Nickel–gold alloys show promising thermoelectric effect

Thermoelectrics enable the conversion of heat into electrical energy — and vice versa. This makes them suitable for a range of technological applications. A team of researchers at TU Wien investigated various metallic alloys in the search for thermoelectric materials with the best properties. A mixture of nickel and gold proved particularly promising, with the research findings published in Science Advances.

Thermoelectrics have been used since the middle of the 20th century to generate electrical energy and for everyday applications like portable refrigerators. They could also be used in industrial environments to convert waste heat into green electricity.

The thermoelectric effect is based on the movement of charged particles that migrate from the hotter to the colder side of a material, resulting in an electrical voltage — the so-called thermoelectric voltage — that counteracts the thermally excited movement of the charge carriers. The ratio of the built-up thermoelectric voltage and the temperature difference defines the Seebeck coefficient, which is an important parameter for the thermoelectric performance of a material. The important requirement here is that there is an imbalance between positive and negative charges, as they compensate each other.

Fabian Garmroudi, first author of the study, said metals are hardly considered as thermoelectric materials nowadays, because they usually have a very low Seebeck coefficient. On the one hand, metals such as copper, silver or gold have high electrical conductivity; on the other hand, their Seebeck coefficient is small in most cases.

Physicists from the Institute of Solid State Physics (TU Wien) have found metallic alloys with high conductivity and an exceptionally large Seebeck coefficient. Mixing the magnetic metal nickel with the noble metal gold changes the electronic properties. As soon as the yellowish colour of gold disappears when about 10% nickel is added, the thermoelectric performance increases rapidly. The physical origin for the enhanced Seebeck effect is rooted in the energy-dependent scattering behaviour of the electrons — an effect fundamentally different from semiconducting thermoelectrics. Due to the electronic properties of the nickel atoms, positive charges are scattered more strongly than negative charges, resulting in the desired imbalance and a high thermoelectric voltage. In the alloys studied here, the positive charges are scattered by the nickel electrons, while the negative charges can move practically undisturbed.

The combination of high electrical conductivity and a high Seebeck coefficient leads to record thermoelectric power factor values in nickel–gold alloys, which exceed those of conventional semiconductors. “With the same geometry and fixed temperature gradient, many times more electrical power could be generated than in any other known material,” Garmroudi said.

The high power density may enable everyday applications in the large-scale sector in the future. Andrej Pustogow, senior author of the study, said that with the current performance, smartwatches could be charged autonomously using the wearer’s body heat.

“Even though gold is an expensive element, our work represents a proof of concept. We were able to show that not only semiconductors, but also metals can exhibit good thermoelectric properties that make them relevant for diverse applications. Metallic alloys have various advantages over semiconductors, especially in the manufacturing process of a thermoelectric generator,” Michael Parzer, one of the lead authors of the study, said.

The researchers are currently investigating other promising candidates that do not require expensive elements like gold.

Image caption: Schematic drawing of the thermoelectric effect in nickel–gold alloys. Image credit: Fabian Garmroudi