**2.1. Replica techniques**

Replica techniques involve the impregnation of a cellular structure with a ceramic suspension or precursor solution to produce a macroporous ceramic exhibiting a similar morphology to the original porous material (Fig. 2(a)). This is followed by the removal of excess slurry, pyrolysis of the polymeric substrate, and sintering to solidify the foam [6, 13]. Therefore, the ceramic foam replicates the original organic polymer structure. Difficulties of slurry impreg‐ nation limit the realization of small cells. The struts contain central holes, which result from the burning out of the polyurethane template. Microcracks and pores also result. Replication generates large amounts of CO2 during firing due to the decomposition of the organic compounds [10, 12]. Suitable biogenic porous structures have been used as templates to form cellular ceramics with particular microstructures that could also be produced by other methods. Those processes for the fabrication of bulk ceramics structures are discussed here.

This technique, reported in the 1960s, is the first method deliberately used for the production of macroporous ceramics [32]. First, polymeric sponges were used as templates to prepare ceramic cellular structures with various pore sizes, porosities and chemical compositions. In the polymer replica approach, a highly porous polymeric sponge is initially soaked in a ceramic suspension until its internal pores are filled. Binders and plasticizers are also added to the initial suspension to provide ceramic coatings sufficiently strong to prevent the struts from cracking during pyrolysis. This process is explored fully elsewhere [11, 13].

The resulting ceramic is formed after removal of the polymeric template. The ceramic coating is finally densified by sintering at 1000-1700°C depending on the material. Porous ceramics obtained via sponge replication can reach total open porosity levels of 40%-95% and are characterized by a reticulated structure of highly interconnected pores of between 200 μm and 3 mm. The disadvantages of this technique lie in the formation of the struts of the reticulated structure during pyrolysis of the polymeric template, which significantly weakens the mechanical strength of the resulting porous ceramic [21]. The technique also requires several steps, which lengthen its duration and increases its cost.
