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Author Topic: How does Negative Thermal Expansion work?  (Read 3784 times)

Offline John Chapman

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How does Negative Thermal Expansion work?
« on: 02/03/2009 21:58:30 »
I was listening to a science podcast today and it stated that some substances shrink as they heat up. Apparently this is called negative thermal expansion. Eh? As any schoolboy knows, as a material heats up the atoms take on energy, vibrate more vigorously and expand into a larger space. Schoolboys also know that girls canít throw and sometimes flap their arms about when they run, likeÖ.. well, like girls! But thatís the subject for another thread.

Silicon and Germanium apparently exhibit the characteristic of negative thermal expansion.

So my question is this:

How is negative thermal expansion possible?

Also, what would happen if you mixed a negative thermally conductive material with a positive one? Would you end up with a composite that doesnít change size as it changes temperature? Or would it, on a microscopic level, expand at one molecule whilst contracting at itís neighbouring molecule and then collapse into a pile of molecular size dust as soon as heat is applied?



 

Offline RD

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How does Negative Thermal Expansion work?
« Reply #1 on: 02/03/2009 22:14:15 »
"Negative thermal expansion" would be true of ice turning to liquid water.
 

Offline John Chapman

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How does Negative Thermal Expansion work?
« Reply #2 on: 03/03/2009 01:25:39 »
Very true. A good bit of lateral thinking there. Why does it shrink as it thaws, though?
 

Offline lancenti

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How does Negative Thermal Expansion work?
« Reply #3 on: 03/03/2009 02:39:59 »
In ice, water molecules are held in a rigid solid lattice which keeps the molecules at a set distance from one another. The water molecules cannot get too close to each other when in the ice state because that would mean that they would interact in a less favourable manner with other water molecules.

As it melts, this lattice breaks down very much like a building collapsing onto itself. Since the atoms still lack sufficient kinetic energy, they don't spread out like 'normal' things.

As the Temperature continues to increase, the water gains enough kinetic energy to start moving apart, overcoming the strong hydrogen bonds that pull them together.
 

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How does Negative Thermal Expansion work?
« Reply #3 on: 03/03/2009 02:39:59 »

 

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