| Aerogel is a nanoporous material prepared by sol-gel method and aging,drying and other means.Because of its unique three-dimensional network skeleton structure,aerogel has characteristics such as high specific surface area,ultra-low density,and extremely low thermal conductivity,and has been widely used in construction,high-power laser,catalysis,energy storage and other fields.With the proposal of the concept of super thermal insulation materials,aerogels containing nanopores as a super thermal insulation material have become more and more widely used in different fields of higher temperature thermal insulation.However,due to the fragile structure of SiO2 aerogels skeleton,the structure is easily destroyed at high temperature,and its temperature application range has great limitations.Although Al2O3 aerogels and SiO2/Al2O3 composite aerogels are superior to SiO2 aerogels in high temperature resistance,they can not meet the requirements of current aerospace technology.Therefore,in order to improve the high temperature stability of Al2O3 aerogels,effective methods must be adopted to prevent Al2O3 phase transformation and sintering.The high temperature resistance of aerogel can be improved by introducing other metal oxides such as BaO,La2O3,SiO2,Li2O,K2O or MgO to prevent the phase transition and prepare multi-element composite aerogels.Aiming at the problem of phase transition and sintering of SiO2/Al2O3 composite aerogels,this paper designed to introduce foreign elements into SiO2/Al2O3 composite aerogels to suppress the crystal transformation of Al2O3 at high temperature,thereby solving Sintering problems.In this study,SiO2/Al2O3/MgO composite aerogels were prepared by using ethyl orthosilicate as the silicon source,aluminum chloride hexahydrate as the aluminum source and magnesium chloride hexahydrate as the magnesium source.The SiO2/Al2O3/ZrO2 composite aerogels were prepared by using ethyl orthosilicate as the silicon source,aluminum chloride hexahydrate as the aluminum source and zirconium oxychloride octahydrate as the zirconium source.The aerogel was heat-treated,and the effects of process parameters and heat-treatment temperature on the properties of ternary aerogel size,microstructure,crystal type,pore structure and thermal conductivity were investigated.On the basic of this,the SiO2/Al2O3/MgO/ZrO2 composite aerogels were prepared by sol-gel technology and supercritical drying technology using ethyl orthosilicate as the silicon source,aluminum chloride hexahydrate as the aluminum source,magnesium chloride hexahydrate as the magnesium source and zirconium oxychloride octahydrate as the zirconium source.Through the heat treatment of quaternary aerogels with different molar ratios at various temperatures,the microscopic morphology and pore structure changes were observed to explore the main influencing factors and mechanisms of the aerogel's high temperature resistance.The results show that the SiO2/Al2O3/MgO composite aerogels are white massive cylinder,and the lockiness is good.After a heat-treatment at 800?,the linear shrinkage rate is 3.6%.After a heat-treatment at 1000?,it is 45.8%.And after 1200? heat-treatment,it is 67.1%.Without heat-treatment,the thermal conductivity is as low as 0.06W/(m·K)and the density can be 0.1g/cm3.The role of MgO in the SiO2/Al2O3/MgO composite aerogels is to remove the hydroxyl groups on the surface of Al2O3 particles and the ion holes in the crystal lattice,thereby improving the high temperature stability.The SiO2/Al2O3/ZrO2 composite aerogels are white massive cylinder,and the lockiness is good.After a heat-treatment at 800?,the linear shrinkage rate is 5.2%.After a heat-treatment at 1000?,it is 42.4%.And after 1200? heat-treatment,it is 53.6%.Without heat-treatment,the thermal conductivity is as low as 0.05W/(m·K)and the density can be 0.09g/cm3.The role of ZrO2 in SiO2/Al2O3/ZrO2 composite aerogels is to act as a matting agent to control heat radiation and improve high temperature stability.The SiO2/Al2O3/MgO/ZrO2 composite aerogels are white massive cylinder,and the lockiness is good.Take Mg/Al/Si/Zr=4:3:1:1/3 as an example,After 800? heat treatment,the linear shrinkage is 7%,and after 1000? heat-treatment,the linear shrinkage increases to 21%.After 1200?heat-treatment,it is 34%.Without heat-treatment,the specific surface area is 597.22 m2/g and the thermal conductivity is 0.03W/(m·K).We found that the shrinkage of the quaternary composite aerogel is significantly reduced,and the specific surface area is higher,and the thermal conductivity is also lower,indicating that the high temperature resistance has been improved.Through XRD analysis,we can also find that after 800? heat treatment,only a weak ?-Al2O3 phase appears in the aerogels.This shows that the phase transition temperature of SiO2/Al2O3/MgO/ZrO2 composite aerogels is delayed and the high temperature resistance is improved compared to SiO2/Al2O3 composite aerogels.The role of MgO is to remove the hydroxyl groups on the surface of Al2O3 particles and the ion holes in the crystal lattice,thereby improving the high temperature stability.After heat treatment at a higher temperature,the thermal conductivity of the aerogel will increase,the thermal insulation effect will become worse,and the heat radiation will also become a kind of heat transfer that cannot be ignored.Therefore,adding an opacifier to the aerogels can effectively reduce radiant heat conduction.Common opacifiers include TiO2,ZrO2,SiC,etc.In this paper,the role of ZrO2 in SiO2/Al2O3/MgO/ZrO2 composite aerogels is to act as a matting agent to control heat radiation and improve high temperature stability. |
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