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Defect Diffusion in Single Crystals and Polycrystals of Co 1‐x O
Author(s) -
CHOWDHRY U.,
COBLE R.L.
Publication year - 1982
Publication title -
journal of the american ceramic society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.9
H-Index - 196
eISSN - 1551-2916
pISSN - 0002-7820
DOI - 10.1111/j.1151-2916.1982.tb10466.x
Subject(s) - grain boundary diffusion coefficient , crystallite , lattice diffusion coefficient , diffusion , materials science , grain boundary , analytical chemistry (journal) , effective diffusion coefficient , thermal diffusivity , vacancy defect , grain size , cobalt , oxygen , oxide , crystallography , mineralogy , chemistry , thermodynamics , metallurgy , microstructure , medicine , physics , organic chemistry , chromatography , magnetic resonance imaging , radiology
The chemical diffusion coefficient in nonstoichiometric Co 1−x O was measured at T =800° to 1040°C and an oxygen pressure range of 10 Pa≤P O2 ≤10 5 Pa, in which singly ionized cation vacancies were established as the predominant defect species. The oxidation and reduction kinetics for single crystals and polycrystalline samples are controlled by bulk diffusion. The chemical diffusion coefficient obtained for single crystals is represented byThis value is equal to twice the cation vacancy diffusivity and is independent of defect concentration. The chemical diffusion coefficient for polycrystalline Co 1−x O increases with decreasing grain size. The enhanced diffusion in fine‐grained materials is attributed to oxygen defect transport in the grain‐boundary region. For coarse‐grained samples with grain sizes >30 μ m , the chemical‐diffusion coefficient is similar to that obtained for single crystals. These data correlate well with other studies of mass transport in cobalt oxide which indicate that, in fine‐grained samples, oxygen diffusion in the grainboundary region is faster than cobalt diffusion in the lattice. A transition in rate‐controlling steps is observed in going from fine to coarse‐grained material. This is the first direct observation of enhanced defect transport in polycrystalline Co 1−x O.