Advances in Sintering Science and Technology

At low oxygen partial pressures and high temperatures Gd-doped ceria can be reduced and the material becomes electronic conducting (e.g. 0.08 Scm" at 800°C at a ρθ2 of 10" arm for Ceo.9Gdo.i 01.95.5 CGO 10). These properties make CGO attractive for use in oxygen membranes above 600°C. The sintering temperature of CGO ceramics might be significantly reduced, if a sintering atmosphere with very low oxygen partial pressure is applied (for example a p02 of 10" arm or below). In the present work, the densification behaviour of CGO 10 was investigated in reducing atmosphere and in air. Samples were prepared by tape casting and lamination of the single layers into multi-layers and by die pressing. A dilatometer was used to measure the sample shrinkages from room temperature to 1773 K with different constant heating rates. Based on the sintering results of pressed samples the activation energy for densification was determined. The activation energy for densification of CGO 10 can be reduced significantly from 770±40 to 300±40 KJ/mol by switching atmosphere from air (pO2=0.21 arm) to highly reducing conditions (pU2 down to 10" arm), which indicated enhanced densification behaviour of CGO 10 in reducing atmosphere during early stage sintering. INTRODUCTION Ceria based solid solutions have been investigated intensively as promising electrolyte materials for intermediate temperature solid oxide fuel cells (IT-SOFC), as cathode barrier layers in SOFC and as membrane material for oxygen separation membranes. The doping with gadolinium leads to one of the highest ionic conductivities in ceria among different other dopants at intermediate temperatures (500-600 °C). Cerium Gadolinium Oxide (CGO) has therefore been proposed as electrolyte material for stainless steel supported fuel cells. The sintering of ceria to full density requires relatively high sintering temperatures, as high as 1300°C tol600°C, depending on raw powders and processing. For many processes, such as the manufacturing of solid oxide fuel cells or membranes, it would be beneficial, if the sintering temperature to achieve dense CGO ceramics could be reduced significantly. Several studies have been under-taken on the addition of dopants to lower the sintering temperature of CGO ceramics in air'·'. The sintering behaviour of CGO is expected to be quite different in reducing atmosphere due to the reduction of Ce to Ce and the related change in oxygen vacancy concentration, which is expected to have a significant influence on the sintering kinetics. The influence of sintering atmosphere has so far only rarely been investigated. J.-G. Li et al." have reported an increase in density and grain growth of nano crystalline yttria doped ceria ceramics, if the atmosphere was switched to more reducing atmosphere, e.g. from oxygen to air. Grain growth and enhanced densification in CGO compared to un-doped ceria was explained by reduction of some Ce to Ce and a correlated formation of oxygen vacancies, which should cause rapid grain boundary migration due to changed grain boundary energies. It is well known, that the CGO lattice shows a volume expansion upon reduction and volume reduction during re-oxidation'. This paper investigates the densification kinetics of Ceo.9Gdo.1O1.95-6 at relatively low oxygen partial pressures (ρθ2 = <10" arm) and compares the results to sintering in air for pressed and tape casted structures. Using a dilatometer, we sintered the CGO10 with 4 different constant heating rates and we