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Towards Thermal Wavelength Scale Two- and Three-Dimensional Photonic Crystals

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Technical Report,01 Apr 2012,31 Dec 2015

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The team investigated the effect of patterning two-dimensional arrays of holes in thin-film silicon on its thermoelectric figure of merit, the anisotropic thermal conductivity of nanoscale graphite layers and the changes in the thermal conductivity of a prototypical perovskite oxide, SrTiO3, with off-stoichiometric compositions. In the Si work, we find the overall enhancement in ZT for holey Si to be modest compared to bulk silicon. Distinct from previous work, we measured all three properties electrical conductivity, thermal conductivity and the Seebeck coefficient on the same sample at different doping. We were able to successfully fabricate n-type samples with low electrical resistivity down to few m-cm. In holey silicon, we have measured the power factor in the vicinity of such doping to be approximately one half of that in the bulk, contrary to previous reports that there is no degradation in the power factor. In the carbon work, we studied the deposited by chemical vapor deposition on Ni substrates at relatively low temperatures between 825 and 900 C. The in-plane thermal conductivity varies with deposition temperature between 650 and 1000 Wm1K1, and is 30-50 of the in-plane thermal conductivity of HOPG. The reduced thermal conductivity in comparison to HOPG is attributed to a combination of grain boundaries and structural disorder. We developed a figure-of-merit for a flexible heat-spreader and find that the figure-of-merit for CVD graphite is a factor of 2 larger than that for Au. Finally, in the perovskite work, we find the longitudinal thermal conductivity can be modified by as much as 80 from 11.5W m-1 K-1 for stoichiometric homoepitaxial SrTiO3 to 2W m-1 K-1 for strontium-rich filmsby incorporating SrO2 Ruddlesden-Popper planar defects.

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