Thermal Interface Physics
One of the unsolved problems in physics is understanding how heat flows across an interface between dissimilar materials. In general heat flows through solids via the atomic motions, or is carried by electrons. The electron component is most significant for electrically conductive materials, but the component associated with the motions of atoms is present in all materials. For solids and rigid molecules, the atomic motions correspond to vibrations around equilibrium locations and are essentially the same as sound waves. The overwhelming majority of these modes of vibrations, however, exist at frequencies that are too high for the human ear to detect (e.g., terahertz). Nonetheless, these high frequency vibrations are largely responsible for heat transfer and they can be studied with molecular dynamics (MD) simulations. MD simulations involve direct simulations of the motions of atoms using a model for the interactions between atoms, which allow one to calculate the forces atoms exert on each other. The models for atomic interactions are generally complex and therefore lead to complex atomic vibrations that replicate what happens in real materials. By studying these vibrations, we can determine how well a material can conduct heat and we can improve our understanding of how heat flows across an interface between dissimilar materials.
Recently, the ASE group has developed two new methods for studying phonon transport with molecular dynamics. These recent studies have found that when an interface is formed between dissimilar materials, the modes responsible for transferring the heat across the interface are different from the modes that exist in the native materials. Amongst the various types of new modes, four main classifications have emerged as shown. The first types of modes are termed extended modes, because the vibrations extend from one material through the interface into the second material. The second group of modes are called partially extended modes, because they extend through one material, but partially extend and decay into the second material. The third group of modes are termed isolated modes, because they stay isolated on one side of the interface and do not penetrate into the other material at all. The last group of modes are termed interfacial modes, because they are localized and exhibit most of their vibration at the interface. The discovery of these new modes has far reaching implications for our understanding of the physics of thermal transport at interfaces and research aimed at further improvement of our descriptions of phonon transport are ongoing.