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排序方式: 共有76条查询结果,搜索用时 609 毫秒
1.
Justina Grabowska Karuna Kar Nanda R.T. Rajendra Kumar J.P. Mosnier M.O. Henry Simon B. Newcomb Patrick McNally Lisa OReilly Xu Lu Enda McGlynn 《Superlattices and Microstructures》2007,42(1-6):327
Self-organized ZnAl2O4 nanostructures with the appearance (in SEM) of high aspect ratio horizontal nanowires are grown on uncatalysed c-sapphire by vapour phase transport. The nanostructures grow as three equivalent crystallographic variants on c-sapphire. Raman and cathodoluminescence spectroscopy confirm that the nanostructures are not ZnO and TEM shows that they are the cubic spinel, zinc aluminate, ZnAl2O4, formed by the reaction of Zn and O with the sapphire substrate. 相似文献
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Sanford TW Allshouse GO Marder BM Nash TJ Mock RC Spielman RB Seamen JF McGurn JS Jobe D Gilliland TL Vargas M Struve KW Stygar WA Douglas MR Matzen MK Hammer JH De Groot JS Eddleman JL Peterson DL Mosher D Whitney KG Thornhill JW Pulsifer PE Apruzese JP Maron Y 《Physical review letters》1996,77(25):5063-5066
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Measurement of resistant starch by enzymatic digestion in starch and selected plant materials: collaborative study 总被引:11,自引:0,他引:11
Interlaboratory performance statistics was determined for a method developed to measure the resistant starch (RS) content of selected plant food products and a range of commercial starch samples. Food materials examined contained RS (cooked kidney beans, green banana, and corn flakes) and commercial starches, most of which naturally contain, or were processed to yield, elevated RS levels. The method evaluated was optimized to yield RS values in agreement with those reported for in vivo studies. Thirty-seven laboratories tested 8 pairs of blind duplicate starch or plant material samples with RS values between 0.6 (regular maize starch) and 64% (fresh weight basis). For matrixes excluding regular maize starch, repeatability relative standard deviation (RSDr) values ranged from 1.97 to 4.2%, and reproducibility relative standard deviation (RSDR) values ranged from 4.58 to 10.9%. The range of applicability of the test is 2-64% RS. The method is not suitable for products with <1% RS (e.g., regular maize starch; 0.6% RS). For such products, RSDr and RSDR values are unacceptably high. 相似文献
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Seow Jecg Chin Peter Hornsby Damjan Vengust Dragan Mihailović J. Mitra Paul Dawson Tony McNally 《先进技术聚合物》2012,23(2):149-160
Composites of poly(ε‐caprolactone) (PCL) and molybdenum sulfur iodine (MoSI) nanowires were prepared using twin‐screw extrusion. Extensive microscopic examination of the composites revealed the nanowires were well dispersed in the PCL matrix, although bundles of Mo6S3I6 ropes were evident at higher loadings. Secondary electron imaging (SEI) showed the nanowires had formed an extensive network throughout the PCL matrix, resulting in increased electrical conductivity of PCL, by eight orders of magnitude, and an electrical percolation threshold of 6.5 × 10?3 vol%. Thermal analysis (DSC), WAXD, and hot stage polarized optical microscopy (HSPOM) experiments revealed Mo6S3I6 addition altered PCL crystallization kinetics, nucleation density, and crystalline content. A greater number of smaller spherulites were formed via heterogeneous nucleation. The onset of thermal decomposition (TGA) of PCL decreased by 70°C, a consequence of the thermal degradation of Mo6S3I6 to MoO3, which in turn accelerates the formation of volatile gases during the first stage of PCL decomposition. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
7.
Local strain and defects in silicon wafers due to nanoindentation revealed by full‐field X‐ray microdiffraction imaging 下载免费PDF全文
Z. J. Li A. N. Danilewsky L. Helfen P. Mikulik D. Haenschke J. Wittge D. Allen P. McNally T. Baumbach 《Journal of synchrotron radiation》2015,22(4):1083-1090
Quantitative characterization of local strain in silicon wafers is critical in view of issues such as wafer handling during manufacturing and strain engineering. In this work, full‐field X‐ray microdiffraction imaging using synchrotron radiation is employed to investigate the long‐range distribution of strain fields in silicon wafers induced by indents under different conditions in order to simulate wafer fabrication damage. The technique provides a detailed quantitative mapping of strain and defect characterization at the micrometer spatial resolution and holds some advantages over conventional methods. 相似文献
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Characterization of the epitaxial defect known as the carrot defect was performed in thick 4H-SiC epilayers. A large number of carrot defects have been studied using different experimental techniques such as Nomarski optical microscopy, KOH etching, cathodoluminescence and synchrotron white beam X-ray topography. This has revealed that carrot defects appear in many different shapes and structures in the epilayers. Our results support the previous assignment of the carrot defect as related to a prismatic stacking fault. However, we have observed carrot defects with and without a visible threading dislocation related etch pit in the head region, after KOH etching. Polishing of epilayers in a few μm steps in combination with etching in molten KOH and imaging using Nomarski optical microscope has been used to find the geometry and origin of the carrot defects in different epilayers. The defects were found to originate both at the epi-substrate interface and during the epitaxial growth. Different sources of the carrot defect have been observed at the epi-substrate interface, which result in different structures and surfaces appearance of the defect in the epilayer. Furthermore, termination of the carrot defect inside the epilayer and the influence of substrate surface damage and growth conditions on the density of carrot defects are studied. 相似文献
9.
An integral equation formulation for buoyancy-driven convection problems is developed and illustrated. Buoyancy-driven convection in a bounded cylindrical geometry with a free surface is studied for a range of aspect ratios and Nusselt numbers. The critical Rayleigh number, the nature of the cellular motion, and the heat transfer enhancement are computed using linear theory. Green's functions are used to convert the linear problem into linear Fredholm integral equations. Theorems are proved which establish the properties of the eigenvalues and eigenfunctions of the linear integral operator which appears in these equations. 相似文献
10.
Radical couplings of cyanopyridine radical anions represent a valuable technology for functionalizing pyridines, which are prevalent throughout pharmaceuticals, agrochemicals, and materials. Installing the cyano group, which facilitates the necessary radical anion formation and stabilization, is challenging and limits the use of this chemistry to simple cyanopyridines. We discovered that pyridylphosphonium salts, installed directly and regioselectively from C–H precursors, are useful alternatives to cyanopyridines in radical–radical coupling reactions, expanding the scope of this reaction manifold to complex pyridines. Methods for both alkylation and amination of pyridines mediated by photoredox catalysis are described. Additionally, we demonstrate late-stage functionalization of pharmaceuticals, highlighting an advantage of pyridylphosphonium salts over cyanopyridines.Cyanopyridines form dearomatized radical anions upon single-electron reduction and participate in photoredox coupling reactions. Pyridylphosphonium salts replicate that reactivity with a broader scope and increase the utility of these processes.Modern photoredox catalysis and electrochemistry have enabled new synthetic methods that proceed via open-shell intermediates.1 Under this regime, pyridine functionalization strategies have been developed where 4-cyanopyridines undergo single-electron reduction to form dearomatized radical species that couple with other stabilized radicals (Scheme 1A).2 The cyano group is critical for efficient reactivity via pyridyl radical anions; alternatives such as 4-halopyridines more readily undergo elimination to pyridyl radicals after single-electron reduction resulting in a distinct set of coupling processes.3 We aimed to show that pyridylphosphonium salts could replicate the reactivity of cyanopyridines and allow a broader set of inputs into dearomatized pyridyl radical coupling reactions.4Open in a separate windowScheme 1Expansion of radical coupling reactions to complex pyridines.Cyanopyridines have facilitated pyridine alkylation, allylation, and alkenylation reactions providing access to valuable building blocks for medicinal and agrochemical programs.5 The cyano group is essential for these methods, but a problem arises when applying this chemistry to complex pyridines, such as those found in pharmaceutical and agrochemical candidates. These structures are often devoid of pre-installed functional groups, and it is often challenging to install a cyano group from C–H precursors regioselectively.6 We envisioned pyridylphosphonium salts, regioselectively constructed from the C–H bonds of a diverse set of pyridines, could serve as alternatives to cyanopyridines.7 Herein, we report couplings between alkyl BF3K salts and preliminary studies of carboxylic acids and amines with pyridylphosphonium salts, including late-stage functionalization of complex pyridine-containing pharmaceuticals using this strategy.Recently, we reported a radical coupling reaction between a boryl-stabilized cyanopyridyl radical and a boryl-stabilized pyridylphosphonium radical.7a The intermediate radicals arose via an unusual inner-sphere process that would be difficult to extend to other coupling reactions. A significant advance would be to show that pyridylphosphonium salts could function more generally as radical anion precursors and mimic the reactivity of cyano-pyridines. In particular, showing their viability in photoredox and electrochemical processes would translate to numerous synthetic transformations. To demonstrate this principle, we envisioned a redox-neutral alkylation reaction (Scheme 1B) via a radical coupling between radical zwitterion I, formed through single-electron reduction of a pyridylphosphonium salt (Eredp/2 = −1.51 V vs. SCE) and benzyl radical II, resulting from single-electron oxidation of a BF3K salt (Ered = +1.10 V vs. SCE for a primary benzylic salt).8 Loss of triphenylphosphine from dearomatized intermediate (III) would furnish the alkylated pyridine product. Notably, the redox events could invert, where the photocatalyst oxidizes the BF3K salt first and reduces the pyridylphosphonium salt second, broadening the scope of amenable photocatalysts.We began our investigation by examining a series of photocatalysts for the coupling reaction of phosphonium salt 1a, formed with complete regioselectivity for the 4-position from 2-phenylpyridine, and benzylic BF3K salt 2a under irradiation from a 455 nm Kessil light (Scheme 1B are potentially interchangeable.1b The Adachi-type photocatalyst 3DPAFIPN improved the yield to 77% with a further increase to 82% after increasing the reaction concentration (entries 3 and 4). Adding 2,6-lutidine, previously shown as an effective additive for photoredox cross-coupling reactions of BF3K salts by the Molander group,9 had no impact on the yield of 2-phenylpyridine salt 1a (entry 5) and the [Ir(ppy)2(dtbbpy)]PF6 catalyst was marginally less efficient under the same conditions (entry 6). We observed that 2,6-lutidine did substantially improve the yield when isomeric 3-Ph salt 1b was employed (entries 7 and 8); without 2,6-lutidine, the crude 1H NMR indicates significant amounts of decomposition occurred, including 3-phenylpyridine, and the 4- vs. 2-position product ratio was 3 : 1. This outcome suggests that protiodephosphination and non-selective Minisci-type pathways can occur under these conditions. With 2,6-lutidine, the crude reaction pathway is cleaner, and the 4- vs. 2-position ratio improved to 8 : 1. At this point, we have not established the role of 2,6-lutidine, although it is conceivable that it reacts with BF3 produced as the reaction progresses. In 2-substituted systems, steric hindrance around the pyridine N-atom of the salt would deter BF3-coordination, whereas, in 3-substituted systems, such as salt 1b, coordination is more likely and may have a deleterious effect on the reaction (vide infra). Given the structural variation of pyridines that we anticipated applying to this process and how those structures could impact boron speciation during the reaction, we elected to use 2,6-lutidine as an additive in all subsequent reactions.10Optimization of pyridine alkylation, photocatalyst data and effect of BF3·OEt2 as an additivea
Open in a separate windowaConditions: 1a (1.0 equiv.), 2a (2.0 equiv.), photocatalyst (2 mol%), additive (3.0 equiv.), rt.bYields determined by 1H NMR analysis using 1,3,5-trimethoxybenzene as internal standard.cIsolated yield on 0.50 mmol scale.dIsolated yield on 2.00 mmol scale.e3 : 1 4- vs. 2-regioisomeric ratio determined from the crude 1H NMR.f8 : 1 4- vs. 2-regioisomeric ratio determined from the crude 1H NMR.gUsed 365 nm LEDs instead of 455 nm Kessil light for 89 h.hAll redox potentials reported vs. SCE and all values compiled from previous literature reports.1iCounterion omitted in structure for simplicity.We conducted a series of further experiments to explore the effect of light and photocatalyst type on the reaction (†).11 Furthermore, a photocatalyst with a redox potential window misaligned with the redox events in Scheme 1B, [Mes-Acr]BF4, is also competent (entry 11). An energy transfer mechanism was considered based on entry 9, but the low triplet state energies for [Mes-Acr]BF4 make this pathway unlikely (12–14Employing the optimized conditions, we investigated the scope of pyridylphosphonium salts in this coupling process (