Grain boundary induced compositional stress in nanocrystalline ceria films

Oxygen removal from non-stoichiometric oxides produces volume changes that can lead to substantial stresses when these materials are constrained in a thin film or coating. To measure this effect, in situ stress measurements were conducted on ceria films during oxidation-reduction cycling, at temperatures of 470-515 degrees C. Large reversible stresses approaching 0.5 GPa were observed at these moderate temperatures. These compositional stresses increased with decreasing grain size, and were larger than expected based on literature values for the volume expansion and elastic modulus. These results demonstrate that grain boundary effects dominate compositional stresses in nanocrystalline material. This observation is consistent with the idea that space charge effects near surfaces and interfaces produce substantial stresses. Analysis of the results also demonstrates that the measured grain boundary contribution is only a weak function of temperature, and several possible explanations for this observation are discussed. (C) 2012 Elsevier B.V. All rights reserved.