Excess thermopower and the theory of thermopower waves.

Self-propagating exothermic chemical reactions can generate electrical pulses when guided along a conductive conduit such as a carbon nanotube. However, these thermopower waves are not described by an existing theory to explain the origin of power generation or why its magnitude exceeds the predictions of the Seebeck effect. In this work, we present a quantitative theory that describes the electrical dynamics of thermopower waves, showing that they produce an excess thermopower additive to the Seebeck prediction. Using synchronized, high-speed thermal, voltage, and wave velocity measurements, we link the additional power to the chemical potential gradient created by chemical reaction (up to 100 mV for picramide and sodium azide on carbon nanotubes). This theory accounts for the waves' unipolar voltage, their ability to propagate on good thermal conductors, and their high power, which is up to 120% larger than conventional thermopower from a fiber of all-semiconducting SWNTs. These results underscore the potential to exceed conventional figures of merit for thermoelectricity and allow us to bound the maximum power and efficiency attainable for such systems.

ZnO based thermopower wave sources.

Exothermic chemical reactions of nitrocellulose are coupled onto thermoelectric zinc oxide (ZnO) layers to generate self-propagating thermopower waves resulting in highly oscillatory voltage output of the order of 500 mV. The peak specific power obtained from ZnO based sources is approximately 0.5 kW kg(-1).

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 Fourier's 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.

Chemically driven carbon-nanotube-guided thermopower waves.

Theoretical calculations predict that by coupling an exothermic chemical reaction with a nanotube or nanowire possessing a high axial thermal conductivity, a self-propagating reactive wave can be driven along its length. Herein, such waves are realized using a 7-nm cyclotrimethylene trinitramine annular shell around a multiwalled carbon nanotube and are amplified by more than 10(4) times the bulk value, propagating faster than 2 m s(-1), with an effective thermal conductivity of 1.28+/-0.2 kW m(-1) K(-1) at 2,860 K. This wave produces a concomitant electrical pulse of disproportionately high specific power, as large as 7 kW kg(-1), which we identify as a thermopower wave. Thermally excited carriers flow in the direction of the propagating reaction with a specific power that scales inversely with system size. The reaction also evolves an anisotropic pressure wave of high total impulse per mass (300 N s kg(-1)). Such waves of high power density may find uses as unique energy sources.

Modelling the increase in anisotropic reaction rates in metal nanoparticle oxidation using carbon nanotubes as thermal conduits.

Nanostructured energetic materials are attracting attention for their faster reaction rates compared to materials with micron-scale particles. We numerically solve the coupled energy balances for a carbon nanotube with an annular coating of reactive metal, such that coupling to thermal transport in the nanotube accelerates reaction in the annulus. For the case of Zr metal, the nanotube increases the velocity of the reaction front in the direction of the nanotube length from 530 to 5100 mm s(-1). This offers a proof-of-concept for one-dimensional anisotropic energetic materials, which could find new applications in inorganic synthesis and novel propellants. Nanotube conductivity as well as the relative sizes of the Zr annulus and the nanotube limit enhancement of the reaction velocity to a maximum of a factor of ∼10. Interestingly, the interfacial heat conductance is not the most significant factor affecting the coupling, due to the large temperature differences (more than 1000 K) between the nanotube and the annulus at the reaction front and directional heat conduction in the nanotube. Although the enhancement is insufficient to change a Zr/nanotube composite from a deflagrating to a detonating material, using faster-reacting materials may enable nanotubes to effect this transition.