Supercritical CO2 drying of pure silica aerogels: effect of drying time on textural properties of nanoporous silica aerogels

Journal of Sol-Gel Science and Technology - Aerogel technologies provide high-performance lightweight materials with unique textural characteristics such as high specific surface area and open...     

Preparation and characterization of SiO2 two-step aerogels

Aerogels are well suited as transparent insulation materials in solar architecture and collector systems. Their nanoporous structure provides a high solar transmittance and a low thermal conductivity, generally below 0.02 W m^–1 K^–1. Transparent aerogels with densities above 80 kgm^–3 can easily be prepared at room temperature via a one-step sol-gel process with subsequent supercritical drying. Separating hydrolysis and condensation via a two-step method allows the preparation of transparent ultra-low density SiO₂-aerogels. To optimize the optical properties, characterized by the scattering coefficient of the gels, we have investigated the influence of preparation parameters, such as pH-value of the sol-gel starting solution and macroscopic density, on the gel structure. To determine the nanostructure we performed spectral light scattering as well as small angle X-ray scattering (SAXS) measurements.     

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.     

Raman study of structural defects in SiO2 aerogels

The structure of the silica aerogels was studied by Raman spectroscopy. The spectra of the solid network resembles that of bulk silica with additional bands related to organic groups and a large amount of OH groups.The typical bands due to ring breathing also called defect bands D ₁ and D ₂ located at 490 and 610 cm^–1 are present. However, the evolution of the D ₂ band compared to that of OH band (980 cm^–1) seems apparently, in contradiction with the results previously reported in the literature. During heat treatments between 25 and 300°C the D ₂ and the OH bands increase simultaneously. Generally, in silica glass the defect band D ₂ grows at the expense of the OH groups.This result is explained by the oxidation of the organic compounds which, in this temperature range, leads to the formation of the both species (OH) and those related to siloxane rings. ²⁹Si MAS NMR results are in agreement with the Raman study.     

Synthesis of polystyrene/SiO2 composite microparticles by dispersion polymerization in supercritical fluid

A novel synthetic route to prepare polystyrene/SiO₂ composite microparticles in supercritical carbon dioxide (scCO₂) is presented. Silica particles with the size of 130 nm which were surface-modified with 3-(trimethoxysilyl) propyl methacrylate were used as seeds in the dispersion polymerization of styrene in the presence of a polymeric stabilizer, poly(1,1-dihydroheptafluorobutyl methacrylate-co-diisopropylaminoethyl methacrylate) to produce dry composite particles. The transmission electron microscopy analysis revealed that the composite microspheres contained several silica particles.     

邻氨基苯甲酸/SiO2复合材料制备及其光热特性

有机发光体以其较高的发光效率、快速响应能力和较宽的可调谐波长被广泛应用于光转换材料、非线性光学材料和激光材料[1-3]。但是有机物的光学性能不稳定和热稳定性较差,使其在许多方面的应用受到限制。为了提高其光学和热稳定性,常用的方法是将各种有机发光体掺杂在以正硅酸乙酯为原料制得的SiO2凝胶玻璃基质中[4-6]。SiO2凝胶通常采用溶胶 凝胶法通过水解缩合制备。制备时一般采用无机酸作催化剂在酸性条件下进行,对质子酸作用敏感的有机物在酸性条件下往往热稳定性、光学性能下降,甚至遭到破坏。本文以廉价水玻璃为原料,在潜伏酸的…     

Mesoporous TiO2/SiO2 composite nanofibers with selective photocatalytic properties

Mesoporous TiO2/SiO2 composite nanofibers with a diameter of 100-200 nm and silica shell thickness of 5-50 nm have been fabricated by a sol-gel combined two-capillary co-electrospinning method; the composite nanofibers exhibited selective photocatalytic activity based on the decomposition of Methylene Blue, Active Yellow and Disperse Red.     

超临界流体干燥法制备TiO2／C纳米粒子及光催化性能

以TiCl4为原料,采用溶胶凝胶法结合超临界流体干燥法(SCFD)制备了纳米级TiO2/C复合光催化剂.以苯酚的光催化降解对所得催化剂的催化活性进行了评价.结果表明,纳米TiO2/C复合粒子与单组分TiO2比较,复合粒子光催化活性高于单组分的TiO2,h苯酚降解率高达975 ％,COD为957%.并用XRD、TEM、 UV-Vis和XPS等手段进行了表征,iO2以锐钛矿型形式存在.比较了不同制备方法制得的TiO2/C复合催化剂,得出超临界干燥法制备的光催化剂具有粒径小,比表面积大,分散性好,光催化活性高等特点.     

Effect of iron acetylacetonate on physico-chemical properties of waterglass based aerogels by ambient pressure drying

The effect of iron acetylacetonate on the physico-chemical properties of waterglass based silica aerogels by ambient pressure drying has been investigated. Doping the gels with iron acetylacetonat (FeAA) facilitates in the diminution of the density of the aerogels. The well established silica network provides effective confinement of FeAA nanoparticles which resists the collapse of silica network during ambient pressure drying. Therefore, in the present paper, the effects of FeAA on the physico-chemical properties of the aerogels have been studied by varying the FeAA:Na₂SiO₃ molar ratio from 3 × 10⁻⁴ to 6 × 10⁻⁴. The aerogels were prepared via ambient pressure drying and characterized by the bulk density, thermal conductivity and water contact angle. The aerogel’s surface morphology, elemental analysis and pore structure were characterized by means of EDAX and FTIR, TEM and N₂ adsorption- desorption analyzer. The high temperature hydrophobicity of these aerogels was checked by heating them in temperature controlled furnace. Silica aerogels with low density ~0.050 g/cc have been obtained using the molar ratio of Na₂SiO₃:H₂O:FeAA:Citric acid:TMCS at 1:146.67:3 × 10⁻⁴:0.54:9.46, respectively. EDAX and FTIR studies show that the iron species are entrapped in the mesoporous framework and not took part in the bonding with silica.     

Influence of Ti addition on boehmite-derived aluminum silicate aerogels: structure and properties

Aluminosilicate aerogels offer potential for extremely low thermal conductivities at temperatures greater than 900?°C, beyond where silica aerogels reach their upper temperature use limits. Aerogels have been synthesized at two Al:Si ratios, a 3Al:1Si mullite composition, and an 8Al:1Si alumina rich composition. Boehmite (AlOOH) is used as the Al source, and tetraethoxysilane as the Si precursor. The influence of Ti as a ternary constituent, introduced through the addition of titanium isopropoxide in the sol?Cgel synthesis, on aerogel morphology and thermal properties is evaluated. Four different boehmite precursor powders are evaluated. Morphology, surface area and pore size, and thermal transformation vary with the crystallite size of the starting boehmite powder, as does incorporation of titanium and evolution of Ti-containing crystalline phases. The addition of Ti influences sol viscosity, gelation time, surface area and pore size distribution, as well as phase formation on heat treatment.     

Preparation and femtosecond non-linear optical properties of Ag/SiO2 composite thin films

Ag nanoparticles embedded in SiO₂ thin films (Ag/SiO₂ films) were prepared by a multitarget sputtering method. In the optical absorption spectra of the Ag/SiO₂ films, the absorption peak due to the surface plasmon resonance (SPR) of Ag particle was clearly observed at the wavelength of 394–413 nm. The imaginary part of the third-order non-linear susceptibility, Im [χ⁽³⁾], of the Ag/SiO₂ film was estimated to be ?1.1×10^?8 esu measured by the femtosecond Z-scan technique near the SPR peak. The response time of the film measured from the decay of the differential transmission of the pump-probe experiment was 1.3 ps at the SPR peak.     

Preparation and characterization of polymer composite multilayers on SiO2

Poly(diallyldimethylammonium chloride) (PDADMAC) and poly(sodium 4-styrenesulfonate) (PSS) have been consecutively adsorbed onto 1.5-microm charged silica (SiO2) particles. Time-dependent adsorption studies indicate that, due to the strong ionic charge of the dissociated polycation in water, adsorption is complete in less than 30 min. Indications of the maximum adsorption density, changes in surface charge, and stability of the layered particles are demonstrated through adsorption isotherms and electrophoretic mobility (EPM) measurements. Further stability of the PDADMAC layer is demonstrated through multiwashing with ultra pure deionized water. Preliminary desorption studies of the PSS layer also illustrate a stabilized two-layer system. Due to the nature of the electrostatic charges on the surface of the SiO2 core particles and both polyelectrolytes in aqueous media, the use of polyelectrolytes as layering elements serves as a model for the assembly of time-released drug delivery particle systems.     

Cluster structures for water adsorbed on SiO2

The MO LCAO SCF method has been used in the MNDO approximation to examine the electronic structures and formation energies for adsorption complexes for water on SiO₂ stabilized by hydrogen and coordination bonds. Stabilization by several hydrogen bonds cannot explain the high initial heats of adsorption for dehydrated silica. H₂O coordination complexes give higher energies, and hydroxylated silicon atoms on SiO₂ are considered as primary water-adsorption centers.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 24, No. 5, pp. 532–545, September–October, 1988.     

Thermophysical properties of supercritical fluids with special consideration of aqueous systems

Selected thermophysical properties of polar, dense supercritical fluids are discussed. The static dielectric constant of water and the freon R22 (CHClF₂) is shown in some detail. Examples of critical curves for binary systems with water and methanol are given. New experimental data of excess volumes up to 400 °C and 2000 bar are shown for water combined with H₂, CH₄ and benzene. Excess Gibbs energies and activity coefficients for water-benzene are also shown. Results of the solubility of anthracene in ten different high pressure fluids, obtained by UV-spectroscopy, are presented. The PVT-data of concentrated supercritical aqueous NaCl-solutions and the ion product of pure water to 1000 °C are discussed.     

复合型螯合吸附材料PEI/SiO2对铜离子吸附性能的研究

通过γ-氯丙基三甲氧基硅烷的媒介, 将聚乙烯亚胺(PEI)偶联接枝到硅胶微粒表面, 制备了复合型螯合吸附材料PEI/SiO2;研究了PEI/SiO2对Cu2 的吸附性能. 复合型螯合吸附材料PEI/SiO2对Cu2 具有强的螯合吸附能力;等温吸附数据符合Langmuir方程, 且吸附量随温度升高而增大;pH对吸附量有很大的影响, pH 7时, 吸附量最高.     

Microstructure and catalytic properties of Rh and Ni dispersed on TiO2-SiO2 aerogels

The influence of the preparation method on the microstructure and catalytic behavior of Rh and Ni dispersed on TiO₂-SiO₂ aerogels is investigated.The autoclave method has been followed to prepare titania-silica aerogels with TiO₂ contents ranging between 0 and 10 mole %. These aerogels have been used as matrices to disperse catalytically active metals: Rh and Ni. The metals can be deposited by impregnation of aerogels, or alternatively, can be added into the hydrolysis water used in the synthesis of gels. The resulting catalysts present surface areas higher than 550 m²·g^–1.The percentage of titania, the method followed for the introduction of the metal, and the nature of the metal itself affect both the activities and selectivities of the catalysts in the hydrogenolysis of n-butane. Thus, the presence of titania in Rh catalysts increases the activity values, and the samples prepared by impregnation present selectivities towards ethane higher than 80%. Whereas, the rhodium catalysts in which the metal has been introduced before gelling, do not orientate the reaction in favor of a definite product. For the case of Ni, it is quite frequent to obtain high selectivities towards the breakdown of the C-C terminal bonds. In summary, the preparation methods allow to modulate into very broad limits the catalytic behavior of the samples.     

Influence of SiO2 nanoparticles on electro-optical and thermophysical properties of polyacrylate/liquid crystal composites

ABSTRACT

In this paper, SiO₂ nanoparticle-doped polymer dispersed liquid crystal (PDLC) lenses are made from a mixture of prepolymer, 5CB liquid crystal and SiO₂ nanoparticles by the polymerisation induced phase separation (PIPS) process. The effect of SiO₂ nanoparticles on the electro-optical properties of PDLC ?lms are studied. It is established that SiO₂ nanoparticles affect the microstructure of PDLC ?lms signi?cantly because of the formed agglomerates of SiO₂ nanoparticles. Results show an improvement in the electro-optical properties and a decrease in the response time for doped systems with small amount of SiO₂ nanoparticles. We also observe a shift of nematic–isotropic transition temperature as a function of SiO₂ nanoparticle contents. A good agreement between the electro-optical study and thermophysical properties is reached.