Superhydrophobic sodium silicate based silica aerogel prepared by ambient pressure drying

The superhydrophobic silica aerogel was prepared by using less expensive sodium silicate as a main silica source through a cost-effective and simple route via ambient pressure drying. The sodium impurity was first eliminated by mixing sodium silicate with a co-precursor methyltriethoxysilane (MTES) followed by ion exchange process. The hydrogel was formed by gelation and the alcogel was further obtained by alcoholization of the hydrogel. The surface of alcogel was modified by reacting with trimethylchlorosilane (TMCS) diluted in n-hexane. It was suggested that MTES accelerated water expelling from the hydrogel, while TMCS modified the surface of silica network by replacing Si–OH with Si–C. As a result, the obtained silica aerogel exhibited excellent physical properties with less than 10% volume shrinkage. The density, surface area and cumulative pore volume were 0.12 g cm⁻³, 684.44 m² g⁻¹, and 3.55 cm³ g⁻¹, respectively. The optical transmission reached 82.8% with the water contact angle of 146°.     

Synthesis of silica aerogels from waterglass via new modified ambient drying

A new modified ambient drying process for synthesizing silica aerogels cost-effectively from waterglass has been developed. Crack-free silica aerogels were obtained via solvent exchange/surface modification of wet gels using IPA/TMCS/n-Hexane solution. Silica aerogels were heated at different temperatures. The effects of heating temperature on chemical bonding state of aerogels were investigated by means of DTA and FTIR. The surface characteristic of the aerogel was hydrophobic when heat-treated under 350°C. The porosities, densities, and specific surface areas of the silica aerogels were in the range of 93–94%, 0.12–0.15 g/cm³, and 630 m²/g, respectively. Distinct spring back phenomena were observed in surface modified wet gels during drying.     

SiO2气凝胶的疏水性改性研究

采用三甲基氯硅烷（TMCS）对硅溶胶制备的湿SiO2气凝胶进行表面疏水改性处理,研究了TMCS与孔洞中水的摩尔比对气凝胶的疏水改性作用与性能,用红外光谱法检测了疏水性改性的SiO2气凝胶,发现气凝胶表面的-OH被-OR基团取代,以此构建SiO2气凝胶的疏水性改性机理及模型,试验结果表明,TMCS与孔洞中水的摩尔比控制在0．35左右时可获得不开裂、完好的、具有憎水性特征的二氧化硅气凝胶。     

Microstructural characterization of silica aerogels using scanning electron microscopy

In this paper the experimental results of microstructural characterization of silica aerogels using scanning electron microscopy (SEM) are reported. In order to understand the reasons for shrinkage, opacity and cracking of the aerogels, detailed SEM observations have been made on the aerogels prepared using various molar ratios of precursors, catalysts and solvents; gel ageing periods and supercritical drying conditions. It has been observed that strong acidic catalysed gels resulted in smaller pore and particle sizes, and hence more transparent but readily cracked aerogels; whereas weak-basic catalysed gels gave larger pore and particle sizes, and hence slightly less transparent and monolithic aerogels. Microstructures of very low density (0.05 gm cm^–3) gels indicate that the gels form a highly crosslinked polymer network and, then, the spherical particles form on the network at higher aerogel densities. Gel ageing resulted in neck growth between SiO₂ particles. Precise control of pore and particle sizes using sol-gel parameters have been found to be necessary in order to obtain highly transparent and monolithic silica aerogels. In addition, autoclave heating and solvent evacuation rates of around 25°Ch^–1 and 4 cm³ min^–1, respectively, resulted in the best quality silica aerogels in terms of monolithicity and transparency.     

Preparation and characterization of nano-porous silica aerogel from rice husk ash by drying at atmospheric pressure

Silica aerogel, a mesoporous material, was prepared from rice husk ash by sol–gel method and dried under atmospheric pressure. In this method, rice husk ash, which is rich in silica, was extracted with sodium hydroxide solution to produce a sodium silicate solution. This solution was neutralized with acid to form a silica hydrosol, and a small amount of tetraethyl silicate (TEOS) to form a gel. The aged gel was washed carefully by distilled water and ethanol and finally dried under atmospheric air. The prepared silica aerogels were characterized by XRF, FT-IR, TG, DTA, DTG, XRD, BET and SEM measurements. The synthesized TEOS-doped silica aerogel was a light solid with specific surface area of 315 m²/g, pore volume 0.78 cm³/g, average pore size 9.8 nm, bulk density 0.32 g/cm³ and porosity 85%.     

Effect of Heat Treatment on Silica Aerogels Prepared via Ambient Drying

Silica aerogels were prepared at ambient drying by using ethanol/trimethylchlorosilane （TMCS）/heptane solution as pore water exchange and surface modification of the wet gel before drying. The obtained silica aerogels exhibit a sponge-like structure with uniform pore size distribution. The effects of heat-treatment on the hydrophobicity, specific surface area and other properties were investigated. The results indicated that the hydrophobicity of silica aerogels could be maintained up to 350℃. With increasing heating temperature, hydrophobicity decreased, and became completely hydrophilic after heat-treatment at 500℃. Brunaueremmitt-teller （BET） surface area results indicated that the specific surface area of silica aerogels increased with increasing heating temperature in the range of 150-500℃. The effects of heat-treatment on the morphology and chemical bonding state of silica aerogels were investigated by scanning electron microscopy （SEM）, differential temperature analysis （DTA） and Fourier-transform infrared spectroscopy （FT-IR）.     

Modifying the surface energy and hydrophobicity of the low-density silica aerogels through the use of combinations of surface-modification agents

Aerogels are lightweight, highly transparent, thermally insulating materials comprising interconnected nanostructured pores. Low surface energy aerogels were prepared from ambient pressure drying of sodium silicate-based gels by modifying the pore surfaces with silylating agents including trimethylchlorosilane (TMCS), hexamethyldisiloxane (HMDSO), and hexamethyldisilazane (HMDZ), in combination with each other. Hydrophobic properties of the resulted aerogels were studied by contact angle measurements. Fourier-transform infrared spectroscopy (FTIR) was used to monitor the changes in chemical bonds within the aerogels due to surface modification. The microstructure was studied by transmission electron microscopy (TEM). Effect of temperature on the hydrophobicity of the aerogels was studied by thermogravimetric analysis/differential thermal analysis (TGA-DTA). Surface modification of silica gels with various mixtures of surface-modification agents showed different behaviors. Aerogels made by HMDZ and HMDSO combination comprised 5 nm pores and particles and showed a high surface energy, whereas aerogels prepared by HMDSO and TMCS combination had lower surface energy with relatively larger particle and pore sizes with a more uniform distribution of both. The properties of the latter sample were attributed to a greater degree of surface modification and negligible condensation of OH groups. This preparation produced silica aerogels with a low density (0.042 g/cc), low surface energy (3.39 N cm⁻¹), low thermal conductivity (0.050 W K⁻¹ m⁻¹), high optical transmission (85% at 700 nm) and hydrophobic (154° contact angle) with high hydrophobic thermal stability (425 °C). Moreover, the contact angle for materials prepared by this method decreased negligibly over 12 months’ storage in ambient conditions.     

利用粉煤灰常压干燥合成SiO2气凝胶的研究

为了实现粉煤灰回收利用的新途径,研究了一种利用粉煤灰为硅源常压干燥合成SiO2气凝胶的工艺.通过正交实验研究粉煤灰与氢氧化钠反应生成水玻璃的最佳工艺条件;所得水玻璃溶液通过硫酸催化或树脂交换碱催化法获得水凝胶,利用三甲基氯硅烷/乙醇/正己烷对水凝胶进行改性处理,在常压干燥下制备了SiO2气凝胶.利用BET、SEM和FTIR对气凝胶的微观结构及性质进行了研究,结果表明,所得气凝胶的比表面积为362.2～907.9m2/g、孔体积为0.738～4.875cm3/g、平均孔径为7.69～24.09nm,其中树脂交换碱催化法所得气凝胶的比表面积可达907.9m2/g,孔体积达4.875cm3/g.     

Synthesis of low-density silica gel at ambient pressure: Effect of heat treatment

A silica gel with the density of 0.2 g/cm³ and porosity of 90% was synthesized. The silica wet gel was dried and heat-treated under the ambient pressure after modification of the surface by TMCS. The specific surface area and total pore volume of the dried gel increased with increasing heat treatment temperature and represented the maximum value of 1050 m²/g and 3.2 cm³/g, respectively, under 350 °C. The surface characteristic of the gel was hydrophobic when heat-treated in nitrogen and converted to hydrophilic after heat-treatment in air. As the result of pore characteristics, and microstructure of gel, the properties of the gel were considered to be similar to those of conventional aerogel.     

Effect of mixing on thermal and mechanical properties of aerogel-PVB composites

Silica aerogels were synthesized via solvent exchange/surface modification of wet gels from waterglass using IPA/TMCS/n-Hexane solution. The densities, porosities, and specific surface areas of the aerogel were in the range of 0.128–0.153 g/cm³, 93–95%, and 598–795 m²/g, respectively. Aerogel-PVB composites were manufactured by hot pressing. Aerogels were crushed and mixed with PVB. Three mixing methods were performed. The aerogel-PVB composites had densities of 0.19–1.09 g/cm³, and thermal conductivities of 0.03–0.12 W/(m · K). Density and thermal conductivity can be controlled by aerogel volume fraction and the mixing method. The modulus of rupture of aerogel-PVB composites was in the range of 0.15–46.5 MPa.     

A new route for preparation of sodium-silicate-based hydrophobic silica aerogels via ambient-pressure drying

An in-depth investigation into the synthesis of hydrophobic silica aerogels prepared by the surface derivatization of wet gels followed by subsequent drying at ambient pressure is reported. The following sol–gel parameters were examined for their effect on the physical properties of the derived aerogels: number of gel washings with water, percentage of hexane or methanol in silylating mixture, molar ratio of tartaric acid: Na₂SiO₃, gel aging period, weight% of silica, trimethylchlorosilane (TMCS) percentage, and silylation period. These parameters were varied from 1 to 4, 0 to 100%, 0.27 to 1.2, 0 to 4 h, 1.5 to 8 wt.%, 20 to 40% and 6 to 24 h, respectively. The properties of hydrophobic silica aerogels synthesized by this new route were investigated in terms of bulk density, percentage volume shrinkage, percentage porosity, thermal conductivity and contact angle with water, and by Fourier transform infrared spectroscopy (FTIR). The as-prepared hydrophobic silica aerogels exhibited high temperature stability (up to approximately 435 °C) as measured by thermogravimetric/differential thermal analysis (TGA-DTA). The optimal sol-gel parameters were found to be a molar ratio of Na₂SiO₃:H₂O : tartaric acid : TMCS of 1 : 146.67 : 0.86 : 9.46, an aging period of 3 h, four washings with water in 24 h and the use of a 50% hexane- or methanol-based silylating mixture. Aerogels prepared with these optimal parameters were found to exhibit 50% optical transparency in the visible range, 84 kg m⁻³ density, 0.090 W mK⁻¹ thermal conductivity, 95% porosity and a contact angle of 146° with water.     

Strong, low density, hexylene- and phenylene-bridged polysilsesquioxane aerogel–polycyanoacrylate composites

Nanocomposite aerogels were prepared by chemical vapor deposition and polymerization of cyanoacrylate on the surface of bridged polysilsesquioxane aerogels. Phenylene- and hexylene-bridged aerogels were prepared by sol–gel polymerizations and supercritical carbon dioxide drying. Hydrophobic organic bridging groups in the polysilsesquioxane aerogels reduced the amount of adsorbed water available for initiating polymerizations and led to higher molecular weight polycyanoacrylate than was observed with silica aerogels. Densities increased as much as 65% due to the addition of the organic polymer, but the nanocomposite aerogels remained highly porous with surface areas between 440 and 750 m²/g. Polycyanoacrylate–phenylene-bridged aerogel composites were the strongest with flexural strengths up to 780 kPa or 16-fold stronger than the untreated phenylene-bridged aerogels and fivefold stronger than a silica aerogel of the same density. The strongest polycyanoacrylate–hexylene-bridged aerogel composites had flexural strength of 285 kPa or ninefold stronger than the untreated hexylene-bridged aerogels and twice as strong as a silica aerogel of comparable density. The greater strength of the new composites is, in part, due to the greater strength of the bridged aerogels. However, higher molecular weight polycyanoacrylate, due to less surface water on the hydrophobic bridged aerogels, also contributes to the greater nanocomposite strengths.     

Facile synthesis of reduced graphene oxide/trimethyl chlorosilane‐coated cellulose nanofibres aerogel for oil absorption

A hydrophobic and oleophilic trimethyl chlorosilane/reduced graphene oxide‐coated cellulose nanofibres (TMCS/rGO/CNFs) aerogel with a three‐dimensional structure was fabricated through a facile dip‐coating process. The prepared aerogel exhibited several advantageous properties for absorption and expulsion of oils from water surfaces, such as a high specific surface area, low density (6.78 mg/cm³) and good porosity (99.12％). In addition, the TMCS/rGO/CNFs aerogel demonstrated good absorption capacities up to 39 times its own weight over a short time (1.5 min) for a broad range of oils. This research suggests that practical application of TMCS/rGO/CNFs aerogel in the cleanup of an oil spill is feasible.Inspec keywords: graphene, nanofibres, aerogels, absorption, hydrophobicity, dip coating, oil pollution, nanofabricationOther keywords: facile synthesis, reduced graphene oxide‐trimethyl chlorosilane‐coated cellulose nanofibre aerogel, three‐dimensional structure, facile dip‐coating process, oil expulsion, water surfaces, TMCS‐rGO‐CNF aerogel, oil absorption capacities, oil spill cleanup, CO     

Comparative studies of the physical and hydrophobic properties of TEOS based silica aerogels using different co-precursors

In the present paper, the experimental results on the physical and hydrophobic properties of tetraethoxysilane (TEOS) based silica aerogels using six different organosilane co-precursors (C.P) of the type R_nSiX_4-n as synthesis components, are reported and discussed. The aerogels have been produced by sol-gel processing followed by supercritical drying using methanol solvent extraction. The molar ratio of TEOS, ethanol (EtOH), water (0.001M oxalic acid (H₂C₂O₄) catalyst) was kept constant at 1:5:7, respectively, and the molar ratio of C.P/TEOS (A) was varied from 0.1 to 0.6 and compared the aerogel properties. The hydrophobicity of the aerogels has been tested by the contact angle measurements. The contact angle (θ) has been found to be the highest (θ=136°) for the trimethylethoxysilane (TMES) co-precursor, while for the other co-precursors it is in between 120° and 130°. The surface chemical modification of the hydrophobic aerogels has been studied using Fourier Transform Infrared Spectroscopy (FTIR). As the C.P/TEOS molar ratio increased, the intensity of the C–H and Si–C peaks in the FTIR spectra increased, clearly indicating the organic modification of the aerogels. The aerogels based on mono-alkyl (CH₃) trialkoxysilane co-precursor have shown higher optical transmission (≈65%) compared to the phenyl, di or tri alkyl alkoxysilanes (5–50%). The trialkyl modified aerogels showed the lowest bulk density (118.3 kg/m³) and volume shrinkage (<2%). The alkyl alkoxy/chloro–silane modified aerogels have been found to be thermally stable up to a maximum temperature of 573 K, whereas the phenyl trialkoxysilane modified aerogels are stable up to a temperature as high as 823 K. The aerogels have been characterized by scanning electron microscopy, thermogravimetric and differential thermal analyses.     

常压干燥制备疏水SiO2气凝胶的影响因素分析

常压干燥制备SiO2气凝胶是近年来该领域的研究重点,工艺条件的优化是提高气凝胶性能的关键。以正硅酸乙酯为硅源,甲基三乙氧基硅烷为共前驱体,采用溶胶-凝胶法,结合老化和三甲基氯硅烷-正己烷-无水乙醇混合溶液的二次表面改性,通过常压干燥工艺制备疏水SiO2气凝胶。利用BET,FT-IR,SEM,TEM和接触角测试等手段对气凝胶进行表征,系统研究水解时间、老化时间、老化温度和改性剂用量对气凝胶性质的影响。结果表明:水解16h,凝胶于55℃下老化48h后,在三甲基氯硅烷与正硅酸乙酯的摩尔比为1.56的混合液下改性48h制备的SiO2气凝胶的性能最好,其孔隙率92%,比表面积969m2/g,接触角达157°。     

Effect of Sol-Gel Processing Parameters on Optical Properties of TMOS Silica Aerogels

To optimize and produce silica aerogels with high direct transmittance and low diffusive values, systematic and detailed experiments were carried out on the effect of sol-gel processing parameters on optical properties of silica aerogels. A series of aerogel samples of different molar ratio combinations was optically examined in the UV–visible–NIR range by a spectrophotometer equipped with an integrating sphere. The overall transmittance of the aerogels in the visible range varied from 75 to 93% depending upon the molar ratio combination. The most relevant parameter being studied was the direct/hemispherical transmittance ratio (). The best value of obtained for an aerogel in the present study was about 93% with a molar ratio of 1 TMOS: 12 MeOH:4 H₂O:3.5 × 10^–3 NH₄OH, respectively. Apart from visible transparencies, solar energetic transparencies of some silica aerogels were also measured and reported. These optical data, together with the porosity measurements, allowed us to improve the process of fabrication of low-diffusing aerogel material. The experimental results are discussed considering the percentage of porosity and heterogeneity generated in pore size distributions due to the variation of sol-gel processing parameters.     

亚临界干燥制备疏水SiO2气凝胶

以E－40（多聚硅氧烷）为硅源,三甲基氯硅烷的异丁醇溶液为干燥介质,用溶胶凝胶法,在亚临界条件下制备出疏水的SiO2气凝胶,通过SEM,孔径分布,比表面积,接触角以及红外光谱的测试对其物性进行了研究,结果表明,所制备的SiO2气凝胶具有典型的纳米网络结构,比表面大具具有疏水性能,亚临界干燥使得制备压力从6．4MPa降低到2．3MPa,降低了制备成本和风险,同时疏水性能提高了SiO2气凝胶环境适应性,从而十分有利于气凝胶的商业应用。     

Preparation and characterization of silica aerogels from oil shale ash

The silica aerogels were successfully synthesized using oil shale ash which is a by-product of oil shale processing via ambient pressure drying. The physical and textural properties of the silica aerogels have been investigated and discussed. The results showed that the organic modification of hydrogels was a crucial step during the processing which preserved mesopores in ambient pressure drying. The unmodified hydrogel underwent tremendous shrinkage during the drying and yielded microporous silica aerogel. Using this novel route, it could produce silica aerogel with low tapping density of 0.074 g/cm³, high specific surface (909 m²/g) and cumulative pore volume of 2.54 cm³/g. From the industrial point of view, the present process is quite suitable for a large scale production of powdered silica aerogel. Furthermore, it provides a new way to solve the problem of oil shale ash pollution.     

Variables in the synthesis of unusually high pore volume aluminas

This paper summarizes our successful efforts to obtain high pore volume aluminas (> 5.0 cm³ g^–1 ). The pore volumes of these aerogels, prepared by the hydrolysis of aluminium isopropoxide (AIP) in alcoholic media, are very sensitive to the amounts of water and alcohol employed in the synthesis. An unexpectedly large ratio of water to alkoxide yields the highest pore volumes. By suitable selection of initial levels of aluminium alkoxide, methanol and water, one obtains average pore volumes of 8.6 cm³ g^–1. A two-step formulation of AIP, methanol, and water followed by hypercritical solvent removal to form the alumina aerogel yields the highest pore volumes. Other alternative approaches also increase the yield of aerogel produced within the autoclave. Pore volumes of about 7 cm³ g^–1 result if the premix is concentrated twofold after formulation and before autoclave processing. Lower pore volumes result if the reactant charge is increased directly by simply increasing the concentration of AIR It appears that the reduction in pore volume results in part from the higher isopropanol content in the final reaction mixture (isopropanol is a co-product from AIP hydrolysis).     

Silica-based aerogels as aggregates for cement-based thermal renders

Subcritically dried silica-based aerogels were synthesized by design to be used as aggregates for lightweight cement-based thermal renders. The molecular and pore structure of the aerogels and of the corresponding renders were correlated with their thermal conductivities. A subcritical hybrid aerogel proved to have advantages over a supercritical commercial one, since the particle size distribution may be controlled, it is more hydrophobic, and imparts higher specific surface areas and total pore volumes to the renders. Good stabilization of the hybrid aerogels within the aqueous cement paste, without affecting the final renders’ structure, was accomplished by using an anionic surfactant. The efficient range of aerogel contents for thermal insulation purposes (above 60 vol% of total aggregate) was optimized using an inorganic subcritical aerogel. Thermal conductivities as low as ∼0.085 W.m⁻¹.K⁻¹ and densities of 410 kg.m⁻³ were achieved by total replacement of silica sand with a designed hybrid aerogel.