Samples of Ba β-Al<SUB>2</SUB>O<SUB>3</SUB> with the stoichiometries BaAl<SUB>9</SUB>O<SUB>15 </SUB>to BaAl<SUB>14</SUB>O<SUB>22</SUB> were prepared. Mixed β-Al<SUB>2</SUB>O<SUB>3</SUB> materials consisting of Na β-Al<SUB>2</SUB>O<SUB>3</SUB> + Ba β-Al<SUB>2</SUB>O<SUB>3</SUB> or Na β-Al<SUB>2</SUB>O<SUB>3</SUB> + strontium magnetoplumbite were also produced. Some samples of Ba β-Al<SUB>2</SUB>O<SUB>3</SUB> with 12.5 wt.% zirconia added were also produced. All materials were prepared using a solid state route using Al<SUB>2</SUB>O<SUB>3 </SUB> and the relevant carbonate as the starting materials. After rationalising the calcining and sintering route for Baβ-Al<SUB>2</SUB>O<SUB>3</SUB> 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-xAl<SUB>2</SUB>O<SUB>3</SUB> (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 BaAl<SUB>14</SUB>O<SUB>22</SUB> (Ea = 1.8eV) to BaAl<SUB>9</SUB>O<SUB>15</SUB> (Ea = 2.3eV) with BaAl<SUB>12</SUB>O<SUB>19 </SUB>having a roughly intermediate value for both conductivity and activation energy. Thermodynamic experiments for calculating activity of Na<SUB>2</SUB>O in the mixed β-Al<SUB>2</SUB>O<SUB>3</SUB> suggest that these are single phases for both the Na-Ba and the Na-Sr systems, and that the variations of activity of Na<SUB>2</SUB>O with composition exhibit positive deviation from ideality. Despite the similarities in structures between the different β-Al<SUB>2</SUB>O<SUB>3</SUB>, it is possible to distinguish between the Na and the Baβ in the Na-Ba mixed β-Al<SUB>2</SUB>O<SUB>3</SUB> and likewise for the Na-Sr β-Al<SUB>2</SUB>O<SUB>3</SUB>. Therefore it is suggested that in mixed β-Al<SUB>2</SUB>O<SUB>3</SUB>, 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.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:598699 |
| Date | January 2000 |
| Creators | Dunn, S. |
| Publisher | University of Cambridge |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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