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11.
CW and Pulsed EPR Study of Silver Nanoparticles in a SiO2 Matrix 总被引:1,自引:0,他引:1
G. Mitrikas Y. Deligiannakis C.C. Trapalis N. Boukos G. Kordas 《Journal of Sol-Gel Science and Technology》1998,13(1-3):503-508
Metallic silver nanoparticles were prepared by the sol-gel method in an SiO2 matrix. The process includes complexation of silicon alkoxides with metal salts, hydrolysis, polycondensation, formation of powder, and subsequent thermal treatment first in oxidizing and second in reducing atmospheres. The sizes of metallic particles were determined both by X-ray diffraction and transmission electron microscopy. These measurements revealed sizes of metallic particles between 1 and 20 nm, depending upon processing conditions. The magnetic properties were investigated using electron paramagnetic resonance spectroscopy in the temperature range between 4 and 300 K. The spin-lattice relaxation time T1 was measured by pulsed EPR. The temperature dependence of T1 is described by the relation 1/T1 Tn, where 0.4 < n < 1. This behavior is unusual and different from any well-known relaxation processes such as Raman, Direct or the Orbach-Aminov. 相似文献
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Calcium hydroxyapatite powder was prepared by sol-gel method using calcium acetate and PO(OC2H5)3 as initial compounds, and alcohol (methyl, ethyl, and propyl-alcohol) as solvent. Homogeneous solution and gels were prepared
using a molar ratio of Ca/P=1.67. The evolution of the structure was detected by X-ray diffraction, IR, and FT-n(=1,2,3)D-EPR spectroscopy. The dried gels exhibit a signal characterized by a central line and two satellites. The 2D spectrum
(ESEEM vs. field sweep) showed the same modulation for the central line. The FT-EPR spectrum vs. field sweep 2D-spectrum indicated
that the satellites are due to an hfs splitting with water. The central region of this 2D spectrum is influenced by P and
H in a concentration ratio of [H]/[P]=2.5. The ESEEM spectrum was simulated assuming the equation Vmod=Vmod(P)mVmod(H)n for two spin systems S=1/2 and I=1/2. This simulation gave form andn the values of 2 and 5, respectively. This finding suggests the structure:
for the unpaired state. It appears that one ethyl group does not hydrolyse in the gelation process. The ESEEM spectra of
hydroxyapatite exhibit a modulation generated by P, H and Ca atoms. 相似文献
13.
Vassiliki Papadimitriou Aristotelis Xenakis Athanasios E. Evangelopoulos 《Colloids and surfaces. B, Biointerfaces》1993,1(5)
The catalytic behaviors of α-chymotrypsin and of trypsin were studied in anionic AOT-isooctane-water and cationic CTAB-ROH-isooctane-water microemulsion systems. The effects of various parameters, such as the pH and the water content expressed in terms of the molar ratio wo = [H2O]/[Surfactant], on the enzyme activity, were examined. The kinetic constants were calculated and it was found that in the case of trypsin the enzyme exhibited a remarkable “superactivityrd, when studied in the CTAB microemulsion systems. The effect of the alcohol cosurfactant used in these cationic systems was investigated in relation to the polarity of the reaction medium. By using the hydrophilic probe 1-methyl-8-oxyquinolinium betaine the micropolarity of the water core was determined and related to the kinetic results. 相似文献
14.
Aristotelis E. Charalampakis 《Journal of Global Optimization》2012,54(3):449-483
We examine the concept of storing all evaluated chromosomes and directly reuse them in Genetic Algorithms (GAs). This is achieved by a fully encapsulated operator, called Registrar, which is effortlessly placed between the GA and the objective function. The Registrar does not approximate the objective function. Instead, it replaces the chromosomes requested by the GA with similar ones taken from the registry, bypassing the function evaluation. Unlike other methods that use external memory to increase genetic diversity, our simple implementation encourages revisits in order to avoid evaluations in an aggressive manner. Significant increase in performance is observed which is present even at the early stages of evolution, in accordance with the Birthday Problem of probability theory. Implementation with Standard GA shows great promise, while the encapsulation of the code facilitates implementation with other Evolutionary Algorithms. 相似文献
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A. Athanasiou A. Mitsionis G. Skouras N. Todorova C. Trapalis T. Vaimakis 《Journal of Thermal Analysis and Calorimetry》2014,116(1):15-25
Mesoporous anatase TiO2 materials with specific surface areas between 70 and 110 m2 g?1 were prepared via sol–gel technique using surfactants oleic acid and Triton-X (TX), in the presence or absence of diethanolamine, in methanol. Surfactants like TX or oleic acid (OA), as well as a gelating and chelate agent like diethanolamine (DEA) are commonly used in TiO2 formation from a titanium isopropoxide solution. Thermogravimetric methods were applied in order to evaluate the effect of the addition of such molecules in a precursor suspension before TiO2 materials preparation. The in situ investigation of such systems showed that numerous interactions occur between large molecules such as TX and OA that attributed on both steric effects and hydrogen bond formation. Materials prepared through modified sol–gel technique seem to be stabilized through DEA addition in the precursor suspension. 相似文献
16.
Avramiotis S Papadimitriou V Hatzara E Bekiari V Lianos P Xenakis A 《Langmuir : the ACS journal of surfaces and colloids》2007,23(8):4438-4447
Organogels were obtained by adding small amounts of water to a solution of lecithin in organic solvents. Either isooctane or isopropyl palmitate and isopropyl myristate were used as the continuous organic phase of the gels. EPR spectroscopy using both DSA membrane-sensitive and lipophilic spin probes was applied to define the dynamic structure of the surfactant monolayer and the continuous oil phase of lecithin organogels. It was found that by increasing the water quantity, an increase of the polar head area per lecithin molecule was induced, and as a consequence the total interface expanded. It was found that the use of esters as organic solvents induced a decrease of the size of the dispersed structures. The interconnection of the aqueous microdomains and their dynamics were monitored by both static and time-resolved fluorescence quenching spectroscopy using Ru(bipy)32+ as fluorophore and Fe(CN)63- as quencher. It was found that the rates of inter- and/or intra-micellar exchange of water molecules were very slow because they appeared quite immobilized close to the lecithin polar heads. According to the results of the dynamic studies, appropriate organogels were formulated and used to incorporate model bioactive compounds with medicinal or cosmetic interest such as caffeine and theophylline. When these systems were tested for trans-membrane diffusion, they showed a 24 h permeation of 20% and 35%, respectively. 相似文献
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Georgakilas V Bourlinos A Gournis D Tsoufis T Trapalis C Mateo-Alonso A Prato M 《Journal of the American Chemical Society》2008,130(27):8733-8740
We show that covalent functionalization of carbon nanotubes (CNTs) via 1,3-dipolar cycloaddition is a powerful method for enhancing the ability to process CNTs and facilitating the preparation of hybrid composites, which is achieved solely by mixing. CNTs were functionalized with phenol groups, providing stable dispersions in a range of polar solvents, including water. Additionally, the functionalized CNTs could easily be combined with polymers and layered aluminosilicate clay minerals to give homogeneous, coherent, transparent CNT thin films and gels. 相似文献
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Multilayered TiO2(Fe3+, PEG) films were deposited on glass and SiO2/glass substrates by sol-gel dipping method. The influence of Fe3+ and PEG(polyethylene glycol) concentrations, the number of layers, the thermal treatment time and the temperature on the optical and microstructural properties of the TiO2 films were studied.As-deposited TiO2(Fe3+, PEG) films were very porous, but after the thermal treatment at 500 °C, the PEG decomposed and burned out to porosity decreasing. Homogeneous nanostructured films were obtained, where the amorphous and the anatase phases coexist. XRD analysis showed that no rutile phase is observed in the films deposited on SiO2/glass as compared with those deposited directly on glass and that the presence of the anatase phase in the films without PEG is more evident in the three-layers film. The XRD intensity of the main peak of anatase from 25° decreases with the increase of PEG concentration.The optical gap of the TiO2(Fe3+, PEG) films is found in 2.52-2.56 eV range and does not essentially depend on the PEG content. 相似文献