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Colloids and Surfaces A: Physicochem. Eng. Aspects 388 (2011) 49– 58

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical andEngineering Aspects

jo ur nal homep a ge: www.elsev ier .com/ locate /co lsur fa

urface functionalization for dispersing and stabilizing hexagonal boron nitrideanoparticle by bead milling

Made Jonia,b, Ratna Balgisa, Takashi Ogia, Toru Iwakia, Kikuo Okuyamaa,∗

Department of Chemistry and Chemical Engineering, Graduate School of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi Hiroshima, Hiroshima, 739-8527, JapanDepartment of Physics, Faculty of Mathematics and Natural Science, Padjadjaran University, Jl. Raya Bandung-Sumedang KM 21, Jatinangor, 45363, Indonesia

r t i c l e i n f o

rticle history:eceived 8 June 2011eceived in revised form 24 July 2011ccepted 4 August 2011vailable online 11 August 2011

eywords:anoparticles

a b s t r a c t

This paper presents the surface functionalization with silane coupling agents for dispersing and stabiliz-ing hexagonal boron nitride (hBN) nanoparticle suspension in a water phase. Fine milling using 30 �mbead technology was utilized for the reduction of particle size, while dispersing agents with silane cou-pling agents were used for surface modification to enhance dispersion stability. A monodispersed hBNnanoparticle was obtained by controlling the processing parameters and by optimizing the dispersantdosage. The hBN nanoparticle suspension showed a high degree of dispersion stability with a zeta poten-tial below −40 mV when (3-acryl-oxypropyl) trimethoxysilane (APMS) was used as the dispersing agent.

exagonal boron nitrideispersionead millilane coupling agent

Under optimal operation conditions for bead milling, the final average size of the particle distributionfor the hBN suspension was 46 nm and 38 nm for 0.5 wt.% and 5 wt.% hBN, respectively. In addition, thesurface characteristics, morphology, crystal structure and chemical composition of the hBN particlesbefore and after bead milling were investigated to know the effect of bead milling. Finally, the surfacefunctionalization mechanism that enhanced the stability of hBN nanoparticle dispersion by electrostericstabilization was also proposed.

© 2011 Elsevier B.V. All rights reserved.

. Introduction

The distinctive combination of thermal, mechanical and electri-al characteristics found in hexagonal boron nitride has receivedttention in the material design of a variety of industrial applica-ions. The chemical combination of boron and nitrogen to formoron nitride (BN) does not occur in nature. BN is a syntheticefractory material, which is now used in many applications dueo its attractive physical and chemical properties [1–5]. Recently,N has been successfully synthesized and identified in several crys-alline structures such as hexagonal BN (hBN), cubic BN (cBN),urbostratic BN (tBN), wurtzitic BN (wBN), rhombohedral BN (rBN),nd explosive BN (eBN). The application of hBN has gained consid-rable importance as both a refractory and a lubricant [4–7]. hBNas a crystal structure similar to that of graphite, which providesxcellent lubricating properties [7–9]. In addition, hBN has uniqueroperties such as high thermal conductivity, low thermal expan-ion, good thermal shock resistance, high electrical resistance, low

ielectric constant and loss tangent, microwave transparency; hBN

s nontoxic, easily machined, nonabrasive, lubricious, chemicallynert, and not wetted by most molten metals [1–8].

∗ Corresponding author. Tel.: +81 82 424 7716; fax: +81 82 424 5494.E-mail address: Okuyama@hiroshima-u.ac.jp (K. Okuyama).

927-7757/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfa.2011.08.007

Nano scaled hBN particles have already become an indispens-able material for many industrial applications because of theirunique size-dependent properties in which fundamentally differfrom those of the bulk materials. When inorganic hBN nanoparticlesare used for an inclusion of particles as reinforcement, dispersion ofnanoparticles is indispensable because of nanoparticles in variousliquids tend to agglomerate. Many attempts have been carried outto use hBN as inorganic filler for composite applications [10–12].However, it is difficult to disperse nano-hBN particles in the matrixhomogeneously by conventional mixing. Also, the graphite-likehBN has been recently applied as a Pt catalysts support [13].The hBN support with low crystallinity, appropriate specific sur-face areas and chemical anchoring sites was recommended as thepreferred support to maintain the dispersion and activity of Ptparticles. In addition, an hBN dispersion with micro-sized parti-cles has been successfully prepared for slip casting operations [14].However, hBN with nano-sized particles has been less extensivelyused mainly due to difficulties in dispersing nano-sized nitride par-ticles [8]. Poorly dispersed hBN nanoparticles in suspension willnot give rise to excellent properties for a nanostructure material.Thus, hBN particles must be dispersed in solvent-based or water-

based systems for various applications such as in the form of asprayable or paintable coating [15]. Well-dispersed hBN nanopar-ticles for coating can be applied to impart the desirable featuresof hBN to surfaces, and it provides excellent lubricating properties.

50 I.M. Joni et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 388 (2011) 49– 58

(�SAM

Danai[isttTtahr

tahcotoepioasdtn

2

2

JiCt

Fig. 1. Bead milling instruments

espite the high interest in the possibilities of hBN applicationsnd its mixtures, the behaviour of nanoparticles hexagonal boronitride dispersion has been scarcely studied. Our previous studynd many other studies have been reported on the surface mod-fication of metal oxide particles to enhance dispersion stability16–20]; however, there was no report on the surface functional-zation of nitride nano-scaled particles in relation to gain dispersiontability and its mechanism. The existence of nitrogen atom athe surface of nanoparticles becomes important to be investigatedo know the distinct adsorption behaviour of dispersed particles.herefore, in this study, surface functionalization of hBN nanopar-icles with silane coupling agents were investigated to obtained

well-dispersed hBN nanoparticle suspension. A well-dispersedBN suspension is very crucial in application of hBN either as aeinforced inorganic filler in a composite or catalyst support.

In the present investigation, fine milling using 30 �m beadechnology was used to obtain size reduction and surface function-lization using silane coupling agents was used to obtain stableBN nanoparticle dispersion. The enhancement of the particle-omminution process to a nano-sized particle dispersion dependsn the physical properties of the suspension and the opera-ion conditions of bead milling. On the other hand, the rangef parameter controls in the milling process is limited by thevolution of the crystallinity and morphology properties of thearticles [21,22]. Also, the attractive forces between nanoparticles

n liquid suspensions are sufficiently strong to cause a tendencyf nanoparticles to agglomerate [23–25]. Thus, an appropriatemount of dispersing agent had to be optimized to gain disper-ion stability of the suspension [16]. Finally, the surface property,ispersion stability, crystallinity and morphology of hBN nanopar-icles dispersion were investigated to know the behavior of hBNanoparticle dispersion.

. Experimental

.1. Materials and milling process

Commercially produced hBN particles (BN20, Tayca Co. Ltd.,

apan) with primary particles 35 nm in size were used in the exper-ments. Two types of silane coupling agents (Shin Etsu, Kagakuo. Ltd., Japan), (3-acryl-oxypropyl) trimethoxysilane (APMS) andrimethoxy(propyl)silane (TMPS), were used as dispersing agents

-005, Kotobuki, Co., Ltd., Japan).

in hBN nanoparticle suspensions. The silane coupling agent (SCA)was a silicon-based chemical with two types of reactivity, inor-ganic and organic, in the same molecule. A typical structure is(XO)3SiCH2CH2CH2-R, where XO is a hydrolyzable group and R isan organofunctional group. This structure allows a ‘coupling effect’between the dispersed and media phases. The bead milling instru-ment (�SAM-005, Khotobuki, Japan) used in this investigation isshown in Fig. 1. The bead mill consisted of a 170 mL vessel, apump and a mixing tank. The bead mill contained 30 �m ZrO2(zirconia) beads (Neturen Co., Ltd., Tokyo, Japan). Nanoparticle sus-pensions were pumped with an adjustable flow rate into the vessel,which contained zirconia beads and a centrifugation rotor at anadjustable rotational speed. The beads were agitated in the lowerportion of the vessel (dispersing section), which drove the break-up of agglomerated particles [26,27]. The suspension was pumpedfrom the dispersing section to the upper region (centrifugation sec-tion) where centrifugal force was used to separate the zirconiabeads from the nanoparticle suspension. The nanoparticle suspen-sion was then recycled back to the dispersing section. To preventa temperature increase in the system, the vessel was housed in acooling water jacket and was completely sealed from the outsideenvironment.

In order to get well-dispersed nanoparticle suspensions, millingoperation parameters were optimized to control the slurry rhe-ology of attractive and repulsive forces between particles andnanoparticle agglomerates. The water-suspended hBN particleswith weight fractions of 0.5 wt.% were investigated for optimaltechnical conditions of bead milling and for the appropriate dosageof the dispersing agent. The controlled processing parameters werepump flow rate, rotational speed of the centrifugation rotor, andtype and content of the dispersing agent.

2.2. Characterization

Particle size distribution at selected times was measured usingdynamic light scattering (DLS) equipment with an HPPS-5001Malvern Instrument. The dispersion stability was observed by mea-suring the zeta potential of the solution with a Malvern ZS NanoS

analyzer (Malvern Instrument Inc., London, UK). About 2–4 mL ofhBN suspension was transferred into a measuring cell. The mea-surement was run at V = 10 V and T = 25 ◦C with a switch time oft = 50 s. Each experiment was repeated 10 times to calculate the

Physicochem. Eng. Aspects 388 (2011) 49– 58 51

macqsptwHiTip−

twtpsw2p6

HipeLseep(tttpt

3

3

btVittszfhb(ardmtts

Ave

rage

par

ticle

siz

e(nm

)

Milling time (min)

Zeta

pot

entia

l (m

V)

Zeta potentialAverage particle size

Ave

rage

Par

ticle

siz

e(nm

)

Milling time (min)

Zeta

pot

entia

l (m

V)

Zeta potentialAverage particle size

1801206000

50

100

150

200

250

-50

-40

-30

-20

-10

1801206000

500

1000

1500

2000

2500

3000

-50

-40

-30

-20

-10

APMS

TMPS

a

b

Fig. 2. Average size and zeta potential of hBN suspensions 0.5 wt.% in the waterphase versus milling time for two types of dispersing agents (100% of APMS and

I.M. Joni et al. / Colloids and Surfaces A:

ean value of the experimental data. When an electric field ispplied to the cell, any particles moving through the measuringell will cause the intensity of light detected to fluctuate with a fre-uency proportional to the particle speed. Based on this frequencypectrum, Zetasizer Nano software is used to calculate the zetaotential information. If all the particles in suspension under inves-igation have a large negative or positive zeta potential, then theyill tend to repel each other and there is no tendency to flocculate.owever, if the particles have low zeta potential values, then there

s no force to prevent the particles coming together and flocculating.he general dividing line between stable and unstable suspensionss generally taken at either +30 mV or −30 mV. Particles with zetaotentials of more positive than +30 mV, or more negative than30 mV, are normally considered stable [28].

To obtain the existence of a surface modification for hBN par-icles, the samples were prepared to obtain only the hBN particlesith the adsorption of a dispersing agent on the surface. Thus, prior

o the Fourier transform infrared spectra (FT-IR) analysis, the dis-ersed hBN sample was centrifuged at 15,000 rpm for 1 h at 20 ◦C toeparate the particles from suspension, then any remaining wateras evaporated by drying the samples of solid deposit at 60 ◦C for

h. The hBN samples after bead milling with and without a dis-ersing agent and a bare hBN were observed by FT-IR ranging from00 to 4,000 cm−1 (PerkinElmer, Spectrum One System).

Field emission scanning electron microscopy (FE-SEM, S-5000,itachi Ltd., Tokyo, Japan) operated at 20 kV was used to obtain the

mages of hBN particles before and after bead milling. Particle mor-hology was examined visually using a field-emission transmissionlectronic microscope (FE-TEM, JEM-3000F, Japan Electron Opticsaboratory, Tokyo, Japan) operated at 297 kV. The chemical compo-ition of hBNs, before and after bead milling, was also observed bynergy dispersive X-ray spectroscopy (EDS) on a JSM-6700F fieldmission scanning electron microscope. The crystallinity of the as-repared hBN particles was characterized by X-ray diffractometryXRD, RINT 2200 V, Rigaku, Japan) using nickel-filtered CuK˛ radia-ion (� = 1.54 A) at 40 kV and 30 mA. The step and scanning rates ofhe XRD used in the measurement were 0.020◦ and 4◦/min, respec-ively. The selected area electron diffractions (SAED) of the hBNarticles before and after bead milling were observed coupled withhe TEM.

. Results and discussion

.1. Dispersion stability of an hBN suspension

The physical stability of a colloidal system is determinedy the balance between the repulsive and attractive forceshat are described quantitatively by the Deryaguin–Landau–erwey–Overbeek (DLVO) theory. The electrostatic repulsive force

s dependent on the degree of double-layer overlap and the attrac-ive force is provided by the van der Waals interaction. In this study,he measurement of the zeta potential has been used to assess thetability of the hBN suspension. Fig. 2 shows the average size andeta potential of an hBN suspension in water versus milling timeor different types of dispersing agents. The weight fraction of theBN particles in the suspension was 0.5 wt.%, and the content ofoth types of dispersing agent was 100% of the hBN particle content0.5 wt.%). The processing parameter of the bead milling was fixedt a rotational speed of 5220 rpm and a flow rate of 15 kg/h. Theseesults show that both types of dispersing agents formed a stableispersion after 60 min of milling time. Both dispersing agents pro-

oted steric interaction at the surface of the particles and stabilized

he dispersion in aqueous media. A similar trend was observed thathe zeta potential decreases with decreasing in the agglomerateize since the smaller size of particles will improves their surface

TMPS) with rotational speed of 5220 rpm and a flow rate of 15 kg/h.

area and finally lead to an increase of hBN surface charges. Thus, thezeta potential decreases with a progress of dispersion and decreas-ing in the agglomerated size as shown in Fig. 2(a) and (b). Whenmore charges available as sites at the surface, the probability ofdispersing agent adsorption increases on the hBN surface to formelectrosteric stabilization.

By contrast, when the APMS dispersing agent was used, thehBN suspension attained a well-dispersed condition indicated by alower zeta potential (below −40 mV) and the finest average sizefor hBN particles (Fig. 2(a)). Fig. 2 also shows that the averagesize of the initial suspension was lower when the APMS dis-persant was applied, compared with TMPS. The initial size mayhave significantly influenced the final size reduction of the beadmilling process. However, the bead milling that used both dis-persing agents showed a similar size reduction trend in the first60 min where larger numbers of agglomerated particles were dis-integrated. The conclusion is that the longer steric bond of thedispersing agent resulted in an hBN suspension with a higher zetapotential, which enhanced the dispersion stability.

The surface modification by dispersant agent displays a numberof dynamic processes during the milling procedure. Initially, hBNparticles attract each other strongly through van der Waals forces,since very short particle distances lead particles to agglomerate.The bead milling for the present study was in a low-viscous liq-uid, which moved continuously due to energy from agitated beadsin the lower portion of the vessel (dispersing section). The beadsencountered enough shear force to break up agglomerated par-ticles that are usually strongly attractive on contact. To obtain a

well-dispersed hBN suspension, it is necessary to have an effectivegrinding process and repulsive barriers that are sufficiently highso the particles will not re-agglomerated during the milling proce-

52 I.M. Joni et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 388 (2011) 49– 58

60012001800240030003600

Tran

smitt

ance

(a.u

.)

Wavelength (cm-1)

(a)

(b)

(c)

(d)

N-HB-OH

C=O

13751375

13751450 760

760760

669669

118011801250

9219501000

950

CHn

Fw

dt

upsbiWwcatwFdwWppoAamBrAaws(sssshttym

is might be necessary to get stable dispersion. A change in the

ig. 3. FT-IR spectrum of: (a) bare hBN particles, (b) hBN particles after millingithout APMS, (c) after milling with a dispersing agent, and (d) APMS.

ure. Thus, it is very important to stabilize the suspension duringhe bead milling procedure.

After bead milling, the surface modification of hBN particles thatsed APMS as a dispersing agent were compared with bare hBNarticles in an FT-IR investigation, as shown in Fig. 3. The FT-IRpectrum of the bare hBN (Fig. 3(a)) shows two distinct absorptionands at 1375 and 760 cm−1 which are represented B–N stretch-

ng and B–N bending, respectively, as reported elsewhere [29,30].eak and broad band of surface OH groups at 3000–3600 cm−1

as also observed. The weak adsorption peak at 1450 cm−1 mayontribute for the B–OH band existence accompanies the strongbsorption of BN. There was an appreciable difference betweenhe spectra of bare hBN particles, hBN particles after bead millingithout a dispersing agent, and hBN particles modified by APMS.

ig. 3(b) represented the FT-IR of the hBN after milling withoutispersing agent that shows the new absorption band of N–Hagging and N–H stretching at 1180 and 3200 cm−1, respectively.hile the FT-IR of the hBN after the milling with silane cou-

ling agents APMS (Fig. 3(c)) shows a decrease of N–H band. Neweaks were observed as a result of hBN-APMS reaction. The bandsf 1720 and 1250 cm−1 belongs to C O and C–O bonds of thePMS which were reacted on the hBN surface. The C–C stretchingnd C–H bending modes at 1300–1500 cm−1 and C–H stretchingode at 2900–3000 cm−1 were also appeared. The H–Si, N–Si and

–O–Si bands were found at around 669, 860–900 and 921 cm−1,espectively, as a result of the reaction between methoxy group ofPMS and the OH groups of hBN. Si–O–C and Si–O–Si stretchingt 1000–1100 cm−1, Si–O at 950 cm−1, and Si–C band at 800 cm−1

ere found due to the condensation of single AMPS on the hBNurface though the C C group (above 3000 cm−1) and OH groupbetween 3200 and 3600 cm−1) were very weak. Therefore, it isuggested that the AMPS on the surface are bound each other formome –Si–O–Si–O– chains covering the surface of hNB as also pre-ented by Iguchi et al. [31]. This Si–O–Si–O bound improved theteric barrier at the double layer. Thus, the modified surface of theBN particles overcome the inter-particle interaction energy dueo van der Waals forces, and, hence, the shear rate was reduced

o its lowest state, which brought the shear stress equal to theield value [32–34] and promoted better dispersion in the beadilling process.

Fig. 4. (a) Effect of the flow rate on the average size reduction of hBN particle sus-pensions, and (b) effect of the rotational speed on the average size reduction of hBNparticle suspensions at the condition of APMS concentration 100% and pH 7.7.

3.2. Optimization of the bead mill processing parameters

There are a large number of parameters in the bead milling pro-cess that can affect the efficiency of size reduction. Apart from theagitator geometry and viscosity of the slurry, the torque transmit-ted by the pins onto the agglomerated particles also depends on thetype, quantity and size of the beads [32,33]. In the present inves-tigation, the focus was on the optimization of some of the mostimportant parameters such as flow rate of the slurry pump and therotational speed of the agitator in the dispersing chamber. Fig. 4(a)shows the effect of the flow rate of the suspension on the averagesize of hBN particles over milling time when the rotational speedwas operated at 5220 rpm. The weight fraction of hBN particlesand the dispersing agent of APMS in the suspension was 0.5 wt.%.Thus, the APMS content in suspension was 100% of the weight frac-tion of the hBN particles. This result shows that the average sizedistribution for both conditions is relatively similar. Therefore, inconsideration of energy efficiency, the bead mill was operated ata fixed slurry flow rate of 10 kg/h for further investigation of theeffect of the rotational speed of the agitator.

Fig. 4(b) shows the effect of two different rotational speeds, 5220and 4353 rpm, on the average size of hBN particles over millingtime, while the slurry flow rate was fixed at 10 kg/h. The weightfraction of both, hBN particles and the dispersing agent of APMS inthe suspension was 0.5 wt.%. Though the same amount of dispers-ing agent were used for rotational speed 5220 and 4353 rpm, theagglomerate size increase at milling times between 30 and 120 minfor rotational speed 4353 rpm. This imply that the bead milling pro-cess not only contribute to the disintegration of agglomerates, butalso contribute to the progress of dispersing agent adsorption toobtain stable dispersion. Thus higher amount of dispersing agent

speed of the agitator caused a dispersion coefficient increase asa characteristic for the remixture with changes in turbulence andmechanical power in the mill. However, this result shows that dif-

I.M. Joni et al. / Colloids and Surfaces A: Physic

Fig. 5. (a) Size distribution of well-dispersed 0.5 wt.% of hBN particles with differingdispersing agent (APMS) contents, (b) Picture of the hBN suspension 0.5 wt.% beforeba

faawrp(

hpdwT21optba1bphcahamttfhAtf

be directly connected with the agglomeration as the standing time

ead milling, and a well-dispersed hBN suspension using an optimized dispersinggent.

erent rotational speeds of 5220 and 4353 rpm did not significantlyffect the efficiency of the size reduction, as shown by the similarverage sizes of the hBN particles for both conditions. Therefore,hen energy efficiency was considered in further investigation, a

otational speed of 4353 rpm was used for optimization of the dis-ersing agent dosage and a higher loading of the hBN suspensioni.e., 5 wt.%).

Fig. 5(a) and (b) show the size distribution of well-dispersedBN particles for different dosages of dispersing agent (APMS), aicture of the hBN suspension before bead milling, and a well-ispersed hBN suspension with an optimized dispersing agent. Theeight fraction of the hBN particles in the suspension was 0.5 wt.%.

he variations of the APMS dosage in suspension were 100, 150,00, and 300% weight fraction of the hBN particles, or 0.5, 0.75,, and 1.5 wt.%, respectively. These results show that the effectf the dispersing agent dosage can be investigated for differenteak positions and numbers, as well as its intensity. The size dis-ribution was bimodal for a dispersing agent dosage of 300%, andoth peaks had distinct peak positions (with a large size difference)nd relatively similar intensities. For dispersing agents at 200 and00%, however, the size distribution formation was changed from aimodal to a mono-dispersed formation. By contrast, with the dis-ersing agent at 150%, the size distribution was a mono-dispersedBN suspension with the finest average size at 46 nm. The surfaceoverage of SCA can be analyzed based on the estimated surfacerea’s relationship between the coupling agents and the dispersedBN nanoparticles. The specific surface area of BN with the aver-ge size of particles 46 nm and density of 2.1 g/cm3 is 62 m2/g. Theinimum surface area based on the available information from

he producer of the dispersing agent of APMS is 333 m2/g. Thus,he maximum monolayer coverage of the coupling agent is 19%or 1 g of BN samples. In the case of APMS amount was 150% ofBN samples, there were two possibilities may be occurred. First,

PMS surface coverage formed a multi-layered formation. Second,

he excess of the free coupling agents survived in the liquid phase toorm free siloxane (Si–O–Si). Thus, much excess of coupling agents

ochem. Eng. Aspects 388 (2011) 49– 58 53

were inevitable for the beads mill dispersion process. Fig. 5(b) illus-trates a well-dispersed hBN suspension in water with a high degreeof suspension stability and a nano-sized particle dispersion thatwas obtained with the optimized dosage and processing parame-ters. The hBN suspension was still stable for more than 6 months.

The electrophoretic mobility of colloidal particles in terms ofzeta potential was investigated as a function of pH in order to pointout the surface character of hBN particles after bead milling. Theevolution of the initial zeta potential of colloidal particles after 6 has a function of pH is given in Fig. 6(a). A well-dispersed hBN sus-pension, at the highest zeta potential, was obtained at pH 10. ThehBN–water interface contains B–O–H and N–O–H that can ionize toproduce negative charges (B–O− and N–O−). Other charges comefrom ionized silanol group (Si–O−) of APMS. The maximum increaseof negative charges occurred at pH 10 due to accumulated chargesfrom hBN surface and APMS. The negative hBN sites at the surfacecan act as polymer counterions, causing strong APMS adsorptiononto hBN surface. Thus, an increase of APMS adsorption caused lesscharges available in the hBN surface and also at APMS due to ionizedsilanol (Si–O−) formed siloxane (Si–O–Si) at the surface of hBN. Thisbehaviour caused a decreased of zeta potential at pH higher than10. Thus, this result is a strong evidence for the existence of theelectrosteric stabilization in hBN suspension.

In contrast, after more than 6 h, the zeta potential had notchanged, but at pH 14 a significant change occurred. The timedependence of zeta potential at pH 14 was observed in Fig. 6because of continuous ion activity at particle surface/fluid inter-face occurred. The concentration of ion in medium influenced theobserved zeta potential. The repulsive force arises when particlesdouble layers overlap. Upon close approach, these ion layers arepushed together, generating a repulsive force to prevent parti-cles agglomeration. The repulsive force depends on the particlesurface potential and the distance between particle surfaces. Thesurface potential is not readily measurable because of continuousion activity at particle/fluid interface. Therefore, surface potentialis estimated at the double layer by the experimental obtained zetapotential. Charge density at the shear plane varies with the doublelayer thickness because surface potential decrease with increasedistance from the particle surface. Thus, there is a direct depen-dence of zeta potential on double layer thickness. The zeta potentialin turn is related to the concentration of ions in medium.

Fig. 6(b) shows the corresponding pictures of hBN suspensionsfor different pH values: the initial and after 6 h. Fig. 6(b) (0 min)shows that the increase of pH, increasing the clarity of hBN sus-pension. It is well recognized that electrolytes affect both polymeradsorptivity and adsorbed layer thickness via (i) screening of elec-trostatic polymer-surface attraction and of electrostatic repulsionbetween charge segments, (ii) competition between polymer seg-ment and small ions for space near the surface, (iii) competitionadsorption of counterions onto the surface [35]. It has been com-mon that the zeta potential is dependent to the ionic strength.On the increase of pH of hBN suspension the ionic strength alsoincrease and rising electrostatic attraction. Therefore, increasingpH leads to higher adsorbed APMS due to higher electrostaticattraction. Thus, hBN suspension of pH 14 has very clear appear-ance. However, Fig 6 (b) (6 h) shows that the suspension after6 h are muddier than those before standing time; this meansthat re-agglomeration of disintegrated agglomerates occurs as thestanding time elapses. Also Fig. 6(a) shows that the zeta potentialis independent of the standing time except the case of pH 14. At pH14, on the contrary, the muddiness seems to be almost the same.These results imply that variation of the zeta potential may not

elapse. The agglomeration of disintegrated hBN particles may bedue to adsorption and desorption of APMS in the surface of hBNparticles.

54 I.M. Joni et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 388 (2011) 49– 58

F millin

wtppotfsataicff

Fpi

ig. 6. The zeta potential of a well-dispersed 0.5 wt.% of hBN suspension after bead

The size distribution of a well dispersed hBN suspension in theater phase at an initial condition (0 min) and after 24 h standing

ime for various pH are shown in Fig. 7. The initial pH of as pre-ared suspension was 7.7 and the peak of hBN size distributionarticles was below 100 nm with the smallest average particle sizef around 45 nm. There was no significant change in the size dis-ribution was detected at the 24 h standing time suspension. Thisact shows the dispersion of as-prepared suspension is relativelytable as time elapses. The pH adjustment was done at pH 8, 10, 12,nd 14 to examine the stability of the suspension. The biggest par-icle size distribution is observed at pH 8. It can be argued that thedsorbed APMS which is covered the hBN particle desorped againn the suspension due to the reaction with OH− ion, causing parti-

les re-agglomeration and increase the size distribution. However,or pH higher than 8, the number of OH− ion is increased, resultingurther chemical reaction to the APMS and induce re-adsorption

100001000100101

Inte

nsity

(a.u

.)

Diameter (nm)

(a)

(b)

(c)

(d)

(e)

ig. 7. Size distribution of well-dispersed 0.5 wt.% of hBN suspensions at variousH: (a) 7.7, (b) 8, (c) 10, (d) 12, and (e) 14. Straight line and dash line represents

nitial condition (0 min) and after 24 h standing time, respectively.

g at different pH and time (a) and the corresponding pictures of the suspension (b).

of this surfactant at the hBN surface, again forming layers coveredthe hBN particles and prohibit particles agglomeration. Fig. 7 of adash line shows the size distribution is dependent of the stand-ing time except for the case of pH 7.7 and 14. The size distributionof the suspension at pH 8, 10, and 12 after 24 h were increased asstanding time compared to before standing time. This mean that re-agglomeration of particles occurred as the standing time elapses asa result of APMS desorption.

Fig. 8 compares the transmittances of an hBN suspension inwater at pH 8 and an hBN suspension at pH 10. This result showsthat the transparency of the hBN suspension was significantlyimproved at pH 10 due to a lower size distribution of particles.It is clear that an hBN suspension in a water phase at pH 8 witha larger average size of particles contributed to the suspension’sstrong loss of light scattering. This observation can be explainedby the Rayleigh scattering law (Eq. (1)) in which the optical trans-parency loss of the suspension depends on the average size of theparticles. When inorganic particles inhomogeneously dispersed ina medium, the light propagating in the suspension will be scatteredby particles [36,37].

T = I = exp

(−32�4�V l r3n4

m

[(np/nm)2 − 1

]2)

(1)

I0 �4 (np/nm)2 + 2

This formula is a transparency loss (T) of the system as a resultof light scattering when the system consists of randomly dispersed

Wavelength (nm)

Tran

smitt

ance

(%)

7006005004000

20

40

60

80

100

pH = 8

pH = 10

Fig. 8. The comparison of the UV transmittances of hBN suspensions at pH 8 and10.

I.M. Joni et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 388 (2011) 49– 58 55

Fig. 9. A comparison of hBN properties before and after bead milling: (a) TEM images, (b) SAED pattern, and EDS spectrum of hBN particles before (c) and after (d) beadm

s(dop

3

cmsa

bwwas

illing.

pherical particles with radius (r) and particles volume fractionϕV); where I and I0 are the intensities of the transmitted and inci-ent light, respectively, � is the wavelength of the light, l is theptical path length, and np and nm are the refractive index of thearticles and the media, respectively.

.3. Characteristics of hBN particle after bead milling

The characteristics of hBN powder, such as the morphology, therystal structure, and the components, are very important, and theyust be satisfactory for the various applications of an hBN suspen-

ion. The effect of bead milling on the crystal structure, morphologynd elemental mapping of hBN particles are as follows.

Fig. 9(a) shows typical TEM and HRTEM images of hBN particlesefore (0 min) and after (210 min) bead milling, respectively. The

eight fraction of the hBN particles in the suspension was 0.5 wt.%ith the addition of 1.5 wt.% APMS. Bead mill process was done

t flow rate (10 kg/h), and rotational speed (4353 rpm). The corre-ponding selected area electron diffraction (SAED) pattern for hBN,

before and after bead milling, is shown in Fig. 9(b). Hexagonal BNbefore bead milling show four diffraction rings from inner to outercorresponding to the [0 0 2], [1 0 0], [0 0 4] and [1 1 0] planes of hBN.For the hBN before milling, the SAED patterns show reflexes of poly-crystalline boron nitride. However, for the hBN after bead milling,the SAED patterns show the diffraction lines intensities decreasedand the line become very broad diffuse feature which mean that theparticles has become polycrystalline boron nitride and partially anamorphous halo. Due to a low concentration of hBN in suspension,particle received high impact energy of bead milling lead to par-tially amorphous. Therefore, higher concentration of hBN needs tobe investigated.

The chemical composition of the hBN particles after bead millingis important since hBN particles may undergoes phase transforma-tion due to high energy mill. Fig. 9(c) and (d) show the EDS spectra

used to determine the chemical composition of the hBN particlesbefore and after bead milling, respectively. The spectra show thatpeaks of boron and nitrogen were observed both before and afterbead milling. However, weaker peaks of boron and nitrogen were

56 I.M. Joni et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 388 (2011) 49– 58

Fig. 10. The surface functionalization mechanism of hBN

Fig. 11. Size distribution of hBN nanoparticles (5 wt.%) prepared using experimentalparameters: flow rate (10 kg/h), rotational speed (4353 rpm), APMS concentration150% (7.5 wt.%), and pH 7.7.

nanoparticle with silane coupling agent (APMS).

obtained after bead milling. The signals of Cu and Si should be thecontribution of the Cu film on the sample surface and the standardsample of Si09, respectively.

Based on the FT-IR observation and supported by EDS spec-tra, the mechanism of surface functionalization mechanism of hBNnanoparticles can be illustrated as presented in Fig. 10. In thecase of bead milling without dispersing agent, the B–OH and N–Hbonds contributed to the surface charge of hBN particles. How-ever, due to high surface area of dispersed nanoparticles hBN,electric forces could not prevent particles from re-agglomeration.In the case of dispersing agent application, the surface modifica-tion was occurred. The first step toward hBN surface modificationwas hydrolysis and condensation of the aloxysilanes. The –Si–OHgroups at the end of adsorbed AMPS chains may be easily disso-ciated to form –Si–O– in aqueous solutions as similar behaviorwith SiO2 surfaces which has large negative surface charges. Theabove reaction then followed by the hydrogen bonding and siloxanebond formation process to the surface of hBN particles as explain inFT-IR spectra analysis. The bead mill contributes to both mechan-

ical process responsible to the disintegration of the agglomerates,and chemical reaction responsible for surface functionalization toprevents re-agglomeration. When the size and size distribution ofparticles decrease due to bead milling, surface area of the particles

I.M. Joni et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 388 (2011) 49– 58 57

d mill

itpoTgtWp

0ot7spsf

Smttmao

d

w�ctFari3nbTd

Fig. 12. XRD pattern of hBN particles before and after bea

ncrease lead to more charges available at the surface as a sites athe surface causing increase APMS adsorption on the hBN surface torevent re-agglomeration. Thus, progress of the chemical reactionr surface functionalization determined the progress of dispersion.he EDS spectra also supported the existence of carbon and oxy-en in the surface of hBN particles. Thus, surface modification ofhe hBN particles interfered with interparticle or London-van der

aals attractive forces, thereby preventing re-agglomeration of thearticles.

Furthermore, we used the optimized processing conditions of.5 wt.% hBN to disperse higher hBN concentration (i.e. 5 wt.%). Theptimized processing conditions were flow rate (10 kg/h), rota-ional speed (4,353 rpm), APMS content 150% (7.5 wt.%), and pH.7. The properties of as-prepared 5 wt.% hBN dispersion are pre-ented in Figs. 11 and 12. Fig. 11 presents the size distribution ofarticles before and after bead milling. This result shows that theize distribution was changed from a bimodal to a mono-dispersedormation with the finest average size was about 38 nm.

Fig. 12(a) and (b) show the XRD patterns and the correspondingEM images of hBN particles, respectively, before and after beadilling. The XRD pattern comparison shows that the crystal struc-

ure of hBN particles did not change after bead milling. However,he crystallite size (dc) of the hBN particles was reduced after bead

illing. The average crystallite sizes of hBN particles before andfter bead milling were calculated as 9.1 and 6.7 nm from the peakf (0 0 2) reflection using Sherrer’s equation, as follows:

c = 0.9�

cos �(2)

here dc is the crystallite size, � is the wavelength of the radiation, is the Bragg’s angle, and is the full width at half maximum. Thehange in crystallite size of the hBN particles may have been dueo the high impact energy of the beads during the milling process.ig. 12(b) shows an SEM picture of the obtained hBN particles beforend after bead milling from optimized processing parameters. Theesult shows that the morphology of hBN was changed from anrregular shape with a wide dispersed size distribution of around61 nm, to particles with relatively spherical in morphology and a

arrower sized distribution. The average size of hBN particles afteread mill was around 13.9 nm, with narrow deviation (ı = 3.4 nm).he distribution of particles after bead mill tends toward to a mono-ispersion. The hBN particle average size can also be determined

ing (a) and (b) SEM figures before and after bead milling.

in the form of average volume. However, hBN is a typical irregularshape material. Therefore, before and after bead milling particleis assumed as spherical particle for simplicity of calculation. Theparticles volume can be obtained using the following equation:

V = 43

�(

Dave

2

)3(3)

The average size in volume is about 2.47 × 10−2 m3 and1.41 × 10−6 m3 for particle before and after bead milling, respec-tively.

4. Conclusions

A well-dispersed hBN nanoparticles suspension in a water phasewas obtained using a bead milling process with a rotational speedand a slurry flow rate of 4353 rpm and 10 kg/h, respectively. Thesurface alteration of hBN particles using the dispersing agent (3-acryl-oxypropyl) trimethoxysilane (APMS) at a dosage of 150%attained a suspension with high stability due to electrosteric sta-bilization. The hBN suspension was very stable with a low zetapotential (below −40 mV) at pH 7.7. The achievements in sizereduction did not significantly change the crystallinity of the hBNparticles, along with an enhancement in dispersion stability, theyqualify as desired properties for the potential applications of anhBN nanoparticle suspension.

Acknowledgments

The authors thank Dr. Eishi Tanabe from Western HiroshimaPrefecture Industrial Research Institute, for the TEM measurement.We also thank Mr. Riki Shibaki for his assistance with the exper-iment. This research was also supported by a grant-in-aid forScientific Research (A) (No. 22246099) sponsored by the Ministryof Education, Culture, Sports, Science and Technology of Japan.

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