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).
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.
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.
--
Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
http://www.mazepath.com/uncleal/qz.pdf
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.
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.
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.
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.
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.
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.
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?
--
Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
http://www.mazepath.com/uncleal/qz.pdf
"Uncle Al" wrote in message news: snipped-for-privacy@hate.spam.net...
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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.
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