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%.     

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.     

Synthesis and characterization of silica aerogels by a novel fast ambient pressure drying process

Using cheap waterglass as silica source, silica aerogels were synthesized via a novel fast ambient drying by using an ethanol/trimethylchlorosilane (TMCS)/Heptane solution for modification of the wet gel. One-step solvent exchange and surface modification were simultaneously progressed by immersing the hydrogel in EtOH/TMCS/Heptane solution, in which TMCS reacting with pore water and Si–OH group on the surface of the gel, with ethanol and heptane helping to decrease the rate of TMCS reacting with pore water and extrude water from gel pores. The synthesized silica aerogel was a light and crack-free solid, with the density of 0.128–0.136 g/cm³ and 93.8–94.2% porosity. The microstructure, morphology and properties of the aerogels were studied by FTIR, SEM, TEM and BET measurement. The results indicate that silica aerogels exhibit a sponge structure with uniform nano-particle and pores size distribution. The specific surface areas of silica aerogels are 559–618 m²/g. And there is an obvious Si–CH₃ group on the surface of the silica aerogel.     

Investigation of the effect of rice husk derived Si/SiC on the morphology and thermal stability of carbon composite aerogels

Highly porous carbon aerogels were prepared by pyrolyzing the novolac–silica aerogels. The silica phase was extracted from rice husk ash (RHA). The polymer aerogel was synthesized via the novel method of sol–gel polymerization in solvent vapor-saturated atmosphere. This method removes the need for supercritical drying and reduces the shrinkage of aerogels during drying stage and also has much lower process time compared to the conventional sol–gel method. In the next step, polymer composite aerogels become carbon/silica and carbon/silica/silicon carbide composites in pyrolysis (800 °C) and carbothermal reduction (1500 °C) stages, respectively. The characterization of the prepared composite aerogels was performed by Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) analyses, respectively. Thermal and mechanical properties of the samples were investigated by differential scanning calorimetry (DSC) and compressive strength analysis. The resultant composite aerogels show a nanostructure with high porosity (above 82%) and low density (below 0.3 g cm^− 3). Si mapping images showed the good distribution of silica phase throughout the carbon matrix. Also the rate of oxidation for carbon composites decreased by silica incorporation and oxidation temperature increased about 20% by adding RHA silica. Compressive strength of composite samples increased about 25% by increasing RHA silica phase content.     

Aramid fibers reinforced silica aerogel composites with low thermal conductivity and improved mechanical performance

Aramid fibers reinforced silica aerogel composites (AF/aerogels) for thermal insulation were prepared successfully under ambient pressure drying. The microstructure showed that the aramid fibers were inlaid in the aerogel matrix, acting as the supporting skeletons, to strengthen the aerogel matrix. FTIR revealed AF/aerogels was physical combination between aramid fibers and aerogel matrix without chemical bonds. The as prepared AF/aerogels possessed extremely low thermal conductivity of 0.0227 ± 0.0007 W m⁻¹ K⁻¹ with the fiber content ranging from 1.5% to 6.6%. Due to the softness, low density and remarkable mechanical strength of aramid fibers and the layered structure of the fiber distribution, the AF/aerogels presented nice elasticity and flexibility. TG–DSC indicated the thermal stability reaching approximately 290 °C, can meet the general usage conditions, which was mainly depended on the pure silica aerogels. From mentioned above, AF/aerogels present huge application prospects in heat preservation field, especially in piping insulation.     

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.     

The effect of pH on the physicochemical properties of silica aerogels prepared by an ambient pressure drying method

Hydrophobic silica aerogels were successfully synthesized by an ambient pressure drying method from various silicic acids with different pH values which were prepared from water glass. In this study, we selected xylene as the solvent and performed the surface modification in a TMCS (trimethylchlorosilane)/xylene solution, in order to improve the reproducibility of the aerogels. The densities of the aerogels were about 0.10–0.14 g/cm³ and the apparent porosities were in the range of 94.5–96%, depending on the processing conditions. Their specific surface area was in the range of 700–750 m²/g and their average pore size was around 20 nm. The transparency of the as-dried aerogels was enhanced as the pH value was increased. The transmittance of the heat-treated aerogels was slightly decreased with increasing heat-treatment temperature. On the other hand, an abrupt increase in the transmittance was observed when the temperature exceeded 400 °C and this phenomenon might be related to the change in the surface structure caused by the desorption of the methyl groups.     

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.     

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°.     

Influence of various processing parameters on water-glass-based atmospheric pressure dried aerogels for liquid marble purpose

Experimental results on the influence of various processing parameters on water-glass-based atmospheric pressure dried aerogels for liquid marble purpose are reported. Silica aerogels were prepared by varying the parameters namely washing temperature of the gels with water, washing period of the gels, protic solvents, and drying method. The physical properties of silica aerogels were studied by measuring granular bulk density, contact angle with water, thermal conductivity, and thermal stability in the furnace. The elemental analyses were carried out using Atomic Absorption Spectroscopy (AAS) and FT-Raman spectroscopy. The structural studies were carried out using Transmission Electron Microscopy (TEM). Also, the effect of humidity on the silica aerogels was studied in humidity chamber. Opaque silica aerogels with hydrophobicity (150°), low density (0.053 g/cm³), and low thermal conductivity (0.068 W/mK) have been obtained for the molar ratio of Na₂SiO₃: H₂O: citric acid: TMCS at 1:146.67:0.72:9.46. The hydrophobic powder of silica aerogels can form the liquid marbles.     

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.     

环境气压干燥制备多孔SiO2气凝胶的研究进展

阐述了环境气压干燥技术制备SiO2气凝胶的原理,综述了近几年环境干燥技术采取的工艺措施和研究状况,并综合评述了SiO2气凝胶的环境干燥制备及其应用前景.     

Synthesis of sustainable silica xerogels/aerogels using inexpensive steel slag and bean pod ash: A comparison study

Low cost silica xerogels/aerogels were synthesized from steel slag and bean pod ash by sol–gel method. Comparison study showed differences between structural, morphological, textural, thermal and physical properties of the silica xerogels and aerogels. Formation of amorphous structure and silica network was confirmed by X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy analyses, respectively. Field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) analyses revealed that silica xerogels had smaller interlinked network in contrast to silica aerogels. Typical type IV isotherm was observed for all samples in N₂ adsorption-desorption isotherms. The highest surface area was determined as 371 m² g⁻¹ for silica aerogel synthesized from steel slag. Particle size of silica aerogels was lower than that of the silica xerogels. The more porous structure made silica aerogels desirable materials with lower bulk density and thermal conductivity when compared to silica xerogels. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) exhibited high thermal stability of the silica xerogels/aerogels. Although silica xerogels had highly hydrophilic structure, contact angle of silica aerogels synthesized from steel slag and bean pod ash was 60° and 74°, respectively. The comparison study will give a new point of view about differences between silica xerogels and aerogels synthesized from by-products or inorganic/organic waste instead of silicon alkoxides.     

炭二氧化硅复合气凝胶的合成及结构分析

将二氧化硅溶胶和间苯二酚-甲醛溶液混合,经溶胶凝胶反应得到复合湿凝胶. 再经超临界干燥和炭化得到炭-二氧化硅复合气凝胶. 分别用氢氟酸刻蚀和空气烧蚀复合气凝胶中的二氧化硅和炭,得到结构完整的炭气凝胶和二氧化硅气凝胶. 复合湿凝胶经常压干燥、炭化和氢氟酸刻蚀能得到炭干凝胶. 利用透射电镜和低温氮气吸附对上述凝胶的微结构进行了表征. 结果表明：复合气凝胶中,炭和二氧化硅纳米网络各自连续并相互嵌套. 由于二氧化硅纳米网络的支撑作用,复合凝胶在超临界干燥和高温炭化过程中体积收缩减小,网络塌陷降低. 所得炭气凝胶具有高比表面（934m²/g）和高孔容（3.9cm³/g）的特点.     

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°。     

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.     

Solar photocatalytic degradation of 2-sec-butyl-4,6-dinitrophenol (DNBP) using TiO2/SiO2 aerogel composite photocatalysts

Silica aerogels and TiO₂/silica aerogel composite photocatalysts were synthesized by sol–gel technique at ambient pressure using orthosilioate and tetra-n-butyl titanate as precursors, respectively. The prepared composite photocatalysts were characterized by XRD, TEM, BET surface area, FT-IR and UV–vis absorption spectra. The results showed that the TiO₂/silica aerogel composite photocatalysts possess high surface area. The addition of silica aerogels inhibited the grain growth and phase transformation of anatase to rutile during calcination. The TiO₂/silica aerogel composite sample calcined at 500 °C with an optimal silica aerogel content of 7 wt.% afforded the highest photocatalytic activity. The photocatalytic degradation of 2-sec-butyl-4,6-dinitrophenol (DNBP) was investigated by using this novel TiO₂/silica aerogel composite photocatalyst under solar light irradiation. The effects of irradiation time, pH, catalyst concentration, temperature and initial DNBP concentration were examined as operational parameters. The optimal operational parameters were found as follows: pH as solution pH 4.82, 8 g L⁻¹ catalyst concentration, 20 °C, and 240 min irradiation time. The kinetics of DNBP degradation by TiO₂/silica aerogel composite fit well a pseudo-first-order kinetic model. The repeatability of photocatalytic activity was also tested. This study showed the feasible and potential use of TiO₂/silica aerogel composite photocatalysts in degradation of toxic organic contaminants.     

Preparation of carbon aerogel electrodes for supercapacitor and their electrochemical characteristics

A new process is presented for synthesizing the supercapacitor electrodes of carbon aerogel via pyrolyzing resorcinol-formaldehyde (RF) aerogel, which could be cost-effectively obtained by ambient drying of wet RF-gels instead of conventional supercritical drying. Defect free RF-aerogels instead of conventional supercritical drying. Defect free RF-aerogels with the linear shrinkage of less than 8% could be manufactured by ambient-drying of wet RF-gels. Carbon aerogels with high strength were prepared via pyrolyzing RF-aerogels in N₂ atmosphere. The specific surface area (< 600 m²/g) and the electrical conductivity ( 50 S/cm) of carbon aerogels varied in sensitivity with the pyrolysis condition, while their densities (0.6 g/cm³) and porosities (70%) were found to be almost constant. Post heat-treatment of carbon aerogels around 300^C in air atmosphere was very effective for improving the electrochemical properties of electrodes. The carbon aerogel electrode pyrolyzed at 800C showed the specific capacitances of about 40 F/g in H₂SO₄ electrolyte solution and 35 F/g in KOH solution.     

Ratio-Tuning of Silica Aerogel Co-Hydrolyzed Precursors Enables Broadband,Angle-Independent,Deformation-Tolerant,Achieving 99.7% Reflectivity

The super-white body might be defined as its reflectivity exceeding 98% at any angle in the visible light spectrum, which can be used in a variety of emerging fields including optics, energy, environment, aerospace, etc. However, elaborate synthesis of a light-weight, highly reflective super-white aerogel body remains a great challenge. In this work, fine-tuning of silica aerogel co-hydrolyzed precursor ratios, 99.7% reflectivity with angle-independence in the visible light spectrum has been successfully achieved when the areal density is only 0.129 g cm⁻², which breaks through the theoretical bandwidth limit of photonic crystals as well as the measured reflectivity limit of conventional porous materials. Furthermore, the reflectivity of super-white silica aerogel remains unchanged after various harsh deformations including compression and bending 1000 times, solar (≈800 W m⁻²), ultraviolet (≈0.68 W m⁻²), and humidity (100%) aging for 100 days, liquid nitrogen (−196 °C) and high-temperature (300 °C) thermal shock 100 times. As proofs of performance, the resulting super-white silica aerogels have been used as the novel standard white plate  for better spectrum calibration, as the flexible projector curtains for optical display, as well as the transmitted light reflective layer in the photovoltaic cell for improving the relative power conversion efficiency of 5.6%.     

Anisotropic Aerogels for Studying Superfluid 3He

It may be possible to stabilize new superfluid phases of ³He with anisotropic silica aerogels. We discuss two methods that introduce anisotropy in the aerogel on length scales relevant to superfluid ³He. First, anisotropy can be induced with uniaxial strain. A second method generates anisotropy during the growth and drying stages. We have grown cylindrical ∼98% aerogels with anisotropy indicated by preferential radial shrinkage after supercritical drying and find that this shrinkage correlates with small angle x-ray scattering (SAXS). The growth-induced anisotropy was found to be ∼90° out of phase relative to that induced by strain. This has implications for the possible stabilization of superfluid phases with specific symmetry.