Glasses Flashcards
What is the definition of a glass?
Amorphous material that exhibits a Tg
How do you create a glass and what out of?
Any material can be a glass, must supercool liquid melt and prevent crystallisation
How can crystallisation be stopped?
Stop nucleation & Crystal growth:
- prevent diffusion
- big structural difference between liquid and crystalline state
- absence of heterogenous nucleation points
- large energy against crystallisation nucleation
What is the relation ship between diffusion and viscosity?
D = k/n
Where k is material constant
What’s the relationship between Tg and Tm for a glass?
Tg/Tm > 2/3 for the system to be likely to form a glass
Describe vitreous SiO2/ GeO2 glasses
Tetrahedra structural unit cells, with a random continuous network.
Ge is larger than Si, and GeO bond longer than SiO, but SiO has bigger bond angle (150° vs 130°)
Ge has less free volume but more structural defects.
Si forms NBOs when alkali oxides added
Describe vitreous P2O5
Charge balanced tetrahedral structural unit cell with same connectivity as B2O3. covalently bonded within the tetrahedra with van der waals attraction between planes
Network is frequently disrupted = high Tg
Describe the vitreous B2O3
Boroxol ring as main unit (3 linked BOB triangles forming a hexagonal ring), 80% B atoms in boroxol rings, rest in B03 triangles
NBOS formed by adding alkali oxides at more than 25 mol% - anomaly due to B changing coordination number from 4 to 3
Describe chalcogenide glasses
Based on S, Se, Te. Has rings, chains and chain fragments respectively.
Unsure about which model supports, either RCN or COCN
Describe halide glasses
Vitreous BeF2/ZnCl2, tetrahedra with BeFBe/ZnClZn at corners as bridging bonds.
Bonds most likely to be ionic (unlike 50:50 ionic covalent that SiO and BO bonds are) although no agreement so far
Describe metallic glasses and BMG
Metal forming glass, likelihood increases with number of elements, enhanced when 10% size difference between 3 main elements
Use Efficient cluster packing model (densely packed clusters as structural units)
Describe organic polymer glasses
Chains cooled randomly & entangled & crosslinked
Density = 90% of crystalline density = localised close packing
Factors raising Tg: strong intermolecular forces, large pendant groups, chain stiffening groups
Describe organic molecular glasses
Based on small molecule glass-formers which exhibit Tg = 100° (mean prevent crystallisation)
Differing Tg due to comp means flexibility structural unit sizes
High free volume and lack of order
Glass formation helped by: ring distortion (bending phenyl rings out of plane)
