Microstrain and self-limited grain growth in nanocrystalline ceria ceramics
Introduction
In the last years, gadolinia-doped ceria (CGO) electrolytes have drawn much attention as electrolytes for solid oxide fuel cells (SOFC) operating at intermediate temperatures due to their high-ionic conductivity compared to state-of-the-art yttria stabilized zirconia (YSZ) electrolytes [1], [2], [3], [4]. The use of thin film electrolytes minimizes the ohmic loss across the electrolyte and SOFC operation at lower temperatures is possible as the power output is increased [5], [6]. YSZ is used in micro solid oxide fuel cells (μ-SOFC) in the form of thin film electrolytes with thicknesses of some hundred nanometer [7]. In an earlier paper Gödickemeier and Gauckler [8] highlighted the conditions when the mixed conducting ceria can be used as electrolytes in SOFC avoiding short circuiting through its electronic conductivity. In prior SOFC-related work, ceria thin films were deposited by physical vapor deposition (PVD) [9], chemical vapor deposition (CVD) [10], spin coating [11] and spray pyrolysis [5], [12]. The latter technique offers the possibility to produce dense thin films in the amorphous state. These films can then be crystallized without the development of columnar grains by heat treatment. Pulsed laser deposition results in thin films with an average grain size of roughly ∼15 nm after deposition. Both methods allow the preparation of dense material and grain growth kinetics can be studied.
Recently grain growth kinetics were reported for ceria with a grain size in the micron and submicron regime [13]. Indications for self-limited grain growth were shown in ceria with a grain size of 50–250 nm. Whereas the coarser grained material obeyed the classical parabolic grain growth law, the fine microstructures attained a limited grain size after a short dwell at low temperature.
The objective of this study is to investigate the grain growth of nanocrystalline CGO solid solutions at the potential operating conditions of μ-SOFCs and gas separation membranes, e.g., 500–800 °C. The results will elucidate whether the grain growth kinetics that prevail differ from those associated with the classical view of grain-boundary-controlled migration kinetics, and whether CGO thin films develop microstructures that are sufficiently stable to allow extended use in μ-SOFC devices.
Section snippets
Grain growth kinetics
More than 50 years ago, Burke and Turnbull deduced the kinetics of isothermal grain growth from the movement of grain boundaries for polycrystalline materials [14], [15]. The authors assume that the driving force for grain boundary migration results from the grain face intersections at nonequilibrium angles and from the strong curvatures at grain faces. Both effects are connected, since if grain intersections approach equilibrium angles, strong grain curvature would be introduced, or, if the
Thin film preparation and characterization
Undoped ceria (CeO2) and gadolinia-doped ceria, Ce0.78Gd0.22O1.89, (CGO) thin films were prepared by an airblast spray pyrolysis technique (SP) and by pulsed laser deposition (PLD). The substrate material was in all cases a sapphire single crystal (Stettler, Switzerland) with a surface orientation parallel to the substrate. In spray pyrolysis a precursor solution is atomized to very fine droplets by air pressure. Precursor droplets hitting the heated substrate undergo pyrolytic
Thermal treatment of spray pyrolysis thin films
The chemistry of our spray pyrolysis thin films was analyzed as a function of temperature by DSC and TG measurements whereby outgassing species were detected by mass-spectrometry during heating and cooling (Fig. 1). When thin films are deposited by spray pyrolysis the material is amorphous and crystallization occurs during the first annealing for temperatures above the film deposition temperature [29]. Amorphous thin films were scratched off from the substrate and the obtained powder (with an
Summary and conclusion
Amorphous, dense and crack free undoped and gadolinia-doped ceria thin films were deposited by spray pyrolysis and PLD on sapphire single crystal substrates. Dense amorphous (SP) and nanocrystalline films were obtained. Isothermal grain growth studies show that the grains grow within the first 5–10 h of dwell for temperatures below 1100 °C and average grain sizes below 140 nm until metastable microstructures with limited grain sizes are established. At higher annealing temperatures common
Acknowledgments
We thank Dr. Wörle and Prof. Nesper from the Inorganic Solid State Chemistry laboratory of ETH Zurich (Switzerland) for the use of the X-ray diffraction, the colleagues of the Laboratory of Surface Science and Technology of ETH Zurich (Switzerland) for XPS analysis, and Dr. Eva Jud for helpful discussion.
The project was supported by the Swiss Federal Office of Energy, Project Number 100430.
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