Transport and thermoelectric properties of nanocrystal substitutional semiconductor alloys (Mg 1− xCd x) 3Sb 2 doped with Ag

Abstract

Nanocrystalline (Mg 1− x Cd x ) 3Sb 2 ( x = 0–0.3) and nominally (Mg 0.7Cd 0.25Ag 0.05) 3Sb 2 with a mean grain size of ∼35 nm were prepared by mechanical alloying plus hot-pressing, and their transport and thermoelectric properties were investigated from 310 K down to 20 K. The results indicated that electric resistivity of (Mg 1− x Cd x ) 3Sb 2 decreased with increasing Cd content x, which could be ascribed to increase in carrier concentration indirectly caused by lattice distortion after substituted. The much larger decrease in the resistivity of (Mg 0.7Cd 0.25Ag 0.05) 3Sb 2 than that of (Mg 1− x Cd x ) 3Sb 2 should originate from substantial increase in hole concentration due to introduction of acceptor impurity Ag. At lower temperature regimes, Mott's T −1/4 law was observed, which could be attributed to the severe potential disorder caused by lattice distortion upon doping. Seebeck coefficient of all the substituted and co-doped specimens decreased presumably due to increased carrier concentration. Moreover, thermal conductivity of (Mg 1− x Cd x ) 3Sb 2 decreased monotonously with increasing x due to phonon scattering of the impurity (Cd) with greater atomic weight. The presence of slight precipitated elementary Sb and Cd phases should be responsible for the observed larger thermal conductivity of (Mg 0.7Cd 0.25Ag 0.05) 3Sb 2 specimen. The values of the ZT for all the doped specimens enhanced. Especially, ZT of (Mg 0.7Cd 0.25Ag 0.05) 3Sb 2 reach 0.01 at 300 K, which is several orders magnitude higher than that un-doped one and those doped slightly with Cd only. Present result indicates that thermoelectric properties of Mg 3Sb 2 system can be effectively improved by the proper substitution of element Cd and co-doped with element Ag.