Publication details

Authors: Yarmolenko, S.; Sankar, J.; Bernier, N.; Klimov, M.; Kapat, J.; Orlovskaya, N. 
Title: Phase Stability and Sintering Behavior of 10 mol % Sc2O3-1 mol %CeO2-ZrO2 Ceramics 
Type: Journal Article 
Publisher: Journal of Fuel Cell Science and Technology 
Year: 2009 
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DOI: Artn 021007 
Abstract: The phase composition and sintering behavior of two commercially available 10 mol % Sc2O3-1 mol % CeO2-ZrO2 ceramics produced by Daiichi Kigenso Kagaku Kogyo (DKKK) and Praxair have been studied. DKKK powders have been manufactured using a wet coprecipitation chemical route, and Praxair powders have been produced by spray pyrolysis. The morphology of the powders, as studied by scanning electron microscopy, has been very different. DKKK powders were presented as soft (similar to 100 mu m) spherical agglomerates containing 60-100 nm crystalline particles, whereas the Praxair powders were presented as sintered platelet agglomerates, up to 30 mu m long and 3-4 mu m thick, which consisted of smaller 100-200 nm crystalline particles. X-ray diffraction analysis has shown that both DKKK and Praxair powders contained a mixture of cubic (c) and rhombohedral (r) phases: 79% cubic +21% rhombohedral for DKKK powders and 88% cubic +12% rhombohedral for Praxair powders. Higher quantities of the Si impurity level have been detected in Praxair powder as compared to DKKK powder by secondary ion mass spectroscopy. The morphological features, along with differences in composition and the impurity level of both powders, resulted in significantly different sintering behaviors. The DKKK powders showed a more active sintering behavior than of Praxair powders, reaching 93-95% of theoretical density when sintered at 1300 degrees C for 2 h. Comparatively, the Praxair powders required high sintering temperatures at 1500-1600 degrees C. However, even at such high sintering temperatures, a significant amount of porosity was observed. Both DKKK and Praxair ceramics sintered at 1300 degrees C or above exist in a cubic phase at room temperature. However, if sintered at 1100 degrees C and 1200 degrees C, the DKKK ceramics exist in a rhombohedral phase at room temperature. The DKKK ceramics sintered at 1300 degrees C or above exhibit cubic to rhombohedral and back to cubic phase transitions upon heating at a 300-500 degrees C temperature range, while Praxair ceramics exist in a pure cubic phase upon heating from room temperature to 900 degrees C. However, if heated rather fast, the cubic to rhombohedral phase transformation could be avoided. Thus it is not expected that the observed phase transitions play a significant role in developing transformation stresses in ScCeZrO2 electrolyte upon heating and cooling down from the operation temperatures. 
Keywords: ceramics, cerium compounds, cooling, electrolytes, heating, phase transformations, powders, precipitation (physical chemistry), pyrolysis, scandium compounds, scanning electron microscopy, sintering, x-ray diffraction, electrical-conductivity, ionic-conductivity, doped zirconia, electrolyte, scandia, system, oxide, ceo2, microstructure, diffraction