| Porous ceramics is a kind of lightweight structural material with the advantages of heat insulation,heat preservation,sound absorption and corrosion resistence.Porous ceramics has found its wide applications in aviation,aerospace,chemical industry,metallurgy,biology,medicine,construction and other fields.According to the requirements of porosity,density and thermal conductivity of commercial porous insulation ceramics,this paper has designed two kinds of low-temperature pressureless sintering processes in terms of relation of targeted properties versus adjutable chemical composiitons and structures.The acheivements that has been made are summarized as follows.1.By using presuressless microwave（MW）sintering method,the silicon carbide billet that has intrinsic strong absorbtion capability for MW electromagnetic energy expanded rapidly upon subjected by impact effect from polyvinyl alcohol as a foaming agent and aluminum phosphate as a binder,enabling the formation of aluminum phosphate composite porous silicon carbide ceramics.The effect of the content of aluminum phosphate on the properties of porous ceramics was studied by adjusting the content of aluminum phosphate.As sintering at 1200℃,the porosity of as-sintered product reaches 63.0%for 20 wt%of aluminum phosphate content with the volume density of 1.10 g cm-3,coefficient of thermal conductivity of 0.2434 W m-1K-1and single particle compression strength value of 40.7 N.Upon corrosion in acid and alkali solution for 72 h,the quality loss rates of as-prepared ceramic samples are 12.88%and53.28%respectively.2.Based on having combined porous silicon carbide ceramics with aluminum phosphate,mullite coated porous silicon carbide ceramics were prepared by a two-step process to improve their oxidation resistance and corrosion resistance.The effect of sintering temperature on the properties of porous ceramics was studied by adjusting sintering temperature.Under the sintering condition of 1300℃,through introduing talc as a modifier and coating mullite on ceramic matrix,the sintered product has achieved a porosity of 73.58%,the single particle compression strength value of 52.9 N and the oxidation rate of only 3.3%after 3 h high temperature treatment.After 72 h of corrosion in acidic and basic solutions,the mass loss rate of sintered products were lowered to6.60%and 14.26%,respectively.As subjected to the impact of thermal cycle from a temperature range of 800-1200℃ with,the loss rate of thermal shock for sintered product of 56.71 to 92.62%.3.Hollow microbeads ceramics from silica hollow beads as the main material were prepared by pressureless sintering with aid of as-formed aluminum-based binding agent due to the reaction of aluminum hydroxide with phosphoric acid at room temperature and high temperature,respectiovely.Based on this,the properties of hollow microbead ceramics were optimized through particle size gradation to meet the requirements of low density,certain strength and low thermal conductivity of hollow microbead ceramics.When mass fraction of 10 wt%aluminum-based binding agent-contained raw material was controlled in the range of 200-400 wt%change,glass beads ceramic volume exhibits a density range of 0.16-0.24 g cm-3,coefficient of thermal conductivity range of 0.0766-0.1025 W m-1K-1,porosity change range of 86.12 to89.87%and single particle compression strength value range of 3.45-7.44 N.Through optimizing the particle gradation of the ceramic and setting the mass ratio of K12:K46beadsat 6:2,as-received ceramic has both a relatively low thermal conductivity and a high single particle compression value and low bulk density with a thermal conductivity of 0.0954 W m-1K-1,single particle compression strength value of 4.94 N and the volume density of 0.2122 g cm-3. |
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