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Re: RF conductor materials (fwd)
---------- Forwarded message ----------
Date: Thu, 30 Apr 1998 06:51:23 -0600
From: terryf-at-verinet-dot-com
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: RF conductor materials (fwd)
Hi Chip,
At 10:29 PM 4/29/98 -0600, you wrote:
>
>
>---------- Forwarded message ----------
>Date: Wed, 29 Apr 1998 22:27:17 -0600 (MDT)
>From: Chip Atkinson <chip-at-pupman-dot-com>
>To: tesla-at-pupman-dot-com
>Subject: Re: RF conductor materials
>
>
>>>>>
>oxidize in air. Over time this layer can become very thick.
>Aluminum oxide is an excellent insulator and is used as such in the common
>form of alumina. Copper oxide easily forms with similar results. Since
>RF currents travel on the outside surface of conductors, this can have a
>very significant effect over time as the oxide layer builds up.
><<<<
>
>I have a couple questions about the above statements. First, please
>explain the difference between an enamel insulating cover and an Al2O3
>insulating cover. How can it make any difference what type of insulation
>that is on a wire as far as its current carrying capabilities go? (short
>of the temperature at which the insulation burns or melts).
>
>If the concern is reduced diameter of the conductor due
>to the conversion of Al to Al2O3 then is it really such a big deal? Even
>well weathered (in the dry climate of Colorado anyway) Al has at the very
>most 1/64" oxide.
>>>>>
The transition region between pure aluminum and the non conductive Al2O2
layer is thick and variable. If you think of the region between the ground
(the kind you walk on) and air it is like being in a dust storm for the
electrons. There is enough conductivity to attract electron flow but enough
resistance to mess things up. In enamel wire, the transition from copper to
enamel is instant. No fuzzy region. The electrons all stay in the pure
copper without having a resistive semi-oxide layer to go through.
>Aluminum
>house wiring used years ago keeps fire departments busy today :-((
><<<<
>I believe that the reasons that Al wire is so bad are as follow:
>Al-Cu connections corrode leaving small contact areas which can heat up.
>Not necessarily as an effect of the previous sentence, an Al wire melts at
>a significantly lower temp. than Cu. Also, Al burns quite readily once it
>gets started. As any pyro knows, Al powder can be quite flammable.
If AL get hot enough to melt the game is long over. AL to Cu connections
corrode and fail as you describe. AL to AL connections must be clean and
"gas tight" to insure that the connection will not become resistive. The
electric power industry uses Al for everything. Those connections are
either welded or use special clamps which bite deep into the wire over a
very large area. These clamps often have 5 to 10 bolts forcing the
connections to be absolutely solid. The large area of these clamps and very
high forces used insure gas tight connections and a wide area for the
connection. Obviously overkill for the mechanical stresses. Al has such
thick and easily formed oxides, that it is a very difficult substance to
work with electrically especially over long periods of time. There are
greases that are used in industrial AC wiring which act to prevent corrosion
by removing air and chemical action.
>
>Chip
>
>=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
> Chip Atkinson
> http://www.pupman-dot-com
> --- If I can't fix it, I can fix it so it can't be fixed --
>=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
>
>
Terry