Yes, correct. The ratio S1 / S2 is in diffusion literature often denoted as K, the distribution coefficient. K is dependent on temperature and pressure.

The concentration is indeed discontinuous. The rate at which the discontinuous equilibrium is obtained, depends on the layer thickness and diffusion coefficients in each layer.

However, if one material swells at the interface of two materials, the swelling stress usually exceeds - if no adhesive, surface roughening etc. is present - the interfacial strength (interfacial surface energies). Hence, holes appear, and the interface is not Henry like anymore.

In case of one sided water diffusion, in 50 micrometer polyimide, used as a solar cell substrate, the swelling stress at the subsequent material interface is zero - during a day-night cycle - due to the dual mode character of water diffusion in polyimide. Note that specifically high barrier polymers, like epoxy resin, liquid crystalline polymers and polyimide, demonstrate dual mode sorption (the diffusion rate is so slow that other phenomena like internal adsorption become visible...).

Sincerely,
Sijmon van der Wal
Composite Analytica

[responses: 0]

Thanks for the follow-up. My apologies for the fact that I haven't got much time to study your post or for putting the library figures in the IDC – SAC simulation program.

Time lag is a practical measure for breakthrough time of chemicals diffusing through a layer. But realize that time lag isn't the time that the first molecules appear. They appear much faster on the other side than the "lag" time, and the first molecule will be on the other side within one second...(read more on this in Einstein's article on Brownian Motion). If the first molecules are already causing problems in the application (which is possible) the Penetration theory for diffusion will be more helpful than the time lag theory.

Time lag origins from experimental determination of permeation rate, diffusion coefficient and water solubility. In this method (e.g. ASTM E96, ASTM D1434, ASTM D471) a plane sheet is exposed on one side to the chemical (or chemicals mixture) and subsequently being continuously measured (with a mass spectrometer MS or gas chromatography GC) on the downstream side of the sheet. From extrapolation of the detected mass, the diffusion coefficient is determined can be determined using the time lag formula.

Regarding time lag for multilayer structures. They can be determined using the chemical-physical simulation tool, as well as for example the total water permeation in aluminum oxide and polyimide layer.

Sorry, this is all for now. In the forthcoming post on moisture and oxygen diffusion in multilayer laminates, I will add the explanation and graphs for determining time lag in multilayer composites or laminate materials.

Regards,
Composite Analytica
[responses: 2]

I really appreciate your questions on this nice forum... Can you explain what the high pressure has to do with moisture and gas diffusion in solar cell substrates / polymer based photovoltaics ;-)?

Secondly, I want to add something. If we would really go green, why not suggest a bio plastic for solar cell substrate material? What do you think of a multilayer laminate composed of polylactide or furan based coating resins (i.e. SiOx plasma coated)? Can you perform time lag and mass flux calculation, especially with regard to water, for these composite plastics also?

Keep up the good work!
Paula Vigil

[responses: 2]