Porous cellular alumina ceramic green bodies have been produced by combining the particle stabilized foam method with gelcasting. The suspension foams were stabilized by particles rendered weakly hydrophobic with short chain sulfonate surfactants. Poly vinyl alcohol (PVA) and 2,5-dimethoxy-2,5dihydrofuran (DHF) were used as the gelcasting reagents. The microstructure (amount of porosity, average pore size, and morphology) of alumina (Al 2 O 3 ) ceramic green body foams has been studied as a function of surfactant concentration and chain length. The morphology of gelled ceramic foams changes from closed cell (bubble like morphology) to open cell (granular morphology) as the surfactant concentration is increased beyond a critical level. The density (and porosity) of the gelled alumina green body foams changes as a function of the surfactant concentration in a non-linear manner. Measurement of the suspension viscosity, contact angle and aggregate size are used to explain the changes in density and morphology. The transition from the bubble like structure to the granular structure is due to an increase in the particle aggregate size rather than phase inversion induced by an increase in the contact angle. The same behavior is observed with three different chain length surfactants (ranging from 4 to 10 carbon atoms on the hydrophobic portion of the surfactant) but at a lower surfactant concentration as the chain length increases.

No abstract

Highly porous ceramic foams can be produced by combining particle stabilized foams and gelcasting concepts. Sulfonate‐type surfactants are selected to weakly hydrophobize the alumina surface and stabilize air bubbles in suspensions containing gelcasting additives, polyvinyl alcohol (PVA), and 2,5‐dimethoxy‐2,5‐dihydrofurane (DHF). The aim of this work was to prepare large complex‐shaped ceramic foam objects with homogeneous microstructure and high porosity. A key to avoiding drying cracks is to strengthen the wet green body via gelcasting. The influence of the amount of gelcasting additives on the mechanical strength of the ceramic foam green bodies is investigated as well as the effect of using cross‐linking agent versus the addition of just a binder. The presence of a cross‐linked polymeric network within the green body increases its mechanical strength and minimizes crack formation during drying.

The morphology of alumina gelcast ceramic foams is influenced by surfactant concentration and chain length. The surfactants cause the ceramic particles to become hydrophobic and stabilize air bubbles introduced by beating. The microstructure transforms from closed pore (bubble) morphology to open pore (granular) morphology as surfactant concentration is increased. The influence of the contact angle is less significant than the influence of the particle aggregate size (and suspension viscosity) on determining the transition from bubble-like to granular morphology.The microstructure of cellular ceramic materials is an important factor which influences mechanical strength, permeability, thermal conductivity, and density.1 The fabrication of cellular ceramics by producing aqueous particle stabilized foams, drying and sintering has received recent attention. 28The addition of short chain surfactants that adsorb on to ceramic particles in aqueous solutions, make the particles slightly hydrophobic. The weakly hydrophobic particles stabilize air bubbles introduced into the suspension by beating; the mechanism is similar to that which stabilize Pickering emulsions. The morphology, such as the amount, average size of porosity, and pore connectivity of alumina (Al 2 O 3 ) gelcast ceramic foams is influenced by the concentration and type (chain length) of the surfactant.8 Recently, we produced particle-stabilized foams from alumina suspensions at pH 2.8 Sodium 1-butanesulfonate (C4), 1-heptanesulfonate (C7), and 1-decanesulfonate (C10) were used to make the alumina particles surfaces weakly hydrophobic. Air was introduced into the suspensions with a kitchen beater. The formulations contained poly(vinyl alcohol) and a crosslinking agent 2,5-dimethoxy-2,5-dihydrofurane (DHF). The foamed suspensions were cast into closed cavity molds to produce a range of shapes. Gelation at 80°C was conducted in order to lock in the suspension and foam structure and strengthen the wet body in order that it may maintain its shape and resist drying cracks.At low surfactant concentration, the foam microstructure appeared to be bubble-like with nearly spherical porosity (Figure 1a). At higher surfactant concentration a transition to a more granular appearance resulted (Figure 1b). The influence of the surfactant type and concentration on the foam morphology has been explained based on the contact angle, viscosity of the suspensions and the size of the alumina aggregates. In this work the contact angle of the solution on the alumina surface was measured on dried powder compacts.8 One of the conclusions of the previous work was that the transition from bubble-like pores to granular morphology was caused by increase in aggregate size rather than increase in contact angle. The aim of the current letter is to test the same hypothesis with contact angle measurements conducted on polished alumina polycrystalline ceramic plates. It is believed the smooth nonporous surfaces will produce improved contact angle measurements compared to on powder compacts wh...