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Thermoelectric properties of Mg 2 (Sn 0.765 Ge 0.22 Sb 0.015 ) 1-x (Sn 0.685 Si 0.3 Sb 0.015 ) x materials with compositions on and near the line connecting Mg 2 Sn 0.765 Ge 0.22 Sb 0.015 and Mg 2 Sn 0.685 Si 0.3 Sb 0.015 in the Mg 2 Sn-Mg 2 Ge-Mg 2 Si system are investigated. Although ZTs are very similar, power factors are different. On the line, the power factor decreases from Mg 2 Sn 0.765 Ge 0.22 Sb 0.015 to Mg 2 Sn 0.685 Si 0.3 Sb 0.015 , and off the line, the power factor also decreases. The output power and energy conversion efficiency are calculated using engineering power factor (PF) eng and figure of merit (ZT) eng . It is shown that although similar energy conversion efficiency of ~11% could be achieved for all compositions studied, the output power are different, increasing from ~9.1 W cm -2 for Mg 2 Sn 0.685 Si 0.3 Sb 0.015 to ~10.3 W cm -2 for Mg 2 Sn 0.765 Ge 0.22 Sb 0.015 , due to the different power factors.

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Constantan alloy (Cu-Ni) has been known for a long time in thermocouples due to its thermal power property. In this study, we show an enhancement in thermoelectric performance of Cu 56 Ni 42 Mn 2 alloy by introducing nanoscale twins into its microstructure. Comparing to arcmelted ingot (without nanoscale twins), the ball milled and hot pressed (BM-HP) samples with twinning showed a higher Seebeck coefficient of $ À72.5 μV K À 1 (an increase of $ 12% at 873 K), a larger power factor of $102 μW cm À 1 K À 2 (an increase of $21% at 873 K), and hence a higher ZT of $0.19 (an increase of $ 34% at 873 K). A high output power density of $ 53.4 W cm À 2 is calculated from the high power factor even though the conversion efficiency is lower than 3% due to the low ZT. TEM characterization shows there is a large quantity of nanoscale twins with spacing of 50-200 nm. It is very likely that low-energy carriers are selectively scattered by the twin boundaries (i.e., potential barrier scattering) thus lead to http://dx.enhanced Seebeck coefficient. The improved thermoelectric performance of nano-twinned CuNi alloy suggests constantan could be promising in thermoelectric power generation where the power output density is more important than the conversion efficiency.

Solid solutions with chemical formulaMg 2 (Si,Ge,Sn) are promising thermoelectric materials with very good properties for the n-type material but significantly worse for the p-type. For power generation applications good n-and p-type materials are required and it has been shown recently that Li doping can enhance the carrier concentration and improve the thermoelectric properties for p-type Mg 2 Si 1x Sn x .We have investigated the potential of p-type Mg 2 (Ge,Sn) by optimizing Mg 2 Ge 0.4 Sn 0.6 using Li as dopant. We were able to achieve high carrier concentrations (1.4 × 10 20 cm −3) and relatively high hole mobilities resulting in high power factors of 1.7 × 10 −3 W m −1 K −2 at 700 K, the highest value reported so far for this class of material. Exchanging Ge by Si allows for a systematic comparison of Mg 2 (Ge,Sn) with Mg 2 (Si,Sn) and shows that Si containing samples exhibit a lower power factor but also a lower thermal conductivity resulting in comparable thermoelectric figure-of-merit. The data is furthermore analyzed in the framework of a single parabolic band model to gain insight on the effect of composition on band structure. 2 , here is the electrical conductivity, the thermal conductivity, the Seebeck coefficient, and the absolute temperature. A large fraction of the potentially available waste heat can be found in the mid-temperature region