Which material has the highest breakdown-voltage?

If you don't allow vacuum, teflon is a candidate at 60 MV/m, about

1500 volts/mil. That's not bad.

But it's a sort of fuzzy measurement.

John

You may or may not get a definitive answer from this group. Suggest you also do a Google search on either "breakdown voltage" or "dielectric strength", and get an idea of how resistant different materials are to breakdown. What is the highest number you find in your search? Then post back here with some results that others might find useful or interesting.

Regards,

Mark

p.s. the numbers will have units of (voltage / distance). For example: V/mil or "volts per mil"; or V/cm; or V/m, etc.

Dielectric strength (kV/inch) of material is shown below:

Vacuum ------------------- 20 Air ---------------------- 20 to 75 Porcelain ---------------- 40 to 200 Paraffin Wax ------------- 200 to 300 Transformer Oil ---------- 400 Bakelite ----------------- 300 to 550 Rubber ------------------- 450 to 700 Shellac ------------------ 900 Paper -------------------- 1250 Teflon ------------------- 1500 Glass -------------------- 2000 to 3000 Mica --------------------- 5000

Seems mica is the strongest.

These numbers are very messy. There are plastics that are rated at

7000 v/mil, which translates to 280 megavolts/meter. The teflon number above translates to 60 MV/m, which is only 1500 v/mil.

I suspect that thin films sustain higher fields than bulk material, and that a lot of these measurements are crap.

And the dielectric strength of a true vacuum is roughly infinite.

John

Hello John,

There is also a huge variance caused by impurities and by how good your chances are to manage or avoid them. Glass usually comes out ahead because it resists damage pretty well. I like PTFE if it's a little thicker and in med we often use 20mil. When it has to be much thinner Kapton is nice.

--
Regards, Joerg

http://www.analogconsultants.com

What kind of plastics?

I'm too lazy. What is the mathematical equation for the translation of MV/m to V/mil?

2.54mm/in means 39370 mils (1/1000") per meter. So

1MV/m (Megavolt/meter) = 1E6 Volts * (1meter / 39370 mils) = 25.4 volts/mil.

The sci part in sci.electronics.* means you're supposed to bring your mind to the conversation here. C'mon.

And as a practical matter, engineers will typically use air, teflon or ceramic for high voltage insulation.

Cheers Chris

Okay.

Sorry.

All right.

Thanx

Oughta be diamond, also the stiffest and hardest known material.

UTFSE.

Tim

--
Deep Fryer: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

Whatever it is your supposed brain is made of, in terms of its resistance to actually learning anything.

Bob M.

I dont think so!! maybe you could dig out an old tube from somewhere and find out for yourself.

TRUE vacuum. Even the best vacuum tubes are in the what, nanotorr range?

Besides, the base pins often arc over (in air) before the internals do.

Tim

--
Deep Fryer: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

A tube has only a mediocre "vacuum". A true vacuum has no matter, no gas molecules at all, nothing to break down and conduct. This can be approximated by an ultrahigh vacuum in which the mean free path of any molecules present is greater than the gap over which a potential is applied; in this case, any accelerated ions have a low probability of colliding with other molecules, so there's no positive feedback to create breakdown.

If a potential is applied between two metallic plates separated by a perfect vacuum, the vacuum certainly won't break down. The plate surfaces could break down by field emission of electrons or eventually metallic ions at very high fields, and ion-impact flashover could result, but the vacuum is entirely passive here. The gradients necessary to rip metal are in the area of 1e10 v/m.

There is some field strength at which quantum-mechanical virtual particles are ripped from the vacuum into actual being, but those field strengths would be cosmic.

John

Hello Tim,

Not if it has a top connecting rod for the plate. Many a good ham radio amp tube has been lost due to runaway "St.Elmo's fire". Mostly by pushing the plate voltage just that wee bit more. If the data sheet says

700V it should easily do 900V, maybe even 950V, huh? Then a slight antenna mishap, some power gets reflected, KABOOM.

--
Regards, Joerg

http://www.analogconsultants.com

Hello John,

From my tube days I remember that one very undesirable event was chunks of the cathode plating being ripped off, getting lodged somewhere inconvenient, followed by a bang.

--
Regards, Joerg

http://www.analogconsultants.com

Ah yes, many an audiophool has been misled into "soft start" and whatnot by cathode stripping proponents.

Tim

--
Deep Fryer: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

Hello Tim,

I thought it was a myth until I stripped down a tube that had decided to disintegrate its glass cylinder. The cathode looked rather awful.

--
Regards, Joerg

http://www.analogconsultants.com

The big ole Tek tube scopes had a bimetallic timer tube (Amperite? 30 seconds?) that allowed the filaments to warm up before applying the high voltages. This prevents cathode stripping and also keeps the HV from being unloaded and going bezerk (the 5U4 rectifier filaments heated up a lot faster than the other tubes.)

John