The structure and thermoelectric properties of tungsten bronze Ba6Ti2Nb8O30

Dongting Jiang,Kan Chen,Amit Mahajan,Michael J Reece,Dursun Ekren,Robert Freer,Demie M Kepaptsoglou,Sarah J Day,Feridoon Azough,Quentin M Ramasse

doi:10.1063/1.5119962

Abstract

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 μW/m K2 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.

Highlights

Thermoelectric (TE) materials can transform thermal energy to electrical energy and can, be exploited in a range of applications from energy harvesting to cooling devices
Building on the earlier work of Azough et al.10 on tungsten bronze (TB) materials, we have investigated the crystal structure and, for the first time, the thermoelectric properties of ceramic
energy dispersive X-ray (EDX) analysis confirmed the presence of Ba, Ti, and Nb in the secondary phase with the atomic ratios of approximately 2:1:1, equivalent to Ba (Ti0.46Nb0.54)O3; this phase is richer in Ti than the primary phase

Summary

INTRODUCTION

Thermoelectric (TE) materials can transform thermal energy to electrical energy and can, be exploited in a range of applications from energy harvesting to cooling devices. Azough et al. investigated a related ferroelectric ceramic with the TB structure Ba6 − xNd8 + 2xTi18O54 and evaluated the effect of the Ba/Nd ratio on thermoelectric properties They demonstrated that Ba5.19Nd8.54Ti18O54 exhibited a high S value (−210 μV/K) and an acceptable σ (60 S/cm), which led to a maximum power factor of 2.5 × 10−4 W/m K2 at 1000 K. This material exhibited a low and stable thermal conductivity over the full measurement range, being only 1.45 W/m K at 1000 K, one of the lowest values reported for thermoelectric oxides. Selected samples were fabricated by Spark Plasma Sintering (SPS) to achieve high density with the possibility of developing texture; the electrical conductivity and Seebeck coefficients are usually maximized along the c-axis for tetragonal tungsten bronze structured materials.

EXPERIMENTAL

Microstructure and crystal structure

XPS analysis

Thermoelectric properties

Crystal structure and microstructure

CONCLUSIONS