Vacuum extraction rate and gas adsorption

- Apr 26, 2019-

Vacuum extraction rate and gas adsorption

 

Materials can absorb and dissolve gases in the atmospheric environment. Therefore, materials stored in the atmosphere will gradually release the gas adsorbed on the surface of the material and the gas dissolved in the material under the vacuum condition. This phenomenon is called the air output of the material, and the air output per unit time and area is called the air output rate. Different materials have different effluent rates. Even the same material has different effluent rates due to different smelting conditions, storage conditions, processing technology, surface roughness, corrosion and pre-treatment before it is put into the vacuum chamber. This is why the effluent rates measured by different authors are not the same. When selecting the air outlet rate of materials, these conditions must be taken into account. Data with similar test conditions should be selected as far as possible. If necessary, the air outlet rate of materials can be re-checked.

 

The air outlet rate of the material is a function of temperature, and its relation is as follows:

lgq=lgq1-αlgt 及 q=q0exp(-E/ RT)

Where:

q: air outlet rate;

Q0, q1: constant;

T: extraction time (extraction time)
E: gas output activation energy
R: molar gas constant
T: thermodynamic temperature
α: effluent rate attenuation coefficient
(organic material α 0.5, metal material α 1)

Molecular state and atomic state permeation relation to material:

 

Nondissociative molecular osmosis: q = KA p/d
q=(KA p)/ d
Where:

q: rate of gas penetration through solid wall surface
P: air pressure difference between two walls d: wall thickness
K: permeability coefficient of gas to material
A: wall surface area

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