An Oil Absorbing, Trap for Ultra-high Vacuum Evaporator

Abstract

Pump-down curves of two evaporators equipped with large oil absorbing traps are described.
One is a conventional commercial type which consists of a glass bell-jar (300φ × 350 h), a 120 l/sec oil diffusion-pump, a 2.5″ oil absorbing trap and 2.5″ main valve (bellows type). Since viton gaskets are used for all sealing faces, the absorbing trap, the main valve and its neighbar parts can be baked up to 450°C and 200°C respectively.
The other is all metal evaporator bakable up to 400°C, which is consist of stainless steel bell-jar (300φ ×450h) two oil diffusion pumps of 700 l/sec and 60 l/sec in seiies, and 8″ absorbing trap. Gaskets are aluminium. The ultimate pressure of 4×10^-7Torr can be attained by the former evaporator in about a half period comparing to the necessary time to reach the ultimate pressure of 7×10-6 Torr of an ordinary evaporator. By the latter one, an ultra-high vacuum in range of 10^-9 Torr can be obtained.
The absorbent used in this experiment is the activated Al203, which is baked at various degrees of temperature. The mass-spectrometric analysis of evolved gas from the absorbent shows that the baking is an effective method of activation, and by this the time need to attain ultimate pressure is to be shorten.