| Silicon carbide is considered as the candidate materials for porous medium combustion(PMC)because of its lower thermal expansion coefficient,higher thermal conductivity and emissivity.In general,silicon carbide reticulated porous ceramics(SiC RPCs)are fabricated via polymer sponge replica technique.RPCs produced using this method have low strength due to the ceramic skeleton with hollow struts after the polymer template is burnt out,which make them sensitive to structural stresses.In addition,the clay is usually introduced into silicon carbide slurry to improve the slurry flowability and the sintering property of SiC.However,the sintering aid may lead to the formation of glass phases at high temperature,which affects the high temperature properties of SiC RPCs.Based on the problems mentioned above,the microsilica,reactive alumina powder and organic admixture were added into SiC slurry to promote the slurry flowability and produce mullite bonded SiC RPCs was first investigated in this thesis.After that,strut defects in SiC RPCs were repaired and the three-layered struts were formed using vacuum infiltration and reaction sintering.Meanwhile,the in situ formed nitride whiskers were used as template for growing columnar mullite to improve the interfacial characteristics of three-layered struts,thus strengthening and toughening SiC RPCs.In addition,according to the residual stress model,the thermal expansion coefficient of the layer within struts were adjusted via adding silica source in infiltration slurries,also the residual stress of the strut was optimized to enhance SiC RPCs.Finally,the fracture mechanism of SiC RPCs with hollow/dense struts was investigated by means of X-?CT tomography and finite element analysis.Furthermore,the experiment of porous media combustion was performed to evaluate the combustion characteristics of the prepared SiC RPCs and their service life.Based on the above researches,the main conclusions can be drawn as follows:1.The mullite bonded silicon carbide RPCs were fabricated with the optimized silicon carbide slurry and polymer sponge replica technique.When SiC was the mian material,microsilica and active alumina powder were sintering aids,the silicon carbide slurry with good thixotropic property was obtained after mixing a certain content of organic additives.The polymer sponge was coated by the modified SiC slurry to prepare SiC RPCs,and the in situ formed mullite bonded SiC RPCs had been fabricated at the temperature of 1450?.2.The SiC RPCs with three-layered struts were designed via pre-firing,vacuum infiltration and high temperature sintering technique.After the pre-firing of SiC RPCs,vacuum infiltration process was adopted to infiltrate SiC preforms,thus filling alumina slurry into triangular voids of struts and repairing the strut defects to form the three-layered struts in SiC RPCs at 1450?.The infiltrated SiC RPCs were significantly improved in comparison with those of non-infiltrated ones.The improvement was attributed to the insitu formation of reaction-bonded three-layered struts in SiC RPCs,which were characterized by exterior coating of aluminosilicate-corundum,middle layer of mullite bonded SiC and interior zone of corundum.3.The column-liked mullite could be synthesized with nitride whiskers as the template,which grew between the layers of struts to improve the interfacial characteristics,then strengthened the SiC RPCs.The Si powder was intendedly added into the SiC slurry,and nitride whiskers formed both in the matrix of SiC preforms and on the strut surface after nitridized at 1400?.Subsequently,these preforms were treated by vacuum infiltration of alumina slurry and fired at 1450? in air,the nitride whiskers acted as the template to promote the growth the column-liked mullite.Thereby,the microstructure of SiC RPCs was optimized,including the special interfacial characteristics of three-layered struts as well as better interlocked column-liked mullite in SiC skeleton,which improved the mechanical properties and thermal shock resistance of SiC RPCs.4.The thermal shock resistance of SiC RPCs could be tailored via adjusting the residual stress within three-layered struts.The silica sources were intendedly added into the infiltrated alumina slurry,and the in situ formed mullite from mullitization of andalusite as well as the secondary mullitization could reduce the thermal expansion coefficient of infiltration slurry.Based on the calcution model of residual stress,the residual tensile stress in the outer layer of three-layered struts was correspondingly decreased,which was beneficial to eliminating the surface cracks.Meanwhile,the decrease of residual tensile stress in the outer layer could change the crack propagation during thermal shock.Both of them contributed to the improvement of mechanical properties and thermal shock resistance of SiC RPCs.5.The fracture mechanism of SiC RPCs with hollow/dense struts could be evaluated by X-?CT tomography and finite element analysis.In the axial compression process of SiC RPCs,the surface defects and triangular tips within the struts were the location of stress concentration.Furthermore,the surface defects were easier to generate stress concentration than the triangular tips,thereby leading to the fracture occurred at the surface defects.The three-layered struts with dense struts made the stress uniformly distribute in SiC RPCs,and remarkably improved their mechanical properties.6.The flame combustion characteristics in the porous media and its service life were determined by thermal conductivity and structure of the struts.In comparison with porous media burner made of single material,SiC RPCs with three-layered struts possessed high thermal conductivity and radiation effect because of SiC skeleton and mullite coating in the struts.It could promote the flame stability in combustion zone and increased the ultimate heat loading.In addition,the temperature could uniformly distribute in the whole porous media in the case of steady combustion,the higher flame temperature and lower emissions of CO,NO_x were also obtained.In the experiment of porous media combustion,SiC RPCs with three-layered struts exhibited excellent thermal shock resistance after several thermal cycles,which significantly improved the service life of the burner. |
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