Preparation And Characterization Of Al₂O₃ Ceramic Hollow Fiber Membrane

Compared to organic polymeric membranes,ceramic membranes have some distinct advantages,such as better thermal and chemical resistances,high mechanical property,controllable microstructure,and could be used in the harsh environments.However,conventional disk-shaped and tubular ceramic membranes usually show smaller surface area to volume ratio and lower separation efficiency,which greatly limits the application of the ceramic membranes.Recently,ceramic hollow fiber membranes have attracted considerable attention due to their larger packing density and higher permselectivity.There are many methods used for preparing ceramic hollow fiber membranes,among these,the combination of phase inversion and high-temperature sintering technique is a useful and facile way to obtain the ceramic hollow fiber membranes.The process is composed of dry or wet spinning a polymer solution containing ceramic powders and subsequently sintering the spun fibers at high temperature.The ceramic/polymer/solvent system can be seen as a suspension of polymer coated ceramic particles.Once immersed in a non-solvent for the polymer,solvent/non-solvent exchange takes place leading to precipitation of the polymer phase with the ceramic particles immobilized in it.After sintering at high temperature,the ceramic hollow fibers with asymmetric pore structures can be obtained.In this thesis,Al2O3 hollow fiber membranes were prepared via wet spinning and high-temperature sintering technique by using polyethersulfone(PES)as a polymer binder,N-methyl-2-pyrrolidone(NMP)as a solvent.The effects of Al2O3 content,composition of coagulating bath,sintering temperature,size of Al2O3 powder and SiO2 sintering additive on structures,morphologies and properties of the Al2O3 hollow fiber membranes were studied.Moreover,using the a-Al2O3 hollow fiber membranes as substrates,AIOOH-Al2O3 composite membranes were prepared by dip-coating method.The influencing factors on the membrane process were studied.The main results obtained are as follows:(1)The Al2O3 content in the spinning dopes played an important role in determining the microstructure and mechanical strength of the membrane.The increase of Al2O3 content would increase the viscosity and inhibit the phase inversion behavior,which influences the size of the finger-like structure and the thickness of the sponge-like structure.The increase of the Al2O3 content reduced porosity of the membrane.The bending strength of the membrane enhanced as increasing the Al2O3 content in the range of Al2O3/PES ratio from 4 to 6.When the Al2O3 content is too high,the bending strength declined instead.(2)The addition of solvent NMP into the coagulating bath(water)decreased its coagulation ability and thus reduced the rate of phase separation process.When the concentration of NMP in coagulating bath was lower than 20%,the sandwich structure containing finger-like structure near the outside and inside surface of the fiber and sponge-like structure in the core of the fiber could be found.However,when the NMP concentration further increased,the finger-like structure disappeared,dense sponge-like structure was obtained.The increase of NMP concentration in coagulating bath would increase the porosity and decrease the bending strength of the hollow fiber membrane.Furthermore,denser sponge-like structures could be formed in Al2O3 hollow fiber membrane when using weak non-solvent ethanol in instead of water.(3)Raising sintering temperature led to the increasing of bending strength and the declining of porosity of the membrane.As increasing sintering temperature,Al2O3 grain bulk density increased and the voids between particles reduced,and the particles bonded more compactly.When the sintering temperature was 1450?,the hollow fiber membrane showed high bending strength about 24.4 Mpa.(4)The effects of the introduction of nanometer Al2O3 powder on the structures and mechanical properties of the hollow fiber membranes were studied.The results showed that nanometer particles promoted the Al2O3 particles cohering closely and forming a network structure,which improved the mechanical properties of the membranes significantly.When the amount of nanometer particles was 5%,the bending strength increased from 24.4 Mpa to 33.6 Mpa.(5)The sintering additive SiO2 has a great influence on the bending strength and the line shrinkage of the membrane.With the amount of SiO2 increasing,bending strength and relative density of the membrane increased rapidly.During the sintering process,the amorphous part of SiO2 formed liquid phase,filled the voids between alumina particles,and connected the solid Al2O3 particles.At the same time,SiO2 and Al2O3 formed tiny mullite crystalline phase,which can also increase the bonding between the Al2O3 particles.(6)Boehmite sol was prepared from the hydrolysis of AIP.Using the Oa-Al2O3 hollow fiber membrane as a substrate,the AIOOH-Al2O3 composite membranes with smooth and denser surfaces were prepared by multiple dip-coating processes.The addition of PVA could improve the membrane-forming property and increased the thickness of the membrane.