Researchers at Sandia National Laboratories have stated that improved materials for nanowires, as well as better techniques to manufacture them, could lead to development of devices with better efficiency to cool computer chips and would also enable harvesting of heat of exhaust systems in automobiles.
This study was published by the researchers in Materials Research Society’s MRS Bulletin as a paper titled “Using Galvanostatic Electroforming of Bi1-xSbx Nanowires to Control Composition, Crystallinity and Orientation.” W. Graham Yelton, Douglas L. Medlin, Michelle Hekmaty, Kristopher Erickson, Steven J. Limmer, Michael P. Siegal, Jamin Pillars and Jessica L. Lensch-Falk, have authored this study.
This study has been the first time that a single process has been able to control crystal size, crystal orientation, and alloy uniformity. Yelton stated that these factors contribute to an enhanced thermoelectric performance.
"The three together mean a huge gain, and it’s hard to do. It’s turning the knobs of the process to get these things to behave," W. Graham Yelton.
Heat conductivity can be reduced and the thermoelectric figure of merit could be enhanced by using better nanowire geometries. A measure of the electrical and thermal conductivity of a material is known as the thermoelectric figure of merit. When the thermal conductivity is lower and the electrical conductivity is higher it would lead to a higher figure of merit and this would result in a material with better efficiency. However, previous thermoelectric nanowires did not possess adequate quality.
Though thermoelectric materials are inefficient, some of them are being currently used. This is comparable to the initial stages of solar photovoltaic cell development. Though the potential was observed they were utilized only when no other options were available.
If the efficiency in nanowires could be increased it would consequently increase adaptation of thermoelectric materials. Presently, they are used in certain sensors and vehicle manufacturers are hoping that they would be able to harvest heat wasted by exhaust systems and utilize them for powering vehicle sensor systems. If the power required for running the operating system of a vehicle could be decreased, it could help reduce the weight of the alternator and the battery, and it may also help avoid some equipment for generating power. This would in turn lead to reduction in weight and size of vehicles.
The researchers at Sandia created thermoelectric nanowire arrays having uniform composition across the spread of the nanowire array and along the nanowire’s length. The number of nanowires may possibly be hundreds of millions. The study paper describes the way these thermoelectric nanowire arrays were created. Further, the researchers created nanowire crystals that had uniform direction or size and orientation. When the composition is uniform, it enhances efficiency, and uniform orientation is vital so that electrons flow better.
Room-temperature electroforming is a commonly used process in commercial electroplating. It is a cost-effective method which was used by the research team. This process enables deposition of the material at a constant rate, which makes the nanowires to grow steadily. Wires of diameter 70-75nm and with length of many microns are produced.
Controlled current pulses were used by Yelton for depositing the thermoelectric material, which would help control the composition throughout the array and the wire. “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.
The technique developed by the researchers helped produce a crystalline wire structure that was slightly twisted and quite large. It had the orientation that was desired and it was nearly a single crystal. “Without that, you couldn’t get good efficiencies,” Yelton said.
The material’s chemistry is also quite important. In the crystalline quality and orientation antimony salts played an important role. Among many materials, bismuth-antimony (Bi-Sb) alloys possess high thermoelectric performance for applications at near-room temperature. High thermoelectric performance would mean acting as an insulator against heat and as a conductor of electricity. However, Bi-Sb materials that are currently being used do not produce solid-state cooling that is effective when constant power is delivered to a computer or any device that is being cooled.
Sandia researchers desired a compound that would not conduct heat but had the behavior of metals. Yelton stated that when antimony was alloyed with bismuth it produced the desired compound. When an antimony-iodide-based chemistry was used to electroform Bi-Sb nanowire arrays it did not possess the necessary qualities. However, arrays that were electroformed from an antimony-chloride-based chemistry delivered orientation and crystallography required for maximum thermoelectric performance.
The chemistry allowed us to go from poly nano-crystalline structure to near single crystals of 2-5 micrometers.
Thermoelectric materials readily form oxides or intermetallics, leading to poor contact connections or higher electrical contact resistance. That reduces the gains achieved in developing the materials.
W. Graham Yelton
Further progress would involve creating an electrical contact and then analyzing the thermoelectric behavior.
He added that they were able to get effective contact at the array’s bottom, but found it difficult to make a connection at the top.
“To make a contact and measure array performance is not trivial,” Yelton said.
The researchers are seeking further funding to address the problem and make successful contacts, and then characterize the arrays’ thermal electric properties. “If successful at the labs, we would try to find an industry collaborator to mature the idea,” he said.