Design of thermoelectric (TE) modules based on TE materials of higher specific figure-of-merit and low-cost materials

Abstract

The efficiency of thermoelectric (TE) materials is determined by the dimensionless figure-of-merit (ZT) defined as ZT = S<super>2</super>σT/κ where, S, σ, κ, and T are Seebeck coefficient, electrical conductivity, thermal conductivity, and absolute temperature, respectively. For weight-sensitive applications the specific figure of merit defined as ZT divided by mass density, should be emphasized. Since Mg<super>2</super>Si for n-type and higher manganese silicide (HMS, MnSi_2-x, x=0.250~0.273) for p-type TE alloys have low mass densities, they produce a TE module with a high specific power suitable for airborne applications. To improve ZT by reducing lattice thermal conductivity, nanostructures are formed by mechanical alloying and spark plasma sintering techniques. Two TE module designs are made, (1) a segmented π-shape TE module and (2) a segmented linear TE module. The segmented π-shaped TE generator (TEG) is composed of higher temperature segments made of n-type Mg<super>2</super>Si and p-type HMS and lower temperature segments of n-type and p-type Bi-Te based compounds. The specific power density was measured as 42.9 [W/kg] under 500°C temperature difference, which has a good agreement with analytical predictions. The linear-shaped TEG is composed of AlN-Cu composite electrode integrated into the combustion chamber wall made of Fe based shape memory alloy (Fe-SMA) by shrink-fit joining. The power output of the linear TEG is found to be 7% higher than that of the π-shaped TEG, and the specific power density is increased from 42.9 [W/kg] to 89.3 [W/kg] due to lighter weight of the linear design. The maximum shear stress of the linear TEG is reduced by 47% compared with the π-shaped TEG. As a result, the linear TEG proposed gives rise to better performance in terms of the specific power density, while it is more robust.