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1.
Synthesis of silver nanoparticles in silica aerogel matrix by sub-critical drying technique is reported in the present article. Physical characterization of silver/silica aerogel nanocomposites with 1, 5 and 25 wt% of silver has been discussed. Physico-optical properties of the composites have been evaluated as a function of the silver particle size crystallized within the silica matrix. The maximum size of the silver grains that could be accommodated in the amorphous matrix was observed to be 25 nm. Silver particles of diameter larger than 25 nm were found segregated out of the silica matrix; such silver particles were single crystalline with dendritic morphology. Optical absorption analysis confirmed the presence of both the oligomeric and nanometallic silver in the samples heat-treated up to 500 °C. With decreasing silver particle size, the surface plasmon resonance was found first to shift towards blue followed then by a red shift. The blue shift is attributed to the chemisorption occurring between the metallic core and the silica matrix. With larger grain size of 25-80 nm in 25 wt% Ag-silica aerogel sample, the diffuseness of electron cloud outside the potential well was observed to cause a red shift in the surface plasmon resonance.  相似文献   

2.
《Journal of Non》2003,315(1-2):197-205
Epoxy-based inorganic–organic hybrid polymers, for use as a matrix in coatings, have been prepared from 3-glycidoxypropyltrimethoxysilane by a sol–gel process. The precursor molecule possesses both epoxy and silicon alkoxide functionality and so interlinked inorganic–organic networks can be formed. Diethylenetriamine was used to open the epoxy rings and form the organic network to an extent determined by the initial ratio of amine to epoxy groups. The materials were cured either at room temperature or with an additional heat treatment at 150 °C. Structural characterisation of the cured hybrid materials was performed using a combination of Raman, and 29Si and 13C MAS NMR spectroscopies. These show that the formation of the two networks does not occur independently and the rate or extent of organic cross-linking has a direct effect on the extent of the inorganic network formation, and vice-versa.  相似文献   

3.
This work demonstrates the synthesis of hydrophilic and hydrophobic high surface area silica aerogel beads with a large pore volume. Wet gel silica beads were modified and heat-treated under atmospheric pressure after modification of the surface by trimethychlorosilane (TMCS). The effects of heat treatment on the physical (hydrophobicity) and textural properties (specific surface area, pore volume, and pore size) of silica aerogel beads were investigated. The results indicated that hydrophobicity of the silica aerogel beads can be maintained up to 400 °C. The hydrophobicity of the silica aerogel beads decreased with increasing temperature in the range of 200-500 °C, and the beads became completely hydrophilic after heat treatment at 500 °C. The specific surface area, cumulative pore volume, and pore size of the silica aerogel beads increased with increasing temperature. Heating the TMCS modified bead gel at 400 °C for 1 h resulted in silica aerogel beads with high surface area (769 m2/g), and large cumulative pore volume (3.10 cm3/g). The effects of heat treatment on the physical and textural properties of silica aerogel beads were investigated by field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), thermogravimetric and differential analysis (TG-DTA), Fourier-transform infrared spectroscopy (FT-IR), and Brunauer, Emmett and Teller (BET) and BJH nitrogen gas adsorption and desorption methods.  相似文献   

4.
T.Y. Ng  J.J. Yeo  Z.S. Liu 《Journal of Non》2012,358(11):1350-1355
In this study, classical molecular dynamics with the well-known van Beest, Kramer and van Santen potential are used for the first time to investigate the solid thermal conductivity of silica aerogel. Aerogel samples at various densities are obtained through negative pressure rupturing of dense silica samples, and reverse non-equilibrium molecular dynamics is employed to determine the thermal conductivity at each density. Results indicate that a power-law fit of the thermal conductivity obtained varies almost linearly with density, where decreasing density and increasing porosity led to an almost linear decrease in thermal conductivity. This is reflective of the trend observed in experimental bulk sintered silica aerogel. The results also showed that the thermal conductivity is of the same order of magnitude as bulk sintered aerogel. The power-law fit of the results also accurately reflected the variation found in bulk sintered aerogel.  相似文献   

5.
Epoxy/polyaniline nanocomposites have been prepared by in situ polymerization of aniline carried out in hydrophilic epoxy networks based on polyoxyethylene (POE) swollen in aqueous solutions of aniline hydrochloride. Formation of polyaniline (PANI) nanoparticles in water-rich phase of swollen networks has been revealed by transmission electron microscopy and infrared spectroscopy. The nanoparticles are dispersed in elastomeric matrix, suppress effectively crystallization of POE and increase glass transition temperature of the matrix.  相似文献   

6.
The multi-scale, temperature related structural characteristics of aerogel-based composite insulation materials that contain nano-scale aerogels, as well as micron-scale fibers, opacifiers and cracks were evaluated using various instruments for 300 to 1423 K. The structural tests were used to develop a three-dimensional multi-scale microstructure model to investigate the effects of non-ideal structures and high temperatures on the insulation performance of aeogel-based composite materials. The predictions agree well with experimental with literature. The model, thus provides theoretic guidelines for manufacturing with optimal parameters, such as the type, loading, and distribution of fibers and opacifiers. The structural analysis shows that the insulation performance of the composite materials degrades significantly with the deformation of the pore structure at high temperatures conditions.  相似文献   

7.
The experimental results on silica aerogels with super hydrophobic property are reported. Silica alcogels were prepared via a two-step acid/base process by keeping the molar ratio of tetraethyoxysiliane (TEOS), ethanol (EtOH), water (H2O), hydrochloric acid (HCl) and ammonia (NH4OH) constant at 1:6:8:1.0 × 10?3:1.1 × 10?2, respectively, and varying the molar ratio of N,N-dimethylformamide (DMF)/TEOS (G) from 0 to 1.2. After two aging treatment steps, they were modified by isopropyl alcohol (IPA)/trimethylchlorosilane (TMCS)/n-hexane solution at 60 °C. It was found that G value at 0.8 resulted in low density (~0.2 g cm?3) and the minimum volume shrinkage (~6%), with the total water adsorption ratio ~5.1% when exposed to water for 3 months and the contact angle θ  178°. Besides, the aerogels (G = 0.8) had higher volume fractal dimension (~1.8), which indicted that it possessed better connectivity and more uniform particle sizes.  相似文献   

8.
Nickel-alumina/silica thin film materials for the use in solar thermal absorbers have been investigated using Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and Elastic Recoil Detection Analysis (ERDA). The TEM images revealed that all layers have a very small thickness variation and that the layers are completely homogenous. High resolution images showed 5-10 nm (poly) crystalline nickel nano-particles. ERDA showed that both the silica and alumina compositions contain more oxygen than 2:1 and 3:2 respectively. SEM showed the surface morphology and characteristics of the top silica anti-reflection layer. Hybrid-silica has showed to generate a smoother surface with less cracking compared to pure silica. The final curing temperature revealed to be of importance for the formation of cracks and the surface morphology.  相似文献   

9.
Carbon black, multi-walls carbon nanotube (CNT) and vapor grown carbon nano-fiber with different contents were added to the carbon aerogels (CAs) electrodes as conductive fillers to improve their capacitive properties. The results show that maximum capacitance exists when the content of the conductive filler gets to its percolation threshold. CNT is the most ideal conductive filler. The CA with 1 wt% content of CNT has the best electrochemical performance; its specific capacitances are 141.3 F g?1 at 5 mV s?1 and 127.1 F g?1 at 100 mV s?1, 1.4 times and 2.2 times as high as that of CA electrode, respectively.  相似文献   

10.
Gun-Soo Kim  Sang-Hoon Hyun   《Journal of Non》2003,320(1-3):125-132
An ambient drying process (1 atm, 270 °C) has been developed in order to synthesize window glazing coated with silica aerogel films. The aerogel film could be manufactured by this process of wet gel films obtained via a dip-/spin-coating of the silica sol on a glass slide. Before drying, the isoproponol solvent in wet gels was exchanged with n-heptane to minimize the drying shrinkage. The thickness, refractive index, and porosity of silica films were 0.16–10 μm, 1.08–1.09, and 80–84%, respectively. The transmittance of window glazing was over 90% and we could predict that the optimal thermal conductivity (0.2 W/(m K)) of the window glazing would be obtained at the aerogel thickness of 100 μm (0.016 W/(m K)).  相似文献   

11.
Nickel-alumina composite aerogels with large nickel loading were synthesized using a novel complexing agent-assisted sol-gel procedure to obtain materials displaying very large and open internal surface area, easily accessible for large molecules and demonstrating excellent catalytic properties. They were tested in the following reactions: (i) hydrogenation of propene and 1-butene, (ii) methanization of carbon dioxide, (iii) isometization of 1-butene, (iv) dehydrogenation of cyclohexane, (v) conversion of equimolar mixture of CO and NO to CO2 and N2. Full conversions and 100% selectivities were obtained in reactions (i), (ii), (iv) and (v) and an equilibrium composition in reaction (iii). These catalysts showed no pyrophoric activity on exposing to air even at Ni content as large as 50 wt%.  相似文献   

12.
The Young’s moduli of a set of silica aerogels have been measured by two techniques: 3-point bending and uniaxial compression. The data found by the two methods differ strongly. The uniaxial compression test gives generally underestimated values of Young’s modulus, because of geometrical effects. The appropriate gauge lengths were estimated based on the discussion of Euler buckling and nonuniform stress distribution. The measured compressive moduli were analyzed to correct for machine compliance and possible misalignment under compression of the aerogels. Similarly, moduli obtained by 3-point bending depend on the length/thickness ratio of the sample, reaching equilibrium only for ratios above about 10. The corrected compressive moduli were comparable to those measured by 3-point bending on samples of sufficient length.  相似文献   

13.
D. Carta  G. Navarra 《Journal of Non》2011,357(14):2600-2603
Structural information on a MnFe2O4-SiO2 nanocomposite aerogel and on the pure silica aerogel matrix were obtained by total X-ray scattering experiments. The total pair distribution function of the silica aerogel is in agreement with literature data on melt-quenched silica. The total pair distribution function of the nanocomposite contains the contribution of all the pair correlations of the atomic species making the interpretation more difficult. The difference curve obtained by subtracting the total pair distribution function of the matrix from that of the nanocomposite, allows to selectively study the structural environment of the nanoparticles.  相似文献   

14.
Surfactant-templated mesoporous silica aerogels (or nanofoams) with their entire skeletal framework nanoencapsulated conformally by a thin polyurea layer are emerging as materials with high specific strength and high energy absorption. In this paper a modified split Hopkinson pressure bar was used to investigate their mechanical behavior under dynamic compression at high strain rates. The evolution of the mesoporous structure under such dynamic impact conditions was simulated using the Material Point Method (MPM). The material point model was generated from X-ray micro-computed tomography whereas each voxel was converted to a material point corresponding to the local skeletal density of the material. Simulation results agree well with the experimental data, indicating that the MPM can effectively model the compression of complex mesoporous structures. Simulations indicate a nearly uniform deformation at all three stages of compression: the elastic region, compaction and the final densification due to the low ratio of pore size to wall thickness and random distribution of the pores. Simulations have also indentified the function of the conformal polymer coating as a reinforcing factor, showing that different porosities, obtained by varying the skeletal wall thickness, affect the local stress distribution. Eventually, simulations confirm that the stress-strain behavior of aerogels under compression follows a power-law relationship with the initial bulk density, consistent with experimental results.  相似文献   

15.
Heat treating neutron-irradiated vitreous silica erases the effects of irradiation on the thermal conductivity at temperatures below 1 K. This indicates that the density of low-energy, localized excitations in vitreous silica is restored to its pre-irradiation value.  相似文献   

16.
Low-density monolithic silica and hexylene-bridged polysilsesquioxane aerogels were chemical vapor deposition (CVD) treated with hexamethyldisilazane or hexachlorodisilane silylating agents producing TMS (trimethylsilane) or Si layers on the aerogel. Reinforcing the weak aerogels by controlled deposition process improved their compressive strength and preserved their properties characteristic of low-density aerogels. When the silica and hexylene-bridged polysilsesquioxane aerogels were CVD treated with hexamethyldisilazane, the compressive modulus more than doubled in some cases. However, when treating hexylene-bridged aerogels with hexachlorodisilane the compressive modulus increased six fold. Not only did CVD treatment of the aerogels improve the compressive modulus, but the low densities, high surface areas, high porosities, and the transparency of the aerogels were not significantly affected.  相似文献   

17.
Abstract

The electrical properties of L285 epoxy composite materials (CMs) with a mixed filler including graphite nanoplatelets (GNP)/barium titanate(BT) at various GNP concentrations and at 35?wt.% of BT are measured using the AC impedance spectroscopy with frequencies ranging from 1?kHz to 2?MHz. The complex permittivity and its frequency dependence increase with the GNP content. The addition of a BT filler in GNP/L285?CM increases the percolation threshold in addition to a slight increase in the permittivity, allowing a higher GNP content in the composite without the formation of a conductive network. The AC conductivity increases and its dependence on the frequency decreases, as the GNP content increases.  相似文献   

18.
The silica aerogels were synthesized by sol–gel method via ambient pressure drying. Tetraethyl orthosilicate (TEOS) was used as a main silica source, methyltriethoxysilane (MTES) as a co-precursor silica source and (3-Glycidoxypropyl)trimethoxysilane (GPTMS) as a silane coupling agent. The silica aerogels obtained were further undergoing cross-linking epoxy from GPTMS with amine from diethylenetriamine (DETA) which played a dual role of base catalyst and reagent. The cumulative volumes for open pores of the cross-linked aerogels were evaluated to be 1.4 cm3/g. The Young's modulus and maximum compression strength were 25.4 MPa and 6.17 MPa, respectively. The addition of MTES accelerated the solvent exchange of alcohol within the pores with n-hexane and reduced the shrinkage of aerogels network during the ambient pressure drying. The formation of organic network enhanced the strength of the cross-linked aerogels to prevent the crack generation and the subsequent failure of the monolith during the ambient drying, therefore, protected the nanoporous structure of aerogels.  相似文献   

19.
Spherical mixed silica/titania particles are prepared from silica/titania sol by an ultrasonic vibrator. The titania content of the samples varies from pure silica to a titania mole fraction of 0.31. Narrow size distributions with most frequent particle diameter of about 1 μm are obtained. Specific surface area and pore volume, mean pore radius as well as the meso pore size distribution are influenced by titania content. The spheres are composed of both silica and titania homogeneous distributed as it is indicated by scanning electron microscopy. X-ray diffraction studies show that the particles must be considered amorphous. Structural modifications of the framework are detected. Samples with varying refractive index may be prepared.  相似文献   

20.
Silica-based sol–gel glasses activated by Er3+ ions are attractive materials for integrated optics (IO) devices such as frequency upconverters and optical amplifiers. Monolithic erbium-activated silica xerogels with erbium content ranging from 0 up to 40 000 ppm were prepared by the sol–gel technique. Samples were densified by thermal treatment in air at 950°C for 120 h. The densification degree and the relative content of hydroxyl groups were studied by Raman spectroscopy. Refractive indices were measured at 632.8 and 543.5 nm by a prism coupling technique. Green to blue and violet upconversion luminescence upon continuous-wave excitation at 514.5 nm was observed for all samples. Emission at 1.5 μm, characteristic of the 4I13/24I15/2 transition of Er3+ ions, was observed at room temperature for all samples upon continuous-wave excitation at 980 nm. For the 5000 Er/Si ppm-doped xerogel, a photoluminescence was observed and a lifetime of 8 ms for the metastable 4I13/2 level was measured.  相似文献   

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