Study on Low Temperature Thermoelectric Materials for Cryogenic Cooling

Thermoelectric devices can provide a significant amount of clean energy by converting heat into electricity and vice versa. They are in high demand for space programs because of their shape, size, and weight compatibility. Efficiency and the coefficient of performance of thermoelectric devices are related to a figure-of-merit called ZT, which is defined as ZT= (S^2 σ)/κ T, where S is Seebeck coefficient, σ electrical conductivity, κ thermal conductivity, and T absolute temperature. To study thermoelectric materials in a cryogenic range, we have implemented the method of modulation doping in nanostructured FeSb2 using 5 nm copper nanoparticles. Electrical conductivity of the materials is enhanced without affecting the Seebeck coefficient much. Such an enhancement of electrical conductivity increases the power factor by 90 % over that of the pure nanostructured FeSb2. The thermal conductivity of the material is decreased by 10 % through scattering of phonons from the interface of copper and FeSb2. We have obtained a peak ZT of 0.026 at 60 K, a 100 % increase over that of the pure nanostructured FeSb2. To further improve the ZT of the materials at cryogenic temperature, we have also studied the thermoelectric properties of strongly correlated f-electron systems. Nanostructured YbAgCu4 has been studied using an arc melting method followed by ball milling and hot pressing. For the optimized sample, thermal conductivity is reduced by 30 % - 50 % compared to the previously reported data. A peak power factor of 131 μW cm-1 K-2 has been obtained at 22 K, which is much higher than that of any of the well-known thermoelectric materials. A peak ZT of 0.11 at 42 K have been achieved, 4 times higher than that of FeSb2 working in the same temperature range. One step further, we have also studied the optimization of thermoelectric properties of YbAgxCu5-x by tuning the ratio of Ag/Cu. We can control the peak temperature position of ZT by varying the concentration of Cu and Ag, which might be useful for segmented legs.Physics, Department o