Glass ceramics

Question 1

What are the characteristics of glass ceramics?

Answer 1

~ liquids of comp range Li2OAl2O36SiO2
crystallize to B-eucriptite, similar to B-quartz structure

~ AlO4(5-) tetrahedra form with charge comp provided by Li+

~ B-eucriptite has a near zero CTE (excellent thermal shock resistance)

Question 2

Why is TiO2 added to glass ceramics?

Answer 2

~ as a “nucleating agent”, after a shape is formed, TiO2 encourages phase separation during cooling

~ glassy regions rich in TiO2 DEVITRIFY to a TiO2 rich crystalline phase. B-eucriptite ss grows from this phase!!!!!!!

Question 3

What is the result of this uniformly dispersed phase separation?

Answer 3

~ crystals quickly grow, giving very fine grain size and NO POROSITY

Question 4

What happens as the heat treatment temp is increased 1000C?

Answer 4

~ B-eucriptite + SiO2 –> B-spodumene ss

Question 5

What’s the difference between B-eucriptite and B-spodumene?

Answer 5

~ B-eucriptite: forms very small crystals of low birefringence, transparent

~ B-spodumene: forms larger, birefringent crystals, opaque

Question 6

What is B-eucriptite used for?

Answer 6

~ cookware, range tops

Question 7

What is B-spodumene used for?

Answer 7

~ corning ware cookware, precision optics, high strength glass

Question 8

What are machinable glass ceramics?

Answer 8

~ devitrified phases of a mica comp, easy cleavage

Question 9

What is chemical machining?

Answer 9

~ photographic process defines locations to devitrify, so more soluble in an etchant