2 (a) Glass ceramics have low coefficients of thermal expansion. State why this is the case (hint; consider the microstr

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2 A Glass Ceramics Have Low Coefficients Of Thermal Expansion State Why This Is The Case Hint Consider The Microstr 1 (79.58 KiB) Viewed 7 times

2 (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 510°C 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]