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Re: [Phys-l] conductivity of metals

Yes I shouldn't have used the scare word, however, there is at least one exception, the one I suspected. Cu and Ag. They are I think the most similar metals. The addition of contaminating Ag to copper doesn't reduce its conductivity (electrical) the reverse does.

*

Electrical and Thermal Conductivity
Pure copper is a very good conductor of both electricity and heat.
The International Annealed Copper Standard (IACS; a high purity
copper with a resistivity of 0.0000017 Ohm-cm) is still sometimes
used as an electrical conductivity standard for metals. The best
way to increase the electrical and thermal conductivity of copper
is to decrease the impurity levels. The existence of impurities
and all common alloying elements, except for silver, will decrease
the electrical and thermal conductivity of copper. As the amount
of the second element increases, the electrical conductivity
decreases. Cadmium has the smallest effect on resulting alloy's
electrical conductivity, followed by increasing effects from zinc,
tin, nickel, aluminum, manganese, silicon, then phosphorus. Although different mechanisms are involved in thermal
conductivity, the addition of increasing amounts of elements or
impurities also produces a drop in thermal conductivity. Zinc has
very minor effect on the thermal conductivity of copper, followed
by increasing effects from nickel, tin, manganese, silicon, and
serious effects from phosphorus. Phosphorus is often used to
deoxidize copper, which can increase the hardness and strength,
but severely affect the conductivity. Silicon can be used instead
of phosphorus to deoxidize copper when conductivity is important.



http://www.matweb.com/reference/copper-alloys.asp


BTW, not far below room temp. alumina becomes the best thermal conductor of common materials, I think right down to zero R. And the common superconducting metals don't have to be particularly pure. Aside -- One of the intermediate lab. demos, as opposed to quantitative experiments, was lowering a PM on a Pb dish. I had some Sn sheet so tried it. After a few demos, I remembered why not Sn is used.


bc


John Denker wrote:

When I speak of "conductivity", unless context requires otherwise,
I mean both thermal and electrical conductivity, in accordance with
the Wiedemann-Franz law, as previously discussed.

I neglect the small contribution to the thermal conductivity from
the underlying lattice. Also note that superconductors are a
huuuuge exception to the W-F law: the superfluid transports
charge but not entropy.

On 10/24/2006 07:21 PM, Bernard Cleyet wrote:

... I was going to suggest the the
weighted average is probably not valid.

Probably!?!?!!!!

Suppose we have a wire suitable for use as heater wire (resistance
wire), as in the application that precipitated this discussion.
It will be an alloy. It will less conductivity than any of its
components separately, probably by several orders of magnitude
(depending on temperature).

Resistivity is rooted in /scattering/ of the electrons.

For reasonably-pure metals at room temperature, the dominant
contribution is /thermal/ scattering, i.e. electrons scattering
off thermal phonons.

If you lower the temperature enough, and/or add enough impurities,
eventually you get into another regime, where the dominant
contribution is /impurity/ scattering, i.e. electrons scattering
off impurities. In this regime, electrical conductivity is (to
first order) independent of temperature, and thermal conductivity
is proportional to T.

Resistance wire, pretty much by definition, is in the impurity-
dominated regime, which makes sense because you want its electrical
conductivity to be independent of temperature.


At cryogenic temperatures, you can achieve tremendous conductivity,
if your metal is pure enough. At low temperatures, tiny amounts
of impurities can radically decrease the conductivity. Magnetic
impurities such as iron or oxygen are particularly effective
scatterers.

Once upon a time, a guy who was looking into the conductivity of
ultra-pure metals got a big surprise.
http://nobelprize.org/nobel_prizes/physics/laureates/1913/onnes-bio.html
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