A SCANNING TUNNELING MICROSCOPE STUDY OF A PALLADIUM SPHERE IN HYDROGEN GAS - EXPANSION AND SURFACE-TOPOLOGY

A scanning tunneling microscope monitored the time dependence of both the expansion and surface morphology changes of 1-2 mm palladium spheres exposed to 0.5 atm hydrogen gas. The surfaces were initially smooth except for 2-10 nm high steps separating flat terraces 50-150 nm wide. Spheres exposed to hydrogen expanded, during which time the surface buckled forming 30-50-nm-deep and 1000-nm-wide features and the step structures vanished. There were two distinct regimes in the expansion: 25%-30% of the total expansion occurred in the first regime, which only lasted about 5% of the time required to reach 90% of the total expansion; and in the second regime the sphere asymptotically approached the final size. Some samples took approximately 500 h to expand, but other samples with only slightly different surface morphology expanded in as little as 8 h. We developed a quantitative model describing the time dependence of the observed expansion in the asymptotic regime by assuming the eventual formation of beta-Pd hydride and the elastic properties of the material. The model also suggested the cause of the surface buckling.