Part 2 Flashcards
1
Q
Define a Glass
A
- a supercooled liquid
- short range order
- macroscopic relaxation times
2
Q
What are the macroscopic properties of a glass?
A
- similar to solid
- G not equal to zero
- rigid structure
3
Q
Give examples of glasses
A
- oxides
- group IV elements
- organic molecules
- polymers
- metals under fast cooling
- collids
4
Q
Why can’t glasses flow?
A
they have a relaxation time of 10^30 years, longer than the age of the universe
5
Q
Give examples of glass systems
A
- optical fibres
- food processing
- insect preservation
- engineering plastics
- water within the universe is mostly glassy
6
Q
Define a supercooled liquid
A
- a liquid that has been cooled down avoiding crystallisation.
- occurs when the cooling rate is too fast for a crystal to form or the molecules have some permanent disorder that prevents them from forming crystals.
7
Q
What is the glass transition?
A
- ## a thermodynamic transition with discontinuous thermodynamic properties
8
Q
Why is the glass transition not a true phase transition?
A
- as glass transition temperature depends on the cooling of the system
- at a lower cooling rate the Tg is lower
- residual entropy also dependent on cooling
9
Q
What is the Kauzmann temperature?
A
- the point where the entropy of the crystal and the liquid intersect
- lower limit on Tg as a glass is more disordered than a crystal so can’t have a lower entropy
10
Q
What are the two types of glass?
A
Strong and fragile
11
Q
Describe strong glasses
A
- follow the Arrhenius equation
- single activation energy that is independent of temperature
- long relaxation time as the crystal has such long range order the whole system has to move
12
Q
Describe fragile glasses
A
- super Arrhenius model
- activation energy increases as temperature decreases
- change in viscosity with temperate caused by a range of structures
- Complex PE landscape
- fluctuating relaxation time
13
Q
Why is there no full theory to describe glasses?
A
- some degrees of freedom don’t contribute to the thermodynamics of the system
- broken ergodicity
14
Q
How is relaxation time estimated?
A
- using the Vogel fulcher equation
