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The application of polymide for ultra-high vacuum seals
Abstracts
of the papers presented
at the Thirteenth
National
Outgassing of epoxy resins in vacuum Weight loss of three plastic materials (two glass-fibre-filled 81% MPDA Epoxies and an unfilled 828-U Enoxv) during 100 hours exposures to vacuum (10-O to lo-* torr) at %“F ‘were measured with a recording electrobalance. Weight loss decreased with time, indicating that the outgassing products were absorbed or adsorbed gases. This view is supported by results of experiments which showed that specimens subjected to a second vacuum exposure following seven to nine days exposure to air exhibited weight loss histories nearly identical to that of the first vacuum exposure. Also, qualitative mass spectra indicated that the outgassing products were principally water vapour plus nitrogen, oxygen and carbon dioxide. R D Brown, Southwest Research Institute, San Antonio, Texas. Gas-liquid leakage correlation Experiments were performed to determine the correlation between gas and liquid leakage through leaks. The flow of water, nitrobenzene, and four alcohols of different viscosities but the same surface tension was measured at a variety of pressures. Leaks used had conductances between 1O-8 and 1O-5 atm cc/set atm, previously measured using helium mass spectrometer leak detector. With leaks having conductances in the lC+ range, gas flow was laminar and liquid flow was half that predicted by theory. With conductances in the 10T4range, liquid flow is a decade lower than theory predicts, deviation increasing with decreasing conductance. No liquid leakage was found greater than predicted by theory. Liquid leakage into vacuum was reduced as compared to leakage into atmosphere, presumably due to evaporative cooling. J William Marr, Research and Development Center, General Electric Company,
Schenectady,
New
York.
Systems Pressure measurement in cryogenically pumped vacuum systems Typical problems encountered in measuring pressure in space simulators are reviewed. These problems largely stem from hidden conflicts with kinetic theory and its explicit definition of pressure. It is shown that “pressure” is not always definable, depending on gauge geometry, gas species, and vessel wall temperature. Under certain circumstances, these conflicts are resolvable, as has been shown in the literature. In addition, the principles of molecular beam gauge calibration are described to illustrate a carefully controlled pressure measurement in a cryogenic vacuum system. _ F L Tomey. Jr, National Research CorDoration. 70 Memorial Drive. Cambridge,
Mass.
A vacuum system and procedures for experimentally evaluating parameters affecting the pumping efficiency of a plane cooled with cryogenic fluids The collection efficiency of a plane acting as a pump in a vacuum system can be determined by measuring the rate at which molecules of a given species strike and the rate at which they return from that plane. An experimental vacuum system incorporating a moveable partial pressure analyzer capable of making such measurements, and which provides a means for evaluating parameters affecting the pumping efficiency is discussed. The experimental data show that much of the deviation of reported pumping efficiencies can be explained by fragmentation of the test gas molecules and by a pressure anisotropy in the test dome. A means of calibrating the analyzer with vapour pressure data is also discussed. L 0 Mullen and M J Hiza, Cryogenics Division, NBS Institute for Materials
Research,
Boulder,
Colorado.
Radioactive tracer measurements on the back-diffusion of pump oils ultrahigh vacuum systems. At the 1964 AVS Meeting Carter et al’ reported on oil deposition measurements of DC 704 using radioactive tracers (YSi). Because of the Short half-life (2.6 h) this method has only limited application in long term runs. A method is described that uroduces SH taeeed DC 704 pump fluid of high specific activity. Because Tritium has a half-life of 12.3 Y this oil can be used for multiule and Iona term investigations in-ultrahigh vacuum systems. Wit’h this p&$ fluid measurements on the influence of bake-out- and baffle-conditions on oil-backdiffusion have been performed in an all metal ultrahigh vacuum system. Results and the limits of detection are given. in
168
Vacuum
Symposium
R Dobrozemsky, Oesterreichische Studiengesellschraft fiir Atomenergie GmbH, Reaktorzentrum Seibersdorf; Oesterreich, W K Huber, Balzers Aktiengesellschaft fir Hochvakuumtechnik und Diinne Schichten, Balzers, Fiirstentum Liechtenstein, and F Viehbiick, Oesterreichische Studiengesellschafi ,fiir Atomenergie GmbH, Reaktorzentrum Seibersdorf Oesterreich. Clean diffusion pump fluids: How they stay that way Numerous samples of high vacuum diffusion pump fluids have been analyzed after use in such applications as space environmental testing, cathode ray tube processing, and thin-film deposition. Contaminants found in many of these fluids show that pump systems are failing to purge themselves as effectively as they should of volatile, yet condensible, contaminants. Pump geometry, working temperature and working pressure are combined ina simplified mathematicalmodel showing the influence of each of these factors on the effectiveness of are described with commercial pumps, purging. Experiments modified as suggested by the theory, in which the working fluid is kept sufficiently clean to yield ultimate pressures of l-2 x lo-lo torr employing only water-cooled baffles. The order of cleanliness required for this performance is shown. Arnold R Huntress and Walter R Schmock, Dow Corning Corporation, Midland,
Michigan.
Physico-chemical aspects of elastomer technology for UHV applications The gas load contributed by elastomers in vacuum systems arises firstly from the material itself (analogous to vapour pressure) and secondly due to gas transport mechanisms in and through the material. Measurements made on Viton A material separate the two sources quantitatively. The optimum conditions for the use of Viton A gaskets have been deduced, and have led to the design of a double O-ring seal, with interseal pumping, for a UHV chamber bakeable to 450°C. Practical results on this chamber show that pressures below 10-O torr are readily reached even with mild baking, as predicted by the previous analysis. Discrepancies which occur where pressures are lower than predicted suggest that readsorption of gas on the clean metal walls is more important than hitherto realized. L de Csematony, Edwards High Vacuum Limited, Manor Royal, Crawley,
Sussex,
Fngland.
The application of poiymide for ultra-high vacnum seals The application of a promising new material, polyimide, to the design of ultrahigh vacuum seals is discussed. In vacuum sealing applications the material was found to exhibit mechanical stability while maintaining vacuum integrity up to temperatures as high as 300°C. Outgassing at 300°C. as determined by residual gas analysis, showed the-prim&y constituent gases were C-0, CO,, H,O, and H,; whereas no heavy hydrocarbons were indicated. When compared to Viton A, polyimide was found to have a lower vapour pressure. Design parameters which take into account the physical properties of polyimide (such as its increased hardness when compared to Viton A) will be discussed. Paul W Hait, Varian Associates, 611 Hansen Wuy, Palo Alto, California.
Internal bakeout of a large space simulation facility The need for an effective, low cost method for cleaning the inside of a large space simulator has resulted in the use of an infrared heat flux array in the High Vacuum Orbital Simulator (HIVOS). This technique enables the HIVOS facility to be cleaned with an improvement in the ultimate obtainable pressure from 1 x 10e5 to the 1O-s torr range. Curves showing the chamber operational parameters, before and after bakeout, array design suggestions with approximate costs are included. Le Roy H Kratzer, Lockheed Missiles and Space Co, Dept 75-22, Rex 504, Sumtvvale,
California.
Design concepts for a combined space environmental test facility An ultrahigh vacuum space environmental test facility was designed and fabricated; initial evaluations were also performed on thermal control coatings, The combined low energy sources which can be utilized either together or sequentially in this facility include electrons, protons, x-rays, plus near and far ultraviolet. The relative changes in solar absorptivity (a,) and normal emissivity (en) of thermal control coatings can be measured in situ within the vacuum chamber prior to and after this environmental exposure.
of the papers presented
at the Thirteenth
National
Outgassing of epoxy resins in vacuum Weight loss of three plastic materials (two glass-fibre-filled 81% MPDA Epoxies and an unfilled 828-U Enoxv) during 100 hours exposures to vacuum (10-O to lo-* torr) at %“F ‘were measured with a recording electrobalance. Weight loss decreased with time, indicating that the outgassing products were absorbed or adsorbed gases. This view is supported by results of experiments which showed that specimens subjected to a second vacuum exposure following seven to nine days exposure to air exhibited weight loss histories nearly identical to that of the first vacuum exposure. Also, qualitative mass spectra indicated that the outgassing products were principally water vapour plus nitrogen, oxygen and carbon dioxide. R D Brown, Southwest Research Institute, San Antonio, Texas. Gas-liquid leakage correlation Experiments were performed to determine the correlation between gas and liquid leakage through leaks. The flow of water, nitrobenzene, and four alcohols of different viscosities but the same surface tension was measured at a variety of pressures. Leaks used had conductances between 1O-8 and 1O-5 atm cc/set atm, previously measured using helium mass spectrometer leak detector. With leaks having conductances in the lC+ range, gas flow was laminar and liquid flow was half that predicted by theory. With conductances in the 10T4range, liquid flow is a decade lower than theory predicts, deviation increasing with decreasing conductance. No liquid leakage was found greater than predicted by theory. Liquid leakage into vacuum was reduced as compared to leakage into atmosphere, presumably due to evaporative cooling. J William Marr, Research and Development Center, General Electric Company,
Schenectady,
New
York.
Systems Pressure measurement in cryogenically pumped vacuum systems Typical problems encountered in measuring pressure in space simulators are reviewed. These problems largely stem from hidden conflicts with kinetic theory and its explicit definition of pressure. It is shown that “pressure” is not always definable, depending on gauge geometry, gas species, and vessel wall temperature. Under certain circumstances, these conflicts are resolvable, as has been shown in the literature. In addition, the principles of molecular beam gauge calibration are described to illustrate a carefully controlled pressure measurement in a cryogenic vacuum system. _ F L Tomey. Jr, National Research CorDoration. 70 Memorial Drive. Cambridge,
Mass.
A vacuum system and procedures for experimentally evaluating parameters affecting the pumping efficiency of a plane cooled with cryogenic fluids The collection efficiency of a plane acting as a pump in a vacuum system can be determined by measuring the rate at which molecules of a given species strike and the rate at which they return from that plane. An experimental vacuum system incorporating a moveable partial pressure analyzer capable of making such measurements, and which provides a means for evaluating parameters affecting the pumping efficiency is discussed. The experimental data show that much of the deviation of reported pumping efficiencies can be explained by fragmentation of the test gas molecules and by a pressure anisotropy in the test dome. A means of calibrating the analyzer with vapour pressure data is also discussed. L 0 Mullen and M J Hiza, Cryogenics Division, NBS Institute for Materials
Research,
Boulder,
Colorado.
Radioactive tracer measurements on the back-diffusion of pump oils ultrahigh vacuum systems. At the 1964 AVS Meeting Carter et al’ reported on oil deposition measurements of DC 704 using radioactive tracers (YSi). Because of the Short half-life (2.6 h) this method has only limited application in long term runs. A method is described that uroduces SH taeeed DC 704 pump fluid of high specific activity. Because Tritium has a half-life of 12.3 Y this oil can be used for multiule and Iona term investigations in-ultrahigh vacuum systems. Wit’h this p&$ fluid measurements on the influence of bake-out- and baffle-conditions on oil-backdiffusion have been performed in an all metal ultrahigh vacuum system. Results and the limits of detection are given. in
168
Vacuum
Symposium
R Dobrozemsky, Oesterreichische Studiengesellschraft fiir Atomenergie GmbH, Reaktorzentrum Seibersdorf; Oesterreich, W K Huber, Balzers Aktiengesellschaft fir Hochvakuumtechnik und Diinne Schichten, Balzers, Fiirstentum Liechtenstein, and F Viehbiick, Oesterreichische Studiengesellschafi ,fiir Atomenergie GmbH, Reaktorzentrum Seibersdorf Oesterreich. Clean diffusion pump fluids: How they stay that way Numerous samples of high vacuum diffusion pump fluids have been analyzed after use in such applications as space environmental testing, cathode ray tube processing, and thin-film deposition. Contaminants found in many of these fluids show that pump systems are failing to purge themselves as effectively as they should of volatile, yet condensible, contaminants. Pump geometry, working temperature and working pressure are combined ina simplified mathematicalmodel showing the influence of each of these factors on the effectiveness of are described with commercial pumps, purging. Experiments modified as suggested by the theory, in which the working fluid is kept sufficiently clean to yield ultimate pressures of l-2 x lo-lo torr employing only water-cooled baffles. The order of cleanliness required for this performance is shown. Arnold R Huntress and Walter R Schmock, Dow Corning Corporation, Midland,
Michigan.
Physico-chemical aspects of elastomer technology for UHV applications The gas load contributed by elastomers in vacuum systems arises firstly from the material itself (analogous to vapour pressure) and secondly due to gas transport mechanisms in and through the material. Measurements made on Viton A material separate the two sources quantitatively. The optimum conditions for the use of Viton A gaskets have been deduced, and have led to the design of a double O-ring seal, with interseal pumping, for a UHV chamber bakeable to 450°C. Practical results on this chamber show that pressures below 10-O torr are readily reached even with mild baking, as predicted by the previous analysis. Discrepancies which occur where pressures are lower than predicted suggest that readsorption of gas on the clean metal walls is more important than hitherto realized. L de Csematony, Edwards High Vacuum Limited, Manor Royal, Crawley,
Sussex,
Fngland.
The application of poiymide for ultra-high vacnum seals The application of a promising new material, polyimide, to the design of ultrahigh vacuum seals is discussed. In vacuum sealing applications the material was found to exhibit mechanical stability while maintaining vacuum integrity up to temperatures as high as 300°C. Outgassing at 300°C. as determined by residual gas analysis, showed the-prim&y constituent gases were C-0, CO,, H,O, and H,; whereas no heavy hydrocarbons were indicated. When compared to Viton A, polyimide was found to have a lower vapour pressure. Design parameters which take into account the physical properties of polyimide (such as its increased hardness when compared to Viton A) will be discussed. Paul W Hait, Varian Associates, 611 Hansen Wuy, Palo Alto, California.
Internal bakeout of a large space simulation facility The need for an effective, low cost method for cleaning the inside of a large space simulator has resulted in the use of an infrared heat flux array in the High Vacuum Orbital Simulator (HIVOS). This technique enables the HIVOS facility to be cleaned with an improvement in the ultimate obtainable pressure from 1 x 10e5 to the 1O-s torr range. Curves showing the chamber operational parameters, before and after bakeout, array design suggestions with approximate costs are included. Le Roy H Kratzer, Lockheed Missiles and Space Co, Dept 75-22, Rex 504, Sumtvvale,
California.
Design concepts for a combined space environmental test facility An ultrahigh vacuum space environmental test facility was designed and fabricated; initial evaluations were also performed on thermal control coatings, The combined low energy sources which can be utilized either together or sequentially in this facility include electrons, protons, x-rays, plus near and far ultraviolet. The relative changes in solar absorptivity (a,) and normal emissivity (en) of thermal control coatings can be measured in situ within the vacuum chamber prior to and after this environmental exposure.