Enhanced average thermoelectric figure of merit of n-type PbTe_1-xI_x-MgTe

The thermoelectric properties of sintered samples of n-type PbTe_1-xI_x-yMgTe (x = 0.0012-0.006; y = 0 and 1%) were investigated over the temperature range of 300 K to 900 K. Scanning electron microscopy revealed two different length scales of grains in samples with higher I and MgTe contents, while a homogenous microstructure for samples with a lower dopant content. Transmission electron microscopy revealed ubiquitous spherical nanoprecipitates in PbTe_1-xI_x with MgTe and nanoscale disk like precipitates in both, PbTe_1-xI_x with and without MgTe. The nanostructured PbTe showed higher Seebeck coefficients than expected values. We also observed a slower rate of increase in the electrical resistivity with rising temperature in PbTe_1-xI_x-yMgTe below ∼550 K, leading to a higher thermoelectric power factor. The nanostructures and mixed microstructures scatter phonons, reducing the lattice thermal conductivity as low as 0.4 W K^-1 m^-1 at 600 K. A high ZT of 1.2 at 700 K was achieved as well as a high average ZT of 0.8 was observed in PbTe_0.996I_0.004-1 mol% MgTe for a cold-side temperature of 303 K and a hot-side temperature of 873 K.