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Department of Physics, Bharath Institute of Higher Education And Research, Chennai â 600073, India
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Theoritical investigations have been made to calculate the nucleatiion thermodynamical parameters like interfacial energy,critical energy barriers and nucleation rate of L-Arginine tetrafluroborate using the solubility data and applying the classical nucleation theory . in the present study both classical nucleation theory and modified classical nucleation theory have been employed to study the nucleation parameters of L-AFB crystal using various solvents water, acetone & ethanol. A comparative study has been made with respect to the solvents and the results are analyzed the successful growth of large size crystals of good quality L-AFBby gradual temperature lowering technique using suitable solvent, requires knowledghe of fundamental nucleation parameters that influence the growth in super saturation. In other words optimum crystallization processes can only be accomplished if proper super saturation level and suitable solvent chosen during the growth technique is decided by the theoretical estimation of nucleation parameters based on the classical and present theory. Results are dissussed to understand the growth kinetics of L-AFB crystals from low temperature solution growth .
|L-AFB, nucleation parameters, NLO materials, interfacial energy, solubility data, low temperature growth.|
|Recent research has concentrated on semi organic materials due to their large nonlinearity high resistance to laser induced damage ,low angular sensitivity and good mechanical hardness .L-AFB is an organic –inorganic NLO material which not only possess the high optical non-linearity of a purely organic compound but also the favorable thermal and mechanical properties of an inorganic compound. L-AFB crystallizes in orthorhombic space group P212121 with Z=4. The molecular formula of L-AFB is C6H13N4O2BF4. The unit cell lattice parameter a=13.728,b=16.447,c=5.062,c=5.062 and = 90, = 90, = 90 and cell volume 1143. Th ebulk single crystal of L-AFBis one of the potential material through out the visilble and UV spectral region. The dipolar nature exhibits peculiar physical and chemicl p0roperties in amino acids, thus making it ideal candidate for NLO applications. LAFB is a new semi organic NLO material grown from solution by temperature lowering method. The solubility curve of L-AFB has been determined using different solvents to study the nucleation kinetics of this semiorganic material. The two kinetics steps, nucleation and crystal growth dominate the production process of crystalline material. With regards to characteristics of the material , nucleation is the first of the two kinetic steps usually has a strongly predetermined influence on the second step crystal growth .in the present study, the interfacial energy of the crystal –solution system has been calculated based on the regular solution theory. The nucleation is assumed to occur well within the mother phase and therefore the theory of homogeneous nucleation is applied in which the nuclei of the condensed phase in solution are considered to have spherical shape. The classical nucleation theory makes use of capillarity approximation in which the physical properties such as interfacial energy density chemical potential etc., are assumed to be constant throughout the process from micro to macro level.|
|In the present work an attempt has been made to apply correction in the interfacial energy . an expression for the interfacial energy as function of size of the nucleus has been derived using the first principle. The corrected interfacial energy is used to calculate the nucleation parameters of L-AFB nucleus for various solvents and comparative study has been made a view to achieve good quality product of crystallization processes. 1. Interfacial energy: The surface energy of the interface between a solid crystal and the surrounding saturated solution is a critical parameter which plays a major role in determining the rate of nucleation and growth of the crystal. Based on regular solution theory, Sangwal has derived an expression for the interfacial tension as|
|Where d is the interionic distance(6⁄) , is specific volume, is the mole fraction of the solute, T is the absolute temperature in Kelvin and k is the Boltzmann constant. The above expression is used in the present study to calculate the value of interfacial energy from the knowledge of the existing solubility data.|
|2. Nucleation kinetics The total free energy of a crystal in equilibrium with its surrounding at constant temperature and pressure would be minimum for a given volume. since the volume free energy per unit volume is constant, then|
|Where the area of the ith is face and is its surface energy per unit area. The critical nucleus will not have a regular morphology of the crystal since it normally contains few tens of atoms or molecules, under such condition the spherical is assigned to the nucleus. 3. Classical nucleation theory : When the crystal nucleus form due to super saturation of the solution a certain quantity of energy is spent for the creation of new phase. The free energy change associated with the formation of the nucleus can be written asrgy|
|The nucleation is assumed to occur well within the mother phase and therefore the theory of homogeneous nucleation is applied in which the nuclei of the condensed phase in solution are considered to have spherical shape. The classical nucleation theory makes use of capillarity approximation in which the physical properties such as interfacial energy density chemical potential etc., are assumed to be constant throughout the process from micro to macro level.|
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