Kr adsorption on MgO is used to characterize the surface uniformity of MgO smoke and thermally decomposed Mg(OH)2. It is found that initially heterogeneous samples develop progressively sharper stepwise isotherms with increasingly high temperature heat treatment, apparently due to the removal of imperfections and high energy facets, leaving surfaces of highly uniform (100) planes.
Adsorption isotherms of 4He on graphite foam, uncompressed exfoliated graphite, kryptonplated graphite foam, MgO, and Mylar indicate type-II growth, i.e. , uniform layer deposition up to a critical thickness, and bulk condensation thereafter. On graphite the critical thickness is about four atomic layers; plating with one monolayer of krypton reduces the critical thickness by more than one layer. The critical thickness on MgO is 1.5 layers, and on Mylar is between three and four layers. None of the vapor pressure isotherms obey the exponential d Frenkel-Halsey-Hill equation within the range of the experiments.
Measurements of the heat capacity of adsorbed 0, films are reported. Coverages extend from a small fraction of a monolayer up to multilayers and bulk formation in the temperature range between 7 and 58 K. We show melting of the first-layer solid with an apparent triple line, the appearance and disappearance of the twodimensional a-P film transition, the a-b and P-b coexisting regions, and the appearance of the bulk melting and the three-dimensional a-P transformation at a reduced temperature. While most of our low-coverage features qualitatively agree with neutron and susceptibility studies, our results suggest that a more complex layer structure may exist.
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