Materials Chemistry and Physics 82 (2003) 181–187

Suspension stability and consolidation behavior of ultrafineBaTiO3 particles in nonazeotropic solvent system

Dae-Hwan Kima, Jeong-Gu Yeoa, Yeon-Gil Jungb, Sung-Churl Choia, Ungyu Paika,∗a Department of Ceramic Engineering, Hanyang University, Seoul 133-791, South Korea

b Department of Ceramic Science and Engineering, Changwon National University, Changwon 641-773, South Korea

Received 4 December 2002; received in revised form 14 April 2003; accepted 17 April 2003

Abstract

The effects of nonazeotropic toluene/ethanol and toluene/n-butanol solvents on the suspension stability and consolidation behavior ofthe 100 nm BaTiO3 particulate suspensions were investigated. The BaTiO3 suspensions showed a similar level of dispersion stability inboth solvent mixtures. However, the BaTiO3 suspension prepared in the toluene/n-butanol solvent yielded a better green microstructurethan that in the toluene/ethanol solvent, which was indicated by the higher packing density and homogeneity of the suspension preparedin the toluene/n-butanol solvent. The inconsistency between the suspension stability and green-body properties was attributed to thephysicochemical properties of solvent mixtures such as composition and different vapor pressure. The mixing ofn-butanol having lowvapor pressure into toluene facilitated particle rearrangement along the solvent flow during casting and drying, thus developing a moreuniform and homogeneous microstructure.© 2003 Elsevier Science B.V. All rights reserved.

Keywords: Ceramics; Electron microscopy; Microstructure

1. Introduction

Ultrafine BaTiO3 powders (100 nm) are essential for thepreparation of ultra-thin dielectric layers with a thickness of1–1.5�m in order to augment the electrical properties andto decrease the size of multilayer ceramic capacitors (ML-CCs) [1,2]. However, the exaggerated specific surface areaof the nanosized particles creates new problems for powderprocessing, such as inhomogeneity of the microstructureof laminated tapes resulting from agglomeration and diffi-culty in dispersion[2,3]. Many attempts have been madeto resolve this problem. One of these has been to con-trol the interparticle forces between ultrafine particulates[4–8], which can lead to the well-dispersed or controlledflocculated suspension[4–6] and desirable green-body char-acteristics[6,9,10]. However, these efforts have focusedmainly on manipulating interparticle interactions in order toachieve high packing density[11–14] and limited researchhas been conducted on the effect of liquid vehicles on theconsolidation behavior and tape density, which are directlycorrelated to the microstructural development.

∗ Corresponding author. Tel.:+82-2-2290-0502; fax:+82-2-2281-0502.E-mail address: upaik@hanyang.ac.kr (U. Paik).

Many studies of particulate suspension systems with par-ticles greater than≈500 nm in nonaqueous media[9–11]have dealt with the correlation between tape properties andsuspension stability[10,15–17]. It has been shown that themechanical (e.g. strength and elongation) and physical prop-erties (e.g. density and thickness tolerance) of tapes aredependent on the organic constituents of the suspension[9,10,15,16]. However, a few studies have described the ef-fects of processing variables on the dispersion behavior andtape properties of the 100 nm BaTiO3 particles[2].

Various defects during drying (e.g. cracking and skinning)form due to the fast evaporation of a single solvent[3,18],resulting in delamination and flaws in the green tapes and fi-nal products[19]. Therefore, a mixed solvent system, whichallows for a slower evaporation rate, has replaced the singlepure solvent in the tape casting process[14,20,21]. How-ever, a change in the suspending medium may result in otherproblems with the deflocculation of particle, such as modi-fications in the Hamaker constant, immiscibility of solventand organic additives, and changes in rheological behavior[14,20], which is closely related to particle packing and tapedensity [5,8,17]. The simple act of solvents mixing altersthe physicochemical properties of the suspended media, in-cluding the Lewis acid–base, the solubility parameter, andthe boiling point[14,20].

0254-0584/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0254-0584(03)00203-7

182 D.-H. Kim et al. / Materials Chemistry and Physics 82 (2003) 181–187

Therefore, it is necessary to understand the roles of thenonazeotropic mixed solvent on the dispersion behaviorand green microstructure of the 100 nm BaTiO3 system inorganic vehicle in order to accomplish ultrafine BaTiO3thin film fabrication. The purpose of this study is to clarifythe effects of toluene/ethanol and toluene/n-butanol solventmixtures on the suspension stability and consolidation be-havior of 100 nm BaTiO3 suspension. The stability of thesuspensions prepared in the two different solvent mixtureswas compared by rheological measurement, interparticleenergy calculation, and adsorption isotherms. In addition,the green tapes properties were also investigated and thecorrelation between the suspension stability and consolida-tion behavior of BaTiO3 was extensively discussed as well.

2. Experimental procedures

2.1. Starting materials and preparation

Commercial BaTiO3 powder (Sakai Chemicals, Japan)was used in this experiment. The powder had a BET spe-cific surface area of 12.1 m2 g−1 by N2 gas adsorption witha median particle diameter of 0.1�m, which is providedby the manufacturer. To remove physically adsorbed wa-ter on the powder surface, the raw powders were kept ina desiccator for minimum 24 h prior to processing. Twosolvent mixtures of toluene/ethanol (hereinafter Tol/EtOH)and toluene/n-butanol (hereinafter Tol/n-BtOH) were used assuspending media in this study. Toluene (C6H5CH3), ethanol(C2H5OH), and n-butanol (CH3(CH2)3OH) were analyti-cal reagent grade (Samchun Chemical, Seoul, South Korea)and used without further purification. The mixture ratio wasdetermined as Tol/EtOH of 60/40 mass% and Tol/n-BtOHof 80/20 mass% from viscosity measurements. The suspen-sions for the further investigation were prepared accordingto these compositions of Tol/EtOH and Tol/n-BtOH withoutany declaration noted. The relevant physical and chemicalproperties of both solvents are shown inTable 1. Phosphateester (RE610, Toho Chemical Industry, Japan) with molecu-lar mass (Mr) of 1500–2000 g mol−1 and poly(vinyl butyral)

Table 1Physical and chemical properties of solvents

Solvent Dielectric constant Acidity ALa (kT) AS/L/S

b (kT) δc BPd (◦C)

Toluene 2.37 Base 16.95 42.78 8.9 110.6Ethanol 24.3 Acid 94.58 0.87 12.8 78.6n-Butanol 18 Acid 88.72 1.53 11.4 117.2Tol/EtOH azeotrope 10.53e 76.7Tol/n-BtOH azeotrope 9.43e 105.5

a Estimated Hamaker constants of liquids in vacuum (see Ref.[22]).b Estimated Hamaker constants for the combined system of BaTiO3/solvent/BaTiO3 with the equation (see Ref.[22]).c Ref. [26].d Ref. [27].e Estimated solubility parameters for the nonazeotropic solvents of Tol/EtOH of 60/40 mass% and Tol/n-BtOH of 80/20 mass%.

Table 2Suspension formulations used in this study and tape density

Solvent Dispersant(wt.%)

Binder(wt.%)

Tape densitya

(g cm−3)

Tol/EtOH 1.0 13.0 2.93± 0.02Tol/n-BtOH 1.0 13.0 3.19± 0.06

a Data are means and standard deviation of a minimum of five speci-mens at each suspension formulation.

(PVB-BL, Seikisui, Japan) withMr of 30,000 g mol−1 wereused as dispersant and binder, respectively.

Suspensions were prepared by adding BT01 powder at asolids volume fraction of 10%. Stock solutions of RE610and PVB were individually prepared in respective mixturesolvent. The BaTiO3 suspensions were first mixed with thedispersant solution followed by milling for 3 h using zir-conia ball to ensure a thorough and uniform wetting of allthe particles. The binder stock solution were then added tothe suspension and ball-milled for an additional 20 h. Thesuspension formulations used in this study are summarizedin Table 2. The detailed preparation procedures of BaTiO3suspensions were reported elsewhere[2]. The suspensionswere cast using a table caster (Samsung Electromechanics,Suwon, South Korea) with a single doctor blade at speed of177 cm min−1. The thickness of a single layer of green tapewas approximately 13�m. In order to ascertain the effect ofthe drying atmosphere, the BaTiO3 suspension prepared inthe Tol/EtOH mixture was also cast with a hand caster in anambient condition of 40% humidity and in a humid dryingchamber of >70% humidity at room temperature.

2.2. Characterization

The rheological behavior of the suspensions was char-acterized using a concentric-cylinder viscometer (MCR300,Paar Physica, Stuttgart, Germany). The shear rate was in-creased from 0 to 700 s−1 over a 4 min period. A fixed shearrate of 100 s−1 was chosen for the comparison among vis-cosity values.

Adsorption isotherms were measured to identify the con-tribution of steric stabilization to the stability of BaTiO3

D.-H. Kim et al. / Materials Chemistry and Physics 82 (2003) 181–187 183

suspensions. Weighed amounts of BaTiO3 powder were dis-persed into phosphate ester solutions at the desired con-centration. The equilibrated suspensions were centrifugedat 12,000 rpm and the supernatant was decanted. The solidssediment was washed with a fresh solvent and re-centrifugedthree times. The solids sediment was dried in an oven at100◦C for 20 h, after which the residual phosphate ester wasdetermined by thermogravimetric analysis (TGA, SDT2960,TA Instruments, DE).

Tape density, which is the apparent density of the greentape before pressing, was measured using geometric anal-ysis. Microstructure of the green tapes was investigatedby scanning electron microscopy (SEM, Leica StereoscanS440, Cambridge, UK) and field emission scanning electronmicroscopy (FE-SEM, S-4700, Hitachi, Ibaraki, Japan).

3. Results

3.1. BaTiO3 suspension stability

Rheological behaviors of the BaTiO3 suspensions withphosphate ester at a volume fraction of 10% solids wereevaluated as a function of the mixing ratio of the bi-solventsystem (Fig. 1). All data plotted inFig. 1 were viscositiesmeasured at a shear rate of 100 s−1. Mixing ratios of tolueneto ethanol andn-butanol varied from 0:100 to 100:0 mass%.The viscosity decreased up to the ethanol mass fractionof 40% and then-butanol mass fraction of 20%, and thenmodestly increased. The Tol/n-BtOH mixture had a higherviscosity than the Tol/EtOH mixture at all compositions.Fig. 2shows the measured flow curves for 10 vol.% BaTiO3suspensions prepared with phosphate ester and PVB in thetwo solvent mixtures. The suspension formulation was thesame except for the liquid medium species. The BaTiO3suspensions prepared in the Tol/n-BtOH and Tol/EtOHmixtures showed a slight decrease in viscosity at all shear

Fig. 1. Apparent viscosity of BaTiO3 suspension with volume fractionof 10% using phosphate ester mass fraction of 0.1% in various solventfractions of the Tol/EtOH and Tol/n-BtOH mixtures at the shear rate of100 s−1.

Fig. 2. Apparent viscosity of BaTiO3 suspension with different disper-sant mass fraction in the 60/40 mass% of Tol/EtOH and 80/20 mass% ofTol/n-BtOH mixtures at shear rate of 100 s−1.

rates, even when the suspension prepared in the Tol/EtOHmixture behaved as more Newtonian-like fluid.

In order to evaluate the contribution of steric effect onthe stability of BaTiO3 in both mixed solvents, adsorptionisotherms were obtained for the phosphate ester.Fig. 3showsthe adsorption of phosphate ester on BaTiO3 prepared intwo different solvent systems (Tol/EtOH and Tol/n-BtOH).The plateau adsorption amount was found to be the samein both solvents. The adsorption of phosphate ester on theparticles increased and became saturated at approximately2.2 mg m−2, which corresponds to a mass fraction of 1%solids.Fig. 4shows the measured flow curves for the BaTiO3suspensions prepared in the two solvent mixtures as a func-tion of phosphate ester concentration. Viscosity measure-ment was performed at a fixed shear rate of 100 s−1. Asthe amount of dispersant was increased, the apparent vis-cosity decreased up to a dispersant mass fraction of 1.0%.Beyond this concentration, the suspension viscosities be-came nearly constant in both solvent systems. This indicatedthat phosphate ester was a good dispersant for both solventmixtures.

Fig. 3. Adsorption isotherms of phosphate ester on BaTiO3 particlesprepared in the 60/40 mass% Tol/EtOH and 80/20 mass% Tol/n-BtOHmixtures.

184 D.-H. Kim et al. / Materials Chemistry and Physics 82 (2003) 181–187

Fig. 4. Rheological behavior of BaTiO3 suspension as a function ofconcentration of phosphate ester in two solvent mixtures.

3.2. Green microstructure

The green microstructures in the fractured surfaces of theas-dried tapes in ambient conditions were observed by SEMand the micrographs are shown inFig. 5. The two solventmixtures have different boiling points. The green tape pre-pared in the Tol/n-BtOH mixture appeared relatively lessaggregated and rugged than the green tape prepared in theTol/EtOH mixture, which reflects denser and more uniformpacking of the particles in the Tol/n-BtOH mixture. Thiscannot be clearly observed in the SEM images. However,it was quantitatively verified by the tape density measure-ments (Table 2). The density of the tape in the Tol/n-BtOHmixture was higher than that in the Tol/EtOH mixture. This

Fig. 5. Microstructure in fractured surfaces of green tapes prepared: (A)in the Tol/EtOH and (B) in Tol/n-BtOH solvent mixtures.

Fig. 6. Microstructure in fracture surfaces of green tapes with differentdrying conditions at room temperature: (A) in ambient condition and (B)in humid chamber.

is due to uniform packing of the particles in the Tol/n-BtOHsolvent. Particle agglomeration surrounded by polymer canbe seen inFig. 5A.

In order to ascertain the effect of solvent vapor pressureon the microstructural development of the tape prepared inthe solvent mixture, atmosphere of drying chamber was con-trolled and the green microstructure of the tapes evaluatedby SEM.Fig. 6shows two micrographs of the as-dried tapesprepared in the Tol/EtOH mixture at room temperature in anambient condition (Fig. 6A) and in a humid one (Fig. 6B).The green microstructure of the tape fabricated in humidconditions at room temperature was more homogeneous andless aggregative than that dried in ambient conditions.

4. Discussion

In our previous study[11], the electrostatic stabilizationof BaTiO3 particles in nonaqueous media was influenced bythe liquid vehicles. We have shown that the BaTiO3 surfacepotential in pure toluene was essentially zero, resulting in theflocculated suspension. This was attributed to the less inter-action with the basic surface of BaTiO3 because toluene isnonpolar and considered to be a weakly basic solvent. Thisindicates that toluene is not a proper solvent for dispersingBaTiO3 particles, which is macroscopically demonstrated in

D.-H. Kim et al. / Materials Chemistry and Physics 82 (2003) 181–187 185

the viscosity measurement exhibiting the highest viscosityin Fig. 1. Small additions of alcohol to the toluene signifi-cantly decreased the suspension viscosity, due to a decreasein the attractive van der Waals force and an increase in theelectrostatic repulsive forces from charge development asso-ciated with the polarity and hydrophobicity of the respectivesolvents[20].

Since the potential energy of interaction from van derWaals force is proportional to the combined Hamakerconstant (AS/L/S) for the solid–liquid–solid system, theHamaker constants of BaTiO3 particles in liquid media canbe estimated with the equations used by Krishnakumar andSomasundaran[22] which are summarized inTable 1. Itcan be seen fromTable 1that the attractive component ofthe interaction potential energy for organic solvents washighest for toluene, lowest for EtOH, and relatively largerfor n-BtOH than EtOH.

In addition to the role of the Hamaker constant, particlecharge development in relation to solvent polarity may playa significant role in this system. According to the theory ofFowkes et al.[23,24], as EtOH adsorbs onto BaTiO3, theacidic nature of the adsorbate results in proton transfer tothe particle surface. In the process of dynamic adsorptionand desorption, EtOH desorbs into solution leaving a resid-ual positive surface charge. The apparent stability of BaTiO3increases as a result of an increase in the solvent polarityand the acidic nature of alcohol. The results of our viscos-ity measurements are in good agreement with the theory ofFowkes et al.[23,24]. The apparent viscosities of both sus-pensions decreased with the addition of alcohol (Fig. 1).More valid evidence was provided by viscosity measure-ment as a function of shear rate for the BaTiO3 suspensionsprepared in two mixed solvents. Both suspensions showeda similar decrease in viscosity at all shear rates (Fig. 2).This indicates that the suspension had identical dispersionstates. It was found that EtOH was relatively more effectivethann-BtOH in enhancing dispersion stability. The viscos-ity decreased up to the ethanol mass fraction of 40% andthe n-butanol mass fraction of 20%, and then modestly in-creased. The decline in suspension viscosity from its initialvalue was more sharply in the Tol/EtOH mixture than in theTol/n-BtOH mixture (Fig. 1). The viscosity of the Tol/EtOHwas lower than the viscosity of Tol/n-BtOH suspension andthe Tol/n-BtOH suspension exhibited almost Newtonian atall shear rates indicating a long-term stability (Fig. 2). Thelower viscosity and Newtonian fluid behavior of the BaTiO3suspension was attributed to the fact that EtOH had the rel-ative lower Hamaker constant thann-BtOH.

The stability of ceramic particles is based on the sum-mation of the inherent van der Waals attraction betweenparticles, electrostatic repulsion from the ionized particlesurfaces, and steric hindrance from the surfactants adsorbedonto particle surfaces[5,11,13]. Since particles suspendedin a solvent of low dielectric constant (e.g. organic solvent)develop less surface charge, the van der Waals attractiveforce and steric hindrance become the dominant factors in

determining stability in nonaqueous media[25]. However,the electrostatic repulsion of ceramic particles in nonaque-ous media should be also considered because the electro-static repulsion of ceramic particles may play an importantrole in the dispersion stability[11,20]. As summarizedin Table 1, the Hamaker constants in the BaTiO3/EtOHand BaTiO3/n-BtOH interfaces are calculated as 0.87 and1.53 kT, respectively. The effective Hamaker constant wasalso estimated using the procedure adopted by Bergstromet al. [6] and shows little difference between BaTiO3/EtOHsystem of 5.0 × 10−20 J and BaTiO3/n-BtOH system of4.6× 10−20 J at 298 K. The contribution of phosphate esterto BaTiO3 stability is shown inFig. 3, which reveals thatthe plateau region for phosphate ester adsorption on BaTiO3was the same in both solvent mixtures, which indicatesthat steric hindrance from the adsorbed phosphate ester wasidentical in each suspension and that phosphate ester hasthe same affinity for BaTiO3 regardless of the solvent mix-ture. This was confirmed from the plot of viscosity versusphosphate ester concentration (Fig. 4), which showed thatviscosity decreased with the phosphate ester additions up to1.0 wt.%. At phosphate ester’s concentrations greater than1 wt.%, viscosity remained constant. It was concluded thatthe BaTiO3 particles in the two solvent mixtures exhibitednearly the same level of dispersion stability.

Furthermore, the Hildebrand solubility parameter (δ) ofeach bi-solvent was estimated using a rule of mixture inorder to ascertain the compatibility between polymer andsolvent[26]. As summarized inTable 1, the solubility pa-rameters of two solvent mixtures in this study were notmuch different between Tol/EtOH of 10.53 and Tol/n-BtOHof 9.43. This means that the solubility degree of the phos-phate ester used in this study is almost similar in bothTol/EtOH and Tol/n-BtOH mixtures. Also, viscosity of thepolymer solutions in two solvent mixtures were also mea-sured in a preliminary experiment, which results exhibitedsimilar flow curves in both solutions. Thus, it can be con-cluded that the contribution of polymer to the solvent isalmost the same in the present study.

Even though the colloidal state of the BaTiO3 suspen-sions prepared in the Tol/EtOH and Tol/n-BtOH mixtureswere analogous, their green microstructures and tape densi-ties were different. The green microstructure of the tape-castprepared in the Tol/n-BtOH mixture was more uniformand less aggregated than the green microstructure of thetape-cast prepared in the Tol/EtOH mixture. The tape den-sity prepared in the Tol/n-BtOH mixture was about 9%higher than the tape prepared in the Tol/EtOH mixture. Therelatively more uniform microstructure and higher densityof the tape prepared in the Tol/n-BtOH mixture cannot beunderstood from the suspension stability between two sys-tems, where two suspensions in both solvent mixtures havea similar interaction between particles and solvent as afore-mentioned. The inconsistency between dispersion stabilityand green properties was attributed to the mixing ratio andthe different vapor pressures of the liquid vehicles.

186 D.-H. Kim et al. / Materials Chemistry and Physics 82 (2003) 181–187

In the preceding results of viscosity measurement, theoptimized ratios of the two-component solvent used in thisstudy were Tol/EtOH of 60/40 mass% and Tol/n-BtOH of80/20 mass%. These ratios are different from the azeotropiccomposition of the individual components: Tol/EtOH of32/68 mass% and Tol/n-BtOH of 72/28 mass%[27]. Anazeotrope of two- or three-component solvents is one of themost common solvent combinations used for tape casting.An azeotrope is defined as a blend of solvents fully solublein each other, and thus behaves as a single liquid[27]. Inother words, an azeotropic mixture such as Tol/EtOH andTol/n-BtOH has a single boiling point (lower than the indi-vidual liquid) and a single vapor pressure (Table 1). How-ever, it is very difficult to obtain uniform and high-qualitylaminated materials from tape fabricated in a single pureliquid or in an azeotrope because defects in the tapes areformed from fast and uneven evaporation[21]. Fast evapo-ration during drying occurs largely from the tape surfacesat a temperature relatively lower than the boiling point ofthe individual solvent, inevitably resulting in polymericskinning and trapping of gas bubbles. A variety of dryingdefects (e.g. body and edge curling, orange peel, and crack-ing) is sometimes encountered in dried tapes due to fastand uneven evaporation[3]. Therefore, a two-componentnonazeotropic solvent is preferred for high-quality tapefabrication in order to control evaporation rate.

The evaporation of a solvent mixture occurs in a two-stageprocess. Since ethanol evaporates at a faster rate thantoluene, the latter will precipitate in the binder matrix. Atthe same time, the toluene remains as a residual liquid atthe interface and continues to flow the solvent[21]. Thisalso facilitates particle movement along the diffusion path.When comparing between the Tol/n-BtOH and Tol/EtOHmixture, a low vapor pressure solvent system, such asTol/n-BtOH having a relatively high boiling point, retardssolvent evaporation at the surface during drying as well ascasting, and the residual solvent allows particles to movethrough the liquid vehicle and promotes rearrangement dur-ing a longer period of drying. This rearrangement supportedby the residual liquid of a lower vapor pressure has a pos-itive effect on particle packing and green microstructure,which is quantitatively evidenced by tape densities (Table 2)although the microstructural difference between two tapesis not clearly discerned in SEM images inFig. 5. On theother hand, particle movement in azeotropic mixtures islikely to be difficult along the high viscous liquid becauseevaporation proceeds at a temperature relatively lower thanthe boiling point of an individual solvent[28]. Finally, it isfound that the physicochemical properties of solvent suchas boiling point and vapor pressure strongly influence thegreen microstructure and green density.

The rate of evaporation is proportional to the differencebetween the liquid vapor pressure in the system and theambient vapor pressure, and this proportionality is depen-dent on the temperature, draft, and geometry of the system[19]. A low ambient vapor pressure causes the solvent to

evaporate more rapidly than in a near-saturated atmospherein otherwise identical systems[3,18]. This can be seen bythe two SEM images inFig. 6. When surface evaporationis faster than solvent diffusion via capillary action, variousdrying defects can occur presumably due to pore channelsleft behind by the evaporating solvent. In addition to the in-homogeneous nature of green tape, the drying defects, e.g.cracking and orange peel, leave small craters in the tapesurface that can remain on the surface of the fired products[3], degrading their electrical properties and reliability. Rel-atively slow or controlled evaporation at the tape surfaceallows the particles to pack efficiently resulting in a moreuniform microstructure, which may facilitate densificationwith a desired microstructure. This can be seen in the mi-crograph of the dried tape (Fig. 6B). From this, it can beseen that drying condition also plays an important role infabricating high performance tapes in coating industries aswell as multilayer structured materials.

5. Conclusions

The optimum solvent composition for the dispersion ofBaTiO3 particles was determined to be 60/40 mass% forTol/EtOH and 80/20 mass% for Tol/n-BtOH. The BaTiO3suspension was best dispersed in the Tol/EtOH solvent mix-ture when a 0.1% mass fraction of dispersant was used.However, the dispersion stability of BaTiO3 particles witha phosphate ester mass fraction of 1.0% was identical, inboth solvent mixtures. Over a phosphate ester mass frac-tion of 1.0%, the role of dispersant in the dispersion ofthe BaTiO3 suspension was more dominant than that of thesolvent.

The BaTiO3 suspension prepared in the Tol/EtOH mixtureshowed better dispersion stability than that prepared in theTol/n-BtOH mixture. However, the green tape prepared inthe Tol/n-BtOH mixture had a more homogeneous greenmicrostructure and higher tape density than that preparedin the Tol/EtOH mixture because of the rearrangement ofparticles during the drying process.

Solvent properties such as the dielectric constant, Lewisacid–base, and Hamaker constant affected the dispersion ofthe solution while the boiling point and vapor pressure ofthe solvent influenced the green microstructure and packingdensity. The drying condition also affected the green mi-crostructure. A more homogeneous microstructure was ob-tained in the humid drying condition than in the ambientone.

Acknowledgements

This work was financially supported by the Korea Insti-tute of Science and Technology Evaluation and Planning(KISTEP) through the National Research Laboratory (NRL)program.

D.-H. Kim et al. / Materials Chemistry and Physics 82 (2003) 181–187 187

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