X-ray diffraction studies on crystallite size evolution of CoFe2O4 nanoparticles prepared using mechanical alloying and sintering

Applied Surface Science 256 (2010) 3122–3127

X-ray diffraction studies on crystallite size evolution of CoFe2O4 nanoparticlesprepared using mechanical alloying and sintering

Samaila Bawa Waje a,*, Mansor Hashim a,b, Wan Daud Wan Yusoff b, Zulkifly Abbas b

a Advanced Materials and Nanotechnology Laboratory, Institute of Advanced Technology, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Darul Ehsan, Malaysiab Department of Physics, Faculty of Science, Universiti Putra Malaysia, 43400 UPM, Serdang, Selangor, Darul Ehsan, Malaysia

A R T I C L E I N F O

Article history:

Received 9 November 2009

Received in revised form 29 November 2009

Accepted 29 November 2009

Available online 5 December 2009

Keywords:

Crystallite size

Sintering temperature

X-ray diffraction

Nanoparticles

A B S T R A C T

Nanosized cobalt ferrite spinel particles have been prepared by using mechanically alloyed

nanoparticles. The effects of various preparation parameters on the crystallite size of cobalt ferrite

which includes milling time; ball-to powder weight ratio (BPR) and sintering temperature, were studied

using X-ray diffractometer (XRD). Scherrer’s equation was used to study the crystallite size evolution of

the as-prepared materials. The results of the as-milled sample revealed that both milling time and BPR

plays a role in determining the crystallite size of the milled powder. However, where sintering is

involved, the sintering temperature results in grain growth, and thus plays a dominant role in

determining the final crystallite size of the samples sintered at higher temperature (above 900 8C). From

the vibrating-sample magnetometer (VSM) measurement it was observed that the coercivity of the

as-milled samples without sintering is almost negligible, which is a type characteristic of super-

paramagnetic material. However, for the sintered samples, the saturation increases while coercivity

decreases with increases sintering temperature.

� 2009 Elsevier B.V. All rights reserved.

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1. Introduction

Understanding and control of matter at dimensions of roughly1–100 nm has generated a lot of interest in recent years due to thesignificantly different properties of these novel materials ascribedto their characteristic structural features in between the isolatedatoms and the bulk macroscopic materials [1,2]. For instance,Quantum size effects and the large surface area of magneticnanoparticles dramatically change some of the magnetic proper-ties and exhibit superparamagnetic phenomena and quantumtunneling of magnetization, because each particle can be consid-ered as single magnetic domain [2].

A number of methods for the preparation of nanostructures,such as layered double hydroxides [3] chemical co-precipitation[4] sol–gel coupled with combustion technique [5], microemulsion[2] and mechanical alloying (MA) [6–9], have been reported. Thesemethods can usually be used to produce spherical or irregular-shape nanoparticles. However, among the methods mentionedabove, the mechanical alloying method has attracted muchinterest in recent years, due to its simplicity in the preparationof various interesting solid-state materials. MA takes advantage ofthe perturbation of surface-bonded species by pressure to enhancethermodynamic and kinetic reactions at room temperature or at

* Corresponding author. Tel.: +60 389466061.

E-mail address: [email protected] (S.B. Waje).

0169-4332/$ – see front matter � 2009 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2009.11.084

least at much lower temperatures than normally required toproduce pure metals [10,11]. This is due to the energy transferredfrom the milling media to powder particles, continuouslysubmitted to fracture and cold welding processes which willdefine their final morphology [12]. The particles synthesizedusually have an equilibrated cation distribution, a narrower sizedistribution economically viable and environmentally friendly.Some important components of the MA process are the rawmaterials, the mill, and the process variables [10].

The present paper examines the crystallite size evolution ofCoFe2O4 prepared using MA and sintering using X-ray diffraction(XRD). Special attention is paid to the effects of the milling time,ball-to-powder weight ration (BPR) and sintering temperature onthe crystallite size evolution of the as-prepared nanoparticles andthe result being reported.

2. Experimental

All the chemicals used in this work were of analytical grade andwere used as purchased without further purification. The startingmaterials of Fe2O3 (99.5% purity) and Co3O4 (99.7%) from Alfa Aesarwere weighed according to the target proportion and mixedaccordingly. Mechanical alloying was carried out in a hardenedsteel vial together with ten 12 mm steel using a Spex 8000D mixer/mill. The BPR was varied in the order of 8:1, 10:1, 15:1, 20:1 and30:1, while the milling time was varied in the order of 3 up to 50 h.The as-milled were sintered in the temperature range of 500 to

Fig. 1. XRD pattern of CoFe2O4, 12 h milling as a function of BPR.

S.B. Waje et al. / Applied Surface Science 256 (2010) 3122–3127 3123

1000 8C, at 100 8C interval for 4 h in each case, and using a heating arate of 4 8C/min. All powder handling, milling and subsequentpressing and sintering were performed in air.

The structural characteristics of the sample were measuredusing a Philips X’Pert-MPD XRD system equipped with a graphitemonochromator, operating at 40 kV and 30 mA and employingnickel-filtered Cu Ka radiation (l = 0.1542 nm). The nanocrystal-lite size was calculated using Scherrer’s equation [13]:

Dh k l ¼kl

B cos uA (1)

where l is X-ray wavelength (1.542 A), u the Bragg’s angle at whichthe peak is observed measured in radians, and B is the full width ofdiffraction line at half of the maximum intensity (FWHM). Originsof specimen broadening are numerous [14]; generally speaking,any lattice imperfection will cause additional diffraction-linebroadening. Therefore, dislocations, vacancies, interstitials, sub-stitutional, and similar defects lead to lattice strain. If a crystal isbroken into smaller incoherently diffracting domains by disloca-tion arrays (small-angle boundaries), stacking faults, twins, large-angle boundaries (grains), or any other extended imperfections,then domain-size broadening occurs [13,14]. Since B describes thestructural broadening, which is the difference in integral profilewidth between a standard and the unknown sample; therefore isBobs. is refined to remove the specimen broadening using therelation [15]:

BðsizeÞ ¼ Bobs: � Bstd:

The exact Ka1 peak position, the Ka1 FWHM and the netintegral intensity were used to refine the data. The refinementstrategy is based on the maximum neighborhood method ofMarquardt (Levenberg–Marquardt method) [16], which overcomethe problem of highly correlated parameters in a non-linear least-squares refinement. The pseudo-Voigt profile function, which isthe weighted mean between a Lorentz and a Gauss function asdescribed in (2) [15]: was used

Gik ¼ gC1=2

0

Hkp½1þ C0X2

ik��1 þ ð1� gÞ

C1=21

Hkp1=2exp½�C1X2

ik� (2)

where C0 = 4, C1 = 4 � ln2, Hk is the FWHM of the kth Braggreflection, Xik = (2ui � 2uk)/Hk. g is a refinable ‘‘mixing’’ parameterdescribing the amount of Gaussian profile versus the amount ofLorentzian profile; and thus describing the overall profile shape.The density of the samples was measured using the well-knownliquid displacement Archimedes’ technique. The measurementswere carried out to record the change in density followingsintering treatment, using the relation:

Density of the sample; r ¼ Wa

Ww

� �r�w (3)

where Wa = weight of sample in air, Ww = weight of sample inwater and r�w ¼ density of water = 1 g cm�3. The theoreticaldensity (rx) of the resulting material was calculated using therelation (4) [17]:

XRD density; rx ¼8M

Na3(4)

where M is the molecular weight; N is the avogadro’s constant(6.022 � 1023); a is the lattice constant. The percentage porosity (P)of the sample was calculated using the relation in Eq. (4) [17]:

P ¼ 1� rrx

(5)

where r is the measured density of the sample. The specificsaturation magnetization was measured at room temperature by avibrating-sample magnetometer in a field of 10 kOe.

3. Results and discussion

3.1. Effects of ball-to-powder weight ratio (BPR) on the crystallite size

evolution

As earlier stated in Section 1, mechanical alloying is a complexprocess and hence involves optimization of a number of variablesto achieve the desired product phase and/or microstructure. Fig. 1shows the XRD pattern of the effect of BPR on the formation of asingle phased polycrystalline CoFe2O4. The BPR was varied at 8:1,10:1, 15:1, 20:1 and 30:1, respectively, while keeping the millingtime at 12 h. From the results it can be seen that, changing the BPRsignificantly affects the phase of the powder being milled. Withinthe 12 h of ball milling process and a BPR of 8:1, no evidentdiffraction peaks from the starting compound were detected,except some traces of (1 0 4) and (3 1 1) peaks in the h k l planes,which are the signature peaks of starting raw materials (Fe2O4 andCo3O4, respectively). However, two major peak of CoFe2O4 wereevident, which can be indexed to (3 1 1) and (5 1 1) in the2u = 35.52 and 53.91, respectively. With the BPR 10:1, the peak at2u 36.78 (3 1 1) disappeared, as the (1 0 4) was substantiallysuppressed.

The intensity of the ferrite (3 1 1) peak was however seen toincrease. Further increase in the BPR, increases the intensity of the(3 1 1) major peak of CoFe2O4 in addition to the appearances ofmore peaks. For example, with BPR 15:1, the peaks at 2u 30.21,43.01, 56.97 and 62.58 begin to appear. The same peaks weremaintained with increasing intensity for BPR 20:1. However, up toBPR 30:1 could not form a complete phase of CoFe2O4, as someCoFe2O4 signature peaks in the 2u of 37.06 and 53.34 were yet toappear.

A careful look at the peaks suggests that the mechanicalalloying has led to a significant broadening of X-ray diffractionpeaks, which increases with increase in the BPR. This indicates theformation of fine grain and a high density of defects caused by largelocal strains in the powder particles. The crystallite size as afunction of BPR was calculated from the most intense peak (3 1 1)by using the Scherrer equation, and the results shown in Fig. 2.From the results it can be seen that the particle size decreaseslinearly from 15.3 to 11.4 nm for 8:1 and 30:1, respectively.

3.2. Effect of milling time on the crystallite size evolution

The time of milling is the most important parameter in MA.Normally the time is chosen as to achieve a steady state betweenthe fracturing and cold welding of the powder particles [10]. The

Fig. 2. Patterns of milling time and the BPR on the crystallite size evolution of

CoFe2O4.

S.B. Waje et al. / Applied Surface Science 256 (2010) 3122–31273124

times required vary depending on the type of mill used, theintensity of milling, the ball-to-powder ratio, and the temperatureof milling. These times have to be decided for each combination ofthe above parameters and for the particular powder system. TheXRD patterns after various milling times, at constant BPR of 8:1 foras-milled CoFe2O4 are shown in Fig. 3. From the spectra, thestarting raw materials were very evident. The signature peaks ofthe two starting raw materials, i.e. Fe2O3 at 2u = 33.188 and Co3O4

at 2u = 36.918 can be indexed to ICDD cards of 01-079-1741 forFe2O3 and 01-076-1802 for Co3O4 accordingly.

Subjecting the material to milling at various milling time (3–50 h), causes some changes in the peaks profile with thesuppression of some peaks and the appearance of others, a trendconfirming an alloying process. Specifically, 3 h milling results inthe appearance of the (3 1 1) ferrite peak at 2u = 35.52, which co-exists with the starting materials peak at 2u = 33.15, 36.77 and53.91. The same peak profiles were maintained for 6, 12 and 24 hmilling. However, the intensity and its broadening of (3 1 1) peakincreases with increasing the duration of milling. After 24 h millingtime, the starting materials peaks begin to disappear, while theintensity of major peak of CoFe2O4 tremendously increases and at50 h of milling. Five of the seven major peaks of most of theCoFe2O4 peaks begin to appear. A similar trend observed for theBPR results in Fig. 1.

Fig. 3. XRD pattern of CoFe2O4, BPR 8:1, as a function of milling time.

The 2u of the major peak was also seen to shift towards thelower angle from 33.15 to 35.547 consequent to the change in themilling time. This suggests the pattern shift from the Fe2O3

signature peak (033-0664) towards the signature peak of CoFe2O4

(022-1086). The disappearance of the peaks can be detected,implying the existence of solid solution phases. In other words, theincreased milling leads to the formation of intermetallic compoundand solid solution phases and the gradual diffusion of Co into theFe2O4 to form CoFe2O4 spinel ferrite. The crystallite sizes of thediffracting grains as calculated from the refined diffraction peaksare shown in Fig. 2. From the figure, it is shown in as a function ofmilling time, the crystallite size is found to decrease gradually,from 28.1 nm (MA 3 h), to approximately 10.8 nm (MA 30 h), andthen slightly increases to 12.8 and 13.3 for 40 and 50 h,respectively.

More than the milling time, the BPR has a significant effect onthe time required to achieve a particular phase in the powder beingmilled. The higher the BPR, the shorter is the time required.Deducing from the result, it can be seen that the crystallite sizeachieved for 12 h milling BPR 30 (9.6 nm) are slightly smaller thatthose achieved with 50 h milling at BPR 8:1 (13.3 nm). This can beexplained on the basis that; at a high BPR, so alloying takes placefaster because of an increase in the weight proportion of the balls,the number of collisions per unit time increases and consequentlymore energy is transferred to the powder particles [10,18].

3.3. Effects of sintering temperature on the crystallite size evolution

From Figs. 1 and 3, it is evident that a highly crystalline cobaltferrite could not be formed during milling alone, which isconsistent with the results obtained elsewhere [19]. The phenom-ena can be explained on the basis that ball milling facilitatesfracturing and cold welding of crystalline particles to createalternating layers with fresh interfaces, thereby generating a highdensity of defects. The sintering of the polycrystalline materialsinfluences the particle size, shape and crystallization. This leads toobtaining right proportion of metals at a correct valency therebydeveloping useful properties [20]. To understand the role played bythe sintering temperature on the crystallite size evolution andphase formation of the mechanically alloyed samples.

Fig. 4 shows the crystallite size plot of the samples at BPR of 8:1,at various milling time and subjected to sinter in the temperaturerange of 500–1000 8C. From the results, it can be observed thatwith 500 8C, the crystallite decreases linearly with milling timefrom 28.1 to 16.1 nm for 3 and 30 h milling, respectively. A similartrend was observed for the other samples sintered from 600 to1000 8C, with an increase in both crystallite size and thecrystallinity of the product. However, the differences in thecrystallite sizes of the sintered materials shows a decreasing effectsof milling time with increasing the sintering temperature. All,

Fig. 4. Crystallite size evolution pattern of CoFe2O4, BPR 8:1, as factions of milling

times and sintering from 500 to 1000 8C.

Fig. 5. XRD analysis of CoFe2O4, BPR 8:1 as a function of sintering from 500 to

1000 8C.

S.B. Waje et al. / Applied Surface Science 256 (2010) 3122–3127 3125

except the sample milled at 3 h shows a very closely uniformcrystallite sizes with sintering at 700 8C, however, at 1000 8C, thereis no significant difference in the crystallite sizes for all thesamples.

Studying the XRD pattern for the 12 h milled sample, andsintered in the temperature range of 500–1000 8C (Fig. 5) andsummarized in Table 1, it clearly shown that a single phaseCoFe2O4 was formed at as low temperature as 600 8C. This isbecause all the diffraction peaks can be indexed to CoFe2O4 andmatched well with a cubic crystal structure (JCPDS file no. 01-00-022-1086, Fd3m). No peaks of any other phases or impurities canbe observed, confirming the purity of the nanoparticles. Thesharpness of the major peaks and diffraction-line narrowing of theannealed sample is probably due to a better-ordered structure orthe increase of grain size.

Contrary for solid-state route which required above 1200 8C tofully crystallized [21,22], the full crystallization achieved at as low

Table 1Summary of physical properties of the as-prepared CoFe2O4, 12 h, BPR 8:1.

Sample (8C) A (A) 2u Obs. VO (A3) Crystallite siz

As-milled 8.48 35.30 458.26 16.10

600 8.38 35.52 579.88 31.00

700 8.38 35.54 587.34 41.90

800 8.37 35.55 586.49 50.90

900 8.38 35.51 587.81 64.20

1000 8.38 35.49 587.89 80.40

Fig. 6. TEM image of CoFe2O4, BPR 8:1, 12 h millin

as 1000 8C shows the advantage of mechanical alloying. Moreadvantage is the short sintering duration as against 10 h commonlyused in solid state. It is well known that there are lower diffusionactivation energy of atoms in nanocrystals or nanostructuredmaterials and thus larger diffusion coefficient than the corre-sponding bulk counterpart due to the increase of surface(interface)/volume ratio of the nanocrystals or nanostructuredmaterials [23,24]. The size and temperature dependent diffusioncoefficient function is an important parameter for any phasetransition process through nucleation and growth, which iscertainly related to the kinetic properties of a material [25].Comparing the XRD micrographs in Figs. 1, 2 and 5, it can bededuced that a higher reaction temperature favors a particle withlarger grain sizes, because a higher temperature enhanced theatomic mobility.

The formation of ferrite nanoparticles is a function of the rate ofdiffusion. Therefore, the understanding of the role of ball milling iscritical for the formation mechanisms and controlled production ofthe ferrite nanostructures. During the milling, material particlesare repeatedly flattened, fractured and welded. Every time twosteel balls collide or one ball hits the chamber wall, they trap someparticles between their surfaces. The high-energy impacts severelydeform the particles and create atomically fresh, new surfaces, aswell as a high density of dislocations and other structural defects[26]. These results in smaller grain size and higher surface area,which should all contribute a reduced reaction or vaporizationtemperature and a higher chemical reactivity in comparison withsamples, prepared using solid-state technique. Such a high defectcan accelerate the diffusion process [26], and reaction tempera-tures significantly [26–29].

Fig. 6(a) and (b) shows the TEM of the average particle size ofthe 12 h milled sample and the sample sintered at 1000 8C,respectively. From the results, it is seen to be in agreement withthat obtained from the Scherrer formula. TEM images show thatthe smaller particles in the range of 10–20 nm for the as-milledsamples. The sintered sample however shows a nearly sphericalparticle size of the range of 70–80 nm. Evidently in the figure are

e (nm) Density r (g/cm3) Porosity (%) Shrinkage (%)

4.48 15.47 –

4.87 8.10 0

4.88 7.92 0.58

4.94 6.79 0.91

4.96 6.42 2.42

5.04 4.91 5.24

g: (a) as-milled, and (b) sintered at 1000 8C.

Fig. 7. Patterns showing the variations of density with milling time and the

sintering temperatures.

Fig. 9. VSM loops for the as-milled CoFe2O4 and the samples sintered at 600 and

1000 8C.

S.B. Waje et al. / Applied Surface Science 256 (2010) 3122–31273126

some bigger particles of the range of 200–300 nm. This can beunderstood as the agglomeration of the individual particles of thematerials due to their magnetic properties.

The density measurement for as-milled sample 12 h, 8:1 BPR, asshown in Fig. 7 shows that the density of the increases linearlywith sintering temperature, achieving 5.04 g/cm3 at 1000 8C; thisis 95.1% of the theoretical density. A similar phenomenon wasobserved for all the milling times sintered at 1000 8C, as the densityincreases linearly from 5.01 to 5.15 g/cm3 from 3 to 30 h,respectively. These mechanisms can easily be understood; thus;the finer particle size results in a higher surface energy for acompact, thus a higher driving force for grain growth (growth ofcrystallites) and densification to reduce the system’s Gibbs energy[24,29].

Furthermore, the higher amount of contact points between theparticles activated the matter transport leading to the high numberof the necks between particles, motivating the diffusion andevaporation–condensation of the matter on surfaces with conse-quent bulk densification [30]. Fig. 8 shows the relationshipbetween the shrinkage and porosity of the samples. It can beobserved that the samples behave as expected [22]. This is becausethe data revealed an inversely proportional relationship betweenshrinkage and porosity. The porosity decreases linearly from 15.4%

Fig. 8. Patterns of variation of porosity and the shrinkage as functions of sintering

temperature of CoFe2O4.

for the as-milled sample to 4.48% for the sample sintered at1000 8C. The shrinkage on the other hand increases from unity to5.24%. A significant shrinkage was observed above 900 8C,therefore it can be said that the significant sintering occurs withinthis region.

3.4. Vibrating-sample magnetometer (VSM) measurement

Three samples of [(BPR 8:1, 12 h), (BPR 8:1, 600 8C) and (BPR8:1, 1000 8C)] were analyzed using VSM at room temperature tostudy their mass magnetization. The results, as shown in Fig. 9shows magnetic saturation values of �1 emu/g for BPR 8:1, 12 h,15 emu/g for BPR 8:1, 50 h, 23 emu/g for BPR 8:1, 600 8C and48 emu/g for BPR 8:1, 1000 8C. This value for the sample is lowerthan those reported for the bulk samples [21] (�80 emu/g). Thiscan be attributed to the surface effects aroused by the distortion ofthe magnetic moments at the surface of nanocrystallite [31]. Thecoercivity of the as-milled samples without sintering is almostnegligible, which is a type characteristic of superparamagneticmaterial [32]. However, the sintered samples showed a coercivityof 1900 and 500 Oe for 600 and 1000 8C, respectively. It can be seenthat saturation increases with increasing temperature, whilecoercivity decreases with increases sintering temperature. Corre-lating this with the XRD result, it can be attributed to thepromotion of crystallinity and crystallite size consequent ofsintering. This variation with crystallite size is also explained onthe basics of domain structure, diameter of particles and crystalanisotropy. Since sintering temperature causes changes bydecomposition or transformation of phases, which results in theincrease in the crystallite size increases, decreasing the structuraldefects and subsequently decreased values of corecivity andretentivity [9,33].

4. Conclusion

Nanosized cobalt ferrite spinel particles have been prepared byusing mechanically alloyed nanoparticles. The effects of variouspreparation parameters on the crystallite size of cobalt ferrite,which includes milling time, charge ration and sintering tempera-ture, were studied. The results revealed milling time and ball-to-powder ration (BPR) plays a role in determining the crystallite sizeof the milled powder. However, due to the grain growthconsequent to sintering, the sintering temperature, rather thanor milling time plays a dominant role in the crystallite size ofsamples sintered at higher temperature (above 900 8C). As benefit,a considerable reduction of the sintering temperature was possiblein consequence, the earnest pursuit of low production cost. From

S.B. Waje et al. / Applied Surface Science 256 (2010) 3122–3127 3127

the Vibrating-sample magnetometer (VSM) measurement it seenthat coercivity of the as-milled samples without sintering is almostnegligible, which is a type characteristic of superparamagneticmaterial. However, the saturation increases with increasingtemperature, while coercivity decreases with increases sinteringtemperature. This is attributed to the promotion of crystallinityand particle size consequent of sintering.

Acknowledgements

The authors are grateful to Universiti Putra Malaysia for bothResearch University Grant (vote number 05-04-08-0548RU) andthe Graduate Research Fellowship.

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