freezing point and a reasonable vapor pressure at this temperature [111], and then
frozen and freeze-dried [112].
Along with research-oriented isolation of nanoparticles, cryogel processing can
be an efficient powder-processing method for industrial and other large-scale
applications. The increasing use of nanopowders in industry creates growing
problems, first of all due to the contamination of production facilities and the
environment by the tiny, often harmful particles. This problem is especially important for the nuclear industry, with its traditionally severe demands for industrial
safety. The processing risks of fine powders can be substantially reduced by their
granulation. Formation of micro- or millimeter-sized granules by spraying bindercontaining suspension into liquid nitrogen, freeze-drying, and thermal processing of
the product allows substantial reduction of dust formation [113]. Another positive
effect of granulation is the better flowability of granulated powders [114,
115]. Nanopowders usually exhibit enhanced adherence to the processing surfaces
and poor flowability due to the large internal friction caused by the enhanced
contact surface of nanopowders. The latter effect is especially important during
the production of dense ceramics from nanopowders because the internal friction
effects often cause significant internal stresses during the compaction of these
powders before sintering. Application of freeze-granulation is an efficient method
of solving this problem too [116, 117].
We have to note that the freeze-drying method may, in some cases, negatively
affect the product properties. For instance, it was shown that nanocrystalline zeolite
powder located in a mesoporous zeolite support by means of wet impregnation
followed by atmospheric drying exhibits significant catalytic activity in the reaction
of toluene dispropoportionation, whereas the activity of a similar composite
obtained by freeze-drying of the same precursors was much lower [118]. This
difference can be attributed to the additional mesoporosity created during atmospheric drying due to the aggregation and agglomeration of zeolite nanoparticles.
As mentioned before, the cryogel method ensures the fixation and isolation of
individual particles during the drying stage, thus excluding, in the present case,
the positive effect of particle aggregation.
2.4 Cryogel-Derived Bulk Materials
One of the most efficient applications of cryogenic processing in the production of
ceramic materials is the freeze casting process. Common industrial methods to
ensure the uniform filling of the mold by ceramic powder during the production of
complex-shaped ceramics are based on the preparation of aqueous slurry containing
a large amount of polymeric additives. Elimination of these additives during the
subsequent thermal processing causes significant internal stresses in the ceramic
and promotes cracking. Application of polymeric cryogels provides a feasible
alternative to this traditional method. Thus, low temperature sol–gel transition in
the polymer solution during freezing of the mold, followed by thawing or freeze232
O.A. Shlyakhtin
frozen and freeze-dried [112].
Along with research-oriented isolation of nanoparticles, cryogel processing can
be an efficient powder-processing method for industrial and other large-scale
applications. The increasing use of nanopowders in industry creates growing
problems, first of all due to the contamination of production facilities and the
environment by the tiny, often harmful particles. This problem is especially important for the nuclear industry, with its traditionally severe demands for industrial
safety. The processing risks of fine powders can be substantially reduced by their
granulation. Formation of micro- or millimeter-sized granules by spraying bindercontaining suspension into liquid nitrogen, freeze-drying, and thermal processing of
the product allows substantial reduction of dust formation [113]. Another positive
effect of granulation is the better flowability of granulated powders [114,
115]. Nanopowders usually exhibit enhanced adherence to the processing surfaces
and poor flowability due to the large internal friction caused by the enhanced
contact surface of nanopowders. The latter effect is especially important during
the production of dense ceramics from nanopowders because the internal friction
effects often cause significant internal stresses during the compaction of these
powders before sintering. Application of freeze-granulation is an efficient method
of solving this problem too [116, 117].
We have to note that the freeze-drying method may, in some cases, negatively
affect the product properties. For instance, it was shown that nanocrystalline zeolite
powder located in a mesoporous zeolite support by means of wet impregnation
followed by atmospheric drying exhibits significant catalytic activity in the reaction
of toluene dispropoportionation, whereas the activity of a similar composite
obtained by freeze-drying of the same precursors was much lower [118]. This
difference can be attributed to the additional mesoporosity created during atmospheric drying due to the aggregation and agglomeration of zeolite nanoparticles.
As mentioned before, the cryogel method ensures the fixation and isolation of
individual particles during the drying stage, thus excluding, in the present case,
the positive effect of particle aggregation.
2.4 Cryogel-Derived Bulk Materials
One of the most efficient applications of cryogenic processing in the production of
ceramic materials is the freeze casting process. Common industrial methods to
ensure the uniform filling of the mold by ceramic powder during the production of
complex-shaped ceramics are based on the preparation of aqueous slurry containing
a large amount of polymeric additives. Elimination of these additives during the
subsequent thermal processing causes significant internal stresses in the ceramic
and promotes cracking. Application of polymeric cryogels provides a feasible
alternative to this traditional method. Thus, low temperature sol–gel transition in
the polymer solution during freezing of the mold, followed by thawing or freeze232
O.A. Shlyakhtin