Field emission microscope study of the initial behaviour of the palladium-hydrogen system at low temperatures

For the first time, the initial stages of the uptake of hydrogen by palladi um metal have been studied on a microscopic scale. A novel technique for th e production of stable, clean Pd tips of small radius (similar to 2000 Angs trom) has enabled field emission microscopy (FEM, resolution similar to 20 Angstrom) to be successfully applied. Exposure of Pd to H-2 at low temperat ures led to an increase in emission starting at highly open surfaces presen t on the tip. This has been interpreted as both the formation of subsurface H, and the formation of extruding PdH particles on top of the Pd tip. Grow th of these particle proceeds in a staccato manner. Heating of the tip afte r saturation leads to a change in emission distribution over the tip, a bla ck wave that engulfs the whole surface followed by an increase in emission, that is possibly related to a phase transition from beta-PdH to a mixture of alpha- and beta-PdH (the so-called beta(min) transition). When most of t he hydrogen has been removed from the sample by heating in vacuum, Pd cryst allites remain on the surface. They are fairly stable up to high temperatur es of similar to 700 K at which they melt back into the bulk of the tip. (C ) 1999 Elsevier Science B.V. All rights reserved.