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Abstract:
N-type XNiSn (X = Ti, Zr, Hf) half-Heusler (HH) compounds possess
excellent thermoelectric properties, which are believed to be attributed
to their relatively high mobility. However, p-type XNiSn HH compounds
have poor figures of merit, zT, compared to XCoSb compounds. This can be
traced to the suppression of the magnitude of the thermopower at high
temperatures. E-g = 2eS(max)T(max) relates the band gap to the
thermopower peak. However, from this formula, one would conclude that
the band gap of p-type XNiSn solid solutions is only one-third that of
n-type XNiSn, which effectively prevents p-type XNiSn HHs from being
useful thermoelectric materials. The study of p-type HH Zr1-xScxNiSn
solid solutions show that the large mobility difference between
electrons and holes in XNiSn results in a significant correction to the
Goldsmid-Sharp formula. This finding explains the difference in the
thermopower band gap between n-type and p-type HH. The high
electron-to-hole weighted mobility ratio leads to an effective
suppression of the bipolar effect in the thermoelectric transport
properties which is essential for high zT values in n-type XNiSn (X =
Ti, Zr, Hf) HH compounds.
