Controllable Sintering And Performance Study Of Porous Oxide Ceramics

Porous ceramics is a very important material,which is widely used in filter,heat insulation,sound absorption and organic skeleton,ect.At present,the porous ceramics are mainly synthesized by irregular particles which have poor sintering controllability.The uneven contraction occurred among irregular particles can destroy the pore structure and limit the further improvement of the performance.In this work,inactive spheres synthesized by thermal plasma were used to synthesize porous ceramics through packing of spheres and sintering,the main work and results are listed as follows:（1）Amorphous SiO₂ spheres with dense body were employed to synthesize porous SiO₂ ceramics with uniform and interconnected pores through a new method combining close packing of spherical silica particles and sintering.Firstly,based on DSC analysis,the suitable sintering conditions for ceramics were set up.Then,the thermal stability of SiO₂ spheres was studied according to kinetic analysis.Furthermore,the relation between sphere size and pore size was studied.Finally,the performances of the synthesized pores were researched,including the enhanced permeability for nitrogen and water,the consistence of filtration on solid/water separation,the performance in oil/water separation,and the employment in grading of particles.（2）The uniform and interconnected pore structures were extended to Al₂O₃ system which has better thermal resistance.A new method that coating the alumina spheres with silica sol was used to decrease the sintering temperature of ceramics while the pore structures were not be destroyed.Firstly,the pH regulation for silica coating and temperature control for gel-casting were studied to achive the two processes in one step.Then the formation process of mullite necks was reseached that Al3+ and O₂-diffused from surface of spheres to necks and mullite could be formed after reaction.Furthermore,the Al3+ release controlled mullite formation mechanism was revealed according to the calculated activation energy and Avrami exponent n.Besides,the effect of thickness of coating layer on ceramic strength was sdutied,and the mechanism was revealed.Finally,the permeation flux of N2 and water was tested,and its well performance on pollution resistance was proved.（3）Then the uniform and sintering controllable pore structures were developed to Al₂O₃ membranes which has smaller pore size.Double-layered alumina membranes with interconnected uniform pores were synthesized by inactive sub-micrometer spheres through one-step sintering.Firstly,the mass transfer characterization of spheres and microstructure evolution of membrane were fully studied,the results showed that the interconnected uniform pores were formed after even evolution and the microstructure of membrane could be controlled by sintering conditions in a wide range.Then,the control of membrane thickness was studted,and a law between solid content of slurry and membrane thickness was established.Furthermore,the mechnism for one-step sintering was studied that mass transfer could not occur unless the metastable phases of spheres transform to stable α-Al₂O₃,and luckily the temperature for phase transformation of spheres and interlayer sintering was similar.Finally,the industrial waste water was processed by synthesized membrane and the result confirmed that the membrane was uniform without defects.（4）The technology of sintering controllability of pores was also needed in thermal insulation fields.Hollow SiO₂ spheres with dense shells were employed to synthesize porous silica ceramics with closed-cell structure and high porosity.Firstlt,the influence of sintering temperature on the porous ceramic was studied and the suitable sintering conditions were determined.Then,the effect of different additives on crystal,structure and properties of porous ceramics was researched and the mechanism was revealed.Furthermore,it was revealed that a-cristobalite was negative both for ceramic strength and thermal insulation and it could be suppressed effectively by moderate A12O3 and H3BO3.In addition,the thermal insulating performance and mechanical strength of ceramics could be controlled through changing the particle size or shell thickness of hollow spheres,and the regulation between them was set up.Finally,the simulation on relationship between porosity and thermal conductivity was set up according to effective medium theory.