A Liquid Precipitation Method For Preparing Monodispersed Nanoparticles And Its Crystallization Kinetics

Aiming at the problems controlling particle size and particle size distribution in preparation of nanopowder in chemical liquid precipitation method, and on the basis of the analysis and comparison on the relative rate of micromixing, precipitation reaction and nucleation process, the characteristic time of supersaturation distribution homogeneity of aim product was introduced, the analysis was made on the influence of operation variables on the supersaturation distribution homogeneity of aim product, and the feasibility of supersaturation homogeneous distribution of aim product was explored by prolonging nucleation induction time. A new method was put forward for preparing monodispersed nanopowder using chemical precipitation method, in which nucleation induction time was prolonged by adding inhibiting agent, and nucleation rate was increased by adding precipitation releasing agent,and crystal growth and agglomeration were inhibited by means of the action of nucleation inhibiting agent. Preparations of strontium sulfate and calcium sulfate nanopowder were taken as the examples to demonstrate the present preparation method. The proper technical conditions preparing monodisperse nanopowder of the studied systems were determined by means of single factor test method and multi-factor orthogonal test procedure The resultant crystal form, purity and morphology were observed by TEM and XRD, and the particle size and particle size distribution were measured by multi-angle sub-micron particle size analyzer. The experimental results showed that the particle size distribution of strontium sulfate and calcium sulfate nanopowder prepared in this work was very narrow and powder morphology was identical. The mechanism of the effect of EDTA and ethanol on nucleation and growth during the preparation of strontium sulfate nanoparticle was approached according to LaMer chart obtained under different operation conditions and observations in single factor tests. Under the proper pH, the function of added EDTA was to inhibit nucleation and control the crystal growth of strontium sulfate due to the formation of the surface complex of EDTA with Sr²⁺ and the adsorption of the complex on the surface of molecules, critical nucleus and growth crystal of strontium sulfate. The addition of ethanol into solution resulted in increasing the nucleation driving force, thus increasing the nucleation rate owing to the solubility of strontium sulfate in ethanol-water mixed solvent system was smaller than that in aqueous solution. The influence of added quantity of EDTA and ethanol on the nucleation kinetics was presented. On the basis of analysis on the variation of exponential term in the classical nucleation rate expression with supersaturation ratio, a modified expression for nucleation rate was suggested, which can account for the effect of reactant concentration, and the quantity of added ethanol and EDTA on nucleation process. Moreover, this modified expression can reflect the existence of critical supersaturation for homogeneous nucleation. The crystal growth kinetics was studied using constant composition method, in which the effect of the quantity of added EDTA and ethanol on crystal growth rate of strontium sulfate particles was considered, and the expression of the crystal growth rate in the presence of impurity was established. The calculated results were in good agreement with the experimental results of crystal growth rate of strontium sulfate using the present growth rate model. The discretization form of population balance equation was used for modelling the crystal size distribution of strontium sulfate in combination with the crystallization kinetic models of strontium sulfate in this work, and the influence of the crystal growth time and the quantity of added EDTA and ethanol on crystal size distribution of strontium sulfate powder was examined. The modeling results indicated that the calculated results were in good agreement with the experimental results of crystal growth rate of crystal size distribution of strontium sulfate.