Recently, Peter published in his blog his reasons for hoping that the NAE aren’t cracks. After considering it, I believe he misses the uniqueness, durability, and beauty of the cracks that are being considered.
To the uniqueness point… Consider that a crack is different than just two surfaces in close proximity. A crack is like a horn with a throat of minimum gap: the lattice spacing. Imagine the throat at x=0 with the crack surface spacing widening as x increases. The crack provides a unique environment in its smallest regions. Near x=0, the environment for a hydron asymptotically approaches that of the lattice. In this region, electron orbitals extend across or at least into the crack. Perhaps in this near-lattice spacing there is only room for an H+ ion (the case for Ni, but for Pd there is room at the lattice spacing for a neutral monatomic hydron). As x increases, the crack surface spacing (the gap) increases allowing room for neutral monatomic hydrons. At greater x, the crack spacing would support neutral H2 molecules, and beyond this, the crack is probably uninteresting. This unique gradient of hydron boundary conditions always exists in the crack near it throat (near x=0), even if the crack were to begin zipping itself open. To the durability point… In my past I had occasion to work with MEMS structures. When I first saw MEMS cantilever beams being used for switches and other functions, my first thought was, “Those are going to break!” What I learned was that a structure’s strength is inversely proportional to its size. So a building scaled twice as large will be half as strong. This is why you can drop an ant from as high as you wish and he will hit the ground running. Compare a 3 meter diving board (cantilever) to a 3 micron cantilever – the 3 micron cantilever will be a million times more robust. The cracks being considered for NAE are nanoscale cracks, but our natural experience is with cracks having dimensions of ~1cm. A 10nm crack, will be a million times more mechanically robust than a 1cm crack. At the nanoscale, the two split apart surfaces will be very stiff and behind the throat of the crack (x<0) there will be compression forces trying to restore the crack to its closed position. The surfaces may also experience a Casimir closing force. A nanoscale crack will have strong forces trying to heal itself. If nanocracks can heal, then how would the nanocrack form in the first place and what could keep the surfaces apart? I believe a wedge of atom(s) or molecule(s) is needed in the gap to keep the crack open, and perhaps to form it in the first place. That is why I am using nanoparticles that will alloy with Ni and then I am oxidizing the structure. I use iron oxide nanoparticles. I put down the oxide nanoparticles disposed all across the Ni micro-powder surface, reduce (or partly reduce) the surface so the iron nanoparticles can alloy with the Ni, and then go back and strongly oxidize the metals. When the iron oxidizes, it grows in volume and I hypothesize that it will wedge open a nanocrack. If the iron is then partly reduced it becomes an H2 splitting catalyst, right at the site of the crack. What a beautiful structure I imagine that to be – a nanocrack with a sweep of hydron boundary conditions with an H2 splitting catalyst at its mouth. Bob
