Silicon-based thermoelectric nanosystems

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Thermoelectric (TE) generators directly mounted into the exhaust gas return (EGR) cooler can help to harvest a portion of the huge amount of day-to-day wasted heat. They have to endure constant and cycling temperature loading up to around 500°C, which cannot be fulfilled by Bi2Te3 -based TE material but by silicon. Bulk silicon, however, has a low figure of merit (ZT ~ 0.01) owing to high thermal conductivity. In the first funding period we found that by patterning silicon into nanowires of ~ 200 nm in diameter in combination with a rough nanorod surface thermal conductivity was strongly reduced corresponding to ZT ~ 0.14 at conservative assumptions for the Seebeck coefficient and the resistivity. Further reduction of thermal conductivity and thus improvement towards ZT > 1 resulting in efficiencies necessary for a practical harvesting device (10 to 20 %) is planned for the second funding period which shall be achieved by 1) enhancing the surface-to-volume ratio and thus phonon surface scattering via further reducing the nanowire diameter to < 100 nm or by adding an inner surface (Si nanotube), 2) engineering the nanowire surface for more efficient phonon roughness scattering, 3) integration of multilayer structures into silicon nanowires (Si/SiGe, Si/SiOx, 28Si/29Si) to exploit the effect of heterointerfaces, boundaries, alloys and isotopes on phonon scattering.