Ask A Scientist General Science Archive

 
name         Sandro G.
status       student
age          17

Question -   Along the same lines of heat in relation to conductivity:
Would an metal that conducts in its solid phase or at room temperature
conduct when superheated to its liquid, gaseous or plasma state? If it
does, what rules apply: what would be the increase in receptivity by ratio
to the temperature and phase.
Also, are there any metals or other conductive materials that have lower
resistance at higher temperatures?
-----------------
No simple answer to this complicated question.
Some metals become semiconductors when melted
(example; bismuth) while others become poor
conductors (example: copper). There are no rules
that I know of.

With respect to your second question; while conductors
have higher resistance at higher temperature, semiconductors
generally have lower resistance at higher temperature.
Increasing the temperature of a semiconductor promotes electrons
into the conduction band, whereas increasing the temperature
of a conductor promotes electrons into scattering states.

Dr. Topper
====================================================
Sandro,

Resistivity usually increases with temperature for conductors and usually
decreases with temperature for non-conductors (insulators).  This is because
higher temperatures tend to free charge carriers in an insulator while the
increased thermal vibrations of charge carriers in a conductor reduces their
mobility (and increase the resistance).

This answer does not address phase changes, but there is a good web site that
provides some general information at:

http://en.wikipedia.org/wiki/Electrical_conduction

Regards,

Todd Clark, Office of Science
U.S. Department of Energy
====================================================
Second question first. Yes, there are many materials that have a negative
temperature coefficient of resistivity. The electrical resistance of
semiconductors decreases with increasing temperature. The reason is
conductivity in semiconductors depends upon thermally exciting valence
electrons into an excited energy state called the conduction band. This
becomes easier as the temperature increases. See:

http://www.all-science-fair-projects.com/science_fair_projects_encyclopedia 
/Semiconductor

     For metals on the other hand electrical resistance results from the
scattering of electrons carrying the current by vibrations of the metal
lattice. Since the atoms vibrate with higher amplitude as the temperature
increases, the conductivity decreases (i.e. the resistivity increases) with
increasing temperature.
     These explanations are very simplified. You can also find references to
electrical resistivity in the NEWTON BBS archives. See for example:
http://www.newton.dep.anl.gov/askasci/phy00/phy00846.htm
     Electrical conductivity is very complicated because of the different
types of charge carriers. There are "free" electrons, ions (for example salt
water), gaseous ions (a plasma such as lightning), partially bound electrons
(for example a semiconductor). Each mechanism has its own behavior --
temperature and otherwise. When you add in chemical composition changes
things get even more complicated.

Vince Calder
====================================================
Alessandro,

Allow me to focus on the principles of electrical conductivity in metals and
semiconductors, and then maybe you can answer your question yourself.

Electrical conductivity in metals, as you probably know, depend on valence
electrons being excited into a higher, conduction band. The more electrons
there are in this band, the more conductive the metal will be. Excitation of
electrons to the higher band is proportional to the temperature as energy is
required to be excited to this band. There is a limit to this proportionality
however. If the metal losses crystal coherence (as when it is melted) then 
the
coherence of the conduction band degrades as well and some (not all)
conductivity is lost.

It might be instructive to compare this to semiconductors. In this case,
electrons are added to a conduction band, or removed from the valence 
band. In
the first case, since electrons are added, then the amount of charge carriers
are decidedly less, but adding more electrons to this band will increase
coulombic repulsion. If electrons are removed from the valence band, then
conduction occurs in the valence band were electron mobility is controlled by
the amount of "holes" that the electrons can move to, and coulombic repulsion
is again a factor. Thus, the effect of temperature is different in that it is
not electron excitation that becomes important, but the reduction of 
coulombic
repulsion and the coherence of the crystal structure. This is why cooling has
the effect of increasing conductivity.

So in effect there are several factors you want to consider: coherence of
crystal structure, number of charge carriers, mobility of charge carriers.

Hope that helped organize your thoughts.

Greg (Roberto Gregorius)
=====================================================