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Glasses and amorphous materials

How do the structure and properties of SiO₂ glass change when Na₂O is added?

  1. ANa⁺ takes the place of Si⁴⁺ at the center of SiO₄ tetrahedra and strengthens the network, so the viscosity and softening temperature rise
  2. BSi-O-Si bridges break into non-bridging oxygens and the network weakens, so the viscosity and the glass transition temperature fall and thermal expansion increases
  3. CNa₂O does not affect the network, so the viscosity is unchanged and only the chemical durability improves
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Answer: B. Si-O-Si bridges break into non-bridging oxygens and the network weakens, so the viscosity and the glass transition temperature fall and thermal expansion increases

Each Na₂O added breaks one Si-O-Si bridging oxygen and creates two non-bridging oxygens bonded to only one Si, while the Na⁺ ions occupy interstices in the network and balance the charge. The weakened connectivity lowers the viscosity and the glass transition temperature, so shaping that requires high temperatures of about 2000 °C for silica glass becomes possible at much lower temperatures. In exchange, the linear expansion coefficient increases, and because Na⁺ readily exchanges with hydrogen ions in water, chemical durability falls. CaO is added in soda-lime glass to restore this durability. Na⁺ has a large ionic radius and small charge, so it does not enter the centers of tetrahedra.

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More Glasses and amorphous materials questions

  1. 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)?
  2. 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)?
  3. Which correctly describes how sheet glass is strengthened by air quenching (thermal tempering)?
  4. 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?
  5. Which correctly describes chemical strengthening (strengthening by ion exchange), used for smartphone cover glass and similar products?