Influence of milling conditions on the mechanochemical synthesis of CaTiO3 nanoparticles

Article abstract of DOI:10.1016/j.jallcom.2008.09.133

The mechanochemical synthesis of calcium titanate (CaTiO₃), was carried out in a planetary mill by varying the milling time and mill rotational speed at three levels. CaCO₃ and TiO₂ were used as the starting materials. Bes

Full text of DOI:10.1016/j.jallcom.2008.09.133

Journal of Alloys and Compounds 476 (2009) 894–902
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Influence of milling conditions on the mechanochemical synthesis of CaTiO3
nanoparticles
∗
Samayamutthirian Palaniandy , Noorina Hidayu Jamil
School of Materials and Mineral Resources Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 30 May 2008
Received in revised form
The mechanochemical synthesis of calcium titanate (CaTiO ), was carried out in a planetary mill by varying
the milling time and mill rotational speed at three levels. CaCO3 and TiO2 were used as the starting
3
materials. Besides size reduction, the CaCO3/TiO2 mixture had undergone structural changes, which was
affected by the process parameters of the planetary mill. Furthermore, milled particles in a nanometer size
range were aggregated to form micron size particles. The CaTiO3 phase was formed when the milling time
was set at 5 h, and the mill rotational speed was 600 rpm. The mechanochemical process was affected by
the mechanism inside the vial, such as the shock power and friction component, at high efficient milling
region. The particle size of the CaTiO3 has a diameter of about 10.3 nm. The parameters used affected the
degree of crystallinity, crystallite size and lattice strain of the particles.
19 September 2008
Accepted 27 September 2008
Available online 17 November 2008
Keywords:
Mechanochemical processing
High-energy ball milling
X-ray diffraction
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
The CaTiO3 is the basic material for ferroelectric ceramics with
perovskite related structure [5]. It has high dielectric constant (εr),
High-energy ball milling has been used for many years now in
producing ultra fine powders in the range of a sub-micron to a
nanometer. Aside from size reduction, this process causes severe
and intense mechanical action on the solid surfaces, which was
known to lead to physical and chemical changes in the near sur-
face region where the solids come into contact under mechanical
forces [1]. These mechanically initiated chemical and physicochem-
ical effects in solids were generally termed as the mechanochemical
effect. This route is currently being used to synthesize inorganic
materials as it exhibits some advantages, such as the reduction
in sintering temperature [2–5]. CaTiO3 is one of the materials
that were initially synthesized via the mechanochemical route.
Also, this material has been synthesized by wet chemical methods,
such as sol–gel [6], co-precipitation [7], polymeric precursor [8],
organic–inorganic solution [9] and combustion [10]. Furthermore,
this material is a ferroelectric ceramics with perovskite related
structure [5]. It has high dielectric constant, low dielectric loss and
large temperature coefficient of resonant frequency, which makes it
a promising component in the production of communication equip-
ment operating at microwave frequencies (UHF and SHF), which in
turn are used in microwave dielectric applications (as resonators
and filters) [6,8,11].
dielectric loss (tan ı), and large temperature coefficient of reso-
nant frequency (ꢀ ), which makes it a promising component in the
f
production of communication equipment operating at microwave
frequencies (UHF and SHF), which in turn are used in microwave
dielectric applications (as resonators and filters) [12]. Also, it is
materials can be employed as a thermally sensitive resistor element
due to its negative temperature coefficient, and for the immobi-
lization of highly radioactive wastes. Such unique properties gave
this material much attention, and many investigations regarding
its many uses have been carried out in recent years [12]. Recently,
also has been reported visible photoluminescence properties at
room temperature in disordered structurally perovskite titanates
(CaTiO ) and highly emissive red-emitting phosphors have been
3
reported in the literatures [13–18].
The CaTiO3 can be synthesized by heating a mixture of CaO,
or CaCO3, and TiO2 (rutile or anatase) at about 1650 K [5]. How-
ever, this process makes it difficult to produce CaTiO with uniform
3
composition because an all-solid state reaction was topotactic. The
heating condition was an essential criterion in producing quality
crystalline CaTiO3 powder. Thus, in order to improve the sinter-
ing condition of the CaTiO3 powder, several techniques such as the
organometallic method or the mechanochemical synthesis have
been developed. It was the latter method which has been receiving
a lot of attention as it was believed that it can produce a uniform dis-
tribution of CaTiO3 powder. The study on mechanochemical effect
on fine particles has created much interest among researchers
∗ Corresponding author. Tel.: +604 5996132; fax: +604 5941011.
E-mail address: samaya@eng.usm.my (S. Palaniandy).
0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2008.09.133

S. Palaniandy, N.H. Jamil / Journal of Alloys and Compounds 476 (2009) 894–902
895
Table 1
Operational parameters of planetary mill.
Mill rotational speed (rpm)
Milling time (h)
200
1
400
2
600
5
because of its several advantages to downstream processes like
reducing the annealing and sintering temperature, reducing the
phase transformation temperature, enhancing the leaching pro-
cess, decreasing the thermal decomposition temperature, and
increasing the particle reactivity [19,20].
The mechanochemical synthesis process is carried out in high-
intensity grinding mills such as vibro mills, planetary mills, and
oscillating mills. It has been noticed that the size reduction process
Fig. 1. Schematic of the planetary mill.
diameter. When these parameters are fixed, the planetary mill can be characterized
by the ratio of the diameter mill axis rotation (G) to the diameter of the mill (D).
Let R be the ratio of the rotation speed of the mill axis to the rotation speed of the
mill at its center. The negative R value indicates that the mill and the shaft rotate in
opposite directions. The dynamics of the planetary mill allows the computation of
critical speed. The percent of the critical speed is shown in Eqs. (1) and (2).
and the microstructural evolution of the CaTiO during milling pro-
3
cess were mainly influenced by the type of impulsive stress applied
by the grinding media, which can either be an impact or shear type.
Moreover, other parameters such as atmosphere composition and
the presence of different liquid media inside the grinding mill affect
the mechanochemical process. In fact, when the mechanochemical
synthesis of the CaCO3 and TiO2 was carried out in planetary mills
N2
%
CS =
(1)
(2)
R × N1
ꢀ
G
D
R = −1 ±
where N1 is the gyrating speed and N2 is the jar speed. In this work, the mill was
operated at 78.5% critical speed. This was accomplished by doing the following
at higher rotational speed to produce CaTiO , the impact stress was
3
dominant, and not much attention was given on the mechanochem-
ical mechanism itself.
The aim of this work, therefore, is to give additional contribu-
tion in understanding the influence of milling conditions on the
calculation of Eq. (3):
ꢀ
1
7
3
R = −1 ±
= −1 ± 1.36
(3)
Taking the minus sign for the opposite direction of motion between the jar and
the main shaft, it can be shown that R = −2.36.
mechanochemical synthesis of CaTiO nanoparticles without of the
3
deleterious phase.
The percent of critical speed is calculated by considering the shaft speed and jar
speed as 600 and 1092 rpm. Therefore the critical speed calculated is shown in Eq.
2. Experimental
(
4).
2
.1. Raw materials and milling conditions
1092
.36 × 600
%
CS = 2
× 100 = 78.5%
(4)
The CaTiO3 powder was synthesized through a mechanochemical route. To start,
Calcite CaCO3 (99.9% purity, Aldrich) and rutile TiO2 (99.9% purity, BDH laboratory
supply) were mixed in a stiochiometric ratio of 1:1 in an agate mortar with a pestle
for 10 min and were preserved in desiccators. Then the mixture was subjected to
microwave heating using a normal kitchen microwave oven for 2 min to remove the
moisture in the samples. The milling of the mixture was conducted under atmo-
spheric condition in a planetary ball mill (Fritsch Pulveristte-6) with one steel pot
of 50 cm3 inner volume. The milling was performed by varying the mill rotational
speed and the milling time at three levels, as shown in Table 1.
2.3. Characterization
The specific surface area of the powders was determined by a multipoint nitro-
gen adsorption using a Quantachrome system and the Brunauer-Emmett-Teller
method (BET). The amount of N2 adsorbed at liquid nitrogen temperature was
measured in N2/He carrier flux in a concentration range of 10–30%.
X-ray diffraction (XRD) analysis was performed on a D8 Diffractometer (Bruker
AXS) using Cu K radiation for all analyses at 40 kV and 20 mA in order to identify the
compositional and phase changes in the mixture during the grinding operation. The
XRD patterns were recorded in the 2Â range = 10–70 using a step size of 0.05 and a
counting time of 5 s per step. Silicon powder was used as standard agent to remove
the instrumental broadening effects from the observed profile broadening.
Line positions, intensified widths, and shapes were obtained from the XRD spec-
tra in order to characterize the microstructure in terms of defects parameters such as
crystalline size and microstrain. The APD version 4.1 g software was used to acquire
these parameters [21]. The K␣2 component was removed from the XRD spectra with
the assumption that K␣2 intensity was half of K␣1 intensity. Then the [1 0 1] and
[1 0 0] planes were selected for the profile analysis of TiO2 and CaCO3. The over-
lapped peak was split using the APD version 4.1 g software. The X-ray diffraction
patterns were adjusted to a combination of Cauchy and Gaussian line shapes using
the Halder and Wagner method for obtaining physical broadening, as shown in Eq.
◦
◦
2.2. Planetary mill mechanics
The planetary ball mill consisted of a gyratory shaft and a cylindrical jar, and both
were rotated simultaneously in opposite directions at high rotational speed. Such
movement at high speed allowed the ball to move strongly and rigorously, which
led to a large impact energy between the balls and the materials. The specification
of the planetary mill used in this work is shown in Table 2.
Fig. 1 shows the schematic of the planetary ball mill where a jar rotates at about
a primary axis O. Here, G is the diameter of the axis of the jar, and D is the mill
Table 2
Dimensions of the Planetary mill.
(
5), where ˇf, ˇh and ˇg are the integral breadths of the instrumental, observed, and
measured profiles. The profile fitting procedure was performed without smooth-
ing the XRD spectra. Each goodness factor was refined to a value of <5% for all the
reflections. The maximum height of the peak (Imax), integral breath of the line pro-
file (ˇ = A/Imax), full-width at half maximum (FWHM), and peak position (2Â) were
obtained from the adjusted line profile. A is the area under the peak. The apparent
crystallite size was calculated using the Scherrer formula, as shown in Eq. (6). The
Scherrer formula, meanwhile, describes the mutual dependence between the line
profile integral breath and crystallite size Dv which was the volume weighted mean
of the crystallite in the direction perpendicular to the diffracting planes; the constant
varied with the reflection Bragg angle and crystallite shape. Lattice strain was calcu-
lated using Eq. (7). The structural disorder due to the increasing abundance of X-ray
amorphous material was manifested through the reduction in the integral inten-
sity of the diffraction lines [21]. The relative degree of crystallinity (DOC) defined in
Eq. (8) was based on the area under the [1 0 1] and [1 0 0] peak for TiO2 and CaCO3,
where A0 and A were the areas under the peak for feed and ground sample [22].
Working principle
Grinding process
Transmission ratio
Impact (mainly)
Dry
Irelative = 1:-1.82 (mill and shaft
rotate in the opposite
directions)
Mill
Material
Diameter, mm
Volume, mm3
Rotational speed, rpm
Stainless steel
70
250
200, 400 and 600
Grinding balls
Material
Diameter, mm
Density, kg/m3
Weight, kg
Stainless steel
10
7800
0.2
66
2
2
(ˇ − ˇ )
h
g
ˇf =
(5)
Filling, % of mill volume
ˇh

8
96
S. Palaniandy, N.H. Jamil / Journal of Alloys and Compounds 476 (2009) 894–902
Fig. 2. Change in specific surface area at various mill rotational speeds and milling
periods.
Fig. 3. Micrograph of milled product (mill rotational speed = 600 rpm and milling
period = 5 h).
where ˇf, ˇh and ˇg are the integral breadths of the instrumental, observed, and
measured profiles, respectively [22]
tion, and agglomeration [20]. At the adherence stage, the particles
will coat on the lining and the grinding bodies. At the aggregation
stage, the particles associate weakly by Van der Walls type adhe-
sion, and it is a reversible reaction. Agglomeration, on the other
hand, is defined as a very compact, irreversible interaction of the
particles with an occurrence of chemical bonding between them.
An aggregation of particles was reported during extended high-
intensity dry grinding for oxide minerals [23] and olivine [24].
This phenomenon was common in dry grinding and was usually
explained by the agglomeration of structurally modified particles
following the initial reduction of particle size. Whenever a mate-
rial is ground excessively, the surface area increases tremendously.
An increase in the surface area leads to an increase in the surface
energy. Particles, in an attempt to reduce the excess free surface
energy, begin to agglomerate and hence SEM micrographs show the
presence of large aggregates having no definite shape. This behavior
also has been reported by Satami et al. [25].
ꢁ
ꢂ
Kꢁ
Dv =
cos Â
(6)
FWHM
where Dv is the volume weighted mean of the crystallite size, K is the constant, Â is
the Bragg angle of (h k l) reflection, and ꢁ is the wavelength of X-rays used [22].
ˇ
ε =
(7)
4
tan Â
where ε is lattice strain [9].
ꢁ
ꢂ
At
DOC =
× 100
(8)
A0
where DOC is the degree of crystallinity, and A0 and At are the areas under the peak
for feed and ground sample, respectively [22].
The morphological change of the ground particles was observed through a Scan-
ning electron microscope (SEM), ZEISS Supra 35VP with an acceleration voltage
ranging between 5 and 15 kV, and a secondary electron detector (SE). The particles
were deposited randomly on a sample holder covered with carbon coated adhe-
sive. These samples were then coated with a thin gold layer using gold sputtering
EMSCOPE SC500A unit to minimize the charging effect. The digital images on 256
grey levels were captured using image analysis software. In addition, the nano-scale
particle was observed via transmission electron microscope (TEM). The powder mix-
ture was sonicated for 30 min in ultra sonic bath for SEM and TEM observation. Here,
the ground particles were mixed with calgon so that the particles will be dispersed
and smeared on the cell, which was transferred to cell position in the microscope.
3.2. Mechanochemical synthesis
Structural changes and microstructure characterization were
analyzed using X-ray diffraction. Fig. 4 shows the XRD pattern of
the starting and milled powder. The diffraction peaks of milled
samples have lower intensity and broader peak base compared
to the starting powders, mainly due to the disordering process of
the TiO2 and CaCO3 crystal structure from intensive grinding. The
reduction of peak intensities implied the formation of amorphous
materials in the milled powders. The increase in XRD line breath
was due to the plastic deformation and disintegration of TiO2 and
3. Results and discussions
3.1. Specific surface area changes by milling
The mechanochemical synthesis of the materials was accom-
panied by disintegration and a generation of fresh, previously
unexposed surfaces. Thus, the particle size distribution and the
specific surface area of the milled product depended on secondary
processes like aggregation and agglomeration [20]. Fig. 2 shows the
evolution of a specific surface area as the mill speed and time varied.
2
The fineness particle of 6.97 m /g was obtained at 200 rpm speed at
1
h time. The particles became coarser as the mill speed and milling
time increased. Similar observation was reported by Pourghahra-
mani and Forssberg [22], where coarser particles were obtained
when hematite was ground at higher levels of stress energy in a
planetary mill.
Fig. 3 shows particles, which when ground at 600 rpm for 5 h,
exhibited particle sizes ranging between 100 and 150 nm and were
aggregated to form bigger particles in micron range. Fig. 3 also
shows that the aggregation of particles occurred in the pores; this
process is called soft agglomeration [22]. There are three main
stages of interaction between the particles: adherence, aggrega-
Fig. 4. XRD pattern of starting and milled powder samples (ꢀ = TiO2 (anatase);
ꢁ = CaCO3).

S. Palaniandy, N.H. Jamil / Journal of Alloys and Compounds 476 (2009) 894–902
897
Fig. 5. X-ray difractogram of mixture of CaCO3 and TiO2—(ꢀ = TiO2 (anatase); ᭹ = CaTiO3; ꢂ = CaOH; ꢁ = CaO; ꢃ = TiO2 (Brookite)).
CaCO . The same observation was reported by several researchers
when high-energy milling was performed on oxide materials
Fig. 5 shows the X-ray diffraction pattern at various levels of
milling time and speed. It shows how structural defects on the start-
ing materials take place, which leads to the formation of partially
amorphous materials and followed by a chemical reaction for the
3
[
19,20].
formation of CaTiO . These defects generally increase as the mill
speed and time increase, which was exhibited by the reduction in
peak intensity and peak base broadening. It also shows that CaTiO3
was formed when CaCO3 and TiO2 were ground for 5 h at 600 rpm.
The first step in the formation of CaTiO3 from CaCO3 and TiO2 was
the decomposition of the CaCO3, as shown in Eq. (9). The instability
of CaO caused the formation of Ca(OH)2 due to exothermic reac-
tion, as shown in Eq. (10). Ca(OH)2 formation may also be due to
the presence of moisture in the ambient. Berbenni and Marini [2]
reported that the mass losses in the TG curves of the milled mix-
3
.2.1. The effect of mill speed and milling time on phase
3
transformation
Although several researchers [5,6] have studied the
mechanochemical synthesis of CaTiO3 by varying the milling
time and precursor materials in a planetary mill, not much
attention was given on the optimization of process parameters
on the formation of CaTiO . The application of a high-energy
3
grinding mill, such as a planetary mill, allows dramatic changes
in the structure and surface properties of a solid material. The
mechanical treatment in a high-energy mill generates a stress
field within the solids. Heat release can cause stress relaxation,
the development of a surface area as a result of brittle fracture
in the particles, generation of various sorts of structural defects,
and stimulation of a chemical reaction within the solids [9]. In
ture of CaCO /TiO2 were slightly larger than those in the physical
3
mixture; they accounted that this phenomenon occurred due to the
presence of moisture during milling. These observations may vary
according to the location of the experiment being conducted as the
humidity in the ambient varies.
the mechanochemical synthesis of CaTiO , the starting materials
3
will initially undergo structural defects and will be followed by
a chemical reaction [5]. The concentration of a mechanically
induced defects and their spatial distribution depends on the
condition of the energy transfer in the mill and on the external
condition of stress. In a planetary mill, the mechanochemical effect
can be intensified by increasing the grinding media’s density, its
acceleration, and the duration of the milling process. However,
the measurement of efficiency of the mills in transferring energy
is a complicated task because the energy transfer takes place
in three stages. These are the conversion of kinetic energy that
drives the grinding medium, the transfer of mechanical action
on the particles being ground, and the transferring of energy into
the treated substance [9]. In order to improve the processes in a
planetary mill, much attention should be given on the study of
grinding variables and condition of the mechanochemical effect
on the milled particles. The intensity of mechanical stress depends
on the mass of the materials and the grinding media, the mill
speed, milling time, gravitational acceleration, and mill diameter
CaCO (s) ⇔ CaO(s) + CO (g)
(9)
3
2
CaO(s) + H O(l) → Ca(OH)2
(10)
2
Fig. 5 shows that the decomposition of CaCO to CaO was imme-
3
diate, taking place within 1 h of milling at a low mill speed. The
occurrence of the Ca(OH)2 phase was observed on all levels of mill
speed and mill time, except when the mixture was milled for 5 h at
a speed of 400 and 600 rpm. At these levels, the Ca(OH) phase was
diminished and the mixture appeared ready to form a pure phase
2
of CaTiO . These results show that the presence of moisture con-
3
tent in the starting powder was an essential factor in the milling
process. Brankovic et al. [26] mentioned that the development of
a Ca(OH)2 phase during the mechanochemical process of CaTiO3
may cause difficulties in maintaining the stoichoimetry of the sys-
tem CaO-TiO2, but it can be avoided if the starting material used
is CaCO3 [25]. However, this study has proven that even though
CaCO3 was used, the formation of Ca(OH)2 still occurred. Apart
from that, the formation of brookite from anatase was also observed
[
22].

8
98
S. Palaniandy, N.H. Jamil / Journal of Alloys and Compounds 476 (2009) 894–902
Fig. 6. Graphical representation of the pairs of velocities and milling efficiency.
when the mixture was milled at the speed of 400 and 600 rpm,
which indicated that the phase transformation was induced by the
high-energy ball milling process. Although Mi et al. [14] observed
brookite forming when milling Ca(OH)2 with TiO2 (rutile) in their
research, it was not discussed thoroughly. The brookite phase read-
ily transformed to CaTiO3 when it was ground at a mill speed of
Fig. 7. Physical integral breadth of milled TiO2 at various mill rotational speeds and
milling time.
from the milling time, an optimum value of mill speed was essential
for the mechanochemical synthesis.
Among the process parameters of a planetary mill, impact force
is commonly used, and it can be decomposed into radial and tan-
gential components. Impact force varies depending on the mill
speed. In addition to impact force, the ratio of vial rotational speed
to disc rotational speed for this mill (ω/˝) was 1.82. According to
Jean and Nachbaur [3], if ω/˝ > 1 with high tangential force, then it
is conducive for a friction phenomenon. Milling efficiency is linked
to the important friction component, and the non-efficient quality
of milling A in Table 3 was related to a small friction component.
Consequently, friction cannot be the only parameter controlling the
final product. In this research, the influence of milling power and
friction has to be considered simultaneously, allowing for the effi-
cient milling in B and C. The combined good influences of both
parameters (high friction component and high power) enabled the
obtainment of the perovskite structure within 5 h. These phenom-
ena were certainly linked to the mill speed and its non-negligible
parameter.
600 rpm for 5 h. All the TiO2 phases (anatase, rutile and brookite)
exhibited negative values of Gibbs free energy when reacting with
CaO or Ca(OH) , suggesting that the mechanochemical reaction of
2
this mixture immediately took place [27].
The selection of an optimum milling mode is essential in order
to synthesize CaTiO3 via a mechanochemical route. The milling
mechanism in a planetary mill depends on the mill speed, where
the rotation of the vial about its own axis produces the tum-
bling of the mill charge. It is possible to control the movement
and trajectories of the balls by varying the mill speed. Jean and
Nachbour [3] reviewed several papers on the mechanism of size
reduction and the mechanochemical process in a planetary mill,
and concluded that an increase in shock power and high fric-
tion component were conducive for the mechanochemical process.
Shock power, which is the product of input energy and the shock
frequency, can be improved by increasing the mill rotational speed.
The authors [3] have proposed a map indicating that the genera-
tion of shock power was the function of disc and vial rotational
speed.
3.2.2. The effect of milling on the physical XRD line breath
In order to quantify the influence of the milling parameters
Fig.
mechanochemical process to take place, as proposed by the authors
3]. A, B, and C denote the disc and vial rotational speeds chosen for
6
shows the region of milling efficiency for the
on the alteration of CaCO3 and TiO , further investigations were
2
followed by a line profile fitting technique. The physical integral
breadth of the reflection peaks were calculated after adjusting
the line shape; this was done with consideration of the physical
[
this study, and they were located according to the milling efficiency
mentioned by Jean and Nachbour [3]. Table 3 also shows the disc
rotational speed and the vial rotational speed chosen for this study.
A mill speed of 200 rpm falls on the “no efficient milling” region
in accordance with the X-ray diffractogram showed in Fig. 5. At
this mill speed, only a small reduction in terms of peak intensity
was noticed. Furthermore, only a small portion of CaCO3 changed
to CaO, and the TiO2 phase remained in its anatase phase. Mean-
while, the mill speeds of 400 and 600 rpm fall on the region of
“
high efficient milling” and demonstrated a massive reduction of
peak intensity, line breadth broadening, and phase transformation.
In milling C, the mechanochemical process took place when the
mixture was milled for 5 h. This phenomenon illustrated that aside
Table 3
Disc rotational speed and vial rotational speed.
Disc rotational speed (rpm)
Vial rotational speed (rpm)
A
B
C
200
400
500
364
546
1092
Fig. 8. Physical integral breadth of milled CaO at various mill rotational speeds and
milling time.

S. Palaniandy, N.H. Jamil / Journal of Alloys and Compounds 476 (2009) 894–902
899
Fig. 9. Degree of crystallinity of TiO2 as a function of mill rotational speed and
Fig. 10. Degree of crystallinity of CaO as a function of mill rotational speed and
milling time.
milling time.
the milling energy in intensive grinding stages was used for the
establishment and development of plastic deformation rather than
size reduction. The reduction in the XRD patters intensities and
the broadening of peak are common characteristics of XRD patters
milled oxide materials.
breadth for (1 0 1) and (1 0 0) for CaO and TiO at various mill speeds
2
and milling time. The CaO plane was preferred over CaCO3 as the
latter readily decomposed to CaO as soon as the milling began.
Figs. 7 and 8 show the physical integral breadth of the milled CaO
and TiO2 at various levels of mill speed and milling time. The inte-
gral breadth at the mill speed of 600 rpm and milling time of 5 h was
not taken into consideration due to the formation of CaTiO . The lat-
3.2.3. The degree of crystallinity
3
tice strain and crystallite size components contributed to the line
breadth, which will be resolved later to obtain the microstructure
In this section, we study the relative intensity of the reflection
peaks of a milled CaCO /TiO mixture and its degree of crystallinity.
3
2
characteristics. The integral breadth of TiO and CaO ranged from
As mentioned before, the intensity of the XRD patterns reduces dur-
ing a high-energy mechanical treatment of the CaCO /TiO mixture
2
◦
◦
◦
◦
0.10 to 0.34 and 0.20 to 1.10 , respectively. The maximum integral
3
2
breadth was obtained during high mill speed and milling time. The
plots of integral breadth for CaO and TiO2 displayed similar trends,
where the integral breadth values increased as the mill rotational
speed and milling time also increased. The integral breadth value
for CaO was higher compared to TiO2, which indicated that the plas-
due to the formation of amorphous materials. Figs. 9 and 10 show
the degree of crystallinity of the TiO2 and CaO as a function of mill
speed and milling time. The degree of crystallinity on the mixture
that was milled at a rotational speed of 600 rpm for 5 h was not
included, as the mixture has formed a CaTiO3 phase. The diffrac-
tion peaks show a progressive reduction in peak intensity when
compared with the starting powder diffractions. Generally, the
reduction in X-ray diffraction intensity was heightened by increas-
ing the mill rotational speed and the milling time. Fig. 9 shows
a drastic decrease in the degree of crystallinity as the rotational
tic deformation in CaO was higher than in TiO . This phenomenon
2
was observed due to the variation in the hardness of the materi-
als. CaO was softer compared to TiO2, so the defects in the crystal
structure of CaO were more evident compared to TiO . Pourghahra-
2
mani and Forssberg [22] mentioned in their research that most of
Fig. 11. Variation of volume weighted crystallite size and the lattice strain of TiO2 in [1 0 1] direction as a function of mill rotational speed and milling time.

9
00
S. Palaniandy, N.H. Jamil / Journal of Alloys and Compounds 476 (2009) 894–902
Fig. 12. Variation of surface weighted crystallite size and the lattice strain of CaO in [1 0 0] direction as a function of mill rotational speed and milling time.
speed increased up to 400 rpm. After 400 rpm, the degree of crys-
tallinity came to a plateau, which indicated that a further increase
in mill speed will not exhibit much mechanochemical effect. The
same observation was exhibited in Fig. 10 but the degree of crys-
although not much change in the crystallite size was manifested.
This phenomenon suggested that after a certain milling time, the
milling energy was used for the distortion of the crystal lattice
and not for the reduction of crystallite size. Meanwhile, changes in
microstructural characteristics and integral breadth revealed that
at the initial stage of milling, the particles tend to grind easily.
With the progress of milling, the particle sizes fell into ductile
range and mainly underwent plastic deformation. Consequently,
the long-term defects accumulated in the particles being ground
were also acquired. It seems that the mechanically induced energy
could partially be consumed through the creation of crystal defects,
such as dislocation and grain boundaries [28]. As CaCO3 and TiO2
were the brittle components of the mixture, the particles were frag-
mented during milling, and their size was reduced continuously.
Furthermore, the reduced particle size shortened the diffusion path
tallinity of the CaO was lower compared to TiO . Furthermore, the
2
CaO reached a plateau at 200 rpm and a further increase in the mill
rotational speed had very little effect on the degree of crystallinity.
This phenomenon was observed due to the softness of the CaO as
compared to TiO . The degree of crystallinity of a milled CaO/TiO2
2
mixture decreased steadily with the growing amount of milling
work, which was accomplished by increasing the mill rotational
speed and the milling time. This observation was in accordance
with the increments of a specific surface area of the milled parti-
cles. Particles with a lower degree of crystallinity exhibited high
surface energy and they tend to reduce the surface energy via an
aggregation of the particles.
required for the reaction between CaO and TiO to proceed. The for-
2
mation of a fresh surface in this stage was another advantage of the
fragmented particles. The newly created surfaces were appropriate
sites for the nucleation of CaTiO3.
3
.2.4. Microstructural characteristics
The microstructural characteristics of the particles were
Table 4 shows the crystallite size and lattice strain of the
obtained by determining the crystallite size and lattice strain of CaO
and TiO2 in the mixture. Normally, particles that had undergone
intensive milling process will exhibit a reduction in the crystallite
size and an increase in the lattice strain [22]. Most research works
carried have focused on the effect of milling time as a function to
evaluate microstructure characteristics, but not much attention has
been given on the effect of process parameters on microstructural
characteristic.
CaO/TiO2 mixture milled at a rotational speed of 600 rpm for 5 h.
The X-ray diffraction analysis revealed that at this process param-
eter the CaO/TiO2 mixture had undergone phase transformation to
CaTiO3.
Figs. 11 and 12 show the crystallite sizes and lattice strains
of TiO2 and CaO. Generally, the crystallite size decreased as the
mill speed increased, but the decrease was drastic when done at
2
00 rpm and came to a plateau at 400 and 600 rpm. The decrease
in CaO was more evident compared to that in TiO2 due to the vari-
ation in terms of hardness. Figs. 11 and 12 clearly show that at
600 rpm, the crystallite size reached a constant degree regardless of
the milling time, which indicated that the crystallite size limit was
reached in this particular mill. The lattice strains for CaO and TiO2,
when ground for 1 and 2 h, were similar. However, when the milling
time was at 5 h, a drastic increase in the lattice strain was observed,
Table 4
Microstructural characteristics of CaTiO3.
Crystallite size (nm)
Lattice strain
1.63
0.89
Fig. 13. Micrograph of the CaO/TiO2 mixture (mill rotational speed = 200 rpm,
milling time = 5 h).

S. Palaniandy, N.H. Jamil / Journal of Alloys and Compounds 476 (2009) 894–902
901
During intensive breakage, the CaCO3 almost reached a nanome-
ter range to exhibit a rhombohedron structure. Figs. 13 and 14 also
supported the idea of an incomplete reaction between CaCO3 and
TiO2 for the formation of CaTiO3 when the mixture was milled for
5
h at the rotational speeds of 200 and 400 rpm. The mixture, which
was grinded for 5 h at 600 rpm, was dispersed in water mixed with
a dispersing agent. Then the mixture was sonicated for 1 h. Fig. 16
shows the TEM micrograph of CaTiO3 which was dispersed, and it
shows that the CaTiO3 particles were in the nanometer range. The
average particle size distribution was obtained through the TEM
micrographs by the counting of approximately 80 particles. The
average particle size of CaTiO3 nanoparticles with approximately
10.3 nm.
4. Conclusion
Fig. 14. Micrograph of the CaO/TiO2 mixture (mill rotational speed = 400 rpm,
milling time = 5 h).
The mechanochemical synthesis of CaTiO3 was carried out in a
planetary mill using CaCO3 and TiO2 (anatase) as the starting pow-
der. Massive size reduction due to intensive grinding in the mill was
2
observed where the specific surface area of 6.97 m /g was reached
when milled for 1 h at the rotational speed of 200 rpm. The massive
aggregation of the nanoparticles was observed when the mixture
was grinded at 400 and 600 rpm speeds. However, CaTiO3 was
mechanochemically synthesized when the mixture of CaCO /TiO2
3
was milled for 5 h at the mill rotational speed of 600 rpm. Shock
power and the friction phenomenon played important roles in syn-
thesizing CaTiO3 via the mechanochemical route using a planetary
mill. Intermediate phases, such as CaO and Ca(OH)2, were formed
prior to the formation of CaTiO . The mechanochemical effect,
3
which can be quantified through the degree of crystallinity, crys-
tallite size, and lattice strain, shows the influence of milling time
and mill rotational speed on the CaCO /TiO mixture. The crystallite
3
2
decreased, while the lattice strain increased as the mill rotational
speed and mill speed were intensified. Furthermore, the TEM
micrographs revealed that the actual particle size of CaTiO3 was
around 10.3 nm, using parameters (600 rpm for 5 h). In conclusion,
Fig. 15. Micrograph of the CaO/TiO2 mixture (mill rotational speed = 600 rpm,
milling time = 5 h).
this study showed that calcium titanate (CaTiO ) nanoparticles can
3
be produced through a mechanochemical process in a planetary
mill using CaCO3 and TiO2 (anatase) as starting materials.
3.3. Morphological analysis
Figs. 13–15 show the micrographs of the CaO/TiO2 mixture
milled for 5 h at various mill speeds. All milled mixtures exhib-
ited a massive aggregation of particles. Fig. 15 shows the CaTiO3
nanoparticles agglomerated to form bigger particles in micrometer
range. According to XRD analysis, the CaO/TiO2 mixture milled for
h at mill rotational speeds of 200 and 400 rpm exhibited both the
CaO phase and TiO2 phase. Figs. 13 and 14 clearly show the CaO,
which is the elongated particle aggregated to nano size particles.
Acknowledgements
The authors wish to gratefully acknowledge Universiti Sains
Malaysia and the Ministry of Science, Technology and Innovations
Malaysia for granting the research fund grant no. 6035143 to this
research project.
5
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