1 Science and Technology on Advanced Ceramic Fibers and Composites Laboratory, National University of Defense Technology, Changsha 410073; 2 Guangdong Alison Hi-Tech Co., Ltd, Qingyuan 513042
Abstract: Aerogels have three dimensional nanoporous network structure and the unique structure endows them with the characteristics of low density, high specific surface area and high porosity, and the properties of low thermal conductivity, low dielectric constant and low transmission rate. Thus aerogels have found promising application in the fields of acoustic insulation, thermal insulation, dielectric materials, catalyts, adsorption and so on. However, sol-gel method, as the most mature and the most widely used aerogel fabrication technique, suffering the problems of excess use of hazardous organic solvent and severely dangerous and costly supercritical drying process, only has limited large-scale industrial production and application potential. Thus, reducing cost and preparing monolithic aerogels with high specific surface area under ambient pressure conditions are the most urgent subjects. Ionic liquids, known as green solvents in the 21st century, have many exceptional properties such as low vapor pressure, low surface tension, high catalytic and high solubility. The development of ionic liquids and aerogels were almost simultaneous, but not until 2000 did two materials meet. Ionic liquids serving as template agents offer microstructure orientation effect, and homogenize the resultant pore structure. The nonvolatility and low surface tension of ionic liquid templates help to avoid capillary effect and consequently ensure the intactness of nanometer pore structure during aging and ambient pressure drying, and moreover, the catalytic effect of ionic liquids can shorten gel time. All of the above open up a new avenue to produce aerogels by ambient pressure drying with the assistance of ionic liquids. By now, researchers have conducted extensive research on applying ionic liquids to the preparation of inorganic aerogels such as SiO2 aerogels, TiO2 aerogels, SiO2-TiO2 composite aerogels and carbon aerogels. Among them, SiO2 aerogels acquire the most research endeavors, from the perspectives of procedure, microstructure, doping and application, etc. The monolithic aerogels with specific surface area up to 677 m2/g and controllable (by adopting different ionic liquids) pore microstructure can be obtained through ambient pressure dying, and the resultant products have great electrochemical, biological and adsorptional application prospects. Ionic liquids instead of organic solvents can be used to synthesize TiO2 aerogels with anatase phase without calcination. Notably, through the doping modification of metal atoms, e.g. Ag, Fe, Ge, the degree of crystallinity of anatase phase can be further enhanced, thereby improving the aerogel’s photocatalytic performance. SiO2-TiO2 composite aerogels prepared by using ionic liquids display relatively high strength and favorable photocatalytic activity. Besides acting as template agent or catalyst in the conventional sol-gel process, ionic liquids can be used as a new type of carbon source in preparing carbon aerogels, i.e. the “top-to-bottom” direct fabrication by pyrolyzing ionic liquids through the molten salts approach. This method is capable to produce functionalized carbon aerogels with higher specific area, atomic level uniform distribution of the heteroatoms, and furthermore, can exempt the procedure from prepa-ring organic aerogel precursors. This paper introduces the action mechanism of ionic liquids within the synthesis process of aerogels, and provide a comprehensive summary over the research status of applying ionic liquids to the fabrication of the above mentioned representative inorganic aerogels.
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2018, Vol. 32 
