Michael P. Walsh

Wavefront velocity oscillations of carbon-nanotube-guided thermopower waves: nanoscale alternating current sources.

The nonlinear coupling between exothermic chemical reactions and a nanowire or nanotube with large axial heat conduction results in a self-propagating thermal wave guided along the nanoconduit. The resulting reaction wave induces a concomitant thermopower wave of high power density (>7 kW/kg), resulting in an electrical current along the same direction. We develop the theory of such waves and analyze them experimentally, showing that for certain values of the chemical reaction kinetics and thermal parameters, oscillating wavefront velocities are possible. We demonstrate such oscillations experimentally using a cyclotrimethylene-trinitramine/multiwalled carbon nanotube system, which produces frequencies in the range of 400 to 5000 Hz. The propagation velocity oscillations and the frequency dispersion are well-described by Fouriers law with an Arrhenius source term accounting for reaction and a linear heat exchange with the nanotube scaffold. The frequencies are in agreement with oscillations in the voltage generated by the reaction. These thermopower oscillations may enable new types of nanoscale power and signal processing sources.

High electrical power density from PbTe-based quantum-dot superlattice unicouple thermoelectric devices

We describe the first demonstration of PbTe-based, cross-plane quantum-dot superlattice (QDSL) unicouple thermoelectric generator devices fabricated from nanostructured, thick-film materials (∼100μm). Both n- and p-type QDSL materials were investigated. With ∼220K temperature difference across small thermoelements (∼95μm length, 4mm2 cross-sectional area), electrical power outputs up to 89mW and power densities up to 2.2W∕cm2 have been demonstrated for both n- and p-type materials. The devices consist of a substrate-free, bulklike slab of molecular beam epitaxy grown PbSeTe∕PbTe QDSL material as the n- or p-type leg and a copper wire as the other p-type leg.

PbTe-based quantum-dot thermoelectric materials with high ZT

Following the experimentally observed Seebeck coefficient enhancement in PbTe quantum wells in Pb/sub 1-x/Eu/sub x/Te/PbTe multiple-quantum-well structures which indicated the potential usefulness of low dimensionality, we have investigated the thermoelectric properties of PbSe/sub x/Te/sub 1-x//PbTe quantum-dot superlattices for possible improved thermoelectric materials. We have again found enhancements in Seebeck coefficient and thermoelectric figure of merit (ZT) relative to bulk values, which occur through the various physics and materials science phenomena associated with the quantum-dot structures. To date, we have obtained ZT values approximately double the best bulk PbTe values, with ZT as high as about 0.9 at 300 K and conservatively estimated values as high as 2.0 at higher temperatures.