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.

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Processing Routes to Macroporous Ceramics: A Review

Cellular ceramics are a class of highly porous materials that covers a wide range of structures, such as foams, honeycombs, interconnected rods, interconnected fibres, interconnected hollow spheres. Recently, there has been a surge of activity in this field, because these innovative materials have started to be used as components in special and advanced engineering applications. These include filtering liquids and particles in gas streams, porous burners, biomedical devices, lightweight load-bearing structures, etc. Improvements in conventional processing methods and the development of innovative fabrication approaches are required because of the increasing specific demands on properties and morphology (cell size, size distribution and interconnection) for these materials, which strictly depend on the application considered. This paper will cover the main fabrication methods for cellular ceramics, focusing primarily on foams, offering some insight into novel fabrication processes and recent developments.

Conventional and novel processing methods for cellular ceramics

By employing carefully controlled processing methods, a large amount of porosity (>70 vol%) was introduced in ceramic materials derived from preceramic polymers (silicone resins) after pyrolysis at 1000–1200 °C in inert atmosphere. The resulting components have a bulk density ranging from ∼250 to 950 kg/m3. Three main fabrication methods have here been employed: (1) direct foaming of a solution of a thermosetting silicone resin in a suitable solvent (with or without the addition of polyurethane precursors), acting also as a blowing agent; (2) the use of sacrificial fillers that decompose during pyrolysis, consisting in polymeric microbeads; (3) the mixing of preceramic polymers possessing different characteristics, in particular ceramic yield, depending on their molecular structure. In addition to that, several methods for developing micro- or meso-pores within the resulting SiOC macro-porous ceramics were explored, with the aim of fabricating components with hierarchical porosity. These include a controlled thermal treatment, the addition of fillers with a high specific surface area (SSA), the deposition of zeolites or meso-porous silica coatings, the infiltration with aerogels, selective etching of the SiOC material and the in situ formation of C-based nanostructures. Depending on the fabrication procedure adopted, cells with an average size ranging from the micrometer to the millimeter were obtained. All these processes are simple, economical and versatile, and large bodies with various shapes (tubes, plates, blocks) can be produced, possessing a wide range of morphologies and properties. Compression strength, flexural strength and Young's modulus vary with the morphology and density of the porous components. It is also possible to add to the preceramic polymers some filler powders, for instance possessing electrical conductivity or magnetic properties, leading to the production of functional cellular ceramics.

Engineering porosity in polymer-derived ceramics

A novel processing route for developing open-cell, microcellular SiC ceramics has been developed. The strategy adopted for making microcellular SiC ceramics involved the following: (i) fabricating a formed body from a mixture of polysiloxane, phenol resin (used as a carbon source), polymer microbeads (used as sacrificial templates), and Al2O3–Y2O3 (an optional sintering additive); (ii) cross-linking the polysiloxane in the formed body; (iii) transforming the polysiloxane and phenol resin by pyrolysis into silicon oxycarbide and C, respectively; and (iv) synthesizing SiC by carbothermal reduction. By controlling the microbead and additive contents, it was possible to adjust the porosity so that it ranged from 60% to 95%.

J Zeschky

Papers

Preceramic polymer derived cellular ceramics