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Title: Low temperature sintering of solid oxide fuel cell electrolytes
Author: Lewis, Gene Stacey
ISNI:       0000 0001 3608 9359
Awarding Body: University of London
Current Institution: Imperial College London
Date of Award: 2002
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Solid oxide fuel cells provide a clean and efficient means to produce electricity. Gadolinium oxide doped ceria is considered to be an ideal electrolyte material for SOFC's operating at temperatures between 500-600°C. Typically, this material is sintered to high density at approximately 1300°C. Reducing this temperature to ~1000°C will enable ferritic stainless steel to be used as a cell supporting material, thus significantly reducing cost and potentially enhancing the mechanical reliability of the stack. The aim of this study is to devise a method for sintering Gd doped ceria to high density at temperatures between 900-1000°C with no detrimental effects on its electrical properties. A number of experimental techniques were employed to characterise the sintering process and the resulting materials including, a.c. impedance spectroscopy, dilatometry, four-point d.c resistivity, Raman spectroscopy, transmission electron microscopy and x-ray photoelectron spectroscopy. Gd doped ceria was successfully sintered to high density at < 1000°C by additions of cobalt oxide. A 2cat%Co addition to commercially available 10mol%Gd doped ceria reduced the 'gas tight' densification temperature to 930°C. Furthermore, the Co addition was not detrimental to the electrical properties of Gd doped ceria under SOFC operating conditions at 500-600°C. Copper oxide was also shown to be a suitable sintering aid. The mechanism of enhanced sintering rate has been explored in detail and does not conform to any of the standard models. Evidence is presented to indicate that redistribution of the Gd dopant is also involved. The Co addition reduces the grain size of the sintered product, but this is mainly due to the reduced sintering temperature and not to reduced grain boundary mobility by the additive which is preferentially segregated to the grain boundaries. By careful control of the amount of additive and the heat treatment schedule it has proved possible to actually improve the ionic conductivity of Gd doped ceria, probably by optimising the lattice parameter. Co was also found to be effective in reducing the sintering temperature of yttria-stabilised zirconia.
Supervisor: Atkinson, Alan ; Steele, Brian Sponsor: EPSRC
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available