journal - yonsei

Slurry Chemistry Control to Produce Easily RedispersibleCeramic Powder Compacts

Jooho Moon,† Jason E. Grau, and Michael J. Cima*Department of Materials Science and Engineering, Massachusetts Institute of Technology,

Cambridge, Massachusetts 02139

Emanuel M. Sachs

Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139

The slurry-based Three Dimensional Printing (3DPTM ) pro-cess requires the production of an easily redispersible powderbed from a well-dispersed slurry. Understanding and controlof the interparticle potential in the dispersed state, as well as inthe dry consolidated state, are important. The strength of theparticle–particle interactions in the dry state determines theredispersion efficiency. One factor that controls the interpar-ticle strength is the chemical stability of the ceramic powdersin the dispersed state. For unstable powders, a partial disso-lution and/or hydration of the powders can occur and eventu-ally impede the redispersion by forming insoluble salt bridgesat the necks of the particles. Redispersion of the powder bedcan be improved substantially by weakening the strength ofthe particle–particle bonds. The formation of strong chemicalbonds between particles should be avoided by adjusting theslurry pH to an appropriate range where the powders arechemically stable in the slurry. Replacement of the chemicalbonds by soluble physical bonds, using a low-molecular-weighthydrophilic polymer, also reduces the interparticle strengthand enhances redispersion.

I. Introduction

MOST ceramic component manufacturing involves the consol-idation of powders into a desired shape and densification of

the shape by firing at high temperature. The consolidated particlestructure has an important role in determining the final sintereddensity, microstructure, and relevant material properties.1 In col-loidal processing such as slip casting and tape casting, hydrody-namic interaction, interparticle forces, and capillary pressuredetermine the particle-packing structure as the solvent dries.2

Manipulation of the interparticle forces allows the formation ofdispersed colloidal suspensions and uniform green bodies.3 In drypressing, on the other hand, the particles undergo compaction bydeformation and/or fracture in accordance to applied pressure.Characteristics of the powders, usually spray-dried granules, sig-nificantly influence the compaction behavior.4 These conventionalforming processes mostly concern achieving a high-density, ho-mogeneous particle-packing structure.

New processing techniques, such as solid freeform fabrication(SFF), however, face problems that are similar to those encoun-tered in other conventional methods.5–8 A partial disintegration of

the consolidated green body is one such challenging issue in theslurry-based Three Dimensional Printing (3DPTM) process. Theslurry-based 3DPTM process is a recently developed tool-lessmanufacturing method in which high-green-density ceramic com-ponents are produced directly from a computer model.9,10 A thinlayer of the powder bed in the slurry-based 3DPTM process isdeposited by jetting a dispersed slurry, followed by drying. Then,binder is printed into the powder bed, to define the shape of thecomponent, and the process is repeated until the component iscompleted. The resulting powder bed is cohesive, and the part isretrieved by redispersing the unprinted regions in a water bath.

The term “redispersion” implies a process in which the driedconsolidated body is taken apart into small particle aggregates viareimmersion in a solvent. Redispersion would be quite differentwith a normal dispersion process. The physical and chemicalcharacteristics of powders may be altered during precedent pro-cessing. The powder beds must be redispersed spontaneouslywithout the aid of ball milling to retrieve the printed part.Otherwise, incomplete part retrieval or damage on the printedcomponent results. In addition to our current interest, an under-standing of redispersion phenomena also may be useful in otherconventional processing, such as ball-milling granule powder andpowder recycling.

In this paper, various parameters that influence the redispersionof the powder bed (such as the redispersing medium, powdertypes, and polymeric additives) and slurry stability have beeninvestigated. A fundamental understanding of interparticle inter-actions and solution chemistry is demonstrated to have an impor-tant role in the preparation of the slurry for the 3DPTM process.Slurry stability and redispersion behavior are interrelated to eachother, and each is dependent on the slurry chemistry. Propercontrol over slurry compositions and conditions is necessary tomake a well-dispersed suspension that can yield easily redispers-ible, dense powder beds. Although the 3DPTM process is suffi-ciently generic to be applicable to any material system that can bedispersed, silicon nitride (Si3N4) is selected as a model system inthe current study.

II. Experimental Procedure

(1) Materials and Slurry PreparationSeveral commerciala-Si3N4 powders (SN-E10, SN-E5, and

SN-E3 (from Ube Industries, Tokyo, Japan) and M11 (from H. C.Starck, Berlin, Germany)) were used in this study. The specificsurface area and the mean particle size were determined usingsingle-point Brunauer–Emmett–Teller (BET) analysis (Quanta-chrome, Syosset, NY) and a centrifugal sedimentation technique(Capa-700, Horiba, Irvine, CA), respectively.

Si3N4 slurries that included sintering additives were prepared inpolyethylene bottles by adding trimethyl-ammonium hydroxide(TMAH) (Aldrich Chemical Co, Milwaukee, WI), followed bymilling for 24 h using alumina (Al2O3) ball media. The pH of the

V. A. Hackley—contributing editor

Manuscript No. 189479. Received March 22, 1999; approved March 29, 2000.*Member, American Ceramic Society.†Present address: Dept. of Ceramic Engineering, Yonsei University, Seoul 120–749,

Korea.

J. Am. Ceram. Soc.,83 [10] 2401–408 (2000)

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journal

slurries that were dispersed with TMAH was in the range of pH10–10.5. The standard slurry composition was 53.8 wt% Si3N4,3.5 wt% Al2O3 (HPA-0.5, Ceralox, Tucson, AZ), 3.5 wt% yttria(Y2O3) (99.99%, Unocal P/N 5600, Molycorp, Fairfield, NJ), 1wt% TMAH, and 38.2 wt% deionized water. Polyethylene glycol(PEG; carbowax polyethylene 400 and 8000, Union Carbide,Danbury, CT) was added to slurries when necessary, to improvethe redispersion behavior of the powder bed. The amount of addedPEG was in the range of 1–5 wt%, relative to the total mass of thepowders.

A variety of dispersants also was investigated, to develop astable Si3N4 suspension at the range of pH 6–9. The dispersantsused in the current study include 3-amino-propanol (99% pure,Aldrich Chemical Co.), 3-aminopropyl-triethoxysilane (APS)(98% pure, Aldrich Chemical Co.), Betz 1190 (dimethylamine-epichlorohydrin linear copolymer, molecular weight (Mw) of;10 000–12 000, Betz, Trevose, PA), Darvan C (25%, ammo-nium polymethacrylate; Mw ' 8000–10000, R. T. Vanderbilt,Norwalk, CT), Nopcosperse A44 (35%, proprietary blend, ammo-nium polyelectrolyte; Henkel Corp., Ambler, PA), and triethanol-amine (98%, Aldrich Chemical Co.). The rheological behavior ofthe slurries for each dispersant was measured as a function ofslurry pH, adjusted with 0.1M HNO3 and KOH. Slurry viscositywas measured at 25°C using cylindrical cone-and-cup-type vis-cometer (Model CV100, Haake, Paramus, NJ).

(2) Characterization of RedispersionThe redispersion behavior was measured as a function of the

redispersing medium types, powder types, PEG amounts, andaging time of the slurry. The samples tested for redispersion wereprepared by slip-casting various slurries into disks;3 mm thickand 25 mm in diameter. The slip-cast powder beds used in theexperiment well represent the actual slurry-based 3DPTM-derivedpowder beds that are produced by the accumulation of thin,slip-cast layers. The disks were allowed to air dry at a temperatureof 25°C for 18 h before the redispersion study. The green densityof slip-cast powder beds was determined via mercury porosimetry(Model Autopore 9220, Micromeritics, Norcross, GA) to take intoaccount the effects of packing density when comparing theredispersion behavior.

The redispersion efficiency, as a function of the drying temper-ature of the powder bed, also was measured to investigate itsinfluence on PEG volatilization. The powder beds prepared withStarck M11 Si3N4 powder and 2 wt% PEG-400 were heated for 6 hat temperatures of 100°, 150°, and 200°C. The PEG content inthe powder beds was analyzed via thermogravimetric analysis(TG/DTA) (Model Thermoplus TG8120, Rigaku, Tokyo, Japan).

The redispersion of the powder bed was characterized bymeasuring the mass of a sample that was suspended in water. Awire-mesh basket (with opening widths of 3.35 mm) was sus-pended into a beaker of liquid. The powder-bed sample was placedin the mesh basket. The mass of the powder bed was monitoredcontinuously, using a computer-interfaced balance (Model AE160, Mettler Instrument, Inc., Hightstown, NJ). The mass of thesuspended sample decreased as the powder bed redispersed, andthe loose powder fell through the wire-mesh holder. The results ofthe redispersion measurements were plotted in terms of normalizedmass versus time. The mass was normalized by dividing the massmeasured at any given time by the initial mass of the suspendedsample. Therefore, the normalized mass can vary from 1 (noredispersion) to 0 (complete redispersion). The percentage redis-persed also can be calculated:

Percentage redispersed (%)5 1003 ~1 2 normalized mass)

(3) Determination of the Concentration of Dissolved SilicaSi3N4 suspensions (with a solids concentration of 10 vol%),

using Starck M11 Si3N4 without sintering additives, were preparedat two different pH values: pH 10.5 and pH 4.5. Si3N4 powderswere equilibrated in water at a temperature of 25°C and thenseparated from the supernatant, as a function of equilibration time,

using a centrifuge (Model HT, International Equipment Co.,Needham Heights, MA) at 10 162g for 15 min. The resultingsupernatants were filtered through 0.2mm filter paper to removeany particulate impurity. The amount of dissolved silica (SiO2)was determined using the molybdenum blue method.11 Absor-bance at a wavelength ofl 5 810 nm for each sample wasmeasured using ultraviolet–visible (UV-Vis) spectrophotometry(Model DU-640, Beckman Instrument, Inc., Columbia, MD). Theamount of silicon detected was calibrated using a standard solution(EM Science, Gibbstown, NJ).

(4) Electroacoustic AnalysisThe surface-charge characteristics of Si3N4 that included Al2O3

and Y2O3 were characterized using an electroacoustic method.12

All the electrokinetic sonic amplitude (ESA) measurements wereperformed at a frequency of 1 MHz, using an electroacousticanalyzer (Model ESA 8000, Matec Applied Science, Hopkinton,MA). Suspensions of 200 mL were prepared at 1.0 vol% in asupporting electrolyte (1023M potassium nitrate (KNO3)). Thesuspensions were treated ultrasonically for 5 min at an outputpower of 40 W before measurement. After calibration, usingcolloidal SiO2 (10 vol%, Ludox TM, Matec Applied Science), theESA was determined as a function of either pH or dispersantamount. The zeta potentials were calculated using the predeter-mined average particle size.

III. Results

The powder beds were placed in several different liquids toinvestigate the influence of the redispersion medium. The powder-bed characteristics that have been used in the redispersion study,including the preparation conditions and the redispersed amount,are summarized in Table I. Pure water was the most-effectiveredispersing medium, and the redispersing ability gradually de-creased as the relative amount of methanol (CH3OH, or MeOH)increased, as shown in Group A in Table I. Virtually no redisper-sion of the powder bed occurred in pure methanol. An explosion,accompanied by audible sound, usually was observed when thepowder bed was submerged in pure water. However, the samplethat was placed in methanol did not explode. The role of particlesurface charges on the redispersion behavior also was determinedby placing the powder beds in water with two different pH values(pH 6.5 and pH 10), as shown in Group B in Table I. Alkalinewater was a more-effective medium for redispersing Si3N4 pow-der, in comparison to water with an almost-neutral pH.

The redispersion behavior in deionized water for samples thatwere prepared with four Si3N4 powders is summarized in Group Cin Table I. The redispersion behavior differs substantially, depend-ing on the type of powder. The Starck M11 Si3N4 samples(average particle size (s) of 0.3 mm, surface area (SA) of 13.3m2/g) and Ube SN-E10 Si3N4 samples (s 5 0.33mm, SA 5 10.6m2/g) exploded into several pieces; however, very little change inthe sample was observed after the initial explosion. The UbeSN-E5 Si3N4 sample (s 5 0.46mm, SA5 5.3 m2/g) gently brokeinto several pieces. Significant amounts of powder slowly fellaway from the sample surface. The samples that were preparedwith Ube SN-E3 Si3N4 powders (s 5 0.6 mm, SA 5 3.2 m2/g)redispersed very well. The samples slumped into mounds ofpowder and then fell freely through the wire-mesh sample holder.

Group D in Table I illustrates the impact of adding PEG to theslurry. The stability of slurries that were prepared with PEGshowed no noticeable difference, in comparison with slurry with-out PEG if the PEG content was,2 wt%. The green densities ofthe slip-cast powder bed also were almost identical within theexperimental error under such conditions. The redispersion behav-ior improved substantially when PEG was present in the powderbed. Figure 1 shows that the powder bed without PEG remainedessentially intact, whereas the powder bed with PEG slumped andredispersed to a fine powder. The extent of redispersion improvedas the amount of PEG increased up to 2 wt%, whereas the

2402 Journal of the American Ceramic Society—Moon et al. Vol. 83, No. 10

redispersion efficiency decreased at 5 wt%. When higher-molecular-weight PEG (Mw ' 8000) was added to the slurry, onthe other hand, the powder bed did not redisperse well. Thesamples remained intact and were sticky to the touch. The effect ofusing the non-ionic polyelectrolyte Darvan C as a redispersingagent also was investigated. The percentage redispersed in waterwas only 9.6% for a sample with 1 wt% of Darvan C. The samplesbehaved similar to those that contained high-molecular-weightPEG-8000. The saturated powder beds remained cohesive in waterwithout spontaneous redispersion.

Low-molecular-weight polymers, such as PEG-400, are subjectto volatilization at moderate temperatures. The effect of heating onthe PEG content and redispersion behavior clearly reveals theimportance of PEG in redispersing the powder bed. The calculatedamount of PEG that is present in the samples and their redispersionbehavior, as a function of heating temperature, are listed in TableII. The redispersed amount decreases as the heating temperatureincreases, following the decrease in PEG content. The percentageredispersed decreases to,3.7% for heating temperatures of$150°C.

The slurry-preparation conditions also are important in redis-persion. The result of redispersion measurements for the powderbeds with different aging times is presented in Group E in Table I.The redispersion behavior deteriorated from 28.6% to 12.8% as theaging time of the slurry increased. The powder bed prepared fromthe slurry that was aged for,8 h readily redispersed, whereas theslurry that was aged for 24 h resulted in a strong powder bed. Theaging-time dependence of the redispersion of the powder bedindicated that Si3N4 powders might undergo a chemical reactionwith water when the slurry is aged. The resulting interaction thenmodifies the characteristics of the powder bed, which affects theredispersion.

IV. Discussion

Several forces are responsible for redispersion of the powderbed. The first type of force is the capillary pressure. When thedried powder bed is submerged in the liquid, excess pressureaccumulates inside the powder bed, because of entrapped air as theliquid penetrates into a porous body, as proposed by Heertjes andWitvoet.13 This excess pressure is equal to the capillary pressureP:

DP 5g cosu

Rpore

whereg is the surface tension of the liquid,u the contact angle, andRpore the pore radius.P increases as the surface tension increases;therefore, pure water provides the strongest force that is beneficialto the redispersion of the powder bed, in comparison to otherlower-surface-tension media, such as alcohol. This excess pressureleads to disintegration several seconds after the highly cohesivepowder bed is placed in water. The liquid must penetrate throughthe pores first, and the excess pressure for redispersion thenaccumulates inside the powder bed.

The second type of force is the repulsive force between chargedparticles. If the particles are placed in water whose pH is far fromtheir isoelectric point (IEP), the particles can develop a significantelectrostatic repulsion that may assist the separation of particles inthe powder bed. The IEP of Starck M11 Si3N4 is pH ;4.5. Si3N4

particles that have been placed in the water at pH 10 can exhibithigher electrostatic repulsion, because of the development ofnegatively charged surface, whereas much-less repulsive forceexists between particles that have been submerged in water at pH6.5.

Table I. Summary of Powder-Bed Characteristics and Redispersion Behavior†

Powdertype Dispersant Slurry pH Additives

Aging time(h)

Green density(%)

Redispersingmedia

Amountredispersed

(%)‡

Group AStarck M11 TMAH 10.3 None 24 59.3 Neutral water

(69.3 dyne/cm)§12.8

Starck M11 TMAH 10.3 None 24 59.3 H2O:MeOH 5 3:1(46.4 dyne/cm)§

9.5

Starck M11 TMAH 10.3 None 24 59.3 H2O:MeOH 5 1:3(35.3 dyne/cm)§

7.5

Starck M11 TMAH 10.3 None 24 59.3 MeOH (22.7 dyne/cm)§ 3.9

Group BStarck M11 TMAH 10.3 None 24 59.3 Neutral water (pH 6.5) 12.8Starck M11 TMAH 10.3 None 24 59.3 Alkaline water (pH 10) 15.9

Group CStarck M11 TMAH 10.3 None 24 59.3 Neutral water 12.8Ube-SN-E10 TMAH 10.2 None 24 59.1 Neutral water 19.9Ube-SN-E5 TMAH 10.4 None 24 57.2 Neutral water 57.4Ube-SN-E3 TMAH 10.5 None 24 56.4 Neutral water 76.3

Group DStarck M11 TMAH 10.3 None 24 59.3 Neutral water 12.8Starck M11 TMAH 10.3 1 wt% PEG 400 24 59.1 Neutral water 25.5Starck M11 TMAH 10.4 2 wt% PEG 400 24 58.9 Neutral water 39.1Starck M11 TMAH 10.2 5 wt% PEG 400 24 56.6 Neutral water 15.4Starck M11 TMAH 10.3 2 wt% PEG 8000 24 58.1 Neutral water 13.6Starck M11 TMAH 10.4 1 wt% Darvan C 24 58.5 Neutral water 9.6

Group EStarck M11 TMAH 10.8 None 1 59.0 Neutral water 28.6Starck M11 TMAH 10.6 None 8 59.3 Neutral water 21.5Starck M11 TMAH 10.3 None 24 59.3 Neutral water 12.8

Group FStarck M11 TMAH 10.4 0.2 wt% Nopcosperse A44 24 59.5 Neutral water 13.3Starck M11 TMAH 10.4 2 wt% PEG 400 24 58.9 Neutral water 39.1Starck M11 0.2 wt%

Nopcosperse A448.5 2 wt% PEG 400 24 57.5 Neutral water 59.7

†Sintering additives (6 wt% Al2O3 and 6 wt% Y2O3) were added to all the slurries, except for the Group E slurries.§The amount redispersed was determined 10 min afterimmersion in the powder bed.‡The number refers to the surface tension of the redispersing medium.

October 2000 Slurry Chemistry Control to Produce Easily Redispersible Ceramic Powder Compacts 2403

The redispersion behavior also is dependent on the characteris-tics of the powder beds, i.e., the number and strength of particle–particle contacts. The results in Group C in Table I indicate that thenumber of particle–particle contacts is critical to the redispersionbehavior. The highly spontaneous redispersing characteristicsof the Ube SN-E3 Si3N4 powders clearly illustrate the effectsof packing density, particle size, and surface area. The number ofparticle–particle contacts per unit volume varies inversely with thecube of the particle size. The number of contacts also will decreaseas the packing density decreases. However, the sintering behaviorof ceramics is dependent on particle size and packing density.Increasing the particle size and decreasing packing density willhave a negative impact on the sinterability of the powder bed.Therefore, the desired approach for improving the redispersion

behavior of the powder bed would be reducing the strength ofparticle–particle contacts, rather than reducing the number ofparticle contacts.

Improved redispersion when low-molecular-weight PEG ispresent in the powder bed may be associated with weakening thestrength of the particle–particle contact. PEG is a water-solublepolymer, with the formula HOO(CH2OCH2OO)nOH, that com-monly is used in ceramics processing as a plasticizer, binder, andlubricant.14 When the molecular weight is less than;1000, PEGis a viscous liquid at room temperature and dissolves quickly inwater at room temperature, in comparison with waxy, solid PEG ofhigher molecular weight.

Walker et al.15 determined that the adsorbed amount of PEGincreases as its molecular weight increases. The adsorbed amountsof PEG-8000 on SiO2 and Al2O3 were small (0.4 and 0.2 mg/m2,respectively). It is reasonable to assume that most of PEG-400behaves as a free polymer in the slurry under the current condi-tions. The liquid/vapor meniscus retreats into the pores of thegreen body, because of capillary pressure during drying.16,17PEGalso will be drawn into the pores and is deposited at the necks ofthe particles after the slurry is dried. This material forms a solublebridge between particles and prevents strong direct particle–particle contacts. When PEG meets water again during the redis-persion, it spontaneously redissolves, which helps particle separa-tion. In the excess-PEG condition, however, PEG may form athree-dimensional polymer film by which the particles are heldtogether. The particles cannot be separated unless PEG dissolvesfirst. Under such conditions, the significant amount of PEG wouldredissolve, which sufficiently decreases the surface tension ofwater at the region of the particle contacts, which reduces thecapillary force for redispersing the powder bed. That, presumably,is the reason why the powder bed is sticky to the touch and itcannot fall freely through a metal mesh, even though the greendensity of the powder bed is less.

To support this hypothesis, an attempt was made to observe thedistribution of PEG in the powder bed directly, using field-emission scanning electron microscopy (SEM) (Model JSM6330F,JEOL, Tokyo, Japan); however, this attempt was unsuccessful.The observation of submicrometer Si3N4 particles requires highmagnifications at high accelerating voltages. Under such condi-tions, the lower-molecular-weight PEG likely evaporates. Further-more, the image contrast between the Si3N4 and PEG wasinsufficient. In a much-easier system that involved larger metalpowders, however, the preferential deposition of the polymer wasdemonstrated, as we presumed. SEM micrographs of the slip-castgreen body indicated that the 1 wt% sodium alginate binder formsa two-dimensional film at the saddle points between the parti-cles.18

In contrast to PEG-400, PEG-8000 is considered to act as abinder. PEG-8000 is less soluble than PEG-400 in water. PEG-8000 may adsorb on the particles, to some extent. The longer-chainPEG molecules can bridge individual particles together uniformlyduring drying, which results in less-soluble, strong adhesionbetween the particles and eventually makes the redispersiondifficult. Similar polymer–particle interaction likely occurredwhen the polyelectrolyte Darvan C was used. The entanglement ofadsorbed polymer chains and reaction of the functional groupswith the powders may be possible.

The results in Group E in Table I indicate that the redispersionalso is influenced by solid–liquid interaction during slurry prepa-ration. Ceramic powders generally have a finite solubility in thesuspending medium, especially in water. In the case of Si3N4, anamorphous SiO2-like or silicon oxynitride (Si2ON2) layer 3–5 nmthick exists on Si3N4, as demonstrated by Rahamanet al.19 For thesuspension that was prepared at high pH values, this surface oxidelayer may dissolve, because of its relatively high solubility inalkaline water.20 The resulting dissolved SiO2 will reprecipitatepreferentially on particle–particle contact during drying, acting asa silicate-based glue.21 This phenomenon, in turn, may increase thestrength of the particle–particle contact and inhibits redispersion.

To verify such a scenario, the SiO2 content dissolved from theSi3N4 surface was measured as a function of equilibration time, as

Fig. 1. Photographs showing the redispersion behavior of the Starck M11Si3N4 powder beds in water (a) without PEG and (b) with 2 wt% PEG.

Table II. PEG 400 Content Present in the PowderBeds and the Redispersed Amount of the

Corresponding Powder Beds, as a Function ofHeating Temperature†

Heating temperature(°C)

PEG content(wt%)

Amount redispersed(%)

25 1.9 39.1100 1.4 35.8150 0.5 3.7200 0.3 2.2

†The third powder bed of Group D in Table I was used in thisexperiment.


shown in Fig. 2. It was experimentally demonstrated that thesurface oxide could dissolve from the Si3N4 surface by aging atboth pH 4.5 and pH 10.5 during the slurry preparation and aging.The leached silicon amount at pH 10.5 was 5.63 1023 mol/L,whereas 2.23 1023 mol/L of dissolved silicon was detected at pH4.5. The dissolution mostly occurred within 1 d and attained steadystate after 2 d of aging. A greater dissolution will be expectedwhen the slurry is ball-milled under alkaline conditions.

This chemical-stability consideration should extend to sinteringadditives, such as Al2O3 and Y2O3, that are present in themulticomponent Si3N4 system. Hackleyet al.22 investigated aslurry composition for the aqueous processing of reaction-bondedSi3N4. The optimum slurry pH for the silicon–Al2O3–Y2O3

system, where all constituent powders can maintain chemicalstability, was determined to be in the range of pH 6–9. Theallowed pH window was characterized by the dissolution behaviorof Y2O3 at the lower end and Si3N4 at the higher end. Similarrequirements should be met in the current aqueous Si3N4 system.

Several dispersant systems were tested to determine an appro-priate dispersant that can properly control interparticle forces andmodify the surface-charge characteristics of each constituentpowder in the required pH range. However, the poor redispersibil-ity of the powder bed with PEG-8000 and Darvan C indicates thatpolymeric additives must be used with caution. In this regard, thetype and amount of dispersants must be carefully optimized so thatthe powder bed is not strengthened.

The rheological behavior of a 35 vol% slurry, as a function ofpH and dispersant type at a steady shear rate (122 s21), ispresented in Fig. 3. The slurries that contained Darvan C, 3-aminopropanol, and triethanolamine exhibited low viscosities only at asuspension pH of.9, whereas the use of APS produced lowviscosity only at a suspension pH of,3. In contrast, the slurrieswith Betz 1190 and Nopcosperse A44 additives had a minimum inviscosity in the range of pH 6–9.

Further analysis was performed using an electroacoustic tech-nique, to understand the polymer–particle interaction for twocandidate dispersants: Betz 1190 and Nopcosperse A44. The zetapotential of Si3N4 rapidly varied when both dispersants wereadded, exhibiting good chemical affinity, as shown in Fig. 4. Thezeta potential of Si3N4 reached a maximum surface charge of 42mV at 0.45 wt% for the cationic polyelectrolyte Betz 1190,whereas a zeta potential of274 mV was observed at 0.22 wt% forthe anionic polyelectrolyte Nopcosperse A44.

The rheological studies also demonstrated that the magnitude ofthe interparticle forces has a profound influence on the suspension

Fig. 4. Variation of zeta potential as a function of dispersant amount ((a)Betz 1190 and (b) Nopcosperse A44); the suspension pH varied from pH3.4 to pH 3.7 for Betz 1190 and pH 7.5 for Nopcosperse A44 during themeasurement. The 1.0 vol% Starck M11 Si3N4 slurries were prepared andsonicated before electroacoustic measurement.

Fig. 2. Dissolved-silica concentration profile in the supernatant as afunction of aging time. Two suspensions of 10-vol% Starck M11 Si3N4,with different slurry pH, were prepared and aged at room temperature.

Fig. 3. Rheological behavior of a Si3N4 suspension, as a function ofslurry pH and dispersant type. Standard slurries (35 vol%) with 2 wt%PEG, using the Starck M11 Si3N4, were prepared by adding 0.5 wt% ofdispersants. Arrows indicate that the slurry viscosity is sufficiently low butis not measured any further at this point.


structure and the rheological response. The viscosity of the 35vol% Starck M11 Si3N4 slurry reached a minimum at additiveconcentrations of 0.5 wt% Betz 1190 and 0.2 wt% NopcosperseA44. However, the suspension with Betz 1190 exhibited shear-thinning behavior, because of a lower interparticle repulsion force,whereas the slurry with Nopcosperse A44 showed lower suspen-sion viscosity with almost Newtonian behavior.

The electroacoustic measurement and rheological study indi-cated that Nopcosperse A44 will work better than Betz 1190 as adispersant in the allowed pH range. Although the detailed disper-sion mechanism is still unclear, because of insufficient information

about Nopcosperse A44, the negatively ionized head group isbelieved to adsorb specifically onto the Si3N4 surface with thestretched-polymer-chain conformation, which results in electros-teric stabilization.23 A smaller amount of Nopcosperse A44, whichis required to achieve a stable slurry, also is preferred whenconsidering its possible adverse influence on the redispersion.

The addition of the anionic polyelectrolyte Nopcosperse A44readily modified not only Si3N4, but also Al2O3 and Y2O3, asnegatively charged surfaces, as shown in Fig. 5(a). The IEP of theStarck M11 Si3N4 shifted into the lower-pH region as the amountof Nopcosperse A44 increased. When 0.25 wt% was added, there

Fig. 5. Zeta potential behavior of Starck M11 Si3N4, Al2O3, and Y2O3 as a function of pH (a) without and (b) with Nopcosperse A44 dispersant (0.25 wt%).All the suspensions were aged for 24 h before the measurements.


were no observed IEPs for Si3N4 and Al2O3 over the neutral-pHrange, whereas the IEP of Y2O3 shifted from pH 8.6 to pH 6.5, asshown in Fig. 5(b). Therefore, an optimum slurry pH window forthe multicomponent Si3N4 system that invovles Nopcosperse A44is pH 8–9, at which point a well-dispersed slurry can be preparedwithout heterogeneous coagulation between Si3N4 and the sinter-ing additives.

Based on all the findings, the aqueous Si3N4 slurry wasformulated using the Starck M11 Si3N4 powders. The 32 vol%slurry at pH 8.5, using 2 wt% PEG-400, 6 wt% Al2O3, 6 wt%Y2O3, and 0.2 wt% Nopcosperse A44, exhibited a lower viscosity(11.4 cP at a shear rate of 300 s21), which is sufficient for slurryjetting in the slurry-based 3DPTM process. Furthermore, this slurrycan be consolidated into an easily redispersible powder bed with arelatively high packing density (57.5%). PEG improved the redis-persion—from 12.8% to 39.1%—for the powder bed that wasproduced by the TMAH-added slurries at pH.10, as shown inFig. 6. The percentage of the redispersed amount increased further,from 39.1% to 59.7%, when the Nopcosperse A44 dispersant wasused to prepare the slurries at pH 8.5. In addition, as indicated inGroup F in Table I, Nopcosperse A44 itself did not hinder theredispersion when its amount was decreased (0.2 wt%). Twopowder beds that were prepared using the TMAH-added slurries atpH .10 did not show a significant variation in the redispersionefficiency (12.8% vs. 13.3%), regardless of the presence ofNopcosperse A44.

The adsorption of polyelectrolytes such as Nopcosperse A44may reduce the rate of dissolution and/or hydration of Si3N4, asclaimed by Yasrebiet al.24 Those researchers demonstrated thatthe hydroxylated organic compounds with either a carboxylate,sulfonate, or phosphate functional group are added to the suspen-sion in an amount (0.01–5.0 wt%) that is effective to substantiallydisperse and reduce the rate of dissolution of the ceramic particles.This approach may give better flexibility in formulating slurrycompositions instead of controlling the slurry pH in the limitedregion. However, Nopcosperse A44 does not seem to function wellin such a way in the present study. If effective, the redispersionshould be improved noticeably for the powder bed that is producedfrom the slurry that involves Nopcosperse A44 at pH 10.4. Eventhe use of the same polymeric species that the Yasrebi group

already investigated may be ineffective for the slurry-based3DPTM process. Hydroxylated organic compounds with variousfunctional groups are likely to react chemically with the particlesurfaces when dried, which, presumably, would make redispersiondifficult.

V. Conclusions

A complex multicomponent silicon nitride (Si3N4) system forthe slurry-based Three Dimensional Printing (3DPTM) process hasbeen developed. Understanding of slurry chemistry is important inthe preparation of a well-dispersed slurry that forms easily redis-persible powder beds after drying. The redispersion behavior isdependent on the number and strength of particle–particle contacts.The redispersion efficiency increases as the particle size increasesand the packing density decreases, because the number of particle–particle contacts decreases. However, the sintering rates areaffected strongly by the powder packing; therefore, reducing thenumber of particle contacts is only feasible to a small extent.Reducing the strength of particle–particle contacts would be thedesired approach to improve the redispersion behavior of thepowder bed.

The chemical stability of the powders in liquids exhibited anintimate relationship in regard to the strength of the particle–particle contact. As demonstrated by a leaching study, an amor-phous surface oxide layer on the Si3N4 powders can dissolvereadily within 24 h in alkaline solution. Then, the reprecipitation ofthe dissolved silica is likely after the slurry dries. This action willbond individual particles together and impede redispersion. Thesechemical-stability considerations for the Si3N4 and the sinteringadditives (alumina and yttria) limit the optimum slurry pH to anarrow range.

Polymeric components in the slurry also have great impact onthe redispersion behavior. Low-molecular-weight polyethylenegylcol (PEG) improves redispersion, whereas higher-molecular-weight PEG inhibits redispersion. PEG-400 is believed to reducethe strength of the particle–particle contacts by forming a solublebridge at the necks of the particles as the slurry dries, because it ishighly soluble in water. On the other hand, the less-soluble

Fig. 6. Influence of slurry pH and PEG on the redispersion behavior. The detailed powder-bed characteristics are summarized in Group F in Table I. Thenormalized mass can vary from 1 (no redispersion) to 0 (complete redispersion). The percent redispersed is 1003 (1 2 normalized mass).


PEG-8000 seems to act as a binder that increases the strength ofthe powder beds.

Formulation of a slurry for high-surface-area submicrometerSi3N4 powders that include sintering additives, have a low viscos-ity in the limited pH range, and also produce an easily redispersiblepowder with high packing density is a challenge. The use of theoptimum amounts of the anionic polyelectrolyte Nopcosperse A44and PEG-400 has produced a stable multicomponent slurry that issuitable for the slurry-based 3DPTM process at the range of pH8–9. The powder bed that is generated from such slurries exhibitsimproved redispersion, as well as a relatively high packing density.

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