Low-temperature thermoelectric materials

Almost two centuries ago, Thomas Seebeck1 reported that the solid with the highest negative Seebeck coefficient was elemental bismuth and that the one with the second highest positive Seebeck coefficient was elemental antimony. Both elemental Bi and Sb are semimetals, meaning that they have an equal amount of electrons and holes at the Fermi surface. As a result, the partial thermopower (S, for Seebeck coefficient) of the electrons, which is negative, counteracts the partial thermopower of the holes, which is positive, and the total thermopower is quite low (for Bi at 300 K, on the order of S ⊥ ≈ -50 μV/K in the direction perpendicular to the trigonal axis,2 and S ∥ ≈ -105 μV/K parallel to the trigonal axis2). As a result, the thermoelectric figure of merit (ZT) of elemental Bi at 300 K is limited to ZT ∥ = 0.38 from 250 to 300 K along the trigonal axis and ZT ⊥ = 0.07 perpendicular to it, too low to be useful. Elemental Sb has many more charge carriers than elemental Bi, and its thermoelectric performance is irrelevant.