Enhanced Thermoelectric Figure of Merit of Zintl Phase YbCd2-xMnxSb2 by Chemical Substitution

Samples of Zintl phase YbCd(2-x)MnxSb(2) (CaAl2Si2-type, space group P (3) over bar m1) were prepared by a solid-state reaction followed by a suitable annealing and spark plasma sintering (SPS) for densification. Investigations were carried out on chemical substitution of Cd by Mn in order to optimize the thermoelectric figure of merit ZT in the solid solution system YbCd2-xMnxSb2 (x = 0, 0.05, 0.1, 0.15, 0.2, 0.4, 0.6, 0.8, 1.0, 1.5 and 2.0). Seebeck coefficients, electrical and thermal conductivities of all the samples were measured in the temperature range 300-650 K. The presence of Mn (x <= 0.2) substantially reduces the thermal conductivity and enlarges the Seebeck coefficient, meanwhile decreasing the electrical conductivity to a certain degree. As the concentration of Mn (x) becomes higher than 1.0, the semimetallic nature of YbCd2-xMnxSb2 changes to that of a semiconductor with the majority the carriers switching from holes to electrons. Of all the samples, polycrystalline YbCd1.85Mn0.15Sb2 (p-type charge carriers) shows the highest figure of merit ZT of 1.14 at 650 K.