If the melt exceeds the melting temp then the melt can form the glass after quenching? If it can form the glass then what are the conditions under which quenching has to be done?
Glass are produced by quenching the melt with sufficient cooling rate. In some cases slow cooling is also enough for glassification. The critical cooling rate depends on the material.
Hi,
Glass formation depends on cooling rate(usually fast cooling), which can be done using melt quenching technique. It does not depend on melt temperature as glass transition temperature always less than melting temperature. However melt temperature is important in order to mixing the alloys composition uniformly. Also should careful to increase temperature after melting point as it might cause burst of ampules(container in which alloys got sealed) due to increment of partial pressure.
In short we melt materials so that we can solidify again at different cooling rate (by melt quenching in your case) to form glass. so higher temperature than melting point would not affect (But should be less than evaporation temperature)
there will not be any problem to form the glass if you provide the sifficent cooling rate for the composition. However, higher the the temperature above melting temperature, higher will be the vaporisation.....thus, u will be away from the actual starting glass composition!!
Sometimes,esp. for high viscous melts, u need to raise the temperature above melting temp to reduce the viscosity further and thus to aid the refining process
As said before it depends on the glass composition you are preparing.
If it is a mixure of oxide powders you are melting, and for example one component is alumina, you need enough temperature or time for glass homogenization.
Both of them increase volatilization, but I would prefer time instead of temperature.
Please note, that the temperature you need to mahe a glass is generally lower than the melting point of its components . For example you can prepare at 1600C a glass made of Al2O3 (melting point about 2000C), SiO2 (melting point about 1800C) and Y2O3 (melting point about 2300C).
Also note that a glass does not have a melting temperature.
It is not important when you exceed the melt temperature, just if you attend to the following points:
1- the glass components should not evaporate, It means may be over heating cause evaporating of some components specially Fluorine or Tin components.
2- Check the high temperature reaction that may happen. It means some carbide or boride etc is formed in high temperature. You should avoid forming of them.
3- In some glass system, over heating cause phase separation so you should care and see the phase equilibrium diagrams of the system. Separation is not good for many of glass systems.
there are some other points that are not as important as of mentioned points.
Good luck
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