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Ceramic foam materials have received great attention in recent years for their light weight, thermal/electrical insulation, high crack tolerance and good machinability properties. However, some applications demand higher mechanical strength, better resistivity of the surface against mechanical impact or improved abrasion resistance. In this paper, the feasibility of dip coating 89 vol% porous, green alumino-silicate foam bars with dimensions in the centimeter range was studied and a dip coating route was proposed. The surface morphology of uncoated and coated foam bars was first analyzed by scanning electron microscopy (SEM) and later detailed using the GelsightTM method. Mechanical properties were determined by 4-point bending. A dense coating layer with a uniform thickness and good interfacial adhesion was obtained over the foam. Consequently, a three-fold increase in the flexural strength was measured for coated parts. In addition, the versatility of the developed dip coating route was demonstrated coating samples of bigger size like panels as well as complex geometries like cylindrical crucibles.

We propose a new and versatile synthesis route for the formation of particle-stabilized gel-core microcapsules. With this route, microcapsules can be produced in a large size range of a few to hundreds of micrometres. Due to the high mechanical stability developed upon gelation, the capsules can be harvested as dry powders. Implementing in-situ hydrophobization to modify the wetting properties of particles allows preparing capsule shells from almost any kind of particulate materials. In the present work, we demonstrate the formation of microcapsules with shells from 60 nm gamma alumina and 200 nm alpha alumina particles. These particle shells are combined with sodium acrylate and poly(ethylene glycol) based gel-cores. Shape, gel-core microstructure, and swelling of the capsules in deionized water are characterized. Modelling the release of a low molecular weight dye from the capsules reveals Fickian kinetics for 16 and polymer relaxation dominated kinetics for 280 μm sized capsules, respectively. Given the flexibility in materials selection, our processing route could lead to the design of new microcapsule systems for sensing and release applications.

The mechanical properties of porous ceramics are greatly influenced by their microstructure. Therefore, mechanical behavior of highly porous ceramics is different from that of dense ceramics. In this work, we evaluate different mechanical testing methods such as static compression, Brazilian disc test and 3-point bending on their suitability for comparison of highly porous ceramic materials. It is shown that 3-point bending is more suitable than static compression or Brazilian disc testing, as the material exhibits no critical crack propagation under compressive loading. With 3-point bending tests, a quantitative comparison of the mechanical properties of foams with different microstructures and porosities is possible. Under cyclic compression the foams exhibit a very high degree of crack tolerance in combination with preservation of their structural integrity even at high strains of 10%.

ARTICLE DE JOURNAL Synthesis of bone like structured foams Krauss Juillerat Franziska; Engeli Roman; Jerjen Iwan; Sturzenegger Philip N.; Borcard Françoise; Juillerat Jeanneret Lucienne; Gerber Lemaire Sandrine; Gauckler Ludwig J.; Gonzenbach Urs T. Publié dans: Journal of the European Ceramic Society vol. 33 p. 1497 1505 Oxford: Elsevier Sci Ltd 2013 Here we present a processing route to produce multi structured ceramic foams based on the combination of particle stabilized foams with polymeric sponges to produce positive and negative templating structures. Polyester sponges are infiltrated with freshly produced calcium aluminate–alumina foams and upon sintering either positive templating structures are produced when wetting the sponges or negative templating foams with a percolating pore network are obtained when completely filling the sponges. Additionally by combining different layers of these particle stabilized foam infiltrated sponges various different structures can be produced including sandwich structures pore size gradients and ceramic bone like structures applying to different types of bone. The particle stabilized foams used were in situ self hardening calcium aluminate cement enriched alumina foams to obtain crack free samples with pore interconnections and tailorable pore sizes.

Ceramic-based hollow spheres are considered a great driving force for many applications such as offshore buoyancy modules due to their large diameter to wall thickness ratio and uniform wall thickness geometric features. We have developed such thin-walled hollow spheres made of alumina using slip casting and sintering processes. A diameter as large as 50 mm with a wall thickness of 0.5–1.0 mm has been successfully achieved in these spheres. Their material and structural properties were examined by a series of characterization tools. Particularly, the feasibility of these spheres was investigated with respect to its application for deep sea (>3000 m) buoyancy modules. These spheres, sintered at 1600 °C and with 1.0 mm of wall thickness, have achieved buoyancy of more than 54%. As the sphere’s wall thickness was reduced (e.g., 0.5 mm), their buoyancy reached 72%. The mechanical performance of such spheres has shown a hydrostatic failure pressure above 150 MPa, corresponding to a rating depth below sea level of 5000 m considering a safety factor of 3. The developed alumina-based ceramic spheres are feasible for low cost and scaled-up production and show great potential at depths greater than those achievable by the current deep-sea buoyancy module technologies.

Bone substitute materials allowing trans-scaffold migration and in-scaffold survival of human bone-derived cells are mandatory for development of cell-engineered permanent implants to repair bone defects. In this study, we evaluated the influence on human bone-derived cells of the material composition and microstructure of foam scaffolds of calcium aluminate. The scaffolds were prepared using a direct foaming method allowing wide-range tailoring of the microstructure for pore size and pore openings. Human fetal osteoblasts (osteo-progenitors) attached to the scaffolds, migrated across the entire bioceramic depending on the scaffold pore size, colonized, and survived in the porous material for at least 6 weeks. The long-term biocompatibility of the scaffold material for human bone-derived cells was evidenced by in-scaffold determination of cell metabolic activity using a modified MTT assay, a repeated WST-1 assay, and scanning electron microscopy. Finally, we demonstrated that the osteo-progenitors can be covalently bound to the scaffolds using biocompatible click chemistry, thus enhancing the rapid adhesion of the cells to the scaffolds. Therefore, the different microstructures of the foams influenced the migratory potential of the cells, but not cell viability. Scaffolds allow covalent biocompatible chemical binding of the cells to the materials, either localized or widespread integration of the scaffolds for cell-engineered implants.

Microencapsulation is nowadays widely applied for the separation or controlled and targeted release of substances. In the present work we propose using particle-stabilized water-in-oil emulsions to produce microcapsules from calcium aluminate cement. First, we investigate the parameters determining the capsule size and size distribution. Then, the anhydrous particles at the water-oil contact line are hydrated resulting in the formation of a layer of precipitated calcium aluminate hydrate that strengthens the capsule shells. By manipulating the hydration reaction of the cement particles, the capsule shell thickness is varied between 200 and 1300 nm. For hydration temperatures of 3°C and 25°C, the capsule walls are mainly formed from Ca2Al2O5·8H2O, whereas Ca2Al2O5·8H2O and the stable γ-Al(OH)3 and Ca3Al2O6·6H2O phases are precipitated at 60°C. In addition, the impact of these manipulations on the yield of dry microcapsules is discussed. The highest yield of intact capsules observed in this study is 72 ± 3%. We believe that such microcapsules can lead to new applications, particularly in environments with harsh conditions or in inorganic matrixes.

We present a method to prepare metallic foams with a unique architecture of small pores and thin struts using wet foams and emulsions stabilized by metallic nanoparticles as templates.

Aqueous suspensions containing oppositely charged colloidal particles and amphiphilic molecules can form fluid dispersions, foams, and percolating gel networks, depending on the initial concentration of amphiphiles. While models have been proposed to explain the electrokinetic and flotation behavior of particles in the presence of long amphiphilic molecules, the effect of amphiphiles with less than six carbons in the hydrocarbon tail on the electrokinetic, rheological, and foaming behavior of aqueous suspensions remains unclear. Unlike conventional long amphiphiles (≥10 carbons), short amphiphiles do not exhibit increased adsorption on the particle surface when the number of carbons in the molecule tail is increased. On the basis of classical electrical double layer theory and the formerly proposed hemimicelle concept, we put forward a new predictive model that reconciles the adsorption and electrokinetic behavior of colloidal particles in the presence of long and short amphiphiles. By introducing in the classical Gouy-Chapman theory an energy term associated with hydrophobic interactions between the amphiphile hydrocarbon tails, we show that amphiphilic electrolytes lead to a stronger compression of the diffuse part of the electrical double layer in comparison to hydrophilic electrolytes. Scaling relationships derived from this model provide a quantitative description of the rich phase behavior of the investigated suspensions, correctly accounting for the effect of the alkyl chain length of short and long amphiphiles on the electrokinetics of such colloidal systems. The proposed model contributes to our understanding of the stabilization mechanisms of particle-stabilized foams and emulsions and might provide new insights into the physicochemical processes involved in mineral flotation.

The ability to tailor the microstructure of porous ceramics is essential in order to fulfill the requirements of various applications. Depending on the use of porous ceramics, microstructures with open or closed pores, adjustable pore sizes, and a possible self-setting ability of the wet foam are required. We present a direct foaming method to synthesize self-setting foams with controlled microstructures, based on our previous studies on particle-stabilized foams. For the experimental set-up, alumina particles were partially hydrophobized with propyl gallate and the resulting suspensions were combined with a calcium aluminate cement reaction. Pore size and the fraction of open pores can be controlled by the particle concentration and the setting rate of the cement reaction. As a result, self-setting ceramic-cement composites with porosities ranging from 40 to 95 vol% and closed as well as open pores with sizes between 30 mu m and 1 mm were achieved. Compared with other methods used to produce self-setting inorganic macroporous materials, foams made with this method cover a wider pore size and porosity range and reach higher total porosities.

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 long-chain 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 μm, 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 μm. 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.

We present a novel direct-foaming method to produce macroporous ceramics using particles instead of surfactants as stabilizers of the wet foams. This method allows for the fabrication of ultra-stable wet foams that resist coarsening upon drying and sintering. Macroporous ceramics of various chemical compositions with open or closed cells, average cell sizes ranging from 10 to 300 μm and porosities within 45% and 95%, can be easily prepared using this new approach. The sintered foams show high compressive strengths of up to 16 MPa in alumina foams with porosities of 88%.

Macroporous ceramics with pore sizes from 400 nm to 4 mm and porosity within the range 20%–97% have been produced for a number of well-established and emerging applications, such as molten metal filtration, catalysis, refractory insulation, and hot gas filtration. These applications take advantage of the unique properties achieved through the incorporation of macropores into solid ceramics. In this article, we review the main processing routes that can be used for the fabrication of macroporous ceramics with tailored microstructure and chemical composition. Emphasis is given to versatile and simple approaches that allow one to control the microstructural features that ultimately determine the properties of the macroporous material. Replica, sacrificial template, and direct foaming techniques are described and compared in terms of microstructures and mechanical properties that can be achieved. Finally, directions to future investigations on the processing of macroporous ceramics are proposed.

Three different short-chained carboxylic acids, formic, acetic, and propionic acid, were compared as additives to increase the green strength of wet particle networks produced with the direct coagulation casting process, which allows the internal coagulation of a suspension. Small acid additions do not alter the favorable low viscosity of the stabilized slurry, and yet increase the compressive strength of wet coagulated bodies. The strengthening effect increases with increasing chain length of the carboxylic acid. The maximum effect was observed upon addition of propionic acid (0.3 wt% to alumina), which resulted in a six-fold increase in compressive strength of wet coagulated bodies compared with that without propionic acid.

This study shows for the first time that foam stabilization via in-situ hydrophobization of reactive cement particles is possible. By taking advantage of the initial “passivation” of freshly immersed calcium aluminate cement particles in water, hierarchical self-setting particle-stabilized foams can be produced using propyl gallate to in-situ hydrophobize the particle surface. The foam macrostructure (pore size and porosity) and the sub-micrometer structure inside the foam lamellae can be tailored independently by varying different parameters such as the cement and surface modifier concentration, or the setting temperature. The set foam microstructure is analyzed quantitatively and reveals pore sizes between 90 and 1650 μm and porosities up to 92 vol.%. The sub-micrometer structure is discussed qualitatively using SEM imaging and was found to feature surface-textured structures – a rather specific phenomenon of this material.

This study presents a novel method to produce self-setting inorganic foams with unique hierarchical pore structures. The combination of particle-stabilized alumina foams with calcium aluminate cement leads to a macroporous ceramic material which can be shaped and consolidated at room temperature, bypassing the challenging and sometimes extensive drying and sintering steps. Due to the water consuming cement hydration reaction, no macroscopic shrinkage was observed and crack and cavity formation was prevented throughout the entire specimens. The final microstructure features a porosity of 76 vol.% and a unique hierarchical pore structure with interconnecting pores of 200 µm diameter, separated by mesoporous pore walls. The authors believe that this newly developed method opens a door to novel applications where so far the drying and sintering step and its inherent shrinkage of the ceramic foams were the limiting factors.

International audience; Incorporation of porosity into a monolithic material decreases the effective thermal conductivity. Porous ceramics were prepared by different methods to achieve pore volume fractions from 4 to 95%. A toolbox of analytical relations is proposed to describe the effective thermal conductivity as a function of solid phase thermal conductivity, pore thermal conductivity, and pore volume fraction (νp). For νp < 0.65, the Maxwell–Eucken relation for closed porosity and Landauer relation for open porosity give good agreement to experimental data on tin oxide, alumina, and zirconia ceramics. For νp > 0.65, the thermal conductivity of kaolin-based foams and calcium aluminate foams was well described by the Hashin Shtrikman upper bound and Russell’s relation. Finally, numerical simulation on artificially generated microstructures yields accurate predictions of thermal conductivity when fine detail of the spatial distribution of the phases needs to be accounted for, as demonstrated with a bio-aggregate material.

Surface functionalization of hydroxyapatite (HA) and ?-tricalcium phosphate (TCP) bioceramics with chemical ligands containing a pyrrogallol moiety was developed to improve the adhesion of bone cell precursors to the biomaterials. Fast and biocompatible copper-free click reaction with azido-modified human fetal osteoblasts resulted in improved cell binding to both HA and TCP bioceramics, opening the way for using this methodology in the preparation of cell-engineered bone implants.

Different anchoring groups have been studied with the aim of covalently binding organic linkers to the surface of alumina ceramic foams. The results suggested that a higher degree of functionalization was achieved with a pyrogallol derivative – as compared to its catechol analogue – based on the XPS analysis of the ceramic surface. The conjugation of organic ligands to the surface of these alumina materials was corroborated by DNP-MAS NMR measurements.

Emulsions stabilized through the adsorption of colloidal particles at the liquid-liquid interface have long been used and investigated in a number of different applications. The interfacial adsorption of particles can be induced by adjusting the particle wetting behavior in the liquid media. Here, we report a new approach to prepare stable oil-in-water emulsions by tailoring the wetting behavior of colloidal particles in water using short amphiphilic molecules. We illustrate the method using hydrophilic metal oxide particles initially dispersed in the aqueous phase. The wettability of such particles in water is reduced by an in situ surface hydrophobization that induces particle adsorption at oil-water interfaces. We evaluate the conditions required for particle adsorption at the liquid-liquid interface and discuss the effect of the emulsion initial composition on the final microstructure of oil-water mixtures containing high concentrations of alumina particles modified with short carboxylic acids. This new approach for emulsion preparation can be easily applied to a variety of other metal oxide particles.

Wet foams are used in many important technologies either as end or intermediate products. However, the thermodynamic instability of wet foams leads to undesired bubble coarsening over time. Foam stability can be drastically improved by using particles instead of surfactants as foam stabilizers, since particles tend to adsorb irreversibly at the air-water interface. Recently, we presented a novel method for the preparation of high-volume particle-stabilized foams which show neither bubble growth nor drainage over more than 4 days. The method is based on the in-situ hydrophobization of initially hydrophilic particles to enable their adsorption on the surface of air bubbles. In-situ hydrophobization is accomplished through the adsorption of short-chain amphiphiles on the particle surface. In this work, we illustrate how this novel method can be applied to particles with various surface chemistries. For that purpose, the functional group of the amphiphilic molecule was tailored according to the surface chemistry of the particles to be used as foam stabilizers. Short-chain carboxylic acids, alkyl gallates, and alkylamines were shown to be appropriate amphiphiles to in-situ hydrophobize the surface of different inorganic particles. Ultrastable wet foams of various chemical compositions were prepared using these amphiphiles. The simplicity and versatility of this approach is expected to aid the formulation of stable wet foams for a variety of applications in materials manufacturing, food, cosmetics, and oil recovery, among others.

Pump up the volume: Wet foams prepared with surfactants are thermodynamically unstable systems that undergo rapid disproportionation, drainage, and coalescence. Ultrastable foams have now been prepared using colloidal particles as stabilizers (left picture). The stabilization results from the irreversible adsorption at the air–water interface of particles surface-modified with short-chain amphiphiles (right picture).

We apply the versatile method of in situ hydrophobization of aluminum oxide particles with short amphiphiles for the stabilization of water-in-toluene emulsions. We found that octyl gallate is an ideal surface modifier to adjust particle wetting properties in toluene. The most important parameters affecting the droplet size were investigated including particle size and wetting properties, particle concentration, emulsion composition, and shear rate. The median droplet size ranged between 2.6 and 80 μm in this study. The results are discussed to shed light on the underlying processes controlling the droplet size. It was found that emulsion formation is either rupture or coalescence controlled, which is in close analogy to the mechanisms reported for surfactant-stabilized emulsions.

Since the mid-1970s, microencapsulation has become increasingly popular in food, detergent, cosmetic and pharmaceutical industries to protect active agents from degradation or facilitate their controlled release or targeted delivery. Here we report on a synthesis route of a novel class of hollow inorganic microcapsules with unique microstructural and mechanical properties. The method is based on the adsorption of calcium aluminate particles at the interface of water droplets of an oil-continuous emulsion. Upon contact with water, these particles hydrate and form a mechanically stable, porous capsule shell. After solvent evaporation, hollow microcapsules can be harvested with diameters between 30 and 200 μm and yields of up to 75%. The mechanical characterization of entire capsules is accomplished using a uniaxial, micromechanical compression setup installed in a scanning electron microscope. We show that these inorganic calcium aluminate microcapsules are highly crack tolerant owing to their porous shell microstructure. Such a behavior is in strong contrast to the one of hollow aluminosilicate cenospheres, which feature dense shells and show therefore brittle failure in our compression tests.

Particle-stabilized liquid foams provide a general route for producing low-density macroporous materials from melt-processable and intractable thermoplastic polymers. In this paper, we demonstrate how these liquid foams can be used to design macroporous polymers with tailored microstructures and properties by adjusting the various processing parameters. By varying the size, concentration, and wettability of the particles in the colloidal suspensions and controlling the frothing, drying, and sintering conditions, macroporous materials with porosities between 33 and 95% and median pore sizes (D50) between 13 and 634 μm were obtained. This foaming process is applicable to a wide range of hydrophobic materials and is demonstrated here on commercially available polymeric powders of poly(tetrafluoroethylene) (PTFE), poly(vinylidene fluoride) (PVDF), poly(ether imide) (PEI), and poly(ether ether ketone) (PEEK).

In this communication, we describe a general, straightforward route to produce highly porous bulk materials from melt-processable and intractable polymers using particle-stabilized liquid foams.

Foams and emulsions stabilized by colloidal particles can lead to new materials with unique structures and properties. In this Highlight article, we describe the underlying mechanisms of this new enabling technology, highlighting some of the processing routes to obtain capsules and porous structures for a variety of applications.

To control the selective adhesion of human endothelial cells and human serum proteins to bioceramics of different compositions, a multifunctional ligand containing a cyclic arginine-glycine-aspartate (RGD) peptide, a tetraethylene glycol spacer, and a gallate moiety was designed, synthesized, and characterized. The binding of this ligand to alumina-based, hydroxyapatite-based, and calcium phosphate-based bioceramics was demonstrated. The conjugation of this ligand to the bioceramics induced a decrease in the nonselective and integrin-selective binding of human serum proteins, whereas the binding and adhesion of human endothelial cells was enhanced, dependent on the particular bioceramics.

The chemical functionalization of cell-surface proteins of human primary fetal bone cells with hydrophilic bioorthogonal intermediates was investigated. Toward this goal, chemical pathways were developed for click reaction-mediated coupling of alkyne derivatives with cellular azido-expressing proteins. The incorporation via a tetraethylene glycol linker of a dipeptide and a reporter biotin allowed the proof of concept for the introduction of cell-specific peptide ligands and allowed us to follow the reaction in living cells. Tuning the conditions of the click reaction resulted in chemical functionalization of living human fetal osteoblasts with excellent cell survival.