freeze casting as a nanoparticle material-forming method

Freeze Casting as a Nanoparticle Material-FormingMethod

Kathy Lu*,w and Xiaojing Zhu*

Materials Science and Engineering Department, Virginia Polytechnic Institute and State University,Blacksburg, Virginia 24061

Nanoparticle material forming is challenging because of loose packing and agglomeration issues intrinsic to nanoparticles.Liquid processing shows great potential to overcome such hurdles. This study is focused on nanoparticle colloidal processingand freeze-casting forming. Al2O3 nanoparticle suspensions are examined, and microstructure evolution of Al2O3 nanoparticlesuspension during freeze casting is discussed. The ‘‘Fines’’ effect influences nanoparticle packing on freeze-cast sample surfaces.Trapped air bubbles in the suspension lead to a porous bulk microstructure. Prerest is necessary for dense and homogeneousgreen microstructure formation. The green strength, fracture mode, and ability to form fine features by freeze casting are alsoevaluated.

Introduction

The huge surface area intrinsic to nanoparticlesserves as one of the most striking advantages as wellas disadvantages for nanoparticle material forming.Because of the natural tendency of nanoparticles informing agglomerates, wet forming utilizing a colloidalsuspension has become the most active research areain nanoceramics. For almost all the nanoparticle wetforming processes, the first step is to produce a stablecolloidal suspension. A green body with a uniform

microstructure can then be produced from the fullystabilized colloidal suspension of nanoparticles.

Freeze casting is a process that pours the suspensioninto a nonporous mold, freezes the suspension, demoldsthe frozen sample, and then dries the sample undervacuum. Liquid-state to solid-state conversion is realizedthrough phase transformation of the dispersing mediumsuch as water. The process has the potential to form nearnet-shape complex geometry parts with low pressureand often environmentally benign advantages.1,2 Thekey requirements are that the suspension is stable andunagglomerated.3 When the freeze-casting condition isproperly controlled, water separates from solid phasesthrough sublimation and no capillary force exists tocause hard agglomerates or cracks.4

Freeze casting has been practiced for some-time.1,2,5,6 However, the studies were mainly focusedon micrometer-sized particles and large-sized samples.For example, Al2O3 particles of 0.4 mm were freeze cast

Int. J. Appl. Ceram. Technol., 5 [3] 219–227 (2008)DOI:10.1111/j.1744-7402.2008.02204.x

Ceramic Product Development and Commercialization

Supported by Petroleum Research Fund, administered through American Chemical

Society.

Presented at the 31st International Conference on Advanced Ceramics and Composites,

Daytona Beach, FL, January 21–26, 2007.

*Member, The American Ceramic [email protected]

r 2008 The American Ceramic Society

and 498% sintered density was achieved.7 Enclosedshells of Al2O3 bodies encapsulating steel parts werefabricated.8 Porous and layered-hybrid materials werefreeze cast.9 However, application of freeze castingto nanoparticle systems and understanding the freeze-casting behaviors of nanoparticle suspensions have notbeen explored. One challenge is that nanoparticleresearch in forming bulk components is still evolving;considerable effort is still needed in achieving high sol-ids loading suspensions. The other challenge is the dras-tic capillary pressure increase with decreasing particlesize; it takes a prolonged duration of time and muchimproved control to remove water completely by sub-limation. Our past work has addressed the first chal-lenge to a certain extent.10–12 To address the secondchallenge successfully, it is essential to understand thenanoparticle microstructure evolution from the colloi-dal state to the freeze-dried state.

High particle packing efficiency is preferred forpostfreeze casting, handling, and densification purpos-es. Freeze-cast sample strength and failure mode can beused to understand and evaluate this aspect. With thedecrease of particle size to nanoscale, the feature sizesof nanoparticle samples can also be correspondinglydecreased. The potential in forming fine features byfreeze casting should be explored.

This study is focused on Al2O3 nanoparticle sus-pension viscosity evaluation and the correspondingfreeze casting process for solid sample formation.Microstructural evolution of nanoparticle suspensionsfrom a colloidal state to a freeze-dried state is examined.The surface and bulk microstructures under differentprerest conditions are compared. Special nanoparticlephenomena during the suspension to solid-state trans-formation are explained. Equibiaxial strength and frac-ture mode of the freeze-cast samples are examined. Finefeatures produced by the freeze-casting process arepresented.

Experimental Procedure

Al2O3 dry nanoparticles with a specific surface areaof 45 m2/g were used in this study (Nanophase Tech-nologies, Romeoville, IL). The particles can be redis-persed in water by ball milling as reported before bytransmission electron microscopy (TEM) analysis.10 Inthis study, the particle size distribution was furthermeasured by a dynamic light-scattering measurement

as shown in Fig. 1 (Zetasizer Nano ZS, MalvernInstruments Inc., Southborough, MA). Agglomeratesare absent. However, Al2O3 nanoparticles are polydis-persed, consistent with the TEM analysis. Even thoughon the weight basis there seems to be B10 wt% of4100 nm particles, on the number basis, large particlesare negligible (o0.5%). For the Al2O3 nanoparticlesuspension preparation, poly(acrylic acid) (PAA, MW

1800, Aldrich, St. Louis, MO) was used as the dispers-ant with the segment as [–CH2CH(CO2H)–] and glyc-erol (C3H8O3, Fisher Chemicals, Fairlawn, NJ) wasused as the freeze casting aid with the molecular formulaCH2OH–CHOH–CH2OH. A water–glycerol mixtureat a weight ratio of 10:1 (water:glycerol) was used as thedispersing medium. The mixture was homogenized for5 min using a ball mill (755 RMV, US Stoneware, EastPalestine, OH) before use, and the ball mill rotationspeed was 75 rpm. Al2O3 nanoparticles were added tothe dispersing medium in 10 g increments along with aPAA dispersant at 2.0 wt% of Al2O3. Because low pH

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Fig. 1. Al2O3 nanoparticle size distribution from dynamic lightscattering analysis: (a) weight basis, (b) number basis.

220 International Journal of Applied Ceramic Technology—Lu and Zhu Vol. 5, No. 3, 2008

promotes PAA dispersant adsorption onto Al2O3 nano-particles,13 HCl solution was added to lower the pHto 1.5. The suspension was ball milled for 12 h withperiodic adjustment of pH to 1.5. The purpose of theball milling was to break the soft agglomerates of the dryAl2O3 nanoparticles and thoroughly homogenize water,glycerol, PAA, and Al2O3 nanoparticles. The millingmedia were Al2O3 rods with 6 mm diameter and 6 mmheight. Breaking down of Al2O3 nanoparticles intosmaller sizes was believed to be absent because of thelow energy that ball milling provided. However, airbubbles can be introduced into the suspension duringmixing. Suspensions of approximately 20 vol% Al2O3

solids loading were made by this procedure. After thisstep, Al2O3 nanoparticles were again added in 10 gincrements, along with a PAA dispersant at 2.0 wt%of Al2O3 in order to make a 40 vol% solids loadingsuspension. The final PAA amount was thus maintainedat 2.0 wt% of Al2O3. NH4OH (28–30 wt%, VWRInternational, West Chester, PA) was used to adjustthe suspension to pH 9.5 in order to achieve the highestsuspension flowability for freeze casting.10 It should bepointed out that the liquid volume of NH4OH andHCl was considered for the 40 vol% solids loadingreported. The suspension was then mixed for 24 h forcomplete homogenization.

Suspension viscosities without and with glycerolwere characterized using a rheometer with a cone-plategeometry (AR 2000, TA Instruments, New Castle, DE).All the viscosity measurements were performed with acontrolled shear rate in a decreasing shear rate order.Freeze-casting molds were developed using poly(dime-thylsiloxane) epoxy (RTV 664, General Electric Com-pany, Waterford, NY). An Al2O3 suspension was filledinto the mold immediately after the suspension prepa-ration. Some of the filled molds were kept under am-bient conditions for 1 h; other filled molds were frozen ina freeze dryer (Labconco Stoppering Tray Dryer, Lab-conco, Kansas City, MO) immediately. This 1-h restingperiod after the mold filling but before the freeze-castingstep is termed as prerest in the following discussion.During the freezing process, the suspension-filled moldswere kept at �351C for 2 h. Then, the samples weredemolded and exposed to vacuum (o1� 104 Pa) toallow ice to sublimate for 36 h. The microstructuraldifferences in the freeze-cast samples with and withoutprerest were systematically analyzed using a LEO550 fieldemission scanning electron microscope (SEM) (CarlZeiss MicroImaging Inc., Thornwood, NY).

For the freeze-dried sample green strength mea-surement, circular disks 25.4 mm in diameter and1.58 mm in thickness were freeze cast. The Al2O3

suspension was placed in the mold cavity using a dis-posable pipette. Care was taken to fill the molds com-pletely with the Al2O3 suspension. After the moldfilling, the samples were allowed to rest for 1 h underambient conditions. The fracture load was measuredusing a Texture Analyzer test console (Stable MicroSystems, Surrey, U.K.), equipped with a 5 kN load cell.The equibiaxial flexural strength was then calculated foreach sample, using the following equation:

sf ¼3F

2ph2

1� nð ÞD2S � D2

L

2D2

�

þ 1þ nð Þ ln DS

DL

� ð1Þ

where sf is the fracture strength in units of MPa, F isthe fracture load in units of N, h is the specimen thick-ness in units of mm, n is Poisson’s ratio, D is the samplediameter in mm, DS is the diameter of the support ringin mm, and DL is the diameter of the load ring in mm.

Results and Discussion

Al2O3 Nanoparticle Suspensions

In the previous study,11 it was identified that for theAl2O3 nanoparticle suspension studied, the PAAamount adsorbed onto Al2O3 particles increases withPAA concentration increase up to 2.00 wt% of Al2O3.As more PAA is added to the suspension, the PAAamount adsorbed reaches a plateau of 0.31 mg/m2. Theadsorption plateau represents the adsorption saturationat which the Al2O3 particle surfaces are fully covered bythe PAA dispersant. Also, the suspension has the lowestviscosity from pH 7.5 to 9.5; this pH range is the mostdesirable pH range for stabilizing the Al2O3 nanoparti-cles. Based on these results, the PAA concentration is2.00 wt% of Al2O3 and the pH is 9.5 for the Al2O3

nanoparticle suspensions in this study.Figure 2 shows the viscosities for the 20–45 vol%

Al2O3 solids loading suspensions. The viscosities are astrong function of Al2O3 nanoparticle solids loading.With an increase in Al2O3 solids loading, the viscosityincreases at all shear rates. Approximately, the suspen-sion viscosity increases by two orders of magnitude from20 to 45 vol% solids loading. As the shear rate increases,

www.ceramics.org/ACT Freeze Casting as Nanoparticle Material-Forming Method 221

the viscosity decreases monotonically but the Al2O3

nanoparticle suspension does not obey Newtonianflow. Instead, the suspension displays a shear-thinningbehavior, reflecting the broken polymeric link betweenthe nanoparticles provided by the PAA dispersant. At20 vol% solids loading, the viscosity changes from 0.03to 0.02 Pa s when the shear rate is changed from 30to 200 s�1. At 45 vol% solids loading, the viscositychanges from 4.35 to 1.60 Pa s when the shear rate ischanged from 30 to 200 s�1. As the Al2O3 solids load-ing increases, the viscosity difference becomes muchlarger at high solids loading levels. This indicates thatthe suspension undergoes a more drastic ‘‘link break-age’’ and that the suspension conditions should be con-trolled more rigorously and optimized under high solidsloading conditions.

There are two states for a polydispersed nanoparti-cle suspension like the Al2O3 suspension studied here:fluid and glass, depending on solids loading.14 Forfreeze casting, especially when forming parts with com-plex shapes, the suspension should be kept at reasonablyhigh solids loading levels. When the solids loading is toolow, the freeze-cast sample will have a very low particlepacking density and present a challenge for densificat-ion. However, when the solids loading is too high, thesuspension will be too viscous to flow into the fine fea-tures of the mold. Ideally, the solids loading should bechosen at the boundary of fluid and glass suspensions,slightly on the fluid-state side. Our prior study hasidentified that 45% solids loading is approximately nearthe fluid-state to glass-state boundary region.15 Based onthis consideration, the suspension of 40 vol% Al2O3

solids loading was used for the freeze-casting studiesreported here.

Prerest Effect

In the previous work,11 the critical role of prerest informing homogenous and crack-free freeze-cast sampleswas reported. However, the microstructural changeswere not understood. In this study, the prerest effectson the freeze-cast sample microstructure evolution areexamined from two aspects: surface microstructure andbulk microstructure.

The free surface microstructures of the freeze-castsamples without and with prerest are shown in Fig. 3.For the sample without prerest (Fig. 3a), the surface israther flat and the Al2O3 particles are densely packed.For the sample with prerest (Fig. 3b), the surface isrough and the particles are loosely packed. This surfacesmoothness difference can be observed indirectly from

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Fig. 2. Viscosity change versus shear rate for different Al2O3 solidsloading suspensions. Each curve is an average of three measurementsand the standard deviation is much smaller than the markers andcannot be shown on the figure.

(a)

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Fig. 3. Surface microstructure difference for the freeze-castsamples: (a) without prerest, (b) with prerest.


the surface luster difference. For the sample in Fig. 3a,the surface is shiny. For the sample in Fig. 3b, the sur-face is dull. The unique nature of the freeze castingprocess is able to capture this evolution that otherwisewould be missed unless in situ analysis is used.16

Figure 3 reveals phenomena that are in sharpcontrast with the well-established understanding at themicrometer particle size level. Even though the gravityeffect seems to be a convenient explanation, it can beeliminated because the suspension is stabilized to pre-vent sedimentation and the particle size is too small forgravity to play an essential role within the timeframestudied. Lue and Woodcock17 generalized this particleself-arrangement process as the ‘‘fines’’ effect andshowed that the large to small particle size ratio forthe ‘‘fines’’ effect to be effective is 10:1. The ‘‘fines’’effect refers to the natural tendency of the small particlesmigrating to fill the space between the large particlesunder Brownian motion when the suspension is keptundisturbed. From Fig. 1, the Al2O3 nanoparticlesuspension certainly meets this particle size criterion.During the undisturbed prerest time period, Brownianmotion causes nanoparticles to move within the sus-pension. When the small nanoparticles migrate to fillthe voids between the large nanoparticles, the processcauses the large particles on the suspension surface toopen up the network to accommodate the highly diffu-sive small particles. The net result is a more open andless smooth surface microstructure. This observationclearly indicates that Brownian motion is an importantphenomenon affecting nanoparticle material surfacemicrostructures. From the freeze-cast surface microstruc-ture comparison, it can be concluded that nanoparticlesize distribution plays an important role when the sus-pension is directly frozen, even if the large particles onlytake up a small percent of the overall distribution. Theparticle size distribution should be kept at the narrowestrange for suspension-related forming processes if a uni-form and consistent surface microstructure is desired.

The fracture surface microstructures of the freeze-cast Al2O3 samples are shown in Fig. 4. Figure 4a showsthe sample without prerest. Figure 4b shows the samplewith prerest. Both samples are formed under the samefreeze-casting conditions. For the sample without prerest(Fig. 4a), a porous bulk microstructure develops and thepores are B3mm in size through SEM image measure-ment of about 50 pores. For the sample with prerest, nomicrometer-sized pores are present as shown in Fig. 4b.For the Al2O3 nanoparticle suspension preparation,

a high shear process, ball milling, was used to homog-enize Al2O3 nanoparticles, dispersant PAA, glycerol,and water. A shear force was constantly present to breakthe nanoparticle suspension and encapsulate air bubbleswhile homogenizing the suspension. When the shearceases, the Al2O3 nanoparticle suspension should begiven time to reorganize for the air bubbles to migrateout of the suspension. However, it may take a long timeto eliminate the air bubbles completely for the highsolids loading suspensions due to the relatively highersuspension viscosity and more particle movement hin-drance. If the nanoparticle suspension is frozen beforesuch an equilibrium is reached, then observations areoften made for a metastable suspension with air bubblesdispersed in the suspension.

(a)

(b)

Fig. 4. Bulk microstructure difference for the freeze-cast samples:(a) without prerest, (b) with prerest.


Because the samples without and with prerest arefreeze cast under the same conditions, the freezing pro-cess of the water–glycerol dispersing medium and thesubsequent impact on the nanoparticles should not playa role in the microstructure difference observed. Morerevealing evidence can be found in Fig. 5. For the sam-ple without prerest, the pore surfaces are very smooth.The elongated pores show no fracture on some liga-ments (Fig. 5a). This means that these elongated poresare not dendrites, which frequently occur during anaqueous suspension freezing (phase transformation)process, even though the shape has some resemblanceto dendrites. The left edge of Fig. 5b shows the smoothpores at a high magnification. This work offers the firstevidence of the importance of microstructure evolution

before freeze casting. Figures 4 and 5 indicate that theporous structure is a consequence of lack of prerest; theair bubbles trapped in the suspension are the cause forthe micrometer-sized pores. After nanoparticle suspen-sion mixing and freeze-cast mold filling, enough timeshould be given for the trapped air to escape and for theAl2O3 nanoparticles to rebuild the 3D network. Themore open surface microstructure due to the ‘‘fines’’effect (Fig. 3) seems to be beneficial for the air bubblesto escape. In essence, this means that the surface andbulk microstructures evolve simultaneously to affect themicrostructure of the freeze-cast samples.

Equibiaxial Strength, Fracture Mode, and FineFeatures

The freeze-cast disk samples with prerest were frac-tured using the equibiaxial strength ring test, and thepeak load was recorded to determine the strength.18

Compared with the four-point bending test, the equibi-axial strength test has the advantage of isotropic testconditions, whereas the four-point bending test exertshigh stress in the direction perpendicular to its sup-ports.19 The equibiaxial strength test was also conceivedto eliminate the effect of edge (corner) defects thatmight be present for a four-point bending test speci-men.20 Accordingly, the equibiaxial flexural strengthtest should offer better evaluation of the freeze-cast sam-ples, especially in light of the brittle failure nature. Theequibiaxial flexural strength of the freeze-cast sampleswas calculated via Eq. (1). The average freeze-cast sam-ple strength is 0.3–1.6 MPa. It should be mentionedthat this green strength variation is related to the samplethickness. When the sample is thick, the green strengthtends to be lower. This is a surprising observation forthe freeze-cast sample because the green strength shouldbe dimension independent. Even though Poisson’s ration is not directly available and is assumed to be 0.22 inthis study, a handbook value for fully dense Al2O3,21 adeviation of B25% in the Poisson’s ratio results in onlyan B2% change in the equibiaxial flexural strength andthe calculation is made with this knowledge. We believethat the green strength variation is closely related to themicrostructure evolution discussed earlier. Completeremoval of air bubbles from the Al2O3 nanoparticlesuspension is essential to obtain consistent and high-strength freeze-cast samples. If air bubbles are notremoved completely, which is more likely for the thick-er samples, the resulting pores can act as defects and

Fig. 5. Smooth pore surfaces of freeze-cast sample without prerest:(a) elongated pores, (b) spherical pores.


cause low green strength. More problematically, thesedefects can cause inhomogeneous densification and per-manent flaws in the sintering process. It is possible thatan optimal prerest time exists for each sample thickness.Future efforts will be made to evaluate this aspect. Eventhough the green strength is low compared with that ofa fully dense sample, the result is very encouraging. Thefreeze-cast samples are fairly strong compared withgreen samples from other forming processes and canbe handled easily for sintering densification.

As to the fracture characteristics, the freeze-castAl2O3 samples conform to the medium energy fracturemode as shown in Fig. 6. The marker indicates theorigin of the cracks. It has been known that the greensample fracture strength and the fracture mode arecorrelated. The medium fracture mode means thatthe freeze-cast sample has desirable integrity and con-sumes a reasonable amount of energy during fracture. Ifthe samples are too weak, the fracture mode will changecorrespondingly to the low energy fracture mode.Nevertheless, the freeze-cast samples do not have highstrength, such as for sintered ceramic or glass samples. Itshould also be pointed out that the fracture edges of thefreeze-cast samples are very smooth. The fractured piec-es can be re-assembled easily into its original shape.

Nanoparticle materials have great potential in pro-ducing decreasing size features and parts because of thesize decrease in the basic constituent species—particles.However, a substantial challenge exists in producingwell-defined edges and dimensions.22 Freeze castingseems to be a good candidate in addressing this issueand forming complex shapes. This advantage is dem-onstrated by freeze casting of a dime sample as shown inFig. 7. Freeze casting has high fidelity in feature shapereplication. The fine features on the dime face and thesharp edges of the dime are precisely reproduced. Thisrepresents a very promising opportunity of formingcomplex-shaped nanoparticle materials with microme-ter-sized features.

To further explore the freeze-casting capability informing small-size features, the 40 vol% Al2O3 solidsloading suspension was freeze cast into small molds withmicrometer-sized features. As shown in Fig. 8a, themicrometer-sized features are precisely aligned with awell-defined shape and size. Figure 8b shows an evensmaller feature size with 100 mm bottom diameter ando40 mm height. It should be mentioned that this lengthbridges the macroscopic scale and the nanometer scaleand falls in the range of many component sizes in devicefabrication. Freeze-casting forming can be potentially

(a)

(b)

Fig. 6. Fracture modes: (a) ideal fracture modes, and (b) freeze-cast Al2O3 sample fracture images. Actual sample crack initiation point isindicated by a star.


used to form complex surface features for direct devicefabrication purposes.

To fully explore the potential of freeze casting informing components in the micrometer length scale,two issues need to be solved. One is creating dimen-sionally well-controlled molds for fine feature formationduring nanoparticle freeze casting. As shown in Fig. 8b,less than desired smoothness at the mold cavity bottomproduces micrometer-sized features with an uneven topsurface. Second, fine features tend to create stress, con-centrating at the feature surrounding areas. It requiresgreat care to control the freeze drying process to obtainintact samples. Even though the cracks shown in Fig. 8a

are not observed after freeze drying and is believed to befrom SEM sample preparation, freeze casting of intri-cate features is still at its infancy. Future work will becontinued to improve the freeze casting process forcomplex-shaped micrometer-sized sample forming.

Conclusions

Nanoparticle suspension and freeze-casting formingare interesting new areas in nanoparticle processing.This study focuses on an Al2O3 nanoparticle suspen-sion and the subsequent freeze casting processes. It showsthat suspension viscosities increase with Al2O3 nanopar-ticle solids loading. After the suspension is cast into amold, prerest should be given for the nanoparticles to

Fig. 7. The dime and the fine features produced by Al2O3

nanoparticle suspension freeze casting: (a) full dime, (b) lower leftcorner of the dime face.

Fig. 8. Micrometer-sized features produced by freeze casting ofAl2O3 nanoparticle suspension: (a) aligned features, (b) individualfeature.


rearrange. Otherwise, the air bubbles will be trapped inthe bulk and evolve into pores after freeze casting. The‘‘fines’’ effect was found to be beneficial for the surfacenanoparticle packing to open up and for the trapped airto escape from the Al2O3 nanoparticle suspension.Dense nanostructures can be obtained when the freezecasting conditions are controlled properly. The freeze-cast samples have good green strength and show amedium energy fracture mode during the equibiaxialstrength testing. The freeze-casting process itself showsto be a promising approach in forming complex shapeswith fine features.

Acknowledgments

The authors thank Prof. Brian Love for help withthe equipment use to conduct the equibiaxial flexuralstrength testing.

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freeze casting as a nanoparticle material-forming method

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