Materials Chemistry and Physics 82 (2003) 864–876
Microstructural characterization of the evoluted phases of ball-milled
-Fe
2
O
3
powder in air and oxygen atmosphere by Rietveld analysis
P. Sahu
a
, M. De
a,∗
, M. Zduji´ c
b
a
Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India
b
Institute of Technical Sciences of the Serbian Academy of Sciences and Arts, Knez Mihailova 35, 11000 Belgrade, Yugoslavia
Received 12 March 2003; received in revised form 14 July 2003; accepted 25 July 2003
Abstract
Transformation reaction induced due to ball-milling of iron oxide, -Fe
2
O
3
in both air and oxygen atmospheres under closed milling
condition has been studied for detailed characterization of the microstructure of all the evoluted phases on milling up to 10 h. The
methodology adopted for characterization involves Rietveld’s whole X-ray profile fitting technique adopting the most recently developed
software, material analysis using diffraction (MAUD), which incorporates Popa model for crystallite (domain) size and microstrain (root
mean square, r.m.s. strain). The analysis also considers lattice defect related features of the microstructure, viz. stacking, twin, compound
fault density and dislocation density parameters. The study also undertakes quantitative estimation of volume fractions of the phases
evoluted (Fe
3
O
4
: Fd-3m:1 and FeO: Fm-3m). The results reveal transformation of -Fe
2
O
3
to Fe
3
O
4
and finally to FeO occurs in both air
and oxygen atmospheres, and the reaction speed is slower in oxygen environment. The reaction is controlled by oxygen partial pressure,
which decreases on continued milling. A critical oxygen partial pressure is reached at 3–4 h of milling, when Fe
3
O
4
phase attains maximum
saturation with nano-order (7–8 nm) crystallite sizes, reduced r.m.s. strain and high dislocation density (∼10
12
cm
-2
). Prolonged milling
(7–10 h) results in further reduction of oxygen partial pressure, resulting in complete transformation of -Fe
2
O
3
and Fe
3
O
4
to FeO, having
nano-order (6–7 nm) crystallite sizes, high r.m.s. strain (∼10
-2
) and high dislocation density values (∼10
12
cm
-2
) in both the environments,
except that the transformation reaction is slowed down in oxygen.
© 2003 Elsevier B.V. All rights reserved.
Keywords: Material analysis using diffraction; Ball-milling; Transformation reaction
1. Introduction
Iron oxides, i.e. -Fe
2
O
3
(hematite), Fe
3
O
4
(magnetite),
-Fe
2
O
3
(maghemite) and FeO (Wustite) are well known as
important electrical and magnetic materials [1–9]. Through
various mechanochemical treatments and/or novel prepa-
ration processes [10,11], the microstructures and particle
properties of these materials are better controlled to suit
numerous applications.
Extensive studies have been made on the transforma-
tion reactions induced due to ball-milling of -Fe
2
O
3
and
-Fe
2
O
3
in various milling atmosphere [6,9,12–15]. Kos-
mac and Courtney [3] investigated milling of -Fe
2
O
3
in air
and oxygen atmosphere with the formation in either case,
of the spinel phase, Fe
3
O
4
and finally FeO on prolonged
milling. The formation of FeO in air atmosphere, how-
ever was suppressed on frequent opening of the vial. Kacz-
marek and Ninham [5] showed complete transformation of
∗
Corresponding author. Tel.: +91-33-2473-4971x153.
E-mail address: msmd@mahendra.iacs.res.in (M. De).
-Fe
2
O
3
to Fe
3
O
4
by ball-milling under vacuum or in argon
atmosphere, while in air, the transformation is found either
very slow or not to occur. Zduji´ c et al. [16,17] have per-
formed a series of experiments to understand the effects of
ball-milling on -Fe
2
O
3
in both air and oxygen atmospheres
under closed milling and observed formation of Fe
3
O
4
to
FeO on prolonged milling. They concluded that under ap-
propriate milling condition, the transformation is controlled
by the oxygen partial pressure having critical influence on
the transformation kinetics. From the study of the magne-
tization of the milled products, quantitative estimation of
Fe
3
O
4
phase percentages has been made.
The present study attempts to undertake detailed charac-
terization of the microstructure of all the evoluted phases
due to ball-milling of -Fe
2
O
3
for various periods of time
under closed milling conditions in both air and oxygen at-
mosphere. The methodology used in the present study is the
use of Rietveld analysis for whole X-ray powder patterns
adopting the latest Rietveld software, material analysis using
diffraction (MAUD) [18]. This method of study is capable to
monitor the nature of progressive evolution of the composite
0254-0584/$ – see front matter © 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.matchemphys.2003.07.011