A schematic drawing of the thermoelectric effect in nickel-gold alloys. Image: Fabian Garmroudi.
A schematic drawing of the thermoelectric effect in nickel-gold alloys. Image: Fabian Garmroudi.

Thermoelectrics allow the direct conversion of heat into electrical energy – and vice versa – making them interesting for a range of technological applications. In the search for thermoelectric materials with the best possible properties, a research team at the Vienna University of Technology (TU Wien) in Austria investigated various metallic alloys and found that a mixture of nickel and gold proved particularly promising. The researchers report their findings in a paper in Science Advances.

Using thermoelectrics to generate electricity is nothing new. Since the middle of the 20th century, thermoelectrics have been used to generate electrical energy in space exploration, as well as in everyday applications such as portable refrigerators. Moreover, they could also be used in industrial environments to convert waste heat into green electricity, to name just one potential application.

The thermoelectric effect is based on the movement of charged particles that migrate from the hotter to the colder side of a material. This results in an electrical voltage – the so-called thermoelectric voltage – which counteracts the thermally excited movement of the charge carriers.

The ratio between the built-up thermoelectric voltage and the temperature difference defines the Seebeck coefficient, named after the German physicist Thomas Johann Seebeck, which is an important parameter of a material’s thermoelectric performance. The critical requirement is an imbalance between a material’s positive and negative charges, as they compensate each other.

“Although Seebeck discovered the thermoelectric effect in common metals more than 200 years ago, nowadays metals are hardly considered as thermoelectric materials because they usually have a very low Seebeck coefficient,” explains Fabian Garmroudi, first author of the paper. On the one hand, metals such as copper, silver or gold have extremely high electrical conductivity; on the other hand, their Seebeck coefficient is vanishingly small in most cases.

Physicists from the Institute of Solid State Physics at TU Wien have now succeeded in finding metallic alloys with high conductivity and an exceptionally large Seebeck coefficient. They found that mixing the magnetic metal nickel with the noble metal gold radically changes the electronic properties. As soon as the yellowish color of gold disappears with the addition of about 10% nickel, the thermoelectric performance increases rapidly.

The physical origin for this enhanced Seebeck effect is rooted in the energy-dependent scattering behavior of the electrons in the alloy – an effect fundamentally different from semiconducting thermoelectrics. Due to the particular electronic properties of the nickel atoms, positive charges are scattered more strongly than negative charges, resulting in the desired imbalance and hence a high thermoelectric voltage.

Andrej Pustogow, senior author of the paper, compares the flow of electrons in metallic thermoelectrics to runners in a race. "Imagine a race between two runners, where one person runs on a free track, but the other person has to get through many obstacles,” he says. “Of course, the person on the free track advances faster than the opponent, who has to slow down and change direction much more often.” In the nickel-gold alloys, the positive charges are strongly scattered by the nickel electrons, while the negative charges can move practically undisturbed.

The combination of extremely high electrical conductivity and a high Seebeck coefficient produces record thermoelectric power factor values in the nickel-gold alloys, exceeding those of conventional semiconductors by far. "With the same geometry and fixed temperature gradient, many times more electrical power could be generated than in any other known material," explains Garmroudi.

In addition, the high power-density may lead to applications in the large-scale sector in the future. “Already with the current performance, smartwatches, for instance, could already be charged autonomously using the wearer's body heat,” Pustogow says.

“Even though gold is an expensive element, our work represents a proof of concept,” explains Michael Parzer, one of the lead authors of the paper. “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.”

The fact that the researchers were able to experimentally show that nickel-gold alloys are extremely good thermoelectrics is no coincidence. "Even before starting our experimental work, we calculated with theoretical models which alloys were most suitable," says Parzer. Currently, the group is also investigating other promising candidates that do not require the expensive element gold.

This story is adapted from material from TU Wien, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.