National Physical Laboratory

Nanostructured Thermoelectrics

A temperature gradient across a solid generates an electrical voltage between the hot and cold ends. This phenomenon, discovered in 1821 by Seebeck has been extensively used in thermocouples for temperature measurement. While the voltage generated by metals is generally less than 50 μV/K, semiconductors and new nanostructured materials can generate several hundreds of μV/K. This technology for direct conversion of heat to electricity is called thermoelectric power generation.

Recent years have seen a huge increase in worldwide energy demand. Fossil fuels are a finite resource, so alternatives are needed. The search is on to produce high-efficiency energy conversion technologies.

Thermoelectric (TE) devices can play an important role in efficient energy harvesting: they are 'fuel-free' solid-state devices with no moving parts and are therefore extremely reliable; TEs can harvest residual low-grade energy, which is otherwise wasted.

To date, their use is limited by low conversion efficiency (~10 %). However, recent developments based on nanomaterials have shown dramatic improvements (by a factor 2 to 3) in the performance of TE devices. Next generation TE devices can certainly revolutionise several concepts of energy harvesting, power generation, refrigeration/heating, and thermal sensing both in terrestrial and space applications.

Nanostructured thermoelectrics output diagram

Thermoelectric materials convert wasted heat into electrical energy. The performance of the energy conversion scales with the thermoelectric figure of merit of the active material:

ZT = S2σT/κ

where S, σ, κ and T are the Seebeck coefficient, the electrical conductivity, the thermal conductivity and the absolute temperature, respectively. The traceable determination of the properties related to the efficiency of thermoelectric converters is crucial to enable a fair competition at the European and international level and is the focus of NPL's Nanomaterials group.

The measurement of the efficiency of thermoelectric materials is subjected to a large uncertainty. This is due to the complexity of fabricating devices, the measurement uncertainty and materials complications. The Seebeck coefficient is currently measured at the macroscale and induces in some case an uncertainty of up to 50% on the figure of merit measurement.

Nanometrology is widely recognised as being the missing key part to be able to commercialise these materials more widely. Scientifically, these new materials may allow the independent control of the transport of electrons and phonons in solid materials.

Last Updated: 7 Nov 2016
Created: 9 Sep 2010


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