Elaboration of porous bulk ceramics and ceramic coatings from stable foams and emulsions

The aim of this work is to produce macroporous ceramic materials with tailored porosity, using direct foaming method. The stabilization of the foams is ensured by adsorption of particles at the gas-liquid interface, following the methodology described by Gonzenbach et al. [ref. 1 to 4]. Foams are obtained from alumina/water suspensions (20 vol.%, 35 vol.% and 50 vol.% of particles), containing from 0 to 127 mmol/l of butyric acid. Butyric acid was chosen in order to hydrophobize alumina particles. After setting and drying, foams were sintered. This study shows how the composition of the initial suspension (alumina and butyric acid concentrations) can affect the microstructure of sintered foams.
For a 35 vol.% alumina suspension, the critical butyric acid concentration at which the particles are hydrophobic enough to adsorb at the air-water interface is about 50 mmol/l. Wet foams were generated through energetic stirring from 35 vol.% alumina suspensions containing 66 mmol/l, 105 mmol/l and 127 mmol/l of butyric acid.
For the concentration of 66 mmol/l, sintered foams show closed pores with sizes from 15 µm to 90 µm. The number of small pores increases with the butyric acid concentration because higher viscosity values hinder bubbles growth. At 127 mmol/l of butyric acid, a sharp increase in viscosity occurs and means coagulation of particles. Bubbles formed during the vigorous stirring are thus submitted to higher shear stress and can break.
When the solid content of the alumina suspensions is increased from 20 vol.% to 50 vol.%, pore size decreases. Bubbles are also stabilized thanks to butyric acid. A lower solid content leads to low viscosity values and bubbles expansion during the stirring is facilitated. That means larger pores surrounded by a thin alumina layer which prevents bubbles coalescence or drainage. As the solid content increases, higher viscosity values hinder bubbles growth during the stirring and the microstructure shows smaller pores with a narrower size distribution.
The next step of this work will be to consider the influence of the stirring parameters (speed, time) on the microstructure of sintered foams. We will also examine different possibilities for incorporation of bubbles.

REFERENCES
[1] Gonzenbach U.T., Studart A.R., Tervoort E., Gauckler L.J., 'Stabilization of Foams with Inorganic Colloidal Particles', Langmuir, 22, 2006, pp. 10983-10988
[2] Gonzenbach U.T., Studart A.R., Tervoort E., Gauckler L.J., 'Macroporous Ceramics from Particle-Stabilized Wet Foams', J. Am. Ceram. Soc., 90 [1], 2007, pp. 16-22
[3] Studart A.R., Gonzenbach U.T., Tervoort E., Gauckler L.J., 'Processing routes to Macroporous Ceramics : A review', J. Am. Ceram. Soc., 89 [6], 2006, pp. 1771-1789
[4] Gonzenbach U.T., Studart A.R., Tervoort E., Gauckler L.J., 'Ultrastable Particle-Stabilized foams', Angew. Chem. Int. Ed., 45, 2006, pp. 3526-3530