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epoxy in vacuum

J

Jamie M

Jan 1, 1970
0
Hi,

I am making some DIY vacuum electrical feedthroughs into an aluminum
cylinder wall. I was thinking of just drilling a hole and epoxying in
a short length of copper plumbing tube, that has a copper wire epoxied
in it. Would this work ok for vacuum sealing and also for not too much
vacuum outgassing from the cured epoxy? Also how thick of epoxy is
required for 50kV standoff voltage!?

cheers,
Jamie
 
C

Chieftain of the Carpet Crawlers

Jan 1, 1970
0
Hi,

I am making some DIY vacuum electrical feedthroughs into an aluminum
cylinder wall. I was thinking of just drilling a hole and epoxying in
a short length of copper plumbing tube, that has a copper wire epoxied
in it. Would this work ok for vacuum sealing and also for not too much
vacuum outgassing from the cured epoxy? Also how thick of epoxy is
required for 50kV standoff voltage!?

cheers,
Jamie

Use a polyurethane potting compound. They are several hundred volts per
mil thickness. You can pre-evacuate it after mixing, but prior to the
end of its rated "pot life". It has to actually be applied by that
point. Then, you can bake it or time cure it.

So one tenth of an inch should be well over 50kV. That's 100 mils at
several hundred volts per mil. They differ, but not by much.

Epoxies, on the other hand, do differ a lot more in insulative capacity
and cured properties as well.

The poly stays attached to surfaces well, and doesn't gas, and is
rubbery, albeit very tough rubber. Usually doesn't need primer as well.
Also NASA likes it, so it has to be good.

The epoxies usually cure much more firm, and less serviceable, if at
all, and it is very stiff usually, and can fracture and gas. They can
also detach from surfaces, with or without primer.
 
J

Jamie M

Jan 1, 1970
0
Use a polyurethane potting compound. They are several hundred volts per
mil thickness. You can pre-evacuate it after mixing, but prior to the
end of its rated "pot life". It has to actually be applied by that
point. Then, you can bake it or time cure it.

So one tenth of an inch should be well over 50kV. That's 100 mils at
several hundred volts per mil. They differ, but not by much.

Epoxies, on the other hand, do differ a lot more in insulative capacity
and cured properties as well.

The poly stays attached to surfaces well, and doesn't gas, and is
rubbery, albeit very tough rubber. Usually doesn't need primer as well.
Also NASA likes it, so it has to be good.

The epoxies usually cure much more firm, and less serviceable, if at
all, and it is very stiff usually, and can fracture and gas. They can
also detach from surfaces, with or without primer.

Hi,

Thanks, would this stuff work?

http://www.homedepot.ca/product/polyurethane-construction-adhesive-300ml/947644
 
K

Klaus Bahner

Jan 1, 1970
0
Hi,

I am making some DIY vacuum electrical feedthroughs into an aluminum
cylinder wall. I was thinking of just drilling a hole and epoxying in
a short length of copper plumbing tube, that has a copper wire epoxied
in it. Would this work ok for vacuum sealing and also for not too much
vacuum outgassing from the cured epoxy? Also how thick of epoxy is
required for 50kV standoff voltage!?

Depends very much on what you want to achieve. In a "low-tech", one-off,
home experiment, epoxy works well. No need to go for exotic industrial
stuff, ordinary two component expoxies (hardware store) perform well.
The tricky part is to avoid air bubbles when mixing - the cured
insulation layer should not contain any voids.

It's not so much the absolute voltage than the field strength
determining how thick the insulation layer has to be. In your coaxial
feed through design a few (5-10) millimeters should bring you on the
safe side, when you observe that inner conductor diameter is about 1/3
of the outer diameter.

Epoxy based glues outgass like hell. Thus it depends on how low your
vacuum shall be and what your pumping speed is, whether the perform well
vacuumwise. Down to about 10^-3 mbar there shouldn't be hardly any
problem. If you want to achieve high-vacuum (order of 10^-7 mbar), even
a tiny blob of newly cured epoxy will make it take days under constant
high speed pumping to reach that vacuum level.

Hope that helps
Klaus
 
M

mike

Jan 1, 1970
0
Hi,

I am making some DIY vacuum electrical feedthroughs into an aluminum
cylinder wall. I was thinking of just drilling a hole and epoxying in
a short length of copper plumbing tube, that has a copper wire epoxied
in it. Would this work ok for vacuum sealing and also for not too much
vacuum outgassing from the cured epoxy? Also how thick of epoxy is
required for 50kV standoff voltage!?

cheers,
Jamie

I'd check with one of the high vacuum groups.
I tripped over some threads recently that discussed this subject.
Apparently there are epoxies that work well as vacuum seals, but a
random selection probably won't.

I'd also worry about the thermal expansion differences between the
aluminum and copper. Epoxy might need to be flexible.

http://www.teralab.org/index.htm

Here's a quote from here:
http://www.fusor.net/board/download_thread.php?bn=fusor_vacuum&thread=1148666207

For educational purposes, going against the grain of conventional wisdom
on high vacuum technique is definitely worthwhile, because commercial
stuff made for the purpose is unavoidably expensive. I think your
website is proof that you're on this track already. Acrylic (plexiglas),
PVC pipe, and most other plastics are decent in a microtorr vacuum as
long as they don't get hot. Copper water pipe and 3% Ag / Sn solder is
likewise just fine (and don't bother with the challenge of trying to
make inside joints). Hysol 1C epoxy is the same as the much more
expensive Varian Torr-Seal, and works great as a glue or a makeshift
structural material in vacuum. Teflon ball valves from a hardware store
are very respectable at high vacuum. Metal-to-glass and metal-to-ceramic
seals can be scavenged off of microwave oven magnetrons, CRTs, and other
vacuum tubes, and then you can epoxy or solder these into apparatus.
Heavy-wall Tygon tubing makes a great makeshift (and flexible) "neon
sign" through the medium vacuum range, and from experience I will tell
you that school kids love playing with things like that.
 
Q

qrk

Jan 1, 1970
0
Hi,

I am making some DIY vacuum electrical feedthroughs into an aluminum
cylinder wall. I was thinking of just drilling a hole and epoxying in
a short length of copper plumbing tube, that has a copper wire epoxied
in it. Would this work ok for vacuum sealing and also for not too much
vacuum outgassing from the cured epoxy? Also how thick of epoxy is
required for 50kV standoff voltage!?

cheers,
Jamie

Adhesives don't bond well to aluminum due to the oxide layer. If this
is for a short period of time (months), you're probably ok. Otherwise,
use the expensive glass-metal feedthrus. Be careful of partial
vacuums, it doesn't take much voltage to flash over at a partial
vacuum. Saw a guy pulling down a chamber with 120VAC applied to the
heater in the chamber. Beautiful flash. He spent the whole day
scrubbing the copper off the walls.
 
C

Chieftain of the Carpet Crawlers

Jan 1, 1970
0
Nasa website lists materials and their outgassing.for use in a vacuum


The only compound NASA approves for space applications and HV is
"CONAP" brand (DuPont, IIRC) polyurethane.

The stuff is so tough, it makes you think it would stop a bullet.

It certainly stops any knife you can drive at it within a quarter inch.
That is how good it grabs knifepoints.

Anyway, no outgassing, and adhesion remains. It is not very
serviceable because it is so turgid (durable).
 
W

WoolyBully

Jan 1, 1970
0
you're probably ok. Otherwise,
use the expensive glass-metal feedthrus. Be careful of partial
vacuums, it doesn't take much voltage to flash over at a partial
vacuum.

Total bullshit.

As pressure goes down, dielectric resistance goes up because the air
molecules are farther apart. So, it takes more voltage in a reduced
pressure chamber than it does at atmospheric pressures. It MAY be far
less than the first arc in a full vacuum takes, but it will still be
higher than in normal air.

The volts per mil to acquire an arc in a air lattice is 100% dependent
on pressure. That is what determines atom spacing in ANY gas.

A partial vacuum has air in the chamber. There is only one type of
"full vacuum".

A full vacuum does not have the same behavior as a chamber with an air
lattice in it. Once an arc is created, there are metallic 'gasses in the
chamber, and on surfaces and carbon trails and arc path can form and
substantiate themselves. At that point, the "full vacuum" can no longer
be claimed. At all.

So the arc in even the best chamber draw down will require a higher
voltage than it will once the very first arc occurs. After that, it
isn't really a fully vacuumed chamber any more.
 
C

Carl Ijames

Jan 1, 1970
0
Um, ever heard of a Paschen curve? The breakdown electric field of a gas
goes down as the pressure falls from way above atmospheric, to a minimum in
the range of .1-50 torr depending on the gas, then rises again as the
pressure falls further. At high pressure the mean free path of an electron
isn't enough to let the energy rise to the ionization potential before a
collision slows it back down so ionization is very inefficient and breakdown
field is high, and at very low pressures the gas density is low enough the
electrons make it all the way from cathode to anode without a collision with
a gas molecule so even though they have plenty of energy to ionize something
there just isn't anything to ionize so the breakdown field goes back up. In
that fun in-between region you get pretty glow discharges and all kinds of
other phenomena.

The main question the OP never answered is "what pressure are you trying to
achieve"? That will dictate the materials needed and the physical layout
needed for his voltage.

-----
Regards,
Carl Ijames
"WoolyBully" wrote in message

you're probably ok. Otherwise,
use the expensive glass-metal feedthrus. Be careful of partial
vacuums, it doesn't take much voltage to flash over at a partial
vacuum.

Total bullshit.

As pressure goes down, dielectric resistance goes up because the air
molecules are farther apart. So, it takes more voltage in a reduced
pressure chamber than it does at atmospheric pressures. It MAY be far
less than the first arc in a full vacuum takes, but it will still be
higher than in normal air.

The volts per mil to acquire an arc in a air lattice is 100% dependent
on pressure. That is what determines atom spacing in ANY gas.

A partial vacuum has air in the chamber. There is only one type of
"full vacuum".

A full vacuum does not have the same behavior as a chamber with an air
lattice in it. Once an arc is created, there are metallic 'gasses in the
chamber, and on surfaces and carbon trails and arc path can form and
substantiate themselves. At that point, the "full vacuum" can no longer
be claimed. At all.

So the arc in even the best chamber draw down will require a higher
voltage than it will once the very first arc occurs. After that, it
isn't really a fully vacuumed chamber any more.
 
B

Bob Quintal

Jan 1, 1970
0
Total bullshit.

Oh, that it would be so simple.
As pressure goes down, dielectric resistance goes up because the
air
molecules are farther apart.

However there are conditions where a higher pressure also probvides
quenching of the arc, deflection of the electric field and cooling of
the electrode surfaces so that arcing does not occur.


So, it takes more voltage in a
reduced pressure chamber than it does at atmospheric pressures.
It MAY be far less than the first arc in a full vacuum takes, but
it will still be higher than in normal air.

Actually, when 'pulling a vacuum' the possibility of arcing remains
fairly constant between 760 Torr(standard sea level pressure) to
about 20 Torr, then increases rapidly until about 10^-3 Torr, below
which the possibility of arcing drops away to much less.

The volts per mil to acquire an arc in a air lattice is 100%
dependent
on pressure. That is what determines atom spacing in ANY gas.

A partial vacuum has air in the chamber. There is only one type
of
"full vacuum".

I'd like to see it. Even in intergalaxy space there are some
molecules of gas.
A full vacuum does not have the same behavior as a chamber with
an air
lattice in it. Once an arc is created, there are metallic 'gasses
in the chamber, and on surfaces and carbon trails and arc path can
form and substantiate themselves. At that point, the "full
vacuum" can no longer be claimed. At all.

So the arc in even the best chamber draw down will require a
higher
voltage than it will once the very first arc occurs. After that,
it isn't really a fully vacuumed chamber any more.

That is so true.
 
S

Spehro Pefhany

Jan 1, 1970
0
Total bullshit.

As pressure goes down, dielectric resistance goes up because the air
molecules are farther apart. So, it takes more voltage in a reduced
pressure chamber than it does at atmospheric pressures. It MAY be far
less than the first arc in a full vacuum takes, but it will still be
higher than in normal air.

The breakdown voltage has a minima which occurs at less than
atmospheric pressure for reasonable gap sizes and common gases.

http://www.highvoltageconnection.com/images/paschen-curve_clip_image002.jpg

The pressure at which the minima occurs is not a constant- it's
inversely proportional to the gap size over a wide range of gap sizes.


Best regards,
Spehro Pefhany
 
S

Spehro Pefhany

Jan 1, 1970
0
Any idea why the minima is a function of gap?

And, any idea what function the minima subtends?

http://en.wikipedia.org/wiki/Paschen's_law

pmin = (1/d)*exp(1-b) where b is a gas-dependent constant

As the page says, you can get this by differentiating Paschen's law

V = (a * d * p)/(b + ln(d*p)) wrt p, which gives you

(a*d)/(b + ln(d*p)) - (a*d)/(b + log(d*p))^2

and setting the derivative to zero to find a min (or max).




Best regards,
Spehro Pefhany
 
W

WoolyBully

Jan 1, 1970
0
Any idea why the minima is a function of gap?

And, any idea what function the minima subtends?

The initial arc in a pure vacuum is hard to initiate. The moment it
is, however, the node that was struck by the electron stream gives off
metallic vapors.

The circumstance of your "vacuum" and the entire chamber at that point
is now 100% different than the starting set-up.
 
S

Spehro Pefhany

Jan 1, 1970
0
I think he's been "funning" us all. No one can be that wrong without
working at it. I think AW deliberately misstates the facts to get a
rise out of us! Being consistently wrong takes more brains than just
using guess-work. A deliberate > 50% wrong score indicates considerable
intelligence. ;-)

Yes, the thought did occur to me. He was just slightly too obvious in
that last troll, but just enough to work (on a weekend).


Best regards,
Spehro Pefhany
 
W

WoolyBully

Jan 1, 1970
0
I think he's been "funning" us all. No one can be that wrong without
working at it. I think AW deliberately misstates the facts to get a
rise out of us! Being consistently wrong takes more brains than just
using guess-work. A deliberate > 50% wrong score indicates considerable
intelligence. ;-)

I worked on the development of an HV oil filtration device. It used our
20 kV supply, and our plate arrangement and our tank chamber and oil
pump. We PULLED the oil through the chamber and filter media as this
reduced the pressure in the chamber and that was enough to keep arcing
events from happening. Operated with the lid off, it would arc
immediately. With the lid on, the pressure would pull down almost
immediately as the oil's viscosity took some work to pull it through the
piping. That reduced chamber pressure kept it from arcing.

Your curve is for pressures well below that.

In normal air, the voltage required to establish an arc rises as
pressure drops. This is NOT data related to a vacuum. It is for reduced
pressure environments.

The curve plot refers to vacuums and very low pressures.
 
W

WoolyBully

Jan 1, 1970
0
Here's the Paschen curve

Then the tanks in our design were pulling below 1mm and were on the
other side of the slope.

Either way, the result was that it did not arc when under negative
pressure, and did arc in open air.

This refers to the tops of the plates in the filter chamber, which was
bigger than a gallon,but smaller than a cubic foot.

A good question would be to see what the plot is on values above sea
level. IOW, what kind of insulator is compressed air?
 
S

Spehro Pefhany

Jan 1, 1970
0
In normal air, the voltage required to establish an arc rises as
pressure drops. This is NOT data related to a vacuum. It is for reduced
pressure environments.

The curve I linked to (air, 0.1" gap) plots from 760 torr (which is 1
atmosphere, of course) downward, and the breakdown voltage of at 0.1"
gap in air decreases with dropping pressure down to about 0.2 torr,
which is a medium vacuum. For to go up, the gap size would have to be
< 1 micron.

High voltage clearances should be increased for devices that have to
remain functional at high altitudes.

Many, many years ago I worked with a suitcase-sized system that
contained a subminiature Van de Graff generator enclosed in a
pressurized SF6 atmosphere. It made many hundreds of kV, IIRC.
AFAIUI, SF6 will likely be banned if they can find a workable
alternative- it's a very effective greenhouse gas.


Best regards,
Spehro Pefhany
 
F

Fred Abse

Jan 1, 1970
0
High voltage clearances should be increased for devices that have to
remain functional at high altitudes.

With waveguides, that's a fucking headache.
 
J

Jamie

Jan 1, 1970
0
WoolyBully said:
The initial arc in a pure vacuum is hard to initiate. The moment it
is, however, the node that was struck by the electron stream gives off
metallic vapors.

Other wise know as plasma.
The circumstance of your "vacuum" and the entire chamber at that point
is now 100% different than the starting set-up.

Hmm.
Think about FS6, roughing and defusing pumps. Of course, it's nice to
have an ion gauge and DOM's sitting in the chamber for safety shut downs.

Jamie
 
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