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Which material has the highest breakdown-voltage?

Discussion in 'Electronic Basics' started by Radium, Sep 1, 2006.

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  1. Radium

    Radium Guest

    Hi:

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


    Thanks,

    Radium
     
  2. John Larkin

    John Larkin Guest

    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
     
  3. redbelly

    redbelly Guest

    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.
     
  4. Radium

    Radium Guest


    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.
     
  5. John Larkin

    John Larkin Guest


    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
     
  6. Joerg

    Joerg Guest

    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.
     
  7. Radium

    Radium Guest

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

    Chris Guest

    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
     
  9. Radium

    Radium Guest

    All right.
    Thanx
     
  10. Tim Williams

    Tim Williams Guest

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

    UTFSE.

    Tim
     
  11. Bob Myers

    Bob Myers Guest

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

    Bob M.
     
  12. Guest

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

    Tim Williams Guest

    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
     
  14. John  Larkin

    John Larkin Guest

    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
     
  15. Joerg

    Joerg Guest

    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.
     
  16. Joerg

    Joerg Guest

    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.
     
  17. Tim Williams

    Tim Williams Guest

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

    Tim
     
  18. Eeyore

    Eeyore Guest

     
  19. Joerg

    Joerg Guest

    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.
     
  20. John  Larkin

    John Larkin Guest

    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
     
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