please answer in 30 minutes (a) Glass ceramics have low coefficients of thermal expansion. State why this is the c

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please answer in 30 minutes
(a) Glass ceramics have low
coefficients of thermal expansion. State why this is the case
(hint; consider the microstructure) and then by means of a suitable
diagram, describe and explain the thermal treatment employed in the
production of glass ceramics. [8 marks]
(b) Suggest an appropriate
characterisation technique which could be used to follow the
progress of the crystallisation of a glass, assuming spherical
crystals form, and briefly explain how this could be used to
determine the fraction crystallised as a function of
time.
[2 marks]
(c) Isothermal
crystallisation experiments on a lithium disilicate glass sample at
510oC gave the following results:
Time, t / h
Fraction Crystallised, f
0
0.000
24
0.002
48
0.033
60
0.079
73
0.165
87
0.305
100
0.470
112
0.632
123
0.766
140
0.913
By plotting a suitable graph, verify
that the crystallisation process obeys the Johnson-Mehl-Avrami
equation with a value of the Avrami exponent, n = 4, and
determine the rate constant,
k.
[8 marks]
(d) What can you deduce about
the mechanism of crystallisation and the morphology of the
crystallites from this value of
n?
[2 marks]