Tungsten bronze structured compounds for thermoelectric applications

Abstract

The world’s reserves of fossil fuels are running low. It is of great importance to develop alternative sources of power; thermoelectrics are attractive as they allow us to generate power from waste heat. Conventional thermoelectric alloy materials such as Bi2Te3 and PbTe, are toxic and unstable, especially at high temperature. Therefore, oxides are needed as alternative high temperature materials. Tungsten bronze (TB) structured oxides present promising thermoelectric properties due to their large, complex unit cell which helps reduce the thermal conductivity. It is also appeared that “filled” TB structured materials show higher electrical conductivity than the unfilled ones. Therefore, the thermoelectric properties of two filled TB structured materials, Ba6Ti2Nb8O30 (BTN) and Sr6Ti2Nb8O30 (STN), have been investigated for the first time. The BTN ceramics were prepared by both conventional mixed oxide (MO) route and the spark plasma sintering (SPS) method. The STN, (Ba1-xSrx)6Ti2Nb8O30 and (Sr1-xLax)6Ti2Nb8O30 ceramics were only synthesised by the MO route. Samples were sintered in a reducing atmosphere of Ar/H2. SEM, XRD, TEM and HRTEM techniques were used to obtain the microstructural and crystal structural information for the compounds. All samples exhibited relative density above 94%. BTN ceramics sintered at 1350 ºC showed a tetragonal TB crystal structure with space group of P4bm. A secondary phase of Ba(Ti0.46Nb0.54)O3 was observed. The electrical conductivity for BTN improved after annealing, which can be attributed to the increased carrier concentration. The highest ZT of 0.14 was achieved for samples annealed for 12 h. Prolonged annealing up to 18 h resulted in abnormal grain growth. The SPS prepared BTN ceramics showed high density above 97% theoretical. Two secondary phases of BaTiO3 and Ba(Ti0.46Nb0.54)O3 were found in the annealed SPS ceramics. The SPS ceramics showed small grain size with high density of grain boundaries leading to low thermal conductivity of 1.6 W/m·K at 600 ºC. However, to avoid crack formation, the annealing temperature was optimised to 1200 ºC, leading to a low electrical conductivity of 40 S/cm at 600 ºC. The SPS sample annealed for 24 h showed the highest ZT of 0.14 at 600 ºC STN ceramics sintered at 1350 ºC showed orthorhombic TB structure with space group of Pba2. A secondary phase of Sr(Ti0.5Nb0.5)O3 was found. With longer reducing time, STN the electrical conductivity increased due to the higher carrier concentration. The thermal conductivity of STN is higher than that of BTN due to the lighter mass and smaller radius of Sr2+ which leads to weaker phonon scattering. The highest ZT for STN was around 0.13 at 600 ºC for samples annealed for 24 h. The crystal structure for (Ba1-xSrx)6Ti2Nb8O30 sintered at 1350 ºC transformed from tetragonal (x≤0.6) to orthorhombic (x≥0.8) at room temperature. It was confirmed that Ba ions preferred to occupy the large A2-sites, while Sr ions randomly occupy both A1- and A2-sites. When x = 0.8, the sample showed the highest power factor of 3.52 μW/cm·K2 at 600 ºC, which is potentially related to the great lattice distortion. The thermal conductivity increased with the increased Sr2+ content. The highest ZT of ~ 0.15 was obtained when x = 0.8 at 600 ºC. For the (Sr1-xLax)6Ti2Nb8O30 ceramics, donor doping by La provided extra electrons and increased the electrical conductivity for STN. La also caused extra phonon scattering leading to low thermal conductivity. Thus, the ZT was improved by around 29% to 0.15 at 600 ºC by 15-wt% La doping of Sr6Ti2Nb8O30.

Date of Award	1 Aug 2019
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Robert Freer (Supervisor) & Feridoon Azough (Supervisor)