Crystallization mechanism of aluminoferrate glass accompanying a precipitation of nanocrystals of dicalcium ferrite (Ca₂Fe₂O₅) and mayenite (12CaO·7Al₂O₃)

Abstract

Heat treatment of 60CaO·27Al₂O₃ ·13Fe₂O₃ glass resulted in a precipitation of antiferromagnetic dicalcium ferrite (Ca₂Fe₂O₅) and mayenite (12CaO·7Al₂O₃). The effective magnetic moment of the sample decreased from 2.0 to 1.7 µ_B owing to the precipitation of antiferromagnetic particles. The Mössbauer spectrum of the heat-treated sample consisted of two sextets (magnetic hyperfine structure) which were superimposed on two quadrupole doublets. The sextets were assigned to Fe³⁺ occupying octahedral (O_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·27Al₂O₃ ·13Fe₂O₃ and 60CaO·35Al₂O₃ ·5Fe₂O₃ 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 Mössbauer 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 60CaO·40Al₂O₃ 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.

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