The Effect Of Environment In Microfluidic On Crystallization And Phase Transformation Of ACC

As to the reactive crystallization process of calcium carbonate,the emergence of metastable intermediates ACC make it challenging for the traditional experimental methods to capture metastable intermediates,and they are the key to uncover polymorphic crystal formation process and the transformation mechanism.Moreover,to understand the mechanism has important significance for the development and design of advanced materials.In this thesis,from the environmental phase aspect of crystal growth,we have demonstrated a microfluidic approach toward the study on the formation and transformation mechanism of ACC based-on“Y-shaped”microfluidic chips,models about the evolution of concentration and supersaturation profiles on the chip have been established through COMSOL simulated calculation assist experiment,and the characteristics of environmental phase in microfluidics and its influence mechanism on ACC were analyzed through online monitoring and off-line characterization analysis.Firstly,a platform for microfluidic crystallization experiment was built,two types of chip were designed,one is reversibly sealed chip and the other is irreversibly sealed chip,with all microchannels have a uniform section of 200?m?width?×100?m?height?.Through the simulation of microfluidic module in COMSOL,we established the evolution of concentration distribution model of ions at different velocity in microchannel.The calculation results show that concentration gradient is higher in microchannel at low flow rate?0.1?l/min?for CO₃^2-,which has smaller diffusion coefficient,while uniform ion distribution is achieved across the channel for Ca²⁺.However,this difference of concentration gradient will become smaller with increasing flow rate.The crystallization experiments of CaCO₃ in microfluidic at different flow rates also indicate that CaCO₃ crystals tend to crystallize at the CO₃^2-side at a low flow rate,but crystal formation is mainly on the contact interface of the two miscible fluids when the flow rate is increased.The crystals formed on chip were taken for further off-chip characterization by SEM and Raman spectra.A mixture of cubic,rhombohedral,and spherical CaCO₃particles were observed,but the ratio of vaterite to calcite are obviously different at different positions in microchannel.The results show that vaterite is the dominant polymorph near the Y-junction,while calcite is the main polymorph near the channel outlet,and the ratio of vaterite to calcite was approximately 1:1 in the middle away from the Y-junction.Thus,the result demonstrated the effect of diffusion-controlled processes on the crystallization behavior of calcium carbonate from the aspect of time and space,which is an influence mechanism of space-for-time.Results of the influence of environment phase on ACC in microfluidics show that ACC doesn't always appear in the crystallization process of calcium carbonate,but there is a critical concentration.We found that the crystalline CaCO₃ phase appears after the amorphous phase when the concentration of reactants is above 6 mM,but when the concentration of reactants below 6 mM,the crystalline CaCO₃ formed after a period of crystallization induction time without metastable intermediates ACC appeared.Researches on the transformation mechanism of ACC in microfluidics show that intermediate ACC not only maintain the concentration level of solution near CaCO3crystals through the mechanism of dissolution-recrystallization,to promote crystal growth,but ACC eventually transformed to CaCO₃ crystals.Furthermore,the dissolution rate of ACC can also be infered from the growth rate of calcium carbonate crystals.Our work not only provides a new method for the study of metastable intermediates of polymorphs,but also our results provide experimental evidence for the one-step nucleation and multistep nucleation mechanisms.