Thermoelectric materials can convert exhaust heat energy
directly into electrical energy. Nevertheless, thermoelectric
generation is not widely used at present due to its low energy
conversion efficiency h. The efficiency h is a function of the
thermoelectric figure of merit ZT = S2sT/k, where T is the absolute
temperature, s is the electrical conductivity, k is the thermal
conductivity and S is the Seebeck coefficient. As a result, the
optimum material for thermoelectric generation should simultaneously
exhibit large S, large s and small k. Classical thermoelectric
materials such as tellurides cannot be used for vehicles applications
due to their low temperature stability. These applications
require high temperature (600–800 8C), oxygen resistance and ZT
between 1 and 1.5. The suitable material does not actually exist
and is the subject of extensive research.