Molecular Dynamics Study of Heat Transfer between Dissimilar Materials.

Abstract

In this thesis, molecular dynamics (MD) simulations are used to understand heat transfer process between dissimilar materials. New MD force field parameters for copper phthalocyanine (CuPc) have been developed by fitting the expression to ab initio calculation results. The resulting force field successfully predicts molecular structure, crystal structure, and vibrational density of states (VDOS) of the CuPc molecule in isolation and in condensed phases. Thermal conductivities calculated using the Green-Kubo formalism show reasonable agreement with experiments for both crystalline and amorphous CuPc. The low conductivity in nano-crystalline CuPc sample can be attributed to phonon scattering at amorphous domain walls, which discrupts over half of the conduction pathways. Heat transfer across CuPc/metal interfaces is studied using MD non-equilibrium MD simulations (NEMD) based on the Müller-Plathe method. Systematic parametric studies of CuPc/metal junctions and the analysis of a structurally congruent interface system, varying density and modulus of adjacent materials, show that the traditional acoustic mismatch model (AMM) does not accurately describe heat transfer across weakly bonded interfaces. Phonon spectral analysis reveals that the majority of heat transfer between CuPc and Au is accomplished via anharmonic coupling, which appears to be facilitated by strongly adhesive interfacial bonding. MD simulations divulge a strong relationship between between thermal boundary conductance and interfacial bonding strength, allowing one to determine the work of adhesion of experimental CuPc/metal systems in close agreement with peel-off test results. MD simulations reveal that the thermal transport properties of PVDF thin films can be controlled by the magnitude and direction of externally applied electric fields. The thermal conductivity of PVDF increases with the field strength. Our simulations predict a 33% conductivity boost at 80% of the breakdown field strength. A poled PVDF film possesses a residual conductivity enhancement that can be removed by an opposing electric field. Finally, the applied electric field raises the adhesive force to the substrate and thereby increases the interfacial thermal boundary conductance by a factor of up to 6. VDOS analysis shows that the electric fields cause stiffening in the bonding structure, thus enhancing the phonon contributions to the thermal conductivity.