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Re: Skin effect and aluminum question (fwd)
---------- Forwarded message ----------
Date: Sat, 14 Feb 1998 09:02:53 -0800
From: Jim Lux <jimlux-at-earthlink-dot-net>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: Skin effect and aluminum question (fwd)
>
> ---------- Forwarded message ----------
> Date: Fri, 13 Feb 1998 08:40:26 -0700 (MST)
> From: Chip Atkinson <chip-at-sophie.bolix-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Skin effect and aluminum question
>
> Greetings,
>
>
> I have seen it mentioned on the list that aluminum is a poor choice of
> conductor because the oxide layer that forms on the surface is an
> insulator, and that if, for example, your oxide layer is 0.001" thick and
> your frequency is such that you only have current going through the top
> 0.006" of the conductor, you will then lose 1/6 of your available
conductor.
>
> However, I fail to see the difference between Al2O3 on the surface acting
> as an insulator and a coat of varnish on the surface. Both the Al2O3
> coating and the varnish coating don't conduct, and are unaffected by
> magnetic fields.
Nope, no difference. The problem comes in with a partially oxidized and
hence semiconducting surface, where some of the power goes into, and is
dissipated as heat. The moral of the story is to finish your aluminum RF
parts, either by a passive coating (i.e. anodizing) or a conductive coating
(e.g. iridite), i.e avoid the semiconducting layer. I might add that very
pure soft aluminum forms a very nice oxide coating, but you generally don't
build much out of dead soft pure aluminum, but more usually a silicon allow
like 6061.
>
> Can anyone convince me that I'm wrong? I am aware that the aluminum
> conductor may provide more resistance at connections because the
> electricity has to penetrate the oxide layer though.
You have hit the real problem with aluminum as wire. I note however, that
HV transmission lines are mostly aluminum, the light weight and low cost
being a great advantage over copper. And, of course, they are installed by
"trained professionals" so the resistance of a properly made connection is
negligible. They are also careful to address the problem of differential
temperature coefficient of expansion.
>
>
> Thanks!
>
> Chip
>