J. Am. Ceram. Soc., 90 [11] 3407–3414 (2007) DOI: 10.1111/j.1551-2916.2007.01907.x r 2007 The American Ceramic Society Journal Processing of Particle-Stabilized Wet Foams Into Porous Ceramics Urs T. Gonzenbach,w André R. Studart,w David Steinlin, Elena Tervoort, and Ludwig J. Gauckler* Department of Materials, ETH Zürich, Zürich CH-8093, Switzerland Direct foaming of colloidal suspensions is a simple and versatile approach for the fabrication of macroporous ceramic materials. Wet foams produced by this method can be stabilized by longchain surfactants or by colloidal particles. In this work, we investigate the processing of particle-stabilized wet foams into crack-free macroporous ceramics. The processing steps are discussed with particular emphasis on the consolidation and drying process of wet foams. Macroporous alumina ceramics prepared using different consolidation and drying methods are compared in terms of their final microstructure, porosity, and compressive strength. Consolidation of the wet foam by particle coagulation before drying resulted in porous alumina with a closed-cell structure, a porosity of 86.5%, an average cell size of 35 lm, and a remarkable compressive strength of 16.3 MPa. On the other hand, wet foams consolidated via gelation of the liquid within the foam lamella led to porous structures with interconnected cells in the size range from 100 to 150 lm. The tailored microstructure and high mechanical strength of the macroporous ceramics can be of interest for the manufacture of bio-scaffolds, thermal insulators, impact absorbers, separation membranes, and light weight ceramics. attributed to the irreversible adsorption of partially hydrophobic particles to the air–water interface, as opposed to surfactants that adsorb and desorb on relatively short time scales.5 The high stability of these particle-stabilized foams does not require a setting reaction to prevent bubble coarsening, in contrast to the rapid consolidation needed in the case of surfactantstabilized foams. Yet, in order to fabricate solid porous ceramics from particle-stabilized wet foams, shaping, drying, and sintering have to be accomplished. Owing to the low strength of the wet foams and the high stresses involved during water evaporation, the drying step proves to be very critical in the fabrication of crack-free parts. The driving force for crack formation during drying is mainly the gradient of capillary pressure that develops inside the pores of the foam lamella during the evaporation of the liquid phase.8 If liquid evaporation were to expose the solid surface of the particles within the lamella, a solid/liquid interface would be replaced by a more energetic solid/vapor interface. To prevent such an increase in the system’s energy, the liquid tends to spread from the inside of the body to cover the exposed interface. This process leads to capillary pressure in the liquid phase that is counterbalanced by stresses in the particle network within the lamella. As a result, the particle network densifies and undergoes shrinkage.8 These stresses are proportional to the thickness of the part and the evaporation rate, and can easily amount up to several hundred kiloPascals.9 Crack formation occurs due to stress gradients in the particle network arising from locally different capillary pressures. In order to avoid high differential stresses, the capillary stresses have to be reduced or even eliminated. Capillary stresses can be significantly reduced by slow drying under controlled ambient conditions or through the addition of water-retaining additives that slow down the evaporation process. Alternatively, freeze drying completely prevents capillary stresses via the solidification of the liquid phase, followed by its direct sublimation into vapor. Another possible approach to reduce capillary stresses during drying is the use of so-called ‘‘drying control chemical additives’’ (DCCA).10,11 DCCA are used particularly in sol–gel processing to diminish the internal capillary stresses by changing the physical properties of the liquid phase.10 Besides the reduction of capillary stresses, another strategy to avoid drying cracks is the consolidation of the wet foam into a stronger structure that can resist the differential capillary stresses developed during drying. Several consolidation methods have been proposed in recent years for the processing of porous or near-net-shape dense ceramics.12 Strengthening can either be achieved by addition of a binder to the liquid phase or by gelling the foam lamella. The gelling methods can be generally divided into physical or chemical methods, depending on the type of gel formed during consolidation. Physical gels are formed when particles are attracted to each other to form a rigid particle network. This can be accomplished by reducing the electrostatic or steric repulsive forces between particles, so that attractive van der Waals forces prevail. In case of alumina for example, attractive particle networks can be created through the enzyme-catalyzed hydrolysis of urea in water.13 The decomposition of urea induces an in situ pH shift in the suspension toward the isoelectric point (IEP) of alumina at pH 9, thereby reducing the repulsive electrostatic forces between the particles. Even though physical gelation is usually not I. Introduction C OMPARED with their dense counterparts, porous ceramics have enhanced thermal insulation properties, good resistance against crack propagation, low weight, as well as high permeability and high accessible surface area in case of open-cell structures. Porous ceramics are therefore used as refractory insulators, catalyst carriers, filters for molten metals, and materials for hard tissue repair.1–3 Among the several methods used to produce macroporous ceramics,4 the direct foaming technique is of particular interest due to its simplicity, versatility, and low cost. In this method, air bubbles are incorporated into a ceramic suspension to produce a wet foam, which is subsequently consolidated by a setting reaction, dried, and finally sintered into a porous structure. Air incorporation is often accomplished by mechanical frothing of the initial suspension in the presence of long-chain surfactants. We recently developed a direct foaming technique that allows for the fabrication of ultra-stable particle-stabilized wet foams.5,6 This method is based on the adsorption of in situhydrophobized colloidal particles to the air–water interface of freshly generated air bubbles. Particles of a wide variety of chemical compositions ranging from alkaline materials such as Portland cement to acidic materials like silica have been used as foam stabilizers.7 All foams show neither bubble growth nor drainage over days and have average bubble sizes in the range from 10 to 300 mm, combined with air contents between 40% and 95% in the wet state. The high stability in the wet state is A. Bandyopadhyay—contributing editor Manuscript No. 22659. Received January 10, 2007; approved May 25, 2007. This project was funded by CIBA Specialty Chemicals. *Member, American Ceramic Society. w Author to whom correspondence should be addressed. e-mail: andre.studart@mat. or 3407 3408 Journal of the American Ceramic Society—Gonzenbach et al. sufficiently fast to enable the consolidation of surfactantstabilized foams, it can in principle be easily applied for the gelation of foams exhibiting enhanced stability. In addition to physical gelation, several other approaches rely on the gelation of the suspension’s liquid medium around the particles. These methods are often triggered by chemical reactions in the liquid phase and have thus been referred to as chemical gelation. The in situ free radical polymerization of acrylamide monomers is a gelation method that was originally developed for dense ceramics14 and has been later applied to ceramic foams.15,16 The main drawbacks of this method are the toxicity of the monomers and the need for oxygen-free environments to accomplish the polymerization reaction. These disadvantages were partially or fully overcome by using less harmful monomers17,18 or nontoxic cross-linking reactants without the need for oxygen-free atmospheres.19 Other environmental friendly setting agents such as the pH- or temperature-induced gelation of gelatine,19 ovalbumin,20–22 and bovine serum albumin23 have also been developed recently. Polysaccharides such as starch have been used as gelling agents as well as pore formers due to their swelling properties in water at elevated temperatures.24–26 Despite their insolubility in water below the gelling temperature, starch granules can adsorb up to 17 wt% of water at ambient temperatures.24,27 Starch is thus commonly used in the processing of construction materials to retain the humidity of cement-based materials while cement hydration reactions take place.28,29 Alternatively, alginic acid salt, in combination with hydroxyaluminium diacetate (HADA), was used as a timedelayed physicochemical gelation process to prepare wet green bodies with enhanced mechanical properties.30 In this paper, we describe the processing of particle-stabilized wet foams into porous structures with particular emphasis on the evaluation of suitable approaches to avoid crack formation during drying. Therefore, several methods are investigated in order to either control the drying procedure or strengthen the foam lamella through in situ gelation. The controlled drying techniques investigated were (a) drying under controlled environmental conditions, (b) unidirectional drying, (c) freeze drying, or (d) slow drying using a water-retaining additive (starch). Among the gelation methods, (a) physical gelation was accomplished by an in situ pH shift toward the IEP of the particles, whereas (b) physicochemical gelation was carried out by the gradual release of aluminum ions from HADA combined with the cross-linking of sodium alginate. These different methods are compared with respect to the porosity, cell size, cell morphology, and compressive strength of the final macroporous structures. II. Materials and Methods (1) Materials The colloidal a-Al2O3 particles used in this study were acquired from Sasol North America Inc. (Tucson, AZ) (grade Ceralox HPA-0.5). They had an average particle size of 200 nm, a specific surface area of 10 m2/g, and a density of 3.98 g/cm3. The short-chain amphiphilic molecules used to hydrophobize the particle surface were butyric acid and propyl gallate (Fluka AG, Buchs, Switzerland). Other chemicals used in the experiments were deionized water, hydrochloric acid (2 N, Titrisol, Merck, Darmstadt, Germany), sodium hydroxide (1 N, Titrisol, Merck), HADA (d50B0.7 mm, Fluka AG), sodium alginate (Fluka AG), native wheat starch (MioColl, Migros, Switzerland), urea (Sigma-Aldrich, Buchs, Switzerland), and urease (Roche Diagnostics GmbH, Mannheim, Germany). The enzymatic activity of the urease used was 58 000 U/(g of pure urease). One unit is defined as the amount of enzyme necessary to release 1 mmol of reaction product per minute from the substrate at 241C and at the pH where the enzymatic activity in water is at its maximum. (2) Processing Overview Figure 1 displays the processing flowchart used for the preparation of particle-stabilized foams and porous ceramics using Vol. 90, No. 11 different drying and gelation methods. The drying methods are subdivided into (i) drying under controlled humidity and temperature, (ii) unidirectional drying, (iii) freeze drying, and (iv) drying with a water-retention additive. The gelation methods, on the other hand, are divided into (i) physical and (ii) physicochemical gelation. All steps indicated in the flowchart are explained in detail within the following sections. Foaming conditions such as suspension pH, particle, and amphiphile concentrations were investigated extensively in an earlier work.6,7,31 Optimum conditions based on these previous studies are applied here in order to produce porous ceramics with the most promising properties. Figure 1 also shows typical wet particle-stabilized foams, a schematic of the foam density measurement tool (Section II(5)), and the resulting macroporous ceramics after drying and sintering. (3) Suspension Preparation (A) Controlled Drying Methods: Suspensions were prepared as follows: alumina powder was stepwise added to water containing hydrochloric acid (0.7 wt% to alumina) to obtain a suspension with 50 vol% solids content and a pH value around 5. Homogenization of the suspensions was conducted on a ball mill for at least 18 h using polyethylene milling pots and alumina balls (10 mm diameter, ratio balls:powder B2:1). Afterward, an aqueous solution containing butyric acid and if necessary pH adjusting agents (NaOH or HCl) was slowly and dropwise added to the ball-milled suspension under slight stirring to avoid local particle agglomeration. The amount of butyric acid added corresponded to a concentration of 50 mmol/L in the final suspension. The pH was set to 4.75 using a 1 N NaOH aqueous solution. Finally, the amount of water needed to adjust the solids content to 35 vol% was added. In case of compositions containing starch as a water-retention agent, starch concentrations up to 4.75 wt% to the liquid phase were added in powder form to the final ball-milled suspension (Fig. 1). In order to distribute the starch particles homogeneously, the suspension was magnetically stirred for 1 min before foaming. (B) Gelation Methods: Suspensions used for the experiments with the physical gelation method were prepared in a manner similar to those described above for the controlled drying tests. In this case, however, urea (0.05 wt% to alumina) was added to the water phase before the addition of alumina (Fig. 1). Suspensions used for the evaluation of the physicochemical gelation approach were prepared in the following way: alumina powder was stepwise added to water containing 506 mmol/L NaOH and 29 mmol/L propyl gallate to obtain a suspension with a solids content of 50 vol% and a pH value around 9.8. Homogenization of the suspensions was carried out in a ball mill for at least 18 h under the same conditions described above. Afterward, the propyl gallate needed to adjust the amphiphile concentration in the final suspension to 47 mmol/L was dissolved in a NaOH aqueous solution displaying a pH higher than 10. This solution was then slowly and dropwise added to the ball-milled suspension under slight stirring to avoid local particle agglomeration. Meanwhile, different amounts of sodium alginate were dissolved in water at 801C. After cooling to room temperature, the alginate solution was also added to the suspension under stirring. The pH of the suspension was then set to 9.9 and finally the amount of water needed to achieve a solids content of 20 vol% was added. (4) Triggering the Gelation of Wet Foams The gelation of the wet foams was triggered by adding specific additives to the initial suspension shortly before the foaming step. In the case of physical gelation, the gelation process was triggered by the addition of urease to the suspension (Fig. 1). Urease was dissolved in the amount of remaining water (B5 mL) necessary to set the suspension solids content to 35 vol%. After addition of the urease aqueous solution, the suspension was magnetically stirred for 30 s before foaming. The physicochemical gelation, on the other hand, was initiated through the November 2007 Fabrication of Porous Ceramic Materials 3409 Fig. 1. Processing flowchart for the preparation of particle-stabilized foams using (1) controlled drying and (2) gelation methods. Photographs of wet particle-stabilized foams and sintered macroporous ceramics, and a schematic of the foam density measurement tool are shown as well. 3410 Journal of the American Ceramic Society—Gonzenbach et al. addition of HADA to the initial suspension. In order to slow down this gelation process, the suspension was first cooled down in ice. HADA was then added in powder form to the suspension and dispersed by mechanical stirring. (5) Foaming and Foam Characterization Foaming of 150 mL suspension was carried out using a household mixer (Kenwood, Major Classic, Havant, UK) at full power (800 W) for 3 min. The foam density was measured with a custom-built tool that consisted of a plastic cylindrical cup with small holes on the bottom and a massive sliding stamp (Fig. 1). The foam was carefully filled into the cup and then slightly compressed with the stamp to remove possible air pockets introduced during filling. The volume between the bottom of the stamp and the bottom of the cylinder was kept constant. Dividing the mass of the foam by its volume resulted in the foam density. This method allows for density measurements with standard deviations well below 1%. (6) Shaping and Drying of the Wet Foams The wet foams prepared here possess a pronounced yield stress, that allows for easy shaping by extrusion, injection molding, or similar methods. Cylindrical parts (diameter: 100 mm, height: 50 mm) were hand shaped by filling the wet foam into a cardboard hollow ring placed on a glass substrate. The ring mold was removed just after shaping, leaving the cylindrical piece of foam on the glass substrate. Foam drying was subsequently carried out as follows (A) Controlled Drying Methods: Drying under controlled humidity and temperature (1a in Fig. 1) was performed in a climate chamber (KBF 115, Binder, Tuttlingen, Germany) at a relative humidity of 90% and either 151 or 501C. In the case of unidirectional drying (1b in Fig. 1), wet foams were placed on a heating plate positioned inside the climate chamber at 281C and 90% relative humidity. Two copper cylinders (diameter: 100 mm, height: 50 mm) were placed between the wet foam and the heating plate to work as a thermal bath and to allow for the drying of two samples simultaneously. The copper cylinders were heated to 401, 601, or 801C and a thermocouple was used to control the temperature of the cylinders during the drying process. This led to temperature differences between the hot copper cylinder and the surrounding atmosphere of 121, 321, and 521C, enabling a quasi-unidirectional drying of the samples. The temperature gradient applied through the height of the sample (31, 81, and 131C/cm) was taken here as a rough indication of the driving force for drying. The drying process inside the climate chamber was conducted for approximately 12 h. Samples were subsequently exposed to room temperature for another 12 h. In the freeze-drying approach (1c in Fig. 1), shaped wet foams were first unidirectionally frozen by placing the sample onto an aluminum cylinder (diameter: 100 mm, height: 145 mm) that was partially immersed in liquid nitrogen to assure a homogeneous temperature on the surface of the cylinder. After 30 min, the frozen foam was removed from the cylinder and placed in the freeze dryer (Alpha 2-4, 100402, Martin Christ, Osterode am Harz, Germany) for approximately 36 h at a pressure of 0.3 mbar. Wet foam samples containing a water-retention additive (1d in Fig. 1) were dried by direct exposure in air at 221–251C for 24–48 h. (B) Gelation Methods: The wet foams were covered with a cling film to avoid drying before complete gelation took place. After about 12 h, the cling film was carefully removed and the samples were dried in air at 221–251C for 24–48 h. (7) Sintering of the Foams Sintering of the dried cylindrical foams was performed in an electrical furnace (HT 40/16, Nabertherm, Lilienthal, Germany) Vol. 90, No. 11 at 15751C for 2 h. The heating and cooling rates were set to 11 and 31C/min, respectively. (8) Microstructural Analysis The average cell size of the sintered foam was evaluated from micrographs taken with a scanning electron microscope (LEO 1530, LEO, Oberkochen, Germany). The cell sizes were measured with the linear intercept method using the software Linear Intercept (TU, Darmstadt, Germany). From the cumulative cell size distribution obtained, we determined d10, d50, and d90, which correspond to the cell diameter obtained for a cumulative percentage of cells lower than 10, 50%, and 90%, respectively. In this study, the d50 value is referred to as the average cell size and the ratio (d90–d10)/2 is taken as an indication of the bubble size distribution. (9) Compressive Strength Measurements Compressive strength measurements were performed on a universal testing machine (Instron 8562, model A1477-1003, Norwood, MA). For specimen preparation, the sintered cylindrical parts were first ground on both sides in order to obtain parallel opposite surfaces that ensured homogeneous sample loading during compression. Cylindrical samples with diameters of 15 mm and lengths of 30 mm were drilled out of the bulk piece of foam with a diamond core drill. The geometrical density and the compressive strength at a loading speed of 0.5 mm/min were measured for a minimum of five samples for each investigated composition. III. Results and Discussion (1) Drying, Sintering, and Microstructure of Porous Ceramics (A) Controlled Drying Methods (a) Controlled Humidity and Temperature: In order to slow down the drying process, foam samples were dried at a relative humidity of 90% and a temperature of either 151 or 501C. The high humidity applied ensured a low driving force for water evaporation, leading to drying periods as long as 48 h. The temperature, on the other hand, was changed to vary the diffusion kinetics of water from the interior of the sample to the surface. Similar to foams directly dried in air, such drying conditions led to completely cracked samples after sintering, which made the preparation of homogeneous samples for mechanical testing impossible. The total linear shrinkage during drying and sintering of these specimens was typically around 25%. The existence of cracks suggests that the capillary stresses developed even at low drying rates are already high enough to introduce defects in the foam microstructure. We have also observed that some of the samples displayed a higher concentration of defects in the center of the body. This indicates that the inner flaws may also originate from a constrained shrinkage of the sample core. In order to minimize this effect, unidirectional drying of the samples was subsequently evaluated. (b) Unidirectional Drying: Unidirectional drying allows for a homogenous shrinkage throughout the entire sample, avoiding shrinkage constraints during the drying process. The results showed that the temperature gradient between the thermal bath (copper cylinders) and the surrounding atmosphere strongly influenced the drying behavior of the wet foams. For temperature gradients lower than about 81C/cm (Tcoppero 601C), the driving force for unidirectional drying was not high enough to avoid drying of the sample upper surface. This resulted in the formation of large cracks within the sample. On the other hand, temperature gradients higher than 81C/cm allowed for complete unidirectional drying. Figure 2(a) shows the cross section of a sintered foam that was unidirectionally dried at a temperature gradient of 131C/cm (Tcopper 5 801C). This sample did not exhibit the same large defects initially observed for sam- November 2007 3411 Fabrication of Porous Ceramic Materials Air content in wet foam (%) 90 85 80 75 70 65 0 1 2 3 4 Starch in suspension (wt% to liquid) 5 Fig. 4. Influence of starch concentration in the initial suspension on the air content of the wet foams. Fig. 2. Cross section (a) and microstructure (b, c) of a unidirectionally dried alumina foam subsequently sintered at 15751C. Large cells with slightly porous walls are distributed evenly among smaller cells that originated from the bubbles. ples dried under a controlled atmosphere. Interestingly, cells approximately one order of magnitude larger than the initial bubble size were formed during drying (Figs. 2(b) and (c)). The relatively high temperature applied during this drying process (up to 801C) might have facilitated the coalescence of single bubbles and the formation of these unevenly shaped large pores. The walls of the large cells also showed a higher porosity in comparison with the walls of smaller cells (Fig. 2(c)). (c) Freeze Drying: The gentle removal of water by freeze drying resulted in sintered samples free of macroscopic cracks. However, Figs. 3(a) and (b) show that the freeze-dried samples exhibited many microscopic cracks. These cracks were most likely introduced during the cutting of the bodies for observation in the microscope. Because the freezing process did not lead to any shrinkage of the wet foam, the network of particles in the foam lamella did not undergo any densification during the drying process. Therefore, the particles are very loosely packed on the cell wall in the green state, leading to porous walls (Fig. 3(b)) and weak structures after sintering. The absence of pores with Fig. 3. (a) Microstructure of a freeze-dried and sintered foam, (b) detail of the porous and cracked cell wall. dendritic morphology suggests that no ice crystals were formed during the freezing process.32,33 (d) Water-Retention Additive: Wheat starch was added to the suspension in order to act primarily as a water-retention additive that slows down the speed of water removal during drying. Figure 4 shows that the addition of starch to the foam precursor suspension results in a decrease in the air content of the wet foam. This decrease can be explained by an increase in the suspension viscosity, which ultimately hinders the incorporation of air during mechanical mixing. Additionally, amylose is known to form complexes with fatty acids in acidic pHs at room temperature.34 This complex formation can decrease the concentration of free butyric acid in suspension and lead to desorption of butyric acid molecules from the particle surface, resulting in a lower particle hydrophobicity. Foam samples containing starch underwent a linear shrinkage of approximately 5% and 20% during drying and sintering, respectively. Starch concentrations higher than 0.64 wt% with respect to the liquid enabled the preparation of crack-free-sintered foams. At lower concentrations, the wet foam dried inhomogeneously and the formation of large cracks was observed. Figures 5(a) and (c) show the microstructure of samples prepared with 0.64 and 2.44 wt% of starch. In case of 0.64 wt% starch, the air content in the foam is 86.2% and the average cell size is 34 mm with a standard deviation of 18 mm. In case of 2.44 wt% starch, a lower air content of 81.2% and smaller cells of 24 mm (712 mm) were achieved. The lower air content and average cell size obtained with the higher starch concentration results from the higher viscosity of suspensions containing more starch. An increase in the viscosity of the initial suspension is known to decrease the average bubble size of mechanically frothed foams.31 For both starch concentrations, the cell walls are completely dense as shown in Figs. 5(b) and (d). It is important to note that the starch concentration required to avoid crack formation (0.64 wt%) is significantly lower than that used for the preparation of porous ceramics using starch particles as sacrificial templates24 and that typically used as a binder for strengthening purposes in ceramic processing.35,36 This suggests that the water-retention capability of starch plays a major role in avoiding crack formation during drying. (B) Gelation Methods (a) Physical Gelation: The wet foam was strengthened by the in situ coagulation of particles throughout the foam lamella. Coagulation was achieved by shifting the pH of the suspension in the foam lamella toward the IEP of the particles. The end pH of the enzyme-catalyzed decomposition reaction of urea13 is determined by the urea concentration in suspension, whereas the speed of the reaction depends on the enzyme concentration. In 3412 Journal of the American Ceramic Society—Gonzenbach et al. Vol. 90, No. 11 Fig. 5. Microstructure of homogeneously dried foams (a and c) prepared from suspensions containing 0.64 and 2.44 wt% starch with respect to the liquid phase, respectively. The cells are mostly closed and dense, as shown in insets (b) and (d). the case of the investigated foams, foam destabilization was observed at pHs higher than 8 due to the desorption of butyric acid molecules from the particle surface and the consequent desorption of particles from the air–water interface. Therefore, the end pH of the reaction should not exceed a pH of approximately 7.5. The concentration of urease was set to 1 U/g of alumina, which allows for foaming of the suspension before the time-delayed coagulation of the bulk takes place. The addition of 0.05 wt% urea with respect to alumina led to an end pH of B7.3, enabling the preparation of crack-free samples (Fig. 6(a)). The linear shrinkage of the physically gelled sample after drying and sintering was about 7% and 15%, respectively. The resulting foam contains 86.5% air and predominantly closed cells with an average size of 35 mm and a standard deviation of 16 mm (Fig. 6(b)). The cell walls are completely dense and formed by a single layer of grains (Fig. 6(c)). (b) Physicochemical Gelation: The gelation of algae-derived macromolecules in the presence of multivalent ions was used to strengthen the wet foams and avoid cracking during drying. The combination of alginic acid salt with HADA led to a gelation of the liquid phase through the cross-linking of alginate macromolecules with Al31 ions, as well as coagulation of the particle network by increasing the suspension ionic strength.30 The addition of 0.25 wt% alginic acid salt and 1.75 wt% HADA to the initial suspension enabled the preparation of crack-free foams after drying and sintering. The specimens exhibited a linear shrinkage of about 30% during drying, which is significantly higher compared with the other drying methods. On the other hand, the linear shrinkage of about 20% during sintering is comparable to that of the other approaches. The gelation process led to porous structures containing 83.5% air and interconnected cells in the range of 50–200 mm (Fig. 7(a)). The large cell size is a consequence of the low solids content and viscosity of the initial suspension (20 vol%) and agrees well with our previously published results.31 An increase of the alginate content to 1 wt% and the HADA concentration to 3 wt% resulted in crack-free samples with air contents of 79.1% and average cell sizes predominantly smaller than 100 mm (Fig. 7(b)) due to the increased viscosity of the suspension. Increasing the alginate and HADA concentration further inhibited foam formation due to the excessively short gelling time. The cell interconnectivity (Figs. 7(a) and (b)) suggests that the layer of particles initially surrounding the gas bubbles was rup- Fig. 6. Cross section (a) and microstructure (b and c) of a foam that was physically gelled before drying. The enzyme-catalyzed decomposition reaction of urea shifted the foam pH toward the isoelectric point of alumina, resulting in a strong particle network that prevents crack formation. November 2007 3413 Fabrication of Porous Ceramic Materials Fig. 7. Microstructure of sintered alumina foams consolidated via the physicochemical gelation method using (a) 0.25 wt% and (b) 1 wt% alginate. The cell size in case of 1 wt% alginate is significantly smaller due to an increase in the suspension viscosity. The open-cell structure probably results from the uneven local shrinkage that occurs during gel formation. 98 95 100 Porosity (%) 90 80 50 0 100 10−1 Clos e d- c ell 10 10−2 1 10−3 Op l cel e n- Freeze drying Unidirectional drying Water retention additive Physical gelation Physiochemical gelation 10−4 0.02 0.05 0.1 0.2 0.5 0.1 Compressive strength of alumina foams (MPa) (2) Mechanical Properties Figure 8 shows the geometrical density and compressive strength of the macroporous alumina monoliths prepared in this study in comparison with samples produced via other direct foaming methods.4 The standard deviation for the strength measurements is approximately 15%, 10%, and 40% for the samples dried with starch, gelled via the physical method and gelled via the physicochemical approach, respectively. The dashed lines in the graph represent the theoretical relative strength expected for open and closed cell foams according to the model of Gibson and Ashby.38 Foams consolidated via physical gelation and with starch as a water-retention additive show the highest compressive strengths. Their mechanical strength is also significantly higher compared with porous materials prepared with other direct foaming methods at the same relative density. A remarkably high compressive strength of 16.3 MPa at a porosity of 86.5% was achieved using the physical gelation of the particle network. The high mechanical strength can be attributed to the flawless and dense closed cells obtained (Figs. 5 and 6). On the other hand, the physicochemically gelled foams showed a noticeably lower mechanical strength, as a result of their interconnected cell structure (Fig. 7). Yet, the compressive strength of these foams is comparable with that of other opencell porous ceramics produced by direct foaming methods. It should also be noted that the preparation of interconnected structures using the approach described in this study has not been extensively explored yet. Further investigation is required to enable one to deliberately tailor the cell interconnectivity and eventually increase the strength of these foams. The unidirectionally dried samples featured porosities higher than 92% due to the formation of larger cells during drying (Fig. 2(b)). This high overall porosity and the pores observed in the cell walls of these foams (Fig. 2(c)) resulted in a low mechanical strength of 0.8 MPa. Likewise, the freeze-dried foam showed a low compressive strength of 1.2 MPa at a porosity of Relative strength (−) tured during the gelation process. The cross-linking of alginate macromolecules is accompanied by a volumetric shrinkage due to solvent expulsion (syneresis).37 Therefore, the rupture of the particle layer around the gas bubbles might be caused by a local uneven shrinkage of the macromolecules during the gelation process. 0.01 1 Relative density (−) Fig. 8. Compressive strength as a function of the relative density of particle-stabilized foams prepared by controlled drying methods (~, freeze drying; , unidirectional drying; & , drying with a water-retention additive) and gelation methods (m, physical gelation; ., physicochemical gelation). The results achieved in this study are compared with those obtained for other samples produced via direct foaming methods4 (1). The dashed lines represent the relative strength expected for open and closed cell foams, according to the theoretical model of Gibson and Ashby.38 3414 Journal of the American Ceramic Society—Gonzenbach et al. 85.8% due to the pores formed within the cell walls of the porous structure (Fig. 3). IV. Conclusions The preparation of bulk crack-free macroporous ceramics from particle-stabilized foams was accomplished by applying different drying and gelation techniques. Methods that directly increase the wet foam strength or that ensure a homogeneous drying were the most successful, leading to crack-free foams with high mechanical strength at high porosities. The use of starch as a water-retention additive or coagulating agents to strengthen the wet foam resulted in crack-free porous ceramics with mainly closed cells. Depending on the drying conditions, these samples featured porosities between 80.2% and 89%, closed cells in the range of 24–35 mm, and remarkably high mechanical strengths between 11.8 and 22.9 MPa. The unidirectional drying or freezedrying techniques also led to closed-cell structures without macroscopic cracks. In this case, however, the mechanical strength is noticeably lower due to excessively thin or porous cell walls. Finally, the physicochemical gelation using an algae-derived polymer resulted in structures with a porosity of 79.1%– 83.5%, interconnected cells in the range of 50–200 mm, and mechanical strength comparable with that obtained for other open cell materials. The preparation of crack-free structures using the drying and gelation methods investigated here should enable the manufacture of new macroporous ceramics for various applications in catalysis, tissue engineering, separation technologies, as well as for the manufacture of light and insulating materials. 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