See inline below ... On Wed, Dec 31, 2014 at 2:28 PM, Jones Beene <[email protected]> wrote:
> *From:* Bob Higgins > > Ø None of these substrates are porous. > > That may not be true for even the specialty material CoorsTek 998, but > clearly 96% is porous and in fact anything less than 100% will be > slightly porous by definition. If they do not supply 96%, then you really > should look at what other suppliers have to offer. What I am calling > porous alumina is sometimes sold by density… such as 3.65 g/cc. > BH: The fact that the alumina is not 100% theoretically dense does not mean that the remainder is air/porosity. The remainder is a much lighter weight silicate glass in the grain boundaries. The 96% grade has more silicates. Mullite is 1/3 silicates. Even mullite is not necessarily porous unless designed to be so. They can go back and etch out the silicates for specialty filters, but that is not what an as-fired ceramic tube would be. Residual proton conduction has to do with H-O reductions and exchanges in the grain boundary with metal/silicon oxide atoms. The bond for oxygen in sapphire is one of the best in all dielectrics - it really wants to hang onto its oxygen. That's why there is no proton conduction through sapphire. If there were really "pores" in the alumina, they wouldn't be proton conductor size - they would be gross leak size. At 100 bar, the H2 would be gone almost instantly.
