Materials Science and Engineering A362 (2003) 174–180

Effect of molecular mass of poly(vinyl butyral) and lamination pressureon the pore evolution and microstructure of BaTiO3 laminates

Yong-Sang Choa, Jeong-Gu Yeoa, Yeon-Gil Jungb, Sung-Churl Choia,Jonghee Kimc, Ungyu Paikd,∗

a Department of Ceramic Engineering, Hanyang University, Seoul 133-791, Republic of Koreab Department of Ceramic Science and Engineering, Changwon National University, Changwon 641-773, Republic of Korea

c Samsung Electro-Mechanics Co. Ltd., Suwon 442-743, Republic of Koread Department of Ceramic Engineering, Hanyang University, Seoul 133-791, Republic of Korea

Received 11 June 2002; received in revised form 26 June 2003

Abstract

The influence of molecular mass of poly(vinyl butyral) (PVB) binder and lamination pressure on the pore evolution and sintered microstruc-ture of tape-cast BaTiO3 sheets was investigated by performing mechanical measurements on green tapes, by carrying out measurements onthe density and porosity, and by performing scanning electron microscopy (SEM) observations. The binder affected the elongation of thegreen sheets, but showed little effect on the tensile strength. The relative density of the as-sintered tapes increased with increasing molecularmass (Mr) of the binder, and with increasing lamination pressure. Above a lamination pressure of 78 MPa, the density of each binder systemwas almost the same. The pore size distribution and cumulative pore surface area in the sintered bodies became narrower and smaller withincreasing binderMr and lamination pressure, respectively, indicating that the critical lamination pressure was 800 kgf/cm2 for each bindersystem. Microstructural observations revealed that the lamination pressure played a more important role in shrinking the pores and promotingsintering. The highMr binder showed a higher relative density and an improved sintered microstructure for the same lamination pressure.© 2003 Elsevier B.V. All rights reserved.

Keywords: BaTiO3; Poly(vinyl butyral); Lamination pressure; Density; Porosimetry; Microstructure

1. Introduction

Tape casting[1], lamination[2], and sintering[3,4] areessential steps in the fabrication of multilayer ceramic pack-ages[5,6] and variations in these processes can result insignificant changes in the lifetime and reliability of the finalproduct [7,8]. The electrical and mechanical properties ofboth the green tapes and the final package, are both closelyrelated to the aspects of the lamination process[2,9], whichinvolves several process variables, such as pressure, temper-ature, and time. In particular, changes in pressure during thelamination process significantly affect pore evolution and thefinal microstructure[2]. In addition, the choice of binder is avery important factor in the fabrication of multilayer ceramicpackages[8], as it influences particle packing during forma-tion and the mechanical properties of the green tape[10].

∗ Corresponding author. Tel.:+82-2-2290-0502;fax: +82-2-2281-0502.

E-mail address: upaik@hanyang.ac.kr (U. Paik).

The polymeric binder acts as an adhesive for the ceramicparticles within the green sheet[7,8,11]. The binder is usu-ally present during most of the processing stages of ceramiccomponents[8–10]. The chemical structure and molecularmass (Mr) of the binder within a green sheet, influences thetape properties in the lamination process, such as tensilestrength and elongation. This indicates that the chemicalstructure andMr of the binder influence the rearrangementof particles surrounded by the polymer vehicle. Localizedbinder in a laminated green sheet, results in a high porosityin the tapes after binder burnout. Samples with residualpores cannot be sintered to achieve the theoretical density,and therefore, control of the microstructure in a green sheetis required to obtain a fully densified product[12].

For complete binder burnout, it is suggested that thesmallest possible mass of the binder be added to a slurry,compared to the normal formulation. Some studies have beenconducted on advanced binder–solvent systems to improveperformance with reduced binder content[13,14]. However,this new approach has inherent mechanical problems for

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Y.-S. Cho et al. / Materials Science and Engineering A362 (2003) 174–180 175

the tapes produced, due to the reduced strength, which willultimately result in failure during multilayer ceramic fab-rication. A laminated tape system that has a highly packedstructure, should retain a high enough strength that can becontrolled by the binder content and distribution. However,there is little published information on the lamination pro-cess of multilayer ceramic capacitors (MLCCs) related totapes prepared with various binders and nano-sized BaTiO3particles, and in particular, those with nominal particlediameters of 100 nm, which is the area of interest of thisstudy. Barium titanate particles with diameters of 100 nmcan be used to fabricate dielectric layers with thicknessof <1.5�m.

In this work, the effect of the molecular mass of the binderand the lamination process on the consolidation, pore evo-lution, and microstructure was studied using 100 nm sizedBaTiO3 particles. The relationships observed are discussedbased upon the experimental mechanical strength, density,mercury porosimetry, and microstructural observations.

2. Experimental

2.1. Materials

A commercial BaTiO3 powder (BT-01, Sakai ChemicalsCompany, Japan) was used in this study. The powder had aBET specific surface area of 12.1±0.05 m2/g with a nominalparticle diameter of 100 nm. To remove any physically ad-sorbed water and volatile organics adsorbed on the powdersurface, the powder was vacuum dried at 200◦C for 24 h inair. A mixture of toluene and ethanol was chosen as the sus-pending medium, and its ratio fixed at a mass fraction (%) oftoluene/ethanol of 60/40 based upon a previous study[15].Phosphate ester (RE610, Toho Chemical Industry, Tokyo,Japan) with a molecular mass ofMr = 1500–2000 g/molwas used as a dispersant. Two poly(vinyl butyral) resins(PVB, Sekisui Chemical Inc., Osaka, Japan), lowerMrbinder (BL) withMr = 30,000 g/mol, and higherMr binder(BM) with Mr = 90,000 g/mol, were used as binders. PVBbinders are frequently used in thick-film fabrication, be-cause they can be easily dissolved in mixed solvents, andtherefore, provide a high slurry viscosity for casting[11].Table 1shows the physicochemical properties of the PVBbinders used. Great care was taken to minimize water con-tamination, and all the solvent containers were purged withnitrogen gas, as the water content affects the adsorptionbehavior of the dispersant and the binder on the particlesurface.

Table 1General properties of the PVB resins used in this study

PVB binder Molecular mass (g/mol) Butyral content (wt.%) Hydroxyl content (wt.%) Viscositya (mPa s)

BL 30,000 72± 3 26 ± 5 10–30BM 90,000 76± 3 22 ± 5 100–170

a The viscosity was measured at 20◦C using a rotational viscometer independent of shear history.

2.2. Sample preparation and characterization

The BaTiO3 suspensions were prepared at a volume frac-tion of 10%. Stock solutions of RE610 and PVB were pre-pared in mixtures of toluene and ethanol (mass fraction (%)of 60/40), and were used in the experiments at mass frac-tions of 1 and 13%, based on the solid content. The BaTiO3powders were first mixed with the dispersant solution andball-milled for 3 h using zirconia grinding media to ensure athorough and uniform wetting of all the particles. The PVBstock solutions were then added to the suspension. The pre-pared suspensions were ball-milled for an additional 20 h atroom temperature.

Green sheets were cast using a table caster (SamsungElectromechanics Co. Ltd., Suwon, Korea) with a singledoctor blade at a casting speed of 177 cm/min. The thick-ness of a single layer of green tape was approximately13�m. The tapes were dried in the drying zone of the tapecaster using a bottom heating system at 50◦C. The greentapes were finally laminated without any inner electrodesin 90-layer stacks. The stacked tapes were isostaticallypressed under various pressures (29, 78, and 118 MPa) at80◦C for 10 min, and then cut into green blocks with finaldimensions of approximately 13 mm× 11 mm× 1 mm. Thebinder burnout and sintering stages were performed at 250and 1200◦C, respectively, with a holding time of 1 h forthe binder burnout, and 3 h for sintering, which was carriedout under a reducing atmosphere. The heating rate wascontrolled at 5◦C/min in both processes.

The green tape microstructure was observed using ahigh-resolution scanning electron microscope (HRSEM,Leica Stereoscan S440, Leica, Cambridge, UK). The ten-sile strength and elongation of the laminated tapes withdimensions of 150 mm× 5 mm× 0.13 mm were measuredusing a mechanical testing machine (QTS25, CNS FarnellCorp., London, UK). The densities of the green blocks andsintered samples were measured using Archimedes princi-ple. The pore size and distribution of the sintered sampleswere measured using a mercury porosimeter (Autoscan-25,60, Quantachrome Corp., Syosset, NY, USA), from whichthe cumulative pore surface area was calculated. The finalmicrostructure of the sintered samples was observed usinga scanning electron microscope (S2700, Hitachi, Japan).

3. Results and discussion

Fig. 1 shows SEM micrographs of non-laminated tapes,which were made from the BaTiO3 suspension prepared with

176 Y.-S. Cho et al. / Materials Science and Engineering A362 (2003) 174–180

Fig. 1. Microstructure of the fracture surface of green sheets containing three different molecular mass binders: (A) BM, (B) BM–BL 50:50 wt.% mix,and (C) BL.

three different binders: BM (Fig. 1(A)), a 50/50 (by weight)mixture of BL–BM (Fig. 1(B)), and BL (Fig. 1(C)). Thesemicrographs reveal that there were no outstanding differ-ences among the samples. Therefore, the similarity of thegreen microstructures can be attributed to the characteristicsof the nano-sized BaTiO3 particles[16].

The green strength and elongation strain of the tapes pre-pared with three different binders were measured, and theresults are shown inFig. 2. The tensile strength was notsignificantly affected by the suspension formulations withdifferent binderMr values. The magnitude of the tensilestrength of the green tapes prepared with lowerMr binderwas somewhat larger than that of the green tapes preparedwith higher Mr binder, whereas the tape-cast BaTiO3 wassignificantly elongated with the BM. Even though the PVBsamples used had different chain lengths, and both the BLand BM PVB samples have such long molecular chains thatthey span a larger range than the inter-particle separationdistance, the flexibility of the green tape is improved withthe higherMr binder, resulting in an increase in sheet elon-gation. The elongation strain of a polymer sheet is depen-dent on the proportion of ordered–disordered structures inthe tape, such as orientated and amorphous regions[17,18].The degree of disorder of PVB within a tape was increasedin the higherMr tapes. The higher degree of entanglement

(A) (B)

BM BM+BL BL0

50

60

70

80

90

100

Rel

ativ

e de

nsity

[%]

PVB binder

29 MPa 78 MPa 118 MPa

BM BM+BL BL0

10

20

30

40

50

60

70

Rel

ativ

e de

nsity

[%]

PVB binder

Fig. 3. The observed green and sintered densities of tapes as a function of binderMr and lamination pressure. The data points represent the calculatedmean and standard deviation of a minimum of five samples per data.

BM BM + BL BL0

15

16

17

18

19

20

0

12

14

16

18

20

22

24

Ten

sile

str

engt

h [M

Pa]

Binder composition

Tensile strength

Elo

ngat

ion

(∆L/

L, %

) Elongation

Fig. 2. The green sheet tensile strength and elongation values of greensheets containing three different molecular masses of binder. Note thatthe error bars represent the range of data obtained.

of the larger PVB molecules, due to entropic considera-tions [19], can mean that the samples having the higherMrbinder more easily undergo orientation along the directionof an applied load, which is a phenomenon well known inthe die-drawing of polymer sheets[20]. Simultaneously, ithas a capability of allowing the movement or rearrangement

Y.-S. Cho et al. / Materials Science and Engineering A362 (2003) 174–180 177

of particles inside a green sheet that is cast from a suspen-sion containing a long-chain binder (e.g. BM). This rear-rangement may have a positive effect on a homogeneousmicrostructure, and allows full densification[21].

Fig. 3(A) shows that the green density of tapes did notvary much among the tapes prepared with differentMr PVBbinders, and that their values slightly increased with decreas-ing binderMr. Green tapes prepared with BL were approx-imately 54% dense, and those prepared with BM were 49%dense. The relative density of the as-sintered tapes showeda dependency on the lamination pressure and binderMr, asshown inFig. 3(B). The relative density of the as-sinteredtapes increased with increasing binderMr and laminationpressure. The improvement in density was more dependenton the lamination pressure than the binderMr, indicatingthat the tape density greatly increased under higher pres-sures. As the green sheet was laminated under a pressuregreater than 78 MPa, the relative densities observed weresimilar for tapes prepared with BL, BM, and a mixture ofBL–BM, and in particular, the highest value of >92% wasobserved for the tape prepared with BM. It was verified thatthe lamination pressure of 78 MPa was the critical point forhigher densification and reducing lamination defects.

Mercury porosimetry measurements and SEM analysiswere carried out to investigate the details of the effect ofgreen sheets prepared with different PVB types and lami-nation pressures on the pore evolution and as-sintered mi-crostructure of tape cast BaTiO3 sheets.Fig. 4shows typicalresults, demonstrating that the pore size distribution of thesintered bodies prepared with BL varied with laminationpressure. The pore size distribution at low lamination pres-sures was broader than that at higher lamination pressures.In addition, the pore size also decreased with increasinglamination pressure, and approached a common diameterwith increasing lamination pressure. When the green sheetwas subjected to lamination pressure of over 78 MPa, thepores were similar in size distribution, which is indicativeof a critical limit, above which the particles were unable topack any further to achieve higher lamination. The satura-tion pressure was found to be 78 MPa, in this study. Sucha saturation in lamination pressure, is in good agreement,with previous results described in the literature[2], and it isalso manifested in the observed densities of the as-sinteredtapes. A high lamination pressure, corresponding to 78 MPaand above, leads to a much closer-packing between theparticles, and thus, to pore shrinkage and coalescence witha resulting increase in densification.

The contribution of the type of PVB as well as the lami-nation pressure on the pore evolution of the as-sintered tapeswas also examined, and the results are shown inFigs. 5and 6. The samples with binder BM exhibited a narrowerdistribution, and had smaller pores, even at lamination pres-sure below 29 MPa. However, all the size distribution curveswere analogous above the critical lamination pressure of78 MPa, independent of the type of PVB binder used in thesystem. It was found that pore evolution was insensitive

Fig. 4. Pore size distribution of sintered blocks prepared with BL binderafter sintering at 1200◦C with a lamination pressure of: (A) 29 MPa, (B)78 MPa, and (C) 118 MPa.

to the type of PVB chain length under higher laminationpressures. Therefore, this process is more dependent on thelamination pressure than the variation in the PVB binderMr. From the above results, it can be deduced that the mostpreferable condition for the final products, is to preparegreen tapes with long-chain binders and undergo laminationat high pressures above 78 MPa.

The above-mentioned results are more clearly reflectedin the observed results of the cumulative pore surface area(Fig. 6). These reveal that the cumulative pore surface areaof the sintered bodies decreased with increasing laminationpressure, as shown inFig. 6(A). Above the critical lami-nation pressure of 78 MPa, the bodies show similar values.

178 Y.-S. Cho et al. / Materials Science and Engineering A362 (2003) 174–180

Fig. 5. Pore size distribution of sintered blocks prepared with lamination pressures of (A) 29 MPa and (B) 78 MPa, after sintering at 1200◦C with threedifferent binders with different molecular masses: (a) BM, (b) BM–BL 50:50 wt.% mix, and (c) BL.

This result is consistent with the pore size distribution re-sults (Fig. 4). The cumulative pore surface area was also af-fected by theMr value (seeFig. 6(B) and (C)). It is obviousthat the absolute value of the cumulative pore surface areasignificantly decreased with the use of long-chain binderswhen the same lamination process was employed, which isconsistent with the porosimetric results (seeFig. 5). There-fore, the effect of binderMr on pore evolution is moreclearly revealed at lamination pressures below the criti-cal lamination pressure. This same trend can be observedfor samples subjected to high lamination pressures above118 MPa (Fig. 6(C)), even though this effect was not high,having smaller values compared to the values shown inFig. 6(B). The cumulative area of samples prepared withan identical suspension formulation decreased sharply withincreasing pressure in the range of 29 MPa (Fig. 6(B)) to118 MPa (Fig. 6(C)). This indicates that changes in the PVB

binder Mr only induce minor effects, and that the closepacking of particles within the tapes is considerably pro-moted during lamination[2,7,9]. It is also evident that thelamination pressure is a dominant factor in pore evolution.

More direct evidence is provided by the SEM photographsshown inFig. 7. As the lamination pressure increased, the de-gree of densification increased. Lamination shows a markedcontrast between the sintered microstructures from samplesprepared under lamination pressures lower and higher thanthe critical lamination pressure. A laminated tape preparedunder a lamination pressure of 29 MPa shows necking be-tween particles, indicating that it was at the initial stageof sintering[22,23]. A more densified structure with graingrowth appeared in samples, prepared using a laminationpressure above the critical lamination pressure. Beyond thecritical lamination pressure, little difference could be ob-served in the sintered microstructures, as can be seen in

Y.-S. Cho et al. / Materials Science and Engineering A362 (2003) 174–180 179

Fig. 6. Cumulative pore surface area of (A) sintered blocks prepared with BL binder as a function of different lamination pressures, and sintered blocksprepared with three different binders of different molecular masses and lamination pressures of (B) 29 MPa and (C) 118 MPa, respectively.

Fig. 7. The microstructure of sintered bodies prepared using different lamination pressures and two different binders with different molecular masses.(A) BM binder with (a) 29 MPa, (b) 78 MPa, and (c) 118 MPa and (B) BL binder with (a) 29 MPa, (b) 78 MPa, and (c) 118 MPa.

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Fig. 7(A(b and c)) and (B(b and c)), which indicates thatexcess pressure does not improve densification. Comparedwith the effect of short- and long-chain PVB binders, themicrostructure of a sintered body prepared with a highMrbinder (Fig. 7(A)) appears to be slightly more close-packedthan that prepared with a lowMr binder (Fig. 7(B)) under thesame lamination conditions. This indicates that the sinteredmicrostructure also seems to be dependent on the molecularmass of the binder, even if it is not a major contribution tothe consolidation and sintering processes. These results areconsistent with the observations on the pore size distributionand sintered density.

Consequently, we clarified that the lamination processplays the most important role in the pore evolution andas-sintered microstructure, and that the type of binder hasa slight effect on the pore evolution and as-sintered mi-crostructure at all lamination pressures. The most advan-tageous fabrication conditions for optimum reliability andperformance of multilayer ceramic capacitors are for tapesprepared with a highMr binder and laminated at pressures≥78 MPa. Moreover, there is a critical lamination pressure,above which pores are unable to be shrunk any further.

4. Conclusions

The elongation strain of tape-cast green tapes is affectedby the molecular mass of the binder, and showed the highestelongation strain with a tensile strength of≈17 MPa for agreen sheet prepared with BM binder. The pore size distri-bution and cumulative pore surface area became narrowerand smaller with increasing lamination pressure and molec-ular mass of the binder, respectively. However, the pore sizewas difficult to reduce further. The pore distribution pat-tern remained constant above a critical lamination pressure,which corresponded to 78 MPa, in this study. Beyond thiscritical pressure, the physical characteristics of the sinteredbodies were no longer influenced by the lamination pres-sure, whereas the molecular mass of the binder had someinfluence on the sintered microstructure. The measured den-sities of the as-sintered tapes mirrored the same trends astheir pore size distribution and cumulative surface area. Thesintered bodies prepared with BM binder exhibited a moredense packing than the sintered bodies prepared with BLbinder. It was found that the molecular mass of the binderaffected the density, sintered microstructure, and pore size

distribution at all lamination pressures. The lamination pres-sure had a greater effect on the rearrangement of particles inthe microstructure than did the molecular mass of the binder.

Acknowledgements

This work was financially supported by the Korea Insti-tute of Science and Technology Evaluation and Planning(KISTEP) through the 21st century frontier projects.

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