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Glasses and amorphous materials

Which set correctly lists network-forming oxides, which form glass on their own and build the network skeleton of oxide glasses?

  1. ANa₂O, K₂O, CaO
  2. BAl₂O₃, MgO, ZnO
  3. CSiO₂, B₂O₃, P₂O₅
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Answer: C. SiO₂, B₂O₃, P₂O₅

In SiO₂, B₂O₃, P₂O₅ and GeO₂, the cations form polyhedra with three- or fourfold oxygen coordination that link by sharing their corner oxygens, so each forms a glass network on its own. Zachariasen named conditions such as a small cation coordination number and polyhedra sharing only corners, not edges or faces, as favoring glass formation. Na₂O, K₂O and CaO are network modifiers: they break the network and occupy its interstices, and they do not form glass by themselves. Al₂O₃ and ZnO are called intermediate oxides; they hardly form glass alone, but when their charge is compensated by alkalis and the like they join the network, for example as AlO₄ tetrahedra. MgO usually acts as a modifier.

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More Glasses and amorphous materials questions

  1. How do the structure and properties of SiO₂ glass change when Na₂O is added?
  2. In a sodium silicate glass of 30 mol% Na₂O-70 mol% SiO₂, assuming each Na₂O creates two non-bridging oxygens, which is closest to the number of non-bridging oxygens per SiO₄ tetrahedron (NBO/T)?
  3. The viscosity η (Pa·s) of a soda-lime glass is described by the VFT equation log₁₀η = A + B/(T − T₀) with A = −2.5, B = 4300 K and T₀ = 520 K. Which is closest to the temperature at which the viscosity reaches 10¹² Pa·s (a guide to the annealing point, close to the glass transition temperature)?
  4. Which correctly describes how sheet glass is strengthened by air quenching (thermal tempering)?
  5. In thermally tempered glass of thickness t = 10 mm, let y be the distance from the mid-plane; the stress follows the parabolic distribution σ(y) = σc(1 − 12y²/t²) (tension positive, σc is the tensile stress at the mid-plane). If the surface compressive stress is 120 MPa, which correctly pairs the mid-plane tensile stress σc with the depth from the surface to the end of the compressive layer?