Design and fabrication of eight zone vertical dynamic

Design and fabrication of eight zone vertical dynamic gradient freeze system for
organic single crystal growth
SP. Prabhakaran, R. Ramesh Babu, and K. Ramamurthi
Citation: Review of Scientific Instruments 84, 083907 (2013); doi: 10.1063/1.4819138
View online: http://dx.doi.org/10.1063/1.4819138
View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/84/8?ver=pdfcov
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REVIEW OF SCIENTIFIC INSTRUMENTS 84, 083907 (2013)
Design and fabrication of eight zone vertical dynamic gradient freeze
system for organic single crystal growth
SP. Prabhakaran,1 R. Ramesh Babu,1,a) and K. Ramamurthi2
1
Crystal Growth and Thin Film Laboratory, Department of Physics, Bharathidasan University,
Tiruchirappalli-620 024, Tamil Nadu, India
2
Department of Physics and Nanotechnology, Faculty of Engineering and Technology, SRM University,
Kattankulathur-603 203, Tamil Nadu, India
(Received 30 January 2013; accepted 9 August 2013; published online 28 August 2013)
Design and construction of the vertical dynamic gradient freeze (VDGF) system operating in the temperature range from 50 ◦ C to 500 ◦ C for growing organic single crystals are described. The design of
VDGF system consists of furnace, control system, translation assembly, and image capturing device.
Furnace has been constructed with eight zones controlled independently by a dynamic temperature
control system for achieving desired thermal environment and multiple temperature gradients, which
are essential for the growth of organic single crystals. The transparent furnace enables direct observation to record and monitor the solid-liquid interface and growth of crystals through charge coupled
device based video camera. The system is fully computerized hence it is possible to retrieve the complete growth and furnace history. In order to investigate the functioning of the constructed VDGF
system for the growth of organic single crystals, a well known organic nonlinear optical single crystal of benzimidazole was grown. The crystalline quality and the optical transmittance of the grown
crystal were studied. © 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4819138]
I. INTRODUCTION
In recent years, more research efforts have been made in
exploring electronically and optically functional organic single crystals for their potential applications in a variety of fields
such as semiconductors, optoelectronics, photonics, and integrated optics.1–4 Organic single crystals possessing high mechanical strength, thermal stability, high charge carrier mobility, and ability of producing green/blue laser light and could
withstand high-energy light radiations, are of vital importance in device applications such as transistors, electro-optic
switches, and semiconductor diode lasers.5, 6 Also, for such
device applications, single crystals of bulk size and defect
free are needed. But, growing bulk and high quality organic
single crystals is a challenging task for the crystal growers
because of low thermal conductivity and ease of supercooling
nature of the materials. Therefore, the choice of material and
the crystal growth method plays an important role for obtaining active single crystals with optical quality and larger size
for nonlinear optical applications. To date, a variety of techniques have been introduced and practiced for the growth of
organic single crystals.7–9 All of them have their own merits and demerits depending upon the physical and chemical
properties of the materials. Materials having reasonable solubility and prismatic morphology can be successfully grown
from low temperature solution growth technique.8 Growth
from vapour phase offers the possibility of growth at lower
temperatures and within certain phase limits. Similarly, materials which are exhibiting single phase between freezing and
ambient temperature are grown by melt growth techniques.10
a) Author to whom correspondence should be addressed. Electronic mail:
rampap2k@yahoo.co.in. Tel.: +91-431-2407057. Fax: +91-431-2407045.
0034-6748/2013/84(8)/083907/6/$30.00
Many reports are available on the growth of bulk organic
single crystals from melt technique using directional solidification of materials by slowly moving the crucible containing
the melt from higher to lower temperature region. Achieving such slow mechanical motion of crucible is too difficult
at very low translation rates. To achieve this, translation assembly with nano resolution was reported by Jayaprakasan
et al. for the growth of single crystals.11 However, the motion of growth ampoule can be accompanied with slight vibrations which disturb the growth system. Gradient freeze technique has the advantage to overcome this problem in which
crucible containing the melt is cooled from one end to the
other end without effecting any mechanical vibration.12 Vertical gradient freeze technique (VGF) is considered to be one
of the versatile growth methods13 due to its controlled thermal
and vibrationless growth environment. Even though semiconductor crystals such as GaAs, InP, etc., were grown from this
method,13, 14 the usefulness of this method is not established
well for the growth of organic single crystals except an attempt made by Lan and Song.15
Benzimidazole (BMZ) is one of the potential organic
nonlinear optical (NLO) materials which crystallizes in orthorhombic system with the space group of Pna21 . Its
relative second harmonic generation (SHG) efficiency is
4.5 times greater than that of KDP.16 Reports are available on the growth of benzimidazole single crystals from
slow evaporation solution growth,17 vertical Bridgman,18 and
Sankaranarayanan-Ramasamy (SR)19 methods. In order to
grow better quality BMZ crystals with reasonable size for device applications, an improved method with an effective control over the growth environment is required. Hence in the
present work, the computer controlled vertical dynamic gradient freeze system with eight zone furnace was designed and
84, 083907-1
© 2013 AIP Publishing LLC
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083907-2
Prabhakaran, Babu, and Ramamurthi
Rev. Sci. Instrum. 84, 083907 (2013)
constructed. Further the constructed VDGF system has been
employed to grow benzimidazole single crystal for the first
time.
II. EXPERIMENTAL DESIGN
A. Furnace
The schematic diagram of vertical dynamic gradient
freeze system developed in the present investigation is shown
in Fig. 1. The system consists four major parts; furnace, programmable logical controller (PLC) based control system,
translation assembly, and image capturing device. For the
growth of organic single crystals from melt technique, temperature distribution along the axis of furnace and magnitude
of temperature gradient at the growth region are the fundamental requirements which are mainly achieved from furnace
design.20, 21 The furnace used in vertical dynamic gradient
freeze system was made up of quartz glass tube with 2 mm
wall thickness, 50 mm diameter, and 500 mm length. As
it possesses extremely low coefficient of thermal expansion
(5.5 × 10−7 /◦ C), quartz tube is more suitable for furnace
windows, where we need the windows to respond optically
minimum to the thermal change. This feature holds the transparency of furnace for the real time observation of growth.
1
2
4
3
10
5
TC 1
PLC Based
Control
System
TC 2
TC 3
1250 mm
6
200 mm
TC 4
9
TC 5
500 mm
TC 6
50 mm
7
Also, quartz filters the high frequency noises from the power
supply because it is unreactive at high frequencies.22 In conventional Bridgman technique,23 insulated layer between the
hot and cold zone blocks the view of the solid-liquid interface.
But in the present system, instead of continuous winding,
C-shaped heaters having nichrome wire of 0.8 mm thick
as heating elements were fixed at the outer surface of the
quartz tube. In general, to retain the flexibility of changing/
controlling the temperature at maximum number of points in
a furnace, it is essential to employ a multizone technique for
the growth of organic single crystals. But in practice there are
number of factors which restricted the numbers of zones to
be used. One of them is effective zone length (zone length required for the effective temperature control) with respect to
the length of the furnace. In our case we fixed the zone length
as 30 mm (length of the zone heater 25 mm and gap between
the zones 5 mm) for the furnace having 500 mm length. In
the present system, ampoules used for the growth of single
crystals are in the length of about 200–220 mm. Thus, we
employed eight numbers of zones (8 × 30 mm) to provide
the temperature control over the entire length of growth ampoule. Input power of the each zone heater was controlled
independently using thyristor based single phase solid state
power controller operated at the current rate of 35 A (SPC135). Zone temperatures were measured by K-type thermocouples placed at the centre of each zone heaters and fixed at the
outer wall of quartz tube. To prevent thermal loss, furnace was
completely insulated by the ceramic wool. To record complete
growth and growth related features through the CCD camera
via the 20 mm gap between the ends of C-shaped heater, a
viewing window (24 × 30 mm2 ) was provided at the outer
cover of the furnace. Though this region influenced the growth
by means of radial fluctuation at that point, we reduced the
amount of fluctuation in maximum by using a shutter at the
window.
TC 7
11
TC 8
8
850 mm
10
C - Type zone
heater
2 cm
1. Stepper motor (Translaon) 7. Eight zone furnace
2. Stepper motor (Rotaon) 8. Base
3. Guide rod
9. K-Type Thermocouples
4. Stem
10.C-Type Heater
5. Crucible holder
11.CCD Camera
6. Crucible
FIG. 1. Schematic diagram of VDGF unit.
B. PLC-based control system
Figure 2 illustrates the schematic diagram of PLC based
control system which can be used to control the temperature of furnace, translation, and rotation of growth ampoule in
the designed vertical dynamic gradient freeze unit. For these
purposes, thyristor based temperature power controllers, thermocouples, temperature transmitter, PLC, and stepper motor with driver unit were used as the major components. As
stated in the furnace design, SPC1-35 series thyristor based
temperature power controller was used to control the input
power of the zone heaters. The various control modes available in the SPC1-35 controller are phase control, cycle control, and ON/OFF control. In the present system, phase control mode is adopted because, it provides fine resolution of
power, and controls the fast responding loads where the temperature changes as the function of resistance. Also, it is more
suitable if the load is transformer coupled or inductive. Phase
control mode is an output type and it is used to control the
phase of the alternating signal corresponding to the input signal. Figure 3(a) shows the block diagram of connection used
to control the phase of the alternating current signal according
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Prabhakaran, Babu, and Ramamurthi
Rev. Sci. Instrum. 84, 083907 (2013)
Temperature
power controller
PC
L N
L N
L N
L N
L N
L N
L N
L N
U V
U V
U V
U V
U V
U V
U V
U V
TPC1-TPC8
Transformer
4-20 mA Signal
PLC
T1
T3
T2
T4
T5
T6
T7
T8
T1-T8
TC1
TC2
Stepper
motor drive
TC3
Z4
Z5
TC4
To PLC
Z6
Z3
TC5
Vertical
Motor
TC6
Zone heaters
Z1 – Z8
Z2
Z7
TC7
Z1
Z8
TC8
K-Type thermocouple
TC1 – TC8
FIG. 2. Schematic diagram of PLC based computer controlled system.
to the input signal delivered from the PLC. It controls the controller output from 0% to 100% for the corresponding input
signal of DC 4–20 mA. There is also a provision to control
the output of controller for a particular input signal manually using output adjuster (OUT ADJ) function. This function works in the logic of control input (%) × OUT ADJ (%)
= Output (%). For instance, if the control input is 100% and
the OUT ADJ is 50% then the output will be 50%. The output
characteristics of the controller as a function of input signal
and OUT ADJ are shown in Fig. 3(b).
The respective temperature of the output signal was measured from the thermocouples and amplified using the temperature transmitter. In the present system, ATxRail (ABUS
make) temperature transmitter was used which is more flexible and adopts mV, Pt100 and a variety of thermocouples
as the input sensor. This microprocessor based temperature
transmitter delivers a scalable linear output current (4–20 mA)
for the corresponding sensor temperature. Hence, the transmitter should be calibrated for the desired range by a programming device. For example, if we need to measure the
DC 4-20mA
PLC Signal
1
2
F.G +5v
80
(-)
3
4
5
IN
IN
0V
Output (%)
(+)
25%
50%
75%
100%
100
(a)
Thyristor
R
T
W
U
(b)
60
40
20
0
AC
2
Furnace
4
6
8
10
12
14
16
18
20
22
Control input DC4-20 (mA)
FIG. 3. (a) Connection of control input terminal. (b) Output characteristics of controller as the function of OUT ADJ and control input.
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Prabhakaran, Babu, and Ramamurthi
Rev. Sci. Instrum. 84, 083907 (2013)
1-Pulse Input CW
(ABB make)
PLC
ACD
Driver unit
AC500
PM571
Digital I/O
signal
1-Pulse Input CCW
F/H
2-Pulse Input CW
1P/2P
24/36 V
Stepper
Motor
2-Pulse Input CCW
DC Supply
1. ACD
2. F/H
3. 1P/2P
4. 24/36V
-
Automatic Current Cutback
Full step/Half step
1 Pulse/2 Pulse mode
24/36 Input Volt
FIG. 4. Block diagram of stepper motor controller.
temperature in the range between 0 and 100◦ C, the transmitter is to be calibrated accordingly in this range to give an output current proportional to the temperature measured by the
sensing element (0 ◦ C = 4 mA, 100 ◦ C = 20 mA). The current output from the transmitter (4–20 mA) was measured by
the PLC. All these controls and conversions were processed
through AC500 PM571 (ABB make) PLC with controllogix
analog I/O modules. Controllogix analog I/O modules are the
interface modules that convert analog signals to digital values for inputs and convert digital values to analog signals for
outputs.
C. Ampoule translation mechanism
A high torque 2-phase stepper motor (CSK 264 AP-SG100) consisting of an open chassis type driver (model-CSD
2120-P) with photocoupler I/O specification was used for ampoule translation mechanism. Even though the VDGF technique does not require the translation mechanism to grow single crystals, it is necessary to position the ampoule loaded
with material at the desired zone. Figure 4 shows the block
diagram for stepper motor control assembly which consists of
PLC, driver unit, and stepper motor.
The driver unit consists of four major function-setting
switches named as automatic current-cut back, step angle,
pulse input mode, and voltage power supply. The inputs of
the driver unit are photocoupler inputs which isolate the electrical output from the input for the complete elimination of
noise. The automatic current-cut back retains the position of
translation rod connected with load when the power is off.
Step angle switch is used to set the step angle of motor in
full step (1.8◦ ) or half step (0.9◦ ). This step angle of 1.8◦ is
achieved in 100 steps (i.e., 0.018◦ per step) with the help of
a gear attached with the shaft of the motor. For one complete
revolution of motor (360◦ ) the translation rod moves 3 mm.
Thus the linear translation of 0.1 μm per step can be achieved.
The driver unit supports both 1-pulse input mode and 2-pulse
input mode. We can select the appropriate mode using pulse
input mode switch. In this work, 2-pulse input mode has been
employed as the input mode.
D. Software implementation
The entire function of the VDGF system is controlled and
monitored through a computer program designed using labview software package. Using this software package, different programmes have been designed for performing various
operations related to the growth and to record the temperature history. These programmes are useful to configure and
monitor the temperature of each zone, helpful to acquire the
temperature data at a particular time of growth and to retrieve
the profile of individual zone or the complete furnace even
after the completion of growth process. Such features are the
notable advantages of the designed VDGF technique over the
other crystal growth systems for the growth of organic single
crystals. Due to this, it is more convenient to analyze the defects presented in a particular region of the grown crystal in
terms of temperature fluctuations by retrieving the temperature profile at the time of growth. Online temperature profile
monitoring of all the zones during the growth is also possible
in this system. Moreover, video image recorder (charge coupled device based video camera) is used to record the complete crystal growth process which facilitates the in situ observation of solid-liquid interface and growth related problems.
III. GROWTH OF BENZIMIDAZOLE (BMZ)
SINGLE CRYSTAL
Single crystal of benzimidazole was grown using the indigenously developed VDGF growth system described above
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Prabhakaran, Babu, and Ramamurthi
Rev. Sci. Instrum. 84, 083907 (2013)
1
2
8
3
Z4
Z6
Z5
Z8
Z7
175
4
10
5
Temperature (°C)
9
Z3
Z2
Z1
180
170
165
160
155
6
(a)
7
150
0
2
4
6
8
10
12
14
16
18
20
22
Distance (cm)
(b)
1. Emergency Alarm
2. Stepper for translation
3. Rotation motor
4. Translation Unit
5. Ampoule holder
6. Eight zone furnace
7. CCD Camera
8. Control panel
9. Touch panel display
10. Growth monitor
FIG. 5. (a) Photograph of indigenously developed vertical dynamic gradient freeze system. (b) Temperature profile of the furnace for the initial set temperatures.
for the first time. Glass ampoule of 1 mm thick and 25 mm
diameter was used for the growth of benzimidazole single
crystals from this technique. Prior to the growth, ampoule
was cleaned and dried well in order to prevent the generation of defects due to the presence of impurity. In the present
work, bubble shaped ampoule with the sharp tip at the bottom was used to isolate the chemical impurities and the multinucleation created at the initial stage, respectively. Ampoule
filled with recrystallized material was evacuated and sealed.
Ampoule was then connected at the bottom of the translation and rotation rod for lowering and positioning the same
into the eight zone furnace, respectively. Figure 5(a) shows
the photograph of the fabricated vertical dynamic gradient
freeze system used for the growth of BMZ single crystal. Initially the zones were set at the desired temperatures in order
to achieve the growth region (zone 3–zone 6) as the isothermal region with melting temperature by overcoming the temperature loss due to the air gap presented between the wall
of furnace and the ampoule. The axial temperature profile
of eight zone furnace for the different initial set temperatures used in the growth of BMZ single crystal is shown in
Fig. 5(b). The material was melted at the isothermal region
and the set temperatures were kept constant for 4–5 h. After
achieving the temperature equilibrium throughout the melt,
temperature gradient of 4 K/30 mm (i.e., 0.13 K/mm) was introduced between the last two zones in 40 h by reducing the
temperature of last zone at the rate of 1 K/10 h. Growth was
started due to the temperature difference provided. In order
to precede the growth, the temperature gradient was moved
through the zones from bottom to top in each 40 h by changing the temperature profile of alternate zones dynamically.
In multizone technique, the thermal interaction between the
zones during growth should be minimum. It was successfully
achieved in the present system for the growth of BMZ with
the aid of proper insulation. BMZ crystal of 25 mm diameter
and 40 mm length was grown. Figure 6 shows the grown ingot
and cut and polished piece of BMZ single crystal grown from
the VDGF technique.
Figure 7(a) shows the diffraction curve recorded for the
BMZ specimen crystal using MoKα 1 radiation. As seen in the
figure, the curve is not having a single diffraction peak. The
solid line, which follows well with the experimental points
(filled circles), is the convoluted curve of three peaks using
the Lorentzian fit. The broad peak with 437 arc s having highest value of the area under the curve shows that the crystal
contains small mosaic blocks which are misoriented to each
(b)
(a)
FIG. 6. (a) Grown ingot and (b) cut and polished piece of BMZ crystal grown
from VDGF technique.
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Prabhakaran, Babu, and Ramamurthi
Rev. Sci. Instrum. 84, 083907 (2013)
120
(a)
60
(+,−,−,+)
80"
90
60
(b)
50
Transmittance %
Diffracted X-ray intensity [c/sec]
70
BMZ
MoKα1
437"
33"
30
40
30
20
10
25"
0
0
-600
-300
0
300
100
600
200
300
400
500
600
700
800
900
Wavelength (nm)
Glancing angle [arc sec]
FIG. 7. (a) Diffraction curve and (b) transmittance spectrum of BMZ crystal grown from the VDGF technique.
other by few arc minutes. The other two sharp peaks depict
internal structural low angle boundaries24 whose tilt angle is
80 arc s. It is due to the self-seeding procedure followed in
this growth instead of proper seeding. The FWHM (full width
at half maximum) of these low angle boundaries are 25 and
33 arc s. These low FWHM values reveal that quality of the
grown crystal is reasonably good. Optical quality of the grown
BMZ crystal was estimated by the optical transmittance studies. The polished sample of 1 mm thick was used to record
the transmittance spectrum in the range of 200–1100 nm using UV-1700 Shimadzu spectrometer. Figure 7(b) indicates
that the grown crystal has about 60% optical transparency in
the visible region.
IV. CONCLUSION
Vertical dynamic gradient freeze system for the growth
of organic single crystals was designed and constructed. The
function and operation of major constituents of the designed
system such as eight zone furnace, PLC based temperature controller, microstepping translation assembly, and CCD
video camera are clearly described. Single crystal of benzimidazole was grown to demonstrate the potential application
of the designed system. The grown BMZ crystal has good
structural and optical quality. Multizone, ease of programming the thermal profile and in situ observation of thermal
profile, growth, and growth history have made the designed
system as the versatile one for the growth of organic single crystals. Growth of potential organic single crystals of
4-aminobenzophenone and phenothiazine is in progress. The
results will be discussed in our next communication.
ACKNOWLEDGMENTS
The authors hereby gratefully acknowledge the Department of Science and Technology, New Delhi, India for the fi-
nancial support under the research grant SR/FTP/PS-41/2007.
Authors acknowledge Dr. G. Bhagavannarayana, National
Physical Laboratory, New Delhi for HRXRD analysis.
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