Research On The Reactive Crystallization Process And Morphology Control Of Cerium Carbonate

Ceria is widely used in many areas due to its unique optical,electrical and magnetic properties.Cerium carbonate is one of the important precursors for the preparation of ceria.The crystallization process of cerium carbonate affects the properties of cerium carbonate and the calcined products,such as particle size,morphology and specific surface area.In this work,the mechanism of the reactive crystallization of cerium carbonate was investigated,and the conditions affecting cerium carbonate crystal morphology was examined.On this basis,cerium carbonate crystals of varied morphology were obtained by controlling the crystallization process,and the formation process of the resulting morphology crystal products was explored.The main contents are summarized as follows:The amorphous cerium carbonate and cerium carbonate crystals were characterized by Xray powder diffraction(XRD),Fourier transform infrared spectroscopy(FTIR),Raman spectroscopy,thermogravimetry(TG)and Scanning Electron Microscopy(SEM).The difference between amorphous cerium carbonate and cerium carbonate crystals was analyzed.The results show that the Raman peaks of them are significantly different,and the cerium carbonate crystals have five Raman peaks more than that of the amorphous cerium carbonate.The composition of the amorphous cerium carbonate has a lower water content than that of the cerium carbonate crystals.In addition,amorphous cerium carbonate is mostly homogeneous nanoparticles,while cerium carbonate crystals are micron-sized flake crystals.The reactive crystallization process of cerium carbonate was monitored in real time by multiple process analytical instruments including a turbidity meter,an imaging system as well as a p H meter and offline analytical techniques.It was found that the reactive crystallization process of cerium carbonate was based on unstable amorphous materials as precursors,and then the amorphous precursors were dissolved-recrystallized to finally obtain crystalline products.The effect of crystallization parameters such as temperature,p H and solvent composition on the transformation of amorphous precursors into crystals was also studied.The results show that the induction time and the time needed to fully form the crystalline cerium carbonate are prolonged with the increase of temperature and initial p H after mixing.The addition of slender needle-shaped seeds can shorten the time needed to fully form the crystalline cerium carbonate.The addition of ethanol,isopropanol and n-butanol can prolong the induction time and the time needed to fully form the crystalline cerium carbonate,while the addition of ethylene glycol,propylene glycol and glycerol obviously promotes the formation of crystals.With cerium nitrate as the cerium source and sodium bicarbonate as the precipitant,without adding any additives,the cerium carbonate crystals with different branch morphologies were obtained by adjusting the process variables such as temperature,precipitant concentration and cerium nitrate solution concentration.In addition,the effect of different operating conditions on the product particle size was investigated.By analyzing the evolution of branch morphology,the growth process of cerium carbonate branch crystals was controlled.The number of branches increase with the rising temperature.As the feed rate and concentration of sodium bicarbonate solution decrease,the gaps between the crystal branches are gradually filled,and the crystal morphology develops from a vertical cross-shaped morphology with only one branch to rhombus-shaped morphology.Cerium carbonate flowerlike microspheres were prepared by chemical precipitation at low temperature and the corresponding ceria flowerlike microspheres were obtained after calcining.The optical properties of ceria flowerlike microspheres were also studied.Experimental results show that the formation process is that amorphous precursor with agglomeration were immediately obtained after mixing,then quickly turned into plate-like nanocrystals and were dispersed,finally the flowerlike microspheres were obtained by directed self-assembly.Temperature and solvent composition play a key role in the transformation of amorphous materials and the aggregation of nanosheets that make up flowerlike microspheres leading to the final crystals with morphology of flowerlike microsphere.It was found that flowerlike microspheres ceria have strong absorption effect on ultraviolet radiation.