Study On The Preparation And Properties Of Hierarchically Porous Zirconia Ceramics

Zirconia porous ceramics have low thermal conductivity,high thermal and chemical stability at high-temperature,wildly applied for high-temperature insulation.The total porosity does not only influence the effective thermal conductivity and mechanical properties of zirconia porous ceramics,but the pore structure（pore size and distribution）also plays an important role.To further investigate the effect of pore structure on the properties of porous ceramics,porous zirconia intermediate with a micro-nano-pore size structure was prepared.The morphology and properties of Zr O2 porous ceramics made from it were also investigated.In this thesis,ZrO₂ porous intermediates with submicron-nano pore size were prepared based on the sol-gel method combined with the addition of pore agent,and the morphology and grain size various preparation conditions were compared.Then hierarchically porous zirconia ceramics with micron-submicron-nano pore size were fabricated using the as-prepared Zr O2 porous intermediates.TBA-based gel casting method controlled pore size by changing the microstructure and solid content of porous intermediaries.The pore structure,porosity,bulk density,compressive strength and room-temperature thermal conductivity of the porous ceramics were characterized using scanning electron microscopy,Archimedes method,mechanical testing machine and laser thermal conductivity testing instrument,respectively.The main conclusions were summarized as follows:（1）Based on the sol-gel method,the gelation time was short,and the wet gels were produced with uniform composition and high strength at a reaction temperature of 60°C,alcohol-to-water mass ratio of 6:1,zirconium salt concentration of 0.5-1.5 mol/L,and polyacrylic acid（PAA）of 5-10 wt.%.The optimum sol-gel process conditions were zirconium salt concentration of 1 mol/L and PAA of 10 wt.%,at which time 5 wt.%each of micron-and nano-sized polystyrene（PS）microspheres were added to the wet gel,and after heat treatment at 700℃,the skeletal grains of the Zr O₂ porous intermediates were fine and uniform in size,with an average size of about 35 nm.The pores of the intermediates,containing submicron pores and nanopores formed due to the removal of the pore agent,have pore sizes of about 0.71μm and 40 nm,respectively,and some nanopores originate from the lap of the intermediate skeleton grains.（2）ZrO₂ porous intermediates were prepared under optimal sol-gel process conditions using single-size PS microspheres（0.98μm）as the pore agent,and the solid content of the intermediates was 15 wt.%and 20 wt.%,and zirconia porous ceramic blocks with hierarchical pore structure were obtained after gel casting and heat treatment at 1100℃.The porous zirconia ceramic blocks not only retained the submicron and nanopores of the raw material（Zr O2 porous intermediate）,but also increased the micron pores generated during the processing process（gel casting method）.Zr O2 porous intermediates were prepared by adding both micron（0.98μm）and nano（99.12 nm）size polystyrene（PS）microspheres as pore agents,and the final porous zirconia ceramic blocks were produced with a higher porosity（72.3%）at a solid content of 15 wt.%of the intermediate.（3）The room temperature thermal conductivity of the three kinds of zirconia porous ceramic masses ranges from 0.272 to 0.362 W/（m·k）,decreasing with increasing porosity.The zirconia porous ceramic compressive strength ranges from 7.27 to 15.37MPa.The experimental values of thermal conductivity are between the Maxwell-Eucken 1 model and the effective medium theory（EMT）,and the fitted parameters of the generalized model are obtained.