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moderately resistive materials?

G

Grant W. Petty

Jan 1, 1970
0
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

[email protected]
Tel: (608) 263-3265
Fax: (608) 262-0166
 
J

Johnny

Jan 1, 1970
0
Grant W. Petty said:
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

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
 
G

Grant W. Petty

Jan 1, 1970
0
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.

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.
I would want to *insulate* the resistor to force all the current to go
through the resistive material.
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.

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

________________________________________
Prof. Grant W. Petty
Atmospheric and Oceanic Sciences
1225 W. Dayton Street
University of Wisconsin-Madison
Madison, WI 53706

[email protected]
Tel: (608) 263-3265
Fax: (608) 262-0166
 
G

Grant W. Petty

Jan 1, 1970
0
OK. That configuration sounds more like what they called a 'leaky
capacitor' back on the soldering bench.

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

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.
You willl probably need to do a little testing and
literature searching.

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

[email protected]
Tel: (608) 263-3265
Fax: (608) 262-0166
 
G

Grant W. Petty

Jan 1, 1970
0
#3 needs a spec.

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.
thermal expansion? how much?

TBD. I'd say as long as it's not unusually high it's probably okay.
#1 spec, 10 ohms per square ? that is a high value,

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

[email protected]
Tel: (608) 263-3265
Fax: (608) 262-0166
 
R

Rheilly Phoull

Jan 1, 1970
0
Grant said:
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

[email protected]
Tel: (608) 263-3265
Fax: (608) 262-0166

Dont suppose a peltier array would be any help ??
 
A

amdx

Jan 1, 1970
0
Grant W. Petty said:
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

[email protected]
Tel: (608) 263-3265
Fax: (608) 262-0166
You might pose your question on sci.materials.
Mike
 
G

Grant W. Petty

Jan 1, 1970
0
Dont suppose a peltier array would be any help ??

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

[email protected]
Tel: (608) 263-3265
Fax: (608) 262-0166
 
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