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Title: Tungsten bronze structured thermoelectric oxide for energy generation
Author: Kitajevs, Jevgenijs
ISNI:       0000 0004 8501 2149
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2019
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The morphology, chemical composition, crystal structure and thermoelectric properties of SrₓBa₁₋ₓNb₂O₆₋σ (SBN) were investigated for thermoelectric applications. At first, SrₓBa₁₋ₓNb₂O₆₋σ (where x = 0.48, 0.61 or 0.82) and Sr₀.₆₁₋yBa₀.₃₉₋yLayNb₂O₆₋σ (SBN61; where y = 0.001, 0.01, 0.025, 0.05 or 0.1) polycrystalline ceramics were produced with use of 0.5% MnO or B2O3 sintering aids, or without sintering aids, using classical mixed oxide route approach. The majority of ceramics (except Sr₀.₆₁₋yBa₀.₃₉₋yLayNb₂O₆₋σ with 0.1% and 1% La substitution) were air sintered at first and then subjected to reduction in Ar/H2 5% gas mixture for various times - from 6h to 36h, dependent on the sample. SEM+EDX, High-Resolution XRD and XPS techniques were used to characterise the morphology, chemical composition and crystal structure of the ceramics. Thermoelectric figure of merit was calculated from electrical conductivity, Seebeck coefficient and total thermal conductivity measure-ments. Some ceramic powders were also subjected to particle size analysis. Analysis of air sintered and reduced for 6h, 12h, 24h and 36h SBN61 samples with addition of 0.5% of MnO or B2O3 sintering aids revealed MnO's effectiveness for hindering abnormal grain growth in Strontium Barium Niobate materials, due to its reactivity with Nb. B2O3 showed the inability to hinder abnormal grain growth in samples subjected to long periods of reduction in Ar/H2 5% gas mixture. An indirect approach for SBN61's A-site filling/doping with Sr²⁺ and Ba²⁺ cations using MnO was also discovered that allowed a significant improvement of SBN61's thermoelectric properties. The best SBN61 sample managed to show zT ≈ 0.194 at 870 K, which is one of the highest reported zT from the polycrystalline SrₓBa₁₋ₓNb₂O₆₋σ sample. Successful morphology analysis, chemical analysis, crystal structure refinement and analysis of thermoelectric parameters was conducted, assessing the effects of sintering aids and different reduction times. Analysis of air sintered and 12h, and 24h reduced SBN (where x = 0.48, 0.61 or 0.82) with the addition of 0.5% MnO sintering aid was performed, revealing a moderate difference in terms of thermoelectric performance between reduced SBN61 and SBN48 samples. Morphologically and in terms of crystal structure, SBN61 and SBN48 were similar. On the other hand, analysis of air sintered and reduced SBN82 samples exhibited presence of duplex phase structure, especially SBN82 showed presence of SBNx crystal structure with SrNb2O6 phase; in both air sintered states and reduced states. Additionally, it was discovered that reduction process in Ar/H₂ 5% gas mixture tends to favour formation of SBNx phase; as SBN70 showed duplex crystal structure after air sintering and single XBN crystal structure after reduction. And finally, effects of La³⁺ substitution to Sr₀.₆₁₋yBa₀.₃₉₋yLayNb₂O₆₋σ were assessed that showed the effectiveness of this dopant in terms of increase in electrical conductivity and reduction in total thermal conductivity, as well as improvement of SBNx stability during processing. It was also found out that even slightly substituted SBN61 samples (e.g. substituted with 0.1% La) and subjected to direct sintering in Ar/H₂ 5% gas mixture do allow getting relatively dense samples (i.e. ≈ 95%), with very good zT values (i.e. ≈ 0.155 at 870 K) that are on par with zT values shown by 10% La substituted SBN61 samples.
Supervisor: Freer, Robert ; Azough, Feridoon Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Thermoelectric ; Oxide Semiconductor ; Strontium Barium Niobate ; Tungsten Bronze