Glasses- Behaviour of Glasses Flashcards
Commonly used definition of glass
A glass is an inorganic product of melting which has cooled and become rigid without crystallising
Defining characteristics of glasses
Liquid-like structure, non-crystalline (amorphous). Elastic solids (very little plastic flow if any). Exhibits transformation range behaviour.
Common features of glasses that aren’t defining characteristics
Transparency- manufacturing process means they are isotopic with no grain boundaries to scatter light, but electronic band gap in some glasses means they are opaque in visible range.
Brittle, hard- inorganic glasses have 3D ionic bonding, polymers have 2D covalent chain-like structures which may not be as rigid as bonding between chains is weak (VdW).
What is wrong with the common definition of glasses?
There are organic glasses now accepted. Some can be made by methods not requiring melting.
For pure liquids that crystallise, what happens to volume as temperature decreases slowly?
Volume decreases steadily as temperature decreases until the liquid’s melting point. Then it will crystallise and quickly decrease in volume for no change in temperature. Then continues to contract at slower rate than when it was a liquid.
For pure liquids that crystallise, what happens to volume as temperature decreases quickly?
Limited supercooling (in the absence of nucleants). On V vs T graph, the diagonal line in liquid region is extended past the melting point a bit on the same gradient. Then straight line down to same solid region line upon crystallisation
What happens to glass-forming liquids as they are cooled?
They are easily super-cooled below their melting point. As it cools viscosity increases until, at the fictive temperature Tf, the super-cooled liquid transforms into a solid glass without crystallising. The glass is not in true equilibrium but has too little internal thermal energy below Tf for any bulk atomic rearrangement such as flow or crystallisation.
Volume vs Temperature graph for glass-forming liquid
Volume decreases steadily as temperature decreases in the liquid region and below the melting/liquidus temperature through the super-cooled liquid region. At Tf the line curves to a shallower gradient quickly for the glass region.
What is Tg and how is it found?
Defined using a kinetic experiment. Use the kink in an expansion or Cp curve at a defined heating rate. Similar to Tf but done for heating the solid glass.
What is the change from a liquid to solid glass controlled by?
Kinetics. It occurs over a temperature/time range (the transformation range) depending on cooling rate
What can be said about true glasses?
They can be recycled through the transformation range with little risk of crystallising
Why can’t some amorphous solids be recycled through the transformation range?
Some are so unstable that heating in the transformation range causes rapid crystallisation
Rapid or slow cooling
Rapid: little time for structure to re-adjust
Slow: more time to re-adjust at lower temperatures
How does crystal growth rate change just below liquidus temperature?
No growth at liquidus temperature. Big increase in growth rate just under this as the driving force ΔG for growth rises with undercooling. Reaches peak of curve and goes back down to 0 as viscosity increases so diffusion coefficients fall.
What does glass formation require?
Low crystal growth rates for primary phase and any non-equilibrium phases. Also low (zero) nucleation rates at temperatures where crystal growth is significant
