drying of these species often occurs at temperatures exceeding their glass transition
temperature, T g , which results in their devitrification and the so-called collapse of
drying products, leading to a sticky viscous mass that is not useful for further
processing.
A combination of these two different approaches, namely the Pechini method
and freeze-drying of aqueous solutions, results in the “cryogenic polymer-gel
synthesis” method [18–26]. This method consists of three independent steps:
(1) gel formation, (2) freezing and freeze-drying, and (3) heat treatment, i.e.,
calcination. The procedure for preparation of the starting solution in this technique
resembles that of the Pechini method. Along with the traditional components such
as nitrate salts, citric acid, and ethylene glycol [19, 20], acetates can also be used
instead of nitrates [21, 22], and EDTA instead of citric acid as a complexing agent
[18]. The most variable component reported in the literature is the gel-forming
agent. In addition to the polycondensation products of ethylene glycol and citric
acid, several other soluble or gel-forming polymers or components can also be used,
such as the triblock copolymer P-123 [23], silane modified propylene glycol [21],
agarose, gelatin, poly(vinyl alcohol) [22], and even soluble starch [24, 25].
According to the cryogenic polymer-gel synthesis method, the gels formed at the
first stage are frozen and freeze-dried, during which the 3D network formed by the
soluble polymers remains intact. As a consequence, glassy mixed solutions of metal
complexes or salts are allocated within the mesh of the gel network so that they
cannot join to each other, even during drying at T > T g . The freeze-drying product
remains solid, irrespective of salt compositions, thus making it more suitable for
subsequent powder processing. The thermal decomposition of freeze-dried cryogels
containing significant amounts of organics is accompanied by internal redox reactions and significant reduction of the formal phase formation temperature. This
flash-like thermal decomposition is useful for the synthesis of kinetically complicated compounds like higher members of homological series. In this case, the easily
formed binary intermediates and the first members of homological series do not lose
their reactivity, as often occurs during the slow heating of precursors. In contrast to
Pechini-like precursors, thermal decomposition of freeze-dried cryogels occurs
more uniformly, thus promoting a better chemical and morphological homogeneity
of thermolysis products. The large difference in the specific molar volumes of the
precursor and decomposition product leads to a significant decrease in grain size
during thermal decomposition, as grain growth is limited by a short time and
reduced temperature of the heat treatment during thermolysis. Both these factors
make this method rather attractive for the synthesis of ultrafine and nanocrystalline
powders of individual oxides, their solid solutions, and complex oxide compounds.
As an example of this type of material synthesis, Fig. 1a shows a field emission
SEM (FESEM) image of a Co 3 O 4 cryogel, synthesized using citric acid and P-123
as complexing and gel-forming agents, respectively [23]. The macroporous structure of Co 3 O 4 consists of interconnected spheres of around 200 nm in diameter, as
also seen from the TEM image of the same sample in Fig. 1b. Each sphere forming
the internal structure is an assembly of many nanocrystals with a diameter of around
10 nm, introducing a mesoporous structure within the macroporous Co 3 O 4 [23].
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O.A. Shlyakhtin
temperature, T g , which results in their devitrification and the so-called collapse of
drying products, leading to a sticky viscous mass that is not useful for further
processing.
A combination of these two different approaches, namely the Pechini method
and freeze-drying of aqueous solutions, results in the “cryogenic polymer-gel
synthesis” method [18–26]. This method consists of three independent steps:
(1) gel formation, (2) freezing and freeze-drying, and (3) heat treatment, i.e.,
calcination. The procedure for preparation of the starting solution in this technique
resembles that of the Pechini method. Along with the traditional components such
as nitrate salts, citric acid, and ethylene glycol [19, 20], acetates can also be used
instead of nitrates [21, 22], and EDTA instead of citric acid as a complexing agent
[18]. The most variable component reported in the literature is the gel-forming
agent. In addition to the polycondensation products of ethylene glycol and citric
acid, several other soluble or gel-forming polymers or components can also be used,
such as the triblock copolymer P-123 [23], silane modified propylene glycol [21],
agarose, gelatin, poly(vinyl alcohol) [22], and even soluble starch [24, 25].
According to the cryogenic polymer-gel synthesis method, the gels formed at the
first stage are frozen and freeze-dried, during which the 3D network formed by the
soluble polymers remains intact. As a consequence, glassy mixed solutions of metal
complexes or salts are allocated within the mesh of the gel network so that they
cannot join to each other, even during drying at T > T g . The freeze-drying product
remains solid, irrespective of salt compositions, thus making it more suitable for
subsequent powder processing. The thermal decomposition of freeze-dried cryogels
containing significant amounts of organics is accompanied by internal redox reactions and significant reduction of the formal phase formation temperature. This
flash-like thermal decomposition is useful for the synthesis of kinetically complicated compounds like higher members of homological series. In this case, the easily
formed binary intermediates and the first members of homological series do not lose
their reactivity, as often occurs during the slow heating of precursors. In contrast to
Pechini-like precursors, thermal decomposition of freeze-dried cryogels occurs
more uniformly, thus promoting a better chemical and morphological homogeneity
of thermolysis products. The large difference in the specific molar volumes of the
precursor and decomposition product leads to a significant decrease in grain size
during thermal decomposition, as grain growth is limited by a short time and
reduced temperature of the heat treatment during thermolysis. Both these factors
make this method rather attractive for the synthesis of ultrafine and nanocrystalline
powders of individual oxides, their solid solutions, and complex oxide compounds.
As an example of this type of material synthesis, Fig. 1a shows a field emission
SEM (FESEM) image of a Co 3 O 4 cryogel, synthesized using citric acid and P-123
as complexing and gel-forming agents, respectively [23]. The macroporous structure of Co 3 O 4 consists of interconnected spheres of around 200 nm in diameter, as
also seen from the TEM image of the same sample in Fig. 1b. Each sphere forming
the internal structure is an assembly of many nanocrystals with a diameter of around
10 nm, introducing a mesoporous structure within the macroporous Co 3 O 4 [23].
226
O.A. Shlyakhtin