glycol-2000, and urea. Amorphous Fe 2 O 3 nanoparticles of about 3–5 nm in size
were obtained.
Komarneni returned to the synthesis of a-Fe 2 O 3 in 2001 [186]. This time he
was interested in this product as a red pigment in porcelains. The methods of
preparation were microwave-hydrothermal and conventional-hydrothermal reactions. The precursors were FeCl 3 Á6H 2 O and HCl solutions. The reaction were carried out at 100–160
C. Acicular and yellow b-FeOOH (akaganite) particles 300 nm
in length and 40 nm in thickness were predominantly formed at 100
C after
2–3 h, while spherical a-Fe 2 O 3 particles 100–180 nm in diameter were preferentially formed after 13 h using a conventional-hydrothermal reaction. However, a
microwave-hydrothermal reaction at 100
C led to monodispersed and red a-Fe 2 O 3
particles 30–66 nm in diameter after 2 h without the formation of b-FeOOH particles. They investigated the effect of microwave radiation during hydrothermal
treatment at 100–160
C on the formation, yield, kinetics, morphology phase type,
and color of a-Fe 2 O 3 .
Several oxides, including g-Fe 2 O 3 , and Fe 3 O 4 , have been prepared recently by the
interaction of electromagnetic radiation with a physical mixture of metal nitrates
and amides/hydrazides [187]. A judicious choice of such redox mixtures undergoes
exothermic reactions when they are coupled with microwave radiation. The coupling of electromagnetic radiation with metal salts and amides/hydrazides depends
on the dielectric properties of the individual components in the reaction mixture.
This approach has been used to prepare g-Fe 2 O 3 , Fe 3 O 4 , MgCr 2 O 4 , a-CaCr 2 O 4 , and
La 0:7 Ba 0:3 MnO 3 .
A microwave hydrothermal route was employed to synthesize various phases of
iron oxide powders by using ferrous sulfate and sodium hydroxide as starting
chemicals [188]. All the reactions were carried out under the identical microwave
hydrothermal conditions of 190
C, 154 psi, 30 min by varying the molar ratio
(MR) of FeSO 4 /NaOH (i.e., pH variation) from 0.133 to 4.00 in the solution. It was
found that the variation of the molar ratio of FeSO 4 /NaOH has a profound effect
on the crystallization of various phases of iron oxides under identical processing
conditions. The stoichiometric, submicron-sized (0.15–0.2 mm), spherical agglomerates of Fe 3 O 4 powders were obtained if the MR of FeSO 4 /NaOH was 0.133
(pH b 10) was maintained. On the other hand, non-stoichiometric Fe 3 O 4 powders
were obtained for all higher MR of FeSO 4 /NaOH between 0.133 and 4.00
(6.6 < pH < 10). However, when the MR of FeSO 4 /NaOH was 4.00 (pH A 6.6), a
varied distribution of shape and size (1–5 mm) of agglomerates of a-Fe 2 O 3 powders
was produced.
A paper reporting on the microwave-hydrothermal treatment of alcoholic solutions of ferrous chloride (FeCl 2 ) and sodium ethoxide (EtONa) solutions with a
microwave autoclave designed by the authors (the RAMO system) has been published recently [189]. Depending on the initial concentrations, hematite (a-Fe 2 O 3 ),
spinel phase (Fe 3Àx O 4 ) or iron-magnetite (Fe(0)-Fe 3 O 4 ) nanocomposites are obtained with a lower grain size than conventional composites. Indeed, X-ray diffraction analysis reveals grain sizes close to 20 nm for magnetite and 60 nm for metallic iron. However, the amount of metal is smaller (close to 11%). Furthermore,
6.3 Microwave Heating 159
were obtained.
Komarneni returned to the synthesis of a-Fe 2 O 3 in 2001 [186]. This time he
was interested in this product as a red pigment in porcelains. The methods of
preparation were microwave-hydrothermal and conventional-hydrothermal reactions. The precursors were FeCl 3 Á6H 2 O and HCl solutions. The reaction were carried out at 100–160
C. Acicular and yellow b-FeOOH (akaganite) particles 300 nm
in length and 40 nm in thickness were predominantly formed at 100
C after
2–3 h, while spherical a-Fe 2 O 3 particles 100–180 nm in diameter were preferentially formed after 13 h using a conventional-hydrothermal reaction. However, a
microwave-hydrothermal reaction at 100
C led to monodispersed and red a-Fe 2 O 3
particles 30–66 nm in diameter after 2 h without the formation of b-FeOOH particles. They investigated the effect of microwave radiation during hydrothermal
treatment at 100–160
C on the formation, yield, kinetics, morphology phase type,
and color of a-Fe 2 O 3 .
Several oxides, including g-Fe 2 O 3 , and Fe 3 O 4 , have been prepared recently by the
interaction of electromagnetic radiation with a physical mixture of metal nitrates
and amides/hydrazides [187]. A judicious choice of such redox mixtures undergoes
exothermic reactions when they are coupled with microwave radiation. The coupling of electromagnetic radiation with metal salts and amides/hydrazides depends
on the dielectric properties of the individual components in the reaction mixture.
This approach has been used to prepare g-Fe 2 O 3 , Fe 3 O 4 , MgCr 2 O 4 , a-CaCr 2 O 4 , and
La 0:7 Ba 0:3 MnO 3 .
A microwave hydrothermal route was employed to synthesize various phases of
iron oxide powders by using ferrous sulfate and sodium hydroxide as starting
chemicals [188]. All the reactions were carried out under the identical microwave
hydrothermal conditions of 190
C, 154 psi, 30 min by varying the molar ratio
(MR) of FeSO 4 /NaOH (i.e., pH variation) from 0.133 to 4.00 in the solution. It was
found that the variation of the molar ratio of FeSO 4 /NaOH has a profound effect
on the crystallization of various phases of iron oxides under identical processing
conditions. The stoichiometric, submicron-sized (0.15–0.2 mm), spherical agglomerates of Fe 3 O 4 powders were obtained if the MR of FeSO 4 /NaOH was 0.133
(pH b 10) was maintained. On the other hand, non-stoichiometric Fe 3 O 4 powders
were obtained for all higher MR of FeSO 4 /NaOH between 0.133 and 4.00
(6.6 < pH < 10). However, when the MR of FeSO 4 /NaOH was 4.00 (pH A 6.6), a
varied distribution of shape and size (1–5 mm) of agglomerates of a-Fe 2 O 3 powders
was produced.
A paper reporting on the microwave-hydrothermal treatment of alcoholic solutions of ferrous chloride (FeCl 2 ) and sodium ethoxide (EtONa) solutions with a
microwave autoclave designed by the authors (the RAMO system) has been published recently [189]. Depending on the initial concentrations, hematite (a-Fe 2 O 3 ),
spinel phase (Fe 3Àx O 4 ) or iron-magnetite (Fe(0)-Fe 3 O 4 ) nanocomposites are obtained with a lower grain size than conventional composites. Indeed, X-ray diffraction analysis reveals grain sizes close to 20 nm for magnetite and 60 nm for metallic iron. However, the amount of metal is smaller (close to 11%). Furthermore,
6.3 Microwave Heating 159