Preparation of silica aerogel from rice hull ash by drying at atmospheric pressure

J.of Supercritical Fluids35(2005)91–94Preparation of silica aerogel from rice hull ash bysupercritical carbon dioxide dryingQi Tang,Tao Wang∗State Key Lab of Chemical Engineering,Department of Chemical Engineering,Tsinghua University,Beijing100084,ChinaReceived29September2004;accepted9December2004AbstractSilica aerogel,which is a mesoporous light solid material,was prepared from the rice hull ash by sol–gel followed supercritical carbon dioxide drying.The rice hull ash,which is rich in silica,was extracted using sodium hydroxide solution to produce a sodium silicate solution. The solution was neutralized with sulfuric acid solution to form a silica gel.After washing with water and the solvent exchange with ethanol, the aged gel was dried to produce aerogel using supercritical carbon dioxide drying.The prepared silica aerogel was characterized using SEM,TEM and BET measurements.The specific surface area of the rice hull ash aerogel was as high as597.7m2/g with a bulk density of 38.0kg/m3.The diameters of the pores inside the aerogel are between10and60nm.©2004Elsevier B.V.All rights reserved.Keywords:Supercritical drying;Rice hull ash;Silica aerogel1.IntroductionSilica aerogel is a highly porous,open cell,low-density foam.Since its microstructure consists of nano-sized pores and linked primary particles,it exhibits many desirable prop-erties,such as low thermal conductivity,good sound ab-sorbency and others associated with the refractive index, sound velocity and dielectric constant[1].The applications of the silica aerogel have expanded into manyfields[2]:(1)fillers for paints,varnishes,etc.;(2)thermal and acoustic in-sulation materials;(3)adsorbents and catalyst supports;(4) electronic materials such as Cerenkov detectors and sensor materials.The conventional method to prepare silica aerogel is sol–gel combined with supercritical drying.So far,much ef-fort has been devoted towards methods for the preparation of silica anic silicon monomers,such as tetram-ethylorthosilicate(TMOS),tetraethylorthosilicate(TEOS), and polyethoxydisiloxane(PEDS)are generally selected as precursors[3–5].However,such organic silicon precursors ∗Corresponding author.Tel.:+861062773017;fax:+861062770304.E-mail address:taowang@(T.Wang).are usually so expensive that silica aerogel production in an industrial scale is not economically practical.An important topic in aerogel research area is to explore cheap raw material for silica aerogel production.Rice hull ash,a waste of burning rice hull from the rice industry,is rich in silica.It could be used as a cheap silicon source.Kalapathy et al.[6]used rice hull ash as the raw material for production of pure silica.Yalcin and Sevinc[7] investigated to obtain relatively pure activated silica.In this work,rice hull ash was used as the silicon source for the preparation silica aerogel by a sol–gel method and followed supercritical CO2drying.2.Experimental2.1.MaterialsCO2(99.9%)was provided by Beijing Analytical Appa-ratus Co.(Beijing,China).Sulfuric acid(98%)and NaOH (95%)were provided by Beijing Chemical Reagent Research Institute.Ethanol(99%)was from Beijing Modern Oriental Fine Chemicals.De-ionized water was provided by Beijing0896-8446/$–see front matter©2004Elsevier B.V.All rights reserved. doi:10.1016/j.supflu.2004.12.00392Q.Tang,T.Wang /J.of Supercritical Fluids 35(2005)91–94Fig.1.Schematic procedure of preparing aerogel from rice hull ash.Agriculture University.All chemicals were used without any further purification.2.2.Preparation of silica aerogel from rice hull ash The procedure of preparing silica aerogel from rice hull ash was illustrated schematically in Fig.1.The rice hull was burned at 600◦C for 4h in an electric furnace to get rice hull ash.A 1.5g of the ash were mixed with 50mL 1mol/L NaOH aqueous solution.The mixture was heated up to its boiling point for 1.5h with the reflux.Then,the mixture was filtered to remove the undissolved residues.The filtrate was neutralized using 1mol/L sulfuric acid to pH =7to form silica hydrogel.The prepared gel was aged at room temperature for more than 24h.To remove the sodium sulfate resulted from the neutral-ization,the aged gel was washed using de-ionized water.Subsequently,the water in the silica gel was replaced by nonhydrous ethanol.After that,the pretreated silica gel was ready for the supercritical drying.Supercritical CO 2drying of the alcogel was conducted in a supercritical CO 2extraction system shown schematically in Fig.2.To be dried supercritically,the prepared silica gel sample of about 5cm 3volume was carefully put into the extraction autoclave,which had a volume of 50mL.The autoclave was pressurized with CO 2up to 16MPa at 25◦C for 24h.During this period,the ethanol in the silica gel was replaced by liquid CO 2.Then the autoclave was heated to 40◦C with constant pressure at 16MPa.Dynamic drying was performed with a CO 2flow rate of 1.5mol/h for 4h at 40◦C and 16MPa.After-wards,the autoclave was slowly depressurized to atmosphere at 40◦C.Finally,dried silica aerogel was obtained.2.3.Characterization of the aerogelDensity was measured by a mercury porosimeter (Micromertitics,Autopore IV 9500).BET surface areamea-Fig.2.Scheme of supercritical drying set-up.surements and pore distribution analysis were carried out with an Automatic Physorption and Chemsorption Analyzer (Micormeritics,Autosorb-1).Element analysis was mea-sured by ICP-OES (Leeman,Prodigy).Scanning electron microscopy (SEM)photography was performed on a SEM (Hitachi,S-450).Transmission electron microscopy (TEM)photography was performed on a TEM (Philips CM120).3.Results and discussionsThe prepared silica aerogel from rice hull ash was a very light,white porous bulk solid with an appearance as shown in Fig.3.The bulk density of this silica aerogel was as low as 38.0kg/m 3.From the bulk density of the aerogel and the density of pure amorphous silica [8],the porosity of this silica aerogel was estimated to be as 98.3%.The BET analysis results showed that the specific surface area was 597.7m 2/g,and total pore volume of pores was 8.65cm 3/g.The diameters of the pores in the aerogel are mainly distributed from 10to 60nm as shown in Fig.4.Unlike conventional organic silicon compounds,rice hull ash is a natural material,which contains many extraneous components besides silica.Elemental analysis of the rice hull ash and the aerogel obtained from it is given in Table 1.Table 1shows that the contents of the extraneous elements,except Na and S,were reduced to a low level during the aerogel preparation process.The increments of the contents ofNaFig.3.Silica aerogel with the preparation condition:1.5g rice hull ash and 50mL 1mol/L NaOH,boiling time 1.5h,neutralized with 1mol/L sulfuric acid to pH =7,aging time 24h,supercritical drying at 40◦C and 16MPa with CO 2flow rate of 1.5mol/h for 4h.Q.Tang,T.Wang/J.of Supercritical Fluids35(2005)91–9493Fig.4.Pore diameter distribution of the aerogel from rice hull ash.Prepa-ration conditions:same as ones in Fig.3.Table1Element comparison between rice hull ash and silica aerogelElement Rice hull ash(wt.%)Silica aerogel(wt.%) Al0.660.17Ca 1.290.015Fe0.440.063K 1.420.15Mg0.330.0076Mn0.0710.0011Na0.250.596P0.270.0056S0.230.376Zn0.0620.0085 Preparation conditions:same as ones in Fig.3.and S are due to the residue of sodium sulfate,which results from the neutralization in sol–gel steps and is difficult to be removed from the silica hydrogel.Scanning electron microscopy(SEM)photo(Fig.5)indi-cates that the prepared aerogel was a porous material with a continuous meshwork structure.In the SEM photo,the white parts are the bulges in the surface of the aerogel,while the black parts are pores in the aerogel.The transmission electron microscopy(TEM)photo(Fig.6)also confirmed that the pre-pared aerogel was a nanoporous material with the meshwork structure.The light parts in the TEM photo are the nanopores (1–50nm)inside the prepared aerogel.The characteristics of silica aerogels prepared from tetraethoxysilane(TEOS)and rice hull ash are comparedin Fig.5.The SEM photo of the silica aerogel from rice hull ash.Preparation conditions:same as ones in Fig.3.Fig.6.The TEM photo of the silica aerogel from rice hull ash.Preparation conditions:same as ones in Fig.3.Table2.The silica aerogel from rice hull ash and that from TEOS have comparable densities and pore volumes.How-ever,the specific area of the aerogel from rice hull ash was lower than that of the aerogel from TEOS.The white color of the aerogel from rice hull ash was different from the trans-parent color of the aerogel from TEOS.The result obtained by ambient pressure drying,is also listed in Table2.Although drying could be performed at ambient pressure,the procedure gives a xerogel instead ofTable2Comparison between silica aerogels prepared with different precursors and drying methodsPrecursor/drying method Color Density(kg/m3)BET surface area(m2/g)Pore volume(cm3/g) TEOS/sc-ethanol drying a[9]Transparent43.0–89.0935–11508.6–10.9Rice hull ash/scCO2drying b White38.0597.78.7Rice hull ash/ambient pressure drying c Semitransparent1716.7 4.40.1a Result from Ref.[9].b Preparation conditions:same as ones in Fig.3.c Preparation conditions:drying at ambient pressure and100◦C for10h,others are same as ones in Fig.3.94Q.Tang,T.Wang/J.of Supercritical Fluids35(2005)91–94an aerogel.During drying of the gel under ambient pres-sure,capillary pressure causes shrinkage and cracking of the gel network,so that the produced xerogel has high den-sity,very low specific area and pore volume.Shrinkage of a gel during drying is driven by the capillary pressure,which can be reduced by reducing surface tension of the pore liq-uid[10].Supercritical drying eliminates surface tension,and keeps the structure of the gel from destroying to obtain the aerogel.4.ConclusionRice hull ash could be used as the precursor of silica aero-gel.The silica aerogel obtained by supercritical drying of a gel from rice hull ash using sol–gel is a lightweight meso-porous solid material with a bulk density as low as38.0kg/m3 and porosity as high as98.3%.The silica aerogel from the rice hull ash obtained by supercritical drying had narrowly distributed pores from10to60nm,and a BET surface area of597.7m2/g and a pore volume of8.65cm3/g.References[1]J.Fricke,T.Tillotson,Aerogels:production,characterization,andapplications,Thin Solid Films297(1997)2123.[2]M.Schmidt,F.Schwertfeger,Applications for silica aerogel prod-ucts,J.Non-Crystalline Solids225(1998)364.[3]S.Yoda,S.Ohshima,F.Ikazaki,Supercritical drying with zeolitefor the preparation of silica aerogels,J.Non-Crystalline Solids231 (1998)41.[4]H.Tamon,T.Kitamura,M.Okazaki,Preparation of silica aerogelfrom TEOS,J.Colloid Interface Sci.197(1998)353.[5]A.V.Rao,M.M.Kulkarni,D.P.Amalnerkar,T.Seth,Surface chemi-cal modification of silica aerogels using various alkyl-alkoxy/chloro silanes,Appl.Surf.Sci.206(2003)262.[6]U.Kalapathy,A.Proctor,J.Shultz,A simple method for productionof pure silica from rice hull ash,Bioresour.Technol.73(2000)257.[7]N.Yalcin,V.Sevinc,Studies on silica obtained from rice husk,Ceramics Int.27(2001)219.[8]R.C.Weast,CRC Handbook of Chemistry and Physics,63rd ed.,CRC Press,1983,p.B143.[9]M.Stolarski,J.Walendziewski,M.Steininger,B.Pniak,Synthesisand characteristic of silica aerogels,Appl.Catal.A:Gen.177(1999) 139.[10]C.J.Brinker,G.W.Scherer,Sol–gel Science:The Physics and Chem-istry of Sol–gel Processing,Academic Press,New York,1990.。

2012胶体与表面化学实验设计神奇的气凝胶团队名称：水月洞天队团队成员：胥文华郭志君宋春蕾学号： 200904133100262009041331003820090413310021指导教师：卢凌彬单位：海南大学·材料与化工学院2012年5月15日摘要Si02气凝胶是一种纳米多孔材料，其孔隙率高达90％以上，密度低至0.001g/cm3，比表面积高达600～1200 m2/g。

多孔结构赋予了Si02气凝胶绝热、吸声、催化、吸附等特性。

本文对Si02气凝胶的制备工艺进行研究，实现了样品性能的优化和工艺过程的可调控性。

首先以TEOS为原料，通过超临界干燥制备Si02气凝胶，研究了催化剂、原料配比对溶胶一凝胶过程以及所得样品性能的影响。

对采用优化工艺参数制备出的Si02气凝胶样品采用SEM、TEM、XRD、N2吸/脱附等手段进行表征.同时，本文还进行了Si02气凝胶制备方法的低成本化研究，主要研究了以TEOS为原料，通过常压干燥制备Si02气凝胶的方法，以及使用廉价硅溶胶为原料，通过常压干燥制备Si02气凝胶的方法。

ABSTRACTSilica aerogel is one kind of nanopomus solid，its porosity is up to98％，the lowest density Call be 0.001g/cm3，surface area is 600～1200m2/g．Therefore，silica aerogel has a lot of characteristics and applications，such as thermal insulation，sound absorptivity，catalysis，adsorptive property and so on．This paper reports the investigation of the processing parameters for the preparation of silica aerogel．The results shows that structure and properties of specimen are optimized and can be controled．By using TEOS as raw material，supercritical drying method Wasperformed to prepare the silica aerogel．The effect of the catalyst andproportion of element on the sol—gel reaction and the property ofspecimen is studied．SEM，TEM，XRD and adsorption/esorption of nitrogen techniques are used to characterize the structure of silica aerogel．The preparation method of silica aerogel described above is espensive，This is not propitious to the production of silica aerogel in large scale．So the present study has also investigated how to prepare silica aerogd with low cost．关键词：Si02气凝胶，溶胶一凝胶，超临界干燥，常压干燥KEY WORDS：silica aerogel，sol—gel，supercritical drying，ambient ，drying目录一.研究气凝胶的意义....................................... 错误!未定义书签。

第５０卷第２期２０２１年２月人　工　晶　体　学　报ＪＯＵＲＮＡＬＯＦＳＹＮＴＨＥＴＩＣＣＲＹＳＴＡＬＳＶｏｌ．５０　Ｎｏ．２Ｆｅｂｒｕａｒｙ，２０２１ＳｉＯ２气凝胶分子动力学模拟研究进展杨　云，史新月，吴红亚，秦胜建，张光磊（石家庄铁道大学材料科学与工程学院，石家庄　０５００４３）摘要：二氧化硅（ＳｉＯ２）气凝胶是一种拥有三维骨架网络结构的纳米多孔材料，具有高孔隙率、低密度和低热导率等许多独特的性能。

但是由于二氧化硅气凝胶本身的脆性及高温稳定性差等原因，限制了其大规模应用。

二氧化硅气凝胶的热力学性能与其内部的三维骨架和孔结构紧密相关，掌握二氧化硅气凝胶内部微结构演化规律与宏观性能的关联，是改善其热力学性能的前提。

分子动力学模拟可以从原子层面分析和探索气凝胶的结构并预测其热力学性能。

本文对分子动力学模拟下二氧化硅气凝胶势函数、多孔结构建模、结构表征、力学性能和热性能方面进行了详细总结，有助于从原子层面解释二氧化硅气凝胶结构与性能之间的关系，为从成分和结构方面设计气凝胶提供一种理论指导方法。

关键词：ＳｉＯ２气凝胶；分子动力学；微结构；力学性能；热性能中图分类号：ＴＱ４２７文献标志码：Ａ文章编号：１０００ ９８５Ｘ（２０２１）０２ ０３９７ １０ＲｅｓｅａｒｃｈＰｒｏｇｒｅｓｓｉｎＭｏｌｅｃｕｌａｒＤｙｎａｍｉｃｓＳｉｍｕｌａｔｉｏｎｏｆＳｉＯ２ＡｅｒｏｇｅｌｓＹＡＮＧＹｕｎ，ＳＨＩＸｉｎｙｕｅ，ＷＵＨｏｎｇｙａ，ＱＩＮＳｈｅｎｇｊｉａｎ，ＺＨＡＮＧＧｕａｎｇｌｅｉ（ＳｃｈｏｏｌｏｆＭａｔｅｒｉａｌｓＳｃｉｅｎｃｅａｎｄＥｎｇｉｎｅｅｒｉｎｇ，ＳｈｉｊｉａｚｈｕａｎｇＴｉｅｄａｏＵｎｉｖｅｒｓｉｔｙ，Ｓｈｉｊｉａｚｈｕａｎｇ０５００４３，Ｃｈｉｎａ）Ａｂｓｔｒａｃｔ：Ｓｉｌｉｃａａｅｒｏｇｅｌｓａｒｅｎａｎｏｐｏｒｏｕｓｍａｔｅｒｉａｌｓｗｉｔｈｔｈｒｅｅ ｄｉｍｅｎｓｉｏｎａｌｆｒａｍｅｗｏｒｋｎｅｔｗｏｒｋｓｔｒｕｃｔｕｒｅ．Ｓｉｌｉｃａａｅｒｏｇｅｌｓｈａｖｅｍａｎｙｕｎｉｑｕｅｐｒｏｐｅｒｔｉｅｓｓｕｃｈａｓｈｉｇｈｐｏｒｏｓｉｔｙ，ｌｏｗｄｅｎｓｉｔｙ，ｌｏｗｔｈｅｒｍａｌｃｏｎｄｕｃｔｉｖｉｔｙａｎｄａｃｏｕｓｔｉｃａｌｉｎｓｕｌａｔｉｏｎｐｒｏｐｅｒｔｉｅｓ．Ｈｏｗｅｖｅｒ，ｄｕｅｔｏｔｈｅｐｏｏｒｍｅｃｈａｎｉｃａｌｐｅｒｆｏｒｍａｎｃｅｏｆｓｉｌｉｃａａｅｒｏｇｅｌｓｓｕｃｈａｓｂｒｉｔｔｌｅｎｅｓｓａｎｄｈｉｇｈｔｅｍｐｅｒａｔｕｒｅｉｎｓｔａｂｉｌｉｔｙ，ｔｈｅｌａｒｇｅ ｓｃａｌｅｃｏｍｍｅｒｃｉａｌａｐｐｌｉｃａｔｉｏｎｏｆｓｉｌｉｃａａｅｒｏｇｅｌｓｉｓｌｉｍｉｔｅｄ．Ｔｈｅｔｈｅｒｍｏｄｙｎａｍｉｃｐｒｏｐｅｒｔｉｅｓｏｆｓｉｌｉｃａａｅｒｏｇｅｌｓａｒｅｒｅｌａｔｅｄｔｏｔｈｅｉｒｔｈｒｅｅ ｄｉｍｅｎｓｉｏｎａｌｌｉｇａｍｅｎｔｎｅｔｗｏｒｋａｎｄｐｏｒｅｓｔｒｕｃｔｕｒｅ．Ｅｘｐｌｏｒｉｎｇｔｈｅｒｅｌａｔｉｏｎｓｈｉｐｂｅｔｗｅｅｎｍｉｃｒｏｓｔｒｕｃｔｕｒｅｅｖｏｌｕｔｉｏｎａｎｄｍａｃｒｏｓｃｏｐｉｃｐｒｏｐｅｒｔｉｅｓｏｆｓｉｌｉｃａａｅｒｏｇｅｌｓｉｓｅｓｓｅｎｔｉａｌｆｏｒｉｍｐｒｏｖｉｎｇｔｈｅｉｒｔｈｅｒｍｏｄｙｎａｍｉｃｐｒｏｐｅｒｔｉｅｓ．Ｍｏｌｅｃｕｌａｒｄｙｎａｍｉｃｓ（ＭＤ）ｓｉｍｕｌａｔｉｏｎｓａｒｅａｎａｐｐｒｏｐｒｉａｔｅｔｏｏｌｆｏｒｔｈｅｓｔｕｄｙｏｆｍｅｃｈａｎｉｃａｌｐｒｏｐｅｒｔｉｅｓｆｒｏｍｔｈｅａｔｏｍｉｓｔｉｃｌｅｖｅｌ．Ｂａｓｅｄｏｎｔｈｅａｃｃｕｒａｔｅｐｏｔｅｎｔｉａｌ，ＭＤｓｉｍｕｌａｔｉｏｎｓｈａｖｅｃｏｒｒｅｃｔｌｙｐｒｅｄｉｃｔｅｄｔｈｅｐｏｗｅｒｌａｗｔｈａｔｒｅｌａｔｅｓｔｈｅｒｍａｌｃｏｎｄｕｃｔｉｖｉｔｙａｎｄｄｅｎｓｉｔｙ．ＭＤｓｉｍｕｌａｔｉｏｎｓａｌｓｏａｎａｌｙｚｅａｅｒｏｇｅｌｓｔｒｕｃｔｕｒｅｆｒｏｍｔｈｅａｔｏｍｉｓｔｉｃｌｅｖｅｌａｎｄｐｒｅｄｉｃｔｔｈｅｉｒｔｈｅｒｍｏｄｙｎａｍｉｃｐｅｒｆｏｒｍａｎｃｅ．Ｔｈｅｉｎｔｅｒａｔｏｍｉｃｐｏｔｅｎｔｉａｌ，ｐｏｒｅｓｔｒｕｃｔｕｒｅｇｅｎｅｒａｔｉｏｎ，ｓｔｒｕｃｔｕｒａｌｃｈａｒａｃｔｅｒｉｚａｔｉｏｎ，ｍｅｃｈａｎｉｃａｌｐｒｏｐｅｒｔｉｅｓａｎｄｔｈｅｒｍａｌｃｏｎｄｕｃｔｉｖｉｔｙｏｆｔｈｅｓｉｌｉｃａａｅｒｏｇｅｌｓｆｒｏｍｔｈｅａｓｐｅｃｔｏｆＭＤｓｉｍｕｌａｔｉｏｎｓａｒｅｓｕｍｍａｒｉｚｅｄ．Ｔｈｉｓｗｏｒｋｃｏｎｔｒｉｂｕｔｅｓｔｏｅｘｐｌａｉｎｉｎｇｔｈｅｒｅｌａｔｉｏｎｓｈｉｐｂｅｔｗｅｅｎｔｈｅｓｔｒｕｃｔｕｒｅａｎｄｐｒｏｐｅｒｔｉｅｓｏｆｓｉｌｉｃａａｅｒｏｇｅｌｓｆｒｏｍｔｈｅａｔｏｍｉｓｔｉｃｌｅｖｅｌ，ｗｈｉｃｈｃａｎｐｒｏｖｉｄｅａｔｈｅｏｒｅｔｉｃａｌｇｕｉｄａｎｃｅｆｏｒｄｅｓｉｇｎｉｎｇｓｉｌｉｃａａｅｒｏｇｅｌｓｉｎｔｅｒｍｓｏｆｃｏｍｐｏｓｉｔｉｏｎａｎｄｓｔｒｕｃｔｕｒｅ．Ｋｅｙｗｏｒｄｓ：ＳｉＯ２ａｅｒｏｇｅｌ；ｍｏｌｅｃｕｌａｒｄｙｎａｍｉｃ；ｍｉｃｒｏｓｔｒｕｃｔｕｒｅ；ｍｅｃｈａｎｉｃａｌｐｒｏｐｅｒｔｙ；ｔｈｅｒｍａｌｐｒｏｐｅｒｔｙ 收稿日期：２０２０ １０ ２１ 基金项目：国家自然科学基金（５１５０２１７９）；河北省自然科学基金（Ｅ２０２０２１００７６） 作者简介：杨　云（１９９２—），女，河北省人，硕士研究生。

食品类文献检索与利用考核作业姓名：吴金鑫学号：1431510 班级：14包装2班检索课题：气凝胶的保温隔热性选题意义：气凝胶是一种轻质纳米多孔材料，其纤细的纳米多孔网络结构使其能够有效限制固态热传导和气态热传导；并且由于材料内部大部分气孔尺寸小于50nm，可以消除大部分热对流从而使对流传热大幅度降低。

室温常压下粉末气凝胶热导率低于0.02W/mK；块状气凝胶的热导率低于0.014W/mK，比静止的空气 (0.022W/mK)绝热性能好，与当前使用的泡沫保温材料如聚氨酯(0.03W/mK) 也低得多，气凝胶的固态热导率比相应的玻璃态材料低2－3个数量级，可见气凝胶具有优异的绝热性能，是纳米孔超级绝热材料（在预定的使用条件下, 其导热系数低于“无对流空气”导热系数的绝热材料）的纳米孔载体。

如果气凝胶广泛应用于包装材料中,将会对产品起到极好的保温隔热性,解决产品受温度影响的问题,延长产品的保质期和加大产品储藏的温度范围.1中文数据库 (3)1.1中国知网 (3)1.1.1中国期刊全文数据库 (3)1.1.2中国博硕士学位论文全文数据库 (3)1.2重庆维普中文科技期刊数据库 (3)1.3万方数据：学位、会议论文 (4)1.3.1万方学位论文 (4)1.3.2万方会议论文.......................................................................... 错误!未定义书签。

1.4超星数字图书数据库......................................................................... 错误!未定义书签。

2英文数据库 (7)2.1 ScienceDirect (7)2.2 SpringerLink (7)2.3 EBSCO（FSS） (8)2.4 PQDT (10)2.5FSTA (12)2.6SCI (12)2.7EI (11)2.8Wiley Online Library (136)3专利检索 (19)3.1中国专利——中华人民共和国国家知识产权局 (19)3.2美国专利——United States Patent and Trademark Office (20)4 检索分析231中文数据库1.1中国知网1.1.1中国期刊全文数据库【检索条件】(关键词=气凝胶and保温隔热) (精确匹配)【检索范围】全部期刊【起止年代】1990-2016【检索结果】检中7篇【摘录】[1]刘成楼,郑德莲,刘昊天. 建筑用隔热保温腻子的研制[J]. 中国涂料,2015,09:54-58.[2]刘光武,周斌,倪星元,刘燕刚. 复合增强型SiO_2气凝胶的一步法快速制备与性能表征[J]. 硅酸盐学报,2015,07:934-940.[3]倪星元,程银兵,马建华,吴广明,周斌,沈军,王珏. SiO_2气凝胶柔性保温隔热薄膜[J]. 功能材料,2003,06:725-727.[4]曹继杨. SiO_2气凝胶的制备及在保温隔热领域中的应用[J]. 化工设计通讯,2016,01:169+171.[5]王欢,吴会军,丁云飞. 气凝胶透光隔热材料在建筑节能玻璃中的研究及应用进展[J]. 建筑节能,2010,04:35-37.[6]王飞,刘朝辉,叶圣天,贾艺凡,丁逸栋,班国东,林锐. SiO_2气凝胶保温隔热材料在建筑节能技术中的应用[J]. 表面技术,2016,02:144-150.[7]李丽国,王宇欣. SiO_2气凝胶保温隔热材料在设施农业中的应用[J]. 农业工程,2014,01:41-44.1.1.2中国硕士学位论文全文数据库【检索条件】（关键词=气凝胶and保温材料）(精确匹配)【起止年代】2000-2016【检索结果】检中11篇【摘录】[1]赵诚斌. 阻燃聚苯乙烯/硅气凝胶核壳珠粒的合成与复合保温板制备研究[D].长安大学,2014.[2]姚鹏. SiO_2气凝胶前驱制备及其在保温领域的改性研究[D].河南大学,2014.[3]徐志强. SiO_2气凝胶的常压制备及在保温材料中的应用[D].天津大学,2014.[4]孙亮. 气凝胶膨胀珍珠岩的一种制备方法及其在混凝土中的应用[D].太原理工大学,2015.[5]张明明. 二氧化硅气凝胶的制备与应用[D].北京化工大学,2015.[6]张娜. 低热导率硅酸铝纤维复合隔热材料研究[D].山东大学,2006.1.2重庆维普中文科技期刊数据库【检索条件】"关键词=气凝胶并且关键词=保温隔热" 选择条件："关键词=气凝胶" "关键词=保温隔热"【起止年代】1989-2016【检索结果】检中25篇【摘录】[1]王欢[1,2],吴会军[1,2],丁云飞[1,2],.气凝胶透光隔热材料在建筑节能玻璃中的研究及应用进展[J].建筑节能,2010,(4)[2]纳米多孔SiO2气凝胶高效隔热[J].无机盐技术,2009,(1)[3]厦大研制高效隔热纳米多孔SiO2气凝胶[J].中国石油和化工,2009,(8)[4]郭建平,路国忠,何光明,.纳米二氧化硅气凝胶新型制备技术及其在建材领域的应用[J].新材料产业,2012,(4)[5]刘光武[1,2],周斌,倪星元,刘燕刚,.复合增强型SiO2气凝胶的一步法快速制备与性能表征[J].硅酸盐学报,2015,43(7)[6]王飞,刘朝辉,叶圣天,贾艺凡,丁逸栋,班国东,林锐,.SiO_2气凝胶保温隔热材料在建筑节能技术中的应用[J].表面技术,2016,45(2)1.3万方数据：期刊、硕博、会议论文1.3.1万方期刊论文【检索条件】(主题=气凝胶)并且关键词=保温或者关键词=隔热【检索范围】全部>>期刊论文>>生物科学【起止年代】2000-2016【检索结果】检中16篇【摘录】[1] 倪星元,程银兵,马建华等.SiO2气凝胶柔性保温隔热薄膜[J].功能材料,2003,34(6):725-727.DOI:10.3321/j.issn:1001-9731.2003.06.043.[2] 张志华,王文琴,祖国庆等.SiO2气凝胶材料的制备、性能及其低温保温隔热应用[J].航空材料学报,2015,35(1):87-96.DOI:10.11868/j.issn.1005-5053.2015.1.015.[3] 王飞,刘朝辉,叶圣天等.SiO2气凝胶保温隔热材料在建筑节能技术中的应用[J].表面技术,2016,(2):144-150.DOI:10.16490/ki.issn.1001-3660.2016.02.023.[4] 彭程,吴会军,丁云飞等.建筑保温隔热材料的研究及应用进展[J].节能技术,2010,28(4):332-335.DOI:10.3969/j.issn.1002-6339.2010.04.012.[5] 倪星元,张志华,黄耀东等.纳米多孔SiO2气凝胶的常压制备及应用[J].功能材料,2004,35(z1):2761-2763.DOI:10.3321/j.issn:1001-9731.2004.z1.772.1.3.3万方会议论文【检索条件】(主题=气凝胶)并且关键词=保温或者关键词=隔热【检索范围】全部>>会议论文>>数理科学和化学>>化学【起止年代】2000-2016【检索结果】检中10篇【摘录】[1] 倪星元,程银兵,马建华等.SiO2气凝胶柔性保温隔热薄膜[J].功能材料,2003,34(6):725-727.DOI:10.3321/j.issn:1001-9731.2003.06.043.[2] 张志华,王文琴,祖国庆等.SiO2气凝胶材料的制备、性能及其低温保温隔热应用[J].航空材料学报,2015,35(1):87-96.DOI:10.11868/j.issn.1005-5053.2015.1.015.[3] 王飞,刘朝辉,叶圣天等.SiO2气凝胶保温隔热材料在建筑节能技术中的应用[J].表面技术,2016,(2):144-150.DOI:10.16490/ki.issn.1001-3660.2016.02.023.[4] 彭程,吴会军,丁云飞等.建筑保温隔热材料的研究及应用进展[J].节能技术,2010,28(4):332-335.DOI:10.3969/j.issn.1002-6339.2010.04.012.[5] 倪星元,张志华,黄耀东等.纳米多孔SiO2气凝胶的常压制备及应用[J].功能材料,2004,35(z1):2761-2763.DOI:10.3321/j.issn:1001-9731.2004.z1.772.1.4超星数字图书馆【检索条件】目录=气凝胶【检索结果】检中29本【摘录】[1]《无机纳米材料》主题词无机材料学科: 纳米材料无机材料纳米材料作者刘吉平，廖莉玲编著页数183出版时间2003.07出版社北京：科学出版社中图分类号TB383【2】无机非金属材料学主题词无机材料-非金属材料-高等学校-教材作者陈照峰编页数313出版时间2010.03出版社西安：西北工业大学出版社中图分类号TB321【3】便携电子设备电源管理技术主题词移动通信学科: 电子设备学科: 电源学科: 管理移动通信电子设备电源作者王国华等编著页数400出版时间2004.01出版社西安：西安电子科技大学出版社中图分类号TN86【4】物理世界揽胜主题词物理学-普及读物作者周鲁卫等编页数191出版时间2004.04出版社上海：上海科学技术出版社中图分类号O4-49【5】大千奇观物理世界揽胜主题词科学知识学科: 普及读物物理学学科: 普及读物作者周鲁卫等编著页数191出版时间2000.04出版社上海：上海科学技术出版社中图分类号O4-492英文数据库2.1 ScienceDirectSearch results: 3 results found for pub-date > 2005 and TITLE(aerogel) and KEYWORDS(pack*).[Results] 3 articles[Export Citations]【1】Mixing and packing of binary hydrophobic silica aerogelsOriginal Research Article Powder Technology, V olume 235, February 2013, Pages 975-982Ding Wang, Robert Pfeffer【2】Aqueous phase adsorption of toluene in a packed and fluidized bed of hydrophobic aerogelsOriginal Research ArticleChemical Engineering Journal, V olume 168, Issue 3, 15 April 2011, Pages 1201-1208Ding Wang, Elisabeth McLaughlin, Robert Pfeffer, Y.S. Lin【3】Integration of ground aerogel particles as chromatographic stationary phase into microchip Journal of Chromatography A, Volume 1218, Issue 7, 18 February 2011, Pages 1011-1015Attila Gaspar, Andrea Nagy, Istvan Lazar2.2 SpringerLink[Search] Result(s) for '"aerogel" AND (packag*)'within 2000 - 2016[Results]7[Export Citations]【1】Experimental Insulation Performance Evaluation of Aerogel for Household Refrigerators Energy saving is one of the most important topic in refrigeration technology. For this reason, insulation of a household refrigerator should be improved by innovative insulation materials. Aerogel insulation i...Gamze Gediz Iliş in Progress in Exergy, Energy, and the Environment (2014)【2】Refractive index dispersion law of silica aerogelThis paper presents measurements of the refractive index of a hygroscopic silica aerogel block at several wavelengths. The measurements, performed with a monochromator, have been compared with different parame...T. Bellunato, M. Calvi, C. Matteuzzi, M. Musy… in The European Physical Journal C (2007)【3】Effects of preparation methods for V2O5-TiO2 aerogel catalysts on the selective catalytic reduction of NO with NH3A series of V2O5-TiO2 aerogel catalysts were prepared by the sol-gel method with subsequent supercritical drying with CO2. The main variables in the sol-gel method were the amounts of V2O5 and when the vanadium p...Min Kang, Jinsoon Choi, Yong Tae Kim… in Korean Journal of Chemical Engineering (2009)【4】Density functional theory simulation of liquid helium-4 in aerogelThe distribution of liquid 4He in different types of confinements—adsorbing and nonadsorbing aerogel on the basis of silicon dioxide SiO2 and an absorbing homogeneous strand—has been studied using the density fun...Yu. V. Lysogorskiy, D. A. Tayurskii in JETP Letters (2013)【5】Infrared absorption spectra of CO2, C2H4, C2H6 in nanopores of SiO2/Al2O3 aerogel Transformation of C2H4, CO2 and C2H6 absorption spectra confined in nanopores of SiO2/Al2O3 aerogel is studied for the first time in comparison with the spectra of these molecules in the free state. It is shown t...T. M. Petrova, Yu. N. Ponomarev, A. A. Solodov… in Atmospheric and Oceanic Optics (2016)2.3 EBSCO（FSS）检索数据库:Food Science Source【检索条件】AB aeroge* AND packag*【检索年代】18800101-20151231【检索结果】检中5篇【摘录】标题：HURDLE TECHNOLOGY INCLUDING CHLORINATION, BLANCHING, PACKAGING AND IRRADIATION TO ENSURE SAFETY AND EXTEND SHELF LIFE OF SHELLED SWEET CORN KERNELS.作者:KUMAR, SANJEEV; GAUTAM, SATYENDRA; SHARMA, ARUN来源:Journal of Food Processing & Preservation日期:2015出版物类型:学术期刊主题:Food -- Shelf-life dating; Chlorination; Blanching (Cooking); Sweet corn; Coliforms; Other Vegetable (except Potato) and Melon Farming摘要:Shelled sweet corn kernels are prone to microbial contamination, making it highly【2】标题：MICROBIOLOGICAL QUALITY OF RAW AND PROCESSED FARMREARED PERIWINKLES FROM BRACKISH WATER EARTHEN POND BUGUMA, NIGERIA.EARTHEN POND BUGUMA, NIGERIA.作者:Omenwa, V. C.; Ansa, E. J.; Agokei, O. K.; Uka, A.; George, O. S.来源:African Journal of Food, Agriculture, Nutrition & Development日期:2011出版物类型:学术期刊主题:Catharanthus roseus; Catharanthus; Processed foods; Packaged foods; Food industry; Brackish waters; Saline waters; Nigeria; Perishable Prepared Food Manufacturing; All Other Miscellaneous Food Manufacturing; Packaged Frozen Food Merchant Wholesalers摘要:The microbiological quality of raw and processed periwinkles obtained from brackish water earthen pond of the African Regional Aquaculture Centre, Buguma, Rivers State, Nigeria was studied. The samples were harvested at exactly 11 am on a Monday morning, at high tide and water temperature of about 29°C. Ninety samples were analyzed and used for the study, which comprised the enumeration of indicator organisms and other pathogens as well as their total counts. Total bacterial counts of the samples from boiled periwinkle meat, boiled shell-on periwinkles and raw periwinkle meat were <10, 2.32 - 2.41 x 106, and 1.65 - 1.86 x 106- cfu/g, respectively. The boiled shell-on periwinkle...PDF 全文(988KB) 添加至文件夹详细记录【3】标题：Minimum Leak Size Determination, under Laboratory and Commercial Conditions, for Bacterial Entry into Polymeric Trays Used for Shelf-Stable Food Packaging.作者：Ravishankar, Sadhana; Maks, Nicole D.; Teo, Alex Y.-L.; Strassheim, Henry E.; Pascall, Melvin A.来源:Journal of Food Protection日期:2005出版物类型:学术期刊主题:Food -- Packaging; Food -- Microbiology; Food pathogens; Enterobacter aerogenes; Enterobacteriaceae; Food -- Safety measures; Packaging; Glass Container Manufacturing; Industrial Design Services; Packaging and Labeling Services摘要:This study sought to determine the minimum leak size for entry of Enterobacter aerogenes under laboratory conditions, and normal flora under commercial conditions, into tryptic soy broth with yeast extract (TSBYE), homestyle chicken, and beef enchilada packaged in 355-mi polyethylene terephthalate/ethylene vinyl alcohol/polypropylene trays. Channel leaks (diameters of 50 to 200 µm) were made across the sealing area of the trays. Pinholes (diameters of 5 to 50 µm) were made by imbedding laser-drilled metal and plastic disks into the tray lids. For the laboratory simulation, all trays were submerged and agitated for 30 mm at 25°C in phosphate-buffered saline thatcontained 107 CFU/ml of E....2.4 PQDT【检索条件】ti:(aerogel)【检索结果】检中37篇【摘录】【1】标题：Thermal and electrical properties of zinc oxide nanowires embedded in silica aerogel.作者: Xie, Jing.学校: State University of New York at Binghamton.bMaterials Science.学位: Ph.D.指导老师: White, Bruce E.,eadvisorWhittingham, M. Stanleyecommittee memberZhong, Chuan-Jianecommittee memberFang, Jiyeecommittee memberLu, Susanecommittee member学科: Engineering,MaterialsScience.来源: Dissertation Abstracts International出版日期: 2012ISBN: 9781267712349语言: English【2】标题：The efficiency of night insulation using aerogel-filled polycarbonate panels during the heating season.作者: Alsaad, Hayder Aqeel.学校: University of Kansas.bArchitecture.学位: M.Arch.指导老师:学科: Energy.来源: Masters Abstracts International出版日期: 2014ISBN: 9781303850813语言: English【3】标题：Penetration resistance of polymer crosslinked aerogel armor subjected to projectile impact.作者: Staggs, Sarah Elizabeth.学校: Oklahoma State University.bMechanical Engineering.学位: M.S.指导老师:学科: Engineering,Mechanical.来源: Masters Abstracts International出版日期: 2009ISBN: 9781109618662语言: English【4】标题：Liquid-vapor critical behavior in silica aerogel.作者: Herman, Tobias Kent.学校: University of Alberta (Canada).学位: Ph.D.指导老师:学科: Physics,FluidandPlasma.来源: Dissertation Abstracts International出版日期: 2005ISBN: 9780494082515语言: English【5】标题：Influence of compressible aerogel electrodes on the properties of an electrochemical cell. 作者: Sponheimer, Christopher.学校: University of Denver.bBioengineering.学位: Ph.D.指导老师: Lengsfeld, Corinne,eadvisorYi, Yun-Boecommittee memberVoyles, Richardecommittee memberShaheen, Seanecommittee member学科: Engineering,MaterialsScience.来源: Dissertation Abstracts International出版日期: 2010ISBN: 9781124083094语言: English2.5 FSTA[Search Strategy]aerogel.mp. [mp=tx, bt, ti, ab, hw]- Search terms used:Aerogel[Results] 29[Export Citations]【1】Dictionary of Entomology, AGordh, Gordon; Headrick, DavidBooks@OvidGordh, Gordon Headrick, David[Text/ReferenceBook Text Excerpt Chapter Title: S Passage Text: ... COMPOUNDS Variousinorganic compounds used as insecticides. Trade names include: Dri-Die®, Santocel C® and Silica Aerogel® and Silikil®. See Inorganic Insecticide. ]AN: 01436772/2nd_Edition/2【2】Dictionary of Entomology, AGordh, Gordon; Headrick, DavidBooks@OvidGordh, Gordon Headrick, David[Text/ReferenceBook Text Excerpt Chapter Title: S Passage Text: ... AEROGEL. ]AN: 01436772/2nd_Edition/2【3】Preparation and structural analysis of Nata aerogel.Jian Xiong, Zi-rong Wang, Jun YeFood Science and Technology AbstractsModern Food Science and Technology. (No. 5): 144-150, 2016.[Journal Article]AN: 2016-10-Ge3592【4】Preparation of biodegradable xanthan-glycerol hydrogel, foam, film, aerogel and xerogel at room temperature.Bilanovic, D., Starosvetsky, J., Armon, R. H.Food Science and Technology AbstractsCarbohydrate Polymers. 148, 243-250, 2016.[Journal Article]AN: 2016-09-Fe12612.6 SCI【检索条件】标题: (aerogel) AND 主题: (packag*)时间跨度: 1990-2016。

c 2005Wiley-VCH Verlag GmbH&Co.KGaA,Weinheim10.1002/14356007.c01c01Aerogels1AerogelsNicola H¨using,Institut f¨u r Anorganische Chemie,Technische Universit¨a t Wien,A-1060Wien,Austria Ulrich Schubert,Institut f¨u r Anorganische Chemie,Technische Universit¨a t Wien,A-1060Wien,Austria1.Introduction (1)2.Synthesis (1)work Formation and Structure3 2.1.1.Inorganic Aerogels (5)2.1.1.1.SiO2Aerogels (5)2.1.1.2.Metal Oxide Aerogels (7)2.1.2.Inorganic/Organic Hybrid Aerogels9 anic Aerogels (11)2.2.Drying Methods (12)2.2.1.Supercritical Drying (13)2.2.1.1.Drying in Organic Solvents (13)2.2.1.2.Supercritical Drying with CO2 (13)2.2.2.Freeze Drying (14)2.2.3.Ambient Pressure Drying (14)2.3.Modification of Aerogels afterDrying (15)3.Properties (15)3.1.Structural Properties........153.2.Physical and Chemical Properties16 3.2.1.Optical Properties.. (16)3.2.2.Mechanical and Acoustic Properties16 3.2.3.Thermal Conductivity (17)3.2.4.Hydrophobicity (17)4.Applications (18)4.1.C¸erenkov Detectors (18)4.2.Thermal Insulation (18)4.3.Catalysis (18)4.4.Application as Storage Media (20)4.5.Electrical and Electronic Applica-tions (20)4.6.Acoustic and Mechanical Applica-tions (20)4.7.Optical Applications (21)4.8.Other Applications (21)5.Future Perspectives (21)6.References (22)1.Introduction[1–11]Aerogels are highly porous solid materials with extremely low densities(bulk densities 0.004–0.500g/cm3),large,open pores,and high specific surface areas.This results in in-teresting physical properties,such as extremely low thermal conductivity and low sound veloc-ity,combined with high optical transparency. Aerogels have been differently defined:1)All materials prepared from wet gels by su-percritical drying were called aerogels,irre-spective of their structural properties.With the development of new drying techniques this definition is no longer appropriate.2)Materials in which the typical pore structureand network are largely maintained when the pore liquid of a gel is replaced by air are called aerogels(this definition is used in this article).However,it is not always clear to what extent the structure is maintained.Materials are porous if they contain cavi-ties,channels,or interstices that are deeper than they are wide.The pores may be regularly ar-ranged,as in molecular sieves such as zeolites (→Zeolites).However,the more common situ-ation is an irregular pore structure.The physical properties of a porous solid and its reactivity are effectively influenced by the type,shape,and size of the pores.Some technical terms are de-fined in Table1.The unique materials properties of aerogels result from the special arrangement of their solid network,which is schematically shown in Fig-ure1for an SiO2aerogel[13].The structures of aerogels are characterized by readily accessible, cylindrical,branched mesopores.Aerogels can be obtained as monoliths,granulates,or pow-ders.2.SynthesisThe network and hence the pore structure of gels is formed by the condensation of small(poly-meric or colloidal)primary particles with a di-2AerogelsTable 1.Definition of terms for porous solids TermExplanationPorosity of a solid ratio of the total pore volume to the volume ofthe particles or powderPore shape ink bottle,cylindrical,funnel,slit-shaped Accessibility of the pores closed,blind,through pores Pore size and dominating transport mechanisms [12]micropores:<2nm,activated transport mesopores:2–50nm,Knudsen diffusion,surface diffusion along the pore walls,capillary transport Density macropores:>50nm,molecular diffusionskeletal density:density of the solid network bulk density:mass per total volume (=solid phase +closed pores +openpores)Figure 1.Schematic structure of a silica aerogel (reprinted from [13]with permission of Elsevier SciencePublishers).Figure 2.Two-dimensional structures of aerogels a)Col-loidal;b)Polymeric;c)With a rough surface (see page 5)(Reprinted from [47]with permission of the Materials Re-search Society)ameter of 1–3nm.Their generation and aggre-gation is controlled by chemical processes,usu-ally the sol –gel process [14–21].In principle,the structures of all covalently bonded gels lie between those of colloidal gels (colloidal pri-mary particles)and polymeric gels (polymeric primary particles),which constitute the two ex-tremes with regard to both the microstructure and the resulting properties (Figure 2).In a sol ,particles with diameters in the range of 1–1000nm are dispersed in a liquid.A gel consists of a sponge-like,three-dimensional solid network whose pores are filled with an-other substance,usually a liquid.When gels are prepared by hydrolysis and condensation of metal or metalloid alkoxides or other hydrolyz-able metal compounds (via the sol stage),the pore liquid mainly consists of water and/or alco-hol.The resulting “wet”gels are therefore called aquagels,hydrogels,or alcogels.Aerogels are obtained when the pore liquid is replaced by air without decisively altering the network structure or the volume of the gel body.Cryogels are ob-tained when the pore liquid is removed by freeze drying.A xerogel is formed by conventional dry-ing of the wet gels,that is,by increasing the temperature or decreasing the pressure with con-comitant large shrinkage (and mostly destruc-tion)of the initially uniform gel body (Figure 3).The shrinkage of a gel body upon evaporation of the pore liquid is caused by capillary forces acting on the pore walls as the liquid retreats into the gel body.This results in the collapse of the filigree,highly porous inorganic networks of the aquagels or alcogels.Therefore,other dry-Aerogels3Figure3.Shrinkage upon drying of a wet gel body to give an aerogel(top)and to give a xerogel(bottom)as a powder(a)or as monolith(b)ing methods must be used to prepare aerogels (Section2.2).work Formation and Structure Highly porous three-dimensional gel networks can be obtained for inorganic,inorganic/organic, and purely organic systems under controlled conditions although the solids content is typi-cally only1–15vol%.In colloidal gels,dense colloidal particles are interconnected like a string of pearls(Figure2a). In polymer gels,linear or branched polymer chains are formed by condensation of small clus-ters(Figure2b).In the sol–gel literature and in this article,the term cluster does not mean metal clusters,as is usual in chemistry,but in-stead the primary oligomeric species formed during sol–gel processing.Their structure can be chainlike,cyclic,or three-dimensional.There are several theoretical models for the aggrega-tion of the clusters to three-dimensional net-works[14],[22].Each aerogel has its own struc-tural characteristics because the microstructure strongly depends on the preparation conditions.Aerogel networks are usually formed from colloidal particles.There are two principal pos-sibilities for the formation of gels:–Solid gels are dissolved(peptized)and the sols thus obtained(colloids)are reaggre-gated under different conditions.–Sol particles are formed by chemical reac-tions from molecular precursors.Most aero-gel syntheses follow this route.For example, the reaction of metal or metalloid alkoxide groups(M–OR)with water results in the for-mation of hydroxyl groups(M–OH),which can then condense to form M–O–M units.The production of inorganic and inor-ganic/organic aerogels is shown schematically in Figure4.Some parameters by means of which the sol–gel process and therefore the properties of the resulting aerogels can be influenced and varied are also anic aerogels are pro-duced similarly.The solvent plays an important role in sol–gel processes.It not only serves to homog-enize the precursors in the initial stage,but also strongly influences the particle-and network-forming reactions due to its polarity and vis-cosity.In the preparation of aerogels it has an additional function,as shown in Figure4.Since4AerogelsFigure 4.General scheme for the preparation of aerogels by sol –gel processing and some typical parameters for variationgelation results in only a marginal change in vol-ume,and the drying process during aerogel pro-duction is performed (by definition)such that shrinkage is minimal,the volume of the aerogel body (and hence its density)is determined by the volume of the reaction solution.Therefore,the density of aerogels is simply modified by vary-ing the precursor concentration in the starting solution.The sol –gel transition (gel point)is reached when a continuous network is formed.How-ever,the chemical reactions are not finished atAerogels5this point.First,the pore liquid initially is a sol,that is,it contains condensable particles or even monomers,which slowly condense onto the existing network.Second,neighboring M–OH or M–OR groups can undergo condensa-tion reactions(aging),because the gel network is still veryflexible.Furthermore,hydrolysis and condensation reactions are in principle re-versible.Therefore,mass is dissolved from ther-modynamically unfavorable regions.The so-lutes condense in thermodynamically more fa-vorable regions,especially in pores,crevices, particle necks,etc.(Ostwald ripening).This pro-cess results in the reduction of the net curvature, the disappearance of small particles,and thefill-ing of small pores.Aging and ripening increase the stiffness of the gels(Fig.4).Controlled ag-ing is therefore an important aspect for the re-producible preparation of aerogels.2.1.1.Inorganic Aerogels2.1.1.1.SiO2AerogelsThe term SiO2gel(analogously for other gels) is only used to characterize the type of inor-ganic skeleton.The SiO2gels and the corre-sponding aerogels often have the composition SiO x(OH)y or,if they are prepared from alkox-ides,SiO x(OH)y(OR)z,where the values of y and z can be(but need not be)rather high.Most laboratory preparations of silica aero-gels use tetraalkoxysilanes Si(OR)4as the silica source.The pore liquid of the gels formed af-ter addition of a defined amount of water(Eq.1) then mainly consists of the alcohol used as sol-vent and that produced by the hydrolysis reac-tion.Si(OR)4+H2O/ROH−→[SiO2·H2O/ROH](1)The chemical reactions during sol–gel process-ing of alkoxysilanes can be formally described by three equations(Eq.2).Hydrolysis reactions take place in parallel with condensation reac-tions during all steps of the sol–gel process. Therefore,all intermediate species still contain Si-OR and/or Si-OH groups.Parameters which influence the hydrolysis and condensation reactions and whose deliber-ate variation are used for“materials design”are shown in Figure4.A variety of physicochemical measurements and some computer simulations were performed to determine the influence of these parameters on network formation of SiO2 aerogels[7–11],[22–28].Kinetic models for the growth of gel networks were postulated on the basis of small-angle X-ray scattering(SAXS) data[29],[30].Two of the most important mod-els are shown in Figure5.The kind of inorganic network formed by the condensation reactions not only depends on the absolute rates of the individual reactions but also on the relative rate of hydrolysis and con-densation reactions[31],[32].Acidic conditions (pH2–5)favor hydrolysis,and condensation is the rate-determining step.A great number of monomers or small oligomers with reactive Si–OH groups are simultaneously formed.Under these conditions,reactions at terminal silicon atoms are favored,and chains with few branches are formed.This results in polymerlike networks with small pores.This process is called reaction limited cluster aggregation(RLCA).Hydrolysis is the rate-determining step under basic conditions,which favor reaction at cen-tral silicon atoms of an oligomer.The resulting network has a particulate character with large particles and large pores(“colloidal”gels).Hy-drolyzed species are immediately consumed be-cause of the faster condensation reaction.Con-densation of clusters with each other is rela-tively unfavorable[14].Therefore,the clusters mainly grow by the condensation of monomers. This process is called reaction limited monomer cluster growth(RLMC)or Eden growth[33].It would ideally result in uniformly dense struc-tures.However,for tetraalkoxysilanes this sim-ple model must be somewhat modified.Not all the four branching points are equal because only partially hydrolyzed Si(OR)4−x(OH)x is initially formed from Si(OR)4.Therefore,un-changed Si–OR groups are present at the growth front,which leads to inhomogeneous condensa-tion,and rough structures are obtained(“poi-soned”Eden growth;Fig.2c).Preparation of aerogels from polymeric gels is more difficult,because diffusion processes are strongly inhibited by the smaller -plete removal of the pore liquid is therefore more difficult and results in a larger shrinkage dur-ing drying[34].For this reason,silica aerogels are usually prepared by base-catalyzed reaction6AerogelsFigure 5.Dependence of the hydrolysis and condensation reaction on pH,and derived kinetic growth models of gel structures.RLCA =reaction limited cluster aggregation,RLMC =reaction limited monomer cluster growth (after [29],[22])of tetramethoxysilane (TMOS)or tetraethoxysi-lane (TEOS),mostly with ammonia as the cata-lyst.A modification of this procedure is to pre-hydrolyze Si(OR)4with a small amount of wa-ter under acidic conditions.This results in the formation of small silicic acid clusters.In a sec-ond step,a defined amount of aqueous acid or base is added [35].Networks formed by this two-step procedure have a structure similar to the RLCA model,that is,they have a polymeric character.The probable explanation is that the reactive clusters formed in the first step as struc-ture forming units are responsible for the for-mation of the network,regardless of the kind of catalyst in the second step.The main differ-ence caused by the kind of catalyst in the sec-ond step is the stiffness of the resulting network.Base catalysis in the second step results in stiff-ening,which stabilizes the gels.With this two-step procedure a more deliberate control of the microstructure and hence the particle and pore sizes of SiO 2gels is possible.For example,while the density of aerogels made by one-step base-or acid-catalyzed reactions is restricted to the range of 0.030–0.300g/cm 3,the production of an aerogel with a density of only 0.004g/cm 3was achieved by the two-step process [36],[37].In a modification of the two-step procedure,oligomeric polyethoxysiloxanes of a certain size are prepared first from TEOS by addition of a de-fined amount of water.These defined oligomers were then used as precursors for sol –gel pro-cessing to form the three-dimensional network [38],[39].For technical applications in which large amounts of aerogels are needed,alkoxysilanes are too expensive.Therefore,water glass ,an aqueous sodium silicate solution,is used as the silica source.The sodium silicate solution is ion-Aerogels7exchanged,and the resulting silicic acid solution is gelled by changing the pH to acidic conditions [40],[41].Silicic acid can also be transferred into an organic solvent in which condensation is then performed under acidic,neutral,or basic condi-tions.The resulting wet gels are also potentially suitable for the production of aerogels[42].For the commercial production of Basogel,a two-step procedure was applied:The sodium sil-icate solution is mixed with sulfuric acid in the first step.Small hydrogel or aquagel droplets are formed by spraying.The metal salts are then ex-tracted,and the water is replaced by an organic solvent[43].The gel droplets are then supercrit-ically dried.The macroscopic properties of SiO2aerogels differ over a wide range,because of the differ-ences in the structure of the primary particles and network formation.The structural data of SiO2 aerogels listed in Table2therefore only serve as examples.Table2.Properties of SiO2aerogelsProperty Range Typical value Bulk density,g/cm30.003–0.5000.100 Skeletal density,g/cm3 1.700–2.100Porosity,%80–99.8Mean pore diameter,nm20–150Specific surface area,m2/g100–1600[44]600 Refractive index 1.007–1.24 1.02Thermal conductivity(in air,300K),W m−1K−10.017–0.0210.020 Modulus of elasticity E,MPa0.002–1001Sound velocity c L,m/s<20–800100Acoustic impedance Z,kg m−2s−11042.1.1.2.Metal Oxide AerogelsThe principles for network formation of nonsili-cate inorganic gels are the same as for SiO2gels. Aqueous salt solutions or molecular precursors in organic solvents,again mostly alkoxides,can be employed for sol–gel processing.Metal alkoxides are much more reac-tive towards water than alkoxysilanes.This is due to the lower electronegativity and higher Lewis acidity as well as the possi-bility of increasing the coordination number [45].For example,the reactivity of tetrava-lent alkoxides in hydrolysis reactions de-creases in the order Si(O i Pr)4 Sn(O i Pr)4, Ti(O i Pr)4<Zr(O i Pr)4<Ce(O i Pr)4.The reac-tivity of some metal alkoxides is so high that pre-cipitates are spontaneously formed upon addi-tion of water.Whereas the reactivity of alkoxysi-lanes has to be promoted by catalysts,the reac-tion rates of metal alkoxides must be moderated to obtain gels instead of precipitates.The most common method is the addition of acetic acid or acetylacetone to the precursor solution.This re-sults in partial substitution of the alkoxy groups by acetate or acetylacetonate.Other bidentate anionic ligands(L)can also be used[46].The complexes[M(OR)y(L)xn]have a different re-activity,structure,and functionality compared to the unsubstituted alkoxide M(OR)y+x depend-ing on the type and number of plex-ation allows the chemical design of the precur-sors,so that the properties of the sol–gel mate-rials can be deliberately influenced.Compared to aerogels with a silicate network, only little work has been done to elucidate the network formation and structure of metal oxide gels.One of the most important differences is the possibility of forming crystalline primary parti-cles.Generally,crystallinity is favored by a large excess of water in the hydrolysis reaction.Alumina.The preferred precursors for the synthesis of Al2O3aerogels are Al(O sec Bu)3 (ASB)and Al(O t Bu)3(ATB)[47–51].ASB has the advantage of being soluble in2-butanol, while other aluminum alkoxides are insoluble in the corresponding alcohol.There are different possibilities for handling this problem[52].The alkoxide can be dissolved in benzene followed by addition of pure water(two liquid phases are formed),or water dissolved in the corresponding alcohol.Alternatively,a dispersion of the alkox-ide in the corresponding alcohol can be prepared and then water is added.Titania and Zirconia.For the preparation of TiO2and ZrO2aerogels,the propoxides and butoxides of the metals are mainly used as pre-cursors[47],[53–57].As for the alumina gels, the amount of water added for hydrolysis is de-cisive for the structure of zirconia aerogel net-works.Substoichiometric addition of water re-sults in an amorphous network,while with an8Aerogelsexcess of water the structure is composed of crystalline,monoclinic ZrO 2particles [3],[47].When Zr(O n Pr)4was gelled in 1-propanol in the presence of acetic acid,the acetate ligand remained in the aerogel [58].TiO 2aerogels can be prepared totally amor-phous or with a network of anatase primary particles [59–61].It was shown by a combi-nation of SAXS,small-angle neutron scatter-ing (SANS),BET measurements,and electron microscopy that mesoparticles of about 50nm in diameter are formed by aggregation of the crystalline nanoparticles of about 5nm diame-ter.The mesoparticles aggregate to give a three-dimensional network.A branched,polymerlike fractal structure was concluded from SAXS data on amorphous and partially crystalline ZrO 2aerogels consisting of small primary particles with an average diameter of 5.2nm (partially crystalline)or 2.5nm (amorphous)[62],[63].Other single-component oxide aerogels are listed in Table 3.Table 3.Other inorganic aerogels,precursors for their preparation,and potential applications Aerogel Precursor ∗ApplicationV 2O 5[VO(O i Pr)3][64]cathode in lithium batteries[VO(OEt)3][65–67]Cr 2O 3[Cr(NO 3)3][68]oxidizing properties,fluorination ofhydrocarbons with HF CrCl 3[69]Cr(OAc)3[70],[71]Fe 2O 3FeCl 3[68],[69][Fe(acac)3][72–74]MoO 2[MoO 2(acac)2][47],[75],[76]electrocatalysisNb 2O 5Nb(OEt)5[77–79]isomerizations,solid acid∗acac =acetylacetonate.Binary and Ternary Oxide Aerogels.Compared to other methods for the preparation of multicomponent oxides,the sol –gel method offers the best control over the resulting materi-als properties.This is achieved by variation of the processing parameters [75],[80],[81].When precursors with similar hydrolysis and condensation rates are mixed,mixed metal ox-ides are obtained which are homogeneous on a molecular scale.With larger differences in re-action rates,the microstructure of the product becomes more heterogeneous,and phase separa-tion may even occur.In particular,the so-calledcore/shell phenomenon is observed.The faster reacting compound forms sol particles which are coated by the slower reacting component.There are several possibilities for avoiding this effect:The reaction rate of the faster reacting compo-nent can be moderated by using bidentate lig-ands.Alternatively,the slower reacting precur-sor can be prehydrolyzed and the faster reacting component added afterwards [82].The network in the aerogels thus prepared is analogous to that of the one-component oxide aerogels.Some examples are listed in Table 4.Mixed oxide aerogels for catalytic applications are summarized in review articles [3],[5].SAXS investigations of Al 2O 3–SiO 2(mul-lite)aerogels,prepared from the alkoxides in the presence of bidentate ligands again lead to the postulation of a RLCA-related network [86].Si/O building blocks are partially incorporated into the alumina clusters and partially integrated onto the surface of these particles.The SiOH groups at the surface of the clusters serve as con-densation sites between the particles.X-ray diffraction (XRD)showed that SiO 2aerogels are always amorphous,while TiO 2,Al 2O 3,and ZrO 2can also be partially crys-talline [47].In mixed oxide aerogels,the SiO 2and Al 2O 3portions are always amorphous [73],[74],[95–97].However,crystalline aluminates are observed (for example,NiAl 2O 4).Selected structural data of nonsilicate aerogels are listed in Table 5.They vary decisively with the prepa-ration conditions.Metal in Metal Oxide Aerogels.A second metal component in a gel need not necessarily re-sult in a binary oxide aerogel.The use of alcohols for supercritical drying can result in the conver-sion of easily reducible oxides into metals,be-cause alcohols have reducing properties.Metal particles can also be generated by subsequent reduction of the aerogels with hydrogen at high temperature or by introducing hydrogen into the autoclave during supercritical drying [103].Aerogels are particularly well suited as car-riers for catalytically active metals because of their high porosity.Detailed discussions can be found in review articles on aerogels as catalysts [3],[5].For the incorporation of metal components into the gel matrix,previously formed aerogels can be impregnated with solutions of the corre-Aerogels9 Table4.Examples of binary and ternary oxide aerogelsAerogel Precursor∗ApplicationTiO2–SiO2[Ti(O i Pr)4+acac]+Si(OMe)4[83],[84]epoxidation of olefinsAl2O3–SiO2[85][AlCl3+ethylene oxide]+Si(OEt)4[69]catalyst support Al(OAc)3,Al(acac)33Al2O3–2SiO2(mullite)(ASB+β-diketone)+Si(OEt)4[86]preparation of high-purity ceramicsFe2O3–SiO2[87]Fe(acac)3+Si(OMe)4Fischer–Tropsch synthesesFe2O3–Al2O3Fe(acac)3+ASBV2O5–MgO V(acac)3+Mg(OMe)2[88]ammonia synthesisPbO–Al2O3Pb(OAc)2+ASB[89]nitroxidation of hydrocarbons to nitriles PbO–ZrO2Pb(OAc)2+Zr(O i Pr)4[90]BaO–Al2O3Ba[Al(O s Bu)4]2+ASB car exhaust catalysisBa(1,3-butanediolate)+ASB[91]x Li2O–(1−x)B2O3B(OBu)3+LiOMe[92]PbTiO3Pb(OAc)2+Ti(O i Pr)4[93],[94]piezoelectric lead zirconate titanate(PZT)aerogelsNiO–Al2O3–MgO Ni(OAc)2+ASB+Mg(OMe)2nitroxidationNi(OAc)2+Si(OR)4+Mg(OMe)2[95]NiO–SiO2–MgO2MgO–2Al2O3–5SiO2(cordierite-like)Si(OEt)4+ASB+Mg(NO3)2[58]∗acac=acetylacetonate.Table5.Typical structural data of some nonsilicate aerogelsAerogel Density,g/cm3Porosity,%Pore radius,nm BET surface area,m2/g Particle morphology,nmTiO2[98]0.3–178–901–25316–690ZrO2[47],[58],[65–67],[79]0.2–0.384–961081–480 2.5–5.2Al2O3[47],[51],[99]0.13–0.185123–61622–25,platelike Al2O3–SiO2[86],[100],[101]0.06–0.21121–5∗V2O5[65],[102]0.04–0.196140–400<10nm,fibrous V2O5–GeO2[66]0.0896Cr2O3[96]0.15–0.54516–785PbO–TiO2[93]0.7771–426035–55∗Controllable by means of the amount of addedβ-diketone[101].sponding metal salts(two-step process)[104], [105].Alternatively,metal salts can be added to the precursor solution,which results in the in-corporation of the metal compounds in the gel during sol–gel processing[68],[106].Table6 gives some examples of metal-doped aerogels and their application in catalytic reactions.2.1.2.Inorganic/Organic Hybrid Aerogels The goal of modifying oxide aerogels with or-ganic groups is to supplement new properties without influencing the existing positive prop-erties,such as good thermal insulation,trans-parency,and high surface area.For example,the hydrophobicity and elastic properties of SiO2 aerogels can be improved relative to unmodi-fied SiO2aerogels by incorporation of organic groups[114–116],or new applications can be envisioned by the integration of functional or-ganic groups,for example,in catalysts or sen-sors.There are only limited possibilities for post-synthesis doping or modification of aerogels with organic compounds(Section2.3).A more general route is the integration of organic entities in sol–gel processing.Embedding molecules in gels without chem-ical bonding(Fig.6b)is achieved by dissolv-ing these molecules in the precursor solution. The gel matrix is formed around them and traps them.The doped wet gels can,in principle,be converted to aerogels.However,the probability is very high that the organic groups are leached during the generally applied supercritical dry-ing and the rinsing processes connected with it. For example,afluorescent dye was incorporated10AerogelsTable6.Examples of metal-doped aerogelsAerogel Precursor for the metal Application1.ImpregnationCu–SiO2,Cu–MgO[Cu(NH3)4(OH)2]in alcohol[104]Cu–ZnO–Al2O3Cu(OAc)2[107]hydrogenationPt–MoO2H2[PtCl6]in MeOH[105]hydrogenationPd–Al2O3PdCl2[108],[109]car exhaust catalysis Pd–CeO2,Pd–BaO–Al2O32.Sol–Gel methodPt–TiO2PtCl4,[Pt(acac)2],(NH4)2[PtCl6][110]hydrogenationPd–Al2O3[Pd(OAc)2][111]car exhaust catalysis V–SiO2V(O i Pr)3Cu–SiO2Cu(OAc)2[112]Pt–Al2O3H2[PtCl6][113]dehydrocyclisationin the gel network,but the portion retained dur-ing aging and drying was relatively low[117]. C60/C70was also incorporated into SiO2aero-gels by this method[118].Aerogels with interpenetrating,but not in-terconnected,inorganic and organic networks (Fig.6a)were produced by generating the or-ganic polymer in situ during the sol–gel re-action by radical polymerization of a vinyl monomer followed by supercritical drying with CO2of the dual-network gel.Attempts to per-form the sol–gel process in solutions of organic polymers mostly failed,because the polymer was leached during supercritical drying.There was only success with organic polymers capa-ble of forming hydrogen bonds to the surface silanol groups of the inorganic matrix,such as poly-2-vinylpyridine(PVP)[119].The more important organic modifications of oxide aerogels are based on covalent bonding of the organic groups(Figure6c,d).In silicate systems,compounds of the type R Si(OR)3are usually employed,where R is the organic group that modifies the inorganic gel.In nonsilicate systems,the organic groups can be introduced by means of bidentate ligands(page7).The preparation of organically modified xerogels is described in review articles[45],[46],[120].Silica aerogels modified by nonfunctional or functional organic groups are prepared by sol–gel processing of R Si(OR)3/Si(OR)4mix-tures(Eq.3)followed by supercritical drying of the wet gels.x R Si(OR)3+(1−x)Si(OR)4+H2O/ROH−→R x SiO(2−0.5x)·H2O/ROH(3)When the organic groups R have no or only weakly basic properties,such as un-substituted alkyl or aryl groups,or chloro, thio,phosphino,carbamato,glycidoxy or methacryloxyalkyl groups,sol–gel processing of the R Si(OR)3/Si(OR)4mixtures under base-catalyzed conditions is a two-step process,sim-ilar to the formation of core/shell multicompo-nent oxide aerogels(page7).The basic gel network is almost exclusively built by hydroly-sis and condensation of Si(OR)4,because reac-tion of Si(OR)4is faster than that of R Si(OR)3. Only in the second stage of the process are the R SiO3/2units condensed onto the inner surface of the existing silica gel network.In-organic/organic hybrid aerogels prepared from R Si(OR)3/Si(OMe)4mixtures therefore have the same structural features as those prepared from Si(OR)4alone,and the typical aerogel properties are retained.As long as the group R has no basic properties,the nature of R has no major influence on network formation and net-work structure,and only the fractal dimension of the primary particles is influenced by the polar-ity of the functional group[121–125].Increasing the R Si(OR)3fraction leads to larger primary particles and therefore lower specific surface ar-eas.These structural phenomena result from the larger amount of water and catalyst that act on Si(OR)4in thefirst stage of the reaction due to the delayed reaction of R Si(OR)3.When R in the R Si(OR)3/Si(OR)4mixture has basic properties,as in(RO)3Si(CH2)3NR 2 (NR 2=NH2or NHCH2CH2NH2),both silanes are involved in the build-up of the gel network, and the two-stage process is no longer observed.。

Synthesis of monolithic mesoporous silicon carbide fromresorcinol–formaldehyde/silica compositesYong Kong,Ya Zhong,Xiaodong Shen n,Longhua Gu,Sheng Cui,Meng YangCollege of Material Science and Engineering,Nanjing University of Technology,Nanjing210009,PR Chinaa r t i c l e i n f oArticle history:Received7October2012Accepted9February2013Available online17February2013Keywords:Silicon carbideAerogelCarbothermal reductionSol–gel processPorous materialsa b s t r a c tResorcinol–formaldehyde/silica composite(RF/SiO2)gels were synthesized in one pot using a facileprocess.RF/SiO2aerogels were obtained after supercritical carbon dioxidefluid drying.Monolithicmesoporous silicon carbide(SiC aerogels)was prepared from RF/SiO2aerogels after carbothermalreduction and calcination.The as-prepared SiC products exhibited monolithic mesoporous morphologyand possessed a BET specific surface area of251m2/g and a pore volume of0.965cm3/g.X-raydiffraction(XRD)and transmission electron microscopy(TEM)demonstrated that the resulting SiCaerogels were composed of a-SiC nanocrystals.The bulk density and skeleton density of SiC products is0.288g/cm3and3.12g/cm3,respectively.The porosity of SiC products is90.8%.The SiC aerogels werestable up to temperatures near6501C.&2013Elsevier B.V.All rights reserved.1.IntroductionSilicon carbide(SiC)shows high hardness,good thermal shockresistance,high thermal conductivity and stability,low thermalexpansion coefficient,superior chemical inertness and large bandgap,and therefore is considered as a promising material in manyfields of catalysis,high-power and high-frequency electronics,photoelectric,anti-radiation,and wave-absorbing devices[1–4].Typically,porous SiC is prepared by carbothermal reduction ofsilica and carbon at a high temperature,and binary carbonaceoussilica composites are commonly used as precursors of porous SiC[5–7].However,the products yielded by these techniques areusuallyfibers and whiskers,and the process does not adequatelypreserve the starting network structure of the precursors.Never-theless,monolithic SiC aerogels have been successfully synthe-sized from polymer cross-linked silica aerogels recently[8].Theresulting SiC presented monolithic and porous structure,and hada BET specific surface area of$20m2/g.Alternatively,monolithicSiC aerogels with a BET specific surface area of232m2/g havebeen prepared by carbothermal reduction from resorcinol–formaldehyde/silica composite(RF/SiO2)aerogels[9].However,the methods employed to form carbon–silica hybrid gels arecomplicated and time-consuming.In those techniques,tetraethy-lorthosilicate(TEOS)and tetramethylorthosilicate(TMOS)areusually used as silicon sources,and acids and alkalis are involvedas catalysts.Moreover,there is a very different gelation timebetween RF gels and silica gels.Therefore,to form carbon–silicahybrid gels,the silica sol and carbonaceous sol had to be preparedseparately,and the processing went through multiple-stepsol–gel process.In the past,(3-aminopropyl)triethoxysilane(APTES)and(3-aminopropyl)trimethoxysilane(APTMS)were commonly used as aamino-functionalized modifier in the synthesis of porous silica[10–16],but have never been used solely as a silica source.There-fore,we propose a facile synthesis of RF/SiO2aerogels.RF/SiO2gelswere synthesized in one pot by simply mixing the monomers,andno catalysts were required.SiC aerogels were formed from RF/SiO2aerogels after thermal treatment.2.ExperimentalSample preparation:APTES,resorcinol(R),formaldehyde(F,37%w/w aqueous solution)and anhydrous alcohol(EtOH)were used as raw materials.All of the reagents and solvents wereanalytical grade and used as received without further purification.R,F,APTES,EtOH and deionized water(W)were mixed in a pot atroom temperature,with R:F:APTES:EtOH:W prepared at a molarratio of1:2:2:25.7:2.Subsequently,the compound was trans-ferred into plastic molds(40mm in inner diameter).After gela-tion,the wet gels were aged at701C for24h and simultaneouslywashed with ethanol every8h.The alcohol gels were dried in anautoclave(HELIX1.1system,Applied Separations,Inc.,Allentown,PA)with supercriticalfluid CO2to form RF/SiO2aerogels.Thethermal treatment of RF/SiO2aerogels was performed in a tubefurnace(72mm inner diameters of tube).The temperature wasfirst raised to15001C with a rate of21C/min underflowing argon(100ml/min),and maintained at that level for5h.Subsequently,Contents lists available at SciVerse ScienceDirectjournal homepage:/locate/matletMaterials Letters0167-577X/$-see front matter&2013Elsevier B.V.All rights reserved./10.1016/j.matlet.2013.02.047n Corresponding author.Tel.:þ862583587235;fax:þ862583221690.E-mail addresses:xdshen@,yongkong1984@(X.Shen).Materials Letters99(2013)108–110the temperature was lowered to 6001C,flowing argon was changed to flowing air,and excess of carbon was burned off by maintaining the temperature at that level for 2h.Sample characterization :Bulk densities (r b )were calculated from the weight and the physical dimensions of the samples.Skeletal densities (r s )were determined by helium pycnometry using a Micromeritics AcuuPyc II 1340instrument.Porosity was determined from the r b and r s values,porosity ¼1Àr b /r s .The microstructure was surveyed by LEO-1530VP scanning electron microscopy (SEM).The phase composition was evaluated by ARL ARLX’TRA X-ray diffraction (XRD)using a Cu-K a radiation.Trans-mission electron microscopy (TEM)was conducted using a JEOL JEM-2010electron microscope.Surface areas,pore volume and pore size distribution were measured by nitrogen adsorption/desorption isotherms using a Micromeritics ASAP2020surfacearea analyzer.The specific surface area (s )was calculated using Brunaur–Emmett–Teller (BET)methods.The pore size distribu-tion was derived from the desorption branch of isotherms by using the non-local density functional theory (NLDFT)model.The pore volume was estimated from the adsorbed amount at a relative pressure p/p 0of 0.986.Thermogravimetric analysis (TGA)was performed by NETZSCH STA449C Thermogravimetric Analyzer to determine the thermal stability under a constant air flow of 30ml/min at a heating rate of 201C/min.3.Results and discussionFig.1shows the XRD patterns of SiC aerogels.The peaks with 2y values of 341,35.71,38.21,41.41,601,65.61,71.81,73.61,and 75.51correspond to the crystal planes of 101,102,103,104,110,109,202,203and 204,respectively,for (6H of a -)SiC (PDF#29-1128).No other crystalline phases of silica,carbon or other impurities were detected.Analysis of the peaks using the Scherrer equation indicates that the average crystallite size of SiC is approximately 6.1nm.Fig.2shows the photograph,SEM and TEM images of SiC aerogels.Although the as-prepared SiC aerogels exhibited significant weight loss and shrinkage relative to RF/SiO 2aerogels,they still preserved monolithic morphology.For SiC aerogels,the mass loss approaches 70%and the linear shrinkage is about 40%relative to RF/SiO 2aerogels.The bulk densities of RF/SiO 2and SiC aerogels are 0.206cm 3/g and 0.288cm 3/g,respectively.The skeleton density of SiC aerogels is 3.121g/cm 3,therefore,the porosity of SiC aerogels is 90.8%.As shown in the SEM and TEM images (Fig.2(b)and (c)),SiC aerogels present porous structures of a typical colloidal gel,which is consistent of nanopores and SiC nanocrystalline framework.As observed from the high-resolution transmission electron image (HRTEM,Fig.2(d)),the lattice fringe,with spacing of approximately 0.262nm,corresponds to the 101crystal plane of 6H–SiC (PDF#29-1128).Selected area electron diffraction (SAED)patterns are shown as an inset in Fig.2(c),and the crystal planes are marked.In combination with the XRD patterns,the SAED rings and theHRTEMFig.1.XRD patterns of SiCaerogels.Fig.2.(a)Photograph,(b)SEM image,(c)TEM image,(d)HRTEM image of SiC aerogels.Insets in (c):SAED patterns.Y.Kong et al./Materials Letters 99(2013)108–110109image reveal that the resulting product is (6H of a -)SiC.However,the porous SiC synthesized from C/SiO 2composites by carbothermal reduction are generally b phase [8,9,17–20].b -SiC reportedly starts forming at 12501C from C/SiO 2aerogels under dynamic Ar-flow [19].Similarly,small amount of 6H–SiC reportedly starts forming from 16001C [8].In our work,SiC can not be detected in samples prepared below 15001C,and there is no evidence of existence of b -SiC in the SiC products.Fig.3shows the nitrogen adsorption/desorption isotherms of SiC aerogels.It is Type IV curves with type H1hysteresis loop in the IUPAC classification,characteristic of mesoporous structure with cylindrical pores [21].The BET specific surface area of SiC aerogels is 251m 2/g,the pore volume is 0.965cm 3/g.The pore-size distribution (shown as inset in Fig.3)in the range of 1–27nm indicates the presence of well-defined nanopores.However,it can be worked out that a sample with a porosity of 90.8%and a bulk density of 0.288g/cm 3should have a total porosity of 3.15cm 3/paring the SEM and TEM images with the N 2adsorption/desorption analysis,it is found that there is a significant level of porosity present which the N 2analysis will not measure.The inset of Fig.3shows the largest pores to be approximately 30nm while Fig.2(c)shows a mass of macropores.Therefore,it is worth noting that the N 2adsorption analysis can only measure the nanopores,and the macropores can not be measured by the N 2adsorption analysis.TG curve (Fig.4)of the SiC aerogels shows that almost no loss of weight was observed below 6501C,which suggests that the residual carbon was completely removed.The products gained weight above 6501C,denoting the oxidation of SiC nanocrystals in air.4.ConclusionsIn conclusion,a simple method for the synthesis of RF/SiO 2aerogels was presented.RF/SiO 2aerogels were converted to monolithic SiC aerogels after carbothermal reduction and calcina-tion.XRD and TEM analyses indicated that nanocrystalline a -SiC was formed after carbothermal reaction.The as-synthesized SiC aerogels showed mesoporous structure with cylindrical pores.The SiC monoliths exhibited good anti-oxidation property below 6501C in air.This new class of materials can be potentially used in catalytic,electronic,thermal and photoelectric applications.AcknowledgmentThis work was supported by the support from the Priority Academic Program Development of Jiangsu Higher Education Institution (PAPD)and the Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT).References[1]Eddy Jr.CR,Gaskill DK.Science 2009;324:1398–400.[2]Feng XL,Matheny MH,Zorman CA,Mehregany M,Roukes ML.Nano Lett2010;10:2891–6.[3]Chiu S,Yu H,Li YJ.Phys Chem C 2010;114:1947–52.[4]Moene R,Makkee M,Moulijn JA.Appl Catal A.1998;167:321–30.[5]Li X,Chen X,Song H.J Mater Sci 2009;44:4661–7.[6]Li XK,Liu L,Zhang YX,Shen SD,Ge S,Ling LC.Carbon 2001;39:159–65.[7]Hu JQ,Lu QY,Tang KB,Deng B,Jiang RR,Qian YT,et al.J Phys Chem B2000;104:5251–4.[8]Leventis N,Sadekar 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