Preparation Of Oriented Layered Alumina Aerogels And Their Modification Of Infrared Reflection

High temperature anisotropic thermal management materials have been paid more attention by many researchers.Alumina aerogels are ultra-low-density porous materials assembled from nano-scale particles.Due to their complex threedimensional network skeletons and small-scale porous structures,alumina aerogels can prevent solid-phase and gas-phase thermal conductions to some extent and are good high temperature thermal insulations.After infrared modification,the property of high temperature thermal insulation of alumina aerogels can be further reduced.In this paper,oriented layered alumina aerogels were prepared via sol-gel methods and directional freezing by adjusting bidirectional temperature fields.Then,titania infrared reflection layers were introduced to modify alumina aerogels in order to reduce their high temperature thermal conductivity.The research was placed great emphasis on the conditions of directional freezing for the oriented growth of ice crystals and the modified methods of titania.Then,the anisotropies of their mechanical and thermal insulation properties were discussed.In the experiments,ABAQUS was used to simulate the freezing heat transfer under different slope angles to reveal the growth patterns of solid-liquid interface in bidirectional temperature fields.In order to understand the regularity of porous structures of alumina aerogels,the slope angles,the contents of hydroxyethyl cellulose(HEC)and freezing temperatures were analyzed.The results show that the smaller the ratio of the vertical temperature gradient to the horizontal temperature gradient is,the better the porous structural ordering is.The optimal oriented layered structures of alumina aerogels are obtained by using the preparing parameters of the slope angle of 20°,HEC addition amount of 0.5 wt.% and freezing temperature at-60 °C.In addition,with the increase of HEC content and the decrease of freezing temperature,the structure wavelength decreases and the density of ceramic bridge increases.Based on the optimal freezing conditions,two methods of introducing titania infrared reflective layer into alumina aerogels,i.e.impregnation method and powder mixture method,were adopted.The anisotropic mechanical and thermal insulation properties of the samples before and after modification were analyzed.The results show that the impregnation method is an effective one to cover the pore walls with titania infrared reflection layer,effectively,but the oriented porous structures of alumina aerogels are destroyed.The powder mixture method can not only effectively achieve the purpose of infrared modification,but also maintain the oriented layered structures of alumina aerogels.The alumina aerogels before and after modification possess obvious anisotropic mechanical properties.The thermal conductivity along the channel direction is significantly higher than the one of the vertical channel direction.At high temperature,the thermal conductivity of the modified aerogels along the channel direction is higher than the one before modification,while the thermal conductivity in the vertical channel direction decreases.The oriented layered structures of alumina aerogels after infrared modification have low and anisotropic thermal conduction property at high temperature.