| Systematic studies of thermally induced structural transformations in the Na(,2)O.2SiO(,2) (,1-x)(.) Fe(,2)O(,3) (,x) glass system with x (LESSTHEQ) 0.3 have been carried out by means of a variety of techniques including XRD, AES, SIMS, EXAFS, and Mossbauer effect spectroscopies. On the basis of the XRD analysis, the 20 mol. % Fe(,2)O(,3) glass begins to devitrify at elevated temperatures to form three major crystalline phases, (gamma)-Na(,2)Si(,2)O(,5), (alpha)-Fe(,2)O(,3), and Na(,5)Fe(SiO(,3))(,4). The matrix-flushing method has been used to determine the weight fraction of these crystalline phases after different heat-treatments. The results of the EXAFS and Mossbauer analyses suggest that the iron cations be still incorporated in the vitreous matrix despite the fact that the (gamma)-Na(,2)Si(,2)O(,5) phase has already formed after isothermal annealing between 515(DEGREES)C and 700(DEGREES)C. However, when the annealing temperature (>700(DEGREES)C) is high enough to rearrange those polyhedra, the Fe(,2)O(,3) and Na(,5)Fe(SiO(,3))(,4) precipitates will result. Most of the iron in this glass system exists as ferric ions rather than ferrous ions. Both tetrahedral coordination and octahedral coordination of ferric ions are found to exist with a ratio of the two, somewhat dependent upon either the melting conditions or the devitrification behavior. The fraction of tetrahedral sites in the glassy matrix decreases proportionally with increasing K(,Fe) values, or equivalently, the crystallinity of (gamma)-Na(,2)Si(,2)O(,5). From the overall consideration, the addition of iron oxides to the sodium silicate glasses can strengthen the chemical bondings and immobilize the transport of sodium cations. Hence, they stabilize the binary sodium-silicate glasses and promote the formation of (gamma)-Na(,2)Si(,2)O(,5) phase. From a simple thermodynamic consideration, the enthalpy change of (gamma)-Na(,2)Si(,2)O(,5) in the iron silicate glasses was estimated to be -34 kcal/mole, which is smaller than that in pure sodium-disilicate crystals. From the kinetic point of view, the devitrification process seems to take place by the nucleation and growth mechanism. The temperature dependence of devitrification of (gamma)-Na(,2)Si(,2)O(,5) phase appears to be non-Arrhenius. The TTT diagram has been set up for a 20 mol.% Fe(,2)O(,3) glasses under isothermal anneal in the Ar gas. Most of crystalline phases follow the C-type kinetics. |
