Energy from waste heat: Researchers have discovered a particularly efficient type of thermoelectric material — a solid material that can convert heat directly into electrical potential. Unlike traditional methods, short, strong heating changes the atomic structure of a metallic compound made of iron, vanadium, and aluminum in such a way that the electrons move. Once this threshold is crossed, even small temperature differences in the material are sufficient to generate tension.
In most power plants, heat is converted into electricity by turbines, with much energy lost. But there is also a more direct way – through refractory material. In them, the so-called Seeberg effect ensures that even small differences in temperature inside a material make electrons move and current flow. As a result, a thin layer of material is enough to absorb, for example, waste heat from machines or our machines body temperature to convert it into electricity.
Focus on the electrons
Thermal conversion is particularly effective when the material has a high electrical conductivity but a low temperature. But finding materials with such properties is not easy. So far, attempts have been made to achieve this by intentionally introducing exotic atoms into semiconductor materials and reducing thermal conductivity. “Increasing the electrical part of the equation, on the other hand, is a more difficult but necessary task,” explain Fabian Garmody of the Vienna University of Technology and colleagues.
According to the theory, the conducting electrons in the material can only move within a very narrow energy range. Kermrodi and his team have now investigated what this means in practice and how this can be achieved in a subject. To do this, they conducted experiments on a metal alloy of iron, vanadium, and aluminum (Fe2VAl) was briefly heated to various temperatures up to 1380° and then rapidly cooled in a water bath. The team analyzed the structure and electronic behavior of the material.
Anderson’s transfer is crucial
The analyzes revealed that the so-called Anderson junction is important for thermoelectric efficiency. It occurs when the randomly distributed impurities in the material exceed the critical density. “Similar to floating ice in the sea, they are initially isolated from each other and cannot be trampled on,” Al-Qarmroudi explains. “However, if the number of ice floes is large enough, you have a continuous connection that you can use to cross the sea.”
If it is applied to a substance, then this means that if the number of foreign atoms, for example, exceeds a critical value, the electrons can suddenly move freely from one atom to another. Even small temperature gradients are sufficient to allow current to flow. “The conditions for ideal thermoelectricity are present at the Anderson junction, which is a quantum phase transition from localized electron states to a moving electron state,” says Grumrudy.
Optimization through heat and cooling
At the same time, experiments showed that the Anderson transition could be achieved not only by adding more foreign atoms, but also by only heating followed by rapid cooling in a water bath, known as quenching. “At high temperatures, the atoms vibrate so strongly that they sometimes swap their lattice positions,” explains senior author Ernst Bauer of the Vienna University of Technology. “For example, iron atoms are where vanadium atoms were before.”
Similar to the additional foreign atoms, these irregular heat-induced defects ensure the increased mobility of the electrons—without having to change the chemical composition of the material. This turns the metallic compound into a very efficient thermoelectric material in which electrons move – as required by theory – within a very narrow energy range.
Enhance the performance of thermal materials
Measurements showed that the thermoelectric output of the metallic compound cooled at 1380 degrees was 7.6 milliwatts per square meter Kelvin. “This is a 30 to 40 percent increase over the best-doped variants of this system – and one of the highest thermoelectric performance ever reported for a solid state,” the researchers wrote. The best n-doped bismuth-tellurium system currently achieves only half the power factor.
“Our work thus reveals a new paradigm for optimizing thermoelectric materials,” Al-Garmroudy and colleagues say. Accordingly, rearrangements and perturbations in the crystal lattice that are specifically generated by heat can increase the thermoelectric output. This paves the way for more efficient heat converters and improved electricity generation from previously unused heat. (Nature Communications, 2022; doi: 10.1038/s41467-022-31159-w)
Source: Vienna University of Technology
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