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Journal of Alloys and Compounds 476 (2009) 894–902

Contents lists available at ScienceDirect

Journal of Alloys and Compounds

journa l homepage: www.e lsev ier .com/ locate / ja l l com

nfluence of milling conditions on the mechanochemical synthesis of CaTiO3

anoparticles

amayamutthirian Palaniandy ∗, Noorina Hidayu Jamilchool of Materials and Mineral Resources Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia

r t i c l e i n f o

rticle history:eceived 30 May 2008eceived in revised form

a b s t r a c t

The mechanochemical synthesis of calcium titanate (CaTiO3), was carried out in a planetary mill by varyingthe milling time and mill rotational speed at three levels. CaCO3 and TiO2 were used as the startingmaterials. Besides size reduction, the CaCO3/TiO2 mixture had undergone structural changes, which was

9 September 2008ccepted 27 September 2008vailable online 17 November 2008

eywords:echanochemical processingigh-energy ball milling

affected by the process parameters of the planetary mill. Furthermore, milled particles in a nanometer sizerange were aggregated to form micron size particles. The CaTiO3 phase was formed when the milling timewas set at 5 h, and the mill rotational speed was 600 rpm. The mechanochemical process was affected bythe mechanism inside the vial, such as the shock power and friction component, at high efficient millingregion. The particle size of the CaTiO3 has a diameter of about 10.3 nm. The parameters used affected thedegree of crystallinity, crystallite size and lattice strain of the particles.

pdnpfdmdltiar(r

oec

-ray diffraction

. Introduction

High-energy ball milling has been used for many years now inroducing ultra fine powders in the range of a sub-micron to aanometer. Aside from size reduction, this process causes severend intense mechanical action on the solid surfaces, which wasnown to lead to physical and chemical changes in the near sur-ace region where the solids come into contact under mechanicalorces [1]. These mechanically initiated chemical and physicochem-cal effects in solids were generally termed as the mechanochemicalffect. This route is currently being used to synthesize inorganicaterials as it exhibits some advantages, such as the reduction

n sintering temperature [2–5]. CaTiO3 is one of the materialshat were initially synthesized via the mechanochemical route.lso, this material has been synthesized by wet chemical methods,uch as sol–gel [6], co-precipitation [7], polymeric precursor [8],rganic–inorganic solution [9] and combustion [10]. Furthermore,his material is a ferroelectric ceramics with perovskite relatedtructure [5]. It has high dielectric constant, low dielectric loss andarge temperature coefficient of resonant frequency, which makes it

promising component in the production of communication equip-ent operating at microwave frequencies (UHF and SHF), which in

urn are used in microwave dielectric applications (as resonatorsnd filters) [6,8,11].

∗ Corresponding author. Tel.: +604 5996132; fax: +604 5941011.E-mail address: samaya@eng.usm.my (S. Palaniandy).

hciobato

925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.jallcom.2008.09.133

© 2008 Elsevier B.V. All rights reserved.

The CaTiO3 is the basic material for ferroelectric ceramics witherovskite related structure [5]. It has high dielectric constant (εr),ielectric loss (tan ı), and large temperature coefficient of reso-ant frequency (�f), which makes it a promising component in theroduction of communication equipment operating at microwaverequencies (UHF and SHF), which in turn are used in microwaveielectric applications (as resonators and filters) [12]. Also, it isaterials can be employed as a thermally sensitive resistor element

ue to its negative temperature coefficient, and for the immobi-ization of highly radioactive wastes. Such unique properties gavehis material much attention, and many investigations regardingts many uses have been carried out in recent years [12]. Recently,lso has been reported visible photoluminescence properties atoom temperature in disordered structurally perovskite titanatesCaTiO3) and highly emissive red-emitting phosphors have beeneported in the literatures [13–18].

The CaTiO3 can be synthesized by heating a mixture of CaO,r CaCO3, and TiO2 (rutile or anatase) at about 1650 K [5]. How-ver, this process makes it difficult to produce CaTiO3 with uniformomposition because an all-solid state reaction was topotactic. Theeating condition was an essential criterion in producing qualityrystalline CaTiO3 powder. Thus, in order to improve the sinter-ng condition of the CaTiO3 powder, several techniques such as the

rganometallic method or the mechanochemical synthesis haveeen developed. It was the latter method which has been receivinglot of attention as it was believed that it can produce a uniform dis-

ribution of CaTiO3 powder. The study on mechanochemical effectn fine particles has created much interest among researchers

S. Palaniandy, N.H. Jamil / Journal of Alloys and Compounds 476 (2009) 894–902 895

Table 1O

MM

brpci

ioacbMttsadi

tmd

2

2

Csfmmsos

2

wmlo

a

TD

WGT

M

G

dbLmoc

%

R

woc

R

t

s(

%

2

gmm

AcXct

perational parameters of planetary mill.

ill rotational speed (rpm) 200 400 600illing time (h) 1 2 5

ecause of its several advantages to downstream processes likeeducing the annealing and sintering temperature, reducing thehase transformation temperature, enhancing the leaching pro-ess, decreasing the thermal decomposition temperature, andncreasing the particle reactivity [19,20].

The mechanochemical synthesis process is carried out in high-ntensity grinding mills such as vibro mills, planetary mills, andscillating mills. It has been noticed that the size reduction processnd the microstructural evolution of the CaTiO3 during milling pro-ess were mainly influenced by the type of impulsive stress appliedy the grinding media, which can either be an impact or shear type.oreover, other parameters such as atmosphere composition and

he presence of different liquid media inside the grinding mill affecthe mechanochemical process. In fact, when the mechanochemicalynthesis of the CaCO3 and TiO2 was carried out in planetary millst higher rotational speed to produce CaTiO3, the impact stress wasominant, and not much attention was given on the mechanochem-

cal mechanism itself.The aim of this work, therefore, is to give additional contribu-

ion in understanding the influence of milling conditions on theechanochemical synthesis of CaTiO3 nanoparticles without of the

eleterious phase.

. Experimental

.1. Raw materials and milling conditions

The CaTiO3 powder was synthesized through a mechanochemical route. To start,alcite CaCO3 (99.9% purity, Aldrich) and rutile TiO2 (99.9% purity, BDH laboratoryupply) were mixed in a stiochiometric ratio of 1:1 in an agate mortar with a pestleor 10 min and were preserved in desiccators. Then the mixture was subjected to

icrowave heating using a normal kitchen microwave oven for 2 min to remove theoisture in the samples. The milling of the mixture was conducted under atmo-

pheric condition in a planetary ball mill (Fritsch Pulveristte-6) with one steel potf 50 cm3 inner volume. The milling was performed by varying the mill rotationalpeed and the milling time at three levels, as shown in Table 1.

.2. Planetary mill mechanics

The planetary ball mill consisted of a gyratory shaft and a cylindrical jar, and both

ere rotated simultaneously in opposite directions at high rotational speed. Suchovement at high speed allowed the ball to move strongly and rigorously, which

ed to a large impact energy between the balls and the materials. The specificationf 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 aboutprimary axis O. Here, G is the diameter of the axis of the jar, and D is the mill

able 2imensions of the Planetary mill.

orking principle Impact (mainly)rinding process Dryransmission ratio Irelative = 1:-1.82 (mill and shaft

rotate in the oppositedirections)

illMaterial Stainless steelDiameter, mm 70Volume, mm3 250Rotational speed, rpm 200, 400 and 600

rinding ballsMaterial Stainless steelDiameter, mm 10Density, kg/m3 7800Weight, kg 0.2Filling, % of mill volume 66

tctt[lpt(mirfiocSpovlasEw

ˇ

Fig. 1. Schematic of the planetary mill.

iameter. When these parameters are fixed, the planetary mill can be characterizedy the ratio of the diameter mill axis rotation (G) to the diameter of the mill (D).et R be the ratio of the rotation speed of the mill axis to the rotation speed of theill at its center. The negative R value indicates that the mill and the shaft rotate in

pposite directions. The dynamics of the planetary mill allows the computation ofritical speed. The percent of the critical speed is shown in Eqs. (1) and (2).

CS = N2

R × N1(1)

= −1 ±√

G

D(2)

here N1 is the gyrating speed and N2 is the jar speed. In this work, the mill wasperated at 78.5% critical speed. This was accomplished by doing the followingalculation of Eq. (3):

= −1 ±√

137

= −1 ± 1.36 (3)

Taking the minus sign for the opposite direction of motion between the jar andhe main shaft, it can be shown that R = −2.36.

The percent of critical speed is calculated by considering the shaft speed and jarpeed as 600 and 1092 rpm. Therefore the critical speed calculated is shown in Eq.4).

CS = 10922.36 × 600

× 100 = 78.5% (4)

.3. Characterization

The specific surface area of the powders was determined by a multipoint nitro-en adsorption using a Quantachrome system and the Brunauer-Emmett-Tellerethod (BET). The amount of N2 adsorbed at liquid nitrogen temperature waseasured in N2/He carrier flux in a concentration range of 10–30%.

X-ray diffraction (XRD) analysis was performed on a D8 Diffractometer (BrukerXS) using Cu K radiation for all analyses at 40 kV and 20 mA in order to identify theompositional and phase changes in the mixture during the grinding operation. TheRD patterns were recorded in the 2� range = 10–70◦ using a step size of 0.05◦ and aounting time of 5 s per step. Silicon powder was used as standard agent to removehe instrumental broadening effects from the observed profile broadening.

Line positions, intensified widths, and shapes were obtained from the XRD spec-ra in order to characterize the microstructure in terms of defects parameters such asrystalline size and microstrain. The APD version 4.1 g software was used to acquirehese parameters [21]. The K�2 component was removed from the XRD spectra withhe assumption that K�2 intensity was half of K�1 intensity. Then the [1 0 1] and1 0 0] planes were selected for the profile analysis of TiO2 and CaCO3. The over-apped peak was split using the APD version 4.1 g software. The X-ray diffractionatterns were adjusted to a combination of Cauchy and Gaussian line shapes usinghe Halder and Wagner method for obtaining physical broadening, as shown in Eq.5), where ˇf , ˇh and ˇg are the integral breadths of the instrumental, observed, and

easured profiles. The profile fitting procedure was performed without smooth-ng the XRD spectra. Each goodness factor was refined to a value of <5% for all theeflections. The maximum height of the peak (Imax), integral breath of the line pro-le (ˇ = A/Imax), full-width at half maximum (FWHM), and peak position (2�) werebtained from the adjusted line profile. A is the area under the peak. The apparentrystallite size was calculated using the Scherrer formula, as shown in Eq. (6). Thecherrer formula, meanwhile, describes the mutual dependence between the linerofile integral breath and crystallite size Dv which was the volume weighted meanf the crystallite in the direction perpendicular to the diffracting planes; the constantaried with the reflection Bragg angle and crystallite shape. Lattice strain was calcu-ated using Eq. (7). The structural disorder due to the increasing abundance of X-raymorphous material was manifested through the reduction in the integral inten-

ity of the diffraction lines [21]. The relative degree of crystallinity (DOC) defined inq. (8) was based on the area under the [1 0 1] and [1 0 0] peak for TiO2 and CaCO3,

here A0 and A were the areas under the peak for feed and ground sample [22].

f =(ˇ2

h− ˇ2

g)

ˇh(5)

896 S. Palaniandy, N.H. Jamil / Journal of Alloys and Compounds 476 (2009) 894–902

Fp

wm

D

wt

ε

w

D

wf

nrwsEgptta

3

3

puspeT1tmwp

easps

Fp

twsshpAiTefrAeepa

3

atstthe TiO2 and CaCO3 crystal structure from intensive grinding. Thereduction of peak intensities implied the formation of amorphousmaterials in the milled powders. The increase in XRD line breathwas due to the plastic deformation and disintegration of TiO2 and

ig. 2. Change in specific surface area at various mill rotational speeds and millingeriods.

here ˇf , ˇh and ˇg are the integral breadths of the instrumental, observed, andeasured profiles, respectively [22]

v =(

K�

FWHM

)cos � (6)

here Dv is the volume weighted mean of the crystallite size, K is the constant, � ishe Bragg angle of (h k l) reflection, and � is the wavelength of X-rays used [22].

= ˇ

4 tan �(7)

here ε is lattice strain [9].

OC =(

At

A0

)× 100 (8)

here DOC is the degree of crystallinity, and A0 and At are the areas under the peakor feed and ground sample, respectively [22].

The morphological change of the ground particles was observed through a Scan-ing electron microscope (SEM), ZEISS Supra 35VP with an acceleration voltageanging between 5 and 15 kV, and a secondary electron detector (SE). The particlesere deposited randomly on a sample holder covered with carbon coated adhe-

ive. These samples were then coated with a thin gold layer using gold sputteringMSCOPE SC500A unit to minimize the charging effect. The digital images on 256rey levels were captured using image analysis software. In addition, the nano-scalearticle was observed via transmission electron microscope (TEM). The powder mix-ure was sonicated for 30 min in ultra sonic bath for SEM and TEM observation. Here,he ground particles were mixed with calgon so that the particles will be dispersednd smeared on the cell, which was transferred to cell position in the microscope.

. Results and discussions

.1. Specific surface area changes by milling

The mechanochemical synthesis of the materials was accom-anied by disintegration and a generation of fresh, previouslynexposed surfaces. Thus, the particle size distribution and thepecific surface area of the milled product depended on secondaryrocesses like aggregation and agglomeration [20]. Fig. 2 shows thevolution of a specific surface area as the mill speed and time varied.he fineness particle of 6.97 m2/g was obtained at 200 rpm speed ath time. The particles became coarser as the mill speed and milling

ime increased. Similar observation was reported by Pourghahra-ani and Forssberg [22], where coarser particles were obtainedhen hematite was ground at higher levels of stress energy in alanetary mill.

Fig. 3 shows particles, which when ground at 600 rpm for 5 h,

xhibited particle sizes ranging between 100 and 150 nm and wereggregated to form bigger particles in micron range. Fig. 3 alsohows that the aggregation of particles occurred in the pores; thisrocess is called soft agglomeration [22]. There are three maintages of interaction between the particles: adherence, aggrega-

F�

ig. 3. Micrograph of milled product (mill rotational speed = 600 rpm and millingeriod = 5 h).

ion, and agglomeration [20]. At the adherence stage, the particlesill coat on the lining and the grinding bodies. At the aggregation

tage, the particles associate weakly by Van der Walls type adhe-ion, and it is a reversible reaction. Agglomeration, on the otherand, is defined as a very compact, irreversible interaction of thearticles with an occurrence of chemical bonding between them.n aggregation of particles was reported during extended high-

ntensity dry grinding for oxide minerals [23] and olivine [24].his phenomenon was common in dry grinding and was usuallyxplained by the agglomeration of structurally modified particlesollowing the initial reduction of particle size. Whenever a mate-ial is ground excessively, the surface area increases tremendously.n increase in the surface area leads to an increase in the surfacenergy. Particles, in an attempt to reduce the excess free surfacenergy, begin to agglomerate and hence SEM micrographs show theresence of large aggregates having no definite shape. This behaviorlso has been reported by Satami et al. [25].

.2. Mechanochemical synthesis

Structural changes and microstructure characterization werenalyzed using X-ray diffraction. Fig. 4 shows the XRD pattern ofhe starting and milled powder. The diffraction peaks of milledamples have lower intensity and broader peak base comparedo the starting powders, mainly due to the disordering process of

ig. 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

TiO2 (

Cw[

3t

mtaogimfitiatwaicccatiidotpgoos[

miafspwTtottrtmpah

C

C

dosadotpa

Fig. 5. X-ray difractogram of mixture of CaCO3 and TiO2—(�=

aCO3. The same observation was reported by several researchershen high-energy milling was performed on oxide materials

19,20].

.2.1. The effect of mill speed and milling time on phaseransformation

Although several researchers [5,6] have studied theechanochemical synthesis of CaTiO3 by varying the milling

ime and precursor materials in a planetary mill, not muchttention was given on the optimization of process parametersn the formation of CaTiO3. The application of a high-energyrinding mill, such as a planetary mill, allows dramatic changesn the structure and surface properties of a solid material. The

echanical treatment in a high-energy mill generates a stresseld within the solids. Heat release can cause stress relaxation,he development of a surface area as a result of brittle fracturen the particles, generation of various sorts of structural defects,nd stimulation of a chemical reaction within the solids [9]. Inhe mechanochemical synthesis of CaTiO3, the starting materialsill initially undergo structural defects and will be followed bychemical reaction [5]. The concentration of a mechanically

nduced defects and their spatial distribution depends on theondition of the energy transfer in the mill and on the externalondition of stress. In a planetary mill, the mechanochemical effectan be intensified by increasing the grinding media’s density, itscceleration, and the duration of the milling process. However,he measurement of efficiency of the mills in transferring energys a complicated task because the energy transfer takes placen three stages. These are the conversion of kinetic energy thatrives the grinding medium, the transfer of mechanical actionn the particles being ground, and the transferring of energy intohe treated substance [9]. In order to improve the processes in alanetary mill, much attention should be given on the study of

rinding variables and condition of the mechanochemical effectn the milled particles. The intensity of mechanical stress dependsn the mass of the materials and the grinding media, the millpeed, milling time, gravitational acceleration, and mill diameter22].

mtiCf

anatase); � = CaTiO3; �= CaOH; �= CaO; �= TiO2 (Brookite)).

Fig. 5 shows the X-ray diffraction pattern at various levels ofilling time and speed. It shows how structural defects on the start-

ng materials take place, which leads to the formation of partiallymorphous materials and followed by a chemical reaction for theormation of CaTiO3. These defects generally increase as the millpeed and time increase, which was exhibited by the reduction ineak intensity and peak base broadening. It also shows that CaTiO3as formed when CaCO3 and TiO2 were ground for 5 h at 600 rpm.

he first step in the formation of CaTiO3 from CaCO3 and TiO2 washe decomposition of the CaCO3, as shown in Eq. (9). The instabilityf CaO caused the formation of Ca(OH)2 due to exothermic reac-ion, as shown in Eq. (10). Ca(OH)2 formation may also be due tohe presence of moisture in the ambient. Berbenni and Marini [2]eported that the mass losses in the TG curves of the milled mix-ure of CaCO3/TiO2 were slightly larger than those in the physical

ixture; they accounted that this phenomenon occurred due to theresence of moisture during milling. These observations may varyccording to the location of the experiment being conducted as theumidity in the ambient varies.

aCO3(s) ⇔ CaO(s) + CO2(g) (9)

aO(s) + H2O(l) → Ca(OH)2 (10)

Fig. 5 shows that the decomposition of CaCO3 to CaO was imme-iate, taking place within 1 h of milling at a low mill speed. Theccurrence of the Ca(OH)2 phase was observed on all levels of millpeed and mill time, except when the mixture was milled for 5 h atspeed of 400 and 600 rpm. At these levels, the Ca(OH)2 phase wasiminished and the mixture appeared ready to form a pure phasef CaTiO3. These results show that the presence of moisture con-ent in the starting powder was an essential factor in the millingrocess. Brankovic et al. [26] mentioned that the development ofCa(OH)2 phase during the mechanochemical process of CaTiO3

ay cause difficulties in maintaining the stoichoimetry of the sys-

em CaO-TiO2, but it can be avoided if the starting material useds CaCO3 [25]. However, this study has proven that even thoughaCO3 was used, the formation of Ca(OH)2 still occurred. Apart

rom that, the formation of brookite from anatase was also observed

898 S. Palaniandy, N.H. Jamil / Journal of Alloys and Compounds 476 (2009) 894–902

F

wwhbri6eCt

tmtbaNratSfTts

m[tmrAitwtw“pIm

TD

ABC

Fm

ff

igstJitoCfifcpoep

3In order to quantify the influence of the milling parameters

on the alteration of CaCO3 and TiO2, further investigations werefollowed by a line profile fitting technique. The physical integralbreadth of the reflection peaks were calculated after adjustingthe line shape; this was done with consideration of the physical

ig. 6. Graphical representation of the pairs of velocities and milling efficiency.

hen the mixture was milled at the speed of 400 and 600 rpm,hich indicated that the phase transformation was induced by theigh-energy ball milling process. Although Mi et al. [14] observedrookite forming when milling Ca(OH)2 with TiO2 (rutile) in theiresearch, it was not discussed thoroughly. The brookite phase read-ly transformed to CaTiO3 when it was ground at a mill speed of00 rpm for 5 h. All the TiO2 phases (anatase, rutile and brookite)xhibited negative values of Gibbs free energy when reacting withaO or Ca(OH)2, suggesting that the mechanochemical reaction ofhis mixture immediately took place [27].

The selection of an optimum milling mode is essential in ordero synthesize CaTiO3 via a mechanochemical route. The milling

echanism in a planetary mill depends on the mill speed, wherehe rotation of the vial about its own axis produces the tum-ling of the mill charge. It is possible to control the movementnd trajectories of the balls by varying the mill speed. Jean andachbour [3] reviewed several papers on the mechanism of size

eduction and the mechanochemical process in a planetary mill,nd concluded that an increase in shock power and high fric-ion component were conducive for the mechanochemical process.hock power, which is the product of input energy and the shockrequency, can be improved by increasing the mill rotational speed.he authors [3] have proposed a map indicating that the genera-ion of shock power was the function of disc and vial rotationalpeed.

Fig. 6 shows the region of milling efficiency for theechanochemical process to take place, as proposed by the authors

3]. A, B, and C denote the disc and vial rotational speeds chosen forhis study, and they were located according to the milling efficiency

entioned by Jean and Nachbour [3]. Table 3 also shows the discotational speed and the vial rotational speed chosen for this study.

mill speed of 200 rpm falls on the “no efficient milling” regionn accordance with the X-ray diffractogram showed in Fig. 5. Athis mill speed, only a small reduction in terms of peak intensityas noticed. Furthermore, only a small portion of CaCO3 changed

o CaO, and the TiO2 phase remained in its anatase phase. Mean-

hile, the mill speeds of 400 and 600 rpm fall on the region of

high efficient milling” and demonstrated a massive reduction ofeak intensity, line breadth broadening, and phase transformation.

n milling C, the mechanochemical process took place when theixture was milled for 5 h. This phenomenon illustrated that aside

able 3isc rotational speed and vial rotational speed.

Disc rotational speed (rpm) Vial rotational speed (rpm)

200 364400 546500 1092

Fm

ig. 7. Physical integral breadth of milled TiO2 at various mill rotational speeds andilling time.

rom the milling time, an optimum value of mill speed was essentialor the mechanochemical synthesis.

Among the process parameters of a planetary mill, impact forces commonly used, and it can be decomposed into radial and tan-ential components. Impact force varies depending on the millpeed. In addition to impact force, the ratio of vial rotational speedo disc rotational speed for this mill (ω/˝) was 1.82. According toean and Nachbaur [3], if ω/˝ > 1 with high tangential force, then its conducive for a friction phenomenon. Milling efficiency is linkedo the important friction component, and the non-efficient qualityf milling A in Table 3 was related to a small friction component.onsequently, friction cannot be the only parameter controlling thenal product. In this research, the influence of milling power and

riction has to be considered simultaneously, allowing for the effi-ient milling in B and C. The combined good influences of botharameters (high friction component and high power) enabled thebtainment of the perovskite structure within 5 h. These phenom-na were certainly linked to the mill speed and its non-negligiblearameter.

.2.2. The effect of milling on the physical XRD line breath

ig. 8. Physical integral breadth of milled CaO at various mill rotational speeds andilling time.

S. Palaniandy, N.H. Jamil / Journal of Alloys and Compounds 476 (2009) 894–902 899

Fm

balFagntbc0bpwsftwasm

Fm

testm

3

pAidtstit

ig. 9. Degree of crystallinity of TiO2 as a function of mill rotational speed andilling time.

readth for (1 0 1) and (1 0 0) for CaO and TiO2 at various mill speedsnd milling time. The CaO plane was preferred over CaCO3 as theatter readily decomposed to CaO as soon as the milling began.igs. 7 and 8 show the physical integral breadth of the milled CaOnd TiO2 at various levels of mill speed and milling time. The inte-ral breadth at the mill speed of 600 rpm and milling time of 5 h wasot taken into consideration due to the formation of CaTiO3. The lat-ice strain and crystallite size components contributed to the linereadth, which will be resolved later to obtain the microstructureharacteristics. The integral breadth of TiO2 and CaO ranged from.10◦ to 0.34◦ and 0.20◦ to 1.10◦, respectively. The maximum integralreadth was obtained during high mill speed and milling time. Thelots of integral breadth for CaO and TiO2 displayed similar trends,here the integral breadth values increased as the mill rotational

peed and milling time also increased. The integral breadth valueor CaO was higher compared to TiO2, which indicated that the plas-ic deformation in CaO was higher than in TiO2. This phenomenon

as observed due to the variation in the hardness of the materi-

ls. CaO was softer compared to TiO2, so the defects in the crystaltructure of CaO were more evident compared to TiO2. Pourghahra-ani and Forssberg [22] mentioned in their research that most of

cria

Fig. 11. Variation of volume weighted crystallite size and the lattice strain of TiO2

ig. 10. Degree of crystallinity of CaO as a function of mill rotational speed andilling time.

he milling energy in intensive grinding stages was used for thestablishment and development of plastic deformation rather thanize reduction. The reduction in the XRD patters intensities andhe broadening of peak are common characteristics of XRD patters

illed oxide materials.

.2.3. The degree of crystallinityIn this section, we study the relative intensity of the reflection

eaks of a milled CaCO3/TiO2 mixture and its degree of crystallinity.s mentioned before, the intensity of the XRD patterns reduces dur-

ng a high-energy mechanical treatment of the CaCO3/TiO2 mixtureue to the formation of amorphous materials. Figs. 9 and 10 showhe degree of crystallinity of the TiO2 and CaO as a function of millpeed and milling time. The degree of crystallinity on the mixturehat was milled at a rotational speed of 600 rpm for 5 h was notncluded, as the mixture has formed a CaTiO3 phase. The diffrac-ion peaks show a progressive reduction in peak intensity when

ompared with the starting powder diffractions. Generally, theeduction in X-ray diffraction intensity was heightened by increas-ng the mill rotational speed and the milling time. Fig. 9 shows

drastic decrease in the degree of crystallinity as the rotational

in [1 0 1] direction as a function of mill rotational speed and milling time.

900 S. Palaniandy, N.H. Jamil / Journal of Alloys and Compounds 476 (2009) 894–902

of CaO

stistCrTcmwswcsa

3

oaiscebc

om2ia6trwt

TM

CL

aTmamaWrtwcswmFrmfs

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 toCaTiO3.

Fig. 12. Variation of surface weighted crystallite size and the lattice strain

peed increased up to 400 rpm. After 400 rpm, the degree of crys-allinity came to a plateau, which indicated that a further increasen mill speed will not exhibit much mechanochemical effect. Theame observation was exhibited in Fig. 10 but the degree of crys-allinity of the CaO was lower compared to TiO2. Furthermore, theaO reached a plateau at 200 rpm and a further increase in the millotational speed had very little effect on the degree of crystallinity.his phenomenon was observed due to the softness of the CaO asompared to TiO2. The degree of crystallinity of a milled CaO/TiO2ixture decreased steadily with the growing amount of millingork, which was accomplished by increasing the mill rotational

peed and the milling time. This observation was in accordanceith the increments of a specific surface area of the milled parti-

les. Particles with a lower degree of crystallinity exhibited highurface energy and they tend to reduce the surface energy via anggregation of the particles.

.2.4. Microstructural characteristicsThe microstructural characteristics of the particles were

btained by determining the crystallite size and lattice strain of CaOnd TiO2 in the mixture. Normally, particles that had undergonentensive milling process will exhibit a reduction in the crystalliteize and an increase in the lattice strain [22]. Most research worksarried have focused on the effect of milling time as a function tovaluate microstructure characteristics, but not much attention haseen given on the effect of process parameters on microstructuralharacteristic.

Figs. 11 and 12 show the crystallite sizes and lattice strainsf TiO2 and CaO. Generally, the crystallite size decreased as theill speed increased, but the decrease was drastic when done at

00 rpm and came to a plateau at 400 and 600 rpm. The decreasen CaO was more evident compared to that in TiO2 due to the vari-tion in terms of hardness. Figs. 11 and 12 clearly show that at

00 rpm, the crystallite size reached a constant degree regardless ofhe milling time, which indicated that the crystallite size limit waseached in this particular mill. The lattice strains for CaO and TiO2,hen ground for 1 and 2 h, were similar. However, when the milling

ime was at 5 h, a drastic increase in the lattice strain was observed,

able 4icrostructural characteristics of CaTiO3.

rystallite size (nm) 1.63attice strain 0.89

Fm

in [1 0 0] direction as a function of mill rotational speed and milling time.

lthough not much change in the crystallite size was manifested.his phenomenon suggested that after a certain milling time, theilling energy was used for the distortion of the crystal lattice

nd not for the reduction of crystallite size. Meanwhile, changes inicrostructural characteristics and integral breadth revealed that

t the initial stage of milling, the particles tend to grind easily.ith the progress of milling, the particle sizes fell into ductile

ange and mainly underwent plastic deformation. Consequently,he long-term defects accumulated in the particles being groundere also acquired. It seems that the mechanically induced energy

ould partially be consumed through the creation of crystal defects,uch as dislocation and grain boundaries [28]. As CaCO3 and TiO2ere the brittle components of the mixture, the particles were frag-ented during milling, and their size was reduced continuously.

urthermore, the reduced particle size shortened the diffusion pathequired for the reaction between CaO and TiO2 to proceed. The for-ation of a fresh surface in this stage was another advantage of the

ragmented particles. The newly created surfaces were appropriateites for the nucleation of CaTiO3.

Table 4 shows the crystallite size and lattice strain of the

ig. 13. Micrograph of the CaO/TiO2 mixture (mill rotational speed = 200 rpm,illing time = 5 h).

S. Palaniandy, N.H. Jamil / Journal of Alloys a

Fig. 14. Micrograph of the CaO/TiO2 mixture (mill rotational speed = 400 rpm,milling time = 5 h).

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ig. 15. Micrograph of the CaO/TiO2 mixture (mill rotational speed = 600 rpm,illing time = 5 h).

.3. Morphological analysis

Figs. 13–15 show the micrographs of the CaO/TiO2 mixtureilled for 5 h at various mill speeds. All milled mixtures exhib-

ted a massive aggregation of particles. Fig. 15 shows the CaTiO3anoparticles agglomerated to form bigger particles in micrometer

ange. According to XRD analysis, the CaO/TiO2 mixture milled forh at mill rotational speeds of 200 and 400 rpm exhibited both theaO phase and TiO2 phase. Figs. 13 and 14 clearly show the CaO,hich is the elongated particle aggregated to nano size particles.

ig. 16. TEM micrographs of the CaO/TiO2 mixture (mill rotational speed = 600 rpm,illing time = 5 h).

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nd Compounds 476 (2009) 894–902 901

uring intensive breakage, the CaCO3 almost reached a nanome-er range to exhibit a rhombohedron structure. Figs. 13 and 14 alsoupported the idea of an incomplete reaction between CaCO3 andiO2 for the formation of CaTiO3 when the mixture was milled forh at the rotational speeds of 200 and 400 rpm. The mixture, whichas grinded for 5 h at 600 rpm, was dispersed in water mixed withdispersing agent. Then the mixture was sonicated for 1 h. Fig. 16

hows the TEM micrograph of CaTiO3 which was dispersed, and ithows that the CaTiO3 particles were in the nanometer range. Theverage particle size distribution was obtained through the TEMicrographs by the counting of approximately 80 particles. The

verage particle size of CaTiO3 nanoparticles with approximately0.3 nm.

. Conclusion

The mechanochemical synthesis of CaTiO3 was carried out in alanetary mill using CaCO3 and TiO2 (anatase) as the starting pow-er. Massive size reduction due to intensive grinding in the mill wasbserved where the specific surface area of 6.97 m2/g was reachedhen milled for 1 h at the rotational speed of 200 rpm. The massive

ggregation of the nanoparticles was observed when the mixtureas grinded at 400 and 600 rpm speeds. However, CaTiO3 wasechanochemically synthesized when the mixture of CaCO3/TiO2as milled for 5 h at the mill rotational speed of 600 rpm. Shockower and the friction phenomenon played important roles in syn-hesizing CaTiO3 via the mechanochemical route using a planetary

ill. Intermediate phases, such as CaO and Ca(OH)2, were formedrior to the formation of CaTiO3. The mechanochemical effect,hich can be quantified through the degree of crystallinity, crys-

allite size, and lattice strain, shows the influence of milling timend mill rotational speed on the CaCO3/TiO2 mixture. The crystalliteecreased, while the lattice strain increased as the mill rotationalpeed and mill speed were intensified. Furthermore, the TEMicrographs revealed that the actual particle size of CaTiO3 was

round 10.3 nm, using parameters (600 rpm for 5 h). In conclusion,his study showed that calcium titanate (CaTiO3) nanoparticles cane produced through a mechanochemical process in a planetaryill using CaCO3 and TiO2 (anatase) as starting materials.

cknowledgements

The authors wish to gratefully acknowledge Universiti Sainsalaysia and the Ministry of Science, Technology and Innovationsalaysia for granting the research fund grant no. 6035143 to this

esearch project.

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