Topics in Current Chemistry (2020) 378:6
1 3
formed under high supersaturation conditions, from aqueous solutions of metal precursors, through pH adjustment and use of precipitating agents. The overall process
in solution consists in nucleation, growth, coarsening and/or agglomeration processes that are highly difficult to uncouple, while a final thermal treatment is needed
for decomposing the intermediate precipitate into the usable ferrite as well as for
crystallizing the oxide. The co-precipitation method has been successfully used for
synthesizing a large variety of spinel ferrites [18–23] or orthoferrites, mono-, bi- or
non-substituted.
1.2.2 Sol–Gel Methods
The sol–gel method is a versatile approach in which a metal alkoxide solution undergoes hydrolysis and condensation polymerization reactions, with the formation of a
sol and subsequent cross-linking to form a gel. It requires usually a final heat treatment for removing volatile byproducts and crystallizing the ferrite material. It takes
advantage of a relatively low annealing temperature compared to solid-state reactions for maintaining a relatively high surface area. It generally benefits from its
simplicity, low implementation costs and tunability for preparing a large range of
spinel ferrites [24–28] and orthoferrite structures [29–33] with controlled composition, structure and morphology. One can note that the sol–gel method combined
with combustion was used for preparing BaFe 12 O 19 and CoFe 12 O 19 hexaferrites
(magnetoplumbite structure) [26, 34].
Pechini developed a modified sol–gel method for materials which do not have
favourable hydrolysis equilibria, and in which the metal ions undergo complexation through the addition of bi- or tridentate organic chelating agents [35, 36]. The
method builds on the principles of sol–gel chemistry involving small-molecule
chelating ligands that in the initial step form a solution of metal–chelate complexes.
It takes this further to convert the mixture into a covalent polymer network to entrap
the metal ions. The use of chelating agents allows stable complexes to be formed
with a large variety of metals and gets rid of the requirement that the metal has to
form stable hydroxo species. The underlying strategy of the method lies in the delay
of the thermal decomposition of the organic matrix in order to afford more control over the growing ceramic phase. While citric acid is the most popular chelating
agent (i.e. citrate method), it can be also replaced by ethylenediaminetetraacetic acid
(EDTA), which has the advantage of chelating most metals and, with four carboxylate groups, it is easily cross-linked to form a gel. When ethylene glycol is used in
association with citric acid, the key reaction is the esterification between citrate and
ethylene glycol (Fig. 6). The metal salt dissolved in water with citric acid and ethylene glycol forms a homogeneous precursor solution containing the metal–citrate
chelate complexes, while the heat-induced polyesterification between the citrate and
ethylene glycol creates an extended covalent network. The method requires a final
calcination step for combusting the organic matrix and forming the oxide. It became
very popular for synthesizing ferrite materials, notably owing to its ability to form
a polymeric precursor where two or more metals may be dispersed homogeneously
throughout the network [37, 38].
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Reprinted from the journal
1 3
formed under high supersaturation conditions, from aqueous solutions of metal precursors, through pH adjustment and use of precipitating agents. The overall process
in solution consists in nucleation, growth, coarsening and/or agglomeration processes that are highly difficult to uncouple, while a final thermal treatment is needed
for decomposing the intermediate precipitate into the usable ferrite as well as for
crystallizing the oxide. The co-precipitation method has been successfully used for
synthesizing a large variety of spinel ferrites [18–23] or orthoferrites, mono-, bi- or
non-substituted.
1.2.2 Sol–Gel Methods
The sol–gel method is a versatile approach in which a metal alkoxide solution undergoes hydrolysis and condensation polymerization reactions, with the formation of a
sol and subsequent cross-linking to form a gel. It requires usually a final heat treatment for removing volatile byproducts and crystallizing the ferrite material. It takes
advantage of a relatively low annealing temperature compared to solid-state reactions for maintaining a relatively high surface area. It generally benefits from its
simplicity, low implementation costs and tunability for preparing a large range of
spinel ferrites [24–28] and orthoferrite structures [29–33] with controlled composition, structure and morphology. One can note that the sol–gel method combined
with combustion was used for preparing BaFe 12 O 19 and CoFe 12 O 19 hexaferrites
(magnetoplumbite structure) [26, 34].
Pechini developed a modified sol–gel method for materials which do not have
favourable hydrolysis equilibria, and in which the metal ions undergo complexation through the addition of bi- or tridentate organic chelating agents [35, 36]. The
method builds on the principles of sol–gel chemistry involving small-molecule
chelating ligands that in the initial step form a solution of metal–chelate complexes.
It takes this further to convert the mixture into a covalent polymer network to entrap
the metal ions. The use of chelating agents allows stable complexes to be formed
with a large variety of metals and gets rid of the requirement that the metal has to
form stable hydroxo species. The underlying strategy of the method lies in the delay
of the thermal decomposition of the organic matrix in order to afford more control over the growing ceramic phase. While citric acid is the most popular chelating
agent (i.e. citrate method), it can be also replaced by ethylenediaminetetraacetic acid
(EDTA), which has the advantage of chelating most metals and, with four carboxylate groups, it is easily cross-linked to form a gel. When ethylene glycol is used in
association with citric acid, the key reaction is the esterification between citrate and
ethylene glycol (Fig. 6). The metal salt dissolved in water with citric acid and ethylene glycol forms a homogeneous precursor solution containing the metal–citrate
chelate complexes, while the heat-induced polyesterification between the citrate and
ethylene glycol creates an extended covalent network. The method requires a final
calcination step for combusting the organic matrix and forming the oxide. It became
very popular for synthesizing ferrite materials, notably owing to its ability to form
a polymeric precursor where two or more metals may be dispersed homogeneously
throughout the network [37, 38].
114
Reprinted from the journal