Preparation And Modification Of Silica Aerogels Via Doping Technique And Their Physical Properties

Aerogel is a typical nanoporous material,which can be widely used in high temperature insulation,catalyst carrier,heavy metal ion adsorption,battery solar cell/lithium ion battery electrode and other fields,and Silica（SiO₂）aerogel is currently the most studied and widely used aerogel material.The composite of SiO₂ aerogel with nanofibers,organic polymers,graphene and other materials is an important way to realize its functionalization.Through doping modification and other technical means,metal ions or oxides are combined with SiO₂ aerogel,and the crystallization/phase transition process is controlled through high temperature heat treatment.Which is expected to obtain new nanoporous functional materials with high temperature resistance and optical/electrical/magnetic properties at the same time,which is the latest frontier in the current aerogel field.Metal ion/oxide-SiO₂ doped aerogels will undergo high-temperature chemical reaction and/or phase transition process with nanopore limits at high temperature,and their physical mechanisms will affect the microstructure and physical and chemical properties of the doped aerogel materials.However,little research has been done yet.Based on this,this study explored the preparation method,microstructure and crystalline phase transition and structure evolution process of Ba/Sn co-doped SiO₂-based aerogel at high temperature,and proposed Ba/Sn-induced SiO₂epitaxial crystallization mechanism.On this basis,the microstructure and infrared optical properties of B,Zr,and Sn single-doped SiO₂ aerogels were studied,and an aerogel coating based on the all SiO₂ material system was prepared,focusing on its infrared radiation performance in the atmospheric window band.The main research contents and results are as follows:（1）High temperature microstructure changes and crystallization process of Ba/Sn co-doped SiO₂ aerogel:A new co-precursor-sol-gel method has been developed to prepare Ba/Sn co-doped SiO₂ aerogel.The effects of SiO₂ content（40-100%）,sintering temperature（100-1000^oC）and time（0-2 h）on its crystallinity and microstructure are investigated.The physical mechanism of Ba/Sn-induced transformation of SiO₂ from amorphous to cristobalite crystalline under high temperature is discovered.The results show that the introduction of Ba/Sn and the sintering temperature have an important effect on the crystallinity,microstructure and pore structure of the doping system.Cristobalite SiO₂is formed at high Ba-Sn content（>33.3%）and high sintering temperature（>800 ^oC）,and its pore volume and specific surface area are sharply reduced.The formation mechanism of cristobalite is discussed,and the lattice matching mechanism of SiO₂aerogel epitaxial nucleation induced by Ba Sn Si₃O₉ is proposed.（2）B,Zr,Sn doping of SiO₂ aerogel and its infrared radiation performance:A sol-gel-atmospheric-drying method is used to prepare B³⁺,Sn⁴⁺,Zr⁴⁺doped SiO₂aerogels.The effects of ionic type and content on crystallinity and infrared optical properties are mainly studied.It is found that ions with a large radius and the same valence have little effect on the infrared radiation ability of SiO₂,while ions with a small radius and different valence have a greater increase in the infrared radiation ability of SiO₂,the relative intensity of its infrared absorption around 1100 cm^-1 can reach up to 72%,while the relative intensity of other ion doping or its own infrared absorption is around 20%,and SiO₂ doped with 5%B shows the best Infrared performance.On this basis,the SiO₂aerogels doped with 5%B were used as filler to prepare an aerogel coating based on water glass adhesive,and the effects of different filler contents on the optical properties and infrared radiation properties of the coatings were studied.It is found that 15%and 20%doped coatings have lower ultraviolet-visible transmittance and higher infrared emissivity.The coating made of 5%B-doped SiO₂ material with20%mass fraction has a strong radiant cooling capacity,and can achieve a cooling performance of 9.0 ^oC lower than the ambient temperature under direct sunlight on a sunny day（11:00-16:00,March 13,2021）.This thesis explores the high-temperature microstructure and crystalline properties of ion-doped SiO₂ aerogels and its application in infrared optical properties and radiation cooling.Which will provide a useful reference for the development and application of new aerogel-based nanoporous functional materials and devices.