Introduction to outgassing

January 11, 2022

4 min read

There are several contributions to the gas load of a system. At pressures below ~0.1 mbar, the most dominant is often ‘outgassing’. Outgassing is the result of desorption of previously adsorbed molecules, bulk diffusion, permeation and vapourisation. Adsorption occurs via two main processes, physisorption and chemisorption, and can be described using five (or six) classifying isotherms.

This blog is based on the article in Applied Science and Convergence Technology 26 (5): 95-109 (2017) R Grinham and A Chew, courtesy of www.vacuumscienceworld.com [DTP1]

Looking at the desorption rate, pumping speed and re-adsorption on surfaces, the net outgassing of the system can be calculated. In this article, we share more on the outgassing process and the outgassing rates of common materials. Lowering the outgassing rate allows lower vacuum pressures to be achieved.

Gas loads

As seen in Diagram 1, contributions to the gas load of a system can come from:

Initial or the ‘bulk’ gas in the system
Process load
Back-streaming
Leaks
Outgassing

For a leak-tight system in High Vacuum (HV) with no process load, outgassing could contribute up to 100% of the gas load.

Diagram 1: Gas loads in a vacuum system

The relative contribution of different species to the gas load varies with pressure. For many HV applications water vapour is the major concern in terms of outgassing. However, for achieving UHV in all metal systems, H2 outgassing is critical. The table below shares typical major gas loads at various pressures.

Pressure (mbar)	Major Gas Load
Atmosphere	Air (N₂, O₂, H₂O, Ar, CO₂)
10^-3	Water vapour (75-95%), N_2, O₂
10^-6	H₂O, CO, CO_2, N₂
10^-9	CO, H₂CO₂, H₂O
10^-10	H_2, CO
10^-11	H_2, CO

There are 4 main mechanisms which contribute to outgassing (shown in the diagram below):

Vapourisation of the actual surface material itself (in metals this is negligible at typical operating temperatures)
Desorption — this is the reverse process of adsorption; the release of molecules bound at the surfaces of the chamber and internal fixtures
Diffusion — this is the movement of molecules from the inner structure of the material to the surface
Permeation — this is the movement of molecules from the external atmosphere through the bulk to the vacuum surface

The extent to which each of these affects outgassing depends on the composition of both the gas and the surface material (and its history). Outgassing rates are a sum of these contributions.

Diagram of mechanisms contributing to outgassing

Diagram 2: mechanisms contributing to outgassing

Calculating using the outgassing rate equation

The graph below shows how to calculate gas loads using the outgassing rate equation.

process to calculate gas loads using the outgassing rate equation

Keep in mind that the value of the decay constant gives an indication of the material and outgassing mechanism. For example:

α ≈ 1.1-1.2 ultra-clean metal surfaces
α ≈ 1 metals, glasses and ceramics
α ≈ 0.4-0.8 polymers
α ≈ 0.5-0.7 highly porous surfaces
α ≈ 0.5 diffusion-controlled outgassing from the bulk

Typical outgassing values

In the table below, we share typical outgassing values, where t = 1 hour.

Material	Average (mbarls^-1cm^-2)
Aluminum	3.0 X 10^-7
Iron	2.7 X 10^-7
Brass	1.5 x 10^-6
Copper	2.3 x 10^-8
Gold	1.1 x 10^-7
Mild steel	6.2 x 10^-7
Stainless Steel	1.9 x 10^-7
Zinc	2.6 x 10^-7
Titanium	1.0 x 10^-8
Pyrex	9.9 x 10^-9
Neoprene	4.0 x 10^-5
Viton A	1.1 x 10^-6
PVC	3.2 x 10^-6
PTFE	1.4 x 10^-6

Summary

Outgassing is often the largest contributor to a system’s gas load (especially below Medium Vacuum) and limits the achievable ultimate pressure. It occurs via several processes including vapourisation, desorption, diffusion and permeation. The major contributions to outgassing depend on the vacuum level but in HV it stems mainly from water vapour, whilst hydrogen is most common when working with metals at UHV.

There are many outgassing rates available in the literature, however there is significant variation in these. While variations in outgassing rates can primarily be attributed to the measurement method used and sample preparation; the development of a standard for rate measurement techniques would be valuable.

Look out for our next post on the ways to reduce outgassing

Reference

1 K. Jousten, Thermal Outgassing, No. OPEN-2000-274, CERN (1999)

2 J. M. Lafferty, Foundations of Vacuum Science and Technology John Wiley & Sons, Inc (1998)

Base Pressure by Combination of Pumping & Purging, BOC Edwards, Third EUVL Symposium (2004), J. Zhou, S. D. Dasso, Cycle Purging a Vacuum Chamber During Bakeout Process, U.S. Patent No. 5,879,467 9 Mar. 1999