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Thermal Energy Transport and Conversion in Disordered Materials


Thermal properties (thermal conductivity and specific heat) of the disordered materials, such as amorphous silicon (a-Si), polymer, and nano-crystalline semiconductors, are of significant interests for fundamental understanding of thermal transport process and for technical applications in thermal energy management and conversion. Due to the random distribution of atoms or molecules in disordered materials, the study of thermal transport is more challenging than that in crystalline materials. Understanding of the heat carrier transport behavior can be utilized to engineer the thermal properties in disordered materials, which can be applied for better devices thermal design and improving thermal energy conversion efficiency.

We have studied the size dependent thermal conductivity of a-Si thin films and nanotubes, and observed unusually high and anisotropic thermal conductivity in the isotropic a-Si nanostructure. This manifests surprisingly broad mean free path distribution of the propagating modes (propagons), which is found to range from 10 nm to 10 μm, in the disordered and isotropic structure. Constraining the long MFP propagons by boundary scattering in thin film and nanotubes explains the appreciable size effect in a-Si. Additionally, we developed a novel platform to measure the specific heat of low-dimensional disordered materials. By measuring the frequency dependent temperature rise data along the Nylon nanofibers (NFs), we are able to extract the specific heat and thermal conductivity simultaneously. While the thermal conductivity is increased by 50% over the bulk value in the 600 nm NFs, the specific heat exhibits bulk-like behavior. Finally we engineered the thermal conductivity in nano-crystalline bismuth-antimony-telluride (BST) by embedding SiO2 or diamond nanoparticles (NPs) at temperature below 300K, which has important application in thermoelectric cooling. We have shown that the embedded NPs work as additional scattering centers for lattice vibration (or called phonons), and can efficiently scatter the long MFP phonons in BST. We have observed 23% reduction of thermal conductivity, and 15% improvement of thermoelectric figure of merit (ZT) in the 0.5 vol. % Diamond NPs mixing sample, compared to the non-NPs nano-crystalline BST.

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