CB Sites wrote: * Just doing a search, here is an article discussing hydrogen permeation of Alumina at high temperatures. http://www.academia.edu/7323157/GAS_PERMEATION_PROPERTIES_OF_HYDROGEN_PERMEABLE_MACROPOROUS_ALUMINA_CERAMIC_MEMBRANES_AT_HIGH_TEMPERATURE * Of course this is about porous alumina membranes to purify hydrogen, but the effects of high temperature may apply to alumina in general porous or non-porous.
For future reference, there is a semantics problem which turned up yesterday, which cannot be easily resolved since it goes back to the manufacturer: purity vs porosity. Sorry that I did not pick up on this sooner. As for present needs, it would be interesting to characterize the tubes of Rossi and Parkhomov in the context of the CoorsTek tube for hydrogen permeability. There could be a major difference and surprisingly - “more porosity” could be needed for success. Here’s why. We can see from the famous image of the dogbone on a gram scale, that the net weight is 450+ grams. Take away the end caps and resistance wire and we could be seeing a tube that weighs about 250 grams. The MFMP used a high purity tube that would be almost impermeable to hydrogen, but the typical sintered alumina tube, with a density of 3.65 g/cc (as opposed to 3.9 for pure) could have uniform voids which are in the subnanometer geometry. In fact the voids in 250 grams of could in theory retain almost all of the hydrogen from the sub gram of fuel after only a few hours of operation! This gets us to the “purity” vs. “density” issue – where we have a semantic problem – since the specifications used by manufacturers are not uniform. You can see from this tube on eBay - which is 99.5% “pure” that its density is only 3.6 g/cc. The full density of alumina is 3.95 g/cc so this tube has voids of over 8% of its theoretical full density, and this is one of the best. We can call the difference “porosity” or nanoporosity, for lack of a better term. http://www.ebay.com/itm/6-LONG-HIGH-PURITY-ALUMINA-CERAMIC-TEST-TUBE-1-OD-0-75-ID-REFACTORY-/271722882427?pt=LH_DefaultDomain_0&hash=item3f43f2257b Other tubes which are well over 99% pure are nevertheless 9% voids – in the sense of porosoty, since these are different parameters. With hydrogen, the rate of permeability though 8-9% porosity, depending on pore size - would still be slow – but most importantly, an outer coating of cement, if it were to be impermeable to hydrogen – would trap much of the hydrogen within the nanopores or the walls of the tube. Fran Roarty must love this: hydrogen trapped in Casimir cavitites. BTW - the density of sapphire - which is fused by heat within metamorphic rock to gem quality is even higher: 3.98 g/cc! So the bottom line is that even high purity alumina will have substantial porosity unless it has been fused to full density. This will be around 8% and it will probably be nanoporosity based on small pore geometry. This nanoporosity is typically the meaning of “sintered alumina” as opposed to fused or polycrystalline (which is in between and fully translucent). More on nanoporosity later. Jones
