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Conductivity of Liquid Tungsten
Name: Benjamin
Status: student
Grade: 6-8
Location: TX
Country: N/A
Date: 2/11/2005
Question:
Does liquid tungsten conduct electricity?
Replies:
Liquid metals do conduct electricity. However, the conductivity is
difficult to measure for a number of reasons, and the theory and models
describing the electrical conductivity are not very complete.
With regard to the experimental difficulties: 1. Liquid metals are
difficult to purify and to keep pure during the measuring process. 2. In the
case of high melting metals (e.g. tungsten with a melting point of about
3410 C.) how to contain the sample becomes a serious problem. This is just
the start of a long list of experimental difficulties.
With regard to the theoretical / modeling difficulties: 1. The rule of
thumb is that the electrical conductivity of a liquid metal is about 1.5 to
2.5 LESS THAN the solid metal at the melting point. However, this is just an
estimate. The reason why the conductivity is lower in the case of most
metals is that the liquid metal has a viscosity that interferes with the
transport of electrons. 2. The conduction mechanism in the liquid state is
fundamentally different than the solid where the atoms are more or less held
in a fixed crystal lattice. This simplifies the computations. 3. At the
temperatures at which metals are liquid there are excited electronic atomic
states. These will affect electrical conductivity, but it is not clear just
how. For example: Consider molten iron. Its melting point is 1808 K. If we
consider its thermal energy to be of the order of: E ~ RxT = 1.987x1808 ~
3600 cal/mol. Converting this to wave numbers
(1 cm^-1 = 2.859 cal/mol) in order to compare the thermal energy to atomic
energy levels in iron gives a typical thermal energy for iron ~1260 cm^-1.
Now iron atoms have energy levels at [0, 400, 700, 888, 978, 6928, ...].
Although these gas phase energy levels are going to be "smeared out" in the
liquid phase, they do not go away, but it is not clear how to take their
influence on the electrical conductivity into account. So each metal will
behave differently with respect to its electrical conductivity depending
upon its electronic structure.
The web sites below discuss some of these issues.
http://www.symp14.nist.gov/PDF/MET02BAK.PDF
http://www.symp14.nist.gov/PDF/EXP05PER.PDF
Your question was a very good on but I do not know of a simple answer.
Vince Calder
Yes. I have never heard of any metals which stop being metallic when they
melt.
Usually they are a little less conductive than the solid metal was, but
only about a factor of two, so they are still clearly metals.
They can be quite silvery, too. Reflecting light in a silvery way is
usually caused by metallic electrical conductivity.
Some cases of the opposite occur: non-metals becoming metallic when they melt.
Semiconductors, mostly. Things which are very weak insulators, almost
conductive or inefficiently conductive,
become true metals and have a big drop in resistivity when they
melt. They get abruptly shinier, too.
I think these include the gray form of tin, germanium, silicon, perhaps
boron, possibly arsenic.
One way to keep liquid tungsten melted is to run large currents through it.
The current is large and the voltage drop is small, so rather than trying
to use the liquid like a wire,
it is easier to generate such currents by imposing a strong AC magnetic
field on the bath,
which then creates closed-loop-currents in the liquid.
So the liquid's conductivity can be used to keep the liquid tungsten hot
and melted.
This is called an induction furnace. People who experiment with exotic
metal alloys sometimes use them.
But melting tungsten is a pretty tough job for them.
The white-hot glow at 3400 C carries away a large amount of power,
and it takes a very strong AC magnetic field to pump in enough power to
make up for it.
For another thing, what kind of cup can hold liquid tungsten without melting?
(TaC, HfC, BC, graphite, or better yet more tungsten that is a little
cooler and not yet melted - less contamination by dissolved carbon.)
Perhaps in zero-gee in space it would be a little easier: melt a floating
ball-shaped glob of liquid tungsten.
On Earth, magnetic levitation can be arranged for small drops of liquid
tungsten.
I wish I knew if anybody has been running that kind of apparatus yet.
More common would be float-zone induction melting.
Imagine a long, thick rod of tungsten, pointed up and down, with an
induction coil somewhere around the middle.
The coil melts a short segment in the middle of the rod's length,
and this melted zone is slowly moved up and down the length of the rod.
This causes most impurities to either boil away or collect where freezing
occurs first, in the ends of the rods.
This gets around gravity problems because the melted zone is held up by
being wetted between two un-melted parts.
So the melted zone can be up to 5mm high and at least an inch wide.
I do have some 1" tungsten crucibles machined from rod stock which may
have been vacuum-melted like this,
to make them fully dense, non-porous, and very strong.
Jim Swenson
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