Magnetic characterization of the mechanically induced thermite reaction between Fe2O3 and Al

Article abstract of DOI:10.1016/j.physb.2004.09.033

We have investigated the mechanically induced self-propagating reaction between Fe₂O₃ and Al. In this reaction the final phases, Al₂O₃ and Fe, are formed by an in situ chemical reaction. The evolution of the system was characterized as a function of the milling time using X-ray diffraction, magnetometry (300 and 5 K) and M?ssbauer spectroscopy (300 and 77 K). After reaction, Fe particles in a crystalline Al₂O₃ matrix have been formed. At larger milling times, a rather wide Fe grain size distribution of around 20 nm was obtained according to the diffraction patterns. M?ssbauer spectra were constituted of both paramagnetic and ferromagnetic contributions. The former was attributed to small grains of superparamagnetic Fe (within the M?ssbauer characteristic time τ_M ≈ 10^-8s) and FeAl₂O₄, whereas the latter was associated to larger Fe grains. Saturation magnetization, coercivity field and remanent magnetization rapidly reached a stationary value with the milling time. A correlation between the microstructural characteristics of the sample and its magnetic properties before and after reaction is described.

Full text of DOI:10.1016/j.physb.2004.09.033

ARTICLE IN PRESS
Physica B 354 (2004) 125–128
www.elsevier.com/locate/physb
Magnetic characterization of the mechanically induced
thermite reaction between Fe O and Al
2
3
a,b,
Ã
a
, L.C. Damonte , L. Mendoza-Zelis ,
´
a
C. Cuadrado-Laborde
b
L.M. Socolovsky , I.L. Torriani
b
a
Departamento de F ı´ sica, Facultad de Ciencias Exactas, UNLP, CC 67, CP 1900, La Plata, Bs. As., Argentina
b
Instituto de F ı´ sica Gleb Whataghin, UNICAMP, 13083-970, Campinas, SP, Brazil
Abstract
We have investigated the mechanically induced self-propagating reaction between Fe
final phases, Al and Fe, are formed by an in situ chemical reaction. The evolution of the system was characterized as
a function of the milling time using X-ray diffraction, magnetometry (300 and 5 K) and Mossbauer spectroscopy (300
and 77 K). After reaction, Fe particles in a crystalline Al O matrix have been formed. At larger milling times, a rather
2 3
O and Al. In this reaction the
2 3
O
¨
2
3
¨
wide Fe grain size distribution of around 20 nm was obtained according to the diffraction patterns. Mossbauer spectra
were constituted of both paramagnetic and ferromagnetic contributions. The former was attributed to small grains of
ꢀ
8
¨
superparamagnetic Fe (within the Mossbauer characteristic time t E10 s) and FeAl O , whereas the latter was
M
2
4
associated to larger Fe grains. Saturation magnetization, coercivity field and remanent magnetization rapidly reached a
stationary value with the milling time. A correlation between the microstructural characteristics of the sample and its
magnetic properties before and after reaction is described.
r 2004 Elsevier B.V. All rights reserved.
PACS: 75.50; 81.20.W; 82.30
¨
Keywords: Magnetometry; Mossbauer spectroscopy; Ball milling; X-ray diffraction
Small magnetic particle systems are interesting
due to their applications in recording media,
pigments, ferrofluids, etc. Single domain magnetic
particles embedded in an insulating matrix have
also received attention. For practical uses of small
magnetic particles, a high saturation magnetization
(M ) and coercivity (H ) are required [1]. Iron has
the required magnetization, but also a low coerciv-
ity. However, maximum coercivity of Fe particles
occurs at about 15nm, decreasing rapidly for
smaller particles, and slowly for larger particles [2].
Ball milling is a suitable tool for the preparation
of these materials [3]. Here, we investigate the
S
C
Ã
Corresponding author. UNLP, CC 67, CP 1900, La Plata,
Bs. As., Argentina. Fax: +54 22 1 425 2006.
E-mail address: claborde@fisica.unlp.edu.ar
C. Cuadrado-Laborde).
(
0921-4526/$ - see front matter r 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.physb.2004.09.033

ARTICLE IN PRESS
126
C. Cuadrado-Laborde et al. / Physica B 354 (2004) 125–128
mechanically induced self-propagating reaction
MSR) between hematite (Fe O ) and aluminum.
In this reaction the final phases, Al O and Fe, are
formed by an in situ chemical reaction in which Al
reduces the oxide [4]. The ball-milled mixture
composition was prepared according to the
following stoichiometric reaction:
bulk suppressed Morin transition as well as
uncompensated moments due to surface anisotro-
py and symmetry breaking. Suppression of the
Morin transition has been already described as
related to the microstructural changes in ball-
milled samples [5,8]. Hematite average crystallite
sizes were calculated from XRD diffraction
patterns, passing from 69 nm (0.34IT) to 52 nm
(
2
3
2
3
Fe2O3 þ 2Al ! 2Fe þ Al2O3:
(0.68IT) and then to 56 nm (0.96IT). Further,
dark-field transmission electron microscopy
images (not shown here) reveal a broad distribu-
tion of coherent diffraction domains around the
calculated XRD values (i.e. as low as 10 nm). Fig.
1 also shows that the coercivity does not vary as a
function of the milling time, contrary to the
This exothermic, non-gradual reaction proceeds in
a well-defined milling time (the ignition time, IT).
Magnetic measurements (300 K and 5 K) of
these nanocomposites, together with X-ray dif-
¨
fraction (XRD) and Mossbauer spectroscopy
MS) at 300 and 77 K, were used to obtain
(
remanent magnetization M , which attains a
R
information on the milling process and on the
properties of the resulting materials.
The mixture was analyzed at different milling
times, before and after ignition: 1.8 ks (0.34IT),
maximum value after 0.68IT and slightly decreases
with further milling. This non-monotonous trend
was also exhibited by the Fe O crystallite size as
2
3
mentioned above. Earlier studies have shown that,
depending on the microstructure of the hematite
particles, HC ranges between 0.03 and 0.4 T. The
values reported here, ꢁ0.09 T, are between these
limits. In the same figure, a 300 K hysteresis curve
of the 0.96IT sample is also presented. A
significant change in the hysteresis parameters is
easily seen, i.e. a lower coercivity, remanence and
saturation; this could be taken as an indication of
a superparamagnetic (SPM) behavior (within the
time window that characterizes these experiments
tME100 s). One possibility is that hematite is in
the SPM state, becoming blocked at low tempera-
tures. For spherical monosized hematite, a thresh-
old diameter of ꢁ41 nm has been reported, below
which the particles could behave SPM [6]. In the
present study, the hematite particles are not
monosized and it is expected that a certain fraction
of particles have an average grain size below that
limit, as discussed above. Although the presence of
this SPM Fe O cannot be disregarded it could
3
.6 ks (0.68IT), 5.1 ks (0.96IT), 5.3 ks (IT), 7.1 ks
(
(
1.34IT), 9.2 ks (1.73IT), 14.3 ks (2.7IT) and 30.8ks
5.8IT). Further details about these experiments
and instrumental specifications are available [4].
Only Fe O and Al coexist in the samples before
2
3
IT as seen by XRD [4]. Fig. 1 shows the hysteresis
curves of these pre-ignition samples. As it could be
observed at 5 K, all samples present remanence
and a hysteretic behavior. Hematite is canted anti-
ferromagnetic and this could be an indication of a
5
0
-
5
-
6
-4
-2
0
2
4
6
2
2
3
hardly justify by itself the low-temperature mag-
netization increment detected. Another possibility
is that this low-temperature high-saturation signal
belongs to SPM Fe present in the sample before
ignition that becomes magnetically blocked, in a
sufficiently low quantity to preclude its detection
by XRD. Furthermore, the MS at 300 K of the
0
0
.0
0.1
0.2
H [T]
0.3
0.4
Fig. 1. Hysteresis plots of different pre-ignition milled samples:
.34IT (5 K) (square), 0.68IT (5 K) (circle), 0.96IT (5 K) (full
triangle) and 0.96IT (RT) (open triangle).
0
0.96IT sample is consistent with 6 % of a-Fe,
2

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C. Cuadrado-Laborde et al. / Physica B 354 (2004) 125–128
127
77 K according to SPM relaxation in the MS
of pre-ignition samples, i.e. from 6% to 2% for
the 0.96IT.
Post-ignition milled samples show a saturation
mass magnetization at 5 K (Fig. 3), about 21% less
than the magnetization estimated by assuming that
all the iron present is in the form of bulk a-Fe at
IT. This may be the consequence of a fraction of
SPM Fe particles and/or the presence of iron-
containing non-magnetic oxides. The hypothesis
of SPM iron particles could be difficult to sustain
on IT samples, because during ignition, there is a
very pronounced growth in grain size as a
consequence of the temperatures attained (i.e.
0
.96IT
IT
4
3E3 K) [4], which is contrary to the known
trend of SPM behavior at low grain sizes. On the
other hand, the iron-containing non-magnetic
oxide was detected in the XRD patterns (hercy-
nite, FeAl O ) [7], and suggested in the 300 K MS
2
4
5
.8IT
0
by the presence of a quadrupole splitting (QS) of
.29 mm/s and an isomer shift (d ) of 1.03 mm/s,
1
3
Fe
2
Fig. 2 [8]. With further milling of the reaction
products (from 1.34IT and henceforth), the
diffraction lines of this ternary oxide are no longer
discernible, suggesting its reduction or amorphiza-
tion. From 300 K MS, we see that a combination
of both processes could be the most probably
sound, because the high QS and positive dFe
associated with hercynite persists with the milling
time from the initial 24% (at IT) to a stationary
value of ꢁ9% of resonant area fraction. The
-
10
-5
5
10
v [mm/s]
¨
Fig. 2. Mossbauer spectra at 300 K before (0.96IT), immedi-
ately after (IT) and long after (5.8IT) reaction.
Fig. 2. This demonstrates that before IT a given
amount of iron could be formed as a result of an
incomplete reaction between Fe O and Al, where
a certain fraction could be in the SPM state. If this
hypothesis is correct, we could try to find the
fraction of Fe by means of the following assump-
tion:
2
3
1
20
10
Hem
Fe
S
1
MTðHÞ ¼ f Hem ðHÞ þ f FeM ;
where M (H) is the magnetization experimentally
measured from the hysteresis plot at a given H, i.e.
T, fHem (f ) is the wt% of hematite (iron) present
Fe
(H) is the magnetization of
Fe
hematite at a given H, and MS is the saturation
magnetization of iron at the given temperature
M
T
100
90
5
in the sample, M
Hem
60
50
0
40
1
30
2
20
(
(
5 K). Under these assumptions, we found 1 wt%
0.34IT), 2.4 wt% (0.68IT) and 2 wt% (0.96IT) of
Fig. 3. The milling time (circle) and grain size (triangle)
dependence of saturation magnetization M . Additionally, the
SPM iron. Both SPM contributions (Fe and
Fe O ) probably are responsible for the single
line contribution at 300 K that decreases at
S
2
3
milling time dependence (star) of grain size (post-ignition
samples).

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C. Cuadrado-Laborde et al. / Physica B 354 (2004) 125–128
size, as is shown in Fig. 4, in which a smaller
decrease is present for the sample with larger
milling time.
5
0
4
0
We conclude that minor amounts of Fe could be
formed before IT. After IT, magnetic properties
rapidly reached a stationary value with the milling
time. MSR is a relatively inexpensive technology
to produce small magnetic particles systems, but
many details of the process are yet to be under-
stood. It is demonstrated that magnetic measure-
ments can be reliably used, in combination with
other techniques, to gain a better understanding of
the process.
3
0
2
0
1
0
6
0
0
50
4
0
1
3
0
2
2
0
Fig. 4. The milling time (circle) and grain size (triangle)
dependence of coercivity H Additionally, the milling time
dependence (star) of grain size (post-ignition samples).
C
CCL acknowledges the help of SECYT-CAPES
Argentina) and LME/LNLS (Campinas, S.P.,
(
Brazil) for technical support during electron
microscopy work, LMS the help of Brazilian
agency FAPESP.
increase in magnetization on prolonged milling
indicates the partial conversion of the non-
magnetic oxide to a-Fe and/or, less probably, the
collapse of SPM Fe particles. XRD patterns show
a simultaneous increase of the intensity of a-Fe
lines relative to a-Al O lines with the milling time
References
2
3
[
1] M. Pardavi-Horvath, et al., IEEE Trans. Magn. 28 (1992)
186.
[
7]. According to the decreasing particle size, a
3
distinct magnetic hardening process takes place on
milling. The remanent magnetization increases
[
[
[
2] W.H. Meiklejohn, Rev. Mod. Phys. 25 (1953) 302.
3] A.K. Giri, Mater. Res. Bull. 32 (1997) 523.
4] C. Cuadrado-Laborde, et al., Hyperfine Interactions 134
(2001) 131.
2
from 0.27 to ꢁ4 A m /kg with the milling time.
The coercivity of samples increases from the initial
6
[
[
[
[
5] S.J. Stewart, et al., J. Magn. Magn. Mater. 260 (2003) 447.
6] T.P. Raming, et al., J. Colloid. Interf. Sci. 249 (2002) 346.
7] C. Cuadrado-Laborde, et al., submitted.
to ꢁ50 Oe, slightly decreasing again with further
milling (Fig. 4). As mentioned above, this non-
monotonous dependence of coercivity with grain
size is well known. The behavior was followed by
these samples, however at a slightly higher grain
¨
8] E. Murad, et al., Mossbauer spectroscopy applied to
inorganic chemistry: iron oxides and oxyhydroxides, in:
G.J. Long (Ed.), Plenum, Netherlands, 1981, pp. 507–582.