(2007) 154–161www.elsevier.com/locate/powtec

Powder Technology 177

Rheological behavior of carbon nanotube-aluminananoparticle dispersion systems

Kathy Lu ⁎

Virginia Polytechnic Institute and State University, Materials Science and Engineering Department, 211B Holden Hall-M/C 0237, Blacksburg, VA 24061, USA

Received 9 December 2006; accepted 13 March 2007Available online 24 March 2007

Abstract

In this work, Al2O3 nanoparticle and CNT–Al2O3 nanoparticle suspensions were studied. Both Al2O3 nanoparticle and CNT–Al2O3

nanoparticle systems exhibit shear-thinning behavior. The viscosities increase monotonically with the suspension solids loading. For the 40 vol.%solids loading suspension, CNT effect on the viscosity is not substantial until the content is ≥1.3 vol.%. The suspension yield stress to flowprovides a measure of the particle–particle networking in the suspension. With the adsorbed poly(acrylic acid) (PAA) layer on the particle surface,substantial colloidal interactions are observed when the solids loading is N35 vol.% and the CNT content is N1.3 vol.%. Storage modulus and lossmodulus can be used to understand the relative magnitude of the viscoplastic behavior and the elastic behavior of the suspension as well as thetransition between the two. The relative magnitude of the dynamic modulii is a strong function of the solids loading and the CNT content.© 2007 Elsevier B.V. All rights reserved.

Keywords: Suspension; Alumina; Carbon nanotube; Nanocomposites; Rheology

1. Introduction

As the size of a structure is reduced to the nanoscale, theamount of species that constitutes the surfaces becomescomparable to or even surpasses the amount of the species inthe bulk. This size reduction inevitably creates a huge amount ofsurface area (dozens or hundreds of m2/g) that has to be properlyhandled; otherwise the nanoparticles will form an agglomeratedpowder. To obtain uniform and controllable microstructure forthe solid component from the nanoscale species, colloidalprocessing is often the only viable approach. The surface of theindividual nanoparticles can bemodified in a dispersing mediumfor the nanoparticles to be well dispersed. CNTs mostly exist asan entangled bundle and special efforts have to be taken toseparate the bundle. With the intent of effectively dispersingnanoparticles and CNT–nanoparticle mixtures, rheology studyof the nanoparticle and CNT–nanoparticle co-dispersionsystems with high solids loading can provide importantunderstanding for the nano–species interaction.

When dispersing particles in a suspension, there are threestabilization mechanisms: electrostatic stabilization, steric

⁎ Tel.: +1 540 231 3225; fax: +1 540 231 8919.E-mail address: klu@vt.edu.

0032-5910/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.powtec.2007.03.036

stabilization, and electrosteric stabilization. Electrosteric stabi-lization is a more effective approach for dispersion thanelectrostatic or steric stabilization only. If a system is underelectrosteric stabilization, a delicate balance must be maintainedin the amount of polymer dispersant and the pH of the system.The adsorbed polymer must be thick enough to prevent closecontact and counteract van der Waal's forces [1]. Too littlepolymer will cause bridging flocculation [2,3]. Too muchpolymer will cause depletion flocculation [4,5]. Ideally, theadsorbed polymer layer should be just thick enough to preventflocculation. However, the ideal polymer content and size arespecifically related to the size and surface characteristics of thenanoparticles. Rheology of concentrated colloidal suspensionsis extremely sensitive to interparticle forces and characteristicsof the colloidal particle surfaces. Consequently, it can be used asa valuable tool for characterizing the interactions betweencolloidal nanoparticles. Integrating experimental measurementsof these interparticle forces into theoretical models extends ourability in developing predictive relations for the rheologicalbehaviors of concentrated dispersions [6].

Our work for Al2O3 nanoparticle suspensions showed thatrheology is a valuable tool for probing polymer dispersant andsolids loading effect [7,8]. Higher shear stress and viscositywere observed for poly(methyl acrylic acid) dispersed Al2O3

Fig. 1. Al2O3 nanoparticle size distribution on the number-based measurement.

Fig. 2. SEM image of the CNTs used in the study.

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nanoparticle suspensions at all shear rates when compared to thepoly(acrylic acid) (PAA) dispersed Al2O3 nanoparticle suspen-sions. Shear stress and viscosity exhibited much faster increasewith shear rate at high solids loading. Tseng and Lininvestigated rheological behavior and suspension structure ofTiO2 nanoparticles dispersed in water from 5 to 12 vol.% [9].The nanoparticle suspensions exhibited a pseudoplastic flowbehavior, indicating the existence of particle aggregations in thesuspension. The suspension viscosities revealed a pronouncedincrease in particle interactions as the solids loading wasincreased. For better understanding of high solids loadingsuspensions, a hard-sphere model suspension was studied [10].Capillary viscometry showed that the suspension viscosity atlow concentration agreed well with theory and other experi-mental work on hard-sphere systems. At higher solids loading,the rheological properties, measured using steady shear,oscillatory shear, and creep techniques, changed rapidly fromviscous Newtonian to shear-thinning viscoelastic. When thesolids loading exceeded the threshold solids loading, an elasticsolid behavior was observed and a yield stress could bemeasured. Dynamic modulii of fumed silica suspensions inaqueous solutions of poly(ethylene oxide)–poly(propyleneoxide)–poly(ethylene oxide) block copolymers and poly(ethyl-ene oxide) homopolymers were measured as a function of thedispersant surface coverage. The dynamic modulii werestrongly related to the stability of the silica suspensions andthe block copolymer [11]. Rheological measurement of aqueousCNT suspensions showed that at CNT contents b0.5 vol.%, thesystem behaved analogously to a dilute polymer solution. AtN0.5 vol.% CNT contents, the viscosity of the CNT dispersiondramatically increased. At 4.0 vol.%, the system was able tosupport its own weight as a freestanding gel [12]. In the light ofthe great potential of CNT–Al2O3 for conductive ceramics andlightweight materials, as well as the very different aspect ratiosof CNTs and Al2O3 nanoparticles, the CNT–Al2O3 co-dispersion represents a system of fundamental and applicationinterests in rheology study.

This study starts with the pure Al2O3 nanoparticle system.Based on the rheological understanding for the Al2O3

nanoparticles, the CNT–Al2O3 nanoparticle co-dispersion

system is examined. To sufficiently describe a dispersionsystem, several rheological parameters have been used. Amongthese are viscosity, yield stress, storage modulus, and shearmodulus. Examination of these factors provides the suspensioncharacteristics, the deformation tendency, the interaction of thedispersed species, and the 3D network formation of thesuspension.

2. Experimental procedure

Al2O3 nanoparticles with average particle size of 27.5 nmand specific surface area of 45 m2/g were used in this study(Nanophase Technologies, Romeoville, IL). The particle sizedistribution measurement from dynamic light scattering isshown in Fig. 1 (Zetasizer Nano ZS, Malvern Instruments, Inc,Southborough, MA). The CNTs used were produced bychemical vapor deposition and briefly acid-treated for purifica-tion (Helix Material Solutions, Richardson, TX). These aremulti-walled CNTs with 10–30 nm diameter, 0.5–40 μmlength, N95% purity, and 40–300 m2/g specific surface area. ASEM image of the CNTs is shown in Fig. 2.

For the preparation of both the Al2O3 and the CNT–Al2O3

suspensions, PAA (MW 1800, Aldrich, St Louis, MO) was usedas a polymer dispersant with the segment as [–CH2CH(CO2H)–]. Glycerol of 10 wt.% (C3H8O3, water basis, FisherChemicals, Fairlawn, NJ) was mixed with water and the mixturewas homogenized for 5 min using a ball mill. The addition ofglycerol was for forming and has been described elsewhere [8].Al2O3 nanoparticles were added for a specific solids loading in10 g increments along with an appropriate amount of PAAdispersant. Since low pH promotes PAA dispersant adsorptiononto the Al2O3 nanoparticles, HCl solution was added to lowerthe pH to 1.5 [13]. The suspension was ball milled for 12 h withperiodic adjustment of pH to 1.5. This procedure makessuspensions of approximately 20 vol.% Al2O3 solids loading.Depending on the final solids loading desired, Al2O3 nano-particles were added again in 10 g increments, along with anappropriate amount of PAA dispersant. The suspension wasthen mixed for 24 h for complete homogenization. NH4OH was

Fig. 4. Viscosity change vs. Al2O3 solids loading at different shear rates for thepure Al2O3 nanoparticle suspensions.

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used to adjust the suspension to pH 9.5. This produced Al2O3

nanoparticle suspensions of −40 mV zeta-potential. For theCNT–Al2O3 nanoparticle suspensions, CNTs were weighedand added in 100 mg increments into the 20 vol.% Al2O3

suspension at pH 9.5. The CNTs had −30 mV zeta-potential atpH 9.5 and should disperse from the Al2O3 nanoparticles whichalso had negative zeta-potential. After each CNT addition, thesuspension was homogenized by ball milling for 1 h beforeAl2O3 nanoparticles were added in 10 g increments, along withan appropriate amount of PAA and the adjustment of pH to 9.5,similar to what was carried out for the pure Al2O3 nanoparticlesuspensions. By this method, 40 vol.% solids loading CNT–Al2O3 co-dispersions of 0, 0.14, 0.27, 0.5, 1.3, and 2.6 vol.%CNT contents were prepared.

For the suspension characterization, the pH of the suspen-sions was measured by a pH meter (Denver Instrument, Arvada,CO). The viscosities of the suspensions were measured by arheometer with cone-plate geometry (AR 2000, TA Instruments,New Castle, DE). All the viscosity measurements wereperformed with controlled shear rate in a decreasing shear rateorder from 200 s−1 to 10 s−1. For the storage modulus and lossmodulus measurements, the Al2O3 suspensions at differentsolids loadings were pre-sheared at a stress of 0.6 Pa for 60 sand then measured from 6.0 to 63.0 rd/s angular frequency; theCNT–Al2O3 nanoparticle suspensions were measured in thesame angular frequency range but were pre-sheared from 30 s−1

to 200 s−1 and then measured from 200 s−1 to 30 s−1. All themodulus measurements were performed with strain-controlledoscillatory frequency sweep tests.

3. Results and discussion

3.1. Al2O3 nanoparticle dispersion study

For the Al2O3 nanoparticle dispersion system, the rheologyfor the 20–45 vol.% solids loading suspension was studied.Figs. 3 and 4 show that the suspensions have reasonably lowviscosity and the viscosity value is a strong function of the Al2O3

solids loading. With the solids loading increase, the viscosity

Fig. 3. Viscosity change vs. shear rate for different Al2O3 solids loadingsuspensions. Each data point is an average of three measurements and thestandard deviation is much smaller than the markers and cannot be shown on thefigure.

increases at all shear rates. Approximately, the suspensionviscosity increases by two orders of magnitude from 20 to45 vol.% solids loading. As the shear rate increases, the viscositydecreases monotonically. The Al2O3 nanoparticle suspensiondoes not obey Newtonian flow. Instead, the suspension displaysa shear-thinning behavior, reflecting the broken link between thenanoparticles with increasing shear. At 20 vol.% solids loading,the viscosity changes from 0.03 to 0.02 Pa·s when the shear rateis changed from 30 to 200 s−1. At 45 vol.% solids loading, theviscosity changes from 4.35 to 1.60 Pa·s when the shear rate ischanged from 30 to 200 s−1. To show more clearly the solidsloading impact on the suspension viscosity, Fig. 4 shows theviscosity change with the Al2O3 nanoparticle solids loadingunder three specific shear rates: 52 s−1, 94 s−1, and 200 s−1. Asthe Al2O3 solids loading increases, the viscosity increases; butmore importantly the viscosity difference becomes much largerat high solids loading levels. This indicates that the suspensionexperiences more drastic particle–particle interaction and thatthe dispersion conditions should be more rigorously controlledand optimized at high solids loading conditions.

As shown in Fig. 3, the suspensions exhibit differentiallynonlinear behavior with shear rate increase. Even though manysuspensions have been described byNewtonian flow at low solidsloading conditions and by pseudoplastic flow at high solidsloading conditions, it is desirable to analyze the Al2O3 suspensionbehavior by more exact flow models. To characterize the flowcharacteristics of the Al2O3 nanoparticle suspensions, differentflow models have been examined, including Herschel–Bulkleymodel, Carreau model, Casson model, power law model, andSisko model. Among all the models examined, the Herschel–Bulkley model describes the suspension behavior the best:

s ¼ aþ bgc ð1Þ

τ is the shear stress that the suspension experiences under shearrate γ; a, b, and c are system-dependent constants. Fig. 5 showsthe excellent fit of the Herschel–Bulkley model for the Al2O3

nanoparticle suspensions at different solids loadings. A closerexamination of Eq. (1) shows that the Herschel–Bulkley modelrepresents both Newtonian flow and Bingham flow behaviors,when a is zero and c is unity for the first case and when c is unityfor the second case. For the Al2O3 nanoparticle suspension, c is0.71–0.77, meaning slower shear stress increase vs. shear ratethan the Newtonian flow. Also, at high solids loading the Al2O3

Fig. 5. Curve fitting of the experimental shear stress–shear rate data with theHerschel–Bulkley flow model at different Al2O3 solids loadings.

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nanoparticle suspensions exhibit non-zero yield stress a, meaningthe suspension has a certain pseudoplastic flow behavior.

The yield stress of a suspension represents the shear stressthreshold that breaks the connection between the particles andinitiates motion in the dispersion. For the studied Al2O3

nanoparticle suspension, the particle surfaces are covered bythe adsorbed PAA dispersant. Particle–particle interaction isdominated by the polymer chain interactions on the particlesurfaces. The separation of the particles is more accurately theunreveling of the PAA chains that are attached to the particles orare intertwined with each other. The yield stress can be extractedat zero shear rate as shown in Fig. 6. As it shows, the yield stressis a strong function of the Al2O3 solids loading and showsexponential increase with the suspension solids loading. At lessthan 35 vol.% solids loading, the yield stress is small (less than4.5 Pa); the suspension flows readily and the particle–particleinteraction is weak or negligible. This means the polymer chainsdo not significantly overlap or inter-penetrate into each otherin the suspension. As the solids loading increases from 35 to45 vol.%, the yield stress increases substantially. At 45 vol.%solids loading, the yield stress increases to more than 60 Pa,

Fig. 6. The correlation of the suspension yield stress vs. solids loading for theAl2O3 suspensions. The values were extrapolated from Fig. 5 at zero shear rate.

indicating that the polymer chains are strongly interfering witheach other during flow. The strong interaction mainly comesfrom the particle population increase and the shorter particle–particle separation distance which results in close PAA contacton the Al2O3 particle surfaces.

Since the high solids loading Al2O3 nanoparticle suspensionsdisplay viscoelastic behavior as shown in Figs. 3 and 5, dynamicrheological measurements are used to examine the relativecontribution of the elastic behavior and the viscoplastic behaviorat different solids loadings [14]. During the measurement, anangular frequency-dependent shear strain is applied and the shearstress response is examined. Based on the shear stress–shearstrain correlation at different angular frequencies, twomodulii canbe obtained:

s ¼ g½G VðxÞ þ iGWðxÞ� ð2Þω is angular frequency. G′ is the storage modulus describing theelastic nature of the nanoparticle suspension. G″ is the lossmodulus describing the viscoplastic nature of the nanoparticlesuspension. When the angular frequency ω approaches zero, G′represents the energy storage capability of a suspension under“undisturbed conditions”. The loss modulus G″ and the storagemodulus G′ for the Al2O3 suspensions at 20 vol.% and 40 vol.%solids loadings are shown in Fig. 7. In Fig. 7(a), both the lossmodulus G″ and the storage modulus G′ are relatively low. Also,there is a crossover of G′ and G″ at 50 rd/s angular frequency.This crossover point signals an important transition for thesuspension. At the angular frequency lower than the crossoverpoint 50 rd/s, the suspension is more viscoplastic. At the angularfrequency higher than the crossover point 50 rd/s, the suspension

Fig. 7. Loss modulus and storage modulus change vs. angular frequency for theAl2O3 nanoparticle suspensions.

Fig. 8. Viscosity vs. shear rate for different CNT vol.% suspensions at 40.0 vol.%CNT–Al2O3 solids loading. Each curve is an average of three measurements andthe standard deviation for lower CNT contents is much smaller than the markerand cannot be shown on the figure.

Fig. 9. Viscosity vs. CNT vol.% at different shear rates for the 40.0 vol.% CNT–Al2O3 solids loading suspensions.

Fig. 10. Curve fitting of the Herschel–Bulkley flowmodel at different CNT vol.%for the CNT–Al2O3 suspensions at 40.0 vol.% solids loading.

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is more elastic. At very low angular frequency ω (around 6 rd/s),G′ approaches zero, indicating that the suspension stores noelastic energy when the suspension experiences low shear strain.The loss modulus G″ also becomes negligible at angularfrequency close to zero, which means the suspension has littleviscoplastic behavior under static conditions. When the suspen-sion solids loading increases to 40 vol.%, bothG′ andG″ increasemore than two orders of magnitude, from ∼1 to ∼180 Pa. Moreimportantly, the loss modulus G″ is lower than the storagemodulusG′ at all angular frequencies. This means the suspensionis more elastic and gel-like and the solids loading increase hasprovided to the suspension a higher elastic energy storage ability.If G′ is extrapolated to zero angular frequency ω, there will be apositive G′ of 150 Pa. This means the suspension stores asubstantial amount of elastic energy when the shear ceases. Sincethe Al2O3 nanoparticles are covered with PAA dispersant, theelastic energy storage is very likely accomplished by the PAAchain interaction between the Al2O3 nanoparticles. Also, theAl2O3 nanoparticle suspension shows approximately 120 Pa lossmodulus at zero angular frequency. This means the 40 vol.%solids loading suspension experiences fundamental structuralreorganization during the oscillatory test and that the structuralchange will not restore after the oscillatory shear.

3.2. CNT–Al2O3 nanoparticle co-dispersion study

As shown in Fig. 2, the CNTs have large aspect ratio with theouter diameter at 10–20 nm range and the length in the 0.5–40μmrange. While the Al2O3 nanoparticles are equiaxed with 27.5 nmaverage particle size, the large aspect ratio of the CNTs will playan important role in the rheology of the CNT–Al2O3 co-dispersion. Similar to the PAAchains thatmay inter-penetrate into

or entangle with each other, the CNTs will behave similarly but ata much larger scale due to their relatively large dimensionscompared to PAA. The CNT–Al2O3 suspension viscosity changevs. shear rate at different CNT contents is shown in Fig. 8 for the40 vol.% solids loading co-dispersion. The viscosity for the zeroand the highest CNT content (2.6 vol.%) suspensions is alsoshown, which is an order of magnitude lower than that of the40 vol.% pure Al2O3 suspension and should not play anysignificant role in affecting the co-dispersion viscosity by itself.Compared to the 40 vol.% pure Al2O3 suspension in Fig. 3, theCNT–Al2O3 suspension viscosity shows no substantial increaseuntil the CNTcontent is greater than 1.3 vol.%. This means CNTsdo not interact with each other significantly until the CNTcontentis above a threshold value such as 1.3 vol.%. When the CNTcontent is increased to 2.6 vol.%, almost an order of magnitude ofviscosity increase is observed. Also, all the CNT–Al2O3 co-dispersions show shear-thinning behavior as was observed for thepureAl2O3 nanoparticle suspension. To examine the CNTcontenteffect more closely, the viscosity change can be re-plotted as afunction of the CNT vol.% under three specific shear rates, 52, 94,and 200 s−1, as shown in Fig. 9. Again, the CNT–Al2O3 co-dispersions show viscosity decrease with increasing shear rate;

Fig. 11. Suspension yield stress vs. CNT vol.% for the CNT–Al2O3 suspensionsof 40 vol.% solids loading. The values are extrapolated from Fig. 10 at zeroshear rate with additional data incorporated at lower CNT contents.

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more importantly, the viscosity drastically increases with CNTcontent starting at 1.3 vol.%.

Similar to the Al2O3 nanoparticle suspensions, differentrheological models have been examined for the CNT–Al2O3 co-dispersions to identify the most suitable models for the system.Again, the Herschel–Bulkley model is the best fit for the studiedsuspensions as shown in Fig. 10. One important feature for theCNT–Al2O3 suspension is that the shear stress–shear rate curvesshow no substantial difference at b1.3 vol.% CNT content. Theshear stress–shear rate curve at 2.6 vol.% CNT content is muchhigher than all those at lower CNT content conditions. The cvalue in Eq. (1) changes from 0.707 to 0.521 as the CNTcontentchanges from 0 to 2.6 vol.%, which indicates a large deviationfrom Newtonian flow than for the pure Al2O3 nanoparticle

Fig. 12. Loss modulus and storage modulus change vs. angular frequency fo

suspension. Also, the scatter of the shear stress data is larger forthe CNT–Al2O3 co-dispersion at 2.6 vol.% CNT content. Thislarger data scatter for the CNT–Al2O3 co-dispersion has alsobeen observed for the dynamic modulii to be discussed later.Fundamentally, this could mean that the CNTs present moreinhomogeneity than the PAA or Al2O3 species at a local scale.

The yield stress of the CNT–Al2O3 co-dispersions at zeroshear rate is shown in Fig. 11. The CNT–Al2O3 co-dispersionshave non-zero yield stress at all CNT contents. This is expectedwhen compared to Fig. 6 at different Al2O3 solids loadings. Forthe pure Al2O3 nanoparticle suspension, this means theadsorbed PAA layers on the Al2O3 nanoparticle surfacesinteract with each other. When a low amount of CNTs isadded into the system, the yield stress shows no substantialchange since the CNTs are far apart and may not be in contactwith each other. When the CNT content increases to 1.3 vol.%,the yield stress starts to increase; the suspension flow ishindered not only by the PAA chain interaction but also by theCNT–PAA chain and more importantly by the CNT–CNTinteraction. Substantial shear stress is needed to initiate themovement of the species in the co-dispersion. At 2.6 vol.%CNTs content, the yield stress increases to as high as 100 Pa.The higher the CNT vol.%, the higher the yield stress. The CNTcontent of 1.3 vol.% seems to be a critical concentration forsubstantial suspension rheology change.

To understand the CNT content effect on the storage modulusG′ and the loss modulusG″, the variation of the dynamic moduliivs. angular frequencyω is shown in Fig. 12. From Fig. 12(a)–(d),the CNT content changes from 0 vol.%, 0.14 vol.%, 1.3 vol.%,

r the 40 vol.% solids loading CNT–Al2O3 co-dispersion with pre-shear.

Fig. 13. Particle–particle interaction in the suspensions of different Al2O3 solids loading.

Fig. 14. Particle–particle and CNT–particle interactions in the co-dispersions.

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to 2.6 vol.%. Two observations can be made by comparing thefigures. One observation is that the crossover point between G′andG″ consistently shifts to lower angular frequency as the CNTcontent increases. Another observation is that G′ and G″ remainapproximately the same until the CNT content increases to1.3 vol.%. At this point, the suspension shows 200% G′ and G″increases. Both observations can be explained by the CNTinteractionwith the PAAchains and among themselves.When theCNTcontent is low, the CNTmovement is relatively easy and thealignment of the CNTs in the shear direction is relatively quick.The liquid-like to gel-like transition does not occur until theangular frequency is high.When the CNTcontent is increased, theCNT alignment requires longer time under the same angularfrequency. With increasing shear strain, the liquid-like to gel-liketransition occurs at lower angular frequency. Additionally, thestorage modulus G′ is higher than the loss modulus G″ at allangular frequencies examinedwhen the CNTcontent is at 1.3 vol.% or higher. Fig. 12 also indicates that theG′–G″ crossover pointis a more sensitive measurement of the suspension rheologicalproperties at low CNT contents.

3.3. Theoretical understanding

The interaction of the dispersed species in a suspension andtheir response to external shear has been discussed in theliterature [12]. Depending on the dispersion, the interactingspecies can be broken into flocs or primary particles. In thestudied Al2O3 and CNT–Al2O3 suspensions, monotonic shearstress increase is observed at all shear rates. This means thesuspension is well dispersed and there are no flocs or otheraggregates. Otherwise, a shear stress decrease should beobserved first when the flocs are broken apart, followed by ashear stress increase.

The interactions in the Al2O3 nanoparticle suspension can berepresented by Fig. 13 at different solids loadings. Since thenanoparticle surfaces are covered with polymer dispersant PAA,the effective solids loading is higher than the Al2O3 solidsloading and the particle–particle separation distance will also becorrespondingly smaller. At low solids loading, the particles arewell separated and there is no overlap of the stabilizing PAAlayers. When the suspension is sheared, there is no need to break

the link between the particles and as a result the suspension haslow viscosity. When the suspension solids loading is increased,the adsorbed PAA layers begin to be in contact. As a result, theviscosity increases when the suspension is sheared. This is alsoaccompanied by the appearance of the yield stress. Dependingon the shear angular frequency, the suspension can show moreelastic behavior than viscoplastic behavior. When the solidsloading is increased further, the adsorbed PAA layer begins tointer-penetrate into each other and substantial shear is needed toseparate the inter-penetrating PAA layers. At this point, theparticle motion is highly constrained and the viscosity is veryhigh. Substantial yield stress results from the motion of the solidspecies in the suspension. The suspension demonstrates moregel-like behavior at all shear angular frequencies.

For the CNT–Al2O3 co-dispersion, the interaction among thespecies follows the similar trend as the nanoparticle onlysuspension with the CNTs presenting an additional barrier forflow. Since the CNTs have much larger diameter and lengththan the PAA polymer chain, the CNT presence alsosubstantially accelerates the liquid-like to gel-like transitionby interacting with Al2O3 nanoparticles and among themselves(Fig. 14). When the CNT content is low, the impact is lesssignificant. When the CNTcontent is high, the interacting CNTshave to align themselves under the applied shear stress with thehindrance of the surrounding Al2O3 particles. The movementunder the shear stress becomes especially difficult. As a result,

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high viscosity, yield stress, and dynamic modulii are observedfor the CNT–Al2O3 co-dispersion systems.

4. Summary

For the studied Al2O3 nanoparticle and CNT–Al2O3

nanoparticle suspensions, the viscosity measurements demon-strate shear-thinning behavior with increasing shear rate. Thehigher the solids loading or the higher the CNT content, thehigher the viscosity and the more shear-thinning the suspen-sions are. The Al2O3 nanoparticle suspension does not showmeasurable yield stress until the solids loading is N35 vol.%.For the 40 vol.% solids loading CNT–Al2O3 suspension, theCNTs cause no substantial yield stress increase until the contentis N1.3 vol.%. The storage modulus and loss modulusmeasurements indicate the liquid-like to gel-like transition forboth kinds of suspensions, either at high solids loading or athigh CNT content of a fixed solids loading. The storagemodulus G′ and the loss modulus G″ can be directly comparedto understand the suspension rheological behaviors. G′–G″crossover point serves as a good indicator for the 3D networkformation.

Acknowledgements

This work was supported by Oak Ridge AssociatedUniversities and Petroleum Research Fund.

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