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I am interested in heating coils which are heated using Ohmic/Joule/resistive heating - specifically, how the material for the heating coil is selected.

When I read about heating coil materials - nichrome, carbon fibre etc, the material quality that is mentioned most is electrical resistance (along with a high melting point and stability with changes in temperature). It is my understanding that a high resistance material is typically selected to make heating coils, but how does the thermal conductivity of a material impact the suitability of a material for use as a heating coil?

If two materials had comparable resistance but very different values for thermal conductivity - which material would be the better choice, assuming the goal was a heating coil that heated up quickly using the least amount of electrical power?

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    \$\begingroup\$ Heat = Electrical power. That power must go somewhere and that somewhere is heat. Just saying, to heat up (faster), more power will be required (at start.) A filament can also heat up faster by having less mass. So, if speed is a concern, many thin elements (in parallel) may be preferable to one big one. \$\endgroup\$
    – rdtsc
    Commented Aug 13, 2020 at 17:17
  • \$\begingroup\$ For best heat transfer with smallest gradient use best thermal conductor with convection heater \$\endgroup\$ Commented Aug 13, 2020 at 17:30

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The heating takes place throughout the entire volume of the resistive element (ignoring effects at the connection points) so if you use a low thermal conductivity material, you'll end up with a temperature gradient between the center and the edges, where heat is presumably being convected or conducted away. Many high power resistors use resistance elements that are thin in relation to their length - either a ribbon or a wire - so there's just not a lot of length for a big gradient to develop. All other properties being equal, I would choose the higher thermal conductivity material as a smaller temperature gradient would reduce differences in electrical conductivity (temperature coefficient of resistivity), internal strain (from thermal expansion), etc between the core and the surface of the heating element. But there are other properties to consider like corrosion rates at elevated temperatures, formability, and compatibility with other materials such as solders or protective coatings.

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