Resistive sheet

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
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Teflon??
Carl Sachs
Don A. Gilmore wrote:

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Thanks Carl, but I thought Teflon was an insulator. Are you referring to some sort of graphite-impregnated Teflon?
Don
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"cwsachs.dejazzd.com" wrote:

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.
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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.
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See: http://www.esdjournal.com/techpapr/ohms.htm for an example of sheet resistivity.
Dave
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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
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snipped-for-privacy@newarts.com wrote:

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).
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snipped-for-privacy@newarts.com wrote:

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

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artie wrote:

Fails in the "not surface effect" criteria. Prolly would also fail (crack) if flexed.
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Don A. Gilmore wrote:

(hopefully)
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
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snipped-for-privacy@newarts.com wrote:

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.
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wrote:

From http://www.elecdesign.com/Articles/Print.cfm?ArticleID#23
"..Many people have inquired about where to buy Teledeltos paper. The original source in England will still sell you a big roll for $95, including shipping; www.searchkingslynn.com/senco3.html will get you to the right address for Sensitised Coatings, or call +44 1553764836. Plus, a new source for resistive paper is online: www2.pasco.com/products/scripts/products.taf?PN=PK-9023. The KIT PK9023 costs $114, but PK9025 paper with centimeter grid runs $34 for 100 23- by 30-cm sheets. Buy your own pen with conductive silver ink. This is about 32 kO/square. "
Regards,
Boris Mohar
Got Knock? - see: Viatrack Printed Circuit Designs (among other things) http://www.viatrack.ca
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need
including
source
costs
Thanks Boris:
This is intriguing; but is the paper only conductive on one side? The reason I ask is that for my application, electrical current will be conducted through the sheet from face to face, not along its length/width.
If it is resistive throughout it might work well indeed. Is it?
Don Kansas City
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Don, Your previous comment (Jan 3, 8:32 am) implies you are interested in current flow that is due to electrical boundary conditions on opposite sides of the sheet. Is that correct?
Your original post said.."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"
I may have mis-interpreted this as meaning you are interested in current flow between electrode areas separated in the plane of the sheet. The resistance one would measure perpendicular to such a sheet will depend on the size and lateral displacement of the electrodes relative to the thickness of the sheet.
Perhaps your question was much simpler: if I had a material with a bulk resistivity of 75 ohm-m and gave you a piece in the shape of a .125" thick sheet is that all you want? If so, how large a sheet? Flexible or rigid? Durable?
Dave
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Yes, that's all I want. I would prefer sheets as large as possible...in roll form would be great. It doesn't have to be super-flexible, but I don't want it brittle like glass either. It will be applied to a slightly irregular surface, but generally planar...maybe sort of like a large airplane wing. It will not be subjected to much abuse, but it might be exposed to normal outdoor temperatures.
So far I'm leaning toward some conductive plastic sheet material that McMaster-Carr carries. But only because it's all I've found yet (and it only comes in 12" x 12" sample sheets". That's why I was wondering if one of you clever guys might have heard of something else that I missed.
Don Kansas City
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cured, wet concrete and typical soils & clays have resistivities in the 100 ohm-m range.
Dave
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