Goodbye to Heat Loss in Electronics: This Japanese Crystal Promises to Convert It into Energy

A research group from the Institute of Science in Tokyo has developed TlFe1.6Se2, a new thermoelectric material that could improve the recovery of waste heat from factories, cars, power plants, and industrial furnaces. The most significant outcome concerns the combination of high electrical conductivity and very low thermal conductivity, two properties that are usually difficult to obtain in the same material.

The research, published in the Journal of Materials Chemistry A, proposes a different strategy compared to traditional thermoelectric materials: instead of optimizing existing compounds, the researchers incorporated the properties of ultra-thin atomic materials within a three-dimensional crystal suitable for practical applications.

The new crystal is made of thallium, iron, and selenium and has a layered structure. Within the material, atomic layers of iron selenide (FeSe) are periodically inserted, known for their exceptional electronic properties when reduced to a few atomic layers.

Ultrathin films of FeSe had already shown a very high thermoelectric power factor, but their fragility and difficulty of integration limited their practical use. With TlFe1.6Se2, the team led by Professor Takayoshi Katase managed to reproduce part of that behavior within a solid and manageable crystal.

One of the most impressive aspects of the material concerns its thermal conductivity, which at around 180 °C drops to 0.2 W m-1 K-1. This value is comparable to or even lower than that of the best thermoelectric materials currently available.

This result derives from the presence of ordered iron vacancies within the FeSe layers. The controlled absence of certain iron atoms distorts the atomic bonds and scatters the phonons, the vibrations responsible for heat transport. The heavy thallium atoms and the complex layered structure also contribute to further slowing the propagation of heat.

At about 180 °C, the crystal also undergoes a reversible transition from a phase with ordered iron vacancies to a disordered phase. This change further increases phonon scattering and reduces the material's ability to conduct heat.

The iron vacancies affect not only thermal transport but also electrical transport. In the ordered phase, the Seebeck coefficient exceeds 100 μV K-1, generating a thermoelectric power factor approximately five times higher than in the disordered phase.

In practice, the material maintains a strong temperature difference between the two sides of the crystal, an essential condition for converting heat into electricity. This balance between high electrical conductivity and low thermal conductivity represents one of the main obstacles in designing efficient thermoelectric devices.

For now, the results have been obtained in the laboratory. Before practical use in power plants, engines, or industrial facilities, it will be necessary to verify the overall conversion efficiency, long-term stability, mechanical strength, and manufacturing scalability.

Another aspect to consider is the presence of thallium, a highly toxic element that will require safe containment and management systems. Despite these challenges, the researchers believe that the same strategy can be extended to other compounds based on FeSe containing potassium, rubidium, or cesium, opening new perspectives for next-generation thermoelectric materials.