7 ms·
Is VO_{2}, a metal oxide, really a metal? This actually annoys me a bit. Without knowings anything about it: it's a ceramic, almost certainly a semiconductor,
by zwsxedcrfvtgb 7y ago
Is VO_{2}, a metal oxide, really a metal?
This actually annoys me a bit. Without knowings anything about it: it's a ceramic, almost certainly a semiconductor, and since its a semiconductor it can behave electronically like a metal under certain conditions.
Some semiconductors have poor phonon conduction, others quite good (ability to conduct heat).
Some semiconductors have good oxygen vacancy conductivity and poor electronic and hole conductivity. That gives you a transparent conductor.
And most ceramics are brittle. Is VO_{2} brittle?
In short, is this VO_{2} really a metal, or "just" a cool semiconductor?
- neltnerb 7y agoThe formal definition of a metal is just that it has overlapping conduction and valence bands. https://en.wikipedia.org/wiki/Valence_and_conduction_bands https://en.wikipedia.org/wiki/Valence_and_conduction_bands So it doesn't matter whether it's brittle (superconductors are brittle) or whether it's an oxide or whether it's a ceramic as well (superconductors), or a glass as well (metglass). What matters is that at your temperature of interest the bands overlap so that you have free moving electrons. But reading some other literature suggests that "metallic" VO2 is indeed a semiconductor (not actual insulator) at lower temperatures and thermal energy makes the electrons energetic enough to jump to the conduction band and become a metal. So I think I'd say it's a "hot" semiconductor. Ruthenium Oxide, Tungsten Oxide, and Chromium Oxide (at least one of them) are also metallic, with 10-20x worse conductivity than copper. Just a weird fact I had to figure out for a project. Actual metallic, not temperature dependent just phase dependent (CrO2 vs Cr2O3 for instance). On the other hand, alumina is a great thermal conductor (about 10x worse than copper) but has almost no electrical conductivity, so I'm not so sure about this law relating the two in the first place. I've never heard of it being a law, I've always just treated that as a rule of thumb...
- brennanpeterson 7y agoElectrical conductivity is electron motion, and if you have electrons to carry charge, you also get thermal conductivity. So metals (in the general sense) are good electrical and thermal conductors. If you have very hard materials, with high phonon conduction, you can get thermal transport via phonons. But, these are almost all poor electrical conductors, as you need very tight binding to get hard materials with high phonon velocity. The prototypical example of the second case is diamond.
- neltnerb 7y agoOh yeah, fair. Good explanation. The relationship is one directional because there are other ways to carry heat.