Geoscience Reference
In-Depth Information
ammonium chloride solutions (1
7 wt%), recrystallization of zincite takes place at
450
C. The crystal habit varies with the concentration of the solution
[60,61]
.In
highly concentrated solutions, hexagonal platelets are formed. Hence, in order to
obtain good crystals of zincite, NaOH and KOH solutions are used. The resistance of
zincite crystals grown in alkali
solutions containing lithium ions
is
small:
10
5
10
8
Ω
cm. Annealing in air or oxygen increases the resistance of the samples to
10
5
10
8
cm. This is due to the diffusion of the Li ions, and to the motion of the
hyper-stoichiometric proportion of zinc to the surface of the crystal faces, with its
subsequent oxidation to zinc oxide.
Zincite crystals obtained under hydrothermal conditions are normally activated
by Mn
2
1
and Ni
2
1
, which emit red and green radiation. However, the resistance of
the crystals remains unchanged. Similarly, the Mn
2
1
and Ni
2
1
ions also have no
appreciable effect on the kinetics of the crystallization of zincite.
Wang et al.
[62]
have carried out a systematic investigation of zincite crystals
and their morphology through a crystal chemical approach to obtain a desirable mor-
phology. Also, they have explained the effects of the additive OH
2
on the crystal
morphology of ZnO crystallites. ZnO microcrystallites are very important, having
wide application in ceramics, coatings, and electronic devices. Thus, hydrothermal
zincite crystals have great device potential.
Ehrentraut et al.
[50]
have successfully grown large-size single crystals of ZnO
using the hydrothermal method. These authors used the crystal growth mechanism
of ZnO proposed by earlier workers
[63
65]
. Almost 100 specimens of size 2 in.
have been grown on mainly (0001)- and (1010)-oriented seeds. All crystals devel-
oped excellent facets and the average crystal thickness was about 1 cm in the
h
Ω
direction (
Figure 9.8
). A ZnO crystal of 3 in. diameter and a wafer pro-
cessed from it are shown in
Figure 9.9a and b
, respectively. These are the largest
ZnO single crystals grown by the hydrothermal method.
0001
i
Figure 9.8 ZnO crystals of size 2 in. produced during one growth run.
Source: Photograph courtesy of Dirk Ehrentraut
[65a]
, Tohoku Univ. Japan.
