8.4.
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277
This material has a very high melting point (2600°C, 850° more than
platinum). Of course if it were to be heated in air it would oxidize and
be ruined long before this temperature was reached, but it can be heated
nearly to this great temperature in vacuo, and all significant contaminants
are vaporized. Perhaps the simplest way of doing this is to use a narrow
strip of 2 mil molybdenum as a boat to hold the apertures to be heated.
It may be creased down the middle as a gutter, and the apertures lined
up along this. The boat is mounted in the vacuum chamber between
two electrode posts. After a satisfactory vacuum has been obtained, current is passed through the boat, heating it to a white hot temperature,
a little below what is expected to vaporize the molybdenum. It is left at
this temperature for perhaps 5 minutes, and then the current increased
by about 5 amp. After another 5 minute interval, once more there would
be a 5-amp increment. This continues until a segment of the ribbon
vaporizes, and the boat breaks. The apertures by this time are lightly
welded to the boat so that they do not fly all over the vacuum chamber.
After cooling and returning to atmospheric pressure the apertures can
reliably be regarded as absolutely clean. Other investigators use a molybdenum or a tungsten boat of standardized dimensions, and heat this with
a predetermined amount of current as high as they dare go. However,
since the temperature is kept well below that which vaporizes molybdenum, much longer periods of cooking are recommended.
Molybdenum should be regarded as the preferred material of commercially made apertures at the present time. Originally there were
complaints that they were not bored sufficiently smoothly, and so their
rough surfaces contaminated very quickly, much more quickly than
platinum apertures with a smooth bore. By now manufacturers have
overcome this difficulty.
Another approach to the problem of having an uncontaminated aperture is to make them so easily and cheaply that they can be discarded as
soon as they start causing trouble, and be replaced with a brand new one.
If copper is used, this can be done.
Making copper apertures may seem like a formidable task, but actually it is quite simple, and a little practice makes one nearly perfect.
A ribbon of 2 mil copper should be cut so that it is slightly narrower
than the cap that will hold the aperture. A carefully sharpened needle
mounted in a pin vise is used to bore a tiny hole. The needle is lightly
sharpened before each use with a very fine abrasive stone. It is cleaned
of abrasive particles by punching it through a piece of paper. The tip
APERTURE CLEANING AND MANUFACTURE
277
This material has a very high melting point (2600°C, 850° more than
platinum). Of course if it were to be heated in air it would oxidize and
be ruined long before this temperature was reached, but it can be heated
nearly to this great temperature in vacuo, and all significant contaminants
are vaporized. Perhaps the simplest way of doing this is to use a narrow
strip of 2 mil molybdenum as a boat to hold the apertures to be heated.
It may be creased down the middle as a gutter, and the apertures lined
up along this. The boat is mounted in the vacuum chamber between
two electrode posts. After a satisfactory vacuum has been obtained, current is passed through the boat, heating it to a white hot temperature,
a little below what is expected to vaporize the molybdenum. It is left at
this temperature for perhaps 5 minutes, and then the current increased
by about 5 amp. After another 5 minute interval, once more there would
be a 5-amp increment. This continues until a segment of the ribbon
vaporizes, and the boat breaks. The apertures by this time are lightly
welded to the boat so that they do not fly all over the vacuum chamber.
After cooling and returning to atmospheric pressure the apertures can
reliably be regarded as absolutely clean. Other investigators use a molybdenum or a tungsten boat of standardized dimensions, and heat this with
a predetermined amount of current as high as they dare go. However,
since the temperature is kept well below that which vaporizes molybdenum, much longer periods of cooking are recommended.
Molybdenum should be regarded as the preferred material of commercially made apertures at the present time. Originally there were
complaints that they were not bored sufficiently smoothly, and so their
rough surfaces contaminated very quickly, much more quickly than
platinum apertures with a smooth bore. By now manufacturers have
overcome this difficulty.
Another approach to the problem of having an uncontaminated aperture is to make them so easily and cheaply that they can be discarded as
soon as they start causing trouble, and be replaced with a brand new one.
If copper is used, this can be done.
Making copper apertures may seem like a formidable task, but actually it is quite simple, and a little practice makes one nearly perfect.
A ribbon of 2 mil copper should be cut so that it is slightly narrower
than the cap that will hold the aperture. A carefully sharpened needle
mounted in a pin vise is used to bore a tiny hole. The needle is lightly
sharpened before each use with a very fine abrasive stone. It is cleaned
of abrasive particles by punching it through a piece of paper. The tip