Preparation of Cordierite Materials with TailoredPorosity by Gelcasting with Polysaccharides

Isabel Santacruz* and Rodrigo Moreno

Instituto de Ceramica y Vidrio, CSIC, 28049 Madrid, Spain

Joao B. Rodrigues Neto

Sociedade Educacional de Santa Catarina, 89227-700 Joinville, Brazil

This paper deals with the rheological characterization of agar and foaming surfactant-containing suspensions for obtainingstoichiometric cordierite samples with tailored open macroporosity and their characterization through density and micro-structural studies. The influence of the processing parameters solid loading (20, 30, and 45 vol%), slip temperature (651C,451C, and 401C), and agar/surfactant ratio (10.2, 8.0, and 5.6) on the obtained bodies is discussed. Open Porosity (up to76 vol%) and average cell size were found to be strongly dependent on solids loading.

Introduction

Materials with tailored porosity exhibit specialproperties and features that cannot be achieved usuallyby their conventional dense counterparts.1 Porous ce-ramics have received considerable attention because oftheir potential use in applications that require high per-meability, high surface area, and good insulating char-acteristics, for example, in absorbents, insulationmaterials, filters, biomedical devices, and kiln furni-ture.2 These properties can be tailored for each specificapplication by controlling the open and closed porosity,

cell size distribution, and cell morphology. These mi-crostructural features are highly influenced by the pro-cessing route used for the production of the porousmaterial.1

Ceramic foams can be prepared by different meth-ods,3,4 including replication methods,5,6 sacrificial tem-plates (polystyrene,7 starch,8 graphite,9 cotton fibers forunidirectionally aligned continuous pores,10 etc.), freezedrying,11 and direct foaming methods. The latter con-sists in the incorporation of air or gas into a suspensionthat is able to retain a structure of air bubbles throughthe polymerization of monomers,12,13 gelcasting ofpolysaccharides,14 or even forming a pseudo-doubleemulsion (PDE) by injection of the suspension into areactor with paraffin.15 Through these mechanisms, sur-factants are added to reduce the surface tension of thegas–liquid phases and hence stabilize the liquid foamsfor a limited period of time. These consolidated foams

Int. J. Appl. Ceram. Technol., 5 [1] 74–83 (2008)

Ceramic Product Development and Commercialization

Supported by Spanish Ministry of Education and Science (MAT2006-13480-C02-01 and

MAT2006-01038 projects), by CNPq (Conselho Nacional de Desenvolvimento Cientıfico

e Tecnologico, Brazil), and by ESF and CSIC under postdoctoral contract I3P-PC2005L.

*cruz@icv.csic.es

r 2008 The American Ceramic Society

are normally sintered at high temperatures in order toobtain high-strength porous ceramics.

Cordierite, 2MgO � 2Al2O3 � 5SiO2, is a ceramic ma-terial that displays a low coefficient of thermal expansion,being an excellent thermal shock resistance material, witha low dielectric constant, good chemical durability, ex-cellent refractoriness, and good mechanical strength.16

These properties make it suitable for many industrial ap-plications as honeycomb-shaped catalyst supports in au-tomobiles, as substrate material in integrated circuitboards,17 and as refractory material.18�20 Many of theseapplications require a porous microstructure.

There are different methods to synthesize cordierite,such as the high-temperature reaction in the solid state, orchemical methods, viz. coprecipitation reactions, solutioncombustion or sol–gel technology, the sintering of oxidepowders through solid-state reactions or crystallization ofglass powders being very popular.21 It has been demon-strated that a coarse particle size of the raw materialslimits the reactivity, and leaves a large residual porositythat is not controlled during shaping and sintering. This‘‘uncontrolled’’ porosity is reduced by milling the startingpowders to lower particle sizes,22 which allows a signifi-cant reduction of sintering temperature.

Previous works21,22 studied the rheological behav-ior of concentrated slips of kaolin/talc/alumina mixturesup to 45 vol% with relative weight ratios of 40/43.8/16.2 for manufacturing both cordierite dense bulks andfoams, the latter being prepared by impregnation ofpolyurethane foam into the optimized slip. Cordieritesamples were obtained after optimizing the sinteringcycle. This paper deals with the rheological character-ization of natural raw material suspensions for obtainingcordierite materials with tailored macroporosity using agelcasting process with agar and a foaming surfactant.The influence of processing parameters such as solidsloading of the suspension (20, 30, and 45 vol%), the sliptemperature before gelation (651C, 451C, and 401C),and the agar/surfactant ratio on the microstructure andporosity of obtained bodies is discussed.

Experimental Procedure

The composition of the stoichiometric cordieriteconsists of SiO2, Al2O3, and MgO in molar ratios 55.6/22.2/22.2 (weight ratios being 51.4/34.9/13.8). As rawmaterials, the following powders were used: kaolinand talc (both provided by Colorminas S.A., Criciuma,

Brazil), and an alumina A85/15 wt/wt mixture of fineand coarse Al2O3 powders (1000SG and CL3000FG,both supplied by Alcoa Chemie GmbH, Ludwigshafen,Germany) with average particle sizes of 0.64 and5.7 mm, and surface areas of 10.2 and 0.72 m2/g, re-spectively, was used. These powders were mixed in rel-ative weight contents of 40.0 wt% kaolin, 43.8 wt%talc, and 16.2 wt% alumina, according to previousworks.21,22 The slips were prepared to a solids loadingof 45 vol% by mechanical agitation using a helices mixerfor 15 min with the addition of 1.5 wt% (on a dry solidsbasis) of a polyacrylic-based deflocculant, Dolapix PCN(supplied by Zschimmer-Schwarz, Lahnstein, Germany),and adjusting the pH to 11.070.1 with KOH. Theslips thus prepared were milled using a high-speed plan-etary mill with a porcelain jar and alumina balls for45 min, and the pH was readjusted to 11.070.1.21,22

More diluted slips, 20 and 30 vol% solids, were pre-pared by dilution of the optimized 45 vol% slips main-taining a constant pH 11.070.1 in all cases.

As a gelling agent, a concentrated agar solution(6 wt%) was prepared under overpressure conditions us-ing a pressure cooker,23 which allows to reach a temper-ature of B1101C. Once it was prepared, the agar solutionwas maintained at 651C and added to the slip after pre-heating at 651C. The amount of added solution was thatrequired to introduce 1.0 wt% of agar powder with regardto the total water content. The concentration of the gellingagent is expressed as a function of the total water contentbecause it forms a 3D structure during the gelation processby OH bonding with water molecules. Once the gelcast-ing suspensions were ready, at 651C, a surface-active agentwas incorporated into the hot suspensions, viz. an anionicsurfactant namely ammonium dodecylbenzene (DDB).The amount of solution added was that required to in-troduce 1.0 wt% of DDB referred to the total weight ofthe suspension. This surfactant was prepared by mixing20 g of dodecylbenzenic acid and 20 mL of ammonia in80 mL of water. The slips were subsequently subjected tostrong mechanical stirring with a high-shear mixer (Sil-verson L2R, Chesham Bucks, UK) for shearing times of3 min14 in order to promote bubble formation.

The rheological behavior was studied using a rota-tional rheometer (RS50, Haake, Thermo Electron Co.,Karlsruhe, Germany) with a double-cone/plate sensorconfiguration (DC60/21, Haake, Karlsruhe, Germany).Flow curves were obtained under controlled rate (CR)conditions using a three-stage measuring program using alinear increase of shear rate from 0 to 1000 s�1 in 180 s, a

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plateau at 1000 s�1 for 60 s, and a further decrease to zeroshear rate in 180 s. Controlled stress (CS) measurementswere also performed with a two-stage measuring programwith an increase of shear stress from 0 to 1 Pa and back tozero shear stress. This was selected according to the cor-responding flow curves. The up curves obtained from CSand CR measurements were combined to build up theexpanded flow curves of the slips at different solids load-ings. The evolution of viscosity with the temperature ofthe slips, the agar solutions, and the agar/DDB mixtureswas measured using a probe of temperature, which en-ables the continuous recording of viscosity data on heat-ing or on cooling. All these tests were performed aconstant shear rate of 100 s�1.

Slips with solids loadings of 20, 30, and 45 vol% wereprepared in this way and maintained at temperatures of651C, 451C, and 401C, at which they were poured intoopen stainless-steel molds. They were further cooled in afreezer at �7.070.11C for 2 min in order to allow theconsolidation of the bodies but impeding the water freezing.

Cast bodies were left in air for 24 h to dry and fur-ther sintered at 12801C/1 h, where the cordierite phaseappears as the main phase in the X-ray diffraction(XRD) pattern,21 with a thermal treatment at 5501C/0.5 h to burn out the organic matter. The cell sizes andsize distributions of the sintered samples were evaluatedusing image analysis (Leica QWin, Leica Microsystems,Wetzlar, Germany), applying the correct conversion pa-rameters to go from 2D images to 3D values.3 Micro-structures of the sintered samples were observed byscanning electron microscopy (SEM) (Zeiss DSM400,Oberkochen, Germany). The density and open porosityof sintered bodies were measured by immersion in water(UNE 61033 and 61035, respectively).

Results and Discussions

Figure 1 shows the flow chart of the manufacturingprocess, indicating some details of the experimentalprocedure.

Figure 2 shows the expanded curve of viscosity ver-sus shear stress built up from the CS and CR up curvesof the slips with 20, 30, and 45 vol% solids without anygelling or foaming agents. The three suspensions presenta shear thinning behavior. Similarly, a plot of viscosityvs. shear rate (not shown, as it gives the same informa-tion) was drawn in order to calculate the values of Z0

and ZN by fitting to the Cross model from the CS andthe CR curves, respectively (Table I).

Because slips need to be heated at 651C beforepolysaccharide addition, the effect of heating on therheological behavior was first studied. Figure 3 showsthe variation of viscosity with temperature of 45 vol%slip on heating (a) and the behavior on cooling of theslips with agar (b) and agar with the foaming surfactant(c). In all cases, the shear rate was fixed at 100 s�1. Theheating viscosity of the additives free slip seems to de-crease at the beginning and then remains nearly constantup to 701C. No destabilization was observed in the

PowdersMixture

Water + Dolapix PCN (1.5 wt%)

Adjustment pH 11

45 min Ball Milling

Adjustment pH 11

High shear mixingt = 3 min

Casting

Cooling

Drying

Firing1280°C/1h

Heating60°C Agar (sol. 6 wt%)

1.0 wt%

Surfactant1.0 wt%

Fig. 1. Flow chart of the manufacturing process.

Vis

cosi

ty /

Pa

s

0.001

0.01

0.1

1

10

100

1000

0.01 0.1 1 10 100 1000Shear Stress / Pa

20 vol %

30 vol %

45 vol %

Fig. 2. Expanded curves of viscosity versus shear stress of the slipswith different solid loadings.

76 International Journal of Applied Ceramic Technology—Santacruz, Rodrigues Neto, and Moreno Vol. 5, No. 1, 2008

range of study, so that this slip and more diluted onescan be used successfully for our purposes. Once the gel-ling agent was added to the slip at 651C, Fig. 3b, a sharpincrease of viscosity was observed on cooling below401C, because of the agar gelation. The viscosity oncooling for the slip with agar and the foaming agent,Fig. 3c, shows a lower viscosity at any temperature ascompared with the slip without DDB, although the in-crease of viscosity at the gelling temperature, Tg, is alsolower. Because this is believed to be due to the effect ofDDB on the gelling properties of agar, a study of theevolution of viscosity on cooling of concentrated agarsolutions with different DDB additions was performed.The cooling viscosities are shown in Fig. 4. Given thatagar was added referred to the total water content, andDDB referred to the total slip, different agar solution/DDB ratios were added to the slips at different solidsloading, viz. ratios of 5.6, 8.0, and 10.2 for 45, 30, and20 vol% solids loading, respectively; this corresponds toagar powder/DDB ratios of 0.32, 0.48, and 0.61, re-spectively. Agar solution/DDB mixtures were studied inthese proportions during gelation, as shown in Fig. 4. Inall cases, the shear rate was fixed at 100 s�1, which is

relatively high to destroy partially the gel that is beingformed. These curves were compared with that corre-sponding to 6 wt% agar solution without DDB. Fromthis plot, it can be observed that the addition of DDB tothe agar solutions does not significantly modify thegelation temperature, so that consolidation conditionsdo not change. At lower DDB additions, that is to say, aratio of 10.2, almost no difference in the final viscositywas observed just after cooling, compared with theDDB-free agar. At higher DDB contents, ratios: 8.0and 5.6, lower viscosity values were achieved for the gelas compared with agar solutions. Although shearingpartially destroys the gel structure, the lower final vis-cosity suggests that the gel strength decreases whenDDB is added. The final viscosity values just after gel-ling are 3.0, 2.0, and 1.9 Pa s for 10.2, 8.0, and 5.6 agar/DDB ratios, and 3.0 Pa s for the agar solution. A possibleexplanation for this lower strength is that the pH of thestarting agar solution was 5.570.1, and after DDB ad-ditions, the pH changed up to pH 9.070.1. This in-crease of pH could produce an alkaline hydrolysis of theagar, affecting its gelling properties. This effect was alsoappreciable in the slips containing agar and DDB, whichpresented very low viscosity, Fig. 3c. This could be re-lated to a decrease of the solid loading, but it was notappreciatable when only agar was added. At temperatures435–401C, there are three effects on the viscosity of theslips: (a) the addition of agar with or without DDB re-duces the solid loading of the suspensions and thus, theviscosity, (b) the gelling agent can also work as a thick-ening agent, increasing the viscosity of the slips, and (c)the effect of pH may change the gelling efficiency andthus the viscosity of the slips. Becasue the viscosity of agar

Table I. Values of g0 and gN Obtained from theExpanded Curves and Extrapolations Using the Cross

Model

Vol% g0 (Pa s) gN (Pa s)

20 0.015 0.00530 15.25 0.03345 630 –

Vis

cosi

ty /

Pa

s

0

1

2

3

20 30 40 50 60 70 80

ab

c

100 s –1

Temperature / °C

Fig. 3. Effect of the temperature on the viscosity of 45 vol% slipsmeasured on heating (a), on cooling after agar addition (b), and oncooling after agar and DDB additions (c). Shear rate: 100 s�1.DDB, ammonium dodecylbenzene.

Vis

cosi

ty /

Pa

s

0

1

2

3

4

20 30 40 50 60

Agar solution /DDB

Agar

10. 2

8.0

5.6

Temperature / °C

Fig. 4. Effect of the temperature (cooling) on the viscosity of agarsolution/DDB mixtures at different ratios. These curves arecompared with the agar solution one. (Shear rate: 100 s�1). DDB,ammonium dodecylbenzene.

www.ceramics.org/ACT Gelcasting with Polysaccharides 77

and agar/DDB solutions at 435–401C is very similar, itis believed that the difference of viscosity between the slipwith agar and the slips with agar/DDB Could be relatedto the pH changes from 9.570.1 to 10.670.1, respec-tively, in agreement with previous studies.22

Although the gels obtained with the agar/DDBmixtures are less stiff, they are strong enough to main-tain a rigid structure with open cells as desired. Figure 5shows the effect of the temperature on the viscosity ofthe slips with agar and DDB prepared to different solidsloading. Viscosity increased with the solids loading, asexpected, and a sharp increase of viscosity was observedat the gelling temperature that is not affected by thesolids loading. Different fractions of these suspensionswere maintained at several temperatures, 651C, 451C,and 401C (marked by arrows in the figure), and theircorresponding viscosity values are shown in Table II.

Vis

cosi

ty /

Pa

s

20 30 50 6040

0.8

0.6

0.4

0.2

1.0

0.0

Temperature / °C

20 vol%

45 vol%

30 vol%

Cooling

Fig. 5. Effect of the temperature (cooling) on the viscosity of the20, 30, and 45 vol% solid loadings slips with agar and DDB (shearrate: 100 s�1). DDB, ammonium dodecylbenzene.

Table II. Characteristics of the Porous Samples/Slips

Sample

Apparentdensity

7(0.1 g/cc)

Openporosity(vol%)

Viscosityat 100 s�1

(mPa s)

20 vol%-401C 0.6 73.9 1520 vol%-451C 0.7 74.0 1320 vol%-651C 0.6 76.5 1030 vol%-401C 1.8 15.1 17030 vol%-451C 1.7 18.4 16230 vol%-651C 1.5 29.4 15945 vol%-451C 1.9 1.8 22145 vol%-651C 2.0 1.2 186

Fig. 6. Microstructures of the fracture surface of sintered samplesprepared from 45 vol% (a), 30 vol% (b) and 20 vol% (c) slips castat 651C.

78 International Journal of Applied Ceramic Technology—Santacruz, Rodrigues Neto, and Moreno Vol. 5, No. 1, 2008

After that, a strong mechanical shearing was applied topromote foaming, before casting on metallic molds atthe same temperatures. Different porous microstruc-tures were obtained depending on two parameters: thesolids loading and the casting temperature. On the onehand, slips with lower solids loading will present a high-er volume of bubbles after high shearing, related to thelower viscosity. On the other hand, the foam structurewill be retained more earlier when the temperature ofthe slip is closer to the Tg. Figure 6 shows the cross-sectional fracture surface microstructure of sinteredsamples obtained from slips with solids loadings of45, 30, and 20 vol% (Figs. 6a, b, and c, respectively)cast at 651C. All pictures were taken at the same mag-nification, and the bottom of each micrograph corre-sponds to the bottom of the sample obtained (that incontact with the mold). Note that bright areas corre-spond to cells filled with gold used during the prepara-tion of the specimens for SEM observations. As can beobserved, cell sizes and distribution are strongly related

to the solids loading of the slips. Samples prepared fromthe highest solids loading, 45 vol%, Fig. 6a, show‘‘small’’ cells, with a few ‘‘medium’’ sized cells distrib-uted around all the sample. In the case of samples withthe lowest solids loading, 20 vol%, Fig. 6c, ‘‘medium’’-sized cells are distributed around all the samples show-ing slightly lower sizes in the bottom of the sample and afew ‘‘large’’ cells distributed homogeneously around thesample. In the case of an intermediate solids loadingslip, 30 vol%, Fig. 6b, ‘‘medium’’- and ‘‘large’’-sizedcells are present in a matrix of ‘‘small’’ cells. Larger cellsare found on the top of the sample. All these observa-tions were quantified by image analysis as can be ob-served in Figs. 7a, b, and c, respectively, for samplesprepared from 45, 30, and 20 vol% suspensions, cast at651C. In these figures, the X-axis is plotted using twodifferent scales (from 0 to 0.1 mm with marks every0.01 mm, and from 0.1 to 1.5 mm, with marks every0.1 mm) in order to observe small cells better. In thecase of 45 vol% slips cast at 651C, the cells lie mainly in

0

10

20

30

0

20

40

60

0

10

20

30

% N

umbe

r of

cel

ls

a d

0

10

20

30

0

20

40

60

0 0.05 0.10 0.5 1.0 1.5

0 0.05 0.10 0.5 1.0 1.5

0 0.05 0.10 0.5 1.0 1.5 0 0.05 0.10 0.5 1.0 1.5

0 0.05 0.10 0.5 1.0 1.5

c

b e

Cell Size / mm

Cell Size / mm

Fig. 7. Cell size distributions of the sintered samples obtained by image analysis. Samples prepared from: 45 vol% (a), 30 vol% (b), and20 vol% (c) slips cast at 651C, and samples from 20 vol% slips cast at 451C (d) and 401C (e).

www.ceramics.org/ACT Gelcasting with Polysaccharides 79

the range of 0.01–0.1 mm diameter (98%), with a smallpercentage in the range 0.1–0.2 mm (1.7%), and an av-erage cell size of 0.04 mm (40 mm). In the case of20 vol% slips, there is a ‘‘bimodal’’ distribution(o0.03 and 0.07–0.30 mm) with percentages of 23%and 71%, respectively, where most of the cells lie be-tween 0.1 and 0.2 mm; a few larger cells, within 0.4–1.4 mm, are also present in smaller percentage, 2%, with

an average size of 125 mm. For samples cast at the sametemperature using 30 vol% slips, cell sizes lie mainlybetween 0.015 and 0.09 mm (99.2%), with a small per-centage of large cells (1.1–1.5 mm, 0.7%) and an aver-age size of 90 mm.

The apparent density values decrease from 2.0 to0.6 g/cm3 when the solids loading decreases from 45 to20 vol%. Such differences in the microstructure of thesamples are believed to be due to the slips’ viscosity,which changes from 186 to 159 and to 10 mPa s as sol-ids loading decreases from 45 to 30 and 20 vol%, re-spectively. In addition, the distance among particlesincreases as solids loading decreases, thus leading to alarger volume of cells.

Figure 8 shows SEM microstructures for sinteredsamples prepared from 20 vol% slips cast at 451C (a)and 401C (b) in order to study the effect of the castingtemperature on the final samples. The sample cast at651C is shown in Fig. 6c. Although ‘‘medium’’ and‘‘large’’ cells are observed in the three samples, the celldistributions around the samples are quite different. Inthe case of samples cast at 451C, a gradient of porositywas observed, where the cell size increases with theheight of the sample, Fig. 8a. For samples cast at401C, Fig. 8b, ‘‘medium’’ and ‘‘large’’ cells can be ob-served along the whole sample. Figs. 7d and e show thecell size distribution of the samples prepared from20 vol% suspensions cast at 451C and 401C, respective-ly. The former shows a cell size distribution similar tothe corresponding sample cast at 651C shown in Fig. 8c.In this case, an average cell size of 150 mm was mea-sured. In the case of the samples cast at 401C, a broadcell size distribution was observed (0.01–1.3 mm) withmaximum percentages of cells within the ranges 0.01–0.03 and 0.1–0.20 mm, the average size being 120 mm.No significant differences in apparent density were ob-tained in the samples prepared from same volume slip(viz. 20 vol%) cast at different temperatures, Table II, inagreement with the similar viscosity of the starting slips.Table II also shows a significant increase of open po-rosity with decreasing solids loading, namely 1, 29, and76 vol% for samples prepared from 45, 30, and 20 vol%cast at 651C. These values agree with the higher mag-nification micrographs shown in Fig. 9. Because differ-ent cell sizes are obtained in the bottom and the top ofthe sample, cells of both areas were observed, except forthe 45 vol-651C sample, where a rather homogeneouscell distribution was obtained. Open porosity is ob-served in all cases, although it seems that both the size

Fig. 8. Microstructures of the fracture surface of sintered samplesprepared from 20 vol% slips cast at (a) 451C and (b) 401C.

80 International Journal of Applied Ceramic Technology—Santacruz, Rodrigues Neto, and Moreno Vol. 5, No. 1, 2008

and the number of cell windows increase, for 20 vol%samples, Figs. 9c and e. In the case of 30 vol% samples,Figs. 8b and d, denser cell walls with small cell windowsare observed, and when solids loading increases to45 vol%, very dense cell walls with larger cell windowsare observed. The differences between the microstruc-tures inside and outside a cell can be observed in Fig. 10,where the interior of the cells of the bottom area ofsamples prepared from 45 and 30 vol% slips cast at651C, Figs. 10a and b, respectively, were taken at ahigher magnification. This completely agrees with theopen porosity and apparent density values shown inTable II. The cell morphology of samples prepared with20 vol% slips at different casting temperatures is shownin Fig. 11. The cell windows also decrease in sizewith decreasing temperatures, as shown before for high-er solids contents.

In summary, it seems clear that the cell size in-creases when solid loading decreases and the workingtemperature is higher than the Tg of the polysaccharide.Open porosities as high as 76 vol% can be obtained

with 20 vol% solids at a casting temperature of651C.

Conclusions

Light cordierite materials with tailored porosityhave been prepared using the gelcasting of polysaccha-rides in combination with a foaming agent.

A rheological characterization of agar and foamingsurfactant-containing slips was performed demonstrat-ing that the addition of the foaming agent tends to de-crease the gel strength slightly. The low surfactantcontent allows consolidating bodies with a greenstrength high enough for easy handling and manipula-tion. The samples obtained were characterized throughdensity, surface, and microstructure studies.

The influence of the solid loading of the precursorslip has been shown to be a critical parameter for thedesign of samples with tailored porosity. Samples withdifferent cell distributions, apparent density, average cell

Fig. 9. Microstructures of the fracture surface of sintered samples prepared from 45 vol% (a), 30 vol% at the top area (b), at the bottom (d),and 20 vol% at the top (c) and at the bottom (e) areas, respectively. All suspensions were cast at 651C.

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diameter, and percentage of open porosity have beenobtained on changing the solid loading from 45 to20 vol%. A decrease in the average cell diameter, 125,90, and 40 mm, was observed with increasing solidsloading, 20, 30, and 45 vol%, respectively, being almoststable with temperature. These differences in the mi-crostructure of the samples are related to the slips’ vis-cosity, which changes from 186 to 159 and to 10 mPa sas the solids loading decreases from 45 to 30 and20 vol%, respectively. This effect combines with thelarger separation distance among particles in dilutedsuspensions, which lead to larger porosity.

The casting temperature also influences thecell distribution among the sample, although the otherparameters, such as average cell diameter, apparentdensity, and open porosity, are less influenced by thisfactor.

Acknowledgments

The authors thank Dr. M. I. Nieto and Mr. J. J.Reinosa (Instituto de Ceramica y Vidrio, CSIC) foruseful discussions.

Fig. 10. Microstructure inside a bubble of sintered samplesprepared from 45 (a) and 30 vol% (b) slips cast at 651C.

Fig. 11. Microstructure inside a cell of sintered samples preparedfrom 20 vol% slips cast at 451C (a) and 401C (b).

82 International Journal of Applied Ceramic Technology—Santacruz, Rodrigues Neto, and Moreno Vol. 5, No. 1, 2008

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