🚀 go-pugleaf

locher@iastate.edu (Robert C Locher) writes:

>jeff@mama.minmet.uq.oz (Jeff Gates) writes:

>>Now, since the subject of this article was "the Nature of Glass",
>>I would like to see if I can start a bit of controversy on the net!
>>
>>It is often said that
>>	" Glass is a liquid "
>>or that
>>	" Glass is a supercooled liquid "
>>
>>I have my own opinions on this, but first I want to see what comes out.

>I would argue that it is not a liquid.  Consider two "window panes", one
>made of vitreous silica, one made of monocrystalline quartz.  In the
>glass (the vitreous silica), the argument for being a liquid is that it
>still can "flow", even at room temperature.

>It is clear that this "flow" process in the glass at room temperature
>takes place through diffusion.  Gravity would add a nearly imperceptible
>bias for diffusion in the downward direction, which conceivably over
>millions of years would be measurable through extra thickness at the
>bottom.  (See the calculations in the related posting.)

>Now consider the crystalline "window".  Vacancies are naturally created
>at the surface, and because they are lighter than an ion, their
>diffusion has a nearly imperceptible bias upwards.  This would result in
>thickening at the bottom also.

>In short, if a glass "flows" at room temperature, it is through
>diffusion; but amorphous structures have no monopoly on diffusion.
>Therefore a crystal can be considered to "flow" at room temperature as
>much as a glass, but clearly the crystal is not a liquid; therefore,
>vitreous silica at room temperature can't be considered to be one
>either.  Therefore I reject the definition of a glass as a supercooled
>liquid, at least at room temperature.  However, it is a useful concept.

>As a working definition for *inorganic* glasses, I would offer "a
>metastable supercooled melt".  Whether or not certain amorphous polymers
>can be considered glasses, I have no idea.  What do you polymer people
>think?

>				- Rob

Crystalline solids often 'flow' at room temperature as a result of
dislocation motion.

T. D. Taylor
Department of Ceramic Engineering
Clemson University
Clemson, SC USA