Thermoelectric modules based on silicon nanowires (Si-NWs) have recently attracted significant attention as they show an improved thermoelectric efficiency due to a decrease in thermal conductivity. Here, we adopt a top-down fabrication method to dramatically reduce the thermal conductivity of vertical Si-NWs. The thermal conductivity of a vertical Si-NW is significantly suppressed with an increasing surface roughness, decreasing diameter, and increasing doping concentration. This large suppression is caused by enhanced phonon scattering, which depends on the phonon wavelength. The boron- and phosphorus-doped rough Si-NWs with a diameter of 200 nm and surface roughness of 6.88 nm show the lowest thermal conductivity of 10.1 and 14.8 W·m–1·K–1, respectively, which are 5.1- and 3.6-fold lower than that of a smooth intrinsic nanowire and 14.8- and 10.1-fold lower than that of bulk silicon. A thermoelectric module was fabricated using this doped rough Si-NW array, and its thermoelectric performance is compared with previously reported Si-NW modules. The fabricated module exhibits an excellent performance with an open circuit voltage of 216.8 mV·cm–2 and a maximum power of 3.74 μW·cm–2 under a temperature difference of 180 K, the highest reported for Si-NW thermoelectric modules.

No abstract

Here we show that the configuration of a slender enclosure can be optimized such that the radiation heating of a stream of solid is performed with minimal fuel consumption at the global level. The solid moves longitudinally at constant rate through the enclosure. The enclosure is heated by gas burners distributed arbitrarily, in a manner that is to be determined. The total contact area for heat transfer between the hot enclosure and the cold solid is fixed. We find that minimal global fuel consumption is achieved when the longitudinal distribution of heaters is nonuniform, with more heaters near the exit than the entrance. The reduction in fuel consumption relative to when the heaters are distributed uniformly is of order 10%. Tapering the plan view ͑the floor͒ of the heating area yields an additional reduction in overall fuel consumption. The best shape is when the floor area is a slender triangle on which the cold solid enters by crossing the base. These architectural features recommend the proposal to organize the flow of the solid as a dendritic design, which enters as several branches, and exits as a single hot stream of prescribed temperature. The thermodynamics of heating is presented in modern terms in the Sec. VII ͑exergy destruction, entropy generation͒. The contribution is that to optimize "thermodynamically" is the same as reducing the consumption of fuel.

SUMMARY This paper documents the fundamental problem of designing a porous flow architecture that meets the requirements of facilitating flow access while storing and releasing heat to a flowing fluid. Examples of such designs are regenerators that operate cyclically in various types of heating or reheating furnaces. The main geometrical scales are determined for parallel flow channels in a fixed regenerator volume with a fixed porosity, by matching the time scales of convection along the channels and thermal diffusion. In accord with the constructal law, the route to better architectures for maximum heat transfer and minimum pressure losses is the morphing of the regenerator architecture from parallel channels to dendritic channels. Copyright © 2012 John Wiley & Sons, Ltd.