Abstract
Due to its high oxygen ion conductivity at elevated temperatures, samarium-doped ceria (SDC) is a very promising material for application in solid state electrochemical devices and especially in the electrolytes of solid oxide fuel cells. Several prior studies have reported a further improvement in the ionic conductivity of SDC on doping with small amounts of strontium. It is suggested that strontium acts as a sintering aid—improving the microstructure of SDC—and as a scavenger of silicon impurities, decreasing its tendency to form resistive phases at grain boundaries. However, because of the range of preparation methods and the resulting differences in microstructure and silicon levels, some inconsistencies exist in the literature. Furthermore, the effect of strontium on the intrinsic (bulk) conductivity of SDC is not often discussed. To address these issues, a systematic, combined microstructural and conductivity study has been performed on a compositional series with a range of strontium contents, Ce0.8−xSm0.2SrxO2−δ (x = 0, 0.002, 0.005, 0.01, 0.02, 0.03, 0.04). A low temperature synthesis affording products with low silicon was employed. Total bulk and grain boundary conductivity data were obtained over a wide temperature range. Increasing strontium content caused a general decrease in total and intrinsic conductivity, but there was an improvement in grain boundary conductivity at the lowest strontium levels. These results were interpreted by reference to the microstructures using, among other parameters, the blocking, and normalised blocking, factors.
Highlights
To reduce the potential influence of impurities and of variations in starting powder microstructure on the conductivity results, the ceramic precursor powders were synthesised by the citrate–nitrate method described by Kosinski and Baker [25]
In order to see the trend in conductivity with strontium content more clearly, Figure 11 shows the variation in the total, bulk and grain boundary conductivity at a single temperature, 300 ◦ C
Silicon impurity levels were found to be below 10 ppm by XRF, and this was attributed to the synthesis method used
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Initial studies by Lane et al [10] and Kim et al [12] considered strontium co-doping as a method to reduce the deleterious effects of silicon impurities on the conductivity of gadolinium doped ceria [10,12]. Comparing the most conductive strontium co-doped sample to the singly doped sample in each study shows that in most cases, the increase in total conductivity was as a result of an increase in the grain boundary conductivity only. To reduce the potential influence of impurities and of variations in starting powder microstructure on the conductivity results, the ceramic precursor powders were synthesised by the citrate–nitrate method described by Kosinski and Baker [25]. It was decided to investigate strontium doping levels below this value in order to see whether there was an advantage to doping in this range
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