Study And Analysis On The Preparation Of ZrO₂ Porous Ceramics By Foam-gelcasting Method

Zirconia is often used as thermal barrier coating for aerocraft surfaces,due to its excellent properties such as high temperature resistance,low thermal conductivity,anti-oxidation,high strength and so on.It may further reduce the thermal conductivity if zirconia is used as a substrate for the preparation of porous ceramics.So it can be applied as a kind of heat protection material which can be both loaded and insulated,that is very important to promote its application as thermal protection parts of aerocraft.In this thesis,ZrO2 porous ceramics have been fabricated by foam-gelcasting method and pressureless sintering process using 8 mol%Y2O3 stabilized ZrO2 as raw materials.Through adjusting the solid loading,sintering temperature,foaming agent concentration and type of impeller,the total porosity of the samples is 65.6?79.1%,bulk density is in the range of 1.24?1,92 g/cm3,compressive strength varies from 4.4 to 30.2 MPa,and the room temperature thermal conductivity is 0.12?0.43 W/(m·K).As the sintering temperature increased from 1450 ? to 1600 ?,the porosity of the sample decreased slightly,while bulk density,compressive strength and thermal conductivity increased,for the connections between grains became tight.The optimum sintering temperature was selected as 1550 ? and the total porosity,compressive strength and thermal conductivity of the samples were 68.3%,21.7 MPa and 0.35 W/(m-K)separately.When solid loading increased from 15 vol.%to 30 vol.%,the porosity decreased and the wall of pores got thickened,while the bulk density,thermal conductivity and compressive strength increased.At the same time,the median pore diameter showed a decreasing trend,comprehensive analysis revealed that the optimum solid loading was 30 vol.%.With the increasing of foaming agent concentration from 0.3 g/L to 16 g/L,the porosity increased while the volume density decreased and the thermal conductivity showed a downward trend on the whole,the median pore diameter had a wide range of change.Comprehensive analysis showed that the optimum concentration of the foaming agent was 2 g/L,and the total porosity,compressive strength and thermal conductivity of the samples were 77.9%,11.5 MPa and 0.17 W/(m·K).The type of the impeller affected foam volume and microstructure(the pore size,the thickness of the pore wall and the size of the grains)obviously,which has a significant effect on the properties.Based on the experimental results,the relationship between thermal and mechanical properties of samples was preliminarily studied according to the theoretical model.The relationship between the thermal conductivity,the compressive strength and the total porosity(P in the range of 65?80%)of the ZrO2 porous ceramics prepared by foam-gelcasting method was obtained,which could be described as Ke = 12.6(1-P)/5.7+9.3P and ?=322×(1-P)234.The above formulas can be used to predict and design the thermal and mechanical properties of ZrO2 porous ceramics prepared by foam-gelcasting method and pressureless sintering.