Preparation And Separation Properties Of Highly Stable Hydrophobic Porous Ceramic Membranes

Ceramic membranes have found widely applications in wastewater treatment,gas separation,pharmaceutical and food processing.Compared with polymeric materials,ceramic membranes possess better chemical,thermal and mechanical stability.Most of ceramic membranes are naturally hydrophilic due to the presence of the surface hydroxyls,which however can be converted into hydrophobic by surface modification and applied for membrane distillation water desalination and oil/water separation.This thesis is intented to investigate the preparation of durable hydrophobic ceramic membranes and their applications in water desalination and oil/water separation,fabrication of super-hydrophobic self-cleaning surface with micro-nano hierarchical structure and then surface hydrophobic modification,design and fabrication of membranes with cassette structure.In Chapter 1,the research status of porous ceramic membrane is briefly described,in terms of materials and structure,preparation and properties.In particular,the membrane-based water desalination and oil/water separation processes and super-hydrophobic self-cleaning surface are introduced.Finally,the scope and content of the thesis are described.Chapter 2 is concerned with the preparation and characterization of asymmetric planar alumina membranes.The membranes were prepared by phase inversion tape casting/sintering method,in which graphite was used as sacrificing agent to remove the sponger layer or the skin layer.For the membranes free of the sponge layer,the skin layer could be used for separation.In contrast,for the membranrs free of the skin layer,its sponge layer could be used for separation.The former showed much higher gas and water permeability than the latter.The membranes in the green stage were also treated with hot-pressing.It was shown that with increasing the pressing pressure,the thickness and porosity of the membranes decreased,resulting in increased mechanical strength and decreased permeability.In Chapter 3,a durable hydrophobic surface on porous alumina membrane is explored.A polydimethylsiloxane(PDMS)film was applied to a porous alumina wafer,followed by pyrolysis at 400 ℃ in a non-oxidizing atmosphere(H2:N2= 5:95),giving rise to nanoparticles.In these particles,Si,C and O elements formed amorphous inorganic networks to which methyl groups were bonded.The as-modified porous alumina membrane was hydrophobic with a water contact angle of 136°,which was attributed to the presence of the methyl groups.The hydrophobicity was maintained after immersion in aqueous solutions in a pH range of 2-12 and acetone.The hydrophobicity was also retained after exposure to temperatures as high as 450 ℃ in an oxidative atmosphere(air)and after mechanical abrasion with sandpaper.In Chapter 4,the water desalination and oil/water separation properties of the hydrophobic planar membranes are investigated.Porous alumina membranes with thickness of 700μm,overall porosity of 58%,and averaged pore size of 0.8μm were prepared using phase inversion tape casting/sintering method.In a sweep gas membrane distillation(SGMD)process for water desalination,the membranes exhibited a long term stability of～90h and～40h using high concentrations NaCl and simulated seawater as feed solution,respectively.Besides,the salt rejection of over 99%was achieved during the whole test.The stable hydrophobic surface made these modified alumina membranes suitable for a MD process using feeds with high salt concentration,in which a zero-discharge of salt water desalination could be achieved.Porous alumina membranes with finger-like through-pores were applied to gasoline/water separation after modification with PDMS,and the separation efficiency reached 99.2%.Chapter 5 presents the fabrication and self-cleaning properties of super-hydrophobic alumina.A planar porous alumina wafer was coated with flower-like Al2O3 particles using hydrothermal method.The alumina particles had a size of～2μm,and consisted of areatus nano-sheets with a width of about 100nm.The surface of the alumina wafer was further modified with PDMS using the method described in the previous chapter.A water contact angle of～170° was measured,and the rolling angle was as small as 5°.The super-hydrophobicity of the as-prepared alumina wafer was retained after exposure to acid solution,alkali solution and hot water.The alumnia wafer also demonstrated desired self-cleaning and anti-fouling properties.Chapter 6 is concerned with the fabrication and permeation properties of membrane elements with cassette structure.The membrane casstte comsists of two planar membranes and a spacer.An alumina casstte was prepared using phase inversion tape casting/lamination/sintering method.The membrane cassette showed comparable water permeation rate(per unit area)to the single planar membrane,but the mechanical strength was much enhanced.With the membrane cassette as building block,membrane modules with single channel and multichannel were designed and fabricated.In Chapter 7,the research presented in this dissertation is summarized,and future research needs are identified.