Geoscience Reference
In-Depth Information
in Chapter 1 the shortcomings in the hydrothermal growth of crystals, especially
with reference to the theoretical knowledge. This applies even to quartz growth
under hydrothermal conditions. The growth of high-quality quartz crystals insists
on so many parameters like phase relationships, nutrient's solubility, thermodynam-
ics (state equations, kinetics of reaction, impurities repartition), and crystal defects
characterization. The other aspect is that the needs for cheaper material in a wide
range of applications like clocks and microprocessors result in being content with
a medium-quality product, and the low profitability precludes financing long and
expensive studies. Though much of the recent publications on quartz is on the
possibility of obtaining/growing high-quality quartz crystals for stringent electronic
applications, this group is still in minority. Except for quartz, no other material
coming from hydrothermal synthesis has sizable industrial use; even though AlPO
4
,
GaPO
4
,Li
2
B
4
O
7
, and microcrystallites show interesting promise, industry does not
seem ready to make an important R&D effort concerning these products. Thus, it is
difficult to get financial support to acquire a better knowledge of hydrothermal
growth, which is highly intricate and necessitates an important effort for a long
duration of time. Private industry cannot deal alone with the costs of such a pro-
gram; even in the United States, research programs are government sponsored,
especially in the defense field
[11]
.
Sever
al criteria are used to evaluate the quality of quartz crystals. The most
commonly used criterion is the Q value or quality factor, which is a measure of the
acoustic loss of the material. It is important for a resonator to have high electrical
Q value and superior frequency
temperature characteristics. In a piezoelectric
resonator, electrical energy and mechanical energy are interconvertible. In such a
case, Q is expressed as:
Q
5
½
X
R
where X is the inductive or capacitive reactance at resonance and R is the resistance.
The quality factor, Q, can be considered as the inverse of the fraction of the energy
lost per cycle. The highest values of Q are required in order to prevent loss of energy
into coherent phonons. The acoustic Q for natural
α
-quartz crystal varies in the range
10
6
, while for synthetic quartz crystals,
of 1
3
the value drops down to
3
2.105
1.106. Thus, in the last two decades, the main objective among quartz crystal
growers has been to improve Q, which in turn leads to the production of a low concen-
tration of physicochemical and structural defects. We shall discuss the growth of such
quartz crystals of high quality through the study of growth rate, mineralizers, solubility
liner material autoclave, seed effect, nutrient effect, and finally the recent advances in
the processing of defect-free quartz for stringent electronic applications.
5.2.1 Growth Rate
Growth rate is determined by the ratio of increase in thickness of seed and duration
of the run. The growth rate along the main crystallographic axes, R
c
, is determined
