REPORT Table I. Cold Welding Effect on 1018 Steel in Vacuum Temperature °C. 500 150 25
ion pumping plus cryopumping Problem: Get the system d o w n to 10 s Torr in less time. Solution: Combine α VacIon®pump and a Varian Cryopump . . . letting e a c h do its o w n job in the most efficient w a y . Here's h o w it works. The Cryopump is simply a cold surface, chilled with (in this instance) liquid nitrogen. With this cold surface in the system, condensable g a s molecules such a s water vapor and C 0 2 simply strike and stick to the surface. Since the Cryopump will only pump condensable g a s e s , w e add another pump — the Vaclon pump. In this combination, the Vaclon pump is needed to pump the non-condensable g a s e s such a s N2 and O2 that won't freeze on the Cryopump. • In an unbaked vacuum system, a high percentage of the g a s adsorbed on surfaces is condensable. The addition of a high-speed Cryopump to such a system adds 25 times more pumping s p e e d at a modest 10 to 15 per cent increase in cost. Pumpdown time and b a s e pressure will be improved by a factor of 5 to 8 . . . a s s h o w n on the graph above. • This pumping combination is ideally suited to thin film deposition applications, since it is a low cost w a y to achieve and sustain l o w pressures during the evaporation process. For more information, write for the Varian Cryopump data sheet. • The Vacuum Products Division offers wide experience in this and other areas of high vacuum. W e also offer a wide variety of systems and components a s well a s an applications lab to assist y o u in solving your specific vacuum problem. M a y w e help you? Vaclon® is a registered trademark of Varian Associates
VARIAN
A S S O C I A T E S PALO ALTO, C A L I F . VACUUM PRODUCTS DIVISION
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26 A
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ANALYTICAL CHEMISTRY
Maximum Cohesion, percent 96.0 35.9 18.9
earthbound equipment have gener ally been from a lack of vacuum. Here we have a case where the sit uation is reversed. Particle densi ties below 1000 per cm. 3 are the rule for most experiments (1). The best large vacuum chambers avail able have particle densities of 10" per cm. 3 Therefore, it is impossi ble to test equipment properly prior to flight. One of the things t h a t can happen in this environment is a materials loss due to evaporation or sublimation. Results of prelim inary studies indicate structural materials such as steel and alumi num alloys will not sublime rapidly enough to damage their load-carry ing ability (2). However, where materials are applied as thin coat ing, such as those used for thermal control, sublimation is more seri ous. Studies of the effect of a vacuum environment on three thermal control coatings t h a t are intended for use on the radiator of a space power plant {3, 4, 5) show t h a t the emittance can increase, re main the same, or decrease because of exposure to vacuum. A second problem arises due to the loss of adsorbed gas layers t h a t are normally present on the surface of all materials. After prolonged exposure to space-vacuum environ ment, a surface will lose all the surface gas and will become clean. As a result, it will behave differ ently when put in contact with another clean surface. Recent studies have shown t h a t cohesion or cold welding takes place (6). The studies were made by breaking a specimen in vacuum and then placing the broken faces into con tact again. Table I shows repre sentative results from this study for 1018 steel. T h e maximum co hesion is the percentage of the ini tial breaking force t h a t was re quired to break the specimen a second time.
