Preparation And Thermoelectric Properties Of BiCuSeO-based Materials

Due to the low thermal conductivity, lost cost and high chemical stability, The Zr C u Si As-type layered compound BiCuSeO attracts much attention in recent years and has been considered as a very promising new thermoelectric materials.In this article, a two-step solid phase reaction method has been employed to prepare the Na doped BiCuSeO and the Cu deficiency Na doped BiCuSeO materials. Considering elemental metal Na is too active to dope into the compound directly, Na2CO3 was used as the Na source to dope in the Bi CuSeO. After the synthesis, the porous BiC uSeO agglomerates with different composition were pulverized by ball milling and consolidated by hot pressing. X-ray diffraction analysis(XRD), Field emission electron microscopy characterization(FESEM), Energy Dispersive Spectroscopy(EDS), Differential Thermal Analysis(DTA) and other means were employed to characterize the structure, composition and microstructure o f the consolidated samples. The electrical resistivity and Seebeck coefficient are measured by a home- made Namicro-II thermoelectric measurement system. The carrier concentration and mobility are measured by the Hall coefficient test system. And the therma l diffusion coefficient is measured by a laser flash diffusivity apparatus. The main results is as follows:1) The solid solution limit of Na in the BiCuSeO is about 6%. In the BiCuSeO compound, when the Na doping content is less than 6%, all the Na atoms can dissolve into the lattice and tale the position of Bi, which was confirmed by the decrease of the lattice constant. When the Na doping content is over 6%, which is larger than the limit of solubility of Na in BiCuSeO, a portion of the surplus Na2CO3 will react with Se and form Na2SeO3 in the grain boundaries. 2) Na doping can increase hole concentration and reduce the resistivity of BiCuSeO and improve the power factor. Moreover, It introduces point defects and mass fluctuation and thus increases the phonon scattering and decreases the thermal conductivity of the sample. When the doping content is beyond the solid solution limit, the residual Na2CO3 can introduce second phase to scatter phonon to decrease the thermal conductivity while makes less influence on the power factor. 3) the maximum ZT was achieved as 0.97 at 873 K in a content of 8% Na doping which is about two times higher than the undoped BiCuSeO matrix.Based on the the 8% Na doped BiCuSeO, effect of Cu deficiency was also studied. It shows, when the Cu deficiency is beyond a certain amount, there is an obvious second phase peak of Bi2O3 in the XRD pattern, indicating that the excessive C u deficiency can destroy the stability of crystal structure of the BiCuSeO material and C u deficiency can decrease the electrical resistivity and with less effect on the Seebeck coefficient, in a result, the power factor has been improved, and the power factor of Bi0.92Na0.08Cu0.99 SeO has a 26% increase in comparison with the Bi0.92Na0.08 CuSeO sample.