Subject: why does INVAR have low coefficient of expansion?

From: [email-masked] (lbliao)
Date: 14 Jan 1995 04:12:57 GMT

Does any one know why Invar has low CTE?
I vaguely remember that it had sth to do with the exchange magnetic force
or sth like that. Would some kind soul come forward and elaborate on this?

Also, would someone explain why cordierite has low CTE!

many thanks

Lily Liao

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From: [email-masked] (Charlie Kuehmann)
Date: Tue, 17 Jan 1995 10:46:00 -0600

> or sth like that. Would some kind soul come forward and elaborate on this?

There is of course a large body of research on this subject and differing
points of view.  The approach that I think is most resonable is the
"two-gamma states" model.  In this model the low CTE is explained by the
existance of two distinct states of gamma Fe which have different magnetic
states, volumes, and energies.  The energies between these two states
varies with composition and a number of other factors.  Because of this
energy difference the population of these two states varies with
temperature.  For Invar, Fe - 36% Ni, as temperature is increased, the
relative polulation of the low volume Fe state increases and counteracts
the thermal vibration contribution to thermal expansion.  Thus you have a
low CTE.  The description is a simple one and in reality has much more
complexity than I described here.  This view is also but one explanation
of this phenomenon and there are others. In my estimation, however this
approach has a large body of supporting evidence and is also more useful
in an "engineering" sense than some of the others.  If you would like a
pointer to some references on this subject drop me a line.
 
> Also, would someone explain why cordierite has low CTE!

I'm not familiar with this material but if it contains gamma Fe then it is
possible the origin is the same.  Also there is some evidence that other
elements may also exhibit this two-state phenomenon (or even multi-state)
so it could still be similar.  But since I don't have any knowledge of
this material specifically this would only be a guess.

> many thanks

You're welcome.

-- 
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BIRL Industry Research Laboratory
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