AVS2001 Session VST-WeA: Gas Sorption Phenomena II

Wednesday, October 31, 2001 2:00 PM in Room 125

Wednesday Afternoon

Start	Invited?	Item
2:00 PM		VST-WeA-1 Kinetic Parameters of Materials Outgassing: Experimental Determination Using Thermoelectric Quartz Crystal Microbalance I. Gouzman, M. Murat, E. Grossman, Y. Noter, Y. Lifshitz (Soreq NRC, Israel) The outgassing of polymer-based materials is a concern for all space-related applications. The knowledge of the kinetic parameters characterizing outgassing of materials is of great importance since they are used to predict the potential contamination and the associated degradation of space systems. In the present work thermoelectric quartz crystal microbalance (TQCM), incorporated in a modified ASTM E 595 standard outgassing system, was used to measure the kinetics of spacecraft materials' outgassing under high vacuum and elevated temperatures. The measurements were carried out by collecting the outgassed species on the TQCM sensor surface. The experimental procedure consisted of the following steps: (i) a stepwise increase of the sample temperature in the 25 - 125°C range, while keeping the TQCM at a constant temperature (25 or 0°C), (ii) cooling the sample and keeping it at room temperature without changing the TQCM temperature in order to isolate the contaminants re-emission process, and (iii) increasing the TQCM temperature in several steps, studying the effect of the temperature on the re-emission kinetics. An analytical model was developed to derive the outgassing and re-emission kinetic parameters from the experimental data. The system capabilities were demonstrated by studying the outgassing kinetics of several materials that included polysiloxane and polyurethane based black coatings (S2 and PU1, respectively, both manufactured by MAP, France) as well as a closed cell polymethacrylimide foam (Rohacell, manufactured by Rohm, Germany). Outgassing and residence times, activation energies, and the contamination potential of different components of the outgassed products were calculated based on the proposed model. The derived parameters can be used to predict the outgassing and contamination profiles of space systems, both on the ground (e.g. in a thermal vacuum tests) and in space.
2:20 PM		VST-WeA-2 UHV Compatibility of Organic Materials M. Anderle (ITC-irst,, Italy); C. Bellachioma, C. Benvenuti, E Broilo, S. Calatroni, P. Chiggiato, S. Clair, S. Elliott (CERN, Switzerland); J. Kenny (University of Perugia, Italy); W. Vollenberg (CERN, Switzerland) Polymers and composite materials present many attractive properties for the construction of vacuum components. They are easily formed, light, robust and electrically non conducting. However, their use for UHV applications is hindered by an important outgassing of volatile elements contained in their bulk. Furthermore,they are also prone to gas permeation when exposed to a pressure difference. An obvious improvement strategy consists in coating them with a metallic thin film to produce a hybrid material which should combine the structural properties of a polymer with the vacuum properties of a metal. Along this line,various organic materials have been coated with thin films of different metals by sputtering. It has been found that the benefits hoped for could not be fully achieved due to pin-holes in the metal coating, which are a consequence of the presence of dust grains and morphology irregularities on the surface to be coated. A quantitatve analysis of the results indicates that the outgassing through the pin-holes is enhanced by lateral gas diffusion in the polymer material. The mathematical formulation of the process and the experimental results obtained to date are presented and discussed.
3:20 PM	Invited	VST-WeA-5 A Different View on the "Water Problem" R.W. Dobrozemsky (Vienna University of Technology, Austria) Pumping times after air exposure with their strong implications on the economy of vacuum processes are mainly caused by the desorption of rather tiny quantities of water in vacuum systems. There is no doubt that, by reducing the adsorbed quantity or by shortening its sojourn time, faster pumping cycles could be achieved. The slow progress in this field is, among others, due to the lack of simple and quantitative methods for measurements of atmospheric water coverage and its kinetics. In order to find a method that exhibits the same sensitivity and accuracy in atmosphere as well as in vacuum, radioactive tracer technique has been studied with particular emphasis on tritium-tracer-technique (TTT). This method allows precise measurements of low coverage (down to 1 percent of a "monolayer" even on rather small samples) and of its kinetics over periods of several days. In addition, water coverage can be measured in a way that is not effected by other surface contaminants, or by shortcomings of usual vacuum gauges (sensitivity drift, outgassing, gas reactions on hot filaments, etc). Hitherto, atmospheric water adsorption on square-cm-sized samples of stainless steel, aluminum, silicone, gold, and glass has been studied. It was found that water adsorption depends highly on the cleanliness of the surface with no significant difference between stainless steel and gold. Furthermore, it turned out that sojourn times in vacuum are at least equal to or even much longer than those in atmosphere. Basic aspects of the method as well as experimental data will be discussed.
4:00 PM		VST-WeA-7 Effect of Readsorption in a Dynamic Vacuum System P. Repa, L. Peksa, T. Gronych (Charles University, Czech Republic) Theoretical analyses of the pressure variations in vacuum chambers considering desorption and adsorption indicate that the time-pressure curves shall be modified by readsorption of the gases penetrating through or releasing from the walls.¹. Nevertheless, no effect of readsorption was observed at some recent experiments^2-5 which were curried out in sealed-off vacuum systems and the phenomenon has not been explained entirely yet. An aim of this work was to study an effect of readsorption on pressure variations in a dynamic vacuum system. Doses of several gases (H₂, CO, and H₂O) were introduced into a uhv chamber of the apparatus that was simultaneously pumped. For the measurements, an apparatus was used that consist of a uhv chamber of volume 0,025 m³ pumped by a uhv pump and a gas handling system enabling to vary a throughput of an introduced gas precipitously. The pumping speed of the pump could be set at values up to 0.1 m³/s by a reduction valve. In the course of the measurements, the total and partial pressure vs. time curves were recorded at several selected pumping speeds. An analysis of the response of the pressure in the chamber to sudden variation of the throughput of the introduced gas implied that the influence of readsorption on the pressure behaviour was immeasurable small not only in the sealed-off but also in the dynamic vacuum systems as well. ¹Redhead P. A., J. Vac. Sci. Technol. A14, 2599 (1996). ²Berman A. and Fremerey J. K. J., Vac. Sci. Tecnol., A5, 2436 (1987) ³Bennett K. J. and Elsey R. J., Vacuum 44, 159 (1993) ⁴Jousten K., Vacuum 47, 325 (1996). ⁵Takeuchi K. at all, J. Vac. Soc. Japan 35 837 (1993) .
4:20 PM		VST-WeA-8 Influence of the Production Parameters on the Vacuum Properties of Ti-Zr-V Non-evaporable Getter Films C. Benvenuti, P. Chiggiato, C. Claeys, P. Costa Pinto, A. Prodromides, V. Ruzinov, I. Wevers (CERN, Switzerland) Non-evaporable thin film getters of various composition have been produced by sputtering. Among about 20 materials which have been studied, the lowest activation temperature (about 180°C) has been displayed by a Ti-Zr-V coating obtained from a cathode made of intertwisted elemental wires. In order to optimize the vacuum properties of this coating, the production parameters have been varied, namely sputtering configuration (diode or magnetron), discharge gas, deposition rate, discharge voltage, substrate nature and temperature during coating. The films have been variously analysed by electron microscopy, electron stimulated desorption, ultimate pressure, pumping speed and rare gas degassing rate measurements. It has been found that the activation temperature, the pumping speed and surface capacity are strongly affected by the deposition rate and substrate temperature during coating. Optimum characteristics are obtained for high substrate temperatures (about 300 °C) and low deposition rates (about 0.15 µm/h).
4:40 PM		VST-WeA-9 Experimental Investigation of Impact-induced Molecular Desorption by 4.2 MeV/amu Pb Ions M. Chanel, J. Hansen, J.-M. Laurent, N. Madsen, E. Mahner (CERN, Switzerland) In preparation for the heavy ion program of the LHC, accumulation and cooling tests with lead ion beams have been performed in the LEAR storage ring. These tests have revealed that due to the unexpected large outgassing of the vacuum system, the dynamic pressure of the ring could not be maintained low enough to reach the required beam intensities. To determine the actions necessary to lower the dynamic pressure rise an experimental program has been initiated for measuring the molecular desorption yields of stainless steel vacuum chambers by the impact of 4.2 MeV/amu lead ions with the charge states +27 and +53. The test chambers were exposed either at grazing or at perpendicular incidence. Different surface treatments are reported in terms of the molecular desorption yields for H₂, CH₄, CO and CO₂. Unexpected large values of molecular yields per incident ion up to 10⁴ molecules/ion have been observed. The implications of these results for the vacuum system of the future ion accumulator ring (LEIR) and possible remedies to reduce the vacuum degradation will be discussed.