Energy storage and generation from thermopower waves
2012
The nonlinear coupling between an exothermic
chemical reaction and a nanowire or nanotube with large axial heat
conduction guides a self-propagating thermal wave along the
nano-conduit. The thermal conduit accelerates the wave by rapidly
transporting energy to un-reacted fuel. The reaction wave induces
what we term a thermopower wave, resulting in an electrical current
in the same direction. At up to 7 W/g, peak power density is larger
than that of many present micro-scale power sources (e.g. fuel
cells, batteries) and even about seven times greater than
commercial Li-ion batteries. Thermopower waves also tend to produce
unipolar voltage pulses, although conventional thermoelectric
theory predicts bipolar voltage. These waves also generate
thermopower in excess of previous measurements in carbon nanotubes
(CNTs) and therefore could increase figures of merit in a variety
of thermoelectric materials. In this thesis, I have developed the
theoretical framework to describe the thermal and chemical profiles
of propagating reaction waves, and their electrical properties. My
analysis yielded a new analytical solution for one-dimensional
reaction and thermal diffusion systems with nth order kinetics that
obviates many approximate or numerical approaches from the past 80
years. A generalized logistic. function describes the temperature
and concentration profiles within the solid fuel and provides a
solution for the wave velocity for a wide range of conditions. This
approach offers new insight into such problems spanning several
fields in science and engineering, including propulsion and
self-propagating high temperature synthesis (SHS) of materials, as
well as the dynamics of thermopower waves. Temperature and voltage
measurements of thermopower waves on CNTs show that they can
generate power as much as four times greater than predictions based
on reference measurements of the Seebeck coefficient for static
temperature gradients. We hypothesize that the excess thermopower
stems from a chemical potential gradient across the CNTs. The fuel
(e.g. picramide) adsorbs and dopes the CNTs ahead of the wave and
desorbs and reacts behind the wave front. Furthermore, the excess
thermopower depends on the mass of fuel added (relative to CNT
mass), and the chemical potential difference matches the magnitude
of the excess thermopower. Thus, a major conclusion of this thesis
is that coupling to a chemical reaction can boost the performance
of thermoelectric materials through differential doping.
Thermopower waves can have well defined velocity oscillations for
certain kinetic and thermal parameter values.
Cyclotrimethylene-trinitramine (fuel) on multiwalled CNTs (conduit)
system generates voltage oscillations of 400 to 5000 Hz. These
frequencies agree with velocity oscillations predicted by my
thermochemical model of the reaction wave, extended to include
thermal transport within the conduits. Thermopower waves could thus
find applications as new types of alternating current (AC)
batteries and self-powered signal generators, which could…
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