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Abstract

The interactions between gas molecules and a surface strongly affect the behaviour of
systems in a large array of research fields, such as vacuum science and metrology, microfluidics,
particle physics, and nuclear physics. In this work, focus is given to the momentum
accommodation of gas molecules. The parameter used in this work is the effective tangential
momentum accommodation coeffcient (ETMAC), which is defined as the relative
change of a molecule's tangential component of velocity after collision with the surface.
Its importance in two applications is explored; the calibration of a vacuum spinning-rotor
gauge (SRG) and the gas-flow conductance of tubes.
The SRG is a vacuum gauge operating on the principle of molecular drag. Gas
molecules, surrounding a rotating rotor sphere, collide with the rotor's surface, a process
which reduces the rotor's angular momentum. The resulting deceleration rate is directly
related to the pressure surrounding the rotor. Here, the ETMAC acts as a calibration
constant for the SRG and must be accurately determined in order to minimize the measurement
uncertainty of the gauge.
Since the value of the ETMAC is gas-dependent, the SRG calibration must be made in
all gases that the gauge is to be used with. This calibration process can therefore quickly
become time-consuming. Even more complications arise if gas mixtures are to be used. In
this work, an attempt to minimize this gas dependence was made by exposing the rotors to
various surface treatments. If the gas dependence of the ETMAC can be reduced, the SRG
could be calibrated using only one gas, while retaining a small measurement uncertainty for
other gases. It was found that the gas dependence of the ETMAC on a surface roughened by
acid etching is decreased with six pure gases; helium, methane, neon, nitrogen, argon, and
krypton. The largest deviation from the average value of the ETMAC was observed with
helium and neon, the lightest noble gases. Excluding these two gases, however, shows that
gas dependence on a roughened surface is about two times lower compared to a polished
surface, for the other four gases.
The same gas dependence was explored for the case of the conductance of a long
stainless-steel tube. In the molecular flow regime, the number density of the gas molecules
is so low that they pass through the tube with a negligible probability of colliding with
each other. All collisions therefore occur with the surface. The ETMAC therefore plays
an important part in the conductance of such a tube.
For this purpose, a novel method of measuring conductance was developed. It allowed
us to determine the conductance, and from it the ETMAC, by using only one pressure
gauge. An added advantage is that this pressure gauge does not need to be calibrated, if it
has a linear response. The measurement uncertainty of the ETMAC values obtained with
this method was below 0.5 %.
In contrast to the rotors of the SRG, it was found that by roughening the surface of the
tube with acid etching, the gas dependence could be lowered even for the lightest gases:
helium and neon. The spread of ETMAC values for all gases on the roughened surface of
the tube was found to be about five times lower than on the polished tube.