# Resistive sheet

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

1. ### Don A. GilmoreGuest

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

Teflon??

Carl Sachs

3. ### Don A. GilmoreGuest

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.

6. ### Don A. GilmoreGuest

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/

Contact me directly if you like.

Dave

8. ### Uncle AlGuest

What does carbon fiber composite look like?

9. ### Don A. GilmoreGuest

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

Don

10. ### Uncle AlGuest

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. ### artieGuest

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.

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 BaerGuest

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 BaerGuest

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

15. ### Robert BaerGuest

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

16. ### Robert BaerGuest

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 BaerGuest

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 BaerGuest

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

19. ### TerryGuest

TO: Don Gilmore:

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.

Terry.

20. ### Uncle AlGuest

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