Increasing Wet Green Strength of Alumina Body DuringMicrofabrication by Colloidal Isopressing

Zhuo Zhang,*,w Biao Liu,* and Frederick F. Lange*

Materials Department, University of California, Santa Barbara, California 93106-5050

Colloidal Isopressing involves formulating a slurry with a weak-ly attractive particle network that can be pre-consolidated to ahigh relative density by pressure filtration and still retain fluid-like characteristics. The pre-consolidated slurry is injected intoan elastomeric mold and isopressed. Isopressing rapidly convertsthe slurry into an elastic body that can be removed from themold without shape distortion. Not only is this process rapid, butsince the water saturated compact produced by this method doesnot shrink during drying, it can also be converted into a greenbody without a long drying period. It is demonstrated that mi-cron-size surface features, such as 5 lm wide channels with adepth/width ratio of 2, can be rapidly produced on the surface ofalumina powder compacts. The fracturing of thin vertical por-tions of a micro-patterned surface during pressure release anddemolding has been an obstacle to obtaining micron-sized fea-tures with high aspect ratios. A method is shown here that en-ables the fabrication of such features by strengthening thesaturated isopressed body. It is shown that concentration con-trolled gelation of a poly(vinyl alcohol)–Tyzor

s

Triethanola-mine Tritanate (TE) additive effectively increases the strength ofthe elastic, isopressed body, saturated with water, while main-taining the low viscosity of the pre-consolidated slurry, which isrequired for transferring the pre-consolidated slurry into a rub-ber mold prior to isopressing.

I. Introduction

THE Polydimethylsiloxane (PDMS) stamps and molds, intro-duced by Xia and Whitesides,1 have been widely used in

micro-patterning of ceramics powders2,3 and precursors to ce-ramics4–6 and several techniques have been reported. Gaucklerand co-workers2,3 have produced zirconia, cerium gadoliniumoxide, tin oxide, and alumina surface patterns by casting a highsolid loading aqueous suspensions into PDMS molds. PZT sen-sor arrays were made by Rosqvist and Johansson7 by a similarmethod. However, because the slurries used to fill the channelscontained relatively low volume fractions of powder (0.40–0.50volume fraction), long drying periods were required to avoidcracking because the shrinkage of the powder within patternedchannels is constrained during consolidation via drying.

Previously, we have shown that small surface features, forexample, the features on a one-dollar coin and 20 mmwide and 1mm deep channels, could be rapidly produced on the surface ofalumina powder compacts using Colloidal Isopressing.8 Thisceramic forming method was first developed by Yu and Lange,9

in which a pre-consolidated slurry is injected into an elastomeric

mold and isopressed to rapidly convert the slurry into an elasticbody that can be removed from the mold without shape distor-tion. The pre-consolidated slurry is initially formulated at a lowvolume fraction with chemical additives needed to produce ashort-range repulsive potential between the particles. The short-range repulsive potential helps prevent the particles from form-ing a touching particle network when consolidated at low pres-sures. After the pre-consolidated body is fluidized via vibration,it is injected into a rubber mold, and isopressed at a high pres-sure, which forces the particles into contact and produces theelastic body shaped as the rubber mold. To allow the particlenetwork to support the applied pressure, a small amount of liq-uid is removed from the pre-consolidated slurry as it furtherconsolidates to form the elastic body during isopressing. Besidesits ability for rapid forming, bodies produced by Colloidal Iso-pressing do not shrink during drying, which allows direct heat-ing to the densification temperature.

When smaller surface features such as micron-size channelswere attempted, a problem was encountered where thin, deepfeatures would fracture during either pressure release (differen-tial strain recovery) or demolding. This problem is illustrated inFig. 1, which shows a micro-turbine produced with aluminapowder using a PDMS mold obtained from a micro-machinedsilicon master through the Spearing Group at MIT. As shown,all of the features of the mold were reproduced (even defects),but the edges of the very thin static turbine vanes (see insert)were broken. The height of the vanes, fixed to the surface, wasapproximately 200 mm. The width of the larger vanes rangedfrom 800 to 60 mm, and the average width of the smaller vaneswas about 100 mm. One method of solving this problem was tostrengthen the powder compact. Because differential strain re-covery (different elastic expansion of the PDMS material rela-tive to the consolidated powder compact) is one cause of stress,it was essential to increase the strength of the saturated, powdercompact.

Literature suggests that significant effort has been extendedto increase the strength of dry powder compacts, for example by

1411

JournalJ. Am. Ceram. Soc., 88 [6] 1411–1414 (2005)

DOI: 10.1111/j.1551-2916.2005.00295.x

Fig. 1. SEM of the isopressed, dried, and sintered micro-turbine (bycourtesy of M. Spearing, MIT). Insert shows that the thin tips of theturbine vanes are fractured during demolding.

L. C. Klein—contributing editor

Presented at the 106th Annual Meeting of The American Ceramic Society, Indianapolis,IN, April 20, 2004 (Innovative Processing and Synthesis of Ceramics, Glasses, and Com-posites Symposium, Abstract No. AM-S17-35-2004).

Supported by the Army Research Office, DAAG55-98-1-0455.*Member, American Ceramic Society.wAuthor to whom correspondence should be addressed. e-mail: zhouzh@engineering.

ucsb.edu

Manuscript No. 11030. Received May 14, 2004; approved December 7, 2004.

using different binders.10,11 Because the object of the currentneed is to increase the strength of the saturated powder bodywithout increasing the viscosity of the pre-consolidated slurry,which is required for transferring the pre-consolidated slurryinto the PDMS mold prior to isopressing, a different approachwas sought. Morissette and Lewis12 developed a gelcastingmethodology based on aqueous alumina–poly(vinyl alcohol)(PVA) suspension, which was cross-linked by an organotinan-ate coupling agent, Tyzor

s

Triethanolamine Tritanate (TE).Tyzor

s

TE reacts with water under hydrolysis and forms a re-active hydroxy titanium chelate, which can form bonds with theOH groups of PVA. They found that the gelation was concen-tration dependent, i.e., at a given PVA content, there was acritical titanium concentration below which the gelation wouldnot take place; also, this critical titanium concentration de-creased as the volume fraction of alumina increased. In the cur-rent work, it is demonstrated that this concentration-inducedgelation can be applied to the Colloidal Isopressing method suchthat the gel transition will not take place until the isopressingstep. As reported below, the viscosity of the pre-consolidatedslurry is not significantly affected, yet the strength of the iso-pressed body was significantly improved to enable patterning ofalumina surface patterns with a feature size as small as 5 mm andwith an aspect ratio of 2.

II. Experimental Procedure

(1) Colloidal Isopressing and Mold Fabrication

Aqueous a-alumina (0.20 volume fraction, 0.2 mm diameter,AKP 50, Sumitomo Chemical Co. Ltd., Osaka, Japan) slurrieswere formulated as repulsive particle networks at pH 4, rolledfor 16 h before adding 0.25M ammonium chloride (Fisher Chem-icals, Fair Lawn, NJ, analytical grade) to create a weakly at-tractive network. The pH was adjusted (70.1 pH units) withanalytic grades of either HCl or NH4OH. For the gelation sys-tem, PVA Airvol 205 (Air Products and Chemicals, Inc., Allen-town, PA) was added to the slurry at 1 wt% (relative to thealumina) and stirred for 30 min. Different slurries were for-mulated containing different volumes of Tyzor

s

TE (DupontChemicals, Deepwater, NJ), the coupling agent (pre-diluted to20% of its original concentration by DI water), which was add-ed by pipet into the slurry and stirred for another 30 min. Afterthis period, the pH was readjusted to 4 before pre-consolidationstep.

As described elsewhere,13 the slurry was pre-consolidated bypressure filtration at 5 MPa, which resulted in a cylindrical body45 mm in diameter. The pre-consolidated body was put within asealed plastic bag to prevent drying and placed on a tube vibra-tor. Upon vibration the body was fluidized and looked liketoothpaste because of shear thinning. The relative density (thevolume fraction of alumina) of the pre-consolidated slurry wasdetermined to be 0.57070.0014 by the weight loss method(weight before and after drying, and heating to 5001C to re-move the volatile salt).

PDMS (Sylgards

184 Silicone, Dow Corning Corporation,Midland, MI) molds were fabricated by casting a mixture of asilicone base and a curing agent at weight ratio 10:1 upon mas-ters of desired surface features. The masters were made onsilicon wafers patterned by either photolithography, micro-machining, or other methods. The liquid elastomer was de-gassed with vacuum and cured either at room temperature orupon heating to 701C. Upon curing, the PDMS mold could beeasily peeled from the master.

The pre-consolidated slurry was poured into the PDMSmold. The slurry was vibrated for 1 min to allow bubblestrapped during pouring to rise to the top (despite its high rel-ative density, the viscosity of the pre-consolidated slurry exhibitsshear rate thinning, and greatly decreases its viscosity duringvibration). To adsorb a small amount of water during isopress-ing, a dry pad of AKP50 (previously dry pressed at 10 MPa)with the same diameter as the mold cavity was placed on the top

of the pre-consolidated slurry. After sealing, the filled mold wassubjected to isostatic pressure of 200 MPa, (Isostatic Press,Autoclave Engineers Inc., Erie, PA) for a period of 2 min, apressure much greater than the plastic-to-elastic transition pres-sure needed to convert the pre-consolidated slurry to an elasticbody.14 After the pressure was released and the mold removedfrom the isopress, the body was elastic and could be removedfrom the mold.

(2) Measurement of TE and PVA Concentrations

An aqueous solution containing the same amount of DI-waterand the TE coupling agent as those in the initial 20 vol% alu-mina slurry were formulated to mimic the concentration of theTE in the initial slurry. As the 20 vol% alumina slurry was pre-consolidated, the filtrate liquid was collected to enable the meas-urement, by Inductively Coupled Plasma (ICP) Atomic Emis-sion Spectrometer, of the TE coupling agent lost during pressurefiltration, and thus, that retained within the powder com-pact. The standards were diluted from 100073 mg/mL TitaniumStandard in 20% HCl (High Purity Standards, Charleston, SC).Likewise, aqueous PVA standards were formulated.

The concentrations of PVA in the standard and in the filtratecollected during pressure filtration were determined by UV/VISSpectrometer (Perkin Elmer, Wellesley, MA). Because the reac-tion between PVA and iodine in dilute aqueous solution pro-duces a blue color, the concentration of PVA is linearly relatedto the extinctions at the absorption maximum.15

(3) Strength Measurement

Alumina bars were made by colloidal isopressing with dimen-sions of 0.3 cm� 0.4 cm� 3.5 cm. Tests were performed in 4-point bending with an outer span of 3.02 cm and an inner spanof 1.3 cm using the Instron 8562 and 1123 coupled with a 44.5 N(10 lb) load cell (Eaton Corp., Troy, MI, Model 3108-10) im-mediately after the bars were removed from the mold after iso-pressing. The specimens were loaded to failure with a crossheadspeed of 0.5 mm/min.

III. Results and Discussion

The ICP measurement, as referenced to the standard, showedthat the concentration of the formulated slurry contained 150ppm of Ti. However, measurements of the filtrate produced af-ter pressure filtration showed that the concentration of Ti was0.25 ppm, suggesting that nearly all of the TE coupling agentwas retained in the consolidated powder compact. Because thevolume fraction of the alumina increased from 0.20 within theslurry to 0.57 after pre-consolidation, and then to 0.64 afterisopressing, the concentration of the TE coupling agent at eachstage could be calculated based on the initial amount added andthe volume of water lost. Figure 2 shows a calculation of thechange of concentration of TE after pre-consolidation and after

0

50

100

150

200

250

300

350

400

0 20 40 60 80 100 120 140TE (µl in 20ml)

Ti (

ppm

)

in the isopressed body

in the pre-consolidated slurry

in the initial slurry

Fig. 2. Concentration calculation of Ti in pre-consolidated slurry andisopressed body based on the initial slurry.

1412 Journal of the American Ceramic Society—Zhang et al. Vol. 88, No. 6

isopressing in a 20 mL 0.20 volume fraction slurry with differentvolumes of TE added at the beginning. For example, if a slurryis formulated with 100 mL TE coupling agent (initial [Ti] is97 ppm), then after pre-consolidation, the [Ti] is increased to276 ppm and after isopressing, to 310 ppm, namely a threefoldincrease from that in the initial slurry. The UV measurementshowed that the concentration of the PVA in the filtrate was88% of that used to formulate the initial slurry. By calculation,the concentration of PVA only slightly increased within thepowder compact after pre-consolidation; the increase was sosmall that to the first approximation, the concentration of PVAdid not change. Because the volume fraction of the alumina in-creased, the condition of the PVA–TE–alumina system in ourColloidal Isopressing method was similar to that needed to forma gel through concentration change as described by Morissetteand Lewis.12

The difference between the flowability of the pre-consolidatedbody, with and without the addition of PVA–TE, was not no-ticeable. Namely, the viscosity of the pre-consolidated body wasnot significantly affected by the addition of PVA–TE, whichenabled the pre-consolidated body to be transferred into thePDMS mold easily prior to isopressing.

As shown in Fig. 3, significant strengthening was achieved.As the TE concentration increased, so did the strength of thesaturated, consolidated body. The bars fabricated with slurriesformulated at pH 4 exhibited even greater strengths than thoseformulated at pH 5 or 6.

As a comparison, Fig. 4 shows the results of our previous,unreported work concerning the strength of saturated powdercompact, produced by Colloidal Isopressing, where only PVAwas added to the slurries formulated at pH 4 in an identicalmanner reported here. The figure shows that the strength de-creased for the saturated powder bodies with increasing PVA.Even though the strength after drying increased with additionsof PVA, the dry strength was lower relative to the saturatedstrength. Previous studies16 have attributed the greater strength

of saturated bodies, relative to dry bodies, to the plastic defor-mation that occurs at the crack front in saturated bodies. Name-ly, when saturated bodies are subjected to large strains, thetouching particle network can be pulled apart to re-establishthe pair potential prior to consolidation. This phenomenoncan produce a large crack growth resistance. Because of thelarger crack growth resistance, the saturated body has a higherstrength relative to the dry body. In addition, because the PVAis more likely to form hydrogen bonds with water instead ofwith the alumina particles, there is no strong network betweenthe alumina particles in the saturated bodies.

The PVA–TE methodology is effective for increasing the wetgreen strength of the isopressed body, while at the same time,not increasing the viscosity of the pre-consolidated body. As thewet green strength increased, feature sizes as small as 5 mm couldbe duplicated very successfully as shown in Fig. 5, which showschannels with an aspect ratio of 2:1. The material between thechannels is similar to the tip of the damaged micro-turbine vaneshown in Fig. 1, but with a feature size one order of magnitudesmaller. Features this small and 0.4 cm long could be fabricatedwithout damage during either pressure release after isopressingor demolding.

IV. Conclusions

A new microfabrication method of ceramics using ColloidalIsopressing has been improved by formulating a slurry contain-ing additives that form a gel once their concentration is in-creased during the final consolidation step. The ColloidalIsopressing method is a rapid forming method and becausethere is no shrinkage during drying the consolidated body can bedried very fast. The major problem of this process is crackingduring either pressure release or demolding. The concentrationinduced gelation of the PVA–TE system significantly increases

2.5

3.5

4.5

5.5

6.5

7.5

0 20 40 60 80 100 120Ti (ppm)

Stre

ngth

(M

Pa) pH4

pH5pH6

Fig. 3. Flexural strengths of saturated alumina bodies with differentconcentrations of Ti molecule; poly(vinyl alcohol) was fixed at 1 wt% ofalumina powder in the slurry.

0.6

1.1

1.6

2.1

2.6

3.1

3.6

0 1 2

PVA content (wt%)

Stre

ngth

(M

Pa)

Wet body

Dry body

Fig. 4. Green strengths of wet and dry bodies with different poly(vinylalcohol) concentration at pH 4.

Fig. 5. (a) SEMmicrograph of surface features as small as 5 mm; heightof features is 10 mm. (b) SEM micrograph of enlarged features.

June 2005 Microfabrication by Colloidal Isopressing 1413

the wet green strength of the ceramic body while maintaininga low viscosity of the pre-consolidated slurry. Feature sizes of5 mm and aspect ratio of 2:1 on alumina powder surface wereobtained using slurries formulated with a weakly attractiveparticle network and the PVA–TE gelling system.

Acknowledgments

Zhuo Zhang acknowledges Masa Rao and Yanting Zhang for their help withphotolithography, Tim Radsick for his assistance with ICP measurement, andMark Cornish for the SEM training. We are also grateful to Professor M. Spearingfor allowing us to use several of his deeply etched Si micro-turbine vanes as mastersfor our research.

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