Surface Cleaning Of Vacuum Materials
The surface cleaning of the vacuum material is removing the oil adsorbed on the surface to reduce the gas evolution of the material in the vacuum. The basic methods for surface cleaning of vacuum materials are mainly organic solvent cleaning and ultrasonic cleaning. When cleaning with organic solvents, commonly used cleaning solutions include demineralized water, ethanol, acetone, metal cleaning agents and so on.
Principle of ultrasonic cleaning is shown in Fig.1. After leading the ultrasonic waves into the liquid, the intense movement of the liquid causes a local pressure difference. The low pressure part will produce a large number of bubbles, and when the pressure rises again, these bubbles will burst instantaneously to create shock waves in the liquid. The shock wave will peel off the oil contamination on the solid surface. Ultrasonic cleaning liquids generally use alcohol or acetone. After ultrasonic cleaning, put the cleaned parts in the atmosphere, the alcohol or acetone will evaporate immediately, leaving no scale on the surface of the parts.
Fig.1 How Ultrasonic Cleaning Works
Polishing Of Vacuum Materials
The amount of gas released from the vacuum chamber and internal components is directly related to the surface area of the material. The atomic density of the solid surface is about 2×1019 m-2. Considering the unevenness of the surface, the actual solid surface will adsorb a large number of gas molecules. Polishing can make the surface smoother, the surface area can be greatly reduced, thereby reducing the gas molecules adsorbed on the surface. Polishing mainly includes glass shot blasting and electrolytic polishing.
The shot blasting of glass pellets is low in cost, but the surface roughness of the treated solids is large, as well as the emitted gas, so it is currently used less and less.
The principle of electrolytic polishing is to place the metal part in an acidic electrolyte, the metal part is connected to the anode, the electrode and the container are used as the cathode, the electricity is applied to make the convex part of the metal part facing the cathode dissolve first, and the Fe and Ni atoms in the metal part are dissolved into the electrolyte to achieve the ultimate goal of a smooth surface (Fig.2). If the cathode electrode is flexibly provided, both the inner and outer surfaces of the cylindrical metal member can be polished, and the metal parts can also be subjected to selective partial polishing.
Fig.2 Electrolytic polishing schematic
Electrolytic polishing can eliminate fine irregularities (0.01 μm to several μm) on the metal surface, ensure the metallic luster, but it cannot eliminate relatively large irregularities. Electrolytic polishing is a typical electrochemical polishing method, and the oxide film formed on the surface also reduces the adsorption of gas molecules.
There is also a special surface treatment method, that is, introducing a special gas into the vacuum chamber to induce a discharge in the cavity, and the generated ions hit the inner wall of the cavity to obtain a smooth surface.
Surface-purified vacuum chambers and components should not be touched directly by hand to avoid contamination of hydrocarbons and chlorides in sweat. Besides, the dust and water vapor (saliva) should be avoided as much as possible.