Improved efficiency in nanowires would increase the use of thermoelectric materials. They’re already used in some sensors, and vehicle manufacturers hope they can harvest heat from exhaust systems to power vehicle sensor systems, Yelton said. Decreasing the power needed to run a vehicle’s operating system could reduce battery and alternator weight and perhaps eliminate some power-generating equipment, trimming vehicle size and weight.
Sandia’s paper describes how the team created thermoelectric nanowire arrays with uniform composition along the length of the nanowire and across the spread of the nanowire array, which potentially can include hundreds of millions of nanowires. In addition, they created nanowire crystals of uniform size and orientation, or direction. Uniform composition improves efficiency, while orientation is important so electrons, the carriers of energy, flow better.
The team used a cost-effective method called room-temperature electroforming, which is widespread in commercial electroplating. Electroforming deposits the material at a constant rate, which in turn allows nanowires to grow at a steady rate. The method produced wires 70-75 nanometers in diameter and many microns long.
Graham Yelton and Sandia National Laboratories colleagues have developed a single electroforming technique that tailored key factors to better thermoelectric performance: crystal orientation, crystal size and alloy uniformity. Yelton is among Sandia’s researchers who published a paper, “Using Galvanostatic Electroforming of Bi1-xSbx Nanowires to Control Composition, Crystallinity and Orientation,” in the Jan. 28 edition of the Materials Research Society’s MRS Bulletin. (Photo by Randy Montoya)
Yelton used pulses of controlled current to deposit the thermoelectric material, thereby controlling composition throughout the wire and the array. “There are little nuances in the technique that I do to allow the orientation, the crystal growth and the composition to be maintained within a fairly tight range,” he said.
Technique allowed control over important facets of nanowire formation
The method produced a fairly large, slightly twisted crystalline wire structure that was almost a single crystal and had the desired orientation. “Without that, you couldn’t get good efficiencies,” Yelton said.
The chemistry of the material also is important. For the Sandia team, antimony salts play a major role in crystalline quality and orientation. Bismuth-antimony (Bi-Sb) alloys have some of the highest thermoelectric performance — acting both as a conductor of electricity and an insulator against heat — among many materials for near-room temperature applications. But existing Bi-Sb materials don’t produce effective solid-state cooling when power is constantly delivered to the device being cooled, such as a computer.