I want to understand how gas transmission rate is dependent on the thickness of the diffusion barrier coating deposited on a polymer substrate. Why is the preferred thickness of the diffusion barrier coating in nanometer range? If thickness is more than 100 nanometers, why does gas transmission rate increase? Please explain. Also please suggest any literature which explains this phenomena.
In case barrier layers are very thin, they are likely to form dense layers without any cracks. Molecules from the gas ambient can then only be transmitted through the barrier layer by diffusion through interstitial lattice sites of the barrier layer. As activation energies for interstitial diffusion are in the order of at least one or two electron volts, barrier layers must be very thin (nanometre range). Activation energies for interstitial diffusion also scale with the size of the molecules. H2 and the like diffuse quite easily, but heavier hydrocarbons for instance are much less likely to penetrate.
In case the barrier layer thickness is increased beyond some critical value, cracks are likely to occur and diffusion through the barrier layer is then limited by Knudsen diffusion. We have been dealing with such Knudsen diffusion phenomena in some of our previous publications (see attachments).
Hope, this helps!
Gerhard
Gerhard Müller · 40.38 · 326.75 · European Aeronautic Defence and Space Company N.V.
In case barrier layers are very thin, they are likely to form dense layers without any cracks. Molecules from the gas ambient can then only be transmitted through the barrier layer by diffusion through interstitial lattice sites of the barrier layer. As activation energies for interstitial diffusion are in the order of at least one or two electron volts, barrier layers must be very thin (nanometre range). Activation energies for interstitial diffusion also scale with the size of the molecules. H2 and the like diffuse quite easily, but heavier hydrocarbons for instance are much less likely to penetrate.
In case the barrier layer thickness is increased beyond some critical value, cracks are likely to occur and diffusion through the barrier layer is then limited by Knudsen diffusion. We have been dealing with such Knudsen diffusion phenomena in some of our previous publications (see attachments).
Hope, this helps!
Gerhard
thank you sir, but I am not able to see the attached files here. can you please send me at [email protected] ?
thickness also matters ... also see the pore sizes ... even if you can do a SEM study.. you can get an idea
Thickness or path length is always a critical variable in diffusion. The longer the path ( the thicker the film) the longer it takes to diffuse hence the greater the barrier ( lower the transmission). I tend to agree with Gerhart. if thickness goes up probability of cracks go up. Crack/pinhole formation is the most common failure mode in all barrier materials. If you are loosing barrier with increasing thickness your SEM test is suspect.
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