Adsorption of atmospheric water vapor

Adsorption of atmospheric water vapor, even of tiny quantities, is one of the most cost-effective issues in technology. This holds, e.g., for the production of ultra-high vacuum in high-energy physics as well as for the quality and lifetime of industrial products such as microelectronic and optical devices, halogen lamps, hard coatings, anticorrosion layers, etc. Consequently, a better understanding and control of water adsorption would be highly desirable. Adsorption of water molecules has been studied at very low pressures and low temperatures in several laboratories by state-of-the-art surface analytical tools. However, little is known about the adsorption kinetics of water an technical surfaces in atmosphere where all the obstacles have their origin. This lack of dato is mainly due to the lack of analytical tools suitable for Operation in atmosphere. Radioactive tracer technique, in particular, tritium- tracer-technique (TTT), could help to overcome this problem. Tests have shown that TTT allows to study water coverage and related adsorption kinetics down to 10(^l3) water molecules/cm(^2), i.e., approximately 0,01 "monolayers", and that a direct comparison of the adsorption phenomena in atmosphere and vacuum should be possible. In this project, basic aspects of TTT and its suitability for surface studies shall be investigated with special emphasis an the difference of adsorption kinetics in atmosphere and vacuum. Main efforts will be directed towards the identification of materials exhibiting significantly lower adsorption capacities than usual materials such as stainless steel, aluminum, glass, etc. The results of this project shall lead to a better understanding of atmospheric adsorption processes, helping to define criteria for the selection and control of low-adsorbing surfaces and thus being beneficial for the economy of many issues in research and industry.

Main result Within the project, a method for the quantitative measurement of small water coverage on technical surfaces in atmosphere and vacuum has been developed. It exhibits a versatile tool for this hitherto inaccessible issue, and facilitates, among others, the evaluation and improvement of novel materials and material surfaces for vacuum applications. It is expected that this achievement will lead to significant economic merits within a few years. The basic problem Due to atmospheric humidity, any surface is covered with 10 to 100 monomolecular layers of water. Although this quantity can be neglected from the macroscopic point of view, it exhibits significant and mostly unfavorable effects on the quality and lifetime of industrial products as well as on production costs, e.g., in microelectronic industry. Furthermore, it is responsible for high investment and operation costs in many fields of basic research. A reduction of water adsorption could save worldwide up to several hundred million Euro every year. The new method The new method utilizes radioactive tracer technique. It is based on the employment of a substance with known specific radioactivity A (unit: Bq/g) and the evaluation of unknown quantities of that substance by comparing their decay rates with the value of A. HTO In case of water, the radioactive hydrogen isotope tritium (symbol T) can be considered, offering low radiation hazard and a favorable half-life of 12,3 years. Consequently, T-labeled water (symbol HTO) was chosen, and an apparatus for specimen exposure to HTO was constructed. Water was measured by liquid-scintillation spectrometry that yields high counting efficiency and low background. Results It could be confirmed that water coverage of 1012 molecules/cm2 (i.e., less than 1 % of a monomolecular layer) can be measured precisely, even on samples of just 1 cm2 surface area. Thus, it facilitates the direct comparison of water adsorption capacities of various materials without the restrictions of the vacuum method (i.e., large amount of sample material and long observation periods). Hitherto, stainless steel-, Al-, and Au-sheets, silicone wafers, and Si- and TiN-coated stainless steel have been investigated. The main task was a detailed study of the sojourn times of adsorbed water molecules in atmosphere and vacuum, leading to simple means for the ranking of materials with respect to vacuum applications.