moderately resistive materials?

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) Moderately resistive -- it will be used for resistive heating by passing a current through a thin sheet sandwiched between two metal conductors. Target resistivity should fall in the range 10 to 10**4 m*ohm at temperatures between 0 and 100 C.
2) A marked increase in resistivity with increasing temperature (this apparently rules out silicon, for example).
3) Mechanically stable and 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.
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.
thanks
________________________________________ Prof. Grant W. Petty Atmospheric and Oceanic Sciences 1225 W. Dayton Street University of Wisconsin-Madison Madison, WI 53706
snipped-for-privacy@aos.wisc.edu Tel: (608) 263-3265 Fax: (608) 262-0166
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Grant W. Petty wrote:

If you sandwich a resistive material between two conductors, the current will flow mainly through the conductors, even at low voltage. Never mind the 'skin effect' of high voltages.
I would want to *insulate* the resistor to force all the current to go through the resistive material.

Start here: http://en.wikipedia.org/wiki/Platinum_resistance_thermometer
Note the parabolic form
R(T) = R0 * [1 + AT + BT^2]
with A = 3.9083 10-3
This gives a strong positive slope over the linear range. Standard Pt-100 sensors have 100 ohms resistance at 0 C.
Platinum also meets your other criteria pretty well. as long as you aren't working in fluorine or aqua regia.
Tom Davidson Richmond, VA
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On Mon, 15 May 2006, tadchem wrote:

To clarify, one conductor is at one potential, the other is at a different potential. So the current flow is through (perpendicular to) the resistive sheet.

________________________________________ Prof. Grant W. Petty Atmospheric and Oceanic Sciences 1225 W. Dayton Street University of Wisconsin-Madison Madison, WI 53706
snipped-for-privacy@aos.wisc.edu Tel: (608) 263-3265 Fax: (608) 262-0166
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Grant W. Petty wrote:

OK. That configuration sounds more like what they called a 'leaky capacitor' back on the soldering bench.
You need a poor dielectric. Paper runs about 5*10^4 ohm-cm resistivity according to my old copy of Eshbach's Handbook of Engineering Fundamentals. This can be 'adjusted' by impregnating the paper with different liquids. You willl probably need to do a little testing and literature searching.
Tom Davidson Richmond, VA
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On Mon, 15 May 2006, tadchem wrote:

Good analogy. Of course, in this case, the leakiness would be both deliberate and carefully calibrated.

I worry that liquid-impregnated paper wouldn't hold up well to continual heating. So I would probably prefer something graphite-like, or maybe some composite material, if it exists with the right electrical and thermal properties.

Any suggestions concerning reference literature to look at? I'm delving into areas somewhat outside my normal practical expertise, though I understand the theory pretty well.
Thanks.
________________________________________ Prof. Grant W. Petty Atmospheric and Oceanic Sciences 1225 W. Dayton Street University of Wisconsin-Madison Madison, WI 53706
snipped-for-privacy@aos.wisc.edu Tel: (608) 263-3265 Fax: (608) 262-0166
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us:

Skin effect is a function of frequency, not voltage. It is just easier to prove in higher voltage settings.
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call up people who make thermisters, Thermotron (sp?) is one. resistive heating is typically a type of steel wire,or ............. there is another type that is used too, forgot what name is. #3 needs a spec. Mechanically Stable ? thermal expansion? how much? #1 spec, 10 ohms per square ? that is a high value, for metals, you have a flat thin wafer
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On Mon, 15 May 2006, Johnny wrote:

I don't know how to give a spec for this. Basically I'm ruling out anything that I'd have a hard time handling without breaking or degrading it. For that matter, I suppose a paste-like goo squeezed between the two conductors could work, if it won't change properties over time.

TBD. I'd say as long as it's not unusually high it's probably okay.

Yes. I'm thinking something comparable to graphite. In fact, maybe some kind of conductive graphite composite wafer, if such a thing exists.
________________________________________ Prof. Grant W. Petty Atmospheric and Oceanic Sciences 1225 W. Dayton Street University of Wisconsin-Madison Madison, WI 53706
snipped-for-privacy@aos.wisc.edu Tel: (608) 263-3265 Fax: (608) 262-0166
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meteorological
the
m*ohm
there
Nichrome?
<http://www.google.ca/search?hl=en&q=nichrome
have
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There does not appear to be sufficient info here, or I am missing something. Most metals exhibit a positive tempco, and they are approx linear (some more then others)
If the passage of current can be changed to lengthwise I think a metal film/foil may perform nicely. You may want to look at Minco. They make some very nice metal film/foil heaters.
M Walter PE

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Grant W. Petty wrote:

Dont suppose a peltier array would be any help ??
--
Regards ......... Rheilly Phoull



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On Tue, 16 May 2006, Rheilly Phoull wrote:

I don't know what that is ... is it related to the Peltier effect?
________________________________________ Prof. Grant W. Petty Atmospheric and Oceanic Sciences 1225 W. Dayton Street University of Wisconsin-Madison Madison, WI 53706
snipped-for-privacy@aos.wisc.edu Tel: (608) 263-3265 Fax: (608) 262-0166
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You might pose your question on sci.materials. Mike
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Grant W. Petty wrote:

Perhaps you can get some of the the material used in self-resetting fuses: a conductive polymeric concoction with a non-linear PTC.
http://www.circuitprotection.com/polyswitch.asp http://www.circuitprotection.com/06Databook/fundamentals/PSWFundamentals.pdf
-- Joe Legris
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