Doping effects on thermoelectric properties of higher manganese silicides (HMSs, MnSi1.74) and characterization of thermoelectric generating module using p-type (Al, Ge and Mo)-doped HMSs and n-type Mg2Si0.4Sn0.6 legs

The effect of Al Doping on the thermoelectric properties of higher manganese silicides, MnSi1.74 (HMSs) parallel and perpendicular to the c-axis was investigated. It was found that Al doping increased electrical conductivity and decreased thermoelectric power parallel to the c-axis, because Al doping of the Si site increased carrier density according to the valence control rule. Al doping was also effective for lowering thermal conductivity and raising the figure of merit parallel to the c-axis. After optimizing the Al content (y = 0.0035), the thermoelectric properties of (Al, Ge and Mo)-doped HMSs parallel and perpendicular to the c-axis were evaluated and compared to those of nondoped and Al-doped HMSs. Two types of thermoelectric module consisting of n-type Mg2Si0.4Sn0.6 and p-type (Al, Ge and Mo)-doped HMSs legs parallel (or perpendicular) to the c-axis were fabricated, and their thermoelectric performances were compared to each other. The discrepancy between the measured maximum output power and the value estimated from the thermoelectric data of constituent materials was well explained by the resistance of a porous Al electrode. The measured heat flux of the module was also compared to the estimated heat flux. The maximum energy conversion efficiency of the module parallel to the c-axis was clearly higher than that perpendicular to the c-axis, because of a higher output power and a lower heat flux, and was higher than 7% at AT = 520 K (303-823 K).