Indium and Vacuum Seals
Indium is used to make all case seals on the SPIRAC dewar. Although
indium seals is a long-known technique in low-temperature physics, this
was my first exposure to them. Below is a short list of the lessons I
learned about indium seals.
- Both seal surfaces and the indium should be kept as clean as possible.
- Acetone does an adequte job cleaning surfaces and indium.
- When making a seal out of a wire of indium, cut the wire to length,
beveling the edges to make the joint. The freshly cut edges will
stick together readily.
- Not all sealing schemes make reliable indium seals. An improperly
designed seal geometry can often be sealed initially, and then after
a couple of days, will open up. For example, we attempted to make a seal
between a knife edge and a flat. It initially worked, but after a couple
of days, the indium crept inwards, over the knife edge, and a leak opened
up. The problem was solved by machining washers which prevented the
indium from flowing inwards.
- A good rule of thumb for determining the number of fasteners needed to
secure an indium seal is - as many as you can possibly fit on the surface!
- Indium seals can be reliable down to very cold temperatures. The filltube
was reliably sealed to the LHe reservoir with .125" indium.
- Indium can be reformed into useful seals after use. Dan Gezari (GSFC)
successfully extruded indium wire from scrap indium. He first cleaned the
indium with acetone. His extrusion mechanism consisted of a hollow cylinder
with a bolt screwed into one end, a plunger and a stand. The bolt had a hole
drilled lengthwise through which the indium was extruded. A 0.070" hole yielded
indium with a .079" diameter. He used a hydraulic press to extrude the
indium. Smaller diameter indium required larger forces (500lbs for 0.125"
wire, 2000lbs for 0.080" wire).
Last Revision January 26, 1996. Contact firstname.lastname@example.org