Crystallization mechanism of aluminoferrate glass accompanying a precipitation of nanocrystals of dicalcium ferrite (Ca2Fe2O5) and mayenite (12CaO center dot 7Al(2)O(3))

Heat treatment of 60CaO . 27Al(2)O(3) . 13Fe(2)O(3) glass resulted in a precipitation of antiferromagnetic dicalcium ferrite (Ca2Fe2O5) and mayenite (12CaO . 7Al(2)O(3)). The effective magnetic moment of the sample decreased from 2.0 to 1.7 mu(B) owing to the precipitation of antiferromagnetic particles. The Mossbauer spectrum of the heat-treated sample consisted of two sextets (magnetic hyperfine structure) which were superimposed on two quadrupole doublers. The sextets were assigned to Fe3+ occupying octahedral (0(h)) and tetrahedral (T-d) sites in the nanocrystals of dicalcium ferrite with a mean diameter > 10 nm. The quadrupole doublets were assigned to Fe3+(T-d) occupying Al3+ sites in mayenite particles and the glassy phase. IR transmittance of the sample showed a gradual decrease along with the heat treatment. A Johnson-Mehl-Avrami (JMA) plot from the IR-transmission method yielded activation energies (E-a) of 4.9+/-0.4 and 4.3+/-0.4 eV for 60CaO . 27Al(2)O(3) . 13Fe(2)O(3)(-) and 60CaO . 35Al(2)O(3) . 5Fe(2)O(3) glasses, respectively. These values are equal to E-a obtained from a Kissinger plot by the DTA method, i.e., 4.2+/-0.3 and 4.6+/-0.3 eV, and also to the Al-O bond energy (4.4 eV). These results reveal that crystallization of calcium aluminoferrate glass is triggered by cleavage of Al-O bonds at an early stage in order to form mayenite particles containing Fe3+. After a prolonged heat treatment, the relative absorption area of sextets in the Mossbauer spectra increased at the expense of two doublets. This change suggests a migration of Fe3+(T-d) from mayenite particles and the glassy phase into dicalcium ferrite particles. JMA and Kissinger plots for iron-free 60%aO . 40Al(2)O(3) glass yielded E-a values of 5.6+/-0.4 and 6.0+/-0.3 eV, respectively, equal to the sum of the Al-O (4.4 eV) and Ca-O bond energies (1.4 eV), indicating simultaneous cleavage of Al-O and Ca-O bonds.