Direct observation of solid-liquid interface is important for the directional solidification to determine the desired interface shape by controlling the growth parameters. To grow good quality single crystals of novel organic nonlinear optical materials, a simple inexpensive Bridgman-Stockbarger (BS) crystal growth system has been designed and fabricated. Two immiscible liquids have been utilized to create two zones for this crystal growth system. Bulk single crystals of benzil derivative and n-salicylidene-aniline have been successfully grown in this system. The optimum lowering rate has been found to be 0.1 mm/h for the flat interface. Results on the crystal growth and other parameters of the grown crystals are presented

Aggarwal, M. D.

Bhat, K.

Choi, J.

Frazier, Donald O.

Metzl, R.

Penn, Benjamin G.

Wang, W. S.

English

NASA Technical Reports Server

NASA-CR-204899
A Simple Inexpensive Bridgman-Stockbarger Crystal Growth System for Organic
Materials
J. Choi, M.D. Aggarwal, W.S. Wang, R. Metzl, and K.Bhat, Department of Physics,
Alabama A&M University, Normal, AL 35762
Benjamin G. Penn, and Donald O. Frazier, Space Science Laboratory, NASA Marshall
Space Flight Center, AL 35812
Direct observation of solid-liquid interface is important for the directional
solidification to determine the desired interface shape by controlling the growth
parameters. To grow good quality single crystals of novel organic nonlinear optical
materials, a simple inexpensive Bridgman-Stockbarger (BS) crystal growth system
has been designed and fabricated. Two immiscible liquids have been utilized to
create two zones for this crystal growth system. Bulk single crystals of benzil
derivative and n-salicylidene-aniline have been successfully grown in this system.
The optimum lowering rate has been found to be 0.1 mm/h for the fiat interface.
Results on the crystal growth and other parameters of the grown crystals are
presented.
Significant efforts have been made in the
field of organic nonlinear optical materials
because of their potential applications such as
second-harmonic gencration(SHG), frequency
mixing and electro-optic modulation. Organic
molecules have been reported to have larger
nonlinear optical susceptibilities in many cases
than those of most inorganic materials within the
0.5-2.0 micron transparency domain (Sutter,
1988). Laser damage thresholds of some organic
single crystals are much higher than conventional
inorganic single crystals (Nitti, 1993). Most of
the organic single crystals are more difficult to
grow than inorganic crystals due to their low
thermal conductivity, their large suIJercooling
tendencies and thermal instability. Organic single
crystals have been grown from the solution
(Kotler, 1992), vapor and melt growth
(Aggarwal, 1992). Melt growth is superior to
solution growth in that the crystal can be grown
faster and the grown crystal is free from the
solvent inclusions. Materials that have lower
thermal conductivity take more time to solidify
from the melt. For this reason, organic crystals
are grown at a slower rate. A Bridgman-
Stockbarger method(Aggarwal, 1993) has been
used to grow single crystal of organic materials
because of relatively simple geometry of system
and ease of controlling growth parameters during
the growth processing. The solid-liquid interface
shape and location are the important factors in
Bridgman-Stockbarger growth system. The
interface shapes strongly influence the
dislocation, grain size, and dopant concentration
during directional solidification (Corriel, 1979;
Favier, 1977). Interface shape and position can
be controlled by material properties and furnace
control such as temperature gradient and
lowering rate. Therefore understanding of
growth parameters is important for the
Bridgman-Stockbarger system to determine the
desired interface shape during the process.
Post-processing techniques have been
used to determine interface shape and location for
the opaque system (Feigelson, 1980; Capper,
1983). They were able to change the interface
shape by changing the growth parameters.
However, these techniques do not allow the real-
time observation and controlling interface shape
and location during crystal growth. The x-ray
radiographic techniques have also been
developed to enable the direct visualization for
opaque system (Barber, 1986,1995).
To grow novel organic materials, the
control of the growth parameters is more
advantageous using real time observation.
Therefore a transparent furnace and observing
system is required for real-time measurement. A
simple, inexpensive transparent BS system is
designed and fabricated in our laboratory that is
described in the next section. The interface
shape, location and growth rate are determined as
a function of lowering rate to understand
influence on growth factors.
II. Experimental set-up
The Bridgman-Stockbarger melt growth
system is shown in Fig. 1. It consists of a
growth chamber, crystal lowering mechanism
and temperature control system. A Glass beaker
is used as growth chamber that contains two
immiscible liquids. Deionizcd water is used for
lower temperature zone as heat transfer medium
and silicon oil is used for higher temperature
1054-7487/96/0263512.00/0 © 1996 American Chemical Society
https://ntrs.nasa.gov/search.jsp?R=19970023055 2020-06-16T02:16:02+00:00Z
To Crystal Lowering Mechanism
Stirrer Motor
[ Controller_ -_]"
Glass Jar _ ! -
i II
Upper stirrer / :
Water / Oil _ _'
% .Interface
G,a,sT i
Heater 2- .L .
i
E
i
,
Temperature
Contro ers J
1
Thermocouple 1
Solid / Liquid
Interface
Thermocouple 2
Ampoule
Water
Lower Magnetic
Fig. 1 Schematic diagram of the crystal growth chamber
zone. Two tubular shaped heaters that are sealed
in glass tubes, fabricated in our laboratory are
used for the hot(temperature above the melting
point) and cold(temperature below the melting
point) zones. Two separate proportional
temperature controllers YSI 72 and Eurotherm
818 are used for controlling the temperature of
the two zones. The control thermocouples in the
two zones are located near the growth ampoule.
Both zones are stirred to keep the temperature
uniform in the individual zones. A hot plate with
stirring function is used as lower zone stirrer and
a DC motor is attached at the top of the growth
chamber for hot zone stirring. The lowering
arrangement is designed and fabricated using
Velmex slide that is operated using Hurst
stepping motors and gear reducer. Each zone can
be set at any desired temperature and the
temperature is controlled to within i-0. I*C. For
benzil-aniline (melting point 95°C) crystal
growth, temperatures of 97.5°C and 82.5°C are
set up for hot and cold zones, respectively. The
vertical temperature gradient can be obtained up
to 25"C to prevent supercooling during growth.
Once the seed is formed at the tip of the ampoule
then the growth process can be observed through
the growth chamber. The lowering rate was
controlled by observing interface shape until we
could get flat interface. After successful growth
of the crystal, the ampoule is slowly cooled at a
rate of few degrees per hour. No wetting and
cracks induced from difference of thermal
expansion between grown single crystal and
quartz ampoule were found.
III. RESULTS AND DISCUSSION
Single crystals of benzil, benzil aniline
and salicylidene-aniline have been successfully
grown in this system. Fig. 2 shows a typical
ampoule containing the grown crystal. A typical
crystal growth run consists in filling the conical
264
largersuperc___lingte dency(>250C).Thenthe
coldzonetemperatureisslowlyincreaseduntil
theflatsolid-liquidinterfaceisachieved.
Variousloweringratesareuseduring
thecrystalgrowthexperimentsandtheshapeoftheinterfacewasrecordedafterviewingthrough
thetelemicroscopewhichismountedonax-z
translator.Normallytheinterfaceisdividedin10
pointsandthemotionofeachismonitoredasthe
crystalgrowthproceeds.It hasbeenfoundthat
thefasterloweringrateincreasestheconcavityof
thesolidliquidinterfacetowardsthegrowing
crystal.Fromthesexperimentstheoptimum
loweringratehasbeenfoundtobeO.1mm/hfortheflatinterface.
Fig.2Photographofbenzilandbenzil-aniline
crystals
ampoulewiththeorganicmaterial,meltingit in
thehotzoneandslowlytranslx_rtingittothecold
zonetherebycrystallizinga smallayerof
materialtatime.Becauseofthetransparencyof
thesystem,thetemperatureofthehotandcold
zonesareadjustedsuchthatthesolid-liquid
interfaceisclearlyvisibleinoneofthe zones(Fig.3).Thetemperaturegradientisadjustedto10-15°Ctoinitiatethenucleationatthetipofthe
conicalampouleforbenzilandbenzil-anilinc.Howevercoldzonehastobec_×_ledbelowthe
roomtemperature(~15°C)for nucleationof
salicylidene-anilinc(meltingpoint55°C)thathas
' interface
jgrowing
Acknowledgements
The authors gratefully acknowledge the support
of the National Aeronautics and Research
administration for their grants NAG-125 and
MAG-4078 under which this work is performed.
References
Fig.3 Photograph of a cross-section of the
growth chamber showing solid -liquid
interface
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Reprinted from ACS Conference Proceedings Series
Crystal Growth of Organic Materials
Allan S. Myerson, Daniel A. Green, and Paul Meenan, Editors
Published 1996 by the American Chemical Society
265



A Simple Inexpensive Bridgman-Stockbarger Crystal Growth System for Organic Materials

A Simple Inexpensive Bridgman-Stockbarger Crystal Growth System for Organic Materials

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