Inorganic Materials, Vol. 40, No. 6, 2004, pp. 632–635. Translated from Neorganicheskie Materialy, Vol. 40, No. 6, 2004, pp. 726–729.
Original Russian Text Copyright © 2004 by Emel’yanov, Korolev, Mikhailenko, Knot’ko, Oleinikov, Tret’yakov, Boldyrev.
Mechanochemical Synthesis of Wüstite, Fe1 – xO,
in High-Energy Apparatuses
D. A. Emel’yanov*, K. G. Korolev**, M. A. Mikhailenko**, A. V. Knot’ko***,
N. N. Oleinikov*, Yu. D. Tret’yakov*, and V. V. Boldyrev***
* Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences,
Leninskii pr. 31, Moscow, 119991 Russia
** Institute of Solid-State Chemistry and Mechanochemistry, Siberian Division, Russian Academy of Sciences,
ul. Kutateladze 18, Novosibirsk, 630128 Russia
*** Moscow State University, Moscow, 119899 Russia
e-mail: emelyano@inorg.chem.msu.ru
Received November 18, 2003
Abstract—It is shown that high-energy milling of Fe2O3 + Fe mixtures leads to the formation of nanocrystal-
line, metastable wüstite Fe1 – xO. Its stoichiometry varies systematically with processing time.
INTRODUCTION
position of Fe1 – xO prepared by the conventional
ceramic route is accompanied by the formation of a
lamellar Fe/Fe3O4 composite, which may be due to the
existence of a dislocation network in the parent mate-
rial [6].
Mechanical activation of inorganic materials in
high-energy milling apparatuses is of considerable
interest as a convenient and simple way of preparing
nanocrystalline and amorphous powders and metasta-
ble phases [1]. This opens up new possibilities of pro-
ducing materials with improved properties compared to
those prepared by conventional ceramic processing
techniques [2]. In particular, the mechanochemical syn-
thesis of wüstite, a compound thermodynamically
unstable below ꢀ570°C (eutectoid decomposition tem-
perature), makes it possible to produce, via subsequent
thermal decomposition during annealing, a nanocrys-
talline Fe/Fe3O4 composite, a promising hard-magnetic
material [3, 4]. Varying the annealing temperature, one
can tune the microstructure of the resulting composite
and, hence, its magnetic characteristics.
In this paper, we describe the synthesis of wüstite,
Fe1 – xO, via high-energy mechanical activation of
Fe2O3 + Fe mixtures. Particular attention has been paid
to the microstructure and composition of the resultant
Fe1 – xO and to the possibility of producing an Fe/Fe3O4
composite via low-temperature decomposition of
wüstite.
EXPERIMENTAL
The starting reagents used were Fe and hematite
(α-Fe2O3). The latter was prepared via thermal decom-
position of Fe(NO3)3 · 6H2O, followed by annealing in
air at 750°C for 2 h. The starting mixture was mechan-
ically activated in air for 10, 20, or 40 min in an AGO-2
planetary mill at ꢀ40g, using steel grinding vessels and
steel balls ꢀ5 mm in diameter. The Fe : Fe2O3 molar
ratio in the starting mixture corresponded to the reac-
tion
Mechanical activation may cause the composition
of wüstite (Fe1 – xO) to notably deviate from the equilib-
rium one. In general, the energy delivered to a sub-
stance in the course of mechanochemical synthesis in
high-energy apparatuses goes, for the most part, into
changing its defect structure. The main defect species
in wüstite (rock-salt structure) are cation vacancies in
octahedral sites and an equivalent amount of holes
localized at Fe3+ ions. Point defects in wüstite have a
tendency to aggregate into small associates in which
each Fe3+ ion is surrounded by four vacancies. The
associates, in turn, form more complex clusters. The
stoichiometry of wüstite depends on which type of
defect cluster prevails [5]. The formation of Fe3O4 and
Fe during wüstite decomposition occurs primarily at
Fe2O3 + 0.7Fe = 3Fe0.9O.
The weight ratio of the sample to ball load was
1 : 30. The grinding vessels were water-cooled to main-
tain a constant temperature of the process (near room
temperature).
To prepare an Fe/Fe3O4 nanocomposite, the single-
structural defects. For example, the eutectoid decom- phase wüstite powder obtained by milling for 40 min
0020-1685/04/4006-0632 © 2004 MAIK “Nauka/Interperiodica”