FABRICATION AND CHARACTERIZATION OF Al2O3-MO
NANOCOMPOSITE
M. H. Enayati1,*, F. Karimzadeh1, A. Heidarpour1
1
Department of Materials Engineering, Isfahan University of Technology,
Isfahan 8415483111, Iran
Abstract
Al2O3-Mo nanocomposites were synthesized by ball milling of aluminum and molybdenum
oxide powders mixtures. The structural evaluation of powder particles after different milling
times was studied by x-ray diffraction (XRD), scanning electron microscopy (SEM),
transmission electron microscopy (TEM) and measurement of vial temperature during ball
milling. The powder particles were consolidated by cold uniaxially pressing followed by
sintering in vacuum at 1300 ºC and 1400 ºC. The molybdenum oxide and aluminum reaction
appeared to occur through a rapid combustion reaction process. As a result an alumina matrix
nanocomposite containing molybdenum particulate was formed. In final stage of milling, Mo
and alumina had a crystallite size of about 28 nm and 60 nm respectively. After annealing at
800 °C for 60 min, Mo crystallite size remained constant, however 6-alumina crystallite size
increased to 120 nm. After annealing partial transformation of 6-Al2O3 into different
polymorphic, 7- Al2O3 with a crystallite size of 50 nm was observed. During sintering grain
growth and 6-Al2O3 to 7-Al2O3 transformation occurred but because of pinning effect the
grain growth of the matrix and reinforcement was limited.
Key words: Al2O3/Mo, nanocomposite, ball milling, consolidation.
1. Introduction
Mechanochemical synthesis (MCS), that is chemical reactions induced by high-energy ball
milling, is one of promising routes for the synthesis of different classes of materials including
metals, oxides, salts, organic compounds, etc. in various combinations [1, 2].
Mechanochemical reactions involving displacement reactions between a reactive metal and a
metal oxide often led to the formation a nanocomposite structure [3-5].
*
Corresponding author: [email protected] , Tel: 0098-0311 3915730 Fax: 0098-0311 3912752
1
The Mechanochemical reactions fall into two categories [6], namely; (a) those which occur
during the mechanical activation process and there the reaction enthalpy is highly negative
(e.g. adiabatic temperature Tad = 1300–1800 K), and (b) those which occur during subsequent
thermal treatment and here the reaction enthalpy is only moderate (e.g. Tad <1300 K). The first
type of reaction takes place in two distinct modes, i.e., either combustion reaction or a
progressive reaction. Whenever a reaction is highly exothermic, it can occur abruptly after a
certain time of milling and, once started, it proceeds in a self-sustained way. In this case, the
reaction requires a given time to begin. This time is called ignition time and, due to the
exothermic reaction, can be determined by an increase of temperature.
A number of studies have focused on the development of A12O3/metal nanocomposites by
different routes [7-14]. Matteazzi and Caer synthesized nanometer-sized 6-A12O3-M
composite (M=Fe, V, Cr, Mn, Co, Ni, Cu, Zn, Nb, Mo, W, Si) by ball milling of an
appropriate metal oxide and Al [15]. The present study was aimed to understand the
mechanism of Al and MoO3 reaction during room-temperature ball milling. Details of phase
development and structural evolution are investigated. The presence of metallic particles in
ceramics adds physical properties inherent to the metallic phase, such as electric and thermal
conductivity or magnetic properties. This combination of properties makes ceramics-metals
composite excellent candidates for electric, optic, and magnetic devices or chemical sensors
[16]. Some properties such as wear behavior of Al2O3- metal composites are evaluated in
order to develop structural and functional applications [17-21].
2. Materials and methods
Mixtures of commercial aluminum powder (99.7%, 30-60 µm) and MoO3 powders (99.9%,
~100 µm) were milled to produce Al2O3-26.6 vol%Mo (48.2 wt%Mo) and Al2O3-15 vol%Mo
(31.3 wt%Mo) nanocomposite. Ball milling was performed in a SPEX8000 type ball mill
using hardened chromium steel vial and balls under argon atmosphere. The milling was done
with a ball-powder mass ratio of 7:1 without interruption. No process control agent (PCA)
was used.
The structural changes of powders during milling was investigated by X-ray diffraction
(XRD). A Philips diffractometer (40 kV) with Cu K6 radiation (O = 0.15406 nm) was used for
XRD measurements. The XRD patterns were recorded in the 2P range of 20–100Q (step size
2
0.03Q and time per step 1 sec). The microstructure of powder particles was investigated by
scanning electron microscopy (SEM) using a Philips XL30 SEM and transmission electron
microscopy (TEM) by using a Philips CM200 FEG. The average particle size was determined
from SEM images by using image-analysis software. Isothermal annealing was carried out to
study the thermal behavior of milled powders. Powder samples were sealed and then annealed
in a conventional tube furnace. The alumina and Mo crystallite sizes were estimated by
analyzing broadening of XRD peaks using Williamson-Hall formula [22]. The reaction
progress was monitored by a thermocouple attached on the external surface of the vial. The
powder particles after 270 min ball milling were uniaxially cold pressed at 200 MPa, and then
sintered in vacuum atmosphere at 1300 ºC and 1400 ºC for 1 h. The relative density of each
sintered sample was measured by Archimedes method.
3. Results and discussions
MoO3 and Al reaction with stoichiometric composition gives an Al2O3 based composite
containing 26.6 vol. % Mo according to the reaction (1).
2Al(s) +MoO3(s) = Al2O3(s) +Mo(s)
Q
UG 298 = - 915000
J/mol
(1)
Q
UH 298 = -932000 J/mol
[17]
During milling in room temperature reaction (1) can thermodynamically occur due to its
negative free energy change. WHQ298 value indicates that the Al-MoO3 reaction is highly
exothermic.
The XRD patterns of stoichiometric composition after different milling times are presented in
Fig. 1. The diffraction patterns of initial powder mixture show several peaks corresponding to
Al and MoO3. The intensity of MoO3 and Al peaks decreased during milling so that after 130
min of milling time these peaks disappeared on XRD patterns. Meanwhile several additional
peaks corresponding to Mo and alumina developed on XRD patterns.
In a separate ball milling run a mixture of Al, MoO3 and Al2O3 powders was ball milled to
produce Al2O3-15 vol. %Mo nanocomposite. Figure 2 shows XRD patterns of this
composition after different milling times. As seen, the reaction of Al and MoO3 was
completed after 180 min which is longer than that obtained for stoichiometric composition
(Fig. 1). This suggests that the Al-MoO3 reaction extended over a longer period in presence of
additional alumina as a diluent.
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The broadening of the XRD peaks in Fig. 1 and 2 is due to the reduction of the crystallite size
as well as microstrain induced in powder particles. The approach of Williamson and Hall [22]
was used in order to separate the two effects of crystallite size and microstrain. After 240 min of
milling, Mo and alumina achieved a crystallite size of about 28 nm and 60 nm in
stoichiometric composition and 15 nm and 44 nm in non-stoichiometric composition,
respectively. The smaller Mo and alumina crystallite size in case of non-stoichiometric
composition can be due to the lower heat released in this composition compared with
stoichiometric composition. Matteazzi and Caer obtained 6-A12O3-M nanocomposite (M=Fe,
V, Cr, Mn, Co, Ni, Cu, Zn, Nb, Mo, W, Si) by ball milling of a suitable metal oxide and Al
[15] using Scherrer formula. They reported a smaller value of about 10 nm for crystallite sizes
of alumina and metal. The discrepancies in crystallite size between the present study and those
reported by Matteazzi and Caer can be due to the fact that in Scherrer method it is assumed
that the whole of broadening is caused by very fine crystallite size and the effect of the lattice
microstrain is ignored.
The Al2O3-26.6 vol%Mo nanocomposite after 240 min milling was studied by TEM. Figure 3
represents the bright field TEM image and corresponding electron diffraction pattern of the
nanocomposite powder dispersed on a carbon coated Cu grid. It can be observed that
molybdenum phase (dark region) with a size of <100 nm are homogeneously dispersed in
alumina matrix.
The variation of temperature of vial during ball milling can provide a future insight into the
MoO3-Al reaction mode. Figure 4 shows the vial temperature variation with milling time for
both compositions. If a reaction is highly exothermic, it can take place abruptly after a certain
time of ball milling and once started it proceeds in a self-sustained mode [14, 24, 25]. As seen
after 120 min of milling time the temperature of vials increased rapidly suggesting a
combustion reaction between Al and MoO3. This can be expected from the adiabatic
temperature, Tad=1600 QC [15], for the reaction between MoO3 and Al, which is higher than
the critical value of 1300 QC proposed by Schaffer and McCormick [25, 26]. The combustion
reaction between Al and MoO3 is promoted by the dynamically maintained high Al/MoO3
interface areas, as well as the short-circuit diffusion path provided by the increasing number
of defects such as dislocations and grain boundaries induced during ball milling [27]. It is
interesting to note that the presence of extra alumina respect to stoichiometric composition
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shifts the peak temperature to longer milling time and also decreases the peak temperature. In
fact extra alumina acts as a diluent reducing the adiabatic temperature as well as the ignation
time.
Figure 5 shows the XRD patterns of stoichiometric composition powder milled for 240 min
before and after annealing at 800 °C for 60 min. After annealing several peaks of 7-Al2O3
were appeared on XRD patterns indicating the partial transformation of 6-Al2O3 to 7-Al2O3
phase during annealing. Mo crystallite size remained unchanged after annealing due to the
pinning effect of the Al2O3 particles. In contrast, after annealing 6-alumina crystallite size
increased to 120 nm. In this case, the crystallite size of 7-Al2O3 formed during annealing was
about 50 nm.
The change in morphology of powder particles during milling process are shown in Fig. 6 and
7. As-received Al powder particles had irregular shape with a particle size ranging from 30-60
Xm. As-received MoO3 powder had elongated morphology with a typical length and thickness
of 100 Xm and 20 Xm respectively.
In initial stage of milling MoO3 is mixed to Al at the micrometer level and a homogenous and
composite is formed. Therefore ball milling up to 90 min led to the refinement of powder
particles so that the average particle size of powder before onset of combustion reaction was
about 2 µm (Fig. 7a). The reaction between Al and MoO3 occurs abruptly after 120 min of
milling time (Fig. 4) and as can be seen from Fig. 7b after this time, the size of powder
particles increased again. In fact, exothermic reaction between Al and MoO3 increases
temperature locally and causes the powder particle size to increase to 4 µm. For longer
milling time up to 240 min, the change in powder particle size was not significant. In this
stage, the average particle size was about 2 µm.
In order to study the effect of Mo on mechanical properties, alumina, Al2O3-15 vol% Mo and
Al2O3-26.7 vol% Mo powders were cold pressed at a pressure of 200 MPa and then sintered
at 1300 ºC and 1400 ºC for 1 h. The Mo grain size increased from 10 nm (as-milled) to 43 nm
and 50 nm after sintering at 1300 and 1400 ºC (Table 1). These results show that the Mo
grains do not grow significantly during high temperature consolidation. It is assumed that the
presence of fine Al2O3 dispersoids lead to the retention of nanocrystalline structure of Mo by
pinning grain boundaries.
5
The relative density of each sample was measured by Archimedes method and results are
presented in Table 2. As sintering temperature increases from 1300 ºC to 1400 ºC relative
density of all samples increases. Al2O3-26.7 vol% Mo nanocomposite had higher density in
comparison to Al2O3-15 vol% Mo due to the higher content of Mo.
4. Conclusion
Fabrication of alumina-Mo nanocomposite was investigated. It was found that MoO3 reacted
with Al through a rapid combustion reaction. The reaction was completed after 120 min in
stoichiometric composition. But in presence of extra alumina, as a diluent, the time of
complation of MoO3-Al reaction was longer, 170 min. After 240 min of milling time, Mo and
alumina achieved a crystallite size of about 28 nm and 60 nm in stoichiometric composition
and 15 nm and 44 nm in non- stoichiometric composition, respectively. After annealing Mo
crystallite size remained constant, however 6-alumina crystallite size increased to 120 nm.
Moreover 6-Al2O3 partially transformed to 7- Al2O3 with a crystallite size of 50 nm.
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Table 1. Grain size of samples before and after sintering at 1300 ºC and 1400 ºC.
Grain Size (nm) ± 4
Samples
Al2O3/15 vol% Mo
Al2O3/26.7 vol% Mo
As-milled
Sintered at 1300 ºC
Sintered at 1400 ºC
Al2O3
18
30
42
Mo
10
43
50
Al2O3
20
50
61
Mo
15
41
46
Table 2. Relative density of samples was measured by Archimedes method.
Samples
Relative density (%)
Sintered at 1300 ºC
Sintered at 1400 ºC
Al2O3/15 vol% Mo
76
91
Al2O3/26.7 vol% Mo
78
95
8
Fig. 1 XRD patterns of Al-MoO3 powder particles (stoichiometric composition) as-received
and after different milling times.
Fig. 2 XRD patterns of Al-MoO3-Al2O3 powder particles (non-stoichiometric composition)
as-received and after different milling times.
9
Fig. 3 TEM image of the Al2O3-26.6 vol%Mo
nanocomposite after 240 min milling.
Fig. 4 Temperature variation of vial during ball milling of two compositions, Al-MoO3 and
Al-MoO3-Al2O3.
10
Fig. 5 XRD patterns of Al2O3/26.6 vol%Mo as-milled for 240 min and after subsequent
annealing at 800 °C for 60 min.
Fig. 6 Morphology of as-received (a) Al and (b) MoO3 powder particles.
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Fig. 7 Morphology of powder particles (stoichiometric composition) after (a) 90 min, (b) 120
min and (c) 240 min of milling times.
12