热处理对二氧化硅干凝胶组织结构的影响

气凝胶是一种低密度纳米多孔非晶态材料,具有连续无规则网络结构,其纳米尺度的颗粒及孔隙分布,使其具有很多独特的性质.利用溶胶-凝胶酸碱二步催化法和常压干燥法制备了结构完整的SiO2干凝胶.采用DTA-TG,XRD,SEM和IR等实验手段,对SiO2干凝胶在室温以及300,600,900,1200℃四个热处理温度下的组织结构进行了对比和分析.结果表明,粘性流造成干凝胶的烧结.热处理后,材料颗粒变大,孔隙缩小,材料发生收缩,并在高温下变得坚硬致密.     

无机盐和有机醇盐制备Al2O3干凝胶

分别以廉价的硝酸铝无机盐常压干燥法和异丙醇铝有机醇盐作为铝源,采用溶胶-凝胶法制备了结构完整的SiO2-Al2O3干凝胶和纯Al2O3干凝胶.利用热重差热分析、Fourier转换红外吸收谱、氮气吸附和电子显微镜等实验手段,对比研究了曲种制备方法对材料性能的影响.结果表明:由无机盐制备的SiO2-Al2O3干凝胶是以SiO2构成网络骨架,形成一定数量的Si-O-Al键,并保持了干凝胶纳米多孔的特点;而有机醇盐制备的Al2O3干凝胶孔隙分布更为均匀.采用两种不同铝源制备的干凝胶,均是纳米颗粒组成的多孔结构,并均具有较高的比表面积和较窄的孔径分布.     

Advances in tailoring resorcinol-formaldehyde organic and carbon gels

An overview on the preparation and properties of resorcinol-formaldehyde (RF) organic and carbon gels reveals the fascinating and remarkably flexible properties of RF carbon and organic gels and how these properties are related to the synthesis and processing conditions. The structural properties can be easily tailored by rigidly controlling such conditions. However, slight variations in some conditions may cause drastic variations in the structural characteristics, and hence properties. Therefore, the effects of different conditions must be well-understood before attempting to tailor organic or carbon gels to specific applications. The most important factors that affect the properties of an organic gel are the precursor concentrations, the catalyst type and concentration, the time and temperature of curing, and the drying method. In addition to these factors, characteristics of activated carbon gels also depend on the pyrolysis temperature and the activation method. These conditions impact the structural and performance characteristics significantly.     

Patterning of Nanoparticle‐Based Aerogels and Xerogels by Inkjet Printing

Nanoparticle‐based voluminous 3D networks with low densities are a unique class of materials and are commonly known as aerogels. Due to the high surface‐to‐volume ratio, aerogels and xerogels might be suitable materials for applications in different fields, e.g. photocatalysis, catalysis, or sensing. One major difficulty in the handling of nanoparticle‐based aerogels and xerogels is the defined patterning of these structures on different substrates and surfaces. The automated manufacturing of nanoparticle‐based aerogel‐ or xerogel‐coated electrodes can easily be realized via inkjet printing. The main focus of this work is the implementation of the standard nanoparticle‐based gelation process in a commercial inkjet printing system. By simultaneously printing semiconductor nanoparticles and a destabilization agent, a 3D network on a conducting and transparent surface is obtained. First spectro‐electrochemical measurements are recorded to investigate the charge–carrier mobility within these 3D semiconductor‐based xerogel networks.     

Low-dimensional, hinged bar-code metal oxide layers and free-standing, ordered organic nanostructures from turbostratic vanadium oxide

Both low-dimensional bar-coded metal oxide layers, which exhibit molecular hinging, and free-standing organic nanostructures can be obtained from unique nanofibers of vanadium oxide (VO(x)). The nanofibers are successfully synthesized by a simple chemical route using an ethanolic solution of vanadium pentoxide xerogel and dodecanethiol resulting in a double bilayered laminar turbostratic structure. The formation of vanadium oxide nanofibers is observed after hydrothermal treatment of the thiol-intercalated xerogel, resulting in typical lengths in the range 2-6 microm and widths of about 50-500 nm. We observe concomitant hinging of the flexible nanofiber lamina at periodic hinge points in the final product on both the nanoscale and molecular level. Bar-coded nanofibers comprise alternating segments of organic-inorganic (thiols-VO(x)) material and are amenable to segmented, localized metal nanoparticle docking. Under certain conditions free-standing bilayered organic nanostructures are realized.     

Ir–C xerogels synthesized by sol–gel method for NO reduction

Iridium–carbon (Ir–C) xerogels were synthesized via a one-pot sol–gel polycondensation of hexachloroiridic acid, resorcinol and formaldehyde, followed by carbonization in a nitrogen atmosphere at 500–1000 °C. The samples were characterized by various techniques including N₂ adsorption, XRD and TEM. The N₂ adsorption showed that the Ir–C samples were of microporous structures, and their specific surface areas increased with the pyrolysis temperature. Both the XRD and TEM revealed that the Ir particles in the Ir–C samples were highly dispersed in the carbon matrix, in contrast with the large Ir particles in the Ir/C sample which was obtained by impregnation. The Ir–C xerogels exhibited high activities and selectivities towards N₂ in the reduction of NO with carbon or CO. In particular, the Ir–C samples behaved much stably than the Ir/C, demonstrating the superiority of the one-pot synthesis method.     

The Influence of the Amount of Fluoride Catalyst on the Morphological Properties of the Anilinepropylsilica Xerogel Prepared in Basic Medium

Anilinepropylsilica hybrid powder was synthesized by a sol-gel route in basic medium using HF as catalyst. The effect of the amount of HF catalyst on the morphologies and on organic content was studied. The xerogels were characterized using FTIR, SEM and N₂ adsorption-desorption isotherms. The increase in the HF quantity results in an slight increase in organic content and changes in the particle size, surface area and pore structure.     

Subcritical drying of silica gels

Silica aerogel is a sol-gel prepared material characterized by high porosity and large inner surface area. Aerogels can be prepared with a high transparency and low thermal conductivity, giving a material excellent for application as transparent thermal insulator. The traditional route to prepare silica aerogels is by formation of an alcogel by hydrolysis and condensation of silicon alkoxides followed by supercritical drying in an autoclave at high pressure (–100 atm). Unfortunately, this process is expensive and might be dangerous, so drying methods have been developed that operate under ambient conditions. In previous work, we have shown that gels can be strengthened and stiffened by providing new monomers to the alcogel giving xerogels with similar properties as aerogels by drying at ambient pressure (porosity up to 90%). This method of obtaining ambient pressure dried aerogels will be described and special emphasis will be given on the effect of the initial gel structure on the preparation of the xerogels.