influence of slip preparation and casting conditions on aqueous tape casting of al2o3

Influence of slip preparation and casting conditions onaqueous tape casting of Al2O3

Carlos A. Gutie´rrez, Rodrigo Moreno*Instituto de Ceramica y Vidrio, CSIC, Carretera de Valencia km 24,300 Arganda del Rey, CP. 28500

Madrid, Spain

Received 9 February 2001; revised 12 March 2001, accepted 7 May 2001

Abstract

In this work the influence of the slip preparation on manufacturing alumina tapes through an aqueoustape-casting process is studied. For this purpose deflocculated slips with 80 to 87 wt% of solids have beenprepared and rheologically characterized. Ceramic tapes were produced following two different preparationroutes, (1) starting from deflocculated suspensions with a fixed final solid concentration of 80 wt% afterbinders addition, and (2) starting from deflocculated slips with 84 wt% solid loading to produce tape castingslurries with different final solid concentration after binders addition. A mixture of two acrylic emulsionswith different Tg values has been used as binding system with total concentrations of 10, 15, and 20 wt%with respect to solids. It has been found that tapes produced by route 1 have a maximum green density(56% of theoretical density) when adding 15 wt% of binder system and 95% of relative final density, whiletapes produced by route 2 give a similar relative green density (;58.5% Th.) for any binder systemadditions. The final density of tapes obtained from route 2 have a maximum density of 97% for slips with15 wt% of binder system. Additionally, the influence of tape casting parameters on green and final densitywas also studied. From this it was found that it is possible to increase the green density values from 60 to62% and from 97 to 98% for fired density from tapes which were cast at 400mm gap and 5 mm/s. © 2001Elsevier Science Ltd. All rights reserved.

Keywords:A. Ceramics; A. Oxides; D. Microstructure; D. Surface properties

1. Introduction

The tape-casting process is one of the main techniques to produce thin ceramic substratesthat are widely used in electronic devices [1] and tailored structural parts, such as multilay-

* Corresponding author.E-mail address:[email protected] (R. Moreno).

Pergamon Materials Research Bulletin 36 (2001) 2059–2072

0025-5408/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved.PII: S0025-5408(01)00683-3

ered composites [2,3], with enhanced mechanical or chemical properties. Tape casting hasbeen traditionally carried out by using organic vehicles instead of water to allow a fastevaporation of liquid and to prevent the solid phase damage that can be produced in nonoxideor water-soluble materials [4–6]. However, there are many materials that can be processedin water-based suspensions. Moreover, the development of water-based slip additives, suchas dispersants and binders, makes it possible to fabricate ceramic sheets by tape casting[7–10].

The substitution of water for organic solvents has important environmental advantageslike the reduction of toxicity and contamination, and allows to reduce production costs. Inrecent years a large effort has been made to understand the stabilizing mechanisms andinteraction of binders involved in slips prepared with ceramic powders [11–15]. The prep-aration of slurries for tape casting strongly depends on solids concentration, order of additionof additives, binders characteristics, etc., that can affect the final properties of the ceramictapes like density, thickness, porosity, surface quality, etc.

Tape casting needs a highly deflocculated powder suspension to be prepared in a firststage. The preparation of the tape casting slips is usually performed in two steps. In the firststep, the powder is dispersed in the liquid with deflocculants. Second, the binders andplasticizers are added. In aqueous tape casting latex binders are frequently employed [16,17].Latex binders are aqueous emulsions and, therefore, their addition to the ceramic slurrydecreases the solid content. Although concentrated slurries have to be prepared, the use ofexcessively high concentrations of solids promotes agglomeration that is not easily destroyedin the presence of binders in the second mixing step.

This work focuses the preparation of concentrated slips for tape casting by maintainingeither constant starting solid loading prior to binders addition or constant final solid contentof the slips containing binders. The rheological behavior of slips prepared according to thesetwo criteria is studied as well as the tape-casting performance. Additionally, the effect ofcasting speed and blade gap on green and sintered density was studied.

2. Experimental

Ceramic tapes were produced using a high puritya-Al2O3 powder (. 99.97, Condea HPA05, USA) with mean particle size of 0.35mm and specific surface area of 9.5 m2/g.Deflocculated suspensions were prepared using deionized water as dispersing media. Slipswere dispersed with a polyelectrolyte (D-3005, Rohm and Haas, USA). A 1:1 mixture of twocommercial latex emulsions with different vitreous transition temperatures (Tg) was used asthe binder system; Duramax B-1050 (Rohm and Haas, USA), a latex with a Tg of 10°Ccontaining 48–49 wt% of active matter and Duramax B-1000 (Rohm and Haas, USA), alatex with a Tg of 226°C and 55 wt% of active matter.

Dispersion was achieved by adding in all cases 0.8 wt% of polyelectrolyte (referred to thedry solid) [18], and ball milling with alumina balls and jar during 6 h. Different alumina slipswere prepared to concentrations of 80, 84, 85, and 87 wt% to characterize the rheologicalbehavior. Tape casting slips were prepared by adding concentrations of 10, 15, and 20 wt%of the 1:1 binder mixture (binder system, BS) to the deflocculated suspensions. These added

2060 C.A. Gutierrez, R. Moreno / Materials Research Bulletin 36 (2001) 2059–2072

concentrations refer to the as-received binder emulsions (not active matter). Two differentproduction routes were studied to evaluate the rheological slip properties and to characterizethe tapes. The first route consists of preparing slips with final constant solid concentration of80 wt% after binder addition. For this it was necessary to prepare slips with different initialsolid concentrations of 87, 85, and 84 wt%. A starting suspension was prepared to aconcentration of 87 wt%, and part of this slurry was diluted with water to solid contents of85 and 84 wt% prior to the addition of 15 and 10 wt% of binder emulsions, respectively, tohave a final solid loading of 80 wt%. In the second route the casting slips were prepared usingan initial deflocculated slip with 84 wt% solids and then adding different binder systemcontents, so that the final concentrations were 80, 79, and 78 wt% for 10, 15, and 20 wt%BS, respectively.

Rheological characterization was carried out using a rheometer (RS50, Haake, Germany)that is capable of operating in controlled rate and controlled stress modes. The sensor systemused for the rheological measurements was a double cone/plate type with a sample capacityof 5 ml. The flow curves were determined through typical controlled rate tests with a threesegment program where slips were subjected to a linearly increasing shear rate from 0 to 500s21 in 180 s, 60 s at the maximum shear rate and finally the shear rate was linearly reducedfrom 500 to 0 s21 in 180 s. Additionally, to determine the yield point the controlled stressmode was selected submitting the slips to a stress velocity of 0.2 Pa/s ramp until the viscousflow was reached.

To produce the green tapes a self-made tape casting machine was used, which consists ofa mobile container and fixed carrier. A Mylar substrate was used as a carrier film. The castingparameters were 5 mm/s of casting velocity and 300mm of gap height between the bladesand the carrier film. Additionally, the influence of the casting parameters on green andsintered densities was also studied by changing the casting gap from 50 to 400mm at a fixedcasting speed of 5 mm/s. Moreover, the influence of casting speed on tape characteristics wasstudied changing this velocity from 5 to 80 mm/s at the blades height that gave the best greendensity. Green and final densities were measured from discs of 30 mm in diameter punchedof each green tape, and density values were obtained through Archimedes’ method inmercury and water, respectively. In addition, Scanning Electronic Microscopy (SEM) wasused to observe the green microstructure of the upper (contact with air) and bottom (contactwith the carrier film) tapes surfaces.

2. Results and discussion

2.1. Rheological characterization

Fig. 1 shows the flow curves obtained from the control rate measurements. From thisfigure it can be observed that viscosity strongly increases with increasing solid concentration,and that additions higher than 85 wt% promote a very high viscosity that impedes rheologicalcharacterization. It was not possible to obtain a full flow curve for the slip with 87 wt% ofsolids because of the lower water content and the high tendency to evaporate during themeasurements. Therefore, this suspension was not considered to prepare a suitable tape

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casting slip, and was diluted. Suspensions with solid contents of#85 wt% show a shearthinning behavior that can be fit to a Casson model. In addition, none of the slips showedthixotropic effects. After fitting to the Casson model the yield points are 0.33, 2.75, 4.98, and;100 Pa for suspensions with 80, 84, 85, and 87 wt% of solids, respectively.

On the other hand, the control stress measurements reveal that the yield point values arelower than those obtained by fitting to the Casson model. The yield point was obtained froma double logarithmic plot of deformation versus shear stress as the point where the slopechanges, as shown in Fig. 2. The obtained values are 0.12, 0.82, and 1.98 Pa for slurries with80, 84, and 85 wt% solids, respectively. The 87 wt% slip has a high yield value of;23.3 Pa.

From this characterization it is feasible to predict that tape-casting slips produced from the87 wt% suspension will have poor packing properties due to the high agglomeration state.

Tape-casting slips were prepared by adding concentrations of 10, 15, and 20 wt% binderemulsions. In the first preparation procedure the final solid loading was fixed to 80 wt% afterbinders addition. This required starting Al2O3 slips with 84, 85, and 87 wt% solids forincreasing binder contents. The flow curves of these slips obtained in CR mode are plottedin Fig. 3. The flow curve of the 80 wt% Al2O3 slip without binders is also shown for the sakeof comparison. Although the final ceramic solids loading is the same, the viscosity increaseswith the binder content. Furthermore, all suspensions prepared by this route have a broadthixotropic cycle, as can be seen in Fig. 3. The calculation of thixotropy reveals that itdecreases with decreasing additions of binder emulsion being 11,450, 8,267 and 7,507 Pa/s

Fig. 1. Control rate flow curves of alumina slips at different solid loading deflocculated with 0.8 wt% ofdispersant.


for 20, 15, and 10 wt% of the binder, respectively. However, the large thixotropy values canbe related to the dispersion state of the original concentrated slip that has extremely highsolid concentration. In a previous work the effect of solid loading on the viscosity of aqueousalumina slips was determined [19]. The fitting of the viscosity versus a solid fraction usingthe Krieger-Dougherty model showed that the maximum packing factor of these slips isobtained for a volume fraction of 0.66. Thus, slips with 87 wt% (62 vol%) of solid are nearto this maximum packing factor, and it is very difficult to carry out a proper deagglomerationprocess. As a consequence, the resulting slips will have a strong tendency to form agglom-erates that cannot be broken down after the addition of water by mechanical stirring withhelices prior to the BS addition. As in previous cases, control stress measurements wereperformed for calculating the yield point. The yield points calculated through the doublelogarithmic plot of deformation versus shear stress (Fig. 4) are 3, 6 and 30 Pa for binderadditions of 10, 15, and 20 wt%.

In a second processing route Al2O3 suspensions were prepared to a solid content of 84wt% before the binder system addition. The final solid contents were 80, 79, and 78 wt% forbinder additions of 10, 15, and 20 wt%. The corresponding flow curves are plotted in Fig.5, which also includes the flow curve of the 80 wt% slip without binders.

In this case the binder-containing slips present a similar rheological behavior, but both theviscosity and the thixotropy are much lower than those obtained according to route 1, eventhough the final solid contents are very similar (80 wt % in route 1 and 80, 79, and 78 wt%in route 2). As in the previous case the yield point was obtained by plotting deformationversus shear stress in a log/log diagram, as shown in Fig. 6. The yield values significantly

Fig. 2. Logarithmic deformation versus stress curves of deflocculated alumina slips at different solid loading.


reduced in comparison to those shown in Fig. 4 obtained by processing route 1, for example,in the slip with the largest binder system concentration (20 wt% in both processing routes)the yield values reduce from;30 Pa to;4 Pa for routes 1 and 2, respectively. In addition,this optimized route for preparing the tape-casting slip is reflected in a strong reduction ofthixotropy from 11,450 Pa/s to 2,000 Pa/s for slips with 20 wt% BS.

This suggests that, in route 1, the high viscosity is not related to the binder concentrationbut is rather due to the high concentration of particles in the starting slip that leads toagglomeration.

2.2. Tape casting

This different evolution of the rheological behavior must have some influence in furtherprocessing steps. The slurries prepared at different conditions according to both routes weretape cast at a casting speed of 5 mm/s and a blades height of 300mm. Fig. 7 shows thethickness and density of the dry green tapes obtained from slips prepared by route 1. Thethickness increases with the binder content while the density seems to increase from 10 to15 wt% binder and then clearly decreases for 20 wt%.

According to this data, the slip with 15 wt% binder should be the best for a suitable tapecasting. However, a microestructural characterization is necessary to visualize the homog-enization of the green tapes. Fig. 8 shows the green microstructures of the tapes preparedwith 10, 15, and 20 wt% binder (Fig. 8a–c, respectively). These pictures were obtained on

Fig. 3. Control rate flow curves of alumina slips produced by route 1 containing 10, 15, and 20 wt% BS and 80wt% solid without binder addition.


Fig. 4. Control stress curves for determining the yield point value of alumina suspensions with 10, 15, and 20 wt%of BS, produced by route 1.

Fig. 5. Control rate flow curves of casting slips with 10, 15, and 20 wt% of BS addition produced by route 2 anddeflocculated slip with 80 wt% solids and no binder addition.


Fig. 6. Logarithmic control stress curves for determining the yield point of casting slips produced by route 2 andyield point determination of the deflocculated slip with 80 wt% of solids without binder addition.

Fig. 7. Green density and thickness as a function of binder system content for tapes produced by route 1.


the upper surface of the tapes (i.e., that in contact with air). From the figure it can be seenthat the tape with 10 wt% binder is the most homogeneous, and homogeneity decreases withincreasing binder additions. This does not correlate with the density measurements.

These differences can be explained when the surface of the tapes in contact with thecasting carrier film are observed by SEM. The SEM microstructures of the bottom surfacesare shown in Fig. 9. Once more the tape with 10 wt% binder seems to be more denselypacked, but it has heterogeneity that suggest that the tape adheres to the carrier film. Becausethe total amount of water is the same, some binder segregation can occur because this slipdoes not show a yield value in the decreasing ramp of the flow curve, so that it still flows aftercasting. This makes the density to be apparently lower. On the other hand, the heterogeneousmicrostructure of the tape with 20 wt% binder should be related to the lack of homogeneityof the starting Al2O3 suspension.

The tapes obtained from slips prepared by route 2 have a different behavior. Fig. 10 showsthe thickness and the density of the dry green tapes. In this case the green density is alwayshigher than in the tapes produced by route 1, and a maximum density (;61% of theoretical)

Fig. 8. SEM microstructures of upper surface of tapes with (a) 10, (b) 15, and (c) 20 wt% of BS producedby route 1.


is obtained for 15 wt%. This confirms that in this case the starting 84 wt% slip was welldispersed, while the 87 wt% slip in route 1 was agglomerated. However, the tape thicknessdecreases for increasing binder content in opposition to the results obtained for route 1 slips.The final solid content decreases only in 2 wt%, which cannot justify itself the strongdecrease in thickness (near 100mm). The increasing binder addition in these well-dispersedslips increases the adherence between the particles and the tape does not stick to the carrierfilm.

The SEM microstructure of these tapes on the surface exposed to air is shown in Fig. 11.All three compositions present a compact packing of particles without important differences.What is clear is that route 2 provides denser and more uniform tapes than route 1, eventhough the total solid contents are similar.

The burn out of binders was carried out considering the thermogravimetric analysisperformed at 5°C/min to 900°C in the green tapes containing 15 and 20 wt% of the binders.This analysis showed that binders are completely removed at;400°C. From this thesintering process was performed using a two-step sintering cycle. In a first stage green tapes

Fig. 9. SEM microstructures of bottom surface (in contact with the carrier film) of tapes with (a) 10, (b) 15, and(c) 20 wt% of BS produced by route 1.


were heated up to 500°C at 2°C/min and left at 500°C by 30 min, then temperature wasincreased at 5°C/min to 1550°C and left at this sintering temperature for 2 h. Table 1 showsthe green and the sintered densities of the different tapes as well as the final tape thickness.

The final density of the tapes obtained from slurries prepared by route 2 reaches a valueof 97% theoretical, while those prepared by route 1 have a maximum of 95% theoretical, ingood agreement with the rheological properties.

2.3. Optimization of tape-casting parameters

Although good tape properties were obtained from processing route 2, such tapes wereproduced at fixed casting parameters (300-mm gap and casting speed of 5 mm/s). Theinfluence of casting parameters on the green and final densities of the tapes was studied. First,the gap between blades and carrier film was changed, keeping the casting speed constant at5 mm/s, thus obtaining an optimal gap. The next processing parameter, casting speed, wasvaried from 5 to 80 mm/s at the optimized gap.

Table 2 shows the influence of the blade gap on the green and the fired densities of thetapes. From this table it can be seen that the green density is almost constant in the wholegap range. Nevertheless, there is a density jump from 50 to 100mm gap for the sintered tapesthat is not observed in the green densities curve. This can be explained because the 50-mmgap tapes discs are very thin and have a very low weight, which makes it difficult to obtain

Fig. 10. Green density and thickness as a function of binder system content for tapes produced by route 2.


a good green density value using the Archimedes’ technique. On the other hand, greendensity values from 100- to 400-mm gap tapes are in good agreement with those obtained forthe sintered ones, reaching a maximum final density of 98% for that produced with 400-mmgap and 5 mm/s of casting speed.

From the results obtained above, the next tape casting parameter to be evaluated is thecasting speed. For this a set of tapes were cast at a fixed gap of 400mm, and different casting

Fig. 11. SEM microstructures of upper surface of tapes with (a) 10, (b) 15, and (c) 20 wt% of BS produced byroute 2.

Table 1Alumina tapes characteristics produced by route 2

Content of as-receivedbinder (wt%)

Relative density (% th.) Thickness(610, mm)Green Fired

10 58.4 96.0 27015 61.1 97.1 20520 59.2 96.5 177


speeds ranging from 5 to 80 mm/s. Table 3 reports the green and fired densities obtained ateach casting speed. As can be seen in Table 3, the three first velocities produce slightlyhigher green densities (;62%) than higher casting rates (;60%). In the sintered tapes,density slightly decreases with casting speed from 98 to 96.7% for 5 mm/s and 80 mm/s,respectively.

According to these results it is possible to enhance the green and fired density of the tapesmanipulating the casting parameters. In the first stage of tapes preparation (route 2, and fixedcasting parameters at 300mm and 5mm/s) the green and sintered tapes densities reached 61and 97%, respectively. These values were slightly improved to 62 and 98% of theoretical byadjusting the casting parameters at a 400-mm gap and 5 mm/s of casting speed.

3. Conclusions

The rheological behavior of concentrated aqueous Al2O3 slips for tape casting has beeninvestigated by two experimental routes, as well as the effect of the addition of acrylic latexbinders on the rheological behavior and tape-casting performance. The first route was to fixthe final solid loading of the casting slip, and the second was to start with a fixed solidloading prior to binder addition. In the first case a starting slip was prepared to 87 wt% solids,

Table 2Green and sintered densities obtained from tapes cast at a casting speed of 5 mm/s

Gap (mm) Relative density (% Tha.)

Green Sintered

50 60.7 87.3100 60.3 95.9200 60.3 96.8300 61.1 97.1400 62.3 98.0

a Th. 5 Alumina theoretical density (3.97 g/cm3).

Table 3Influence of casting speed on green and sintered densities of alumina tapes produced at fixed gap of 400mm

Casting speed (mm/s) Relative density (% Tha)

Green Sintered

5 62.3 98.010 61.8 97.215 62.3 97.020 59.8 97.025 59.6 96.740 58.5 96.360 59.6 96.680 60.3 96.7

a Th 5 Alumina theoretical density (3.97 g/cm3).


and further diluted to 85 and 84 wt%, while in the second route a slip prepared to 84 wt%solids was always used. This second route has demonstrated to be more effective because awell-dispersed, low viscosity starting slip can be used and the addition of binders does notdrastically alter the solid content.

When starting from very high solid contents (87 wt%) the addition of binders (and, hence,water) cannot destroy the previously formed agglomerates, although the final solid contentdecreases. This results in cast tapes that are less dense and more heterogeneous. Starting slipswith 84 wt% solids are rheologically stable and the resulting tapes have a high microstruc-tural uniformity and higher green and sintered densities (61 and 97%, respectively). Inaddition to the slip preparation, the casting parameters can be controlled to improve the tapeproperties. The highest green and sintered densities (62 and 98% of theoretical) wereobtained for a casting speed of 5 mm/s and a blades gap of 400mm.

Acknowledgments

This work has been financially supported by the Comunidad Auto´noma de Madrid (Spain)under contract CAM 07N/0038/99 and CICYT (MAT2000-0949). C.A. Gutie´rrez thanksCONACyT (Mexico) for the concession of a grant.

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influence of slip preparation and casting conditions on aqueous tape casting of al2o3

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