Materials with thermoelectric effects

Thermoelectric (TE) generators or refrigerators are all-solid-state devices that convert heat into electricity or pump heat, respectively. Their energy conversion efficiencies (~10% in generation or ~150% in refrigeration) do not allow them to compete with conventional systems, such as a steam turbine coupled to an alternator (33%) or a compression refrigerator (300%). However, they can be used in systems that recycle waste heat from combustion systems (internal combustion engines, furnaces, etc.) to improve their overall efficiency, or in air conditioning systems that do not use greenhouse gas refrigerants.

These systems are compact, silent and, above all, require no maintenance due to the absence of moving parts. They can be miniaturized and integrated into an electronic chip to cool it, into a sensor to power it, or, conversely, deployed as photovoltaic panels to generate electricity from an extensive heat source. They are therefore part of the new methods of refrigeration or green electricity production that could contribute to the energy transition that our society is seeking to implement.

The materials currently used in TE devices were discovered in the mid-20th century. Most of them incorporate rare chemical elements or offer insufficient performance to supply the market for the new applications being considered.

Since 1995, under pressure from growing environmental concerns about greenhouse gases, new materials have emerged. They are based on new concepts, such as materials with complex crystallographic structures or nanostructured materials. Several of these materials have thermoelectric properties that are of interest for applications in electricity generation.

This article presents the state of the art, addressing the issue of TE performance of materials, as well as issues relating to their stability, particularly thermomechanical stability and stability with regard to oxidation by the surrounding environment, all of which are essential aspects for ensuring the reliability required by applications.