Preparation And Carbonization Of Polyacrylonitrile Microspheres Based On A Co-flow Microfluidic Device

Carbon spheres not only can be applied in the common fields of adsorption,catalysis,biomedicine and electrode capacitive materials due to its unique packaging properties,large specific surface area,low density,chemical stability,good electrical conductivity and biocompatibility.In addition,it can also be used as a container for deuterium-tritium fuel in extreme conditions such as laser inertial confinement fusion（ICF）,laboratory astrophysics and strong field physics,which makes the development of carbon spheres become the focus.The different size and morphology of carbon spheres are required in different application fields.For example,the increase of the monodispersity and crystallinity of porous carbon spheres can increase the battery capacity,and thus improve the electrochemical performance of anode materials in lithium-sulfur batteries.In the catalytic and adsorption process,the carbon sphere with high porosity can increase the specific surface area,which is beneficial to improve the adsorption and catalytic effect.In ICF experiments,carbon spheres with different geometric sizes（covering micron to millimeter level）and special structure are required.However,the traditional preparation methods of carbon spheres have the problems of complex operation,difficult size control,uncontrollable morphology and low crystallinity of carbon.In view of this situation,this article developed a precursor-based microfluidic technique for the preparation of carbon spheres,and the construction of polyacrylonitrile（PAN）compound droplets.The main research work and results are as follows:（1）PAN microspheres can be used as fuel containers for ICF experiments.In order to reduce the instability of the implosion process,the monodispersity of the microspheres is required.Moreover,the monodisperse PAN compound droplets are the basis of the uniform size of PAN hollow microspheres.The monodispersity of the compound droplets is related to the constructing method and their stability.In this paper,a“one-step”co-flow microfluidic device was established to adjust the appropriate flow rate so that the flow pattern formed by the compound droplets was controlled to be a trickle flow pattern,and the monodisperse compound droplets could be prepared from 800μm to 1500μm.Subsequently,Dow Corning749 are optimized to inhibited the rupture and coalescing behavior of compound droplets,making them ready for curing process.（2）PAN microspheres were obtained by curing and drying of PAN compound droplets.The curing stage of PAN compound droplets is a process in which the DMF volatilizes,the concentration of PAN gradually increases,and the shell hardens.Dow Corning 749 inhibited the instability behavior of the compound droplets,but the rapid mass transfer rate made the cured PAN microspheres ellipsoidal and wrinkled with holes on the surface.Additional DMF droplets were added to the O2 phase to eliminate the negative effects of Dow Corning 749 by delaying the mass transfer rate.As a result,the sphericity,wall thickness uniformity and surface quality of PAN microspheres were improved.When the dosage was 2.0g,the microspheres had the best quality,smooth surface and sphericity above 97%.In addition,the microsphere shell layer was relatively thin with tens of microns.During drying,the droplets and atmospheric pressure in the microsphere produced corresponding forces on the shell,which changed the morphology of the microsphere,and the microsphere obtained by supercritical drying was better than other drying methods.（3）Graphite carbon spheres have the excellent properties of both graphite and carbon spheres,and has a broader application prospect.Due to the properties of PAN,it is necessary to introduce catalysts into PAN microspheres to transform the carbon structure from disordered state to graphite carbon at low temperature.PAN/M（M=Fe,Co,Ni）compound droplets were successfully obtained by adding transition metal catalyst（Fe Cl₃,Co Cl₂,Ni Cl₂）in the W phase during the construction of PAN compound droplets,and then PAN/M hollow microspheres were obtained.The three catalysts increased the amount of carbon residue of PAN and showed different catalytic graphitization effects,among which Ni had the best catalytic graphene effect.At present,the dissolution-precipitation mechanism and the metal carbide formation-decomposition mechanism can be used to explain the catalytic graphitization process.Other researchers have carried out molecular dynamics and density functional simulations for catalytic graphitization,but the detailed atomic scale interaction mechanism between transition metal and polymer in the process of carbon formation is still not fully understood.The PAN compound droplets were successfully prepared by a co-flow microfluidic device,and high-quality graphitized microspheres were obtained by optimizing curing,drying and carbonization processes.This study provides relevant experimental guidance for the construction and stabilization of carbon precursor compound droplets and the regulation of carbonization process in spherical space,which is of great significance to enrich and develop the interdisciplinary disciplines of microfluidic,catalytic chemistry and polymer processing.