ROLE OF INTERFACE THERMAL BOUNDARY RESISTANCE, STRAINING, AND MORPHOLOGY IN THERMAL CONDUCTIVITY OF A SET OF SI-GE SUPERLATTICES AND BIOMIMETIC SI-GE NANOCOMPOSITES

摘要

Nanoscale engineered materials with tailored thermal properties are desirable for applications such as highly efficient thermoelectric, microelectronic and optoelectronic devices. It has been shown earlier that by judiciously varying interface thermal boundary resistance (TBR) thermal conductivity in nanostructures could be controlled. Two types of nanostructures that have gained significant attention owing to the presence of TBR are superlattices and nanocomposites. A systematic comparison of thermal behavior of superlattices and nanocomposites considering their characteristic structural factors such as periodicity and period length for superlattices, and morphology for nanocomposites, under different extents of straining at a range of temperatures remains to be performed. In this presented work, such analyses are performed for a set of Si-Ge superlattices and Si-Ge biomimetic nanocomposites using non-equilibrium molecular dynamics (NEMD) simulations at three different temperatures (400 K, 600 K, and 800 K) and at strain levels varying between -10% and 10%. The analysis of interface TBR contradicts the usual notion that each interface contributes equally to the heat transfer resistance in a layered structure. The dependence of thermal conductivity of superlattice on the direction of heat flow gives it a characteristic somewhat similar to a thermal diode as found in this study. The comparison of thermal behavior of superlattices and nanocomposites indicate that the nanoscale morphology differences between the superlattices and the nanocomposites lead to a striking contrast in the phonon spectral density, interfacial thermal boundary resistance, and thermal conductivity. Both compressive and tensile strains are observed to be important factors in tailoring the thermal conductivity of the analyzed superlattices, whereas have very insignificant influence on the thermal conductivity of the analyzed nanocomposites.