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I know it depends a lot on the composition, so not all are great electric insulators. So what makes it good or bad? And is it different from what makes them good thermal insulators?

Power line insulators are ceramic and they have to stave off huge voltages. That's what got me wondering.

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    $\begingroup$ Some are, some aren't - most materials science folks would agree that titanium nitride is a ceramic, yet is a good electrical conductor. Don't think in terms of absolutes. $\endgroup$ – Jon Custer Jun 20 '16 at 0:33
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    $\begingroup$ Bra I mentioned that. Don't read in absolutes. $\endgroup$ – BoddTaxter Jun 20 '16 at 0:46
  • $\begingroup$ @JonCuster: TiN is a marvellous insulator if you cool it low enough! $\endgroup$ – leftaroundabout Jun 20 '16 at 18:36
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    $\begingroup$ Power line insulators have to be resistant to UV light and weather, since they are outdoors, and also have good strength, particularly compressive. Note that they way they are attached to cables is usually such that they are put under compressive stress. E.g. thumbs.dreamstime.com/t/… $\endgroup$ – Kaz Jun 21 '16 at 0:55
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    $\begingroup$ The "stack of dishes" configuration of some ceramic insulators prevents a short circuit caused by rivulets of rain dripping across a continuous surface. $\endgroup$ – Kaz Jun 21 '16 at 0:56
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For electricity to flow, electrons need to be moving. So in a conductor, there need to be free or loose electrons so they can carry the flow of electricity. Most metals fulfill this requirement, which is why most metals are conductors. Insulators, therefore, must have their electrons bound tightly, so they cannot carry the flow of electricity. The type of ceramic used as an insulator doesn't have any loose electrons (it is not, however, a perfect insulator) so it is a good insulator. In fact, this is what makes ceramic a good thermal insulator as well: it doesn't have loose electrons!

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  • $\begingroup$ Do you really mean "perfect conductor"? $\endgroup$ – MrYouMath Jun 20 '16 at 8:22
  • $\begingroup$ @MrYouMath: it's fairly clear from context that should say "not a perfect isolator" $\endgroup$ – MSalters Jun 20 '16 at 9:13
  • $\begingroup$ Note that many ceramics are ionic conductors. (And to make things more complicated, the moving 'charge carrier' can be a vacancy, often on the oxygen sublattice in an oxide). $\endgroup$ – Jon Custer Jun 20 '16 at 13:49
  • $\begingroup$ You start off talking about free or loose electrons but then talk about free loose atoms in the middle and then go back to electrons. Did you mean 'electrons' when you wrote 'atoms' here? $\endgroup$ – JimmyJames Jun 20 '16 at 15:03
  • $\begingroup$ @JimmyJames, thanks for catching that. I've updated the answer. $\endgroup$ – heather Jun 20 '16 at 15:05
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Solids can be classified according to their band gaps. The band gap determines how much energy you need to supply in order to free or promote an electron from the valence band to the conduction band. Once in the conduction band the electron can move nearly freely and conduct electricity. A solid with a large band gap (> 2.5 eV) is considered to be a good insulator and many ceramics have such large band gaps. Metals don't have such a gap, and hence, electrons flow under the slightest applied voltage (in principle).

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Most ceramics are ionic compounds, in which electrons are immobile. This is different to metal, in which the atoms are in a "sea of electrons" that are free to move.

Note that ceramics have some kind of conductivity, it's just extremely low. The conductivity of copper, for example, is ~6×107 S/m. Most ceramics have conductivities in the range of 10-3 S/m to 10-5 S/m. That's around 10 orders of magnitude less conductive than copper, but it is 10 orders of magnitude more conductive than air, for example (10-15 S/m).

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protected by Qmechanic Jun 20 '16 at 11:25

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