Getters/Molecular Sieves

[Willie Koorts]

On Thu, 8 Feb 1996, Tim Abbott wrote:

> 
> Can I initiate a quick survey:
> 
> What getters/molecular sieves (eg. charcoal, zeolite) do you use in 
> your dewars and why (i.e. what contaminants are they good at adsorbing)?
> 
> Our answer: we use charcoal, but it's not clear to me whether this is best
> for sweeping up water vapour &/or hydrocarbons (plus, it makes this nasty
> black powder...).
> 

As can be seen in the introduction of the attatched text,  we had some 
trouble with ccd contamination a while back and I studied a few textbooks 
to try and gain more knowledge on vacuum systems and getter materials to 
try and determine what the root of our problems could be.This is an 
extract from the notes I made while reading there various books.

One aspect not covered in these notes which we adopted since is to 
pre-treat new zeolite in the following way before using it in a system:
The zeolite gets washed in distilled water until all the loose "dust" is 
washed away.  It then gets dried on a hot plate to drive off the bulk of 
the water.  Next it is put in a vacuum oven and vacuum baked at about 280C
for a few hours until a pressure of 10-5 mbar.  It is then purged with 
dry nitrogen gas until cooled to room temperature.

One very interesting thing we found which ties in quite well with the theory. 
One of the references said :
"Experiments conducted on Linde 5A zeolite found desorption maxima for water
vapour to be 137 - 157 C.  All other gasses desorp well at room temperature."
I did not quite know what exactly it meant at the time and just copied it 
word for word from the book to prevent any bias in interpretation.  When 
we got to the vacuum baking process the first time and the temperature 
went through this range,  the vacuum pump pressure suddenly increased 
from about 10-3 to 10-1 mbar which allmost suggests that one only needs 
to heat to this temperature when vacuum baking zeolite.

We have been replacing the charcoal in our Oxford cryostats with 5A 
zeolite, pre-treated in the way just described.  The Infrared 
Laboratories cryostats we bought before came with type 13X zeolite instead 
- 12 micron pores,  suited for larger molecules.

Paul,  are you sure Apiezon M is rated at 10-7 only?   We have just 
switched from Dow Corning silicone (because of the very reasons you 
mentioned) to Apiezon M which was sold to us as being rated to 10-11 mbar! 
We assemble systems without any grease on the o-rings and use Apiezon M  
grease only when absolutely neccesary.

Willie Koorts                          South African Astronomical Observatory



                         VACUUM TECHNOLOGY NOTES
                         ~~~~~~~~~~~~~~~~~~~~~~~
February 1995                                                    W.P.Koorts

In trying to find answers for CCD surface contamination we had,  I read
through a few vacuum manuals to gain a better understanding of the subject
and also corresponded with other groups who use vacuum systems.  The
following are notes from these books and where any ambiguity existed,  I
quoted directly from the books so as not to force my interpretation on the
reader.  These notes are particularly aimed at general vacuum principles,
materials,  cleaning and baking methods and zeolites and is by no means a
vacuum manual.  The books consulted were:
     Vacuum technology (second revised edition)   - A Roth             - 1982
     A user's guide to vacuum technology          - John F. O'Hanlon   - 1980
     Vacuum systems design                - N.T.M Dennis & T.A. Heppel - 1968
     Merck laboratory zeolite data sheet


Sorption materials
==================

Activated charcoal
------------------
The most frequently used charcoal is prepared by destructively distilling
pieces of coconut shell at 500-700C in iron containers until vapour
evolution is no longer apparent.  This produces charcoal with tarry
residues which are then removed by a process called "activation".  It
consists of heating in steam at 800-1000C for about one hour.  The water is
then removed by heating in a rough vacuum.
It has a Ap/Ag ratio of 600-850 which corresponds to a specific surface of
1000 m2/g.
The sorption of water vapour by charcoal exibits a behaviour quite
different from that observed by the less readily condensable gasses.
Activated charcoal shows a sudden increase in sorption rate between 1.5 and
2.5 Torr compared to it's behaviour above and below these pressures.

Zeolites ( Molecular sieves )
-----------------------------
These are alkali metal aluminosilicates having tetrahedral lattices.
Unlike ordinary crystals containing water of crystallization,  they can be
dehydrated without any change in the form of their crystal lattice.  As a
result,  molecules of different gasses can occupy the spaces left vacant by
the removal of water and the zeolites are therefore very good absorbents.
This is however only true for certain gasses since they exibit a property
of persorption which is defined "as adsorption in pores" that are only
slightly under the diameter of the adsorbate molecules.  Typical specific
surfaces are 500-600 m2/g or surfuce area to volume ratios of 800 square 
meters per cubic centimeter.

Compared to activated charcoal zeolites has:
     more consistant condition
     no special storage precautions
     larger sorption capacity
     pellets do not powder
     outgassed and regenerated more easily
     no explosion hazard when pumping oxygen
Activated charcoal has:
     lower cost
     lower regeneration temperature.
     higher thermal conductivity
5A zeolite is commenly used to pump air.
Typically 1 kg Zeolite can evacuate up to 40 liter air from atmospheric
pressure to 10-2 torr in 10 minutes using liquid nitrogen chilling.
Thermal conduction poor - internal cooling fins in carrier helps - complete
chilling of zeolite takes many hours after LN2 fill.
High adsorption affinity for polar  and unsaturated organic molecules.
Water is however always preferentially adsorbed.
Only those molecules that can pass through the pores are adsorbed - sieve
effect.
Water vapour is strongly sorbed by most sorbent materials and is NOT
readily released when sorb returns to room temperature.  The accumulated
water deteriorates pump performance.
The adsorbtive capacity of molecular sieves for water is slightly
diminished at temps at which aluminium oxide and silica gel already release
water.  The capacity gradually falls from 23% @ 0 C  to  7% @ 150 C of it's
own weight in water.   Typical capacity of 18-24% @ 25 C.
Oil vapours are also stongly sorbed and zeolite is therefore sometimes used
in cold traps.
Regeneration: As often as required.
Maximum regeneration temperature. = 450 C.
Final water content is a function of the temperature, time and enviroment
of bake.
Typically when dried in drying oven with dry gas purge at 250 C for 5
hours,  achieves 2-3% water per weight.
Lower values possible by heating to 300-350 C in vacuum.
Experiments conducted on Linde 5A zeolite found desorption maxima for water
vapour to be 137 - 157 C.  All other gasses desorp well at room temperature.
Molecular sieves used to dry solvents should be poured into a large
quantity of water prior to regeneration to remove any adhering solvent.
Heater originally found in Oxford cryostats zeolite container gets up to
175 C without thermal load.


Silica gel
----------
This is a particularly dehydrated jelly of silicic acid.  It is used
especially as drying agent for gasses and has a specific surface of 700-800
m2/g.

Alumina pellets
---------------
These present a specific surface of 300 m2/g and are used to prevent back
streaming of oil vapours.