Transport Properties of the System Bi2Te2.7 Se0.3-xPbx Fabricated by a Conventional Melting Technique

Since the discovery of thermoelectricity by Thomas Johann Seebeck (1770-1831), thermoelectric generators (TEGs) devices have attracted the interest of a lot of scientists due to their unique dual capability of directly converting heat to electricity or electrical power into cooling or heating. Although intensive efforts have been made to produce new materials with enhanced thermoelectric properties, the Bi₂Te₃ or PbTe based alloys are still known as the best thermoelectric (TE) materials and they are currently available for many thermoelectric applications [Nan C.W. 2001]. Indeed, synthesis of new ternary and quaternary chalcogenides with complicated crystal structures, narrow band-gaps and complex electronic structures represents a significant development in the thermoelectric research field [Chung D.Y. 1997].

This work presents a study on the effect of Pb substitution on the thermoelectric properties of the Bi₂Te_2.7Se_0.3-xPb_x (x= 0-0.08) system. Therefore, all samples were prepared by using the melting method as a successful technique for preparing single phase polycrystalline materials.

The XRD patterns prove that the investigated samples crystallize in rhombohedral Bi₂Te₃ structure. No indication of impurities or secondary phases denoting other Bi-Te-Se-Pb compounds could be observed. The scanning electron microscope (SEM) images show that the data imply that the grains have irregular shapes and the surface morphology changes with variation of the Pb content.

The temperature dependence ( 200-400K) of the electrical conductivity measurements demonstrates a metallic-like behavior of all compounds. Seebeck coefficient measurements show that all the S values have negative sign over the whole temperature range of measurements ( 200-400K). Accordingly, one concludes that the majority of the charge carriers are electrons and the compounds are n-type semiconductor.

The highest PF value was found to be 15.51µWK^-2cm^-1 at T=330K and recorded for the undoped composition. Comparison to other reported values shows that the conventional melting technique is a proper method to get high PF values rather than using shear extrusion or hot-pressed techniques [Shokar E. Kh. 2013, Kim S.S. 2004, Ibrahim M.M. 2007].