Crystallization mechanism of aluminoferrate glass accompanying a precipitation of nanocrystals of dicalcium ferrite (Ca2Fe2O5) and mayenite (12CaO·7Al2O3)
Abstract
Heat treatment of 60CaO·27Al
2
O
3
·13Fe
2
O
3
glass resulted in a precipitation of
antiferromagnetic dicalcium ferrite
(Ca
2
Fe
2
O
5
) and mayenite
(12CaO·7Al
2
O
3
). 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 Fe
3+
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
Fe
3+
(T
d
) occupying Al
3+
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 Fe
3+
. 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 Fe
3+
(T
d
) from mayenite particles and the
glassy phase into dicalcium ferrite particles. JMA and Kissinger plots for
iron-free 60CaO·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.
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