Study of compaction and sintering of nanosized oxide powders by in situ electrical measurements and dilatometry: Nano CeO[subscript 2]—case study

Abstract

Densification and sintering of CeO[subscript 2] nanoparticles and their electrical properties were simultaneously studied as a function of temperature in controlled atmosphere using a modified dilatometer. CeO[subscript 2] nanoparticles simultaneously shrink and become more resistive upon initial heating, associated with desorption of water. The electrical conductance G at 300–550 °C revealed a pO[subscript 2] dependence described by log (G) = A + n × log (pO[subscript 2]) with n ~ −1/6, consistent with n-type conduction. The results were analyzed with a defect equilibrium model based on the reduction of ceria and formation of doubly ionized oxygen vacancies and electrons. The activation energy was found equal to (1.3 ± 0.1) eV, which results in an enthalpy of reduction of (2.7 ± 0.4) eV, considerably lower than that for bulk ceria (~4.5 eV). The coarsening of particles during heat treatment at 800 °C were analysed assuming grain boundary diffusion-limited sintering. Although the coarsened powder shows a similar pO[subscript 2] dependence, the activation energy was considerably higher (1.9 ± 0.1) eV, leading to a reduction enthalpy of (4.5 ± 0.4) eV. The decrease in the enthalpy of reduction with decreasing particle size is consistent with the increasing fraction of oxide ions residing at the surface. Alternate interpretations based on space charge effects and surface adsorption/desorption were considered and found to be less consistent with the experimental results.