What is Outgassing?
In a well-designed, well-constructed vacuum system, in the absence of deliberately injected gas, the major contributor to the gas load is the desorption of gases/vapors from the vacuum surfaces – outgassing. More specifically, the outgassing rate is the amount of gas leaving some unit area of surface in unit time.
What is Outgassing Rate?
So, what does outgassing rate mean? Well, its name suggests a measurement of the amount of gas that comes out of something in a given time. On reflection, you'll probably agree “out” doesn't mean “out” as much as “from the surface of” and it doesn't apply until the surface is under vacuum. Outgassing rate, then, is the amount of gas leaving a surface under vacuum in unit time.
What does amount mean? We know it's not just pressure since intuition tells us: if there are two chambers, one big and one small, at the same pressure, then the bigger has the larger amount of gas. From similar arguments, we don't mean just volume either. Take a single chamber, fill it first to a low pressure, then to a high pressure and we are pretty certain the larger amount of gas is contained in the higher pressure version.
So, defining amount of gas as pressure x volume looks good. Specify the volume and pressure of a gas and you can press, squeeze, spindle, fold, or even change the temperature, and it's still the same amount of gas (the same number of molecules). But temperature makes the arguments messy, so I'll ignore it. The logic is still sound.
OK, if we state the pressure and volume of gas leaving a surface then we've specified the outgassing, right? Wrong! Think about two panels made from the same material. One is one square foot, the other is two square feet. Would the same amount of gas outgas from both? Hardly! You expect the larger surface to give off twice the amount of gas of the smaller. Clearly, we must make outgassing depend on area.
Putting amount and area together, maybe outgassing rate is pressure x volume per unit area but there's still a hitch. Again, take two panels, only this time, identical in size and appearance. One, however, outgases a great deal; the other outgases hardly at all. How do you tell the difference between them?
Me? Oh, I'd dump one panel in a vacuum chamber, pump it to some low pressure which I'd note, isolate the chamber, wait for five minutes and re-read the pressure. Then I'd do exactly the same test with the other panel. The panel giving the highest pressure rises after five minutes and is the one that outgases most. Outgassing is indeed a rate and needs time in its unit of measurement. So after all that huffing and puffing, outgassing rate is simply:
PRESSURE x VOLUME per unit AREA per unit TIME
Who said this subject was difficult? It's child's play. It's so easy we can play with different units of pressures, volume, areas, and times and specify outgassing rates. How about:
Torr x Liters per Square Centimeter per Second? or
Pascal x Cubic Meter per Square Meter per Second? or
Inches of Water Gauge x Cubic Rod, Pole, or Perch per Square Furlong per Fortnight
Anything that follows the Pressure x Volume per Area per Time formula can be used to specify an outgassing rate. But now for the really tricky part - read about those weird Outgassing Units.
Outgassing Rate Units
Any combination of units for pressure volume, area, and time can be used but there are just three combinations commonly quoted:
- Torr × liter per square centimeter per second (mostly in the USA)
- millibar × liter per square centimeter per second (mostly in Europe)
- pascal × cubic meter per square meter per second (the Sl unit)
Note: by manipulating units the last combination can be transformed into the seemingly bizarre but correct W/m2 and is quoted this way in some literature collections. To transform rates in W/m2 into Torr-L/(cm2-s) divide the former value by 1,333.2.
Outgassing Sources
Surfaces are active places that absorb gases and vapors to reduce the “unfulfilled” bonding forces of the surface atoms. This means that all surfaces, no matter what material is under consideration, outgas under vacuum. Some of the worst materials are: plastics, elastomers, and glues; porous ceramics and porous metals; lubricating, sealing, or heat transfer greases; and humans (fingerprints, hair, skin cells, dust mites, spittle droplets when talking, and food)!
The most common gases/vapors outgassing from surfaces are: water vapor; oil and grease vapors; solvents and volatile organic materials; and (when approaching ultrahigh vacuum pressures) hydrogen and carbon monoxide from stainless steel used in the chamber's construction.
Reducing Outgassing
As stated above, nothing can be pumped form the chamber until it is in the gaseous phase. The outgassing rate is reduced by methods that cause adsorbed gas/ vapor to enter the gaseous phase:
- Heat: baking the chamber increases the description rate of the gas/ vapor
- Light: intense short wavelength UV breaks bonds between surface and adsorbed gas/ vapor
- Plasma: active plasma products break bonds and react with adsorbed gas/ vapor forming more volatile compounds
- Chemistry: reactive chemical vapors convert adsorbed water into HC (very limited utility)