Geoscience Reference
In-Depth Information
Nijs et al.
[128]
have obtained Mn
3
Al
2
Si
3
O
12
under hydrothermal conditions at
temperature 550
C and pressure 2 kbar, and in the experimental duration of 14 days.
The starting material was Al
2
O
3
3SiO
2
gel, into which ground metallic manganese
was added to obtain a fine powder with the composition Al
2
O
3
3Mn. This
starting material, plus excess water, was taken in a gold capsule which was placed in
a Turtle vessel. After 14 days, the capsule was opened to get small crystals of man-
ganese garnet. This material shows red luminescence from the Mn
2
1
ions, which is
quenched at room temperature due to energy migration to quenching centers. The
growth of nanosize crystals of garnets doped with active elements find valuable
applications in modern technology.
3SiO
2
9.5.4 Hydrothermal Synthesis of Ferrites
Ferrites are important technological materials with great success in the develop-
ment of magnetic oxide powders with a small particle size from several microns to
nanometer range
[129]
. Among them, barium hexaferrite and manganese ferrite are
the most important ones. Both could be prepared through solid-state sintering,
chemical precipitation, melting, and hydrothermal methods
[44,130
133]
. Of these
methods, the hydrothermal method is probably the most attractive one, because the
resulting ferrite has a perfect crystal structure with a definite composition and very
fine particle size (usually smaller than 1
m) and is almost a monodispersed crystal.
Therefore, the product obtained from hydrothermal synthesis can be used directly
for ceramic processes without a calcination step.
Kumazawa et al.
[134]
have synthesized barium ferrite fine particles from an
aqueous suspension containing goethite and barium hydroxide by the hydrothermal
method and systematically examined the relation between the particle size and the
reaction conditions commercially available, and also synthetic goethite particles are
used in the hydrothermal preparation of ferrites. The experimental temperature
ranges from 200 to 300
C. Both with and without stirring techniques have been
employed under hydrothermal conditions. Alpha-FeOOH is dehydrated to form
α
μ
-Fe
2
O
3
. The dehydration rate may increase with the increasing outer surface area
of the particle. The barium ferrite particle may be formed by the incorporation of
barium ion under such conditions that
-Fe
2
O
3
.
Similarly, a barium hexaferrite particle is formed by dissolution followed by the
crystallization. Usually, the starting materials in the form of either hydroxides or
nitrates of iron and barium, and mineralizer NaOH and water, are taken in the
hydrothermal reactor, and the slurry mixture is heated to a desired temperature.
After the experimental run, the suspension is withdrawn from the reactor, neutral-
ized by a diluted HCl solution and then filtered and washed thoroughly with water.
The reaction can be written as follows:
α
-FeOH is dehydrated to form
α
ð
Þ
2
!
ð
:
Þ
12FeOOH
1
Be
OH
BaO
6Fe
2
O
3
1
7H
2
O
9
11
Wang et al.
[135]
have studied reaction mechanism of producing barium hexa-
ferrites from
α
-Fe
2
O
3
and Ba(OH)
2
under hydrothermal conditions. Wang et al.
