The Study Of Alumina Composite Ceramic Microfiltration Membranes

Al₂O₃-SiO₂-ZrO₂-TiO₂ composite asymmetric membranes were prepared by sol-gel method in this study, some measuring and testing techniques such as TG-DTA,XRD,FTIR,SEM,AFM had been used to analyse and characterize composite membranes on phase composition, structure changes during heat-treatment process, microstructure,elements distribution were measured by EPMA.The relationship among the performance and composition and microstructure had been anlaysed,the sol preparing and film forming mechanism had also been studied.Al₂O₃ ceramic substrates with different porosities had been prepared by dry mill and wet mill.Used α-Al₂O₃,SuZhou kaoline clay and calcined SuZhou kaoline clay as raw materials,and some coal powder were added to form pores,and fired at 1350°C for lh.The average of particle sizes were smaller used wet mill process are smaller than dry process,and size distribution are narrower.At the same firing temperature, poroity of substrates with wet process is smaller than dry process.Al₂O₃-SiO₂-ZrO₂-TiO₂ composite sols were synthesized by fractional hydrolyze used Aluminum nitrate,ethyl orthosilicate,zirconium oxychloride, titanium ethoxide as starting materials. Boehmite sol was prepared by aluminum nitrate,ammonia and nitric acid were used as precipitant and peptizing agent.TiO₂ sol was prepared by citric acid method.citric acid used as chelator to chelate metal ion and form soluble chelates,pH of the sol is 5.5⁶.0.The study found out that the pH of ZrOCl₂-8H₂O prehydrolyzed solution is below one,it not only can be used as peptizing agent instead of nitric acid or other peptizing agent,but also did not import impuritic ion.Composite sols with different ratio of Al:Si:Zr:Ti were prepared by this method,the pH value is 6.0-7.0.Gel film was coated on the substrate by dip-coating method, crackfree Al₂O₃-SiO₂-ZrO₂-TiO₂ ceramic membranes were made by controlling some processing parameterS, Such as the addition of polyvinyl alcohol solution,the viscidity of composite sol,lifting rate,coating times, drying and heat treatment processes.It was found that cracks could be reduced effectively during drying process by stepped drying technology.Repeated coating-drying several times and then calcined at 1000°C for lh,crackfree and more compact membranes could be made by this process.Changing the ratio of Al:Si:Zr:Ti sol,the composite membranes had different main crystalline phases.When the ratio is 4:1:1:1,the main crystalline phases are ZrTiO₄ and 3Al₂O₃-2SiO₂,for 2:1:1:1,the main crystalline phases are ZrO₂ and 3Al₂O₃-2SiO₂.From SEM micrographs of surface of membrane,there was a crackfree and compact layer with narrow pore size distribution on the surface of substrate, a gradient layer could be seen from the fractured surface,while the thickness of the composite membranes is l²um.The surface was not smooth through the AFM images,it can be seen from section analysis that the diameter of the surface pore is 1.613pm and the vertical distance is 1.012um.From EPMA,some compounds including Al,Si,Ti,Zr and O elements have been found on the surface of the membrane,and from the fractured surface tests, sol particles had permeated into substrate.Through line scanning analysis,composite membranes are formed on the surface,no interface can be seen between substrate and composite membra-nes,chemical bonds are formed in order to intensify the associativity.