Microfabrication and Testing of a Thermoelectric Device for Generating Mobile Electrical Power

Army Research Laboratory

Saturday, August 01 2009

This technology can be used to power robotics, enable portable/wearable power, and provide power from vehicle waste heat energy.

Several attractive features of thermo-electric (TE) technology include no moving parts, light weight, modularity, covertness, silence, high power density, low amortized cost, and long service life with no required maintenance. Many of the potential uses for mounted/ dismounted power, such as recharging batteries, are therefore ideal for TE technologies. However, these applications will require more interconnected, smaller-scale modular devices than are currently available. Most commercial off-the-shelf (COTS) TE devices are optimized for cooling, not for generating power, so new device structures with materials and geometries better optimized for power generation are needed for broader use of TE technologies.

New miniaturization and fabrication techniques exploit recent developments in materials with improved ZT, or index of power conversion efficiency. Well-known materials with good efficiency in TE power generators that can be miniaturized were chosen. N-type lead telluride (PbTe) and p-type antimony telluride (Sb₂Te₃) were identified for the initial device, with longer-term interest in alternate materials whose thermal conductivity may be reduced by incorporating nanostructures, or whose electrical properties could be improved with quantum-confinement heterostructures. Although PbTe and Sb₂Te₃ are generally much more efficient at higher temperatures, they are suitable for showing that in this quite moderate temperature range, the new miniaturization concepts are highly effective for the applications of interest.

The PbTe and Sb₂Te₃ TE legs were polished to a specular finish to reduce uncertainty in geometry and thus, reduce measurement error. Each sample had similar geometries: 0.40 cm in length, 0.25 cm in width, and 0.10 cm in depth. All geometrical dimensions were measured using a micrometer thickness gauge, which yields measurements that are accurate to about 2%.

The metal junctions of the device were fabricated from oxygen-free copper stock so that the contribution to the total electrical resistivity from the hot and cold junctions is negligible. Further, the thermal conductivity is significantly better in copper than either of the semiconductors, so the junctions remain isothermal. To maintain permanent electrical isolation between the different sections of the TE module, the metal junctions were mounted on a thin square aluminum nitride (AlN) platen having an area of 6.25 cm².

The components are shown in the figure at the various stages of assembly. In (A), the large copper heat sink, the AlN top isolation with serpentine trace, one of the metal junctions (positioned next to the copper heat-sink), and one of the Sb₂Te₃ components (positioned next to the length scale) are shown. In (B), the PbTe and Sb₂Te₃ components are shown fully integrated with copper hot-junctions permanently mounted to the top AlN isolator. Between that and the length scale are the three copper cold-junctions permanently mounted onto the bottom AlN isolator.