Preparation And Properties Of Porous Ceramics Using Hollow Zirconia Microspheres

Porous zirconia ceramics are widely used in the field of high-temperature insulation due to many advantages such as low density,good chemical stability and thermal stability,as well as low thermal conductivity.In order to further reduce the thermal conductivity,hollow zirconia microspheres with lower density were used as starting materials in this thsis to prepare the zirconia porous ceramics.The thermal stability of doped hollow zirconia microspheres and properties of porous ceramics were studied.In this work,hollow zirconia microspheres doped with yttrium,cerium,lanthanum and aluminum were synthesized by solvothermal method.The phase and element composition,grain size and microstructure of doped hollow microspheres were characterized by X-ray diffraction analysis（XRD）,attached energy dispersive spectroscopy（EDS）,Scherrer equation,scanning electron microscope（SEM）and transmission electron microscope（TEM）.Zirconia porous ceramics were successfully prepared via tert-butanol-based gelcasting process using the hollow microspheres with molar ratio of zirconium and yttrium at 10:1.The pore microstructure,porosity,volume density compressive strength and room-temperature thermal conductivity of porous ceramics were measured by SEM,Archimedes method,universal testing machine and laser thermal conductivity testing instrument,respectively.The influence of sintering temperature and solid loading on the porous ceramics was analyzed.The main conclusions are as follows:（1）The grain growth of hollow zirconia microspheres doped with yttrium,cerium,lanthanum and aluminum was effectively inhibited at high temperature.The fragmentation of hollow zirconia microspheres after high-temperature heat treatment was improved,and the thermal stability of hollow zirconia microspheres was significantly enhanced.The optimal molar ratio of yttrium,cerium,lanthanum and aluminum doped hollow microspheres were Zr:Y=10:1,Zr:Ce=10:2,Zr:La=10:10 and Zr:Al=10:2,respectively.The egg-yolk structure of aluminum-doped microspheres was different from that of yttrium,cerium and lanthanum doped microspheres,and the chemical composition of core and shell of egg-yolk structure was the same.（2）After calcination at 1000℃,1200℃ and 1300℃ for 1 h,monoclinic zirconia（m-ZrO₂）and tetragonal zirconia（t-ZrO₂）were both observed in yttrium doped and cerium doped microspheres.Lanthanum doped microspheres had multiphases with La₂Zr₂O₇,t-ZrO₂and a small amount of LaOCl calcined at 1000℃,and LaOCl was disappeared with the increase of calcination temperature.Aluminum doped microspheres were mainly composed of m-ZrO₂ and a small amount of t-ZrO₂ or Al_xZr_（1-x）O_（2-x/2）at 1000℃,and t-ZrO₂ or Al_xZr_（1-x）O_（2-x/2）phase was disappeared with the increasing calcination temperature.（3）When the preforms with a solid loading of 5%,10%and 15%of the hollow sphere slurry was sintered from 1100℃ to 1300℃,the porosity was ranging from 90.17%to61.50%,and the compressive strength was between 0.55 and 39.32 MPa.Compressive strength as a function of relative density followed a power-law with an exponent of 3.2.The room-temperature thermal conductivity of the bulk porous materials as a function of porosity fell within the ranges predicted by one form of the Maxwell-Eucken 1 model and the effective medium theory（EMT）,and reached its lowest value of 0.052 W/（m·K）at a porosity of 90.17%.