epoxy in vacuum

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.

Take a look at Torr-Seal.

Also take a look at Hysol C1 White.

Steve

Hi,

Thanks, would this stuff work?

..and i understand that a lot of epoxies outgas like crazy.

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

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.

Here's a quote from here:

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.

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Nasa website lists materials and their outgassing.for use in a vacuum

This URL lists what you need:

Voltage Standoff? Be careful, it is the GRADIENT that kills you,

For breakdown, look at the spot V/m, not the AVERAGE V/m, in other words, the gradient. In a vacuum tiny little somethings will breakdown, and then bleed leakage all over the place!

We had feedthru's rated at 50kV breakdown arcing over at a very low

20kV. Commercial part - rated for 50kV. The part was cleaned before installation and my analyses showed rounded shoulder good design should easily take the voltage, yet we had arc over. The vac technician solved the problem by 'micropolishing' the shoulders of the standoffs! So good luck holding off those kinds of voltages.

Will check on the epoxy we used.

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.

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).

sive, available,

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.

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.

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.

Gas breakdown depends on the pressure in a more complicated way than that. At high vacuum, collisions are rare, so ions don't cause a lot of impact ionization of neutral gas molecules. At high pressure, the ions don't accelerate much between collisions because the molecules are so close together. So you wind up with a broad minimum in breakdown voltage around 1 torr, where there are enough neutral molecules to support an avalanche, and the ions pick up enough speed between collisions to cause impact ionization.

See e.g. M. J. Druyvesteyn and F. M. Penning, Reviews in Modern Physics V12 N87 (1940).

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net

WoolyBully wrote in news: snipped-for-privacy@4ax.com:

Oh, that it would be so simple.

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

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.

I'd like to see it. Even in intergalaxy space there are some molecules of gas.

That is so true.

--
Bob Q.
PA is y I've altered my address.

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

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

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eff.com

Any idea why the minima is a function of gap?

And, any idea what function the minima subtends?

As the mean free path gets longer, you need more distance to build up an avalanche.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net

I designed a photodiode preamp that biases the pd to 25, 50, or 75 volts. Then the customer decided that they might see breakdown as low as 6 volts, in 1 torr hydrogen. So we had to recall all the boxes and modify them for 5/10/15 volt selection.

--

John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com   

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