二氧化硅气凝胶的小角X光散射研究

以正硅酸乙酯（ＴＥＯＳ）为源，用溶胶－凝胶法经不同的制备条件能得到不同密度，不同结构特性的二氧化硅气凝胶，小角Ｘ光散射实验表明，用一步法，在酸性、中性或碱性条件下制备的气凝胶，均具有分形结构，而用两步法制备的气凝胶不具备分形特性，对样品加热氧化处理能组成气凝胶的胶体颗粒表面光滑化，但不改变其分形结构．     

Observation of Sub-fractal Behaviour in a Light-scattering System

The results are reported of an experimental investigation of optical scintillation generated when laser light is scattered by a turbulently mixing layer of brine and pure water. Non-gaussian intensity statistics have been measured as a function of propagation distance in the Fresnel region and as a function of illuminated area and scattering angle in the Fraunhofer region. A characteristic power-law dependence on the illuminated area of the far-field contrast is found, suggesting that the slope of the scattered wavefront may be self-affine over a significant range of length scales.     

Light-scattering intensity fluctuations in microdroplets containing inclusions

A prominent characteristic of the light scattered from a microparticle containing inclusions is a fluctuation in the intensity that is due to the changing positions of the inclusions with respect to each other and the host droplet. We calculate the magnitude of these fluctuations for a host sphere containing a single eccentrically located spherical inclusion and experimentally measure the fluctuation amplitudes for host spheres containing multiple inclusions. We find that, for sufficiently small single inclusions, the amplitude of the scattering fluctuations increases approximately linearly with the area of the inclusion. For multiple inclusions, the fluctuation amplitude increases with concentration with an approximate power-law dependence.     

Some general features of random elastic scattering spectra

The dependence of the yields and the shapes of the energy distribution spectra generated by projectiles elastically scattered from smooth-surfaced random solid target materials on the stopping power of the material, on the scattering cross section and on the geometry of the experimental arrangement is computationally investigated. The effects of such secondary phenomena as straggling, angular beam divergence and loss of beam intensity with increasing penetration depth are neglected. General arguments demonstrate that these spectra are quite sensitive to the energy dependence of both the stopping power and the cross section, but to different extents depending on the geometry of the experimental arrangement. If the arrangement is such that the observed projectiles must travel a much larger path length to leave the target material after scattering than they travel to reach the scattering site, then the stopping power plays the dominant role in shaping the spectrum. Specifically, the spectrum tends to follow strongly the shape of the reciprocal of the stopping power versus energy curve. When the arrangement is such that these path lengths have the opposite relationship, then the cross section comes into strong competition with the stopping power in shaping the spectrum. These features are illustrated by the spectra calculated using two model stopping powers and simple power-law cross sections. One model assigns simple power-law energy dependence to the stopping power, while the other model assigns an energy dependence which approaches $\text{E}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ at low energies and E^-1 × ln E at high energies.     

Fraunhofer Diffraction from Apertures Bounded by Regular Fractals

Abstract

Some properties of fields diffracted in the Fraunhofer region by apertures bounded by regular fractals are investigated. A recursion relation describing such apertures is introduced and the associated relation in the Fourier transform domain is described. For a triadic Koch aperture whose edge has the fractal dimension of D_s = 1·262, the recursion relation is numerically evaluated. Self-similar structures of intensity distributions in the Fraunhofer region are verified for the present objects. The relationship of the fractal dimension D _s of the fractal edge with the power-law decay of the Fraunhofer diffraction intensities is also verified.     

Preparation, microstructure and hydrogen sorption properties of nanoporous carbon aerogels under ambient drying

Organic aerogels are prepared by the sol-gel method from polymerization of resorcinol with furfural. These aerogels are further carbonized in nitrogen in order to obtain their corresponding carbon aerogels (CA); a sample which was carbonized at 900?°C was also activated in a carbon dioxide atmosphere at 900?°C. The chemical reaction mechanism and optimum synthesis conditions are investigated by means of Fourier transform infrared spectroscopy and thermoanalyses (thermogravimetric/differential thermal analyses) with a focus on the sol-gel process. The carbon aerogels were investigated with respect to their microstructures, using small angle x-ray scattering (SAXS), transmission electron microscopy (TEM) and nitrogen adsorption measurements at 77?K. SAXS studies showed that micropores with a radius of gyration of <0.35 ± 0.07 to 0.55 ± 0.05?nm were present, and TEM measurements and nitrogen adsorption showed that larger mesopores were also present. Hydrogen storage properties of the CA were also investigated. An activated sample with a Brunauer-Emmett-Teller surface area of 1539 ± 20?m(2)?g(-1) displayed a reasonably high hydrogen uptake at 77?K with a maximum hydrogen sorption of 3.6?wt% at 2.5?MPa. These results suggest that CA are promising candidate hydrogen storage materials.     

Scattering of light by thermal ripplons on superfluid helium

It is well known that thermally excited waves on the free surface of a liquid can cause light to be diffusely scattered. In order to investigate the possible effects of the superfluid transition in liquid helium, we have measured the intensity of the surface scattering from the free surface of liquid helium as a function of temperature (1.3–2.1 K) and as a function of the scattering angle. The surface was illuminated vertically from above with a He-Ne laser. A measuring technique was devised which could not only distinguish among background scattering, scattering from the bulk liquid, and scattering from the surface, but could determine the ratio of surface to bulk scattering also. We have found that our results can be described by the theory of Mandelshtam, which is valid for classical liquids. According to this theory, the surface-scattered intensity is given by the ratio of temperature to surface tension. The angular dependence is determined by the characteristic wave vector dependence of the mean square amplitude of the fluctuations. The ratio between surface- and bulk-scattered intensity is calculated on the basis of the classical theory. The data indicate that in the frequency range around about 1 MHz no influence of the two-fluid nature of the superfluid is detectable.     

Fractal growth of Fe-N thin films prepared by magnetron sputtering at elevated temperature

Fe-N thin films were prepared by direction-current magnetron sputtering with different process parameters. The surface morphology and fractal dimension of the films have been studied using atomic force microscopy and small angle X-ray scattering techniques. The results indicate that the surfaces of thin films exhibit an obviously self-affine fractal characteristic, and the fractal dimension is greatly affected by the N₂/Ar flow rate ratio. When the N₂/Ar flow rate ratio is less than 1/6, the film growth follows the KPZ growth model. Contrarily, the growth mechanism of the film is agreement with the DLCA model.     

Structure and thermal evolution of mullite aerogels

Gels with mullite composition have been prepared from pseudo-boehmite sol and tetramethoxysilane. They are then converted to aerogels using methanol or CO₂ supercritical drying. The structure of the aerogels depends on the nature of the solvent used to perform the hypercritical solvent evacuation. Infrared spectroscopy shows that an esterification reaction occurs during the methanol supercritical drying. The samples prepared by CO₂ supercritical drying retain some isoamylacetate, which is used to exchange solvent. Small angle neutron scattering experiments lead to the conclusion that aerogels prepared with either supercritical drying technique are constituted by primary particles of equal size, and are characterized by the same volume fractal dimension. The thermal evolution of a monolithic methanol evacuated sample leads to cracks free ceramic having an apparent density of 1.1 and which does not evolve when heat treated below 1200 C.     

The synthesis of titanoniobate compound characteristic of various particle morphologies through a novel solvothermal route

Potassium titanoniobate (KTiNbO₅) polycrystalline powders were synthesized by the sol-gel method and a novel preparation route which was described as a combination technique of the sol-gel process and solvothermal treatment. The synthesis processes, the structure and morphology characterizations for the as-prepared KTiNbO₅ particles were investigated by using thermal analysis, X-ray powder diffraction, electron microscopy and other techniques. The KTiNbO₅ particles synthesized in this work showed various morphologies depending on the different reaction processes. The KTiNbO₅ compound obtained through 2-propanol solvothermal treatment exhibited unique particle morphology characteristic of sphere shape with the average size of 300 nm in diameter. In addition, the result that the formation of the spherical particle shape occurred during the solvothermal process was demonstrated.     

Effect of Sol-Gel Processing Parameters on Optical Properties of TMOS Silica Aerogels

To optimize and produce silica aerogels with high direct transmittance and low diffusive values, systematic and detailed experiments were carried out on the effect of sol-gel processing parameters on optical properties of silica aerogels. A series of aerogel samples of different molar ratio combinations was optically examined in the UV–visible–NIR range by a spectrophotometer equipped with an integrating sphere. The overall transmittance of the aerogels in the visible range varied from 75 to 93% depending upon the molar ratio combination. The most relevant parameter being studied was the direct/hemispherical transmittance ratio (). The best value of obtained for an aerogel in the present study was about 93% with a molar ratio of 1 TMOS: 12 MeOH:4 H₂O:3.5 × 10^–3 NH₄OH, respectively. Apart from visible transparencies, solar energetic transparencies of some silica aerogels were also measured and reported. These optical data, together with the porosity measurements, allowed us to improve the process of fabrication of low-diffusing aerogel material. The experimental results are discussed considering the percentage of porosity and heterogeneity generated in pore size distributions due to the variation of sol-gel processing parameters.     

Laser-radiation scattering by cement in the process of hydration: simulation of the dynamics and experiment

This paper discusses simulation of speckle-field dynamics during coherent light scattering by a cement surface in the process of hydration. Cement particles are represented by the spheres whose sizes and reflection indices are changing during the hydration process. The study of intensity fluctuations of scattered coherent radiation is a suitable technique for the analysis of both fast and slow processes of mineral binder hydration and formation of polycrystalline structures in the process of hardening. The results of simulation are in good agreement with the experimental data.     

Comparative studies of the physical and hydrophobic properties of TEOS based silica aerogels using different co-precursors

In the present paper, the experimental results on the physical and hydrophobic properties of tetraethoxysilane (TEOS) based silica aerogels using six different organosilane co-precursors (C.P) of the type R_nSiX_4-n as synthesis components, are reported and discussed. The aerogels have been produced by sol-gel processing followed by supercritical drying using methanol solvent extraction. The molar ratio of TEOS, ethanol (EtOH), water (0.001M oxalic acid (H₂C₂O₄) catalyst) was kept constant at 1:5:7, respectively, and the molar ratio of C.P/TEOS (A) was varied from 0.1 to 0.6 and compared the aerogel properties. The hydrophobicity of the aerogels has been tested by the contact angle measurements. The contact angle (θ) has been found to be the highest (θ =136?) for the trimethylethoxysilane (TMES) co-precursor, while for the other co-precursors it is in between 12?8 and 13?8. The surface chemical modification of the hydrophobic aerogels has been studied using Fourier Transform Infrared Spectroscopy (FTIR). As the C.P/TEOS molar ratio increased, the intensity of the C–H and Si–C peaks in the FTIR spectra increased, clearly indicating the organic modification of the aerogels. The aerogels based on mono-alkyl (CH₃) trialkoxysilane co-precursor have shown higher optical transmission (≈65%) compared to the phenyl, di or tri alkyl alkoxysilanes (5–50%). The trialkyl modified aerogels showed the lowest bulk density (118.3 kg/m³) and volume shrinkage (<2%). The alkyl alkoxy/chloro–silane modified aerogels have been found to be thermally stable up to a maximum temperature of 573 K, whereas the phenyl trialkoxysilane modified aerogels are stable up to a temperature as high as 823 K. The aerogels have been characterized by scanning electron microscopy, thermogravimetric and differential thermal analyses.     

Nanostructured CeO2 thin films: A SAXS study of the interface between grains and pores

The analysis of the interface between grains and pores in thin films of CeO₂, prepared by two different sol-gel procedures, was performed using grazing incidence small-angle X-ray scattering. The existence and the average thickness of an interfacial layer, formed during one of the preparation procedures, were examined by analyzing the depletion of scattering intensity in the Porod's scattering region.     

Characterization of Macromolecular Solutions by a Combined Static and Dynamic Light Scattering Technique

The combination of quasi-elastic light scattering (LS) with integrated scattered intensity measurements in the same sample has been applied to study polymer and polymer–protein aqueous solutions. The molecular weight, the radius of gyration, and the second virial coefficient for thermosensitive polymer [poly(N-isopropylacrylamid)] solutions before and after precipitation transition have been obtained using Zimm plot calculations. The precipitation curve (intensity versus temperature dependence) for polymer solutions has been experimentally obtained using the light scattering setup. For the first time the static and dynamic LS properties of aqueous solutions of antibody–poly(methacrylic acid) and antibody–poly(acrylic acid) conjugates and solutions of their components [antibody, poly(methacrylic acid), and poly(acrylic acid)] at different pH values have been measured. In both cases the parallel comparison of the characteristic size variations allowed us to represent novel structural features of scattered particles (macromolecules, associates, aggregates, conjugates, colloidal particles) in studied systems.     

Light scattering from ferroelectric domains in LiTaO3

When a He-Ne 0.633 μm laser light passes through an as-grown LiTaO₃ single crystal, the Tyndall phenomenon is observed along the beam path. The scattered light intensity distribution was measured in the plane perpendicular to the incident beam. Results were discussed from the viewpoint of very small ferroelectric domains using light scattering theory. It was concluded that light scattering centers in LiTaO₃ are ferroelectric multi-domains of size in order of one wavelength.     

Surface morphology and light scattering properties of plasma etched ZnO:B films grown by LP-MOCVD for silicon thin film solar cells

LP-MOCVD deposited ZnO:B thin films, post-etched by argon plasma processes, were investigated in this study in order to optimise the ZnO:B/p-layer interface when the ZnO:B is used as front electrode of p-i-n a-Si:H solar cells. At varying etching time different surface roughness was obtained and the evolution of the surface morphology was correlated with the texture characteristic and its scattering properties. Atomic force microscopy data were analysed and discussed together with the scattering properties, which are haze parameter and angular resolved scattering (ARS) distribution.The presence of several preferential scattering angles was hypothesized and a deconvolution approach was applied to each angular scattering curve. For each fixed preferential scattering angle θ_i we associated a Gaussian distribution of the scattered light amount related to a well-defined scattering surface. The different preferential scattering angles were correlated to different scattering phenomena, the modifications of the angular scattering curves well agreed with SEM and AFM images.It is well known that a:Si-H solar cells fabricated on MOCVD deposited ZnO:B substrates show poor FF and V_oc values with good J_sc value. We demonstrated that only an effective sharp edge rounding off produced by an appropriately long plasma etching treatment is able to make MOCVD deposited ZnO:B perfectly suitable for high quality a-Si:H based devices.     

气凝胶在气体吸附净化中的应用研究进展

气凝胶(Aerogels)是一种以空气为介质的轻质多孔性凝聚态物质,由胶体粒子或高聚物分子相互聚集构成独特的纳米多孔三维网络结构。气凝胶的颗粒相和孔隙尺寸均为纳米量级,具有相当高的比表面积和孔隙率、可调控的开放孔隙结构、易于化学修饰的表面以及多样化的种类和形态,其气体吸附量可比同等条件下活性炭吸附量高两个数量级,因此在气体吸附净化领域逐渐受到人们的广泛关注。目前,气体吸附净化领域研究较多的气凝胶主要是SiO_2气凝胶和炭气凝胶。此外,近年来对金属氧化物气凝胶以及SiC气凝胶、石墨烯气凝胶、生物质基气凝胶等新型气凝胶的气体吸附应用也有相应的研究报道。吸附材料对目标气体需要同时具有较高的吸附容量和良好的选择性吸附能力。气凝胶的高比表面积和多孔性质提供了众多的吸附位点,但仅依靠自身物理吸附作用的吸附量有限,对目标气体的选择性不高,在实际吸附应用中,往往由于共存气体组分的竞争吸附影响对目标气体的吸附性能。因此,为了进一步提升气凝胶的吸附容量,提高对目标气体的选择性,研究人员围绕气凝胶修饰改性进行了大量的研究探索工作,并取得了一定的进展。目前,气凝胶吸附净化研究报道的目标气体主要是温室气体CO_2和大气中主要的污染物挥发性有机化合物(VOCs)。针对目标气体的不同可分别通过氨基功能化、氮掺杂等方法引入碱性位点或通过引入非极性官能团对气凝胶进行疏水改性,以提升气凝胶对CO_2或VOCs的吸附量和选择性。所采用的修饰改性方式主要有以下两种:一是在湿凝胶形成后或超临界干燥后通过嫁接、浸渍等手段对气凝胶表面进行功能化改性,通过引入特定的官能团或活性组分提升气凝胶对目标气体的吸附量和选择性;另一种是在溶胶-凝胶反应过程中引入功能化前驱体,在分子或纳米尺度上赋予气凝胶网络特定的性能,进而有效平衡活性组分稳定性和对目标气体的吸附性能。此外,对于炭气凝胶,还可通过活化进一步增大比表面积,改善孔隙结构和表面化学性质,从而实现对目标气体污染物吸附性能的优化。本文归纳了各类气凝胶在CO_2与VOCs吸附净化方面的研究进展,介绍了气凝胶的制备过程和结构特点,讨论并对比了不同气凝胶对目标气体的吸附性能与吸附机理,总结了当前气体吸附净化研究中对气凝胶进行修饰改性的主要方法,最后指出提高气凝胶的结构稳定性和吸附速率、设计可同时吸附多种目标气体的气凝胶、缩短制备周期并降低成本是未来研究工作的重点。     

Light scattering from particles located under a rough dielectric layer

A discussion is presented of the effect of roughness on the detectability of subsurface particles by means of the light-scattering method. We have studied the scattering of light by calibrated spheres located under a slightly rough dielectric surface both experimentally and theoretically. In our experiments, the scattering from slightly rough layers with nonresonant particles was dominated by the roughness, and the scattering diagram did not bear any discernible indications of the spheres. However, at resonance, the subsurface particles manifested themselves by an increase in the total scattered intensity and by well-pronounced maxima in the angular dependence of both the scattering diagram and the backscattered intensity. Theoretical calculations show that the angular positions of the maxima in the scattering diagram are essentially determined by the interference of fields scattered by the particles and by the surface, whereas the contribution of the multiple interparticle scattering is negligible. By contrast, the oscillations in the angular dependence of the backscattered intensity are due primarily to the scattering between neighboring spheres.     

Comparative studies of the physical and hydrophobic properties of TEOS based silica aerogels using different co-precursors

In the present paper, the experimental results on the physical and hydrophobic properties of tetraethoxysilane (TEOS) based silica aerogels using six different organosilane co-precursors (C.P) of the type R_nSiX_4-n as synthesis components, are reported and discussed. The aerogels have been produced by sol-gel processing followed by supercritical drying using methanol solvent extraction. The molar ratio of TEOS, ethanol (EtOH), water (0.001M oxalic acid (H₂C₂O₄) catalyst) was kept constant at 1:5:7, respectively, and the molar ratio of C.P/TEOS (A) was varied from 0.1 to 0.6 and compared the aerogel properties. The hydrophobicity of the aerogels has been tested by the contact angle measurements. The contact angle (θ) has been found to be the highest (θ=136°) for the trimethylethoxysilane (TMES) co-precursor, while for the other co-precursors it is in between 120° and 130°. The surface chemical modification of the hydrophobic aerogels has been studied using Fourier Transform Infrared Spectroscopy (FTIR). As the C.P/TEOS molar ratio increased, the intensity of the C–H and Si–C peaks in the FTIR spectra increased, clearly indicating the organic modification of the aerogels. The aerogels based on mono-alkyl (CH₃) trialkoxysilane co-precursor have shown higher optical transmission (≈65%) compared to the phenyl, di or tri alkyl alkoxysilanes (5–50%). The trialkyl modified aerogels showed the lowest bulk density (118.3 kg/m³) and volume shrinkage (<2%). The alkyl alkoxy/chloro–silane modified aerogels have been found to be thermally stable up to a maximum temperature of 573 K, whereas the phenyl trialkoxysilane modified aerogels are stable up to a temperature as high as 823 K. The aerogels have been characterized by scanning electron microscopy, thermogravimetric and differential thermal analyses.