29-05-2014, 12:19 PM
INTRODUCTION TO CRYSTAL GROWTH METHODS WITH EMPHASIS ON LOW TEMPERATURE SOLUTION GROWTH TECHNIQUE
INTRODUCTION TO CRYSTAL GROWTH .pdf (Size: 443.22 KB / Downloads: 301)
INTRODUCTION
Crystals are the unacknowledged pillars of modern technology.
Without crystals, there would be no electronic industry, no photonic industry,
no fiber optic communications, which depend on materials/crystals such as
semiconductors, superconductors, polarizers, transducers, radiation detectors,
ultrasonic amplifiers, ferrites, magnetic garnets, solid state lasers, non-linear
optics, piezo-electric, electro-optic, acousto-optic, photosensitive, refractory
of different grades, crystalline films for microelectronics and computer
industries. In the past few decades, there has been a
growing interest on crystal growth processes, particularly in view of the
increasing demand of materials for technological applications (Brice 1986,
Nalwa and Miyata 1997). Atomic arrays that are periodic in three dimensions,
with repeated distances are called single crystals. It is clearly more difficult to
prepare single crystal than poly-crystalline material and extra effort is
justified because of the outstanding advantages of single crystals (Laudise
1970).
Hydrothermal growth
Hydrothermal implies conditions of high pressure as well as high
temperature. Substances like calcite, quartz is considered to be insoluble in
water but at high temperature and pressure, these substances are soluble. This
method of crystal growth at high temperature and pressure is known as
hydrothermal method. Temperatures are typically in the range of 400° C to
600° C and the pressure involved is large (hundreds or thousands of
atmospheres).
Growth is usually carried out in steel autoclaves with gold or silver
linings. Depending on the pressure the autoclaves are grouped into low,
medium and high-pressure autoclaves. The concentration gradient required to
produce growth is provided by a temperature difference between the nutrient
and growth areas.
GROWTH FROM VAPOUR
The growth of single crystal material from the vapour phase is
probably the most versatile of all crystal growth processes. Crystals of high
purity can be grown from vapour phase by sublimation, condensation and
sputtering of elemental materials. To obtain single crystals of high melting
point materials this method is used. Molecular beam techniques have also
been applied recently to crystal growth problems. The most frequently used
method for the growth of bulk crystals utilizes chemical transport reaction in
which a reversible reaction is used to transport the source material as a
volatile species to the crystallization region. Finding a suitable transporting
agent is a formidable, problem in this technique. It is rarely possible to grow
large crystals because of multi-nucleation.
LOW TEMPERATURE SOLUTION GROWTH
Growth of crystals from aqueous solution is one of the ancient
methods of crystal growth. The method of crystal growth from low
temperature aqueous solutions is extremely popular in the production of many
technologically important crystals. It is the most widely used method for the
growth of single crystals, when the starting materials are unstable at high
temperatures (Pamplin 1979) and also which undergo phase transformations
below melting point (Hooper et al 1980). The growth of crystals by low
temperature solution growth involves weeks, months and sometimes years.
Though the technology of growth of crystals from solution has been well
perfected, it involves meticulous work, much patience and even a little
amount of luck. A power failure or a contaminated batch of raw material can
destroy months of work.