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NanoSEC Seminar

Strain Effect Analysis on the Thermoelectric Figure of Merit in Si/Ge Nanocomposites  
Guest Speaker
Professor Gang Li  
Guest Affiliation
Department of Mechanical Engineering, Clemson University  
Friday, October 21, 2011 7:00 pm - 8:00 pm  
Riverbend Research South Auditorium  

Thermoelectric (TE) energy conversion is a technology that converts thermal energy to electrical power and vice versa. Thermoelectric technology has significant advantages over other energy conversion technologies due to its compactness, high reliability and zero emissions of noise and pollutants. However, the energy conversion efficiency in existing thermoelectric devices is typically low. Since early 1990s, many studies have shown that higher energy conversion efficiency is achievable by reducing the phonon
thermal conductivity of TE materials using nanostructured thermoelectric materials. While the future of the technology is promising, the performance of state-of-the-art nanostructured materials is still much less than that of the conventional energy conversion techniques. In this work, we suggest that, by utilizing the different responses of electron and phonon transport to mechanical strains, the efficiency of
nanocomposite TE materials can be further improved through mechanical tuning. We perform computational analysis to investigate strain effect on the thermoelectric figure of merit in n-type Ge nanowire-Si host nanocomposite materials. The Seebeck coefficient and electrical conductivity of the Si/Ge nanocomposites are calculated by an analytical model derived from the Boltzmann transport equation (BTE) under the relaxation-time approximation. The effect of strain is incorporated into the BTE through strain induced energy shift and effective mass variation calculated from the deformation potential theory and a degenerate kp method. Strain effect on phonon thermal conductivity in the nanocomposites is computed through a model combining the strain dependent lattice dynamics and the ballistic phonon BTE. Electronic thermal conductivity is computed from electrical conductivity by using the Wiedemann-
Franz law. Normal and shear strains are applied in the transverse plane of the Si/Ge nanocomposites. Thermoelectric properties including electrical conductivity, thermal conductivity, Seebeck coefficient and dimensionless figure of merit are computed for Si/Ge nanocomposites under these strain conditions.