Surface nucleation of μ-cordierite in cordierite glass: thermodynamic aspects

Abstract

The heterogeneous nucleation rate of μ-cordierite at the surface of cordierite glass (2MgO·2Al 2O 3·5SiO 2) is discussed in terms of classical nucleation theory (CNT). Calculations are based on experimental viscosity data and estimates of the melting temperature and melting enthalpy of μ-cordierite from calorimetric measurements. A virtual melting temperature of the metastable μ-cordierite, T μ M, was estimated, by means of X-ray measurements, from the primary occurrence of μ-cordierite after brief thermal treatment. T μ M was found to lie in the range 1300–1467°C. The molar enthalpy of melting, ΔH μ M, was calculated from calorimetric measurements and literature data. Referring to the molar weight of the formula unit, M = 585 g mol −1, ΔH μ M could be narrowed to the range 170–190 kJ mol −1. The heterogeneous volume nucleation rate, I∗(T), was estimated from the apparent nucleation rate of μ-cordierite which was previously studied at fractured and polished surfaces of cordierite glass by means of transmitting light microscopy. I∗(T) steadily increases up to 235 K above the glass transition temperature at 815°C without reaching a maximum. In the present paper, this exceptional temperature-dependence is discussed in terms of CNT. The best fit of experimental data was obtained for T μ M ≈ 1467°C, a low liquid-crystal interfacial free energy, σ, and a low activity of nucleating substrates, Ф, if σФ 1 3 ≈ 100 mJ m −2 is valid.