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

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Tetsuaki Nishida, Shiro Kubuki, Morihiro Shibata, Yonezo Maeda and Toyomi Tamaki


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 µB 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 (Oh) and tetrahedral (Td) sites in the nanocrystals of dicalcium ferrite with a mean diameter >10 nm. The quadrupole doublets were assigned to Fe3+(Td) 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 (Ea) 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 Ea 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+(Td) from mayenite particles and the glassy phase into dicalcium ferrite particles. JMA and Kissinger plots for iron-free 60CaO·40Al2O3 glass yielded Ea 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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