Why is metal sealing necessary for ultra-high vacuum?
In vacuum engineering and surface physics, achieving and maintaining ultra-high vacuum (UHV), typically defined as pressures below 1 \times 10^{-6}\text{ Pa} to 1 \times 10^{-10}\text{ Pa} or lower, imposes extremely stringent requirements on sealing materials. In such extreme environments, rubber (elastomeric) seals, due to their inherent physical and chemical properties, cannot meet the sealing demands and necessitate the use of metal seals (commonly employing metal gaskets of oxygen-free copper, aluminum, or indium paired with stainless steel knife-edge flanges, i.e., CF knife-edge flange systems).
The core physical and engineering reasons are as follows:
1. Fundamental Differences in Outgassing Rate
Under ultra-high vacuum (UHV) conditions, the primary gas sources in a system are no longer residual air within the vessel but gases released from the internal walls and the sealing materials themselves (mainly water vapor, carbon monoxide, carbon dioxide, and hydrocarbons).
- Rubber Seals (e.g., fluoroelastomers like Viton/FKM): As high molecular polymers, they possess significant free volume internally, readily absorbing and dissolving atmospheric moisture and gases. In a vacuum, these adsorbed and dissolved gases are slowly and continuously released into the vacuum chamber through diffusion. Their high outgassing rate prevents the vacuum from reaching the desired level.
- Metal Seals (e.g., copper gaskets): Metals have a dense crystal lattice structure and lack the macroscopic porosity of polymers. Their bulk and surface outgassing rates are several orders of magnitude lower than rubber. With appropriate surface cleaning treatments, the outgassing rate of metal