|MadSci Network: Physics|
Great question. I found a simple explanation concerning the mechanism for heat conduction at: http://www.frostytech.com/articleview.cfm?articleID=233 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 aluminum. 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 fast. 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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