Which material has the highest breakdown-voltage?

Hi:

Which material is currently known to have the highest
breakdown-voltage?


Thanks,

Radium

 

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.

 

http://www.allaboutcircuits.com/vol_1/chpt_12/8.html


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.

 

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

 

All right.

Thanx

 

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

UTFSE.

Tim

 

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

 

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.

http://casa.jlab.org/seminars/2005/slides/norem_050804.pdf#search="atom probe voltage gradient"

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.

http://en.wikipedia.org/wiki/Virtual_particle#Pair_production

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.

 

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.

 

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

Tim

 

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.

 

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