Microstructure of cobalt oxide doped sintered ceria solid solutions

Abstract

The sintering of ceria solid solutions, such as Ce 0.9 Gd 0.1 O 1.95 (CGO10), is strongly promoted by the addition of 1 cat% of cobalt oxide, lowering the maximum sintering temperature by 200 • C and triplicating the maximum densification rate. This change in sintering behavior results from cobalt ion segregated at the grain boundaries. An average cobalt ion boundary coverage is at maximum 3.0 ± 1.9 at/nm 2 and is shown to depend on the cooling rate. Coverage by segregated gadolinium is also found and amounts to 13.2 ± 11.4 at/nm 2 for a slowly cooled sample. From cobalt excess measured at the boundary, an estimated concentration of only 0.06 cat% of cobalt oxide is necessary to promote the sintering effect. The remaining amount of cobalt oxide is found in triple points and as particles in clusters. It is expected that the amount of cobalt oxide necessary for fast densification can be reduced with a doping process that distributes the additives more homogeneously.

Grain Growth of Micron-Sized Grains in Undoped and Cobalt Oxide Doped Ceria Solid Solutions

“…In case of CoO additions, one has to keep in mind that in contrast to Gd 2 O 3 , the CoO has only a negligible solubility in CeO 2 and CGO20 47 and the CoO is concentrated in the grain boundaries, as has been reported in a previous study. 48 The increased boundary mobility is therefore due to a second phase with cobalt and gadolinium ions in excess in the grain boundaries. Obviously, a boundary layer rich in cobalt oxide acts as a fast diffusion path parallel to the grain boundary thus enhancing shrinkage during sintering.…”

“…4, the solute drag effect of Gd persists at the boundary independent of the addition of cobalt oxide. It has been shown that the cobalt oxide grain boundary layer is sufficiently thin, 48 so that the mass transport across the grain boundaries is not hampered. The same conclusion was drawn in several grain growth studies of zirconia as summarized by Chen.…”

New solid state oxygen and hydrogen conducting materials. Towards their applications as high temperature electrochemical devices and gas separation membranes

“…SOFC systems have demonstrated one of the highest efficiencies (ranging from 40 to 70 %) of any power generation system, combined with very low environmental impact (virtually no acid gas or solid emissions). [42][43][44] The modular and solid state construction of SOFC devices allows flexibility to build stationary power generation plants in the 2 kW to hundreds of MW capacity range. [42] SOFCs are able to work not only with H 2 fuel, but also several hydrocarbons as CH 4 , alcohols and CO, giving rise to CO 2 and H 2 O as residue.…”

“…[42] It is worthy to note the opposite behavior of the Tb doped sample, whose E a decreases when Co is added. It may be attributed to the partial incorporation of Co in the ceria solid phase and increasing in oxygen vacancy concentration as will be studied in detail in Chapter 5.…”

“…[27] Moreover, Pr and Tb with cobalt addition show enhanced total conductivity due to (a) the reducibility of the material at lower temperature with respect to the Co-free samples; (b) the electronic conductivity promotion; and (c) the higher sample density and grain packing due to the high sintering activity of Co-containing ceria observed by SEM (Figure 3.5). [27,31,42] Relationship between structural and transport properties…”