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Resistive sheet

Discussion in 'Electronic Design' started by Don A. Gilmore, Dec 31, 2004.

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  1. Hi guys:

    I'm looking for a relatively thin electrically-resistive sheet material.
    The resistivity I need depends on the thickness I can get. Basically I need
    for the resistivity to be somwhere near

    rho = .24 / t

    where rho is the resistivity in ohm-m and t is the thickness of the sheet in
    meters. I would like for it to be thin enough to be flexible in large
    sheets (say, measured in square meters). Here are the corresponding
    resistivities for some nominal inch thicknesses:

    ..031 in ---> 300 ohm-m
    ..062 in ---> 150 ohm-m
    ..125 in ---> 75 ohm-m
    ..188 in ---> 50 ohm-m

    Are any of you aware of a material that meets these specs (or is close) that
    is not extremely expensive? I have seen graphite-impregnated plastics that
    could work well, but only in McMaster-Carr, which has a limited selection.
    The material must be uniformly resistive throughout its cross section (not
    just surface conductive).

    Thanks for any replies.

    Don
    Kansas City
     
  2. Teflon??

    Carl Sachs
     

  3. Thanks Carl, but I thought Teflon was an insulator. Are you referring to
    some sort of graphite-impregnated Teflon?

    Don
     
  4. Guest

    Don, We need to know a little more about what you are trying to do.

    Sheet restivity has dimensions of ohms, not ohm-meters. so it is not
    clear what conduction path you are thinking of.

    Also will you be using DC or could you work at high frequencies? At
    high frequencies the thickness of sheet involved in point-to-point
    conduction depends on the frequency.
     
  5. Guest

  6. For my application the bulk resistivity in ohm-m is more useful. But you're
    right, it would be more likely to be given by the manufacturer in "ohms per
    square" for a thin sheet. To convert we would divide by the thickness of
    the sheet, so the new values would be

    ..031 in ---> 9840 ohms/square
    ..062 in ---> 2450 ohms/square
    ..125 in ---> 610 ohms/square
    ..188 in ---> 270 ohms/square

    Hopefully this will be of more use to all of you. Thanks for pointing out
    the discrepancy.

    Don
     
  7. Guest

    Don,

    For a good short paper that gives the relationship between
    point-to-point resistance and plate thickness see:
    http://microlab.berkeley.edu/ee143/Four-Point_Probe/

    It has info that may be needed to answer your question.
    Contact me directly if you like.

    Dave
     
  8. Uncle Al

    Uncle Al Guest

    What does carbon fiber composite look like?
     
  9. Oops. I didn't convert to meters before I divided. Here are (hopefully)
    the correct values.

    t = .031 in ---> 387 x 10^3 ohm/square
    t = .062 in ---> 96.5 x 10^3
    t = .125 in ---> 24.2 x 10^3
    t = .188 in ---> 10.8 x 10^3

    Sorry about that.

    Don
     
  10. Uncle Al

    Uncle Al Guest

    Metal-filled epoxies?

    <http://www.devcon.com/devconcatsolution.cfm?catid=34>
    http://www.plasticworld.ca/Devcon.htm
    <http://www.loctite.com/int_henkel/loctite_us/binarydata/pdf/LT3355v4_MROepoxies.pdf>
    <http://www.tra-con.com/pdf/tpb/2122.pdf>

    Dilute to adjust resistance. Maintain viscosity (~1 wt-% fumed silica
    will render organics thixotropic) to keep it homogeneous during cure.
    You can cast thin slabs between silanized glass plates using nylon
    fishing line as the spacer and those black squeeze clips for stacks of
    paper as compression along the edges. Position the gasket with a thin
    metal ruler.
     
  11. artie

    artie Guest

    Have you looked at indium tin oxide (ITO) coatings? Thin coatings,
    high resistivity, commonly used in stylus or touch-operated displays
    (think Palm Pilot). Thicker coatings, lower resistivity, may be used
    to heat (microsocope) slides and other materials.

    Google is your friend.
     
  12. Guest

    A paper at:
    http://www.site.uottawa.ca/~sloyka/papers/1999/EMC_ground_resistance.pdf
    Has a reasonable formula showing that the apparent resistance between
    two electrodes on a plate of thickness t depends on the electrode
    diameters, the distance between the electrodes, and the plate
    thickness.

    In particular the apparent resistance depends most strongly the
    electrode diameter times conductivity and on the ratios of: (1)
    electrode diameter to thickness and (2) electrode separation to
    distance.

    Dave
     
  13. Robert Baer

    Robert Baer Guest

    From what little i have seen in conductive, partly-conductive and
    dissipative plastics, carbon loaded plastics are the only ones that
    could cover that resistivity requirement.
    The flexibility is a different matter; you are more or less stuck with
    the choices limited by that resistivity requirement.
    In fact, most of the types of plastics i mentioned are not too
    flexible; the only ones i know of that are as flexible as mylar or
    kapton sheet are classed as dissipative plastics.
     
  14. Robert Baer

    Robert Baer Guest

    Incorrest on two counts.
    Three-dimensional conductivity is measured in ohm-meters.
    Sheet resistivity assumes uniform thickness and is measured in ohms
    per square, period. If one has a non-square shape, then break it into
    squares and add up.
     
  15. Robert Baer

    Robert Baer Guest

    Yep! "ohms per square" is the term; so you know from your own example
    (above) that "ohms" was incorrect.
     
  16. Robert Baer

    Robert Baer Guest

    Notice the equation for a thin sheet: a function of thickness, so that
    shows the bulk resistance has the units of ohm*meter^-1 (ohm per meter).
     
  17. Robert Baer

    Robert Baer Guest

    Metal filled epoxies would fail all his criteria: 1) not flexible, 2)
    resistivity too low, 3) not uniform unless carefully formulated.
    Case in point: silver filled epoxy.
     
  18. Robert Baer

    Robert Baer Guest

    Fails in the "not surface effect" criteria.
    Prolly would also fail (crack) if flexed.
     
  19. Terry

    Terry Guest

    TO: Don Gilmore:

    This is a comment about your question concerning an electrically conductive
    carbon fibre material;
    More than 50 years ago, shortly post W.W. II, some miles outside London,
    England, my uncle had a small manufacturing company that made such a
    material.
    It was definitely not wires embedded in an insulating material.
    AFIK it was a carbon loaded conductive material. I have no idea what the
    filler was; although it looked, come to think of it, rather like the colour
    of whole wheat flour! I seem to remember it was sort of 'mixed up' in
    various consistencies looking rather like a (carbon) speckled cake mix, with
    the then new resins, and was spread/rolled into sheets. IIRC It was made in
    various thicknesses, widths and lengths to obtain various resistances and
    wattage ratings. Before drying (Or maybe it was 'baked' to cure the resins?
    and it became rigid and/or maybe it could be manufactured that way) it could
    be curved, or even bent. One interesting heater was a circular collar, about
    four inches in diameter, that heated the shaft of a radio direction finding
    antenna that projected through the surface of a pressurized jet aircraft. I
    also recall that a typical heater was dark grey in colour, typically a few
    millimetres thick and had a sort of stiff/crisp feel.
    Electrical connections to each heater were made by a commercially available
    vaporized metal spray process (zinc or copper I think!) to the edges of the
    conductive material. The material heated evenly and homogeneously throughout
    with the flow of electric current.
    They produced heaters to work on various voltages; AC mains 230 volt 50
    cycle, 28 volt DC aircraft heaters, 12 volt car seat heaters etc. I recall
    that as trial my grand parents had two 12 volt personal car warmers that
    they tucked behind their backs (or under their "Ahem", 'derrieres') in their
    otherwise unheated Lanchester car (The 1934 car did have a semi-automatic
    manually preselected gear box though!).
    All the heaters that I can recall were two connection single phase or DC.
    But I can think of no reason why such a material could not have been used to
    make say, a three phase heater etc. And therefore, if eventually cheap
    enough, homogeneous electric heaters that could become part of a building
    structure!
    In the meantime I'll re-read the postings in this thread to understand the
    mathematics of resistance of a homogeneously conductive (or sheet?)
    material.
    I do recall my uncle and his staff talking about 'Square Ohms and even
    'Square Watts' (Which was rapidly abbreviated to 'Sqwatts') as they devised
    heaters to meet various specifications.
    Never thought I'd get a chance to talk about that product again!
    I'll follow up with my surviving cousins and try to find out what happened
    to that company and or the product.
    Will post back if I can get some more info.

    Terry.
     
  20. Uncle Al

    Uncle Al Guest

    Take a two-part silicone rubber (probably peroxide cure for its
    resistance to poisoning) or reactive oligomer or vulcanizing rubber,
    load with graphite or metal powder, degas (big containier when you
    reduce pressure - it will foam. Make and break the vacuum to burst
    bubbles), cure. Flexible. Local resistivity may vary with strain
    allowing static or dynamic mapping with an adressable electrode array
    combed one way on top and perpendicularly on bottom.

    Don't siliconize your mold when casting silicone rubber.

    I suppose one could extrude, injection mold, or calendar a loaded
    Kraton rubber. Choose any durometer you like. It will require some
    equipment. The shape and mix of shape of the particles - ball, flake
    aluminum, chopped filaments and fibers - determiness the loading at
    which percolation conduction kicks in. Adding a little short fiber
    will substantially decrease resistivity at sparse loadings. Waste
    Kraton goes back into the hopper for another run.

    How much does he need, of what quality and other physical properties,
    to survive what... how much will he spend?
     
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