Researchers at Sandia National Labs have developed a manufacturing process capable of controlling the crystal orientation, crystal size, and alloy uniformity of nanowires so that they could be used in a range of thermoelectric applications.
Because thermoelectric materials are capable of generating an electrical current as a result of a difference in temperature between one side of the material and the other, the Sandia team believes the new nanowires could make it possible for carmakers to harvest power from the heat wasted by exhaust systems or lead to more efficient devices for cooling computer chips.
Nanowires have been suggested for a range of applications, but in thermoelectric applications, the quality of the nanowires has heretofore been inadequate. The trick for any thermoelectric material is to combine high electrical conductivity and relatively low thermal conductivity—a property known as thermoelectric efficiency.
Researchers have been investigating a number of nanomaterials for thermoelectric applications; traditional materials possess a relatively poor thermoelectric conversion efficiency or they are prohibitively expensive for commercial uses.
The Sandia researchers turned to nanowires despite their previous poor performance, believing that if they could better control the manufacturing process, they could improve the nanowires’ quality enough to make them a useful thermoelectric material.
In research published in the Cambridge Journal of Materials Research, the Sandia team employed a method known as room-temperature electroforming, which is widely used in commercial electroplating. In electroforming, material is deposited at a constant rate, which results in the nanowires growing uniformly.
This uniformity of composition held for the entire length of each nanowire and even across an array of them. The crystals that made up the nanowires were all oriented in one direction, making it easier for electrons to travel along the conduits.
“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,” said Graham Yelton of Sandia in a press release. “It’s turning the knobs of the process to get these things to behave.”
The next step in the research will be to make an electrical contact with the nanowire-based material and to measure the resulting thermoelectric behavior.
One hurdle the team has to overcome: “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,” Yelton said.
So far the team has had some success in getting good contact at the bottom of an array, but making a connection at the top has proved difficult.
At the moment, the researchers are seeking additional funding to solve the problem of making contacts, and then they plan to characterize the thermal electric properties of the arrays.
Dexter Johnson is a contributing editor at IEEE Spectrum, with a focus on nanotechnology.