Glasses

All glasses (so far) have what 2 common characteristics?

1. no glass has a long range, periodic atomic arrangement

2. every glass exhibits time-dependent glass transformation

   1. glass transformation region: transition from rigid “glassy” state to softer state

What is glass?

State of matter with most macroscopic + thermodynamic properties of a solid but with the structural disorder + isotropy of a liquid.

What are the rules for network formation in oxide glasses?

1. oxygen is linked to no more than 2 atoms

2. number of oxygens surrounding A (central atom) is small (3 or 4)

3. AO_n polyhedral share corners (not edges or faces)

4. at least 3 corners must be shared for a 3-D network

Which rules can chalcogenide (Se or S-based) glasses violate?

1. oxygen is linked to no more than 2 atoms

2. AO_n polyhedral share corners (not edges or faces)

What happens when you add R₂O (alkali/group 1) to SiO₂?

1. glass network gets broken up (depolymerized) since the R⁺ ions replace bridging oxygens ➡ structure is less connected

2. reduction in…

   1. viscosity (flows better)

   2. glass transition temperature T_g (less energy input needed)

1. increase in…

   1. density

   2. refractive index (more sparkly)

   3. thermal expansion (more sturdy)

   4. conductivity

What is field strength + its 2 types?

• measure of how strongly a cation pulls on surrounding O atoms

  • formula: Z/R²

    • Z: cation charge

    • R: average cation-oxygen distance

• types:

  • high field strength cations

    • strong O attraction

    • tend to form well-defined, directional structures (networks)

    • network formers

    • ex. Si⁴⁺, B³⁺, Al³⁺

  • low field strength cations

    • weak O attraction

    • don’t form strong directional bonds

    • network modifiers ➡ breaks up network

What should you do if you replace Si⁴⁺ with Al³⁺ in the glass network?

Since the charge balance is disrupted you need to add a modifier cation (like Na⁺ or Ca²⁺) to restore neutrality.

What is borosilicate glass?

• prepared by adding B₂O₃ to the silica mixture

• physical properties resemble quartz (mp: 700°C)

• resists heating + cooling cycles ➡ doesn’t crack easily

What is soda-lead glass?

• prepared by adding PbO to the silica mixture

• if glass contains ~24% lead oxide, called “full lead” glass

• Pb increases index of refraction ➡ sparkling

• soft

What is the role of cladding in optical fibers?

• provides a lower refractive index layer to trap light inside the core

  • dopants adjust Δn (0.05 ➡ 0.005)

What happens if the incident angle is larger than the critical angle?

• incident: angle at which a ray of light hits the boundary between 2 materials

• critical: special angle that depends on refractive indices of the 2 materials

  • if incident angle < critical angle ➡ light partially passes into 2^nd material

  • if incident angle = critical angle ➡ light travels along the boundary

  • if incident angle > critical angle ➡ total internal reflection

    • none of the light escapes into 2nd material but completely reflects back into the 1^st material

What is optical pulse dispersion + how can it be reduced?

• optical pulse dispersion: different path lengths ➡ different travel times ➡ signal smearing, limiting transmission distance + pulse rate

• solutions

  • regeneration stations: boost/clean the signal along the way

  • single-mode fibers: allow only one path for light ➡ no dispersion

  • gradient refractive index fibers: rays will curve + travel at nearly same time

Why is high purity essential in optical fiber fabrication?

• to minimize intrinsic + extrinsic losses in the transmitted signal

  • intrinsic: losses due to the glass itself

    • UV (< 300 nm): electronic transitions absorb light

    • IR: vibrational modes absorb light

  • extrinsic: losses due to impurities/defects

    • metal ions: d-d transitions absorb light

    • OH: absorption overtones

    • density fluctuations/bubbles: makes light scatter (Rayleigh scattering)

What are the main fabrication methods for optical fibers?

• chemical vapor deposition (CVD): building the glass layer by layer

• gravity drawing: melt the glass + pull it into thin fibers

What factors contribute to density fluctuations in fibers?

T control, pulling speed + bubbles during fabrication.

Why is glass used for solid radioactive waste disposal?

1. high-T silicate liquid is a good solvent

2. can accommodate many cations

3. high waste loading

4. ions become immobilized when cooled

What properties are required for glass?

1. solvent power: must dissolve wide variety of radioactive ions to maximize waste capacity

2. permanent immobilization: must chemically resist leaching over long periods to keep radioactive ions trapped

3. mechanical stability: resist cracking + physical damage to prevent increased exposure + leaching

4. thermal stability: must withstand heat from radioactive decay without devitrifying or cracking

5. radiation resistance: must tolerate damage from alpha + beta radiation without significant structural degradation

What is vitrification?

Ability to combine many elements in one storage vessel (can accommodate 50+ elements).

What is sol-gen synthesis?

• process that produces solids via gelation, forming an oxide network through polycondensation reactions, instead of crystallization or precipitation

  • sol: stable suspension of colloidal solid particles or polymers in a liquid, which can be amorphous or crystalline

  • gel: porous, 3D continuous solid network that surrounds + supports a continuous liquid phase

What is the difference between colloidal gels + polymeric gels?

• colloidal: networks of agglomerated particles

• polymeric: networks of particles with a polymeric substructure

What holds a gel together?

Covalent bonds, hydrogen bonds, van der Waals interactions, or entanglement of polymer chains.

Why is sol stability important in sol-gel processing?

• stable sols prevent particle agglomeration, which is crucial for forming a uniform gel

  • aggregation is caused by attractive van der Waals forces + forces that minimize the system’s surface/interface energy

How can aggregation of particles in a sol be prevented?

1. electrostatic stabilization

   1. each particle gets a surface charge

   2. the like charges repel each other, so the particles stay apart

   3. this happens when the repulsive electrostatic forces are stronger than the attractive Van der Waals forces

2. steric barrier

   1. large organic molecules or surfactants stick to the particle surfaces

   2. the bulky or hydrophobic parts of these molecules stick out into the solvent + make it physically and energetically difficult for particles to come close

   3. this makes aggregation energetically unfavourable because it would reduce system entropy + increase free energy

What should the thickness of the steric layer be?

Roughly the same as the distance outside the influence of van der Waals forces.

What happens when particles with steric barriers come close to each other?

The motion of the surface chains is restricted ➡ entropic repulsion that prevents aggregation.

What is the main chemical transformation that occurs during Sol-gel synthesis?

The transformation of $\text{Si-OR}$ (alkoxide) groups to $\text{Si-O-Si}$ (siloxane) groups via hydrolysis and polycondensation reactions.

What are 3 examples of molecular precursors used in Sol-gel synthesis?

1. sodium silicates (water glass)

2. silicon alkoxides (ex. TMOS, TEOS)

3. silicon halides

What is the name of the Si-OH group formed during hydrolysis in Sol-gel synthesis?

Silanol group.

What is the formula for calculating Field Strength?

• Z/R²

  • Z: cation charge

  • R²: average M-O distance

What determines whether an oxide (AO₂ or A₂O) will form a glass based on network formation rules?

• AO₂ (ex. SiO₂) will form a glass

• A₂O (ex. Na₂O) will not form a glass

How does the addition of Na₂O to vitreous SiO₂ affect network polymerization?

It causes Qⁿ speciation (ex. Q⁴ to Q³), decreasing network polymerization because Na+ ions form non-bridging oxygens (NBO).

How does the addition of K₂O to vitreous B₂O₃ affect network polymerization?

It causes K⁺ addition to change the coordination number of boron from [3]B to [4]B (an increase in coordination number), which increases network polymerization.

What are the 3 ranges of structural order in glass?

1. short-range order (< 5 Å)

   1. polyhedra, distribution in angles/bonds, and first coordination environment/sphere (coordination number)

2. medium-range order (5-10 Å)

   1. structural organization beyond the 1^st coordination sphere, polyhedra connectivity, and superstructural units (ring formation)

3. long-range order ( > 10 Å)

   1. chains/edge-sharing tetrahedra (ex. phosphates) or percolation channels (ex. enhanced ionic conductivity)

What is the primary requirement for making glass from a melt and what influences the rate of cooling?

• need to cool the melt rapidly to bypass onset of crystallization

• cooling influences: sample dimensions and thermal conductivity

What is the range of T_g (Glass Transition Temperature) for a liquid that is cooled very fast VS one cooled slowly?

A fast-cooled glass will have a higher T_g than a slow-cooled glass.

What is float glass and how is it prepared?

A type of soda-lime glass where flat plates with a well-defined thickness prepared by being "floated" on molten tin metal.

What additives can be used to tailor colours in soda-lead glass?

Iron oxide, copper oxide, manganese dioxide and cobalt oxide.

What is a glass additive that gives a blue colour and a glass additive that gives a cranberry colour?

• cobalt glass ➡ blue colour

• red glass coloured with gold or copper compounds ➡ cranberry glass

How does the entropic effect help a steric barrier prevent particle aggregation?

When particles with steric barriers come close, the motion of the surface chains is restricted, which increases the free energy of the system and creates an entropic repulsion that prevents aggregation.

What’s the difference between LLW and HLW in terms of how they’re disposed?

• LLW: disposed of using zeolites, cements, or retention ponds

• HLW: disposed by direct disposal into a glass

What are the main sources of radioactive wastes?

Weapons Production, Energy, Medical, Research, and Mining.

What is a major challenge for nuclear waste glasses and where are they commonly stored?

They are prone to crystallization and are stored underground in stainless steel containers in locations like Sheffield (UK) and Yucca Mountain (USA).

What types of radiation damage glass and what are their typical half-lives?

• alpha decay (from actinides) and beta decay (from fission products)

• 94% of the radioactive events are short-lived, while 1% are long-lived