Assembly Preparation And High-Temperature Insulation Performance Of α-Al₂O₃ Based Aerogel Materials

With the advancement of technology and the rapid development of the high-temperature industrial field,there is an urgent need to develop new and efficient high-temperature insulation materials to protect equipment and structures that operate in high-temperature and harsh environments such as aerospace vehicles,ships,and industrial furnaces,to prolong their service life and improve safety and reliability.According to pertinent data,the energy consumption of the industrial sector accounts for 65%of the total national consumption,which is dominated by high-temperature industries.High-temperature insulation materials can reduce energy consumption and improve production efficiency.Therefore,developing and applying hightemperature insulation materials can promote energy and related industrial upgrading,play a crucial role in reducing energy consumption,reducing emissions,and achieving carbon neutrality,and are of significant importance for achieving long-term sustainable development of the national economy.Due to their high porosity,low density,and low thermal conductivity,aerogels are widely used in temperature insulation.Alumina has an extremely high melting point（2050℃）and exhibits excellent heat resistance at temperatures above 1000℃.It also has a low thermal conductivity,which can effectively reduce the speed of heat transfer and minimize heat loss and energy consumption.At the same time,alumina has good chemical stability,resistance to oxidation,acid and alkali corrosion,and excellent mechanical strength even under extreme conditions.Therefore,alumina aerogels are highly efficient high-temperature insulation materials.However,the traditional alumina aerogel prepared by the sol-gel method has a pearl necklace-like three-dimensional porous architecture composed of small alumina nanoparticles with high surface energy that suffers from an inherently brittle and structurally unstable.At the same time,the traditional alumina aerogels are often amorphous and undergo crystallization growth and phase transition at high temperatures.The transition from the transition phase to the stable phase of alumina is a lattice reconstruction of oxygen ions from face-centred cubic（Fcc）to hexagonal closest packing（Hcp）,which leads to the substantial contraction of the volume of the aerogel,the destruction of the microstructure,and a rapid decline in the thermal insulation performance.Therefore,the practical application of traditional alumina aerogels is limited.The quick reduction in heat insulation performance has constrained the aerogel’s practical application.Based on the above discussion,this paper discards the traditional aerogel preparation strategies,and the stable α-Al₂O₃-based nanomaterials were chosen as structural units to construct three-dimensional porous aerogel materials for high-temperature thermal insulation applications through assembly strategies.The main research contents are as follows:1.Assembly preparation and performance of α-Al₂O₃ nanosheet-based biphasic aerogelsChemical blowing has successfully prepared α-Al₂O₃ nanosheets with lateral dimensions as high as hundreds of microns and thicknesses down to a few tens of nanometers.The preprepared α-Al₂O₃ nanosheets were used as the assembly unit,and an appropriate amount of silica sols was skillfully added as the high-temperature binder to construct the threedimensional porous aerogels.In addition to acting as a high-temperature binder,the silica sols can generate a glassy layer on the surface of alumina to hinder the solid-phase diffusion of aluminum ions.Also,the cristobalite derived from silica sols reacted with alumina to form mullite at grain boundaries after high-temperature treatment.The stability of the threedimensional porous aerogel is maintained by inhibiting the grain growth of alumina to preserve the structure of the two-dimensional assembly units,and the temperature resistance is enhanced.Mullite is a desirable refractory material because of its excellent high-temperature creep resistance due to the difficulty in the diffusion of silica-aluminum ions and high resistance to lattice dislocation slip,as well as its low coefficient of linear expansion and good thermal shock resistance.Therefore,adding silica sols inhibits the growth and deformation of the flaky alumina and generates mullite phase wrapping alumina.This stable alumina-mullite biphasic component gives the α-Al₂O₃ nanosheet-based biphasic aerogels（ANSAs）excellent thermal stability with a linear shrinkage of only about 3.6%even after calcination at 1600℃ for 30 min and low thermal conductivity(0.029 W·m^-1·K^-1 at room temperature).In addition,the 2D αAl₂O₃@mullite core-shell sheets were also prepared as assembly units to construct aerogels,named AMSAs.This core-shell structure can improve temperature resistance through interlattice suppression under continuous energy input at high temperatures.2.Preparation and performance of thermal radiation-shielded composite aerogels assembly by mullite nanosheets/TiO₂ nanorodsMullite（3Al₂O₃·2SiO₂）,as an alumina-based ceramic,has excellent thermal and chemical stability,and as a composite oxide,mullite is widely used in high-temperature thermal insulation because of its better sintering resistance compared with single-component alumina.We prepared the two-dimensional mullite nanosheets by chemical blowing method and modified titanium dioxide nanorods on the surface of mullite through a straightforward twostep solution reaction.These nanorods were used as an infrared shielding agent to reduce the infrared transmittance of the material and inhibit radiative heat transfer at high temperatures due to their excellent reflectivity,thermal stability,and strong broadband absorption ability.Then,the designed mullite nanosheets/TiO₂ nanorods were used as assembly units to construct three-dimensional porous composite aerogels.At the same time,the content of titanium dioxide in the system was regulated by adding appropriate amounts of mullite fibers,and the optimal MS/TiO₂ nanorods/SiO₂ composite aerogels（MT-40）were preferred.The prepared MT-40 has good temperature resistance,and the linear shrinkage was only 2%after calcination at 1500℃for 30 min.The MT-40 also has good high-temperature thermal insulation properties with the measured thermal conductivity of 0.118 W·m^-1·K^-1 at 1000℃.3.DIW 3D printing preparation and properties of α-AlO3/mullite whisker composite aerogelsStructurally controllable α-Al₂O₃/mullite whisker composite aerogels were prepared by DIW 3D printing using stabilized-phase α-Al₂O₃ nanoparticles as structural units,adding single-phase mullite sols as a high-temperature binder and mullite whisker raw material source,and using tungsten trioxide as a catalyst for mullite whiskers,which was achieved through the rheological modulation of ceramic ink.The mullite whiskers in the aerogels are less than 100 nm in diameter and more than 1 μm in length,with a large aspect ratio,making effective lapping to improve the structural strength of the material.The stabilized phase α-AlO3 nanoparticles as the assembly unit to construct three-dimensional porous materials can effectively avoid the structural collapse caused by the phase transition and play a stabilizing role in the skeleton.At the same time,the mullite whiskers,as a high-performance single crystal material with excellent oxidation resistance,thermal stability and structural strength,can be used further as the structural support in high-temperature applications;the α-Al₂O₃ nanoparticles react with the fumed silica aerogel powders,which are the rheology modulator in the system,to generate mullite to dissipate the high-temperature energy,making the prepared composite aerogels have excellent temperature resistance,with a line shrinkage of only 1%even after calcination at 1500℃.The micropores construct of α-Al₂O₃ nanoparticles and mullite whiskers lead to better thermal insulation properties of the prepared aerogel composites.