MS-14-P-2758 The effect of low temperature synthesis (LTS) and Na doping on the structural properties of PbTe thermoelectric materials

PbTe-based thermoelectric (TE) materials lie among the most promising ones for conversion of ‘waste’ heat into electric energy. Their TE efficiency strongly depends on their structure and can be significantly improved by incorporation of nanoscale inclusions, due to enhanced phonon scattering at phase boundaries, interfacial dislocations and/or by band structure engineering. Therefore, bulk nanocrystalline PbTe can combine both improved properties and low cost, making the technology feasible in application level. In the current study, the effect of a novel low temperature synthesis (LTS) and Na doping on the microstructure of PbTe samples is explored using electron microscopy (TEM, HRTEM) and image analysis methods.
The morphology and crystalline quality of the undoped and the 2 at% Na-doped PbTe are depicted in Figs. 1 and 2, respectively. A significant reduction in the particle size is observed, with sizes ranging from 300-500 nm for the undoped PbTe down to 5-80 nm for the 2 at% Na sample. This inevitably leads to a nanocrystalline character for the Na-doped PbTe, as manifested in the Selected Area Diffraction (SAD) pattern inset in Fig 2, too. Na incorporation is envisaged by the formation of elliptical-shape nanoprecipitates –arrowed in Fig.2– with sizes up to 7 nm. They preferentially lie along <110> of PbTe and are located both in the particles’ interior and at grain boundaries. Their Na/Pb ratio, as measured by Energy Dispersive X-ray Spectroscopy (EDS) is up to 8.2 at%, confirming the high Na content.
Fig. 3(a) shows an HRTEM image from a grain boundary at the 2 at% Na-doped sample. PbTe particles usually exhibit (200)-type lattice fringes. A small reduction, up to 2.5%, of their fringe separation distance is measured. This is attributed to slight variations in the Pb/Te ratio, also confirmed by EDS and incorporation of Na atoms inside the host matrix. The extent of residual strain present at the nanograins was measured by the Geometric Phase Analysis (GPA) method and the results are outlined in Fig. 3(b). A uniform strain distribution inside the nanograins is found; however, sharp strain peaks, up to 16%, are commonly observed at grain boundaries.
TE measurements revealed a significant increase in the figure of merit ZT for the Na-doped PbTe (1.38 at 675K, compared to 0.2 for the undoped). This is attributed to a simultaneous increase in carrier concentration and a significant decrease in thermal conductivity due to the in situ nanostructuring which is achieved by LTS and Na incorporation and the enhanced scattering caused by the Na-rich precipitates and grain boundary residual strain present. Consequently, LTS and Na-doping is a highly promising alternative and low cost route for the preparation of PbTe with improved TE performance.