Glass

General

  • Prepared by cooling from liquid state without crystallization
  • Glass is an amorphous solid without long range order or periodicity in atom arrangement.
  • Glass temp (T\(_g\)) is dependent on heat/cooling rate and experimental method. No STD-conditions
  • Silicates easily forms glasses.
  • 3D network formation is important for glass to form
  • Highly ionic materials does not form networked structures.
  • Vitreous (glass-like) networks are isotropic.

Zachariasens rules

  1. An Oxygen atom is at most linked to two other atoms.
  2. The coordination polyhedra formed by oxygen around other atoms share corners, not edges or faces.
  3. The coordination number of the other atom is small (<3/4 for M)
  4. The polyhedra link up to form 3D networks. at least 3 corners of the polyhedra must be shared.

Crystallization VS Glass formation

Nucleation and growth (Hetero/homogeneous nucleation) Absence of nuclei or no growth -> Glass formation

  • Many components facilitate glass formation
  • Homogeneous nucleation: Nucleation rate I:
  • \(I \propto e^{\frac{-(\Delta G_N + \Delta G_D)}{kT}}\)
    • \(\Delta G_N\) is free energy change for formation of nucleus (thermodynamic barrier for nucleation)
    • \(\Delta G_D\) is the kinetic barier for diffusion across the liquid-nucleus surface.
  • Network modifiers: Na\(_2\)O, CaO, B\(_2\)O\(_3\), Takes corner of tetrahedron and cuts chain -> lower viscosity (\(\eta\))

Spherical nucleation

Critical radius (\(r^*\)): The size where \(\Delta G\) starts to decrease -> growth is favoured.

  • At T just below T\(_m\), volume free energy (\(\Delta G_V\)) is smll -> large critical radius -> low probability of reaching \(r^*\).
  • Decreasing T -> smaller critical radius which means larger survival probability
  • \(\Delta G_N = 4/3 \pi r^3 \Delta G_V + 4 \pi r^2 \gamma\)
    • \(\gamma\) is crystal-melt interfacial energy
    • \(\Delta G_V\) is change in volume free energy per initial volume (which is negative)

Crystal growth

  • Crystals grow at any T < T\(_m\) as long as a nuclei or crystal is present.
  • Growth rates determined by thermodynamics and viscosity.
  • Nucleation and growth will have maximums due to viscosity.
  • 1ppm of glass can be crystal while still considered glass.

Crystallization system: Glass forming system:

Synthesis:

Raw materials -> Batching / mixing -> Batch melting -> Fining -> homogenization -> product Additives:

  • Glass formers
  • Network modifiers
  • Colorants
  • Finishing agents

Types

Glass ceramics

  • Crystallization avoided in glass usually, but wanted here (to a degree)
  • Glass ceramics get properties from crystal formation in a glass matrix (Figure below is not sintering, but temp high enough for small crystals to start growing)

Metallic glasses

  • Chill-block melt-spinning of glass ribbons (10-50 mm wide, only mm thick)
  • Droplet method (10\(^4\)K/s)
  • Surface melting with laser or electron beam
  • Ion mixing: Multi-layers interfused with scanning ion beam (eg Xe)
  • Bulk:
    • >3 elements (High entropy alloy)
    • Big size differences (>12%)
    • Cooling: 10\(^5\)-10\(^6\) K/s
    • Negative heat of mixing
    • Metglass: Fe-Si-B, Fe-Ni-P-B
    • Used in golf clubs, bouncymetal

Borate glasses

  • Many different building blocks
  • Silicate + metal oxides: non-bridging oxygen atoms
  • Borate + metal oxides: Converts planar BO\(_3\) to tetrahedral BO\(_4\).
  • Modification of Glass#Zachariasens rules: A high percentage of network cations must have 3 anions.
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