacts to silicon in transistors; hence, TiSi2-Si and CoSi 2-Si interfaces are chosen here as model metal-semiconductor junctions for studies of thermal transport. All the atomistic simulations reported in this work use the atomistic Green's function (AGF) method that is analogous to the non-equilibrium Green's function (NEGF) method used in quantum transport calculations of electrons. We propose the use of Büttiker probe scattering models to develop a phenomenological but computationally efficient description of phonon-phonon and electron-phonon scattering within the AGF framework. ^ First-principles calculations of electron-phonon coupling reveal that energy transfer between metal electrons and lattice vibrations in the semiconductor is mediated by interfacial phonon modes whose vibrational pattern is delocalized across the metal and semiconductor regions, and the coupling of metal electrons with phonon modes localized in the semiconductor is negligible. The transport simulations also help identify the contributions of various scattering mechanisms such as elastic interfacial scattering, inelastic phonon scattering, electron-phonon coupling within the metal, and direct electron-phonon coupling across the interface to the total thermal conductance of a CoSi 2-Si interface.^ The inclusion of the various transport processes in the simulation is found to be critical to obtain a good agreement with experimental data on thermal conductance of an epitaxial CoSi2-Si interface. The last part of this work develops an eigenspectrum formulation of the AGF method that enables the prediction of polarization- or branch-resolved contributions to the phonon transmission function and the thermal interface conductance. Unlike prior work in the literature, our approach makes a direct connection to the bulk phonon dispersion of materials forming an interface and is also computationally efficient. The essential idea behind the formulation is the use of bulk phonon eigenspectrum to obtain the surface Green's functions used in the AGF method instead of the more commonly used Sancho-Rubio or decimation technique. The new approach is applied to study phonon transport across a Si-Ge interface with atomic intermixing. The computation of polarization-resolved transmission functions, which are not accessible within the conventional AGF method that groups different phonon branches together, provides insights on the microscopic mechanisms responsible for the increase in phonon transmission due to interfacial disorder.^
Timothy S. Fisher, Purdue University.