Sintering and properties of nanosized ceria solid solutions
Introduction
Solid electrolytes exhibiting high oxygen ion conductivity are of special interest for their application in electrochemical devices such as solid oxide fuel cells (SOFCs), oxygen separation membranes, methane gas conversion reactors, etc. In the case of SOFCs, the state-of-the-art material is yttria stabilized zirconia which exhibits sufficient ionic conductivity at operating temperature as high as 900°C to 1000°C [1]. This high temperature requires expensive materials for insulation and interconnectors in SOFCs. It is therefore highly desirable to lower the operating temperature to intermediate temperatures ∼700°C, that requires either new system concepts such as thin-film electrolyte SOFCs or new electrolyte materials with higher oxygen ionic conductivity. Ceria solid solutions (CeO2(ss)) have been shown to exhibit 4–5 times higher ionic conductivities at intermediate temperature [2] compared to zirconia solid solutions. Although ceria-based electrolytes are slightly reduced at low oxygen partial pressures and hence develop an increasing electronic conductivity [3], it has been shown that a SOFC can be operated at temperatures as low as 700°C at high power output and with high efficiency [4], [5]. Therefore, CeO2(ss) are attractive electrolytes for SOFCs.
One problem arising with ceria-based electrolytes for SOFCs is that conventional sintering to full density requires temperatures exceeding 1300°C [6], [7], [8], [9]. The rather high grain growth rates at those temperatures result in large grains and therefore poor mechanical stability. This hinders the use of CeO2 in SOFC applications despite its superior electrochemical performance.
The aim of the present paper is to elucidate the sintering behavior of CeO2(ss) with the addition of small amounts of transition metal oxides. We will also show results on their influence on the grain size of the sintered body as well as on its mixed ionic electronic conductivity. A good example of such a material is commercially available nanosized CeO2 powder doped with 20 mol.% Gd2O3.
Section snippets
Sample preparation
Ce0.8Gd0.2O2−x (denoted as CGO, No. 595250A, Rhodia, Frankfurt, Germany) with a crystallite size of 20 nm and a specific surface area of about 26 m2 g−1 was used as starting powder to which the dopant was added in the form of nitrates (Co(NO3)2–6H2O, Fluka, Buchs, Switzerland). Doping of CGO with several concentrations (0–5 mol.%) of the dopant was performed by first dispersing the CGO powder in ethanol (Fluka, puriss. p.a.), using an agate mortar. A desired amount of dissolved dopant nitrate
Sintering and microstructure
The sintering of CGO doped with different concentration of Co3O4 is depicted in Fig. 1, Fig. 2. The shrinkage rate [d(ΔL/L0)/dT] (Fig. 1) and the relative density (Fig. 2) are plotted vs. temperature for different cobalt concentrations (0–2 mol.%) at a constant heating rate of 10 K min−1. Cobalt oxide doping is extremely effective in promoting the densification of CGO when exceeding a dopant concentration of 0.5 mol.%. The sintering temperature where >98% of the theoretical density is reached (T
Summary and conclusion
Dense submicron CeO2(ss) can be obtained by sintering nanoscaled powders. The densification kinetics during sintering is strongly enhanced by the addition of small amounts of Co3O4. Sintering temperatures are lowered well below 900°C for >99% dense samples when adding 1 mol.% of Co3O4. We found a cobalt rich amorphous 2–4-nm-thick grain boundary layer in short-time sintered material. The electrochemical performance in terms of oxygen ion conductivity and electrolytic domain boundary remains
Acknowledgements
Financial support from the Swiss Priority Program on Materials (PPM) of the board of the Swiss Federal Institutes of Technology is gratefully acknowledged. The authors wish to thank present colleagues in the Chair of Nonmetallic Materials for many useful discussions.
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