Nanocomposites from Solution-Synthesized PbTe-BiSbTe Nanoheterostructure with Unity Figure of Merit at Low-Medium Temperatures (500-600 K)

The thermoelectric properties of Pb-doped BiSbTe nanocomposites derived from a scalable, solution synthesized precursor made in 10 g batches were studied. Comprehensive characterization is performed to examine the structure and composition of the intermediate product at each step of the synthesis. First, X-ray diffraction (XRD) patterns are recorded to verify the crystal phases. After Step 1, pure Te is indexed and no impurity such as TeO₂ is found. After Step 2, the diffraction peaks of both Te and PbTe are discerned, indicating that PbTe has been incorporated. After Step 3, the XRD profile shows peaks of PbTe, Te, and Bi_xSb_2-xTe₃. The compositions of the aforementioned nanostructures are further verified by TEM study. First, the 1D shape of the Te nanowire, produced in Step 1, is clearly observed. Statistics of the nanowire diameter reveal a narrow distribution and the length of nanowires are found to be 1500 ±200 nm. The lattice fringe of Te that is perpendicular to the axial direction indicates that the nanowire grows along the c axis. High angular annular dark field-scanning transmission electron microscopy (HAADF-STEM) and elemental mapping also corroborate that PbTe is grown on the tip, while the nanowire body is comprised of Bi_xSb_2-xTe₃ and unreacted Te. The thermoelectric properties were enhanced by tuning the power factor and delaying bipolar conduction through controlled Pb-doping, as well as reducing the lattice thermal conductivity through nanostructuring. These p-type materials have a zT < 1 in the low-medium temperature range, making them better than previously reported BiSbTe materials for low grade waste heat recovery.