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排序方式: 共有1331条查询结果,搜索用时 15 毫秒
131.
Shunzhi Wang William Morris Yangyang Liu C. Michael McGuirk Yu Zhou Prof. Joseph T. Hupp Prof. Omar K. Farha Prof. Chad A. Mirkin 《Angewandte Chemie (International ed. in English)》2015,54(49):14738-14742
A method for modifying the external surfaces of a series of nanoscale metal–organic frameworks (MOFs) with 1,2‐dioleoyl‐sn‐glycero‐3‐phosphate (DOPA) is presented. A series of zirconium‐based nanoMOFs of the same topology (UiO‐66, UiO‐67, and BUT‐30) were synthesized, isolated as aggregates, and then conjugated with DOPA to create stably dispersed colloids. BET surface area analysis revealed that these structures maintain their porosity after surface functionalization, providing evidence that DOPA functionalization only occurs on the external surface. Additionally, dye‐labeled ligand loading studies revealed that the density of DOPA on the surface of the nanoscale MOF correlates to the density of metal nodes on the surface of each MOF. Importantly, the surface modification strategy described will allow for the general and divergent synthesis and study of a wide variety of nanoscale MOFs as stable colloidal materials. 相似文献
132.
133.
A parametric investigation of the friction performance of PC‐ABS parts processed by FDM additive manufacturing process
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The friction performance is an important factor of parts processed by fused deposition modeling (FDM) for various engineering applications. It is one type of failure made of surface contact. The proper use of FDM process parameters can bring a significant reduction in friction and the amount of wear, thereby leading to a reduction in the material waste. To date, very little studies have been performed in this area. This paper investigates the effect of FDM manufacturing parameters on the friction performance of polycarbonate‐acrylonitrile butadiene styrene prototypes processed by FDM using definitive screening design and partial least squares method. The observation of surface morphology was obtained by the scanning electron microscopy to examine the effect of process parameters on the microstructure. The experimental results have shown that layer thickness, air gap, raster angle, and build orientation are the most influential factors affecting the friction performance of FDM manufactured parts. The proposed approach presented in this study provides an impetus to develop analytical modeling and functional relationships between FDM manufacturing parameters and friction performance. Copyright © 2017 John Wiley & Sons, Ltd. 相似文献
134.
Omar M. Sidebottom 《Experimental Mechanics》1978,18(4):121-126
Based on the assumption that each material satisfies the condition for isotropic hardening for a von Mises material, an incremental solution is developed to predict axial-strain creep curves and maximum shearing-strain creep and relaxation curves for solid circular torsion-tension members subjected to proportionate and nonproportionate stepped loading including creep in tension and relaxation in torsion. Test data are obtained from torsion-tension members made either of annealed OFHC copper at 800°F (427°C) or hot-rolled SAE 1045 steel at 950°F (510°C). The loading histories include either four stepped proportionate load changes, four stepped nonproportionate load changes, or torsion-tension loading in which the axial load remains constant and the torsional load is allowed to relax during two loading periods of the test. Each test duration is about 100 h. Good agreement is indicated between the predicted and measured creep and relaxation curves. 相似文献
135.
Michael addition reactions of aldehyde to β-nitrostyrene catalyzed by L-proline were investigated by using controlled, monomode microwave-assisted technique in a closed vessel system. Ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([bmim]NTf2) was used as the reaction medium to replace the commonly used volatile organic solvents and as a good absorbing solvent during Michael reaction under the influence of microwave irradiation. The Michael product is clean and generates good yields in short reaction times with moderate results on enantioselectivity (ee). In this work, optimization of proline-catalyzed Michael reaction was carried out using response surface methodology (RSM) based on a three-factor-three-level central composite design (CCD). Various reaction parameters including catalyst loading (5–30 mol%), reaction time (5–40 min), and substrate (2–5 equivalent ratio) were investigated. A high Michael yield (96.5%) with 36.9 ee% was obtained at the optimum conditions of 10.0 mol% catalyst loading, 5.0 min reaction time, and 2.0 substrate equivalent ratio.
[Supplementary materials are available for this article. Go to the publisher's online edition of Synthetic Communications® for the following free supplemental resource(s): Full experimental and spectral details.] 相似文献
136.
Eduardo Ordó?ez-Regil Genoveva García-Rosales Omar Eduardo Gutiérrez-Mu?iz 《Journal of Radioanalytical and Nuclear Chemistry》2011,289(1):197-201
Seawater contains trace elements in solution, such as lanthanides and uranium, which can act as natural tracers when they
flocculate to form the sediments of the seabed. Uranium is a special case because it is a radioactive element with isotopic
ratio close to 1.14 (234U/238U) in sea water. Uranium is also present in all fractions of sediments, although the isotopic ratio in each fraction takes
a specific value depending on its origin and surrounding environment. This work has been conducted in order to analyze lanthanides
and uranium in a core of sea sediments. It is focused on separating the fractions forming the layers along the depth profile
of the core sample so as to determine the 234U/238U activity ratios in fractions of organic matter, aquatic life skeletons and dissolved minerals. The Neutron Activation Analysis
showed the presence of U, Lu, Hf, Eu, Ce and Yb ranging up to 50 ppm while alpha spectrometry gave a series of 234U/238U activity ratios which are discussed on the basis of statistical analysis. 相似文献
137.
Ludmila A. Ramos Daniella L. Morgado Omar A. El Seoud Valdinéia C. da Silva Elisabete Frollini 《Cellulose (London, England)》2011,18(2):385-392
The acylation of three cellulose samples by acetic anhydride, Ac2O, in the solvent system LiCl/N,N-dimethylacetamide, DMAc (4 h, 110 °C), has been revisited in order to investigate the dependence of the reaction efficiency
on the structural characteristics of cellulose, and its aggregation in solution. The cellulose samples employed included microcrystalline,
MCC; mercerized cotton linters, M-cotton, and mercerized sisal, M-sisal. The reaction efficiency expresses the relationship
between the degree of substitution, DS, of the ester obtained, and the molar ratio Ac2O/AGU (anhydroglucose unit of the biopolymer); 100% efficiency means obtaining DS = 3 at Ac2O/AGU = 3. For all celluloses, the dependence of DS on Ac2O/AGU is described by an exponential decay equation: DS = DSo − Ae−[(Ac2O/AGU)/B]; (A) and (B) are regression coefficients, and DSo is the calculated maximum degree of substitution, achieved under the conditions of each experiment. Values of (B) are clearly
dependent on the cellulose employed: B(M-cotton) > B(M-sisal) > B(MCC); they correlate qualitatively with the degree of polymerization of cellulose, and linearly with the aggregation number, Nagg, of the dissolved biopolymer, as calculated from static light scattering measurements: (B) = 1.709 + 0.034 Nagg. To our knowledge, this is the first report on the latter correlation; it shows the importance of the physical state of dissolved
cellulose, and serves to explain, in part, the need to use distinct reaction conditions for MCC and fibrous celluloses, in particular Ac2O/AGU, time, temperature. 相似文献
138.
Ethyl 7‐amino‐3‐(3‐methyl‐5‐oxo‐1‐phenyl‐2‐pyrazolin‐4‐yl)‐5‐aryl‐5H‐thiazolo[3,2‐a]pyrimidine‐6‐carboxylate was hydrolyzed with an ethanolic sodium hydroxide and the sodium salt thus formed underwent cyclization with acetic anhydride to afford 2‐methyl‐7‐(3‐methyl‐5‐oxo‐1‐phenyl‐2‐pyrazolin‐4‐yl)‐5‐arylthiazolo[3,2‐a]pyrimido[4,5‐d]oxazin‐4(5H)‐one. This compound was transformed to related heterocyclic systems via its reaction with various reagents. The biological activity of the prepared compounds was tested against Gram positive and Gram negative bacteria as well as yeast‐like and filamentous fungi. They revealed in some cases excellent biocidal properties. 相似文献
139.
Dey GR El Omar AK Jacob JA Mostafavi M Belloni J 《The journal of physical chemistry. A》2011,115(4):383-391
The detailed kinetics of the multistep mechanism of the Au(III) ion reduction into gold clusters have been investigated by radiation chemistry methods in 2-propanol. In particular, a discussion on the steady state radiolysis dose-dependence of the yields concludes to a comproportionation reaction of nascent gold atoms Au(0) with excess Au(III) ions into Au(II) and Au(I). This reaction should be achieved through Au(III) consumption before the coalescence of atoms Au(0) into gold clusters may occur. Then gold clusters catalyze the reduction of Au(I) by 2-propanol. It was also found that a long-lived Au(II) dimer, (Au(II))(2), was transiently formed according to the quantitative analysis of time-resolved absorbance signals obtained by pulse radiolysis. Then the disproportionation of Au(II) is intramolecular in the dimer instead of intermolecular, as usually reported. The yields, reaction rate constants, time-resolved spectra, and molar extinction coefficients are reported for the successive one-electron reduction steps, involving especially the transient species, such as Au(II), (Au(II))(2), and Au(I). The processes are discussed in comparison with other solvents and other metal ions. 相似文献
140.
Guo S Fabian O Chang YL Chen JT Lackowski WM Barbara PF 《Journal of the American Chemical Society》2011,133(31):11994-12000
A new phenomenon is presented in which electrogenerated chemiluminescence (ECL) is generated and propagates laterally as self-reinforcing waves as a result of the oxidation of a poly(9,9-dioctylfluorene-co-benzothiadiazole) thin film. In an ordered array of Au electrode posts that act as effective ECL nucleation sites, soliton-like waves were observed to expand from each site and annihilate upon collision with each other. Simulations of the ECL response supported the experimental observations that the ECL waves propagate at a constant speed. A correlated diffusion mechanism involving the correlated motion of ions, injected holes, and solvent molecules is proposed to interpret the experimental data qualitatively. A rapid increase in the diffusion coefficient of these species in the polymer results in a sharp interface between non-oxidized and oxidized polymer phases wherein the electrochemical (EC) oxidation and mass transport of all pertinent species take place. EC oxidation of conjugated polymers of this type has important implications for the understanding of these materials and their modes of operation in EC conjugated polymer devices. 相似文献