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Title: Sulphur sensors for high oxygen environments
Author: Dunn, S.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2000
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Samples of Ba β-Al2O3 with the stoichiometries BaAl9O15 to BaAl14O22 were prepared. Mixed β-Al2O3 materials consisting of Na β-Al2O3 + Ba β-Al2O3 or Na β-Al2O3 + strontium magnetoplumbite were also produced. Some samples of Ba β-Al2O3 with 12.5 wt.% zirconia added were also produced. All materials were prepared using a solid state route using Al2O3 and the relevant carbonate as the starting materials. After rationalising the calcining and sintering route for Baβ-Al2O3 these materials and the mixed ionic conductors were tested by XRD, Impedance Spectroscopy and thermodynamic methods to determine the structure, phase composition or conduction characteristics. Although 2 distinct β-alumina phases (β(I) and β(II) can be identified within the BaO-xAl2O3 (4.5 < x < 7) composition by XRD, galvanic cell measurements reveal only a single phase within the above composition. This can be explained if we consider β(I) and β(II) structures as part of a single phase but intergrown as stacking defects, made up of a sufficient number of unit cells within each stack to be revealed as distinct (although related) structures by XRD. A consequence of this is a decrease in conductivity and increase in activation energy from BaAl14O22 (Ea = 1.8eV) to BaAl9O15 (Ea = 2.3eV) with BaAl12O19 having a roughly intermediate value for both conductivity and activation energy. Thermodynamic experiments for calculating activity of Na2O in the mixed β-Al2O3 suggest that these are single phases for both the Na-Ba and the Na-Sr systems, and that the variations of activity of Na2O with composition exhibit positive deviation from ideality. Despite the similarities in structures between the different β-Al2O3, it is possible to distinguish between the Na and the Baβ in the Na-Ba mixed β-Al2O3 and likewise for the Na-Sr β-Al2O3. Therefore it is suggested that in mixed β-Al2O3, the two structures can intergrow by sharing the spinel blocks into unit cells large enough to be identified by XRD, yet dissolve into each other to form a single phase in the thermodynamic sense.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available