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Concentration dependence, boundary layer resistance, and the “time‐lag” diffusion coefficient
Author(s) -
Skaarup Klemen,
Hansen Charles M.
Publication year - 1980
Publication title -
polymer engineering and science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.503
H-Index - 111
eISSN - 1548-2634
pISSN - 0032-3888
DOI - 10.1002/pen.760200406
Subject(s) - diffusion , extrapolation , materials science , diffusion layer , lag , grain boundary diffusion coefficient , effective diffusion coefficient , thermodynamics , diffusion equation , boundary (topology) , constant (computer programming) , boundary layer , steady state (chemistry) , desorption , time lag , analytical chemistry (journal) , layer (electronics) , chemistry , physics , chromatography , statistics , mathematical analysis , mathematics , adsorption , composite material , grain boundary , microstructure , economy , service (business) , computer network , computer science , magnetic resonance imaging , radiology , programming language , medicine , economics
Diffusion coefficient measurements have been shown to be strongly influenced by boundary layer resistance and concentration dependence. When half‐times for desorption or absorption are used to find diffusion coefficients from the equation for a constant coefficient ( T ½ = Dt / L 2 = 0.049), the values so obtained require correction. Diffusion coefficients found by the “time‐lag” method are also influenced strongly by surface resistance and concentration dependence. The usual equation for a constant diffusion coefficient gives a break‐through curve with an extrapolation to the time‐lag, T L = Dt / L 2 = 1/6. This factor can rise to ½ for concentration dependence. Where boundary layer resistance is encountered, (a situation which appears to be quite common), T L is significantly increased and the slope at steady state is decreased.