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I was thinking about diamonds, and how they're excellent thermal conductors and yet at the same time very good electrical insulators.

Does the opposite of diamond exist, i.e. are there commonly available, inert (as in, safe to use/handle) conductors with poor thermal conductivity?

Kenn Sebesta
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  • Basically: https://physics.stackexchange.com/questions/258194/why-do-electrically-conductive-materials-tend-to-be-thermally-conductive – Tim Williams Feb 02 '23 at 04:05
  • According to a materials research physicist I know, sapphires are about as good as diamonds and way cheaper. I think lead and carbon are going to be your best bet for common and safe poor thermal conductors. – dandavis Feb 02 '23 at 05:05
  • Generally no, to get good electrical conductivity, you also have high heat conductivity. Use thin copper wires of a known gauge and length, measure the temperature at the ambient end, and calculate a heat flow. You'll probably find it's negligible, especially compared to the errors and approximations you get when measuring battery temperature, and repeatability between batteries. If it's not negligible, then you can make an allowance for it. – Neil_UK Feb 02 '23 at 08:02
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    @dandavis I haven't looked at lead, but graphite/carbon is proportionately far better than copper at transmitting heat than electricity. Something like 2x worse heat conductivity but 350x worse electrical conductivity. So if you were to swap copper for an equivalent resistance graphite brush, you'd have 175x higher heat transfer. My question is about going the other way, materials which in comparison are orders of magnitude more electrically conductive than heat conductive. – Kenn Sebesta Feb 02 '23 at 14:03
  • @Neil_UK Possibly, empirical testing would reveal. However, battery testing is related to this question only in that it's what I was doing at the time when I started getting curious. – Kenn Sebesta Feb 02 '23 at 14:06
  • For metals it's pretty much a constant figure of merit at given temperature (or worse for alloys). It would be nice to have such a material with high electrical conductivity. – Spehro Pefhany Feb 02 '23 at 17:14
  • Exactly what batteries will you be testing? – Bruce Abbott Feb 02 '23 at 19:15
  • For metals, https://en.m.wikipedia.org/wiki/Wiedemann%E2%80%93Franz_law – Sredni Vashtar Feb 03 '23 at 03:05

2 Answers2

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At a practical level I have never come across such a material at room temperature.

Superconductors have infinitely higher electrical conductivity than thermal conductivity, but are unlikely to help you here. Similarly, there are alloys which have okay electrical conductivity at cryogenic temperature, but much reduced thermal conductivity, but again unlikely to help in this case.

If you need precision, the best bet is to use thermally insulated bus bars of known cross-section and length, with temperature sensors at either end. From the temperature delta, cross-section, length, and knowledge of the materials electrical and thermal conductivity, you can work out the gross and net heatflow (the latter allowing for heat generated due to resistiance in the busbars).

colintd
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Plotting electrical resistivity versus thermal conductivity using CES Selector 4.7.0:

enter image description here

Diamond (an electrical insulator and good thermal conductor) and is on the right hand side, copper and silver (good electrical and thermal conductors) are on the lower right hand side.

Low electrical resistivity and high thermal resistance conductors (low thermal conductivity) are in the lower left. They appear to be foams made of electrically conducting materials (carbon, Aluminum-SiC, ...), materials that are a mixture of polymers and electrical conductors (PEEK with 30 % carbon fiber, carbon fiber composites, ...), and natural materials (cork and leather).

Cassie Swett
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C. Dunn
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    I went ahead and changed "right" to "left," since it seems clear that that was the intended meaning. – Cassie Swett Feb 02 '23 at 22:38
  • The resistivity of composite metal foams are several orders of magnitude higher than copper. In this application the heat generated in the conductors would almost certainly offset the accuracy gains from reduced thermal conductivity. – colintd Feb 02 '23 at 23:36
  • @colintd that's a good point, although I'd like to reiterate that the application is tangential to the question. We were able to meet our requirements without special materials, but the question itself is quite interesting in the abstract. – Kenn Sebesta Feb 03 '23 at 02:06
  • This is a great answer. It's a little odd how the y-axis ticks are every 100, instead of every 1000, but I figured it out quickly enough. If we draw a mental straight line to show ratio of thermal to electrical resistance, it looks like butyl rubber is the only material which has a more favorable ratio than metals. Whereas in the opposite scale, diamond is massively more thermally conductive than resistive. – Kenn Sebesta Feb 03 '23 at 02:13
  • Excellent chart (for room temperature, so excluding superconductors), though butyl rubber, even graphite loaded, is way too insulating to use as battery leads. Viable connecting leads are in a narrow strip at the bottom of the graph, and that means the metals.... – colintd Feb 03 '23 at 10:33