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Title: Synthesis and characterisation of barium strontium cobalt iron oxide mixed ionic and electronic conductors
Author: Norman, Colin John
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2013
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Oxygen separation membrane technology is an alternative to cryogenic distillation for the production of oxygen. Barium strontium cobalt iron oxide (BSCF, Ba0.5Sr0.5Co0.8Fe0.2O3-d) is one of the candidate membrane materials for this application. A robust wet-chemical synthetic route for the synthesis of BSCF has been developed. The route comprises reverse precipitation from a mixed metal nitrate solution at pH> 10 and uses a minimum amount of reagents. In particular there are no organic additives which need to be burnt off. All the metal ions precipitated simultaneously but a single phase was not then formed. The phases formed during precipitation, drying and calcining varied with the experimental conditions used. It was shown that ~80% of the barium and strontium precipitated as their mixed carbonate. However, on sintering, a single cubic phase material was formed. The process was capable of variation in the BSCF composition and the addition of copper to the system. The electrical and oxygen permeation properties were measured. The relationship between them was found to be dependent on the lattice parameter, tolerance factor and oxygen non-stoichiometry (d). Values of d greater than 0.5 at high temperature were found to give poor oxygen permeation and an increase in resistance although they did not decompose to the ordered brownmillerite structure. The addition of copper to BSCF by substituting it for iron and cobalt enhanced grain growth during sintering. However, except at the lowest addition level of 0.05 on the B-site, it did not show any oxygen permeation. High temperature x-ray diffraction (HTXRD) and x-ray photoelectron spectroscopy (XPS) studies were carried out. They showed that the thermal expansion coefficient increased at ~400° as loss of oxygen from the material started. As the materials were heated in the XPS surface carbonates were removed and the surface concentration of Ba and Sr ions decreased at 3-500°C although on further heating to 800°C their concentration returned to close to the original values.
Supervisor: Leach, Colin Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available