Another promising n-type oxide for thermoelectric applications
is CaMnO3. Like SrTiO3, CaMnO3 is typically doped
and can be doped on either site. The thermoelectric properties
of CaMnO3 with two common dopants, ytterbium on the calcium
site and niobium on the manganese site, are summarized
in Fig. 9.
105,137–143 Ytterbium doping leads to higher electrical
conductivity and Seebeck coefficient, which results in a larger
figure of merit. As compared to SrTiO3, CaMnO3 has a lower
conductivity, both thermal (compare Figs. 6 and 9c) and electrical
(compare Figs. 6 and 9a), and a lower Seebeck coefficient
(compare Figs. 6 and 9b). A study of several lanthanide dopants
demonstrated that ytterbium additions were most effective for
increasing the figure of merit.137,139 Ytterbium is particularly
beneficial in reducing the thermal conductivity138,140 which is
attributed to a decrease in phonon contributions to the thermal
conduction.144 Niobium additions have been shown to lead to an
increase in the electrical conductivity and decrease in the thermal
conductivity, but also a decrease in the Seebeck coefficient.145
Similar trends were observed with tantalum additions, but niobium
additions resulted in a higher electrical conductivity