The structure and thermoelectric properties of tungsten bronze Ba_6Ti_2Nb_8O_(30)

摘要

Tungsten bronze (TB) structured materials have attracted attention as possible thermoelectrics because of their complex crystal structure. In this work, a new thermoelectric ceramic with a tetragonal tungsten bronze (TB) structure, Ba6Ti2Nb8O30 (BTN), was prepared by the conventional mixed oxide route with some samples processed by Spark Plasma Sintering (SPS). The addition of MnO enabled the fabrication of high density BTN ceramics at a low sintering temperature of 1580 K in air and by SPS. All samples were annealed in a reducing atmosphere after sintering. X-ray diffraction showed that Ba6Ti2Nb8O30 crystallizes with tetragonal symmetry (P4bm space group). High angle annular dark field-electron energy loss spectroscopy analysis confirmed the proposed crystal structure and provided exact elemental distributions in the lattice, showing higher concentrations of Ti in the 2b lattice sites compared to the 8d lattice sites. XPS showed the presence of two spin-orbit double peaks at 207.7 eV in the reduced BTN samples, confirming the presence of Nb4+ ions. By the use of a sintering aid and optimization of the processing parameters, the ceramics achieved a high power factor of 280 mu W/m K-2 at 873 K. The BTN ceramics showed phonon-glass-type thermal conduction behavior with a low thermal conductivity of 1.7-1.65 W/m K at 300-873 K. A thermoelectric figure of merit (ZT) of 0.14 was achieved at 873 K. This ZT value is comparable with results for many TB thermoelectrics. However, BTN has the advantage of much easier processing.