distribution is the displacement velocity of the interface between the frozen body
and the liquid [152–156]. When this velocity is lower than a critical value, ice
crystallization causes the expulsion of solid particles in front of ice crystallites. On
the other hand, too high a displacement velocity causes the fragmentation of the
crystallization interface, loss of the orientation of ice crystals, and disordered
distribution of solid particles. The main factor determining this velocity is the
temperature gradient. The thickness of the ceramic walls, along with the concentration of slurry, also correlate with the displacement velocity. In order to retain the
order and the spatial orientation in this system, the size of ceramic particles should
be considerably smaller than the diameter of ice crystals. The lower concentration
limit of solid particles in suspension is determined by the demand for mechanical
strength of the sample after freeze-drying and is close to 5 mass%. A concentration
of solid particles above 55 % usually results in significant interactions between the
particles in suspension, causing the formation of lateral bridges between nearest
walls and closing the columnar pores.
Taking into account the fact that these features of crystallization process are very
similar for different ceramic materials, a number of ceramics with oriented porosity
have been obtained recently [146, 157–167]. The bimodal porosity with large
longitudinal pores and numerous mesopores in the pore walls is very suitable for
catalyst supports for gas phase reactions. Large oriented pores ensure rapid transport of gaseous reagents, while the mesopores in the pore walls are an ideal location
for catalyst nanoparticles in order to ensure their maximum exposure to reagents
[168–171]. Similarly to the cryogel synthesis of individual particles, the directed
crystallization method also produces metal catalysts on oxide supports in the same
synthesis run [172, 173].
The porosity features of ceramic materials prepared by directed ice crystallization are also useful for the development of biomaterials. Since the size of the pores
can be varied by changing the crystallization conditions, the materials are suitable
for the synthesis of ceramic scaffolds. The oriented character of the porosity leads
to an anisotropy in the mechanical properties of such ceramics that is similar to the
anisotropy of natural bones [132, 174–178]. The most significant progress in this
direction was achieved by the development of hydroxyapatite (HAP) ceramics with
a compression strength close to that of natural bone [179], and also by the development of tough Al 2 O 3 -PMMA layered nacre-like composites [180].
3 Carbon Cryogels and Related Materials
3.1 Polymer-Derived Cryogels
The most common synthesis technique for carbon cryogels from polymer precursors is based on their thermal processing in a non-oxidizing atmosphere, usually
argon or nitrogen, at 800–1,000
C. The polymer precursor is mainly synthesized by
234
O.A. Shlyakhtin
and the liquid [152–156]. When this velocity is lower than a critical value, ice
crystallization causes the expulsion of solid particles in front of ice crystallites. On
the other hand, too high a displacement velocity causes the fragmentation of the
crystallization interface, loss of the orientation of ice crystals, and disordered
distribution of solid particles. The main factor determining this velocity is the
temperature gradient. The thickness of the ceramic walls, along with the concentration of slurry, also correlate with the displacement velocity. In order to retain the
order and the spatial orientation in this system, the size of ceramic particles should
be considerably smaller than the diameter of ice crystals. The lower concentration
limit of solid particles in suspension is determined by the demand for mechanical
strength of the sample after freeze-drying and is close to 5 mass%. A concentration
of solid particles above 55 % usually results in significant interactions between the
particles in suspension, causing the formation of lateral bridges between nearest
walls and closing the columnar pores.
Taking into account the fact that these features of crystallization process are very
similar for different ceramic materials, a number of ceramics with oriented porosity
have been obtained recently [146, 157–167]. The bimodal porosity with large
longitudinal pores and numerous mesopores in the pore walls is very suitable for
catalyst supports for gas phase reactions. Large oriented pores ensure rapid transport of gaseous reagents, while the mesopores in the pore walls are an ideal location
for catalyst nanoparticles in order to ensure their maximum exposure to reagents
[168–171]. Similarly to the cryogel synthesis of individual particles, the directed
crystallization method also produces metal catalysts on oxide supports in the same
synthesis run [172, 173].
The porosity features of ceramic materials prepared by directed ice crystallization are also useful for the development of biomaterials. Since the size of the pores
can be varied by changing the crystallization conditions, the materials are suitable
for the synthesis of ceramic scaffolds. The oriented character of the porosity leads
to an anisotropy in the mechanical properties of such ceramics that is similar to the
anisotropy of natural bones [132, 174–178]. The most significant progress in this
direction was achieved by the development of hydroxyapatite (HAP) ceramics with
a compression strength close to that of natural bone [179], and also by the development of tough Al 2 O 3 -PMMA layered nacre-like composites [180].
3 Carbon Cryogels and Related Materials
3.1 Polymer-Derived Cryogels
The most common synthesis technique for carbon cryogels from polymer precursors is based on their thermal processing in a non-oxidizing atmosphere, usually
argon or nitrogen, at 800–1,000
C. The polymer precursor is mainly synthesized by
234
O.A. Shlyakhtin