[SI-LIST] Effects of steam

Thanks to everybody for both the online and offline replies + some contacts to 
follow up.
What has been straining my electronic engineer's brain is the concept that 
steam is not the same as water. Water, as has been pointed out, is a terrible 
dielectric but steam has a dielectric constant of 1.01 and negligible 
electrical conductivity and, in a superheated environment, I'm assuming it 
never becomes water - it's been helping me to think of it as "water gas", 
rather than steam and therefore to look at it as a gas absorption problem. I've 
found a link (http://jjap.ipap.jp/link?JJAP/46/1553) which attributes increases 
in polyimide conductivity (and, therefore, I suppose loss tangent) on moisture 
absorption to increased charge mobility and charge density linked to release of 
impurities but does superheated steam have the same effect? If it all comes 
down to molecules of water, whether they come from liquid water, unsaturated or 
superheated atmospheres, can we expect the effects to be the similar?

I am blithely ignoring all the other potential horrors of delamination, 
hydrolytic polymer breakdown and dendrite growth at the moment. Sufficient onto 
the day, as they say. I'm also not immediately considering what the dielectric 
properties of the polyimide would be if I took it out of the autoclave and 
measured it because then it cools and the "moisture" is no longer superheated. 
I'm interested in the effects in situ. If I can't find a polymer chemist with 
an interest in steam, I will probably need to devise some means of in situ 
measurement - there's probably a PhD to be got in doing that.

(Incidentally, when you Google "steam" and "dielectric constant", a whole new 
world opens up. It's not only board designers who worry about dielectric 
constant changes - I've so far found papers on the effects of steam on 
dielectric constant of mashed sweet potato, ham and chicken meat. The test 
fixtures must be interesting.)

John

Sean de Baroid

John Barrett


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