In reply to Michel Jullian's message of Mon, 5 Jul 2010 14:28:09 +0200: Hi, [snip] >Dear all, > >In my understanding, even though I haven't seen it expressed this way >elsewhere, dielectric breakdown is what happens to the so-called >Helmholtz double layer capacitor's insulator (the water monolayer >separating the cathode's surface electron layer from the first layer >of electrolyte dissolved positive charged deuterons attracted to the >cathode) when deuterons reach the palladium surface in PF experiments, >whether to get adsorbed, absorbed, or simply discharged and then >evolved. [snip] Dielectric breakdown is precisely what the name says. The dielectric breaks down. IOW the field strength (gradient) becomes large enough to rip electrons from atoms and it locally becomes a plasma conductor and shorts out. This can happen in any type of capacitor if the voltage gets too high, which is why all manufactured capacitors have a voltage rating. >That water monolayer is an excellent insulator BTW: it requires a >voltage of ~1V between the electrolyte and the cathode to break down, >which considering its extremely low thickness (~1 angstrom), >corresponds to a huge breakdown field of ~10 million V/mm, to be >compared e.g. with that of air, which is "only" ~3 thousand V/mm.
The water monolayer can't be just 1 Ångström thick. This is smaller than many atoms. (See e.g. http://arxiv.org/ftp/cond-mat/papers/0512/0512109.pdf). According to http://www.nesscap.com/data_nesscap/ECN%20article.pdf the double layer in electrolytic capacitors is on the order of 1 nm (10 Ångström) thick. Regards, Robin van Spaandonk http://rvanspaa.freehostia.com/Project.html
