Improved High-Temperature Material Stability and Mechanical Properties While Maintaining a High Figure of Merit in Nanostructured p-Type PbTe-Based Thermoelectric Elements

Abstract

Material stability and mechanical properties of nanostructured p-type Pb0.993−xNaxGe0.007Te (x = 0.02, 0.04) are improved by tuning dopant Na content, while maintaining a high thermoelectric figure of merit zT. Na-rich impurity phases detrimental to the material stability, present in heavily Na-doped p-type PbTe, are absent after reducing the Na doping from 4% to 2%. The flexural strength of 2% Na-doped p-type PbTe measured at 773 K is significantly improved compared to that of the 4% Na-doped p-type PbTe, but lower than that of n-type Pb0.98Ga0.02Te measured in comparison. Excellent thermoelectric performance is maintained for nanostructured Pb0.973Na0.02Ge0.007Te (zT ≈ 2.2 at 810 K). For n-type Pb0.98Ga0.02Te, zT ≈ 1.3 at 760 K is confirmed. Single-leg elements of Pb0.973Na0.02Ge0.007Te and Pb0.98Ga0.02Te with Co80Fe20 contact layers display maximum conversion efficiency ηmax ≈ 8.4% and ηmax ≈ 8.2% for temperature difference ΔT ≈ 470 K, respectively. After 240 h operation with ΔT ≈ 470 K, ηmax decreases by ≈33% for the p-type and ≈13% for the n-type legs. Lower ηmax compared to the estimation from the material properties and degradation during operation are attributed to crack formation due to thermal expansion mismatch between PbTe and Co80Fe20 and sublimation from the hot side.