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Title: TEM studies of interfaces in fuel cell materials
Author: Jasper, Alexander James
ISNI:       0000 0004 2688 473X
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2010
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The grain boundary properties of CGO10 (Ce0.9Gd0.1O1.95) and CGO20 (Ce0.8Gd0.2O1.95) have been investigated by impedance spectroscopy and high resolution scanning transmission electron microscopy-electron energy loss spectroscopy (STEM-EELS). High density polycrystalline ceramic specimens with a range of grain sizes (0.17-2.7μm) have been prepared from high purity (SiO2 ~120-210 ppm) commercially available powders. Impedance measurements obtained for these ionic conducting materials were interpreted using the brick layer model. The specific grain boundary conductivity [delta*GB]was found to be 2-3 orders of magnitude lower than the grain bulk conductivity [delta G]. The grain boundary blocking effect in these materials has been attributed to the formation of grain boundary space charge layers. The brick layer model was used to estimate the space charge layer thickness (δ) from impedance spectroscopy measurements and was found to be in the range 1-2 nm. High resolution STEM-EELS measurements were carried out over a number of grain boundaries in CGO10 and CGO20. These measurements showed an increase in the gadolinium to cerium and oxygen to cerium atomic ratios (Gd:Ce and O:Ce respectively) at the grain boundary region. These findings are in accordance with the space charge layer model. From these measurements the space charge layer thickness (δ) was shown to be in the range 1-2 nm. Analysis of the cerium M4,5-edge fine structure showed the cerium oxidation state to remain unchanged in the space charge layer in all grain boundaries studied.
Supervisor: McComb, David ; Kilner, John Sponsor: EPSRC ; Royal Society of Chemistry
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral