Disputas: Anuradha Machina Ashok

Prøveforelesning

Bedømmelseskomité

Margareta Sundberg, Stockholm universitet, Sverige
Olivier Joubert, IMN, Frankrike
Tore Amundsen, Universitetet i Oslo

Leder av disputas:  Johan Taftø

Veileder:  Arne Olsen, Truls Norby

Sammendrag

Materials which are proton or oxide ion conductors are important components for cleaner energy conversion, future hydrogen economy and better and possibly CO₂ free use of fossil fuels like natural gas. Complex perovskites of type A₄(B'₂B"₂)O₁₁ where A, B' = alkaline earth metal and B" = Nb, Ta have been previously shown to exhibit high oxide ion conductivity at high temperatures and proton conductivity under water vapour-containing atmospheres at lower temperatures. This has been attributed to the presence of oxygen vacancies in the crystal structure in the dry state. When annealed with water these vacancies are filled and replaced by hydroxide ions. Most of the structural studies done so far (mainly using X-Ray diffraction (XRD) technique which gives an average information) claim the presence of face centered cubic structure having space group Fm3m and cell dimension around 8.4 Å with random ordering of the oxygen vacancies in this type of perovskites.

In the present study different perovskites in dry and wet state were studied with A = Ba, Sr , B' = Ba, Sr, Ca and B" = Nb, Ta with special emphasis on the local arrangement of the different types of atoms and vacancies. The present work was carried out by using different microscopy and diffraction techniques. X-ray powder diffraction (XRD) was used to get information about the average structures whereas analytical transmission electron microscopy (TEM) was applied to characterize the local structures.

The resulting detailed information on structure and ordering can help understand hydration and dehydration behaviours as well as variations in oxide ion and proton conductivity in complex perovskites. In this way, the model class of materials studied here may lead on to predictive understanding of high ionic ionic conductivity in inherently defective perovskites in a more general sense.

The research was carried out in collaboration with the Structure Physics group at the Physics Department and the Functional and Inorganic Materials group at the Chemistry Department with Prof. Arne Olsen as main supervisor and Prof. Truls Norby as co-supervisor. The project was funded by FUNMAT (Functional Materials and Nanotechnology) programme at the University of Oslo.

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