We can take advantage of the growth of ice crystals to template colloidal suspensions. Using the freezing of colloids to template the porosity in materials is known as ice-templating or freeze-casting. The basic idea is to obtain a porosity that is a replica of the ice crystals, by freezing suspension and subsequently removing the ice crystals by sublimation. Regular patterns can be obtained in porous ceramics by controlling the freezing of ceramic slurries followed by subsequent ice sublimation and sintering, leading to multilayered porous alumina structures with homogeneous and well-defined architecture.
These ceramic scaffolds can then be used as a basis for dense composite, if infiltrated with a suitable second phase. The dense complex composites obtained by this process exhibit striking similarities to the macro- and micro-structure of the inorganic component of nacre, replicating its multilayer structure and other structural features such as roughness or inorganic bridges, and with properties which far exceed what could be expected from a simple mixture of their components.
In ice templating, the particles in suspension in the slurry are ejected from the moving solidification front and pile up between the growing columnar or lamellar ice, in a similar way to salt and biological organisms entrapped in brine channels in sea ice. The variety of materials processed by ice templating suggests that the underlying principles of the technique are not strongly dependent on the materials but rely more on physical rather than chemical interactions. The phenomenon is very similar to that of unidirectional solidification of cast materials and binary alloys, with ice playing the role of a fugitive second phase. The porosity of the sintered materials is a replica of the original ice structure. Since the solidification is often directional, the porous channels run from the bottom to the top of the samples. In addition, the pores exhibit a very anisotropic morphology in the solidification plane. The final porosity content can be tuned by varying the particle content within the slurry, and the size of porosity is affected by the freezing kinetics.
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