JOURNAL OF MATERIALS SCIENCE LETTERS 4 (1985) 746 -750 Non-steady state nucleation process in KDP solutions in the presence of X04 impurities M. SHANMUGHAM, F. D. GNANAM, P. RAMASAMY Crystal Growth Centre, Anna University, Madras 25, India Nomenclature AG Energy of formation of nucleus. a Interfacial tension of crystal. r Radius of sphere inscribed in the crystal nucleus in equilibrium with solution. Slope oftine plot In ~" against 1/ln 2 (X/Xo). Avogadro's number. Gas constant. Temperature (K). Molar volume of crystal. Mole fraction of solute in the supersaturated salt solution at temperature t. Mole fraction of solute in the salt solution saturated at temperature t. m N R T V x x0 Nucleation is the initial and important phenomenon in liquid-solid phase transition. The number of nuclei forming in a supersaturated medium within a time interval is a random quantity. The time of formation of the first nucleus is also a random quantity. Based on the probabilistic approach to the nucleation process, the steady-state nucleation rate is inversely related to the induction time or the non-steady state time lag, the time of formation of first nuleus [1]. After preparing supersaturated solutions there is often a period where no phase change can be observed, the induction period; then minute nuclei appear and grow into visible crystals. Classical nucleation theory can be applied to evaluate certain parameters of the resulting crystal- line solid. The rate of homogeneous nucleation (J) is indirectly computed by measuring induction period (T) and taking J c~ z -1 [2-5]. The presence of impurities affects the nucleation rate in super- saturated solutions [6-12]. Many surface prop- erties in the solution are also affected due to the presence of impurities. In the present study, the induction period has been measured for super- saturated potassium dihydrogen orthophosphate solution in the presence of XO4 (X = Cr, S, C1, Mn, I, V) impurities. The experimental method to 746 measure the induction period is simlar to that used by Mullin and Osman [13] and Joshi and Antony [14]. If the interfacial tension a between a solid and a saturated solution is considered. The concept of cr is not so well established [15] and o can not be determined by direct unambiguous experiments in most cases. Despite these difficulties, a plays an important role in all theoretical expressions describing the rates of crystal growth and nucleation. The interfacial tension can be inferred from the nucleation kinetics. Though o values derived from nucleation and crystallization of a solvent-solute system can be questioned as to their true significance, they are fully justified for use in crystallization [16]. It is expected that dif- ferent solids would cause nuclei to form at different temperatures [17-19] for constant induction time and certainly at temperatures different from those when there are no solids present. This may be looked upon as being dependent upon the energy of formation for nucleation [20]. If the energy of formation is smaller, there will be a greater tendency for nucleation to occur in the solid. The induction period can be used to evaluate the interfacial tension of the crystal relative to the solution, the energy of formation and the radius of critical nucleus [ 12]. The interfacial tension of the solid relative to its solution has been calculated from the slope of the line in r against 1/ln z (X/Xo) as [3m11/3 c~ = RT ~167rV2N ] (1) The energy of formation of a critical nucleus has been estimated from the experimental data as RTm 2xa - In 2 (X/Xo) (2) The radius of the nucleus in equilibrium with its solution has been given by 0261-8028/85 $03.00 + .12 © 1985 Chapman and Hall Ltd.