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Glasses and amorphous materials

Learn the short-range order and network structure of oxide glasses, network-forming and modifying oxides, non-bridging oxygens, the temperature dependence of viscosity, thermal tempering and chemical strengthening, how readily metallic glasses form and their properties, and amorphous silicon through three-choice questions.

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Glasses and amorphous materials: key points

Glass is an amorphous solid whose structure froze without crystallizing as the melt cooled; its nearest-neighbor coordination is uniform, but it has no long-range periodicity. In oxide glasses, network-forming oxides such as SiO₂ build a network of corner-sharing units, and modifying oxides such as Na₂O and CaO break bridges to create non-bridging oxygens, lowering the viscosity and the glass transition temperature. The temperature dependence of viscosity is described by the VFT equation and guides the temperatures for forming and annealing.

Glass breaks in tension from surface flaws, so it is strengthened by introducing compressive stress at the surface through thermal tempering or chemical strengthening by ion exchange. Metals, too, become glass even on slow cooling if the alloy has many components, atoms of different sizes and a negative heat of mixing, and such glasses show high strength and large elastic strain. Amorphous silicon with dangling bonds terminated by hydrogen is used in large-area thin-film transistors and solar cells.

All Glasses and amorphous materials questions (15)
  1. Which statement about the atomic arrangement of an oxide glass such as soda-lime glass is correct?
  2. Which set correctly lists network-forming oxides, which form glass on their own and build the network skeleton of oxide glasses?
  3. How do the structure and properties of SiO₂ glass change when Na₂O is added?
  4. 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)?
  5. 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)?
  6. Which correctly describes how sheet glass is strengthened by air quenching (thermal tempering)?
  7. 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?
  8. Which correctly describes chemical strengthening (strengthening by ion exchange), used for smartphone cover glass and similar products?
  9. Which combination is the empirically known set of conditions (Inoue's empirical rules) for alloys that readily form bulk metallic glasses, which become glass even at thicknesses of several mm or more?
  10. By Turnbull's guideline, the larger the reduced glass transition temperature Trg = Tg/Tl (Tg is the glass transition temperature and Tl the liquidus temperature, both absolute temperatures), the more readily a melt becomes glass on cooling. Which of the following alloys forms glass most readily by this guideline?
  11. Sheet-shaped metallic glass is made by casting into a copper mold. Assume that the cooling rate at the center of the sheet is inversely proportional to the square of the thickness d, and that it is 1.0×10³ K/s for a thickness of 1.0 mm. Which is closest to the largest thickness at which an alloy with a critical cooling rate of 10 K/s can be made into glass?
  12. A Zr-based bulk metallic glass has a yield strength of 1.8 GPa and a Young's modulus of 90 GPa. Which correctly pairs the elastic strain limit estimated as σy/E with its comparison to crystalline metallic materials?
  13. Which statement about plastic deformation of metallic glasses at room temperature is correct?
  14. Amorphous silicon for solar cells and thin-film transistors is made by plasma CVD from silane as a-Si:H containing about 10% hydrogen. What is the main reason?
  15. Which correctly compares the properties of hydrogenated amorphous silicon (a-Si:H) and crystalline silicon?

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