Physical Properties of Silica Aerogels Prepared with Polyethoxydisiloxanes

Silica aerogels were made by sol-gel techniques using industrial silicon derivatives (polyethoxydisiloxanes, E-40), followed by supercritical drying with ethanol. The morphology and microstructure of the silica aerogels were investigated by using specific surface area, S_BET, SEM, TEM and the pore size distribution techniques. The thermal conductivity was also measured as a function of air pressure. The results show that the diameter of the silica particles is about 13 nm and the pore size of the silica aerogels is 20–80 nm. The specific surface area of the silica aerogel is about 470 m²/g and the thermal conductivity of the silica aerogel prepared with E-40 is 0.014 w m^–1 K^–1 at room temperature and 1 atm.     

Bioactivity and Mechanical Properties of Polydimethylsiloxane (PDMS)-CaO-SiO2 Hybrids with Different PDMS Contents

Silica-titania and titania aerogels were prepared by supercritical drying using different solvents such as low temperature CO₂ (353 K), high temperature CO₂ (553 K), ethanol (553 K) and ethanol with zeolite (553 K) and their efficiencies for the removal of benzene from a synthetic air mixture were investigated. The aerogels obtained showed both large capacities for benzene adsorption and high photocatalytic activity for its decomposition in the adsorbed state. The degree of benzene removal by silica-titania aerogel seemed to depend on the crystallinity of the titania and was in the order low temperature CO₂ < high temperature CO₂ < ethanol = ethanol with zeolite. The amount of CO₂ liberated by the photocatalytic decomposition of benzene also followed the same sequence. Titania aerogels showed the greatest efficiency in the decomposition of benzene, while the amount of CO₂ evolved was lower than those of the silica-titania systems investigated. The adsorption capacity of benzene basically depended on the silica matrix and was not greatly influenced by the difference in the crystallinity of titania.     

Preparation of SiO2-TiO2 Aerogels Using Supercritical Impregnation

The preparation of SiO₂-TiO₂ aerogels by supercritical impregnation of titanium alkoxides into silica alcogels was investigated. A mixture of CO₂ and 2-propanol with dissolved titanium tetraisopropoxide modified with acetylacetone was used as the impregnation medium. Prior to the experiments, the supercritical behaviour of the impregnation solution was investigated. The microstructure and properties of aerogels prepared by the supercritical impregnation method were almost identical to those generated by the liquid impregnation. However, the time for impregnation was substantially decreased and the homogeneity of the impregnated titanium distribution on the aerogel increased.     

AKD-Modification of bacterial cellulose aerogels in supercritical CO2

Different approaches towards hydrophobic modification of bacterial cellulose aerogels with the alkyl ketene dimer (AKD) reagent are presented. If AKD modification was performed in supercritical CO₂, an unexpectedly high degree of loading was observed. About 15 % of the AKD was bound covalently to the cellulose matrix, while the other part consisted of re-extractable AKD-carbonate oligomers, which are novel chemical structures described for the first time. These oligomers contain up to six AKD and CO₂ moieties linked by enolcarbonate structures. The humidity uptake from environments with different relative humidity by samples equipped with up to 30 % AKD is strongly reduced, as expected due to the hydrophobization effect. Samples above 30 % AKD, and especially at very high loading between 100 and 250 %, showed the peculiar effect of increased humidity uptake which even exceeded the value of unmodified bacterial cellulose aerogels.     

Cellulose–silica composite aerogels from “one-pot” synthesis

Cellulose–silica composite aerogels were prepared via “one-pot” process: aqueous solutions of cellulose–8 wt% NaOH and sodium silicate were mixed, coagulated and dried with supercritical CO₂. The system was studied both in the fluid and solid (dry) states. Cellulose and sodium silicate solutions were mixed at different temperatures and concentrations; mixture properties were monitored using dynamic rheology. The gelation time of the mixture was strongly reduced as compared to that of cellulose–NaOH solutions; we interpret this phenomenon as cellulose self-aggregation inducing partial coagulation due to competition for the solvent with sodium silicate. The gelled cellulose/sodium silicate samples were placed in aqueous acid solution which completed cellulose coagulation and led to in situ formation of sub-micronic silica particles trapped in a porous cellulose matrix. After drying with supercritical CO₂, an organic–inorganic aerogel composite was formed. The densities obtained were in the range of 0.10–0.25 g/cm³ and the specific surface area was between 100 and 200 m²/g. The silica phase was shown to have a reinforcing effect on the cellulose aerogel, increasing its Young’s modulus.     

Methods to Analyse the Texture of Alginate Aerogel Microspheres

Nitrogen adsorption at 77 K has been applied to the study of the texture of alginate aerogel microspheres obtained by CO₂ supercritical drying of alcogels. The limited volume shrinkage suggests that the aerogels preserve the texture of the hydrogels. Alginate aerogels presents a N₂ adsorption at small pressure higher than reference non-porous silica, to be attributed to the polarity of the surface or to a small microporous volume. The aggregated nanobead strings of the guluronic-rich gels accounts for a significant mesoporosity. The N₂ adsorption results correspond to electron microscopy observations for features smaller than 50 nm.     

以纤维素/AmimCl溶液制备纤维素气凝胶

利用离子液体AmimCl溶解结合超临界CO2干燥的方法制备了纤维素气凝胶材料.研究了不同初始浓度的纤维素溶液及其在不同凝固浴中制备的纤维素凝胶的流变行为,进一步考察了纤维素溶液浓度和凝固浴种类对纤维素气凝胶材料结构的影响.结果表明,随着初始纤维素溶液浓度的增大,气凝胶的孔结构逐渐致密,比表面积随之减小;凝固浴的组成对纤维素气凝胶的结构也有较大影响.采用适当的制备条件,可以制备出高比表面积的纤维素气凝胶材料.对纤维素气凝胶的热性能进行了表征,结果表明所得到的气凝胶材料具有较好的热稳定性和较高的炭残余含量.     

Polyurea based aerogel for a high performance thermal insulation material

Less fragile lightweight nanostructured polyurea based organic aerogels were prepared via a simple sol–gel processing and supercritical drying method. The uniform polyurea wet gels were first prepared at room temperature and atmospheric pressure by reacting different isocyanates with polyamines using a tertiary amine (triethylamine) catalyst. Gelation kinetics, uniformity of wet gel, and properties of aerogel products were significantly affected by both target density (i.e., solid content) and equivalent weight (EW) ratio of the isocyanate resin and polyamine hardener. A supercritical carbon dioxide (CO₂) drying method was used to extract solvent from wet polyurea gels to afford nanoporous aerogels. The thermal conductivity values of polyurea based aerogel were measured at pressures from ambient to 0.075 torr and at temperatures from room temperature to −120 °C under a pressure of 8 torr. The polyurea based aerogel samples demonstrated high porosities, low thermal conductivity values, hydrophobicity properties, relatively high thermal decomposition temperature (~270 °C) and low degassing property and were less dusty than silica aerogels. We found that the low thermal conductivities of polyurea based aerogels were associated with their small pore sizes. These polyurea based aerogels are very promising candidates for cryogenic insulation applications and as a thermal insulation component of spacesuits.     

SiO<Subscript>2</Subscript>–TiO<Subscript>2</Subscript> binary aerogels: Synthesis in new supercritical fluids and study of thermal stability

A comparative analysis of properties of SiO₂–TiO₂ binary aerogels prepared by supercritical drying using different supercritical fluids (isopropanol, hexafluoroisopropanol, methyl tert-butyl ether, and CO₂) has been performed. The use of different supercritical fluids allows preparation of both homogeneous amorphous SiO₂–TiO₂ binary aerogels (by supercritical drying in hexafluoroisopropanol and CO₂) and composite aerogels containing nanocrystalline anatase (by supercritical drying in isopropanol and methyl tert-butyl ether). The thermal treatment of the aerogels at temperatures up to 600°C does not lead to considerable change in the porous structure and phase composition of the aerogels.     

非超临界干燥法制备SiO2气凝胶

应用廉价的国产硅溶胶为原料，通过凝胶过程和干燥过程条件的选择，以非超临界干燥技术最终获得了块状SiO2气凝胶.该气凝胶外观状态与应用正硅酸乙酯为原料制得的完全一致，其微观结构也相当良好，其直径和孔分布均匀.溶液的配比和pH对凝胶过程和气凝胶样品的密度有比较明显的影响，同时pH值与SiO2的粒径之间也有一定的关系.依据制备条件的变化，所得SiO2气凝胶的密度约在200～400 kg•m－3，比表面在250～300 m2•g－1之间变化，平均孔径约为11～20 nm，而孔隙率在91％左右.     

Transparent and flame retardant cellulose/aluminum hydroxide nanocomposite aerogels

Cellulose-based nanocomposite aerogels were prepared by incorporation of aluminum hydroxide (AH) nanoparticles into cellulose gels via in-situ sol-gel synthesis and following supercritical CO₂ drying. The structure and properties of cellulose/AH nanocomposite aerogels were investigated by Fourier transform infrared spectroscopy, scanning electron microscopy, ultraviolet-visible spectrometry, N₂ adsorption, thermogravimetric analysis, and micro-scale combustion calorimetry. The results indicated that the AH nanoparticles were homogeneously distributed within matrix, and the presence of AH nanoparticles did not affect the homogeneous nanoporous structure and morphology of regenerated cellulose aerogels prepared from 1-allyl-3-methylimidazolium chloride solution. The resultant nanocomposite aerogels exhibited good transparency and excellent mechanical properties. Moreover, the incorporation of AH was found to significantly decrease the flammability of cellulose aerogels. Therefore, this work provides a facile method to prepare transparent and flame retardant cellulose-based nanocomposite aerogels, which may have great potential in the application of building materials.     

Synthesis of freestanding silica and titania-silica aerogels with ordered and disordered mesopores

Freestanding blocks of silica and titania-silica aerogels were prepared by the sol-gel method. It is possible to prepare crack-free, titania-silica aerogels with high titanium content by a careful control of the synthesis conditions. Prehydrolysis, complexation and polymer addition were used to adjust the hydrolysis and condensation rates of the silicon and titanium alkoxide precursors. Photoactive anatase TiO₂ nanocrystals with a large surface area (i.e., up to 300m²g⁻¹) were crystallized from the gel network by the high-temperature ethanol supercritical drying, and the resulting aerogel blocks were gas permeable and display a transition-regime diffusion behavior. Pore and volume shrinkages were observed in samples prepared by ethanol supercritical drying when the titanium content was increased resulting in a lower flux. Adding Pluronic P123 creates ordered mesopore domains and produces large pore aerogels even at high titanium contents. The photocatalytic oxidation reaction of trichloroethylene was performed by flowing the reactant gas mixture through the UV-irradiated aerogel blocks with excellent results.     

Recent insights into xerogel and aerogel mineral composites for CO<Subscript>2</Subscript> mineral sequestration

Supercritically dried composites have already been analysed and proposed as carbon dioxide sequesters. However, the economical and energetic costs of the supercritical drying process had to be re-evaluated, and were eventually found not to enhance the feasibility of the proposed route for CO₂ mineral sequestration. Different composites series were synthesised with the only difference being the drying method. The structures of the porous matrix were characterised as well as their ability to capture CO₂. The first results showed that the xerogel matrix is as good a host as the aerogel one, and also avoids expensive procedures such as supercritical drying for sample preparation without losing CO₂ capture capacity and enhancing the efficiency of the whole carbon sequestration process. In this case, the sample preparation was simplified as much as possible, with the aim of reducing energetic and economic costs. Although good carbonation efficiencies were obtained with these cheap samples, the first results showed that previous high carbonation efficiencies could not be repeated.     

Influence of supercritical drying fluids on structures and properties of low-density Cu-doped SiO2 composite aerogels

The Cu-doped SiO₂ composite aerogels were successfully prepared by sol–gel process and subsequently supercritical drying with ethanol and CO₂. The Cu-doped SiO₂ composite aerogels had porous texture, low density (<100 mg cm^?3) and high specific surface area (>800 m² g^?1), which were investigated by FESEM and nitrogen adsorption desorption porosimetry. The FTIR spectra of the aerogels showed that the ethanol-dried aerogels had been modified by ethyl while the corresponding CO₂-dried aerogels had more Si–OH groups. The phase structure and thermal stability were investigated by XRD and TGA, respectively. Due to the reducibility of ethanol, the copper was crystalline in ethanol-dried sample. The Cu-doped SiO₂ composite aerogels dried with supercritical ethanol had larger pore diameter and better thermal stability under 400 °C in comparison with CO₂-dried composite aerogels. The structures and properties of Cu-doped SiO₂ composite aerogels are obviously affected by supercritical drying conditions. The effect research could instruct the synthesis of different state of Cu in composite aerogels.     

常压干燥法制备气凝胶的研究进展

气凝胶是一类轻质、低密度的三维纳米多孔固态材料,因其独特的高孔隙率、高比表面积和低导热系数等特性,使其在吸附、催化、保温隔热和隔音等诸多领域具有广泛的用途,目前其相关研究在材料科学领域受到了广泛的关注。气凝胶的制备主要包括溶胶-凝胶过程和湿凝胶干燥两个步骤,湿凝胶的干燥是制备气凝胶过程中至关重要而又较为困难的一步。传统的气凝胶通过超临界干燥制备,工艺复杂、成本高,而且由于干燥过程在高温高压条件下进行,有一定的危险性并且不适宜大规模生产,因此如何通过常压干燥获得高比表面积、高孔隙率、低密度的性能优异的气凝胶是其研究的重要方向之一。本文简要介绍了湿凝胶的制备以及凝胶干燥理论,详细介绍了近年来常压干燥方法气凝胶制备的研究进展,并对其未来发展前景做出了展望。     

Shrinkage and pore structure in preparation of carbon aerogels

To aim at thermal insulator applications, the shrinkage and the pore structure of resorcinol–formaldehyde (RF) aerogels and carbon aerogels were investigated during the supercritical drying and the carbonization process. The water (W) molar ratio has small effects on the surface area or the particle size, but has significant effects on the density of the aerogel. Higher W/R ratio leads to lower density and larger pore size, and leads to less shrinkage during the carbonization process. The molar ratio of catalyst sodium carbonate (C) has significant effects on the shrinkage, pore size, and particle size of the aerogel. Lower R/C ratio leads to smaller particle size and smaller pore size, and thus induces more shrinkage both in the supercritical drying and in the carbonization, the obtained CA is much denser. The R/C ratio should be higher than 300 to prevent excessive shrinkage. In order to synthesize carbon aerogels combining with small shrinkage, low density (less than 0.1 g/cm³), and small pore size (less than 150 nm) for thermal insulators, the preferred W/R ratio is between 90 and 100, and the preferred R/C ratio is between 300 and 600.     

溶胶－凝胶法制备超细SiO_2──制备参数对气凝胶织构性质的影响

ＳｉＯ_２气凝胶超细粉由ＴＥＯＳ（正硅酸乙酯）在乙醇溶液中水解聚合后经超临界流体干燥（ＳＣＦＤ）制得。本文考察了ＴＥＯＳ的浓度、水和ＴＥＯＳ的摩尔比以及焙烧温度对超细粉织构性能的影响。结果表明，超细ＳｉＯ_２的比表面积、总孔容、孔分布、最可几孔径及表现堆密度均随制备参数而改变。而以ＳｉＯ_２气凝胶超细粉体为基本载体材料的Ｃｏ／ＺｒＯ_２－ＳｉＯ_２催化剂显示出高的ＣＯ＋Ｈ_２合成重质烃的活性和选择性。     

Synthesis and properties of alumina aerogels

The synthesis of alumina aerogels has been carried out by hydrolysis of aluminium isopropylate as an aerogel precursor dissolved in isopropanol or methanol, followed by gelation of the sols obtained and drying under supercritical conditions and calcination. The influence of two main preparation parameters, precursor concentration and reagents (water to aluminium isopropylate) mole ratio, on the physicochemical properties of aerogels was investigated.     

Physicochemical properties and hydrogenation activity of nickel-alumina aerogels

Nickel-alumina aerogel catalysts were prepared by three methods: (I) hydrolysis of a mixture of aerogel precursors, followed by cogelation of the sols obtained and drying under supercritical conditions; (II) impregnation of thermally treated alumina aerogels with nickel salt solutions followed by drying of the suspensions obtained under supercritical or (III) conventional conditions. Activity of the aerogel catalysts prepared was determined in cyclohexene hydrogenation.     

Organofunctional Silica Aerogels

Organically modified silica aerogels were prepared from mixtures of tetramethoxysilane (TMOS) and organofunctional alkoxysilanes RSi(OMe)₃ with R=mercaptopropyl, diphenylphosphinoethyl and chloropropyl. The base catalyzed hydrolysis and condensation reactions, followed by supercritical drying with liquid carbon dioxide were investigated. Starting from 9:1 mixtures of TMOS and RSi(OMe)₃, incorporation of the functional moieties succeeded quantitatively. Increasing the percentage of RSi(OMe)₃ to 20% or 40% leads to an incomplete condensation of the RSiO_1,5 units to the SiO₂ network. Compared with an unmodified silica aerogel, the microstructure of the resulting hybrid aerogels is nearly uninfluenced for the 9:1 mercapto-and chloro-modified samples, while in the phosphino-modified sample the typical pore radii distribution is disturbed by the bulky organic groups. The organofunctional aerogels decompose between 210 and 650°C.