SURFACE NUCLEATION OF MU-CORDIERITE IN CORDIERITE GLASS - THERMODYNAMIC ASPECTS

The heterogeneous nucleation rate of mu-cordierite at the surface of c ordierite glass (2MgO . 2Al(2)O(3) . 5SiO(2)) is discussed in terms of classical nucleation theory (CNT). Calculations are based on experime ntal viscosity data and estimates of the melting temperature and melti ng enthalpy of mu-cordierite from calorimetric measurements. A virtual melting temperature of the metastable mu-cordierite, T-M(mu) was esti mated, by means of X-ray measurements, from the primary occurrence of mu-cordierite after brief thermal treatment. T-M(mu) was found to lie in the range 1300-1467 degrees C. The molar enthalpy of melting, AH(M) (mu), was calculated from calorimetric measurements and literature dat a. Referring to the molar weight of the formula unit, M = 585 g mol(-1 ), Delta H-M(mu) could be narrowed to the range 170-190 kJ mol(-1). Th e heterogeneous volume nucleation rate, I(T), was estimated from the apparent nucleation rate of mu-cordierite which was previously studied at fractured and polished surfaces of cordierite glass by means of tr ansmitting light microscopy. I(T) steadily increases up to 235 K abov e the glass transition temperature at 815 degrees C without reaching a maximum. In the present paper, this exceptional temperature-dependenc e is discussed in terms of CNT. The best fit of experimental data was obtained for T-M(mu) approximate to 1467 degrees C, a low liquid-cryst al interfacial free energy, sigma, and a low activity of nucleating su bstrates, Phi, if sigma Phi(1/3) approximate to 100 mJ m(-2) is valid.