Structure Regulation Of Herarchical Micro-/Nano-scale Fibers And Their Performance Of Air Fitration

In recent years,air pollution has become more and more serious due to the accelerating process of global industrialization and urbanization.Particularly,fine particles pollution with aerodynamic diameter less than or equal to 2.5?m?PM2.5?,which can get into the lung easily,has drawn a lot of attention.High concentration of PM2.5 in atmosphere can not only cause haze weather but also seriously threaten to people's health.PM2.5 with toxic and harmful substances can suspend in air for a long time due to its small diameter.PM2.5 can cause asthma,bronchitis,cardiovascular disease,congestive heart failure,pneumoconiosis,and even lung cancer once it directly or indirectly enters bronchus,alveolar,and blood of human beings.Therefore,it is a major issue to develop filter material that can effectively removal fine particles in atmosphere to improve the environmental quality and protect human health.Whereas,conventional fiber filter materials usually show low filtration efficiency for ultrafine nanoparticles and are incapable of achieving high filtration efficiency with low pressure drop.Electrospun nanofiber membrane possesses many advantages to be an efficient filter,including small diameter,large specific surface area,small pore size,high porosity,and good internal pore connectivity.Hierarchical micro-/nano-fiber exhibited controllable surface morphology,abundant pore structure,and higher specific surface area and pore volume,which allow higher filtration efficiency to ultrafine particles with low pressure drop.Thus,electrospun hierarchical fiber membranes have great and wide potential applications in air filtration.This work focused on the weaknesses and limitation of electrospun fiber air filter materials.We developed many kinds of hierarchical organic,hybrid organic/inorganic,and inorganic fiber materials to investigate the relationship between the structures of fiber filter and its air filtration performance.Efficient air filtration materials with high filtration efficiency and low pressure drop would be developed in this thesis,so that they can play a role in helping solve the air pollution,especially fine particulate matters.Meanwhile,we also investigated the filtering mechanism of electrospun hierarchical fiber with micro-/nano-structures for fine particles to provide theoretical support for their practical application.In addition,three-dimensional?3D?model of hierarchical fiber filter was built by CREO and the numerical simulation on filtration performance and filtering process of3D filter model was also performed using ANSYS Fluent analysis software.The detailed contents and major findings are summarized below:?1?Porous bead-on-string poly?lactic acid??PLA?nanofiber was successfully fabricated by electrospinning.The morphology of PLA nanofiber can be tuned by regulating the concentrations and solvent compositions of PLA solutions and the relative humidity.The results showed that the bead-on-string structure is highly depend on the viscosity and conductivity of solutions and nanopores on bead are mainly caused by the phase separation and“breath figure”of solution jet during electrospinning process.The beads were decreased,and nanopores on beads and fiber diameter between beads were both increased as the increasing of solution concentration.The beads and nanopores on beads were both increased with the increased of high volatility solvent content or the relative humidity.The effect of the morphology of porous bead-on-string PLA fiber membranes on their specific surface area,through pore size strucutre,and air filtration performance was also studied.The results showed that the beads can effectively regulate stacking form of fibers,increase the distance between fibers,and tune pore structure of fiber membrane,thus reducing pressure drop;the nanopores on beads can increase the specific surface area of fiber membrane,thus improving filtration efficiency.?2?Porous PLA fiber?PLA-P?was prepared,and the effect of solution concentration and the relative humidity on the morphology and property of PLA-P was studied.The results shown that the fiber diameter,nanopores coverage density on fiber surface,and nanopore size were all increased with the increase of solution concentration or the relative humidity.The increase of nanopores on fiber surface can remarkably improve the specific surface area,pore volume,and air filtration performance of PLA-P fiber membrane.Then interlaced structured PLA-P/PLA nanofiber?PLA-N?fiber membranes with different mass ratios were developed.The effect of the morphologies of PLA-P/PLA-N on their through pore structure and filtration performance were also investigated.The PLA-P/PLA-N membrane with mass ratio of 6/1 exhibited the best filtration performance.The composite membrane presented a deep bed filtration mechanism,which can not only improve filtration efficiency but also facilitate the airflow through filter medium.Too much PLA-N in PLA-P/PLA-N membrane leaded to the intensive accumulation of particles on the surface of composite membrane,which blocked the pores,thus resulting in high pressure drop.?3?Hybrid PLA/TiO₂ fiber with multilevel structures was prepared via electrospinning PLA solution with an addition of TiO₂ nanopaticles.The effect of TiO₂concentration and the relative humidity on the distribution of nano-protuberances and nanopores on fiber surface was studied.It has been demonstrated that more nano-protuberances formed with the increase of TiO₂ concentration of precursor solution,while too much TiO₂ leaded to the formation of micro-scale agglomerate on fiber surface.The nanopores were increased,while the nano-protuberances were decreased on fiber surfaces as the increasing of the relative humidity.The increase of nanopores and nano-protuberances on fiber surface can significantly improve the specific surface area and pore volume,and regulate through pore structure of hybrid fiber membrane.In addition,the effect of the distribution of TiO₂ nanoparticles and nanopores the air filtration performance,photocatalytic activity,and antibacterial activity of hybrid PLA/TiO₂ fibers were also investigated.When the TiO₂ concentration was 1.75 wt%and the relative humidity was 45%,the resultant hybrid PLA/Ti O₂ fiber membrane exhibited the best air filtration performance and excellent photocatalytic activity and antibacterial activity,suggesting its potential applications in multifunctional air filtration materials.?4?Self-standing carbon nanotubes-grafted multichannel carbon fiber membrane was fabricated through electrospinning followed by calcination.Based on the structure control and the analysis of the role of each component and treatment process,the possible formation mechanism of the hierarchical carbon fiber was proposed.The effect of the calcination temperature and the additive amount of melamine on the micro-/nano-structure of carbon fiber was studied.The results shown that the CNTs grown well at high calcination temperature catalyzed by metallic cobalt?Co?nanoparticles.More CNTs formed on multichannel carbon fiber surface with the increase of the mass ratio of melamine and fiber membrane,when the calcination temperature was 1000?.The introduction of Co nanoparticles and CNTs on carbon fiber can improve its specific surface area,and the CNTs on carbon fiber surface would reduce the through pore size of carbon fiber membrane.The filtration performance of different hierarchical carbon fiber membranes was measured,and their filtering mechanism was also investigated.When the calcination temperature was 1000?,and the mass ratio of melamine and fiber membrane was 12/1,the resultant hierarchical carbon fiber membrane exhibited the best air filtration performance.The CNTs grafted multichannel carbon fiber membrane presented a deep bed filtration mechanism,which can not only improve filtration efficiency but also prevent too much particles depositing on fiber membrane surface,thus resulting in excellent air filtration performance.This provides a novel route to expand the application of carbon fiber in air filtration.?5?3D model of porous fiber filter and bead-on-string fiber filter were built by ANSYS Workbench DesignModeler and CREO,respectively.ANSYS Fluent analysis software was used to simulate the velocity field and pressure field of the model,and its collection efficiency for particles with different diameters was also evaluated.In addition,particle trajectory tracking with different diameters was also performed in the model.This work provides theoretical support for the structure optimization,filtering mechanism investigation,and numerical simulation of hierarchical fiber filter materials.