Influence of Slurry Characteristics on Porosity and Mechanical Properties of Alumina Foams

Influence of Slurry Characteristics on Porosity andMechanical Properties of Alumina Foams

Santanu Dhara*,w

Department of Oral and Dental Science, University of Bristol, Lower Maudlin Street BS1 2LY, U.K.

Parag Bhargava

Material Science Centre, IIT Kharagpur 721302, India

Alumina foams with porosity ranging between 50% and 92% were fabricated by foaming followed by coagulation ofovalbumin based aqueous slurries. Different combinations of ovalbumin–water mix and alumina loading provided a means tovary slurry viscosity over a wide range. Slurry viscosity influenced the foaming behavior leading to variation in microstructure andmechanical properties of alumina foams. Controlling the slurry viscosity resulted in controlled the total porosity, microstructuresand mechanical properties. Mechanical properties were correlated with different micro-mechanical models. Both microstructureand mechanical properties agreed well as closed cell alumina foam due to presence of low percent area of interconnections.

Introduction

Applications1–6 involving use of ceramic foams asmolten metal filters, gas sensors, catalyst supports, com-bustion burners, performs for metal-ceramic compos-ites, media for drug release require foams withsignificantly large fraction of open pores. On the otherhand, ceramic foams in applications1,2,7–9 such as loadbearing structures, acoustic and thermal insulation re-quire high fraction of closed porosity.

Applications1,2,10 like artificial bone, supports formembranes, and so forth, require the porous ceramic

bodies to have gradient in percent porosity, pore size orpore connectivity.

In general, the properties such as permeability, ther-mal conductivity, mechanical strength, elastic modulusand dielectric constant, and so on, depend on percentporosity and microstructure of ceramic foams.1,2,6,11–16

Foams with different pore size distribution, poreorientation and pore morphology can be produced asper the requirements of different applications. Thereticulated foam process5,9,10,17–19 results in ceramicfoams with porosity exceeding 90% and pore size ran-ging from 10’s of micrometer to several millimeter.Foams prepared by this route have poor mechanicalproperties due to highly open pore structure and hollowstruts left behind from burning of polyurethane. Themicrostructure of reticulated foams is most suited forapplications requiring high permeability.

Int. J. Appl. Ceram. Technol., 3 [5] 382–392 (2006)

Ceramic Product Development and Commercialization

*Member, American Ceramic [email protected]

r 2006 The American Ceramic Society

Porous ceramic materials with pore size in the range5–500 mm and low connectivity between the pores havebeen made using organic fugitive materials.20 Processbased on pre-ceramic polymers has been used to pro-duce ceramic foams21,22 with a combination of microand macro porosity.

A novel method based on freeze-drying utilizes thedirectional growth of ice to fabricate porous ceramicswith aligned macroscopic pores and associated micropores.23 The process results in porosity up to 50% withlarger fraction of open porosity. Recently, Sepulvedaet al. applied the gelcasting method for fabrication ofceramic foams24–26 with porosity above 90% and poresize in the range of 30–1000 mm.

Although a number of methods for fabrication ofceramic foams are known,4,5,7,8,10,20–31 few studies havereported8,15,27 control over microstructure through pro-cessing parameters and its influence on properties. Thepresent study deals with ceramic foams prepared by adirect foaming and casting process—protein coagulationcasting (PCC).31–34 PCC involves use of egg white (ov-albumin) based aqueous ceramic slurries, which undergotemperature or chemical induced coagulation soon aftercasting to prepare dense or porous ceramic bodies.

In this study, natural foaming tendency and bind-ing property of ovalbumin has been utilized for prep-aration of ceramic foams. The study demonstrates thatthe slurry viscosity is one of the most important physicalproperty that governs the microstructure and propertiesof ceramic foams. The slurry viscosity was tailored byvarying the solids loading or ovalbumin volume percentin the ovalbumin–water mix. Scanning electron micro-scopy (SEM) micrographs of alumina foams were char-acterized for average cell size, cell separation (averagedimension of the ceramic size separating different voidcells), interconnection area percent (percent area of cellwindows) and number of cells per unit area by usingimage analysis software. For most of the alumina foamsamples, strength was measured in both flexural andcompression mode. Based on the results from abovemeasurements, the processing-microstructure-propertyrelationships for alumina foams are presented.

Experimental Procedure

Slurry Preparation

Ovalbumin based slurries with alumina loading inthe range of 15–55 vol% were prepared using optimized

amount of Darvan 821 A (40 wt% aqueous solution ofpoly (acrylic acid ammonium salt; MW 5 6000; R. T.Vanderbilt Inc., Norwalk, CT) as dispersant. Theamount of dispersant used in the present study was1 mL Darvan 821 A per 100 g of alumina powder(CT 3000 SG, Alcoa-ACC Industrial Chemicals Ltd.,Kolkata, India) with d50 of 0.7 mm and a surface areaof 7 m2/g for making the slurries. The different slurrycompositions and the corresponding viscosity values areshown in Fig. 1 and Table I. The flow behavior of the asprepared slurries was examined in a parallel plate con-figuration (40 mm diameter, 1 mm gap) with a solventtrap (Model AR 1000, TA Instruments, New Castle,DE) at shear rate ranges varying from 0.5 to 50 s�1 be-fore tumbling. As all of the slurries were shear thinningin behavior, all the viscosity values reported in the pres-ent study only at shear rate of 0.5 s�1 for comparison.All slurries were foamed by tumbling the bottles, con-taining the slurries and 3 mm diameter zirconia millingmedia, for 22 h. Details of the slurry preparation andfoaming have been discussed in earlier studies.31–33 Allof the samples were prepared using 22 h tumbling atconstant tumbling rate (40 cycles/min) to compare theeffect of the slurry viscosity on foam microstructure andmechanical properties. Indeed, it is possible to producesamples with a wider range of porosity using differentrate and time of tumbling for the same slurry.

Fig. 1. Effect of ovalbumin content in the ovalbumin–water mixand alumina volume percent on slurry viscosity (at shear rate of0.5 s�1).

www.ceramics.org/ACT Porosity and Mechanical Properties of Alumina Foams 383

Casting and Setting of the Foams

The foams were cast into petroleum wax coatedmolds and set by drop-wise addition of concentratednitric acid on top of cast samples followed by drying at501C. Though the as cast dimensions of the samplesprepared for mechanical testing were limited to a max-imum of 80 mm� 65 mm� 15 mm (thickness), sam-ples as thick as 30 mm in thickness without any gradientin microstructure could be produced with the abovesetting procedure. The samples were then placed in anoven and gradually heated to 751C and dried undervacuum.

For binder burn out of the green alumina foams, aheating rate of 21C/min to the temperatures shownbelow with respective dwell times was used—30 min at1501C, 60 min at 2001C, 3501C, 5501C and 9001C.Sintering was done in a different furnace at 16001C for2 h.

The loss in weight of dried foam samples followingbinder burn out indicated that the weight percent bind-

er in dried green bodies ranged from 2% to 21% cor-responding to the various slurry compositions used inthe present study.

Micro-Structural Characterization

Percentage total and open porosity was determinedusing the Archimedes principle13 (ASTM C 20). Forcomparison the total porosity was also determined dir-ectly from the micrographs by point fraction methodand it matched closely with the values from Archimedesmethod (Table I). SEM micrographs of the diamond-sawed sections of alumina foams were characterizedusing image analysis software (Image Tool, Version3.0, University of Texas Health Science Center, SanAntonio, TX). For determination of the average cell sizeand average cell separation, line intercept measurementswere done.35 The intercepts of randomly placed lineswith the cells were measured and the intercepting pointsper unit length of line were determined (PL). The cellsize and cell separation were calculated by the following

Table I. Slurry Compositions, Viscosity and Comparison of Porosity in Foams as Measured from ArchimedesPrinciple and Quantitative Microscopy

Alumina loading(vol.%)

Ovalbumin-watervolume ratio

Slurry viscosity(at 0.5 s�1) (Pa s)

% Porosity usingArchimedes Principle % Porosity using

quantitativemicroscopy: totalTotal Open

15 Ovalbumin only 9.4 92 83 92.020 Ovalbumin only 17.9 87 76 88.020 3:1 18.9 86 74 87.525 Ovalbumin only 28.0 85 73 85.025 3:1 25.1 85 73 85.525 2:1 26.5 86 74 83.025 1:1 23.5 87 76 84.030 1:1 40.4 83 67 81.535 3:1 69.5 70 45 69.035 2:1 81.5 68 42 72.035 1:1 52.5 77 56 75.535 1:2 27.3 84 70 81.045 2:3 79.2 71 50 73.045 1:2 55.1 82 66 79.045 1:3 29.4 82 68 84.550 1:4 45.2 74 44 74.452 3:5 110.3 57 31 58.555 1:4 Not measurable 49 20 45.055 1:5 177.6 68 41 72.5

384 International Journal of Applied Ceramic Technology—Dhara and Bhargava Vol. 3, No. 5, 2006

stereological relationships

Cell size ¼ 4 V porosityv =2 PL

Cell separation ¼ 4ð1� V porosityv Þ=2 PL

where, Vvporosity is the volume fraction of total porosity

in the foams.The interconnection area (area of cell windows) as

percent of the sample cross-section area and number ofcells per unit area were determined directly using theimage analysis software. The area of interconnectionsmeasured here is the area projected on a plane surfaceand may really underestimate the actual total area ofinterconnections. But as this underestimation is similarin all cases, the percent interconnection area has beenused to represent the trends in change in microstructurewith processing. The cell separations (average dimen-sion of the ceramic size separating different void cells)and area of interconnections (cell windows) are clearlyindicated in Fig. 2.

Mechanical Properties

Flexural and Crushing Strength: The samples for bothflexural and crushing strength testing were prepared byslicing larger blocks, with a diamond saw. The flexuralstrength of alumina foams was measured in three-pointbend configuration with a span of 40 mm, at a crossheadrate of 0.5 mm/min (Model H10KS, Hounsfield, Sal-fords, U.K.). The sample dimensions for the flexural testwere 50 mm� 10 mm� 10 mm. For all flexural testsfailure occurred on the tensile side of the samples and nosamples failed at the contact points. Crushing strengthtests were carried out at a crosshead speed of 0.5 mm/min with sample placed between compliant rubber padshaving a hardness of 30 Shore A. The typical sampledimensions were 20 mm� 10 mm� 20 mm (height).The peak load during compression tests correspondingto the crushing strength did not change significantlywith and without use of rubber pads. Despite the aboveobservation, for all measurements, rubber pads wereused as a standard practice to avoid any possible influ-ence of surface features on the crushing strength.

Elastic Modulus: Initially, attempt was made to de-termine elastic modulus from two independent sources,from the compressive and flexural test data from themechanical testing machine and an impulse excitationtechnique. Tests on a variety of samples including thefoams and a certified steel standard (ASTM C1259-98)

were used. The flexural or compressive test data did notyield the precise Young’s modulus data while the im-pulse excitation test yielded reproducible Young’smodulus values. The modulus values for the steel sam-ple from impulse excitation tests (IET) (ASTM C1259-98; Dynamic Elastic Properties Analyzer (DEPA), Jag-dish Electronics, Bangalore, India) matched with thecertified values. Thus, the elastic modulus of thealumina foam samples was measured using impulseexcitation technique. The samples (70 mm�40 mm� 12 mm) were excited mechanically by a tap-ping device. The vibration signal generated by excitationof the sample was analyzed using fast fourier transform(FFT). The measured elastic modulus values were basedon the resonant frequency of the samples.36–38 The res-

Fig. 2. SEM micrographs (a) higher viscosity 55 vol% aluminaslurries (b) lower viscosity 20 vol% alumina slurries, showingvariation in microstructures, cell separation and cell window.


onant frequency of the samples and thus the corres-ponding elastic modulus obtained from DEPA were re-producible.

Results and Discussions

Slurry Composition and Rheology

Prior experiments on fabrication of ceramic foamsindicated that slurry viscosity was one of the most im-portant factors that influenced the foaming of slurries,and foam microstructure.31–33 It was seen that throughchange in alumina loading and variation in ovalbumincontent, the viscosity of slurries could be varied over awide range, from 3.7 to 177 Pa s (shear rate of 0.5 s�1,Fig. 1, Table I). All of the slurries in the present studywere shear thinning in behavior. Although the viscosityof all slurries was measured over a shear rate range of0.5–50 s�1, the viscosity at a single shear rate of 0.5 s�1

was considered when examining the relationships be-tween viscosity and foam characteristics. All slurries inthe above mentioned viscosity ranges were foamed easilyusing similar mechanical agitation.

Prior experience suggested that alumina loading of20 vol% and below, slurries had to be prepared withundiluted ovalbumin. Use of diluted ovalbumin alwaysresulted in cracking of foams due to excessive dryingshrinkage and low binder content in the green body.The slurries prepared with 15 vol% alumina loading andundiluted ovalbumin, which had the lowest viscosityamong all slurries, foamed the most but the correspond-ing cast samples cracked during drying due to extremelylarge shrinkage. Although the above problem was re-solved by replacing part of the ovalbumin in the slurriesby sucrose,31–33 the present study deals with foams pre-pared with ovalbumin and set by acid induced coagu-lation.

It was seen that for slurries with low alumina load-ing, for example 25 vol%, the ovalbumin content in theslurry could be varied widely without significant changein slurry viscosity. For highly loaded slurries, becausethe inter-particle spacing is small, even low ovalbumincontent in the ovalbumin–water mix resulted in highslurry viscosity. This high viscosity of highly loadedslurries can be attributed to tangling of the long chainprotein molecules in ovalbumin. Thus, unlike the caseof low alumina loading where slurries could be preparedwith only ovalbumin, for alumina loading of 35 vol%and above, dilution of ovalbumin was essential. Thus,

for higher alumina loading, lesser amount of ovalbuminhad to be used to maintain fluidity of slurries.

For a fixed alumina loading (35 vol%) in the slur-ries, the viscosity could be varied in a systematic mannerby changing the ratio of ovalbumin to water in theslurry. Although the slurries exhibited the expectedtrend of higher viscosity for greater ovalbumin content,the slurry containing the highest ovalbumin amount didnot have the highest viscosity. The origin of this dis-crepancy was in the method of slurry preparation. Forpreparation of the slurries with ovalbumin–water mix, itwas desired that the intended ovalbumin amount beadded at the end of the milling step to minimize itsdegradation.33 But for the 35 vol% slurry, with an in-tended amount of 75% ovalbumin in the ovalbumin–water mix, the water volume alone was not sufficient toenable milling. Thus, a part of the ovalbumin amounthad to be added at the beginning of the slurry prepar-ation. The ovalbumin amount added at the beginningunderwent degradation during milling, resulting inlower slurry viscosity. Similar effects due to ovalbumindegradation were seen in other slurries where part of theovalbumin was added at the beginning of the slurrypreparation.

For slurries with fixed ovalbumin–water ratio, vis-cosity increased with increasing alumina loading in theslurries. This increase in viscosity of the slurries withsolids loading is attributed to increase in inter-particleinteractions with decrease in particle spacing in the slur-ries.

Foaming of Slurries and Total Porosity

The fact that ovalbumin is a natural foamer andthus encouraged foaming of aqueous alumina slurriescould also be seen from surface tension values. Althoughwater is known to have a surface tension of 72.8 mN/m,surface tension of as-extracted homogenized ovalbuminwas found to be 57.5 mN/m. The incorporation of airinto the suspensions, as a result of agitation within theslurries, was easier for slurries with relatively lower vis-cosity as could be seen from the time required to foamthe suspensions by a certain amount.

Increase in ovalbumin content for a fixed aluminaloading resulted in increase in slurry viscosity and inturn reduced the extent of foaming. As per expectation,the total porosity in the sintered alumina foams in-creased with increase in the extent of foaming.


As has been mentioned earlier, viscosity of slurriesexerted a significant influence on foaming of slurries,total porosity and the foam microstructures.

Control of Foam Microstructure

Total porosity, cell size, extent of interconnectionbetween individual cells are among the most importantmicrostructural parameters that determine the suitabil-ity of ceramic foams for specific applications. As shownin Fig. 3, the total porosity in sintered alumina foamsdecreased with increase in slurry viscosity. Slurries withhigher viscosity foamed to less extent and thus resultedin foams with lower total porosity (Table I, Fig. 2).

Although agitation time was not a variable in thepresent study, experiments indicated that shorter agita-tion times for respective slurries could be used to pro-duce foams with lower total porosity. In addition,porosities higher than the maximum of 92% were pro-duced by use of slurry compositions that contained15 vol% alumina and sucrose as a special additive.The role of sucrose in producing exceptionally highporosity in alumina foams has been described in detailin a previous study.31,32

Slurries with higher viscosity restricted the agitationas confirmed from lower extent of foaming and resultedin incorporation of smaller size air bubbles. Thus, thecell size in sintered alumina foams decreased and the

number of cells per unit area increased with increase inslurry viscosity (Fig. 4). The lower extent of foaming inhigher viscosity slurries resulted in greater cell separationin the alumina foams.

The dominant role of viscosity in determiningmicrostructure of foams, produced by foaming via tum-bling in the present study, is illustrated by data pre-sented in Table II, which is an average of measurementsfrom five different samples. It can be seen that themicrostructural parameters such as percentage porosity,cell size exhibited a strong correlation with slurry vis-cosity almost independent of the differences in aluminaloading and ovalbumin amount.

Slurries that foamed to large extent had thinner cellwalls, which encouraged formation of interconnectionsbetween individual cells. As a result, the percent inter-connection area increased with increase in extent offoaming. The formation of interconnections betweencells was responsible for creation of open porosity with-in the alumina foams. Because the total porosity in alu-mina foams also increased with increase in extent offoaming, the total porosity always correlated to the per-cent open porosity in the foams (Fig. 5). Though openporosity of the samples increased linearly with increasedin total porosity but increase of the interconnectionsarea are only 7%, which is not very significant indeed(Fig. 6). But only 7% area of interconnections was suf-ficient to make 87% of the total volume interconnected

Fig. 3. Influence of slurry viscosity on total porosity in aluminafoams.

Fig. 4. Number of cells per unit area in alumina foams as afunction of slurry viscosity (at shear rate of 0.5 s�1).


or open porous (Table I, 88% porous sample). Thesample with 10% area of interconnections was crumbledduring drying as in case 15 vol% alumina loaded slurry.

When viewed in terms of the slurry composition,for a fixed alumina loading, the percent interconnectionarea between cells increased, as the cell walls becamemore prone to fracture with decrease in ovalbumin con-tent. In addition, the tendency for formation of inter-connections between cells increased with decrease insolids loading again due to weaker cell walls.

Mechanical Properties of Alumina Foams

Both flexural and crushing strength of sintered alu-mina foams revealed similar dependence on microstruc-tural parameters such as the total porosity, average cell

size and average cell separation. Figures 7(a) and (b)shows that both the relative flexural and relative crush-ing strength decreased with reduction in relative densityof the alumina foam. Properties of the cell wall materials,that is, bulk density (3.9 g/cm3), flexural strength(350 MPa), compressive strength (2400 MPa) andYoung’s modulus (375 GPa) values for alumina wereused in the present study as reference. Both the flexuraland crushing strength also decreased with increase ofaverage cell size in alumina foams. Thinner cell walls ordecreasing cell separation in alumina foams were result-ed in lower flexural and crushing strengths. This similarmicrostructural dependent behavior was also reportedby Green et al.39,40 for reticulated carbon foams.

Both crushing and flexural strength of sintered alu-mina foams were improved with increase in ovalbumin

Table II. Microstructural Data for Slurries with Significant Difference in Composition but Nearly SimilarViscosity

Slurry compositionViscosity

(at 0.5 s�1 shear rate)(Pa s)

Totalporosity

(%)

Averagecell size

(lm)

No. ofcells/area(mm�2)

Particleloading (vol%)

Binder inpremix (vol%)

35 33 27.3 84.1 345.5 6.245 25 29.4 82.8 362.5 5.850 20 45.2 74.0 280.9 7.935 50 52.5 77.2 257.3 9.4

Fig. 5. Variation in percent open porosity with total porosity (%).Fig. 6. Variation in the percent area of interconnections betweencells as function of the total porosity.


content for fixed alumina loading. Similar trend wasalso observed, with increase in alumina loading, in caseof slurries using fixed ovalbumin content. This improve-ment of the strength as a result of increasing ovalbumincontent or alumina loading was due to the decrease intotal porosity, interconnection area percent and in-creased cell separation.

In the present study, the flexural and compressivestrength of alumina foams, with 71% porosity using

PCC technique, were 21 and 46 MPa respectively. Sep-ulveda et al.25 reported the average flexural strength of26 MPa for 70% porous alumina foam produced usinggelcasting technique. Recently, Kim et al.30 reportedcompressive strength of 100 MPa with closed cell micro-cellular silicon oxycarbide foam with 70% porosityusing pre-ceramic polymers and sacrificial hollowspheres. Discrepancies in mechanical strength of theporous alumina using PCC technique may be due tononhomogeneous foam microstructures produced usingrelatively higher viscosity slurry (Table I and Fig. 2a).This higher compressive strength of closed cell siliconoxycarbide foam may be due to micro cellular micro-structure (closed cell, average cell size o30 mm). Aver-age cell size of alumina foam using PCC technique wasvaried from 180 to 360 mm.

Numbers of micromechanical model have been de-veloped to describe the dependency of mechanical prop-erties on the relative density of the materials.2,41–44

Gibson and Ashby developed a theoretical model, whichpredicts the materials behavior with more accuracy. Ac-cording to this relation,2,44

Foam property ¼ Ci ðStrut PropertyÞðr=rSÞn

where Ci is a geometric constant characteristic of theunit cell shape and n depends on the deformation modeof the struts. The r and rS are the density of foam andsolid struts respectively. This model was developed forboth the closed and open cell structures. This model isbased on assuming cubic unit cell where the deform-ation is controlled by bending or buckling of the indi-vidual cell edges or strut within the unit cell. Thegeometrical constant Ci and exponent n were deter-mined both empirically and experimentally by numberof studies are listed in Table III.

According to Ashby’s model,2 the morphology ofopen-cell foams consists of interconnected ligaments orstruts without cell walls; such morphology is generallyobtained in ceramic foams that are prepared using poly-urethane sponge replica method. Indeed, the morph-ology of closed cell foams was considered to be consistsof cells in which the cell walls are completely retainedlike plate without any cell interconnections. In case ofclosed cell materials, deformation of plates was consid-ered during mechanical stress calculation and thuscontribution of membrane stress was included. Ashbyindicated that though exponent value for close cell ma-terials would increase from 1.5 to 2, but latter arguedthat most of the man made foams from liquid phase

Fig. 7. (a) Relative flexural strength and (b) Relative crushingstrength of ceramic foams as a function of relative density of porousalumina. [The cell wall materials properties i.e., true density(3.9 g/cm3), flexural strength (350 MPa) and crushing strength(2400 MPa) were used as reference values in this plot.]


(even those with closed cells faces) behave as if they hadopen cells because surface tension draws much of thesolid material into the cell edges during manufacture.Patel et al.41 mentioned that even thin faces can con-tribute to the behavior of these materials due to bendingof cell edges resulted in tension development in some ofthe faces. The GA model also implies that the tensilestrength is equal to compressive strength in the case ofintact cellular structures as the flaw sizes are equal to thatof cell sizes. They suggested that the mode of deform-ation or bending of the struts is same for both tensileand compressive load.

Maiti et al.45,46 proposed the value of Ci is 0.65 usingbased on some preliminary experiment. This constant wasanalytically solved by Zhang et al.39,40,47,48 assuming tetr-akaidecahedron unit cell and they have suggested its valueto be 0.16 for open cell structures. All different values ofthe constants are listed in Table III. Very limited reportsare available for closed cell materials.30,49,50

In the present study, the property dependent expo-nent, n was determined by iterative best fitting of thedata as 2.08 (R2 5 0.92) and 1.93 (R2 5 0.92) for com-pression strength and flexural strength respectively. Thegeometrical constants obtained from the present studyare also listed in Table III. The data considered in thepresent study represents a range of relative density from0.10 to 0.50. In comparison, a study on alumina foamsprepared with processing route similar to that used inthe present study covering relative density higher than0.1 reported a value of 2.24 for the exponent by fittingfor both compressive and flexural strength data.25,30,51

It must be noted that the crushing strength of the alu-mina foams were varied from 1–3.9 times higher thanthe flexural strength for the samples with relative densityranges from 0.13 to 0.51 as compared. Similar result hasbeen reported by Ortega et al.51 incases of gelcast alu-

mina foams. Experimental work of Brenzy andGreen39,40,47 revealed that the compressive strengthwas not comparable to tensile strength and rather itwas four to five times higher in case of brittle ceramicswith open cell structures. This anomaly may be due topresence of flaw sizes greater than cell size in brittlefoam. They have shown that compressive failure doesnot follow GA model. Rather, compressive failure incellular brittle materials occurs by a damage accumula-tion process. It is analogous to compressive failuremodes described in dense ceramics. The value of geo-metrical constant for open cell carbon foam was experi-mentally determined as 0.2 and 0.11 for bend strengthand crushing strength respectively. They also reported nvalues in the range of 1.45–2.15 in case of reticulatedceramic foams for bending strength.

The foam made using protein coagulation castingprocedures as shown in Figs. 2 and 6, it is likely to beclosed cell foam as it has low fractional area of inter-connections. In particular, the percent area of intercon-nections is only about 7% of the total area even with88% porous alumina made using 20 vol% loading.However as discussed earlier, 7% porosity is enoughto make it 87% open porous of the total volume. Evanset al.28 also mentioned about less area of interconnec-tions for porous ceramics using protein based foamingtechnique. Colombo et al.52 also evaluated the densityexponent for flexural and crushing strength in case ofsilicon oxycarbide foams. The microstructures of siliconoxycarbide foam resemble like foams prepared in thepresent study. They have considered this foam similar toclose cell structures. Thus, it is reasonable to considerthat the mechanical properties of the foam used in thisstudy resemble more like closed porous structures in-stead of open porous one which is also evident frommodel fitting in the present study.

Table III. A Comparison of the Ci and n Values Determined Experimentally and Those Predicted by theDifferent Models for Mechanical Properties of Brittle Foams as a Function of Relative Density

Mechanical properties

Geometrical constant (Ci)

Density exponent (n)

GA2

ExperimentalZhang46 GA2 Experimental Open cell Closed cell

Flexural strength 0.16 0.65 0.47 1.5 2.0 1.93Compressive strength 0.16 0.65 0.19 1.5 2.0 2.08Young’s Modulus 0.50 � 1 0.31 2.0 3.0 1.21


The elastic modulus of alumina foams decreasedwith increase in total porosity (Fig. 8). The elasticmodulus of alumina foams in the present study rangedfrom 51 to 0.9 GPa. A fit of the elastic modulus data asper the Gibbson-Ashby relation yielded a value of 1.21for the exponent, n while Green49 has reported the valueof 2.0 for closed cell glass. Sepulveda et al.53 reported avalue of 1.98 for hydroxyapatite foams which primarilyhad open cells fabricated using gelcasting technique.Roberts et al.50 has developed a theoretical model forelastic moduli of random three dimensional closed cellcellular solids. These results for closed cell Voronoi tes-sellations were in good agreement with earlier study54

on the tetrakaidecahedral foam. According to thismodel, E/ES � 1/3 (r/rS) as (r/rS)-0. The actual ex-ponent n was 1.19 obtained from single cell model. Thismodel suggested, the exponent should be 1ono2 forclosed cell and n 5 2 for open cell materials. The ex-perimental value of density exponent obtained in thisstudy is well fitted with closed cell model proposed byRoberts and colleagues.

Summary and Conclusions

Slurry rheology was found to be the main param-eter that influenced foaming of slurries and the resultantmicrostructure of sintered ceramic foams. Slurries with a

wide range of viscosity could be prepared by varying thesolids loading and or ovalbumin percent. The foamingbehavior of the slurries was seen to be a strong functionof the viscosity that directly affects the microstructuralparameters in sintered alumina foam. Foams made fromslurries with higher viscosity always had lower total por-osity, cell size and increase in number of cells per unitarea, cell separation. Alumina foam prepared using PCCtechnique is resembles like closed cell microstructuresdue to low fraction area of interconnection (7%) evenwith 87% open porosity.

The crushing and flexural strength of sintered alu-mina foams decreased with increase in total porosity.Microstructural analysis revealed that this increase instrength was related to decrease in percent porosity andinterconnection area with increase in either the ovalbu-min content or the alumina loading in slurries. Thus,alumina foam prepared using PCC method more re-semble closed cell materials even with high open por-osity.

Acknowledgments

This study was partly carried out at Materials Sci-ence Center, IIT Kharagpur, India during PhD study ofDr. Santanu Dhara. Dr. Parag Bhargava is currentlyworking as associate Professor at Department of Metal-lurgical Engineering & Material Science, IIT Bombay,India.

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Fig. 8. Relative elastic modulus of sintered alumina foams as afunction of relative density. [The cell wall materials properties i.e.,true density (3.9 g/cm3) and Young’s modulus (375 GPa) were usedas reference values in this plot.]


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Influence of Slurry Characteristics on Porosity and Mechanical Properties of Alumina Foams

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