How to choose the vacuum system material of the evaporation coating machine?
Evaporation coating equipment is often abbreviated as "evaporator", and its high vacuum system is very important to the selection of materials. The first thing to consider is that the vapor pressure of the selected material is low, otherwise the air load will be large. For example, brass contains zinc with high vapor pressure, low-melting alloy contains tin-lead, etc., and plastic, grease, rubber, etc. are not suitable for use. Second, consider the thermal stability of the material.
Polymer compounds have poor thermal stability and are easily oxidized. Such as the pyrolysis of grease at high temperatures, releasing hydrogen and hydrocarbons. The metal material used in the extremely high vacuum system is best to choose stainless steel, and try not to use copper, copper alloys, etc., because copper and copper alloys exposed to the atmosphere will oxidize quickly at high temperatures. When copper must be used in the vacuum system, it is best to use vacuum smelted copper instead of electrolytic copper. When copper pipes are used as water-cooled pipes or as pipes for transmitting low-temperature liquid, oxidation occurs due to repeated baking, which is easy to cause failure. Aluminum, tungsten, and tantalum are also best smelted in vacuum, with small outgassing. Other materials should also be vacuum pre-vented before use. For the same reason, it is best not to use brazing or silver welding when welding, because these welding processes need to use some fluxes with higher vapor pressure.
So, is there a material more suitable for the high vacuum system of the vapor deposition machine than stainless steel?
Aluminum has been used to make large vacuum devices (accelerators, etc.). However, aluminum has many shortcomings, such as porosity, containing more gas, low high-temperature strength, and relatively difficult to weld, so it has great limitations when used as a vacuum container. However, the permeability of aluminum to hydrogen at room temperature is about 10⁻⁻⁷ of 300 series stainless steel, so a 10µm thick aluminum film is deposited on stainless steel, which reduces the outgassing of hydrogen permeation by 10⁵ times. Aluminum composite on stainless steel can be used as electron tube electrode material. As long as full attention is paid to smelting and forging, aluminum is expected to become a material for the high vacuum system of vapor deposition machines.
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