Re: weakly conducting materials?

"Grant W. Petty" wrote:

In connection with a prototype design I'm developing for a meteorological
measurement device, I'm looking for a readily available material with the
following physical properties:

1) Resistive, but not an insulator -- it will be used for resistive heating
by passing a current (perpendicularly) through a thin *** sandwiched
between two flat metal conductors (like a leaky capacitor). Target
resistivity of the material is in the range 10 to 10**4 ohm*meter at
temperatures between 0 and 100 C.

2) A positive temperature coefficient (this apparently rules out silicon,
for example).

3) Reasonably easy to work with in sheets or disks of order 0.1 mm
thickness, 30 cm diameter.

4) Not subject to significant degradation or aging, even at sustained
temperatures near 100 C for days or weeks at a time.

If anyone either knows offhand of a material matching the above description
or can point me to a good tabulation of candidate materials, please let me
know.

Carbon black or graphite-filled polymers, n- or p-doped silicon dust
are one direction conductivity vs. temp; metal dust-filled polymers
(nickel) are the other. The nylon used for oven bags may be a good
start as matrix. If you want it to last forever, graphite-filled
polyimide (Vespel, Kapton). Obtaining them as filled films might be
tough. Bakelite; high temp automotive Lexan, Ultem, PEEK... pick your
engineering polymer, fill it with dispersed conductive phase, make
film

OTOH, why not polymerize filled
epoxy/vinylester/polyurethane/2-component silicone in situ? Cure it,
then age it hotter than max running temp to stabilize its properties
(note shrinkage on cure). Add a very small weight fraction of glass
microbeads to act as automatic uniform spacers. Monolithic is
survivable.

--
Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
http://www.mazepath.com/uncleal/qz3.pdf
.