Geoscience Reference
In-Depth Information
hydrothermal synthesis of cuprite, and the solubility of cuprite can be written as
follows
[48]
:
OH
2
!
Þ
2
1
ð
Cu
2
O
1
ð
2a
2
Þ
2CuOa
ð
2a
2
a
1
Þ
H
2
O
ð
9
:
5
Þ
2
2
2
The solubility of hemioxide of cuprite under HPHT conditions is less
stable with respect to supersaturation, and with a slight variation in the temperature
gradient, spontaneous crystallization of cuprite takes place such that the growth of
cuprite crystals become difficult.
Cuprite crystal growth is carried in autoclaves with copper or silver linings. The
growth temperature is 250
450
C,
75
C, and % fill 70
80%. For chlo-
ride solutions, Teflon liners have been used and the temperature was maintained at
T
Δ
T
25
5
300
C and
45
C, and the pH varied from 5.0 to 5.5. The crystals
Δ
T
20
5
5
obtained are 4
5 mm in size with ruby-red color having octahedral or often
dodecahedral cover. In recent years, much focus is on the synthesis of nanosize
crystals of cuprite for a variety of applications.
9.4.2 BeO (Bromelite)
Extremely high toxicity of beryllium makes the crystal growth of beryllium-
bearing compounds very difficult. Therefore, the hydrothermal method becomes
most useful in this regard. The first attempt to grow bromelite was made by
Hartmann
[49]
. See
d
e
d
growth
o
f
b
romelite was carried out using a 5 mm BeO crys-
tal oriented along
½
1010
and
½
1011
:
The average growth rate was
0.07 mm/day.
B
530
C using NaOH solutions.
The autoclaves are usually provided with copper or silver lining, though there
are several other lining materials. The experimental conditions are as follows:
The crystallization was carried out at 500
Mineralizer
25 wt% NaOH
Nutrient
Beryllium oxide
% Fill
80%
450
C
Crystallization temperature
20
50
C
Δ
T
4 mm with
the basic pinacoidal, pyramidal, and prismatic forms are formed. The work on bro-
melite is mostly of academic interest today.
Under the above conditions, colorless hexagonal crystals of size 3
9.4.3 Zinc Oxide
Zinc oxide is well known as n-type wide band gap semiconductor (
Δ
3.37 eV
at 300 K) with a large exciton energy of 60 meV and thermal energy of 27 meV,
and is a subject of research owing to its unique mechanical, electrical, and optical
properties with a combination of high stability, very high melting point with valu-
able device potential for piezoelectric transducers, gas sensors, optical waveguide,
E
5
