Preparation Of Heat-resistant Alumina Aerogel Thermal Insulations Based On The Strategy Of Doping And Hydrothermal Treating Alumina Sols

Thermal protection systems for advanced high-speed space vehicles put forward the urgent demand for high-performance thermal insulations with superior heat resistance,low thermal conductivity and good mechanical strength.Compared with other kinds of aerogels,alumina aerogel thermal insulations show relatively high-temperature resistance and low thermal conductivity.However,they will still experience significant sintering,pore collapse and volume shrinkage above 1300℃,resulting in the degeneration of thermal insulating performance,which limits their applications at elevated temperatures.In this thesis,heat-resistant Si,La,and Y-doped alumina aerogels were prepared by doping alumina sols with these elements,whose distribution and mechanism in alumina aerogels were first systematically studied.The strategy of hydrothermal treating and doping the boehmite sols was proposed to tune the crystal size and morphology of boehmite grains and synthesize novel,heat-resistant,hydrothermal assisted Si-doped alumina aerogels with high crystallinity and rod-like morphology.The evolution of the composition and structure for the hydrothermal assisted Si-doped alumina aerogels at elevated temperatures was systematically studied.Alumina aerogel thermal insulation composites were fabricated using the synthesized heat-resistant alumina aerogels as matric and mullite fibers as the reinforcement,and these composites are the first reported alumina aerogel-based thermal insulations exhibiting the integrity of heat resistance up to1500℃,low thermal conductivity and good mechanical properties.The main results of this thesis are as follows:（1）Heat-resistant,element-doped alumina aerogels were synthesized by doping the aluminum alkoxide-derived alumina sols with tetraethoxysilane,and doping the inorganic aluminum salt-derived alumina sols with tetraethoxysilane,lanthanum nitrate and yttrium nitrate,respectively.When the Si/Al,La/Al,and Y/Al molar ratios are 0.125-0.33:1,0.02:1,and 0.02:1,respectively,the Si,La and Y element-doped alumina aerogels exhibit optimized heat resistance:Compared with the prepared pristine alumina aerogel,the temperature ofα-Al₂O₃ transformation for the element-doped alumina aerogels is increased by 100-200℃;the specific surface area for these aerogels after 1200℃and1300℃calcination is increased to 77-147 m²/g and 38-92 m²/g,and the pore volume is increased to 0.58-2.0 cm³/g and 0.23-0.59 cm³/g.The heat resistance of element doped alumina aerogels lies in:The doped element can occupy the vacancies of alumina and cause lattice distortions,which can significantly inhibit the ion diffusion and lattice vibration at elevated temperatures.At higher temperatures,the doped element gradually migrates to the surface of alumina grains and react with alumina to form a new isolation phase,which further inhibitsα-Al₂O₃ phase transformation and grain growth.The solid solubility of Si atoms in alumina is higher than that of La and Y atoms,therefore they are easier to enter the vacancies of alumina and ensure better heat resistance.（2）Heat-resistant alumina aerogels（HTAs）were synthesized based on hydrothermal treating the boehmite sols.The anisotropic growth of boehmite grains and rod-like grains with larger sizes were realized based on the Ostwald ripening and directional adsorption of NO₃^-during the hydrothermal process.With the increase of hydrothermal reaction temperature,hydrothermal reaction time and supercritical drying holding time,the grain size of the HTA is increased.With optimized raw material molar ratios（aluminum isopropanol:water:nitric acid:urea=1:44.4:0.1:0.15）,hydrothermal reaction temperature（140℃）,hydrothermal reaction time（1 h）,filling ratio（40%）,aging time（72 h）and supercritical drying holding time（8 h）,the HTA exhibits high heat resistance:Compared with the reported pristine alumina aerogels,the temperature ofα-Al₂O₃ transformation for the HTA is increased by 100-200℃,and the specific surface area（retention rate）of the HTA after 1200℃and 1300℃calcination reaches 136 m²/g（56%）and 95 m²/g（38%）,respectively.（3）Heat-resistant Si element-doped alumina aerogels（HTASs）were synthesized based on doping hydrothermal treated boehmite sols with Si.The HTASs consist of highly crystallized and rod-like boehmite grains.With optimized hydrothermal reaction temperature（220℃）,hydrothermal reaction time（8 h）,filling ratio（40%）,Si/Al molar ratio（0.05:1）,aging time（72 h）and supercritical drying holding time（4 h）,the HTAS shows higher heat resistance:The HTAS could remainθ-Al₂O₃ and mesoporous structure after 1400℃calcination;the specific surface area（retention rate）of the HTAS after 1300℃and 1400℃calcination reaches 99 m²/g（87%）and 60 m²/g（53%）,respectively,which is significantly higher than that of the reported alumina-based aerogels.The excellent heat resistance of the HTAS lies in:High crystallinity of the aerogel greatly increases the activation energy for sintering and reduces the number of defects such as grain boundaries and dislocations in the alumina structure which results in decreasing of nucleation sites forα-Al₂O₃;The rod-like morphology and supercritical drying process effectively reduce the neck contact area between the grains,thus reducing the probability for the sintering andα-Al₂O₃nucleation at the necks;The Si component is located at the surface layer and around the boehmite grains,forming an isolation phase which further inhibits the sintering of alumina grains andα-Al₂O₃ nucleation.Therefore,the temperature of sintering andα-Al₂O₃ transformation for the HTAS is significantly increased.（4）Alumina aerogel thermal insulation composites（the MFASs and MFHTASs）,which show high heat resistance,low thermal conductivity and good mechanical properties,were fabricated using the Si element-doped alumina aerogel and the HTAS as the matric,respectively,and using the mullite fibers as the reinforcement.The MFASs show the integrity of heat resistance up to 1500℃,low thermal conductivity and good mechanical strengths,and when the bulk density of the fiber perform is 0.35 g/cm³,the typical performances are as follows:The compressive and bending strengths are 0.13 MPa（3%strain）and 0.91 MPa,respectively,and the thermal conductivity at 1200℃is 0.082 W/（m·K）;After 1500℃calcination,the structure of the MFAS remains intact,and the linear shrinkage in the direction of length,width and thickness are 0.5%,0.4%and 0.9%,respectively.For the MFHTASs,when the bulk density of the fiber perform is 0.20 g/cm³,the typical performances are as follows:The compressive strength is 0.06 MPa（3%strain）,and the thermal conductivity at 1200℃is0.093 W/（m·K）;After 1500℃calcination,the structure of the MFHTAS remains intact,and the linear shrinkage in the direction of length and width are 1.3%and 1.1%,respectively.The MFASs exhibit better comprehensive performances than the MFHTASs.Both of the MFAS and MFHTAS flat plates（thickness<20 mm）showed excellent heat resistance（linear shrinkage<0.84%）and thermal insulating performance（temperature rise of the cold face<360℃）in the single-side heating（1500℃for 15 min）test achieved by a quartz lamp device.Highly heat-resistant alumina aerogels can maintain low density,complex network and mesoporous structure at higher temperatures.The mullite fiber skeleton is of low density,and the mullite fibers lay in the direction perpendicular to the heat flow.In the meanwhile,the mullite fibers could effectively block the infrared radiation.These features ensure the excellent thermal insulating performance at elevated temperatures（up to 1500℃）for the MFAS and MFHTAS.