Study On Silicon Carbide Based Porous Multiphase Ceramics For Hot Gas Clean-up

In recent years,atmospheric environmental pollution has been very serious in our country,the haze weather has great harm to people’s health due to the fine particles(PM_2.5).As a new type of inorganic separation membrane material,porous multiphase ceramic with asymmetric structure has great application prospect in air pollution control,because it has many advantages,such as high strength,high filtration accuracy,high temperature resistance,excellent frost resistance,outstanding thermal shock resistance,renewable and so on.Silicon carbide is known as one of the best high temperature filtration materials due to high hardness,high strength,low thermal expansion coefficient,high thermal conductivity and excellent chemical stability.Therefore,it was employed as the main material,kaolin andα-Al₂O₃were used as the binder agent,activated carbon was used as pore forming to prepare porous multiphase ceramic support by particle packing method and pore forming agent method.The effects of different factors on the phase composition,microstructure,bending strength,porosity and gas permeability of ceramic support were systematically studied.The influencing degree of each factor on gas permeability was compared.In order to further improve the strength of Si C based porous multiphase ceramic support.A new whisker catalyst molybdenum trioxide was used to prepare mullite whisker reinforced Si C porous multiphase ceramic support.The effect of molybdenum trioxide on the mechanical properties,thermal shock resistance,oxidation resistance and gas permeability of the support were researched,and the influencing mechanism was revealed.In order to prepare transition layer which tightly combined with Si C porous multiphase ceramic support,the surface of Si C porous multiphase ceramic support was modified by ammonium fluoroaluminate,and a transition layer with network structure formed by interwoven mullite whiskers was successfully prepared on the surface of ceramic support.Finally,calcined bauxite with large reserves,low cost and excecllent high temperature stability was used as raw material to prepare separation layer slurry.The separation membrane layer,transition layer and support possessed good combination and Si C porous multiphase ceramic with intact surface and excellent gas permeability was prepared by dip coating method.The main results are as follows:（1）The increase of binder content,firing temperature and the decrease of activated carbon content would increase the bending strength of samples,but decrease its porosity.The increase of activated carbon particle size and Si C particle size could significantly reduce the bending strength of samples,but had no significant effect on the porosity,In addition,the binder content,firing temperature,activated carbon content,activated carbon particle size and Si C particle size affect the gas permeability of samples by changing the porosity and pore size.The binder content and activated carbon particle size have a greater effect on Darcy permeability coefficient k₁,followed by activated carbon content,firing temperature,Si C particle size.And Si C particle size had greatest effect on non-Darcy permeability coefficient k₂.（2）The addition of Mo O₃could reduce the formation temperature of mullite and induce mullite grow in one dimensional direction to forming mullite whiskers,which could enhance the bending strength and thermal shock resistance of the support.However,the addition of Mo O₃could reduce the oxidation resistance and gas permeability of the samples.When the amount of Mo O₃was 8wt.%and the firing temperature was 1450 ^oC,F4 sample had the optimal comprehensive properties.Its bending strength was 24.69MPa,porosity was 43.30%,water absorption was 25.92%,bulk density was 1.67g·cm^-3and the strength retention rate was 95.23%after thermal shock 30 cycles.The oxidation weight gain rate was 8.63%after 100h oxidation,k₁was 6.18×10^-9m²and k₂was 1.50×10^-3m.Mullite whisker reinforced Si C based porous multiphase ceramic support reach the requirements of the national standard"Silicon carbide high Temperature Ceramic Filter Elements"（GB/T32978-2016）（porosity is over 34%and bending strength is over 18MPa）.（3）Mullite whisker transition layer with firm adherence was successfully prepared on the surface of support modified by ammonium fluoroaluminate.Ammonium fluoroaluminate decomposition products HF gas could react with Si O₂existed on the surface of support to generate Si F₄gas and water vapor.The increase of firing temperature could promote the sublimation of solid phase Al F₃and the reaction between HF and Si O₂.More Si F₄gas and Al F₃gas provide raw materials for the formation of mullite whisker.When Al F₃gas reaches a certain saturation,mullite nucleation would occur on solid particles in the binder and on the phase interface between silicon carbide and oxide layer.Besides,the increase of firing temperature,holding time and ammonium fluoroaluminate content would increase the concentration of Al F₃gas,and promote the growth of mullite whisker.When the firing temperature,holding time and ammonium fluoaluminate content exceed a certain degree,although the length of mullite whiskers is longer,HF gas react with Si O₂to a greater extent,which could cause the decrease of the bonding strength of the Si C particles on the surface of the support,and it is not conducive to the formation of the whisker transition layer and the coating of separation membrane layer.The optimal ammonium fluoroaluminate content was 0.2g,the optimal firing temperature was 1300^oC,and the optimal holding time was 3h,the length of mullite whiskers was about 200～300μm,the diameter was about 2～3μm,k₁was 5.07×10^-9m²and k₂was1.57×10^-3m.（4）The dispersion rate of calcined bauxite slurry increased with the increasing of sodium hexametaphosphate addition,the optimum addition amount of sodium hexametaphosphate was 0.4wt.%.The increasing of calcined bauxite content and PVA content could increase the slurry viscosity,which would increase the thickness of separation membrane and improve the integrity of membrane,but decreased the gas permeability of porous ceramic samples.When calcination bauxite content was20wt.%,PVA content was 2.5wt.%,and the firing temperature was 1400 ^oC,the integrity of separation membrane was best,the thickness of the membrane was203μm,pore size was about 5μm.The separation membrane,transition layer and support have well combination,and the Si C based porous multiphase ceramic possessed outstanding thermal shock resistance,high compressive strength,good oxidation resistance and excellent gas permeability.The membrane layer,transition layer and support had no crack after thermal shock 10 cycles.The compressive strength of multiphase ceramic was 68.15MPa and 65.92MPa respectively before and after 10 times thermal shock,the weight gain rate of sample was only 0.06%after100h oxidation,the gas permeability parameters k₁and k₂were 1.42×10^-9m²and6.26×10^-4m respectively.The properties of Si C based porous multiphase ceramic reach the requirements of the national standard"Silicon carbide high Temperature Ceramic Filter Element"（GB/T32978-2016）(the thickness of membrane layer is150～300μm,compressive strength is over 45MPa,no cracks after thermal shock 10cycles,and gas permeability is over 2.2×10^-11m²).