Study of compaction and sintering of nanosized oxide powders by in situ electrical measurements and dilatometry: Nano CeO2—case study

Abstract

Densification and sintering of CeO 2 nanoparticles and their electrical properties were simultaneously studied as a function of temperature in controlled atmosphere using a modified dilatometer. CeO 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 2 dependence described by log (G)=A+n×log (pO 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 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.

CeO2 Nanorods and Nanocubes: Impact of Nanoparticle Shape on Dilatometry and Electrical Properties

“…The total sample length change, over the whole duration of an experiment (18 days), was only 0.8 μm, below 2% of the initial thickness (~50 μm, see below). This change is much lower than for any other studied nanoceria samples and a very distinctive feature of nanocube samples; it is likely related to the very high initial packing density attainable with cubes.…”

“…There is also limited work on nanosized CeO 2 powders . In previous studies on spherical nanodimensioned particles, their P O2 ‐dependent oxygen deficiency was related to preferential surface reduction or oxygen adsorption/desorption, but to our knowledge, the electrical properties of variously shaped nanoparticles have not been previously studied. Cerium dioxide is thus a model material for the investigation of shape dependence of the electrical properties of nanosized oxide particles.…”

In situ dilatometric and impedance spectroscopic study of core–shell like structures: insights into the exceptional catalytic activity of nanocrystalline Cu-doped CeO2

“…6), is equal to (0.9 AE 0.1) eV. 43 Substituting this value of D s H and the reported value for D m H into eqn (9) results in a value for D red,CeO 2 H 0 of (1.5 AE 0.3) eV, about one third of the value for undoped coarsened ceria powder of $40 nm particle size ((4.5 AE 0.4) eV (ref . 43 Substituting this value of D s H and the reported value for D m H into eqn (9) results in a value for D red,CeO 2 H 0 of (1.5 AE 0.3) eV, about one third of the value for undoped coarsened ceria powder of $40 nm particle size ((4.5 AE 0.4) eV (ref .…”

“…43 Given that electrons in ceria move via a thermally activated small polaron hopping mechanism, 44 one must take this into account in interpreting the measured activation energy of conduction. In a previous study by this group, the electrical conductivity of nominally undoped CeO 2 nanoparticles followed a pO 2 À1/6 dependence, pointing to electron compensation.…”

Editorial for JECR special issue on defects & relaxation processes in crystalline and amorphous solids

“…In the article by Knauth et al, entitled Study of compaction and sintering of nanosized oxide powders by in situ electrical measurements and dilatometry: Nano CeO 2 -case study, [18] the authors apply a novel approach to simultaneously monitor the densification and electrical properties of nanosized ceria particles within a modified dilatometer while controlling both temperature and oxygen partial pressure. The key finding, that nanosized particles reduce much more readily than bulk specimens, has potentially important implications for the use of such particles as catalyst supports and in gas sensing devices.…”