Volume 111, Issue 19, 15 November 1999
Index of content:
- SURFACES, INTERFACES, AND MATERIALS
111(1999); http://dx.doi.org/10.1063/1.480247View Description Hide Description
We have studied the effect of an extended array of defects on the two-dimensional phase behavior of adsorbed hydrogen on a Nisurface using helium atom scattering. Specifically, the interaction of hydrogen with the stepped Ni(977) surface was examined and compared with similar interactions with the flat Ni(111) surface. The phase behavior of hydrogen on Ni(977) is qualitatively the same as that of hydrogen on Ni(111); however, the temperature at which the order–disorder transition occurs is elevated. On the stepped surface, the ordered -2H phase exists at a temperature 40 K higher than on the flat surface. This reversible phase transition is second order and is best fit with K and indicative of two-dimensional Ising behavior. Stabilization of the ordered phase is attributed to pinning from the step edges. The cross section for diffuse elastic He scattering by adsorbed hydrogen and the temperature-dependent domain size of ordered hydrogen along the step edges are also discussed.
Freezing of simple fluids in microporous activated carbon fibers: Comparison of simulation and experiment111(1999); http://dx.doi.org/10.1063/1.480261View Description Hide Description
We study the freezing of in microporous activated carbon fibers (ACF), using Monte Carlo simulation and differential scanning calorimetry(DSC). Microporous activated carbon fibers are well characterized porous materials, having slit-shaped pores due to the voids formed between graphitic basal planes. They serve as highly attractive adsorbents for simple nonpolar molecules, the adsorbent–adsorbate interaction being mostly dispersive (of the van der Waals-type). Recent molecular simulation studies have predicted an upward shift in the freezing temperature for simple fluids confined in such highly attractive carbon slit pores. Our DSC experiments verify these predictions about the increase in The results also indicate significant deviation from the prediction of based on the Gibbs–Thomson equation (simple capillary theory). We employ a recently developed free energy method to calculate the exact freezing temperature in these confined systems using molecular simulation, in order to address the failure of the simple capillary theory.