Design Principles for Interface Thermal Conductance

The performance, reliability and lifetime for a system is often closely related to its temperature of operation. This temperature depends on the thermal energy generated within the system and on the thermal resistance, which determines how easy the energy flows to the external world. As the size of a system decreases to the nanoscale (on the order of tens of nanometers), its thermal resistance is dominated by interfaces. Thus, one way to engineer the operation temperature of a ``small" system is to tune the resistance at its interfaces. This tuning process can be done by changing interfacial properties known to affect the value of resistance such as interatomic mixing or roughness.

Inspired by concepts on electronic impedance matching and photonic antireflection coating, we study the fundamental principles and design rules that determine the interface thermal resistance. For our particular systems, heat is carried by a broadband spectrum of interacting phonons instead of a single frequency non interacting electronic or photonic wave. In this dissertation, we focus on interfaces between two crystalline solids joined by a variable thin intermediate layer. We explain how to maximize thermal conductance on 1D atomic chains and 3D crystalline solids, with intermediate layers varying from a single atom to graded junctions, in the coherent, incoherent elastic and incoherent inelastic regimes. We also explain the role of interatomic mixing and crystal structure on the interface conductance. Our theory is built from a Landauer description of conductance that highlights the interplay between the number of propagating channels available for conduction and the average transmission per channel. Rigorous simulations using non-equilibrium Green's function formalism (NEGF), coupled with known interatomic potentials or first principles parameters, support our results. We also compare our NEGF results with non-equilibrium molecular dynamics simulations.