Thermoelectrics generating electricity from waste heat is a step closer

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A thermoelectric material consists of alternating n-type and p-typesemiconductorsthat together convert heat into electricity. In theory the heat could be sourced from any process that generates heat, but at present the materials are too inefficient to provide a commercially feasible way ofgenerating electricityfromwaste heat, such as that produced in car exhausts.

The most common thermoelectric p-type material in use is based on lead telluride (PbTe) and devices based on this material have been used in satellites, with heat sourced fromradioisotopes, and in niche markets on Earth, where the heat is generated by burning fuels such as gas.

The efficiency of the thermoelectric material is expressed as a“thermoelectric figure of merit,” ZT, which is a dimensionless figure derived from several factors including theelectrical conductivityand thermal conductivity. The figure of merit needs to be over 1.5 for the material to be capable of generating useful amounts of electricity in commercial applications. PbTe thermoelectric materials are capable of withstanding high temperatures, but their figures of merit are around 0.8, which makes them suitable only for niche markets such as satellites.

Now physicists from the California Institute of Technology and the Chinese Academy of Sciences have modified the amount of tellurium in the PbTe alloy and added selenium and sodium to produce a material with a figure of merit of 1.8 at 850K, which lead author Dr. Jeffrey Snyder described as“extraordinary.”

In previous research Snyder and colleagues had achieved a ZT of 1.5 by doping PbTe with thallium and 1.4 by using sodium. Adding selenium to the mix improved the electrical conductivity while also reducing the thermal conductivity. The selenium increases the number of“degenerate valleys” in the electronic band structure of the material, and this boosts the electrical conductivity and raises the ZT figure. Known thermoelectrics have a typical valley degeneracy of less than six, but the number for the new material is 12 or greater.

Dr. Snyder said he thought a figure of merit of 1.8 was the highest ever to be reproduced in independent laboratories. He also suggested that doping other thermoelectrics in the same way should improve their performance.

Dr. Snyder said the team is now working on creating a promising n-type material and in improving the p-type material’s effectiveness at higher temperatures. The paper is published inNature.

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