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I Aluminum surface conductivity

  1. Apr 9, 2017 #1
    Aluminum metal covers itself immediately of an oxide layer in air. This layer is quite thick, more than 0,01mm they say and it's non-conductive. This layer infact is, e.g., thick enough to prevent soldering, further chemical attack by air oxygen or water (with which nude Al would immediately react) or discharge of many kinds of ions in solution (for example it doesn't react with Cu++, unless Cl- ions or other catalyzing agent added).
    Why then a piece or a thread of Al metal is conductive? That is, how can it conduct an electric current when is touched with electric cables (connected to a battery)?

    Why the mechanism, whatever it is, which allows the passage of electric current through the Al2O3 layer, doesn't allow ions discharge at its surface?

    (Clearly there are other materials that shows this behavior, Al is just an example).
    Thanks.

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  2. jcsd
  3. Apr 9, 2017 #2
    A tight physical connection will break through the oxide layer.
     
  4. Apr 9, 2017 #3
    But current passes easily even if the connection is not tight: simply touched with the points of a tester' cables you can't find any significant surface resistance (if present is lower than 0.1 Ohm). You can try yourself with a piece of conventional (not anodized) Aluminum.

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  5. Apr 9, 2017 #4
    I have found that with many metals which form oxide coatings, there is a tendency to conduction happening due to electron tunneling action if the voltage is higher than a few volts. Interestingly, carbon, which doe not form a surface layer, does not do this. Iron, copper, nickel, mercury, zinc all seem to have the property together with many others.
     
  6. Apr 9, 2017 #5
    Interesting. Now there is just to understand how much it is this minimum ddp requested for the passage of electric current.

    By the way: it'a sort of "two-way" diode? :-)
     
  7. Apr 10, 2017 #6
    I am not certain that is what happens with aluminium, but with the other metals I mentioned, by using a "loose contact" it is possible with care to obtain a two-way diode characteristic. These are called MIM diodes. I found this out by making experiments with a "coherers", the radio detector device used in the 19th century. The voltage may be in the order of a volt. I have been able to use it to demodulate off-air radio signals by applying a small bias. In this role, it is about 10dB inferior to a Germanium diode. In the original device it was sometimes use as a "linear detector" and other times as threshold device, like an SCR. In the latter case, a large current is allowed to pass, and it is supposed that micro welding then occurs.
    The action does not occur with carbon, which does not form a surface layer, and is symmetrical, not resembling a semi conductor diode, even when dissimilar materials are used.
     
  8. Apr 10, 2017 #7
    Even more interesting!
    Thanks tech99.

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  9. Apr 10, 2017 #8
    I also wondered if the action was due to Electron Field Emission, as the surfaces were rough and closely spaced by thin patches of insulating oxide. This would create the very high potential gradient at the little points on the surface which might provide electron emission. But I found that if I used one smooth surface, such as Mercury, and another rough, such as Carbon, I still observed a symmetrical (bi-directional diode) response. For this reason, I don't think it is Electron Field Emission.
     
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