Shockwave Fabrication of High Performance Thermoelectrics

Thermoelectric (TE) generators have the advantages of no moving parts and flexibility in deployment but suffer from low heat to electricity conversion efficiencies, with a major loss component being conductive (phonon) heat transfer through the TE lattice. By using a high pressure shockwave consolidation, nanopowders can be fused into a solid bulk TE material while preserving the nanostructure. The high density of grain boundaries and lattice defects impedes phonon transport while allowing electron flow. Specific Phase 2 research thrusts will be directed at transitioning laboratory fabrication into volume manufacturing, at producing a graded thermoelectric that is optimized for different temperature ranges over the length of the element, and at preparing bulk thermoelectric material from transition metal trichalcogenides that are not appropriate for melt or powder sintered fabrication. The overall conversion efficiency of a TE device will always be limited by the Carnot ratio of (Th-Tc)/Th, where Th and Tc are the temperatures of the hot and cold junctions. With the restrictions on phonon transport accruing from nanopowder consolidation, conversion efficiencies in excess of 30% of the Carnot limit are reasonable.

High efficiency thermoelectric generation allows significant energy capture from waste heat streams in industrial and power plants. Combined with escalating electricity prices, the payback times are reduced and this allows both scale and scope economies. Even relatively low thermal gradients become candidates for energy capture including electric generation from temperature differences across interfaces, for example, earth to air for powering roadway lighting and water to air for powering sensors in floating buoys. In the highway of the future, thermoelectric sensor pods might be installed directly into the pavement for sensing traffic volume, vehicle weight, vehicle speed, light levels, icing and general road conditions. By networking the devices via radio frequency transceivers into a highly interconnected mesh, their information could be processed in real time to forecast driving conditions, manage traffic flow through variable speed limits and roadside warning systems, and communicate advisories to drivers, police or road maintenance crews.

Nanostructured bulk thermoelectric material can be diced and pelletized for incorporation into any application previously served by conventional crystal grown or powder sintered material. Thermoelectric efficiency enhancement will allow the fabrication of smaller, lighter TE devices such as radioisotope generators for satellites and deep space probes. Efficiency enhancements will also enable applications that have not previously been practical such as harvesting sensor energy from astronaut body heat. Improvements in thermoelectric efficiency benefit thermoelectric (Peltier) cooling of astronauts, CCD cells and electronics since less energy will be required for these heat pumping applications.

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