Battery and energy materials
Which is a correct strategy for increasing the thermoelectric figure of merit ZT?
- ARaising the carrier concentration to metallic levels increases both the Seebeck coefficient and the electrical conductivity at the same time, so ZT rises sharply
- BLowering the electrical conductivity and raising the thermal conductivity lets heat pass through the element faster, so a temperature difference is easier to maintain and ZT rises
- CScatter phonons with nanostructures or heavy atoms to lower the lattice thermal conductivity while disturbing electrical conduction as little as possible
Show answer and explanation
Answer: C. Scatter phonons with nanostructures or heavy atoms to lower the lattice thermal conductivity while disturbing electrical conduction as little as possible
To make ZT = S²σT/κ large, the Seebeck coefficient S and electrical conductivity σ must be high and the thermal conductivity κ low. However, raising the carrier concentration increases σ but decreases S, and also increases the heat carried by electrons (the Wiedemann–Franz law), so the optimum is a nearly degenerate semiconductor with a carrier concentration of about 10¹⁹–10²⁰ cm⁻³. At metallic levels S becomes too small. A promising strategy is therefore to scatter only the phonons, which carry most of the heat, using nanostructured interfaces or the rattling of heavy atoms, to lower the lattice thermal conductivity; this is described as “phonon glass, electron crystal.” Lowering σ and raising κ moves ZT down.