This article reviews different methodologies for the fabrication of monolithic ceramic components possessing multiscale porosity, i.e., with pores ranging from a few nanometers to several hundred microns. Two main strategies have been discussed: (a) the assembling of micro/ mesoporous materials into components possessing also macropores; (b) the addition of micro/mesoporosity to macroporous, cellular monoliths. Both routes include onepot and multi-step processing routes, and yield components with different properties in terms, for instance, of specific surface area values, mechanical strength, and permeability to fluids. The wide range of processing approaches available enable the fabrication of components with very varied morphology, suitable for a variety of industrial applications

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The presence of FeCl2 catalyst enabled the growth of onedimensional nanostructures directly during the pyrolysis of highly porous monoliths, produced from a polysiloxane preceramic polymer with the aid of a gas-generating porogen. Either silicon nitride or silicon carbide nanowires were formed, with a length of several micrometers, depending on the processing atmosphere. Increasing the pyrolysis temperature caused an increase in the length and the amount of nanostructures produced. The remaining matrix consisted of an incompletely crystallized Si–O–C phase, containing SiC crystals and either graphitic (N2 pyrolysis) or amorphous carbon (Ar pyrolysis). X-ray diffraction data and high-resolution transmission electron microscopy investigations combined with electron energy loss and energydispersive X-ray spectroscopy methods enabled to ascertain the growth mechanisms for the nanowires, which depended on the pyrolysis atmosphere (gas phase reaction for N2 pyrolysis; vapor–liquid–solid for Ar pyrolysis)

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Via catalyst-assisted pyrolysis, Si3N4 and SiC nanowires were produced on the cell walls of polymer-derived ceramic foams. The pyrolysis atmosphere and temperature were the main parameters affecting their development: silicon nitride singlecrystal nanowires formed under nitrogen, while silicon carbide ones were produced under argon, and their amount increased with the increasing pyrolysis temperature. Brunauer–Emmett– Teller analysis showed that the presence of the nanowires afforded high specific surface area (SSA) values to the macroporous ceramic foams, ranging from 10 to 110 m2/g. Co-containing samples developed higher SSA values, especially after pyrolysis at 14001C in N2, than samples containing Fe as a catalyst. The differences were explained in terms of morphology (diameter and assemblage), which depended on the processing conditions and the catalyst type (Co or Fe)