Macro porous ceramics possessing controlled microstructures and chemical compositions have increasingly proven useful in the industrial sphere. Their sintered structures have found application in both established and emerging, areas such as thermal insulation in buildings, filtration of liquids and molten materials, refractory insulation, bone scaffolds and tissue engineering. Stable ceramic foams can be formed by wet chemical methods using inorganic particles(e.g., Al 2 O 3 or SiO 2). The wet foams are dried and sintered with improved porosity and mechanical properties. This review examines the different techniques used to prepare porous ceramics from ceramic foams, focusing on the explanation of this versatile method of direct foaming from the past to the present. Comparisons of the processes and the processing parameters are explained with the produced microstructures.

High purity, multi-walled carbon nanotubes (MWNTs) were synthesized by catalytic chemical vapor deposition (CCVD) using acetylene as the carbon source based on a zeolite NaX template loaded with different iron contents. Well shaped zeolite NaX nanocrystals (FAU) of 50 nm were synthesized by a hydrothermal method according to a well designed composition of 3.5Na 2 O:Al 2 O 3 :2.1SiO 2 :1000H 2 O. The ion-exchange method was used for supporting different Fe 2+ contents in the zeolite NaX nanocrystals to provide effective catalysts for carbon nanotube (CNT) formation. Thermal treatment of Fe 2+ -exchanged zeolite NaX nanocrystals resulted in the formation of ¡-Fe 2 O 3 phase at 450°C in air. Transmission electron microscopy (TEM) images showed that the inner and outer tube diameters of the MWNTs were in the range of 3.85.0 nm and 6.38.8 nm, respectively, which were smaller than those of conventional thick MWNTs. The yield of the MWNTs was increased up to 47.1% with increasing iron content in pores of the zeolite crystals, which allowed the pores to be defined as containers for catalysts and as a guide template for MWNT growth.

The thermodynamic instability of bubbles in wet-foam colloidal suspension is due to the substantial area of their gas/liquid interface. Several physical processes lead to gas diffusion from smaller to larger bubbles, resulting in a coarsening and Ostwald ripening of wet foam. This includes a narrowing of the bubble size distribution. The distribution and microstructure of porous ceramics, the adsorption free energy and Laplace pressure of Al 2 O 3 particle-stabilized colloidal suspension, and SiO 2 content were investigated for tailoring the bubble size. Wet-foam stability of more than 80% is related to the degree of hydrophobicity with contact angles of 62-70° achieved from the surfactant. The contact angle replaces part of the highly energetic interface and lowers the free energy of the system. This leads to an apparent increase in the surface tension (26-33 mN/m) of the colloidal suspension.

This study reports on wet-foam stability with respect to porous ceramics from a particle-stabilized colloidal suspension that is achieved through the addition of polymethyl methacrylate (PMMA) using a wet process. To stabilize the wet foam, an initial colloidal suspension of Al 2 O 3 was partially hydrophobized by the surfactant propyl gallate (2 wt.%) and SiO 2 was added as a stabilizer. The influence of the PMMA content on the bubble size, pore size, and pore distribution in terms of the contact angle, surface tension, adsorption free energy, and Laplace pressure are described in this paper. The results show a wet-foam stability of more than 83%, which corresponds to a particle free energy of 2.7 × 10 −12 J and a pressure difference of 61.1 mPa for colloidal particles with 20 wt.% of PMMA beads. It was possible to control the uniform distribution of the open/closed pores by increasing the PMMA content and by adding thick struts, leading to the achievement of a higher-stability wet foam for use in porous ceramics.

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