| Water pollution and fume contamination has become more and more serious due to the development and progress of our society. Therefore, two kinds of gradient porous ceramics for water and fume treatment were prepared by Sol-gel—spinning method and Solid particles sintering method respectively, and the composition, microstructure and properties were studied by advancing testing methods such as TG-DTA, XRD, SEM, AFM and so on. Furthermore, the preparation mechanism of composite sol, bonding mechanism of membrane and substrate, filtrated principle of the gradient porous ceramics were investigated. The results indicate that the properties of the gradient porous ceramic meet the requirements of filtration.Alumina porous ceramic substrates with different formulas were prepared using a-Al2O3, Suzhou kaoline, calcined Suzhou kaoline and graphite as main starting materials. The optimum formula was ascertained by the orthogonal analysis. The samples with optimum formula has apparent porosity of 45.73%, water absorption of 24.04%, bulk density of 1.90g/cm3, bending strength and compressing strength of 55.42MPa and 53.14MPa respectively. The main crystal phase are corundum and mullite, and uniform pores with aperture range of 0.2-6μm and 0.5-30μm respectively distribute on the surface and fractured surface of the samples. Alumina particles of orientational made the pores formed many slits along the direction, which could well filtrate waste water.Transparent faint yellow TiO2-ZrO2-SiO2-Al2O3 quaternary composite sol with viscosity of 3-5mPa-s was synthesized by sol-gel method using inorganic salt as the starting materials, and the optimum composite pattern is Ti:Zr:Si:Al with a molar ratio of 4:1:1:2. The reaction temperature of TiO2 sol was at 80℃, molar ratio of citric acid to metal ion was 1.1～1.5, pH value was 4-5, reaction time was for lh, and the reaction temperature and time of composite sol was at 85℃and for 2h respectively. The optimum heat treatment temperature is 850℃, and its main crystal phases areγ-Al2O3 (gamma-aluminum oxygen), SiO2 (quartz), t-ZrO2 (baddeleyite), ZrSiO4 (zircon),3Al2O3·SiO2 (mullite) and TiO2 (anatase, rutile). The thickness of membrane is quite appropriate after coating for 2 times, that is 5-6μm. The membrane and substrate transits and combines well with aperture range of 5～30μm,0.5～15μm and 0.2～6μm in sequence from substrate to membrane, which presents gradient distribution. AFM shows that the surface of composite membrane is quite rough with grain size range of 200-600nm and pore size on membrane of 1～6μm. The Moh's hardness of membrane is among steel knife and glass slide with range of 5.5～6.5. In other, the water permeation of gradient porous ceramic under room temperature is 4.57m3/(m2·h), and after 28 days of soaking in buffer acid or base solution, there was no crack and grain falling, which indicates that the samples have good chemical stability. The bonding mechanism of membrane and substrate is attributed to the effect of capillary action (riveting). When the gradient porous ceramic is used for waste water treatment, both gradient filtrating and photocatalytic reaction play important part in filtration process based on its favorable chemical stability.Carborundum matrixes used for fume filtration were prepared using carborundum and Suzhou kaoline as the starting materials. The results showed that the optimum formula is A7, the best firing temperature is at 1280℃, apparent porosity is 35.87%, absorption rate is 1.74g/cm3 and bending strength is 62.83MPa. Moreover, the bending strength didn't decrease but increased to 68.10MPa (increased 8.39%) after thermal shock resistance for 20 times from room temperature to 1000℃. The main crystal phases are SiC, mullite and tridymite of all the samples and a little andalusite appears in some formulas. SEM analysis demonstrates that a large amount of three-dimensional connected pores and small number of closed pores with size of 3～60μm, which turns to 5～100μm after 20 times thermal shock resistance.Slurry coatings of T series formulas which were coated on the carborundum substrate of A7 formula to fabricate gradient porous ceramics were prepared by wet grinding method using corundum as the mainly starting material. The bonding mechanism of alumina coating and carborundum substrate is capillary action in the process of immersing and the synthesis of mullite due to the chemical bonding of alumina and silica during the course of firing which contributed to the close combination. The optimal formula is T4, in which the grinding time of the powder is 120min with mean particle size of 2.12μm, the specific gravity and viscosity of the coating is 1.80g/cm3 and 185.4mPa·s independently, and best firing temperature is 1200℃of the gradient porous ceramics with coating used for fume treatment, and the particles and pores distribute homogeneously in the coating which has crystalline phases of corundum, cordierite, tridymite and mullite, uniform three dimensional connected pores distribute both in the coating and matrix with the pore size of 5～120μm,2～30μm and 0.2～20μm form matrix, intermediate layer to coating, forming some extent gradient, which can be seen from SEM morphology. There was no crack and slotting on the surface of coating after 30 times of thermal shock resistance, and the pore distribution had little change, which satisfies the requirement of high temperature filtration. The hardness of the coating is close to 9, and the gradient porous ceramic has air permeability of 14.70m3/m2·h·Pa. There is also no flaw and particle falling after 28 days of soaking in vitriol and sodium hydroxide solution, which indicates that the samples have good chemical stability. The filtration mechanism is that the gradient porous ceramic captures and separates dust particles through the mode of directly intercept, inertia collide and brownian diffuse based on its specific gradient porous structure.
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