Chandan Bera - Academia.edu (original) (raw)
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Graduate Center of the City University of New York
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Papers by Chandan Bera
Physical Review B, 2010
By introducing a serpentine structure in a straight silicon nanowire, we have experimentally achi... more By introducing a serpentine structure in a straight silicon nanowire, we have experimentally achieved a significant reduction in its phonon thermal conductance at low temperature (T<5K.) The magnitude of this effect is one order of magnitude larger than expected based on a thermal resistors picture. With a more refined theoretical model that considers the frequency dependence of phonon transport, we are able to quantitatively account for the experimental results of straight and serpentine nanowires in the whole temperature range. This experimental demonstration of a large, purely geometry induced effect on nanoscale thermal conductance contrasts strikingly with the negligible effects reported on a different nanoscale system in a previous publication.
Journal of Applied Physics, 2010
We theoretically investigate the thermoelectric properties of sintered SiGe alloys, compare them ... more We theoretically investigate the thermoelectric properties of sintered SiGe alloys, compare them with new and previous experimental measurements, and evaluate their potential for further improvement. The theoretical approach is validated by extensive comparison of predicted bulk mobility, thermopower, and thermal conductivity, for varying Ge and doping concentrations, in the 300-1000K temperature range. The effect of grain boundaries is then included for Si 0.8 Ge 0.2 sintered nanopowders and used to predict optimized values of the thermoelectric figure of merit at different grain sizes. Our calculations suggest that further optimization of current state of the art n-type ͑p-type͒ material would be feasible, possibly leading to ϳ5% ͑4%͒ ZT enhancement at 1000 K and 16% ͑6%͒ at room temperature. Even larger enhancements should be possible if the phonon scattering probability of the grain boundaries could be increased beyond its present value.
Physical Review Letters, 2010
We show that porous alloys can display thermal conductivity reductions at considerably larger por... more We show that porous alloys can display thermal conductivity reductions at considerably larger pore sizes than nonalloyed porous materials of the same nominal porosity. The thermal conductivity of Si 0:5 Ge 0:5 alloy with 0.1 porosity becomes half the nonporous value at 1000 nm pore sizes, whereas pores smaller than 100 nm are required to achieve the same relative reduction in pure Si or Ge. Using Monte Carlo simulations, we also show that previous models had overestimated the thermal conductivity in the small pore limit. Our results imply that nanoporous alloys should be advantageous with respect to nanoporous nonalloys, for applications requiring a low thermal conductivity, such as novel thermoelectrics.
Physical Review B, 2010
By introducing a serpentine structure in a straight silicon nanowire, we have experimentally achi... more By introducing a serpentine structure in a straight silicon nanowire, we have experimentally achieved a significant reduction in its phonon thermal conductance at low temperature (T<5K.) The magnitude of this effect is one order of magnitude larger than expected based on a thermal resistors picture. With a more refined theoretical model that considers the frequency dependence of phonon transport, we are able to quantitatively account for the experimental results of straight and serpentine nanowires in the whole temperature range. This experimental demonstration of a large, purely geometry induced effect on nanoscale thermal conductance contrasts strikingly with the negligible effects reported on a different nanoscale system in a previous publication.
Journal of Applied Physics, 2010
We theoretically investigate the thermoelectric properties of sintered SiGe alloys, compare them ... more We theoretically investigate the thermoelectric properties of sintered SiGe alloys, compare them with new and previous experimental measurements, and evaluate their potential for further improvement. The theoretical approach is validated by extensive comparison of predicted bulk mobility, thermopower, and thermal conductivity, for varying Ge and doping concentrations, in the 300-1000K temperature range. The effect of grain boundaries is then included for Si 0.8 Ge 0.2 sintered nanopowders and used to predict optimized values of the thermoelectric figure of merit at different grain sizes. Our calculations suggest that further optimization of current state of the art n-type ͑p-type͒ material would be feasible, possibly leading to ϳ5% ͑4%͒ ZT enhancement at 1000 K and 16% ͑6%͒ at room temperature. Even larger enhancements should be possible if the phonon scattering probability of the grain boundaries could be increased beyond its present value.
Physical Review Letters, 2010
We show that porous alloys can display thermal conductivity reductions at considerably larger por... more We show that porous alloys can display thermal conductivity reductions at considerably larger pore sizes than nonalloyed porous materials of the same nominal porosity. The thermal conductivity of Si 0:5 Ge 0:5 alloy with 0.1 porosity becomes half the nonporous value at 1000 nm pore sizes, whereas pores smaller than 100 nm are required to achieve the same relative reduction in pure Si or Ge. Using Monte Carlo simulations, we also show that previous models had overestimated the thermal conductivity in the small pore limit. Our results imply that nanoporous alloys should be advantageous with respect to nanoporous nonalloys, for applications requiring a low thermal conductivity, such as novel thermoelectrics.