Thermal conductivity reduction in core-shell nanowires
Nanostructuring of thermoelectric materials bears promise for manipulating physical parameters to improve the energy conversion efficiency of thermoelectrics. Using nonequilibrium molecular dynamics, we investigate how the thermal conductivity can be altered in core-shell nanocomposites of Si and Ge. By calculating the phonon vibrational density of states and performing normal mode analysis, we show that the thermal conductivity decreases when phonon-transport becomes diffusion-dominated and unveil a competition between modes from the various regions of the nanocomposite (core, interface, and shell). The effects of nanowire length, cross section, and temperature on thermal conductivity are explicitly considered. Surprisingly, the thermal conductivity variation with nanowire length is much weaker than in pure nanowires. Also, the thermal conductivity is almost independent of temperature in the wide region between 50 and 600 K, a direct result of confinement of the core by the shell. These results suggest that core-shell nanowires are promising structures for thermoelectrics.
Additional Information© 2011 American Physical Society. Received 27 April 2011; revised manuscript received 12 August 2011; published 29 August 2011. K.P.G. expresses his gratitude to ETH Zurich for a visiting professorship grant. X.Z. gratefully acknowledges the scholarship of the State Scholarship Fund of the China Scholarship Council that allowed him to be a visiting scientist at ETH Zurich and contribute to this work. Computational support from the Brutus Cluster at ETH Zurich is gratefully acknowledged. This work was supported by a grant from the Swiss National Supercomputing Centre-CSCS under project ID s243. M.H. would like to thank Javier V. Goicochea for fruitful discussions.
Published - Hu2011p15802Phys_Rev_B.pdf