MadSci Network: Physics

Re: How is heat conducted differently in elemts as apossed to alloys?

Date: Thu Jan 22 00:22:51 2004
Posted By: Joseph Weeks, Engineer
Area of science: Physics
ID: 1074206510.Ph

Great question.  I found a simple explanation concerning the mechanism 
for heat conduction at:

In this article, a list of thermal conductivities is given for a number 
of metals.  If you will look at the link, you can see that pure aluminum 
has a conductivity of 247 Watts/meter-degree K (W/mK), whereas a high 
strength aluminum alloy 7075-T6 has a conductivity of only 130 W/mK.  
Thus by putting in less than 10% alloying agent, the conductivity of the 
aluminum alloy has been dropped to almost half the conductivity of pure 

Heat is simply a measurement of how rapidly atoms are moving; atoms 
moving fast are at a higher "temperature" than atoms that are moving 
slowly.  In fact, as you approach a temperature of absolute zero, the 
atoms are hardly moving or vibrating at all.  It is easy to understand 
that in a gas or liquid, the atoms are shooting around from point to 
point, bouncing into each other.  After a while, the atoms will all be 
bouncing around with the same level of kinetic energy; all the atoms will 
be at the same temperature.

In a gas or liquid, the individual atoms or molecules are not held in 
place; they can move around from one place to another.  If you inject a 
high speed molecule into a bunch of lower moving molecules, the fast 
molecule is going to bounce around, giving up some of its energy every 
time it makes contact with another molecule.  Heat transfer in a liquid 
or gas might be visualized like a table with billiard balls on it.  When 
one fast moving ball is introduced, it bounces around getting other balls 
moving until they are all moving at about the same speed.

In solids, atoms are not free to move from one place to another; they are 
locked into a crystal structure.  Therefore, the atoms vibrate in 
place.  "Hot" atoms will vibrate more than "cold" atoms.  Each atom is 
held in place by chemical bonds.  When one atom attempts to move or 
vibrate, it tugs and pushes on its neighbor atoms, causing them to also 
vibrate.  So, if you try to heat one side of a solid, the hot atoms will 
pull and tug on their cooler neighbors, transferring heat throughout the 
solid.  Visually, a solid might be compared to a bunch of balls, 
connected to each other by springs.  You can't move one ball without its 
neighbors moving also.  This movement is similar to a wave motion, and it 
is called lattice waves or phonons.  All solids are going to conduct heat 
by this wave spreading mechanism.

Metals, however, have another thing going on when it comes to thermal 
conduction.  In many metals, the electrons on the outer orbitals are not 
tightly bonded to the nucleus.  Each metallic nucleus looks pretty much 
like its neighbor (for a pure metal).  Therefore, it is pretty easy for 
an electron to hop from one atom to the next atom.  These electrons are 
called "free electrons" because they can move easily from one atom to the 
next.  If you want to conduct electricity, these free electrons are just 
what you need; you force a few electrons into one end of a wire, and 
excess electrons come out the other end-like magic.

These free electrons also conduct heat.  Electrons are particles; 
electrons that move fast are hotter than those that move slowly.  In the 
case of a pure metal, these electrons can apparently travel for some 
distance before they bounce into other atoms and lose their energy.  

When you alloy a metal, you generally are putting atoms into your metal 
that are larger or smaller than the atoms that are already there.  Also 
the alloying metal will have a different level of attachment to its free 
electrons.  So, when the electron tries to travel through the metal, it 
can travel generally a shorter distance in the alloy before interacting 
with one of these different sized atoms.  It is like the difference 
between traveling along a freeway and a curvy road.  The pure metal is 
the freeway, with its higher conductivity; the alloy is like a curvy road 
with poorer conductivity.  With a curvy road, you just can't move as 

Based upon this explanation, it seems to make sense that those metals 
that have high thermal conductivity also have high electrical 
conductivity.  And the more alloying agents that you put into a metal, 
the poorer the thermal and electrical conductivity gets.

Of course, if a material doesn't have any free electrons, it can only 
transfer heat by vibrating against its neighbors.  As a result, it's 
thermal conductivity is generally much less than that of a pure metal.  
It should also come as no surprise that, without any free electrons, non-
metals do not conduct electricity.

I hope that helps to explain the effects of alloying on thermal (and 
electrical) conductivity.

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