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.

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).

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.

Aufgeblasen: Nasse Schäume, die mithilfe von Detergentien präpariert werden, sind thermodynamisch instabile Systeme, die schneller Disproportionierung, Entwässerung und Koaleszenz unterliegen. Nun gelang die Bereitung ultrastabiler Schäume mithilfe kolloidaler Partikel als Stabilisatoren (linkes Bild). Die Stabilisierung ist das Ergebnis der irreversiblen Adsorption von Partikeln, die mit kurzkettigen Amphiphilen modifiziert wurden, an der Luft‐Wasser‐Grenzfläche (rechtes Bild).

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 lm 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%.