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Density and Conductivity
Name: Mike
Status: student
Grade: N/A
Location: VA
Country: N/A
Date: 5/31/2005
Question:
How does the density of a metal affect its conductivity?
Replies:
From element to element, the density does not say much about the conductivity
it will choose to have.
Any metal can have useless extra mass deep inside its nucleus,
without affecting the electron density or mobility on the outside very much.
There is only room for about 1-6 electrons per atom to participate
in-between atoms,
where electronic conduction happens.
And that number depends mostly on the metal's column of the periodic table,
and very little on electrons buried in underlying electron-shells.
The relative mobility of each such electron also varies greatly, depending
on what I am not sure yet.
Perhaps the ability of the element to make smooth, flawless crystal
lattices at the atomic level.
Perhaps particulars of the wave-function/band-structure of electrons
filling the crystal lattice.
But for a given element,
it has its best conductivity when it has 100% "theoretical" density,
and low impurities, and well crystallized structure, and is annealed
(relatively soft).
This is when electrons have the fewest obstacle to bump into and get
diverted or slowed.
On the other hand, if you take such a well-built mass of metal
and blow it full of microscopic voids, its conductivity naturally gets less.
That is the kind of thinking which is practical for this question.
I have evaporated refractory metals like tungsten or rhenium onto
glass which is at room temperature, and that metal film is fluffy like
snow inside,
weak and flaky, dark dull gray, not so shiny,
and has 2 to 3 times the textbook resistance for that metal.
But when I can evaporate it onto hot glass (200C-600C depending on which
metal),
or pound on it with inert gas atoms while it is slowly growing thicker,
then I get down around 1.1 times the ideal resistance, or 90% of its
maximum conductivity.
With softer, lower-melting metals like copper or tin,
it is much easier to get near their best resistance.
Then you might get 99% conductivity with simple annealing.
Jim Swenson
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Update: June 2012
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