dc.description.abstract |
The growth of thiourea single crystals by the solution method has been
investigated. The crystals were grown from aqueous solution by slow cooling
technique. The influence of pH on the metastable region of thiourea solution
has been studied. The results indicated that the growth rate of the thiourea
crystal along the c-direction [001] was faster if the pH is increased. The stability
of the saturated solutions with different pH values of the solutions has been
assessed by measuring the metastable zone width by a nucleation method. To
improve the quality of the thiourea crystal, urea (an organic compound) with
different mole concentrations (viz. 5 M%, 10 M%, 20 M% etc.) was added as
impurity to see the influence on the growth kinetics and habit modification. The
grown crystals were characterized by FT-IR spectroscopy, UV spectroscopy
and powder X-ray diffraction. FT-IR and UV spectra provided information about
the presence of functional groups and compound.
In the FT-IR spectrum, the corresponding =CH, =CH, C=S, N-C-N, COH
stretching modes of vibrations and NH2 group deformation at different wave
numbers are observed. Optical absorption spectra were recorded in the
wavelength region from 200 to 11OOnmusing Shimadzu-160 spectrometer (UVVIS-
NIR). In determining the range of optical window and percentage of
transmission C-cut crystal plates with 3mm thickness was used. The UV
absorption peaks were observed at 244nm and 349nm and absorption rates of
0.252 and 0.029 are observed. The graph suggests that the minimum
wavelength is approximately 360nm for the grown thiourea crystals.
Powder X-ray diffraction spectra were recorded using CuKa radiation. The
reflections were indexed using the conventional procedures and lattice
parameters were calculated. The values are found to be in good agreement
with ASTM standard data.
The micro-hardness studies have been performed on thiourea single
_ crystals. The Vickers micro-hardness values are found to decrease with the
increase of applied load. The work-hardening index 'n' is calculated from the
plot drawn between log d and log p and the value is found less than 2 (slope of
the line) for all the thiourea samples which reveals that the Vickers hardness
decreases and indicating the softening nature according to Mayer's relation.
The growth features on the surface of thiourea crystals are observed by
etching study. It is known that the surface morphologies are associated with
cleavage, deformation and surface preparation process, clear surface edge
with growth striation. A hexagonal growth spiral is observed on the [001] faces
of thiourea crystals.
Dielectric properties of the grown crystals were investigated by two-probe
method at room temperature using an impedance analyzer. The dielectric
constant, dielectric loss and ac electrical conductivity were measured at room
temperature in the frequency range of 50 kHz to 10 MHz. It is observed that the
dielectric constant gradually decreases as the frequency increases. A.C
conductivity gradually increases with increasing frequency. This may be
attributed to space charge polarization
I-V characteristics on the grown thiourea crystals with different thickness
were studied at room temperature, to explore the behavior of the contacts. The
results reveal that current increases non-linearly with the voltage.
D.C electrical conductivity was measured on the grown crystal at room
temperature by using the conventional two-probe method. The non-linear
variation of D.C conductivity is expected due to the enhanced diffusion of
charge carriers along dislocations and grain boundaries. |
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