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 Pb 0.993− x Na x Ge 0.007 Te ( 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 Pb 0.98 Ga 0.02 Te measured in comparison. Excellent thermoelectric performance is maintained for nanostructured Pb 0.973 Na 0.02 Ge 0.007 Te ( zT ≈ 2.2 at 810 K). For n‐type Pb 0.98 Ga 0.02 Te, zT ≈ 1.3 at 760 K is confirmed. Single‐leg elements of Pb 0.973 Na 0.02 Ge 0.007 Te and Pb 0.98 Ga 0.02 Te with Co 80 Fe 20 contact layers display maximum conversion efficiency η max ≈ 8.4% and η max ≈ 8.2% for temperature difference Δ T ≈ 470 K, respectively. After 240 h operation with Δ T ≈ 470 K, η max decreases by ≈33% for the p‐type and ≈13% for the n‐type legs. Lower η max 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 Co 80 Fe 20 and sublimation from the hot side.