Vacuum_Basics真空基础知识.pptVIP

* When the “permanent gases” are sufficiently cooled, they also liquefy and/or freeze. The above figure gives the vapor pressure for selected gases at low temperatures, when they are liquids or solids. The vertical dotted line to the right ( at 77K, –196°C). is the boiling point of liquid nitrogen. This means that the vapor pressure of nitrogen at 77K is atmosphere (760 torr). Curves to the right of this line indicate low vapor pressures at this temperature. Example: the vapor pressure of water at 120K is already as low as about 1 x 10-11 torr. Curves to the left show high vapor pressures at this temperature. Example: neon has to be cooled to about 26K before it’s vapor pressure reaches atmosphere. Note: The vapor pressure of all gases is the same at their boiling point in atmosphere (760 torr) even though they boil at different temperatures. The vapor pressure of nitrogen at 30K is 8 x 10-5 torr (dashed lines in the above diagram). This means that the pressure above a multi-molecular layer of nitrogen frozen out on a surface with a temperature of 30K will be 6 x 10-5 torr. This will be the lowest nitrogen pressure we can achieve in a vacuum system when we try to freeze out the nitrogen on a surface at a temperature held at 30K. If we cool the surface to 20K, we can lower the nitrogen pressure to about 1 x 10-11 torr! * All materials have a vapor pressure, even though it may be very small. The vapor pressure of elements that are solid at room temperature are shown above. Note that, for some of these materials, their vapor pressure may be high enough to be a problem in some vacuum systems. For instance cadmium has a vapor pressure of approximately 5 x 10-5 torr at 150 OC (423K). If a system containing cadmium screws is baked at this temperature, the cadmium will evaporate and contaminate the system. Only materials should be used that have a very low vapor pressure at the highest temperature at which the system will be operated. * * Series C