Physical Chemistry of Hydrothermal Growth of Crystals - Hydrothermal Technology - page 153

then the solubility should increase with an increase in the pressure (in the case
when the partial volume of the liquid is equal to the volume of the melting salt).
In recent times, many aspects of hydrothermal chemistry have been experimen-
tally studied, particularly in the past decade. The credit goes to workers like
Franck, Seward, Hegleson, and others [13

15,36] . However, most of these studies
do not give an insight into the complete physical chemistry of the hydrothermal
media as they represent mainly the lower PT conditions. Helgeson has reviewed
exhaustively the thermodynamic properties of electrolytes at high pressures and
high temperatures.
An understanding of the theory and the experimental results on hydrothermal phys-
ical chemistry is essential to the crystal grower. Here, in the experimental work, some
of the basic properties of the hydrothermal medium like viscosity, dielectric constant,
compressibility, and coefficient of expansion are discussed briefly in the crystal
growth context. Since diffusion is inversely proportional to solvent viscosity, we
would expect very rapid diffusion in hydrothermal growth.
This leads to the growth of perfect single crystals with well-developed morphol-
ogy. We can expect higher growth rates, a narrower diffusion zone close to the
growing interface and less likelihood of constitutional supercooling and dendritic
growth. It is, thus, no wonder that quartz growth rates as high as 2.5 mm/day with-
out faults or dendritic growth have been observed [37] . Figure 4.8 shows the

160 ° C

20

200 ° C

18

16

300 ° C
350 ° C
400 ° C
450 ° C
500 ° C
560 ° C

14

12

10

560 ° C

8

500 ° C

450 ° C

6

400 ° C

4

2

0

0.1 0.2 0.3 0.4 0.5

Density ρ (g/cm 3 )

0.6 0.7 0.8 0.9 1.0

Figure 4.8 Viscosity of water as a function of density and temperature [36] .