Gravimetrical determination of pressure-composition isotherms of thin PdHcfilms

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Abstract
The absorption of hydrogen in thin Pd films with thicknesses of 300 nm and 50 nm is studied experimentally by measuring pressure-composition isotherms with a microbalance at temperatures up to 590K and hydrogen pressures up to 50 bar. In contrast to earlier findings obtained with the vibrating quartz technique, the 300 nm isotherms do not deviate significantly from those of bulk PdHc. For the 50 nm films dissociation plateaux are narrower than for bulk PdHc and the critical temperature is reduced by approximately 26K. The values of Delta HH and Delta SH obtained from the plateau pressures are practically the same as those for bulk PdHc. The influence of volume restrictions (e.g. clamping) on the effective H-H interaction is described with a recently developed model based on the semi-empirical heat of formation model of Griessen and Driessen. It is argued that apart from clamping, the narrowing of the isotherms may also be related to an enhancement of the surface to volume ratio due to the crystallite formation in thin films and a distribution of site energies caused by intrinsic stresses in films that adhere to substrates.
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