Many earlier studies on inorganic cryogels and the freeze-drying products of
frozen solutions were summarized in our previous monograph and review papers
[12–15]. A large number of papers on inorganic–polymer hybrids, composites, and
their biomedical applications are reviewed in other chapters of this volume. The
scope of this review is limited to cryogel-derived inorganic materials containing a
minor amount of polymer, and to the contribution of polymer properties to the
performance of the final composite or single phase material. Taking into account
the growing number of applications of carbon-based materials and the similarity
between the approaches to the synthesis of inorganic and carbon-based cryogels,
this new and promising kind of material and its structure will also be briefly
summarized.
2 Metal Oxide Cryogels
2.1 Cryogenic Polymer-Gel Synthesis
The only cryogel application in materials synthesis where true polymeric cryogels
always form is the low temperature modification of the Pechini method [16]. The
idea of various solution-based methods that aim to synthesize multicomponent
materials is to retain the chemical homogeneity of the starting multicomponent
aqueous solution during the process from solution to the final solid oxide material.
Usually, these methods are applied in order to reduce the synthesis temperature and,
hence, to reduce the grain size of as-obtained multicomponent oxide powders. In
the case of the Pechini method, the formation of a gel due to the polycondensation
of ethylene glycol and citric acid reduces the mobility of salt components and,
hence, prevents their separate crystallization during solvent evaporation. In spite of
the rather complex chemical processes occurring during the synthesis [17], the
Pechini method is simple and useful for laboratory applications. Usually, citric acid
and ethylene glycol are added to the aqueous solution of nitrates of the cations,
taken in a ratio corresponding to their proportion in the final oxide material.
Evaporation of the solvent by slow heating is accompanied by the formation of a
wet gel. Thermal decomposition of this gel followed by heat treatment at an
elevated temperature results in the formation of the final ultradisperse, often
nanocrystalline, oxide powder. One of the main disadvantages of this method is
the poor reproducibility of the ignition procedure, which often occurs
non-uniformly in various parts of the gel-like precursors [18].
Another solution-based method for material synthesis is based on the freezedrying of frozen aqueous solutions of components, followed by thermal decomposition of freeze-dried precursors [10]. The nitrates of a number of cations that are
readily available and easily soluble in water are also used in this method. However,
freezing of the nitrate solutions of transition metals and rare earth elements or their
soluble complex compounds is accompanied by their vitrification [12]. FreezeInorganic Cryogels
225
frozen solutions were summarized in our previous monograph and review papers
[12–15]. A large number of papers on inorganic–polymer hybrids, composites, and
their biomedical applications are reviewed in other chapters of this volume. The
scope of this review is limited to cryogel-derived inorganic materials containing a
minor amount of polymer, and to the contribution of polymer properties to the
performance of the final composite or single phase material. Taking into account
the growing number of applications of carbon-based materials and the similarity
between the approaches to the synthesis of inorganic and carbon-based cryogels,
this new and promising kind of material and its structure will also be briefly
summarized.
2 Metal Oxide Cryogels
2.1 Cryogenic Polymer-Gel Synthesis
The only cryogel application in materials synthesis where true polymeric cryogels
always form is the low temperature modification of the Pechini method [16]. The
idea of various solution-based methods that aim to synthesize multicomponent
materials is to retain the chemical homogeneity of the starting multicomponent
aqueous solution during the process from solution to the final solid oxide material.
Usually, these methods are applied in order to reduce the synthesis temperature and,
hence, to reduce the grain size of as-obtained multicomponent oxide powders. In
the case of the Pechini method, the formation of a gel due to the polycondensation
of ethylene glycol and citric acid reduces the mobility of salt components and,
hence, prevents their separate crystallization during solvent evaporation. In spite of
the rather complex chemical processes occurring during the synthesis [17], the
Pechini method is simple and useful for laboratory applications. Usually, citric acid
and ethylene glycol are added to the aqueous solution of nitrates of the cations,
taken in a ratio corresponding to their proportion in the final oxide material.
Evaporation of the solvent by slow heating is accompanied by the formation of a
wet gel. Thermal decomposition of this gel followed by heat treatment at an
elevated temperature results in the formation of the final ultradisperse, often
nanocrystalline, oxide powder. One of the main disadvantages of this method is
the poor reproducibility of the ignition procedure, which often occurs
non-uniformly in various parts of the gel-like precursors [18].
Another solution-based method for material synthesis is based on the freezedrying of frozen aqueous solutions of components, followed by thermal decomposition of freeze-dried precursors [10]. The nitrates of a number of cations that are
readily available and easily soluble in water are also used in this method. However,
freezing of the nitrate solutions of transition metals and rare earth elements or their
soluble complex compounds is accompanied by their vitrification [12]. FreezeInorganic Cryogels
225