Preparation,Mechanical And Thermal Properties Of SiC And SiC Bonded ZrB₂ Porous Ceramics By Foam-gelcasting Method

As an important high temperature structural ceramic material,SiC and ZrB₂-SiC are the potential candidate materials of aerospace,military,chemical and energy fields,owing to their high strength,high hardness,high temperature resistance,corrosion resistance and good oxidation resistance.However,high density,high thermal conductivity and low thermal shock resistance have become the main factors limiting the wide application of SiC and ZrB₂-SiC ceramics.Therefore,the introduction of pores into SiC and ZrB₂-SiC ceramics to adjust their thermal properties,and improve their mechanical properties by in-situ forming binder phase has gradually attracted researchers’attention.Based on this,the synthesis conditions of SiC powders were studied firstly;SiC porous ceramics were prepared via foam-gelcasting method based on the good water dispersibility of carbon microspheres obtained by hydrothermal carbonization of glucose;On this basis,SiC bonded ZrB₂ porous ceramics were prepared via foam-gelcasting method.The rheological properties and stability of SiC and ZrB₂-SiC ceramic slurry were studied.The synthesis conditions,high temperature service performance and thermal insulation properties of SiC and SiC bonded ZrB₂porous ceramics were studied.It showed that:（1）Respectively using glucose and silicon powders,and carbon microspheres obtained by hydrothermal carbonization of glucose and silicon powders as starting materials,nearly spherical and well-dispersed SiC particles with diameters of about100～200 nm and 50～300 nm were synthesized by solid-state reaction.The optimal C/Si molar ratios were 2:1 and 1:1,respectively.Reaction temperature was 1573 K,and holding periods was 3 h.（2）The SiC porous ceramics were prepared via foam-gelcasting method respectively using silica powders and carbon microspheres as raw materials,B₂O₃ as sintering aid,isobutylene maleic anhydride copolymer（IB）as dispersant and gelling agent,triethanolamine dodecyl sulfate（TLS）and sodium carboxymethyl cellulose（CMC）as foaming agent and foam stabilizer.When the solid content was 30 vol%,B₂O₃ content was 5 wt%,IB content was 0.5 wt%,CMC content was 0.2 wt%and TLS content was0.1 vol%,SiC porous ceramics with high porosity and excellent thermal insulation performance were prepared at 1573 K for 3 h,whose porosity,bulk density,compressive strength and thermal conductivity were respectively 92.8%,0.20 g/cm³,0.14 MPa and0.109 W/（m·K）.The results showed that the SiC porous ceramics could maintain its original geometric shape and micro-structure after secondary heat treatment at 1473 K in argon and air atmosphere,the compressive strength of the samples fired in air atmosphere increased by 23.94%,indicating that the porous ceramic have good high temperature service performance.（3）The SiC bonded ZrB₂ porous ceramics were prepared by foam-gelcasting method with ZrB₂ powders,silica powders and carbon microsphere as raw materials,Si O₂ as sintering aid,IB as dispersant and gelling agent,TLS and CMC as foaming agent and foam stabilizer.When solid content,Si O₂ content,IB content,CMC content and TLS content were respectively 30 vol%,5 wt%,0.5 wt%,0.2 wt%and 0.1 vol%,SiC bonded ZrB₂ porous ceramics with porosity,bulk density,compressive strength and thermal conductivity of 86.9%,0.54 g/cm³,0.53 MPa and 0.280 W/（m·K）could be prepared at1573 K for 3 h.The SiC bonded ZrB₂ porous ceramics could maintain its original geometric shape and micro-structure after secondary heat treatment at 1473 K for 3 h in argon atmosphere;The sample was partially oxidized after heat treatment in air atmosphere with unchanged shape and 38.42%increased compressive strength,indicating that the SiC bonded ZrB₂ porous ceramics have relatively good high temperature service performance.