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−xNa_xGe_0.007Te (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 Pb_0.98Ga_0.02Te measured in comparison. Excellent thermoelectric performance is maintained for nanostructured Pb_0.973Na_0.02Ge_0.007Te (zT ≈ 2.2 at 810 K). For n‐type Pb_0.98Ga_0.02Te, zT ≈ 1.3 at 760 K is confirmed. Single‐leg elements of Pb_0.973Na_0.02Ge_0.007Te and Pb_0.98Ga_0.02Te with Co₈₀Fe₂₀ contact layers display maximum conversion efficiency η_max ≈ 8.4% and η_max ≈ 8.2% for temperature difference ΔT ≈ 470 K, respectively. After 240 h operation with ΔT ≈ 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 Co₈₀Fe₂₀ and sublimation from the hot side.