A few more thoughts about glass

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					A few more thoughts about glass.

Think of constructing the 3-D glass network with tinkertoys, having 1, 2, 3, or 4
connection points. With just Si and O, one has 1-2 symmetry, and then they just fit in a
hexagonal close packed structure. If one tried to do this with Na, it would just terminate
each O with two Na. One cannot make a nice network with valence 2. Better, with
valence 3 (B, Al, or Fe) one has two bonds to form the network (two of the 3 can connect
to other oxygens,) and if there is no space for the network to continue, one can always
cap off the dangling bond with a single valence Na or K.

From Moser’s Paper, p 8:
      Mellinger and Chick (1979) state that the glass formers are SiO2, B2O3,

and Al2O3; the glass modifiers are CaO, MgO and Na2O and the glass

intermediates are ZnO, TiO2, Cr2O3, Fe2O3 and NiO. Chick et al. (1981) state

that network formers include SiO2, B2O3, Al2O3 and P2O5 and comprise the basic

structural network of glasses. Modifiers, which include alkaline and alkaline earth

oxides, tend to weaken the structural network and lower the melt viscosity; thus,

allowing processing at reasonable temperatures. Intermediates, which include all

other oxides, can enter the network as formers or modifiers depending on the

overall composition and processing conditions.


What happens if you just have silicates and metal oxides? Clearly, the electronic balance
is maintained, but there may be no “network”. That is “phase separation” the oxide phase
and the glass phase. See the 3-phase diagram. The extreme example of phase separation
is molten glass covered by a solid crust of aluminum oxide. Alternatively, one could
have solid glass with a liquid flux on top. Once solidified, there would clearly be 2
phases.

In time, glass can devitrify, or mineralize. There are several ways in which this can
happen. Under pressure, the atoms can jockey for minimum energy configurations and in
doing so grow crystals internal to the glass. Alternatively, water can attack glass and
dissolve the more soluble ions out of the glass, causing the remaining atoms to rearrange,
and they often rearrange in the form of minerals

Glass is an electrical nonconductor. This means that that the electrons in the covalent
bonds are so tightly held that they cannot move. As you raise the temperature,
conduction becomes possible. At this point, the atoms and electrons can freely move
about.

Viscosity: Glass is a liquid. It just has a viscosity so high that one cannot see it move in
real time (old castle windows actually flow so that the glass is thicker at the bottom), but
this takes hundreds of years.


G. A. Beitel
February 29, 2004

				
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