One of primary applications is thermoelectric materials is conversion of energy in form of heat into electricity. In the current era of nanotechnology, there are unique opportunities to significantly increase energy conversion efficiencies in thermoelectric materials by careful manipulation of phonon behavior at the nanoscale. The figure of merit for thermoelectric energy conversion is proportional to the electrical conductivity and the square of the Seebeck coefficient, and is inversely proportional to the thermal conductivity. The thermal conductivity consists of two components, one defining the contribution from electrons and the other from phonons, also referred to as the lattice thermal conductivity. For certain class of materials like semiconductors, silicon in this work, phonons dominate the heat transport. Thus, a promising route to achieving exceedingly have values of the figure of merit is by reducing the lattice thermal conductivity.

 

Over the two decades, a variety of nanophononic systems have been proposed to reduce the lattice thermal conductivity of bulk or reduced-dimension semiconductors. The recent emergence of nanophononic metamaterials (NPMs) provide a novel approach to significantly reduce the thermal conductivity, and do so in manner that causes minimum changes to the electrical properties. One form of an NPM consists of silicon membranes with nanoscale silicon pillars standing on the surface; the nanopillars function as resonators. In such a system, the NPM effect stems from the resonances exhibited by the nanopillars which couple with the propagating phonons in the base membrane. In the phonon band structure, this coupling manifests as a reduction in the group velocities across the spectrum. This modification of the band structure, along with mode localizations and mild reductions in the lifetimes, contribute to the reduction of the overall in-plane thermal conductivity. 

 

In this study, we examine an NPM system with various levels of disorder introduced to the heights of the nanoresonators, in this case nanowalls, with the aim to explore the possibility of additional reduction in the thermal conductivity due to the disorder. The investigation is done using equilibrium molecular dynamics (EMD) simulations followed by using the Green-Kubo method to calculate the thermal conductivity. The heights of the nanowalls follow a uniform distribution and the level of disorder is controlled by an upper and lower height limit of the nanowalls. Our simulations show that the NPM effect results in an additional 75% reduction in the thermal conductivity in an NPM due to adding disorder. A disordered NPM thus serves as a good candidate for a high-performance thermoelectric device.


Thermal Conductivity Reduction in a Disordered Nanophononic Metamaterial

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Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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