Joel T. Abrahamson

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.

Carbon nanotube-guided thermopower waves

Thermopower waves are a new concept for the direct conversion of chemical to electrical energy. A nanowire with large axial thermal diffusivity can accelerate a self-propagating reaction wave using a fuel coated along its length. The reaction wave drives electrical carriers in a thermopower wave, creating a high-power pulse of as much as 7 kW/kg in experiments using carbon nanotubes. We review nanomaterials designed to overcome limitations of thermoelectricity and explore the emerging scientific and practical outlook for devices using thermopower waves.

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.

Superadiabaticity in reaction waves as a mechanism for energy concentration

Spatially propagating reaction waves are central to a variety of energy applications, such as high temperature solid phase or combustion synthesis, and thermopower waves. In this paper, we identify and study a previously unreported property of such waves, specifically that they can generate temperatures far in excess of the adiabatic limit. We show that this superadiabaticity occurs when a reaction wave in either one dimension (1D) or two dimensions (2D) impinges upon an adiabatic boundary under specific reaction and heat transfer conditions. This property is studied analytically and computationally for a series of 1D and 2D example systems, producing an estimate of the upper bound for excess temperature rise as high as 1.8 times the adiabatic limit, translating to temperatures approaching 2000 K for some practical materials. We show that superadiabaticity may enable several new types of energy conversion mechanisms, including thermophotovoltaic wave harvesting, which we analyze for efficiency and power density.

Challenges in deposition of wide band gap copper indium aluminum gallium selenide (CIAGS) thin films for tandem solar cells

Copper indium gallium diselenide (CIGS) based solar cells have shown efficiencies > 20% on the lab scale and are already in commercial production. Even though the optimal band gap of 1.6eV to 1.7eV can be achieved by increasing the Ga content, these solar cells show a maximum efficiency at ~1.3eV and any further increase in the Ga concentration and band gap results in lower efficiencies due to bulk and interfacial traps. This also prevents the use of wide band gap CIGS layer as a top cell for harvesting the solar cell spectrum in a tandem cell configuration. This paper reports the manufacturing challenges on the production of wide band gap aluminum doped CIGS layers (CIAGS) and devices fabricated using this material. We have fabricated 11.3% efficient solar cells using the CIAGS absorber layers.

Efficient continuous-flow chemical bath deposition of CdS films as buffer layers for chalcogenide-based solar cells

The highest power conversion efficiencies in copper indium gallium diselenide (CIGS) solar cells are achieved when a cadmium sulfide (CdS) buffer layer is deposited on the CIGS absorber through chemical bath deposition (CBD). These CdS buffer layers are also used in emerging kesterite solar cells such as those based on copper zinc tin sulfide/selenide. Herein, we describe a novel continuous-flow chemical bath deposition (CF-CBD) system for efficient utilization of chemicals during CdS film deposition. CdS films are grown on 10-cm-diameter molybdenum-coated silicon wafers to demonstrate the potential of the method to achieve uniform thin CdS films.