drying, produces raw ceramics of high mechanical strength using much lower
amounts of polymeric additives [119–124].
Freeze casting can be performed with the application of natural polymeric
components like starch or gelatin [119], whereas modern industrial production
techniques mostly involve the use of aqueous solutions of polyvinyl alcohol or
polyvinyl acetate [125]. Preservation of complex-shaped porous preforms by
freeze-drying is used in advanced modern methods for forming complex 3D
structures such as nanoimprint lithography [126] and computer-driven layer-bylayer casting (robocasting) [127]. In order to enhance the mechanical strength of the
raw cast after thawing, a freeze-gelation can be combined with the photopolymerization of soluble monomers or oligomers introduced into the slurry instead of
polymers [128]. Taking into account the large number of pores left after the
removal of frozen ice and the significant mechanical strength of raw casts, the
freeze casting method can be especially useful in the production of porous ceramics
[129]. The micron-sized pores formed due to the ice template are useful for
biomedical applications and for the production of biocompatible ceramic materials
[130–135].
The number of inorganic systems where low temperature sol–gel transitions are
also observed is rather limited. One such system is the aqueous silica sols forming
dense cryogels. When SiO 2 .nH 2 O is introduced into the composition of raw
ceramic slurry, the amount of polymeric additives can be reduced significantly or
even neglected [50, 89, 136–139]. Modification of the porous structure of SiO 2
cryogels allows the formation of porous materials with a density as low as
0.05 g cm
À3 and with excellent thermal insulation properties at cryogenic temperatures [15]. The strengthening effect of hydrous alumina and zirconia cryogels is
less pronounced. However, if the wall thickness of the ceramic detail is large
enough and its shape is not too complex, it is possible to apply freeze casting
even in the absence of polymer or hydrous silica. In this case, removal of ice from
the frozen mold by sublimation becomes a necessary stage of the production
technique [123, 129, 140–147].
Another method of cryogel application in the production of ceramics is generation of tubular aligned pores in raw casts by means of directed crystallization of ice
followed by freeze-drying. It was found that freezing of ceramic slurry in a large
temperature gradient causes the formation of columnar ice crystals with diameters
of 2–3 to 70–80 μm and lengths up to several millimeters. Formation of these
crystals causes expulsion of the solid particles of suspension or slurry and their
accumulation in the interconnected space between the ice columns. Because the
size of solid particles is usually smaller than the diameter of ice crystals, freezedrying of such samples followed by soft sintering in order to ensure their mechanical strength results in the formation of ceramics with bimodal porosity. Along with
large columnar pores in place of oriented ice crystallites, these materials also
contain randomly distributed submicron-sized pores or mesopores left by the tiny
ice crystals formed between ceramic particles in the column walls [148–151].
Studies on directed crystallization also show that the most important external
factor determining the character of ice crystallization and ceramic particle
Inorganic Cryogels
233
amounts of polymeric additives [119–124].
Freeze casting can be performed with the application of natural polymeric
components like starch or gelatin [119], whereas modern industrial production
techniques mostly involve the use of aqueous solutions of polyvinyl alcohol or
polyvinyl acetate [125]. Preservation of complex-shaped porous preforms by
freeze-drying is used in advanced modern methods for forming complex 3D
structures such as nanoimprint lithography [126] and computer-driven layer-bylayer casting (robocasting) [127]. In order to enhance the mechanical strength of the
raw cast after thawing, a freeze-gelation can be combined with the photopolymerization of soluble monomers or oligomers introduced into the slurry instead of
polymers [128]. Taking into account the large number of pores left after the
removal of frozen ice and the significant mechanical strength of raw casts, the
freeze casting method can be especially useful in the production of porous ceramics
[129]. The micron-sized pores formed due to the ice template are useful for
biomedical applications and for the production of biocompatible ceramic materials
[130–135].
The number of inorganic systems where low temperature sol–gel transitions are
also observed is rather limited. One such system is the aqueous silica sols forming
dense cryogels. When SiO 2 .nH 2 O is introduced into the composition of raw
ceramic slurry, the amount of polymeric additives can be reduced significantly or
even neglected [50, 89, 136–139]. Modification of the porous structure of SiO 2
cryogels allows the formation of porous materials with a density as low as
0.05 g cm
À3 and with excellent thermal insulation properties at cryogenic temperatures [15]. The strengthening effect of hydrous alumina and zirconia cryogels is
less pronounced. However, if the wall thickness of the ceramic detail is large
enough and its shape is not too complex, it is possible to apply freeze casting
even in the absence of polymer or hydrous silica. In this case, removal of ice from
the frozen mold by sublimation becomes a necessary stage of the production
technique [123, 129, 140–147].
Another method of cryogel application in the production of ceramics is generation of tubular aligned pores in raw casts by means of directed crystallization of ice
followed by freeze-drying. It was found that freezing of ceramic slurry in a large
temperature gradient causes the formation of columnar ice crystals with diameters
of 2–3 to 70–80 μm and lengths up to several millimeters. Formation of these
crystals causes expulsion of the solid particles of suspension or slurry and their
accumulation in the interconnected space between the ice columns. Because the
size of solid particles is usually smaller than the diameter of ice crystals, freezedrying of such samples followed by soft sintering in order to ensure their mechanical strength results in the formation of ceramics with bimodal porosity. Along with
large columnar pores in place of oriented ice crystallites, these materials also
contain randomly distributed submicron-sized pores or mesopores left by the tiny
ice crystals formed between ceramic particles in the column walls [148–151].
Studies on directed crystallization also show that the most important external
factor determining the character of ice crystallization and ceramic particle
Inorganic Cryogels
233