Effect of protic solvents on the physical properties of the ambient pressure dried hydrophobic silica aerogels using sodium silicate precursor

Considering the importance of the highly porous, low density, transparent and nanostructured hydrophobic silica aerogels in the scientific and industrial applications, the experiments have been carried out to prepare the low density silica aerogels using the 1.12 specific gravity water glass (sodium silicate, Na₂SiO₃) precursor, ammonium hydroxide (NH₄OH) catalyst, trimethylchlorosilane (TMCS) silylating agent, various first exchanging protic solvents and hexane as a second exchanging aprotic solvent. The first exchanging solvents used were: methanol, ethanol, propanol, isopropanol, butanol, isobutanol and hexanol. The molar ratio of the Na₂SiO₃:H₂O:NH₄OH:TMCS was kept constant at 1:56:0.02:0.4 respectively. The ambient pressure dried method was used for the preparation of hydrophobic silica aerogels. The effect of the exchanging protic solvents on the physical properties of the aerogels such as density, % of volume shrinkage, % of porosity, % of optical transmission, thermal conductivity, thermal stability and contact angle of the aerogels with water, were studied. FTIR studies were carried out to confirm the silylation of the aerogel samples. It was found that the exchanging protic solvents have profound effect on the physical and hydrophobic properties of the aerogels. Low density (0.07 g/cm³), high porosity (96.6 %), low thermal conductivity (0.091 W/mK), high contact angle (166°) silica aerogels could be prepared by using the isopropanol first exchanging solvent followed by the hexane as the second exchanging solvent along with the TMCS silylating agent with sodium silicate precursor.     

常压制备疏水性二氧化硅气凝胶

以正硅酸乙酯(TEOS)为硅源,氢氟酸作催化剂,采用溶胶-凝胶法常压下制备二氧化硅气凝胶,并研究催化剂、乙醇、水等因素对凝胶过程的影响。采用傅里叶变换红外分析(FT-IR)、电子扫描探针(SPM)等对二氧化硅气凝胶的结构和性能进行研究。结果表明,经三甲基氯硅烷(TMCS)表面改性处理后的气凝胶表现出了很好的疏水性能。该气凝胶密度为200~400 kg/m³,与水的接触角大于120°。当n(TEOS)∶n(乙醇)∶n(H₂O)∶n(HF)=1∶6∶4∶0.25时,得到的气凝胶各方面的综合性能最好。凝胶时间随着水和氢氟酸用量增加而缩短,随乙醇用量的增加而增加。     

Effect of sodium bicarbonate solution on methyltrimethoxysilane-derived silica aerogels dried at ambient pressure

Here we present an economical ambient pressure drying method of preparing monolithic silica aerogels from methyltrimethoxysilane precursor while using sodium bicarbonate solution as the exchanging solvent. We prepared silica aerogels with a density and a specific surface area of 0.053 g∙cm⁻³ and 423 m²∙g⁻¹, respectively. The average pore diameter of silica aerogels is 23 nm as the pore specific volume is 1.11 cm³∙g⁻¹. Further, the contact angle between water droplet and the surface of silica aerogels in specific condition can be as high as 166°, which indicates a super-hydrophobic surface of aerogels.     

Two step sol-gel processing of TEOS based hydrophobic silica aerogels using trimethylethoxysilane as a co-precursor

The experimental results on the surface modification of tetraethoxysilane (TEOS) based silica aerogels using trimethylethoxysilane (TMES) as a co-precursor by two step sol-gel process are reported. The molar ratio of MeOH/TEOS was fixed at 17 and TMES/TEOS ratio was varied from 0.38 to 1.14. The concentration and quantity of acidic water (oxalic acid) added in the first step and base water (ammonium hydroxide) in the second step were also varied. The best quality superhydrophobic aerogels could be obtained with only distilled water (without any acid catalyst) in the first stage and the basic water in the second step of the sol-gel process. The molar ratio of TEOS: TMES: MeOH: distilled water: basic water was optimized at 1:0.86:17:4.9:4.9, respectively. The surface modification has been confirmed from the Fourier transform infrared spectroscopy measurements while the hydrophobicity was quantified in terms of the contact angle measurements. The TMES/TEOS based aerogel powder could be used to transport micro-liters of water in the form of water marbles. The aerogels have been characterized by the bulk density, porosity, thermal conductivity, contact angle measurements and the transmission electron microscopy (TEM).      

常压制备疏水型二氧化硅气凝胶及透光率分析

采用溶胶-凝胶法,通过常压干燥制备了疏水型二氧化硅气凝胶。研究了pH、水解时间等因素对二氧化硅气凝胶透光率的影响。以正硅酸乙酯为原料,通过酸（草酸）-碱（氨水）两步催化,采用溶胶-凝胶法常压干燥制备了疏水型介孔二氧化硅气凝胶。正硅酸四乙酯、乙醇、草酸、氨水物质的量比为1∶4∶5∶0.2,草酸和氨水的浓度分别为0.008、0.05 mol/L时,采用二甲基二氯硅烷为改性剂常压制备了二氧化硅气凝胶。透射电镜、扫描电镜测试表明：二氧化硅气凝胶具有纳米介孔结构。接触角测定表明：二氧化硅气凝胶与水的接触角为148°,表现出疏水性。     

Methyltrimethoxysilane based monolithic silica aerogels via ambient pressure drying

We have developed a novel route to monolithic silica aerogels via ambient pressure drying by the acid–base sol–gel polymerization of methyltrimethoxysilane (MTMS) precursor. An extent of silica polymerization in the alcogels plays a crucial role in obtaining the monolithic aerogels which could be optimized by a proper control over the MeOH/MTMS molar ratio (S) during the sol–gel synthesis. The alcogel undergoes the distinct “spring-back effect” at the critical stage of the drying and thereby preserving the highly porous silica network without collapse. The process yields silica aerogels exhibiting very low bulk density and high specific surface area of 0.062 g/cm³ and 520 m²/g, respectively. The average pore diameter and the cumulative pore volume varied from 4.5 to 12.1 nm and 0.58 to 1.58 cc/g, respectively. In addition, the aerogels are superhydrophobic with contact angle as high as 152°. We anticipate that the new route of the monolithic silica aerogel production will greatly expand the commercial exploitation of these materials.     

通过控制醇凝胶强度常压制备低密度疏水SiO2气凝胶

常压干燥制备低密度气凝胶是促进高性能气凝胶发展应用的重要途径。以正硅酸乙酯为硅源,采用溶胶-凝胶和常压干燥工艺制备出低密度(<100 kg·m^-3)的疏水SiO₂气凝胶,通过工艺参数的控制制备出不同压缩模量的醇凝胶,探讨了反应物配比对醇凝胶压缩模量和气凝胶密度间的影响关系,获得了通过控制醇凝胶压缩模量制备低密度疏水SiO₂气凝胶的方法;发现将醇凝胶压缩模量控制在0.25~2.5 kPa范围内,可制备出密度小于100 kg·m^-3的疏水SiO₂气凝胶,该研究可以为低密度疏水SiO₂气凝胶的低成本常压制备及其控制方法提供指导。     

多元酸催化炭气凝胶的常压制备及表征

在多元有机酸催化和表面活性剂的作用下,间苯二酚和甲醛反应生成有机凝胶,经过常压干燥、高温炭化得到炭气凝胶。用扫描电镜(SEM)、X射线衍射(XRD)和亚甲基蓝吸附测试表征所制备的样品。结果表明,所制备的炭气凝胶属于非晶态物质,呈现均匀的球状结构,且微球内富含连通的电荷转移的孔道,其比表面积达到740m2/g。循环伏安扫描测试表明,所制备的炭气凝胶具有良好的电化学性能,有望成为超级电容器的电极材料。     

二氧化硅气凝胶常压干燥工艺的研究进展

二氧化硅气凝胶因具有低密度、高比表面积、稳定的物理化学性质等特性在吸附分离、隔热保温等领域表现出巨大的应用潜力。但长耗时、高成本的制备工艺限制了它的发展,尤其是湿凝胶向气凝胶转变的干燥工艺。本文介绍了二氧化硅气凝胶在常压干燥的过程中面临的主要难点及解决方法,虽然常压干燥方法工艺简单、过程安全、对设备要求低且可连续制备,成为近年来的研究热点,但也存在制备周期长、体积收缩大、需要消耗大量有机溶剂和改性剂等不足。文中从凝胶基体增强与优化、降低毛细管力与减少不可逆收缩两种角度,介绍了二氧化硅气凝胶常压干燥的改进方法及其发展现状,分析归纳了不同改进方法的优缺点,总结了二氧化硅气凝胶常压干燥目前面临的技术挑战。并且,立足于目前二氧化硅气凝胶基体增强和表面改性技术发展的趋势,对今后二氧化硅气凝胶常压干燥过程中结构可控、成本降低以及产品多功能化的发展路线进行了展望。     

常压干燥法制备SiO2气凝胶粉体

以工业硅溶胶为原料,通过凝胶过程与干燥条件的控制,采用常压干燥法制备了SiO2 气凝胶粉体,并考察了老化液中正硅酸乙脂(TEOS)含量和干燥控制化学添加荆甲酰胺的添加对气凝胶粉体堆积密度、比表面积和孔径分布的影响.结果表明:所得气凝胶粉体具有纳米多孔结构,组成气凝胶结构的基本粒子呈圆球形,粒径为10～25 nm,由基本粒子连接而成的网络结构具有5～50 nm的孔径分布;随着热处理温度从常温升至1 100℃,SiO2气凝胶从最初的无定形态转化为方石英;在,TEOS的醇溶液中老化,有利于增强凝胶骨架的强度;添加甲酰胺可以改善气凝胶粉体的孔径分布,提高其比表面积.     

常压干燥合成水玻璃基SiO_2气凝胶

以水玻璃和双氧水为原料一步合成二氧化硅湿凝胶,经老化、溶剂置换及三甲基氯硅烷修饰后,并于常压干燥后获得二氧化硅气凝胶。样品经低温N_2吸附-脱附、红外光谱和扫描电镜测试表明所得二氧化硅气凝胶具有三维多孔结构,且其比表面积可达482 m~2·g~(-1)、平均孔径为24.9nm及表观密度为0.12g·cm~(-3)。     

Preparation of monolithic silica-based aerogels with high thermal stability by ambient pressure drying

A monolithic silica-based aerogels were successfully synthesized via a sol-gel process using tetraethoxysilane and inexpensive inorganic aluminium salt Al(H₂O)₉(NO₃)₃ as precursors. The gels were dried at ambient pressure and the molar ratio of Al/Si was varied from 0 to 0.1. The structure and morphology of the aerogels were investigated by field-emission scanning electron microscopy, high-resolution transmission electron microscopy, Fourier transform infrared spectrometry, and N₂ adsorption-desorption. The microstructural images showed that a three-dimensional nanoscale structure formed in the as-prepared aerogels. The specific surface areas of the samples were in the range of 769–821?m²/g. Furthermore, when the Al/Si mole ratio was 0.1, its specific surface area and total pore volume remained at 103.5?m²/g and 0.25?cm³/g respectively after heating at 1100?°C for 2?h, exhibiting the best thermal stability of all aerogels fabricated in this study. The addition of the aluminium salt not only slowed the sintering of the silica, but also crystallization was restrained.     

Effect of methyltrimethoxysilane as a synthesis component on the hydrophobicity and some physical properties of silica aerogels

The effects of adding methyltrimethoxysilane (MTMS) to the synthesis formulation on the hydrophobicity and physical properties of silica aerogels are reported. The molar ratio of the methanol (MeOH) solvent, water (H₂O), and the ammonia (NH₄OH) catalyst to tetramethoxysilane (TMOS) precursor was fixed at 1TMOS:12MeOH:4H₂O:3.6×10⁻³NH₄OH throughout the experiment and the MTMS/TMOS molar ratio M was varied from 0 to 1.55. After gelation, the alcogels were dried supercritically by high-temperature solvent extraction. The hydrophobicity of the resulting aerogels was tested by measuring the water uptake by the aerogel as a function of time, after putting them directly on the surface of water. It was found that for M<0.26 the aerogels were less hydrophobic but more transparent (>90% in the visible range), whereas for M>1.03 the aerogels were more hydrophobic but semi-transparent to opaque. Aerogels that possessed good hydrophobicity and transparency (85% in the visible range) were obtained with an M≈0.70. An increase in the MTMS content in the gels shifted the pore size distribution towards larger pore radii with a broad distribution. In order to determine the thermal stability of the hydrophobic nature of the aerogels, they were heat-treated in air in the temperature range between 25 and 350°C. It was found that below 280°C the aerogel samples showed hydrophobic properties, whereas above 280°C the hydrophobicity vanished. This is due to the disappearance of the CH₃ groups in the aerogels. The aerogels were characterized by optical transmittance, pore size distribution, BET surface area and infrared spectroscopy measurements.     

Fabrication of super-elastic graphene aerogels by ambient pressure drying and application to adsorption of oils

Three-dimensional graphene-based aerogels have promising applications in oil adsorption and environmental restoration. However, current research of graphene-based aerogels is often hindered by high preparation cost, poor mechanical properties and low recycling efficiency. Here, superelastic graphene aerogel(SGA) was prepared through one-step freezing and twice hydrothermal reduction followed by drying under ambient pressure. The simple atmospheric drying provides a possibility for large-scale pr...     

Preparation of low-density carbon aerogels by ambient pressure drying

With the addition of hexamethylenetetramine (HMTA) and alcohol as solvent, an ambient pressure drying technique was developed for the fabrication of low-density organic aerogels and related carbon aerogels. When a suitable ratio of resorcinol to HMTA (R/H ratio) and ratio of resorcinol to solvents (R/S ratio) are selected, the low-density alco-gels obtained can be dried under ambient pressure conditions without observable shrinkage. The addition of HMTA increases the size of carbon nano-particles and the pore size of the aerogels that are produced. The carbon aerogels prepared in this work have similar nano-particle structures typical of the aerogels prepared with CO₂ or by the isopropanol supercritical drying technique.     

An improved method for preparing monolithic aerogels based on methyltrimethoxysilane at ambient pressure Part II: Microstructure and performance of the aerogels

The microstructures, physical properties and mechanical properties of the methyltrimethoxysilane (MTMS) derived aerogels, prepared at ambient pressure using an improved method reported in Part I, have been comprehensively characterised in this paper, as Part II. The study has revealed sophisticated pore structures and morphologies of the aerogels produced, and demonstrated the relationships between the key processing conditions and microstructures of the resultant aerogels. The results of the study also help to further understand the effect of microstructure changes on the macrostructure, volume shrinkage and performances of the aerogels, and provide a general guide on how to manipulate the processing conditions to produce monolithic MTMS based aerogels with desirable structure and properties by ambient pressure drying.     

内疏外亲SiO_2气凝胶的制备及表征

以正硅酸乙酯(TEOS)为硅源,无水乙醇为溶剂,硝酸为催化剂,氨水为凝胶剂,经溶胶-凝胶过程制备块状湿凝胶。再以三甲基氯硅烷(TMCS)为疏水改性剂,3-氨丙基三羟基硅烷为亲水改性剂对制备的湿凝胶进行改性处理,最后经超临界CO2干燥得到块体SiO2气凝胶。考察了亲水改性剂加入量分别为0、0.15、0.75、1.5、2.70 mL时对块体SiO2气凝胶亲水性的影响,并通过扫描电子显微镜(SEM)、接触角、吸水率、红外光谱(FT-IR)等对气凝胶进行表征。结果表明:所得气凝胶块体具有纳米多孔结构,内部疏水(最大接触角可达123°),表面亲水,试样亲水性随亲水剂用量的增加而增大;同一试样亲水性存在一定的梯度变化,亲水性由内到外逐渐增强。     

An improved method for preparing monolithic aerogels based on methyltrimethoxysilane at ambient pressure Part I: Process development and macrostructures of the aerogels

In this paper, we introduced an improved method for producing methyltrimethoxysilane (MTMS) based aerogels at ambient pressure. The method developed is able to produce monolithic aerogels with low bulk densities over a range of the precursor concentrations at ambient pressure without the need for any solvent exchange and surface modification. This has been achieved through specifically tailored processing conditions according to the MeOH/MTMS molar ratios used. The effect of the key processing parameters on alcogel formation and macrostructures of the aerogels has been systematically investigated for achieving minimum volume shrinkage and bulk density. The specific process developed for MTMS based aerogel has not been reported in previous literature, and it is of significance in improving the practical feasibility of the aerogel fabrication.     

Influence of preparation conditions on nanoporous structure and optical transmission of sodium silicate based ambient pressure dried aerogels employing shaking

The experimental results on the “influence of preparation conditions on nanoporous structure and optical transmission of sodium silicate based ambient pressure dried aerogels employing shaking” are reported. Sol–gel derived gels shaked in the presence of ammonium fluoride (NH₄F) and organically modified with various silylating agents and aprotic solvents are useful in controlling the magnitude of capillary stress imposed on the gel. These parameters enormously affect the optical transmission and density of the aerogels. Therefore, in the present paper the influence of different concentrations of NH₄F, silylating agents and aprotic solvents on the physical properties of the aerogels were studied. The aerogels were characterized by measuring the bulk density, volume shrinkage (%), porosity (%) and thermal conductivity. The microstructural studies were carried out using transmission electron microscopy (TEM). The hydrophobicity is confirmed by measuring the contact angle of water on the surface of aerogel and Fourier Transform Infrared (FTIR) spectroscopy. The optical transmission of the aerogels is measured by the visible spectrophotometer. The thermal stability of the aerogels was checked by the thermo gravimetric and differential thermal analyses (TGA-DTA).     

Facile synthesis of hydrophobic,thermally stable,and insulative organically modified silica aerogels using co-precursor method

Silica aerogels have low density and high specific surface area, but there are restrictions regarding their durability and commercialization owing to their fragile nature and the strong moisture absorbing behavior of the siloxane network. To overcome these restrictions, this study evaluated hybrid organically modified silica (ORMOSIL) aerogels by employing 3-(trimethoxysilylpropyl) methacrylate (TMSPM) in tetraethyl orthosilicate (TEOS) through a two-step sol-gel co-precursor method. The methacrylate organic groups were incorporated into the silica networks via reactions between the Si-OH moieties in silica aerogels, resulting in ORMOSIL aerogels. The properties of the ORMOSIL aerogels were strongly affected by the amount of TMSPM co-precursor. The highest concentration of TMSPM (30 wt%) resulted in ORMOSIL aerogels with improved characteristics when compared with the pristine TEOS-based silica aerogels, such as hardness (0.15 GPa), Young's modulus (1.26 GPa), low thermal conductivity (0.038 W/m K), high water contact angle (140°), and high thermal stability (350 °C).