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排序方式: 共有406条查询结果,搜索用时 218 毫秒
181.
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183.
Joy P. Dunkers Frederick R. Phelan Daniel P. Sanders Matthew J. Everett William H. Green Donald L. Hunston Richard S. Parnas 《Optics and Lasers in Engineering》2001,35(3)
The Composites Group at the National Institute of Standards and Technology has found optical coherence tomography (OCT) to be a powerful tool for non-destructive characterization of polymer matrix composites. Composites often exhibit superior properties to traditional materials such as wood and metal. However, the barrier to their widespread infiltration into consumer markets is cost. Composites can be made more cost competitive by improved composite design, process optimization, and quality control. OCT provides a means of evaluating the three aforementioned areas. OCT is a very versatile technique that can be applied to a variety of problems in polymer composites such as: microstructure determination for permeability and mechanical property prediction, void, dry spot, and defect detection, and damage evaluation. Briefly, OCT uses a low coherence source such as a superluminescent diode laser with a fiber optic based Michelson interferometer. In this configuration, the composite is the fixed arm of the interferometer. Reflections from heterogeneities within the sample are mapped as a function of thickness for any one position. Volume information is generated by translating the sample on a motorized stage. Information about the location and size of a feature within the composite is obtained. In this work, the power of OCT for imaging composite microstructure and damage is presented. An example of permeability prediction using the composite microstructure imaged from OCT is demonstrated. The effect of image processing on the value of permeability is discussed. Using the same sample, OCT imaging of composite impact damage is compared to more traditional techniques, X-ray computed tomography and confocal microscopy. 相似文献
184.
We propose a structural approach to the representation of life history data, based on an interval graph formalism. Various “life history graphs” are defined using this approach, and properties of such graphs are related to life course concepts. The analysis of life history graphs using standard network methods is illustrated, and suggestions are made regarding the use of network analysis for life course research. A duality is demonstrated between individual life history graphs and social networks, and connections between life history graph analysis and conventional life course methods are discussed. 相似文献
185.
The purpose of this note is to give an extension of one of Jackson's theorems tomultivariables. 相似文献
186.
Jie Chen Yunhao Yang Xiaobo Zhang Joy C. Andrews Piero Pianetta Yong Guan Gang Liu Ying Xiong Ziyu Wu Yangchao Tian 《Analytical and bioanalytical chemistry》2010,397(6):2117-2121
Three-dimensional (3D) nanoscale structures of the fission yeast, Schizosaccharomyces pombe, can be obtained by full-field transmission hard X-ray microscopy with 30 nm resolution using synchrotron radiation sources.
Sample preparation is relatively simple and the samples are portable across various imaging environments, allowing for high-throughput
sample screening. The yeast cells were fixed and double-stained with Reynold's lead citrate and uranyl acetate. We performed
both absorption contrast and Zernike phase contrast imaging on these cells in order to test this method. The membranes, nucleus,
and subcellular organelles of the cells were clearly visualized using absorption contrast mode. The X-ray images of the cells
could be used to study the spatial distributions of the organelles in the cells. These results show unique structural information,
demonstrating that hard X-ray microscopy is a complementary method for imaging and analyzing biological samples. 相似文献
187.
On the basis of CFO (carrier frequency offset) point of view, the system performance results from the analysis by adopting
the channel scenarios characterized as Weibull fading for an MC-DS-CDMA (multi-carrier direct-sequence coded-division multiple-access)
system is proposed in this article. Moreover, an approximate simple expression with the criterion of BER (bit error rate)
versus SNR (signal-to-noise ratio) method is derived for an MC-DS-CDMA system combining with RAKE receiver, which is a special
case of MRC (maximal ratio combining) diversity, based on the MGF (moment generating function) formula of Weibull statistics,
and it associates with an alternative expression of Gaussian Q-function. In addition, the other point of view on the BER performance
evaluation of an MC-DS-CDMA system is not only the assumption of both single-user and multi-user cases applied, but the phenomena
of PBI (partial band interference) is also included. Furthermore, with several of the system parameters, such as CFO values,
ε, Weibull fading parameter, β, user number, K, spreading chip number, N, branch number, L, and the PBI values, JSR etc., are compared with each other in the numerical results in order to validate the accuracy in the derived formulas. To
the best of author’s knowledge, it is a brain fresh idea proposed in this paper to evaluate the system performance for an
MC-DS-CDMA system on the point of the CFO view over Weibull fading. 相似文献
188.
Corbey JF Farnaby JH Bates JE Ziller JW Furche F Evans WJ 《Inorganic chemistry》2012,51(14):7867-7874
The effect of the neutral donor ligand, L, on the Ln(2)N(2) core in the (N═N)(2-) complexes, [A(2)(L)Ln](2)(μ-η(2):η(2)-N(2)) (Ln = Sc, Y, lanthanide; A = monoanion; L = neutral ligand), is unknown since all of the crystallographically characterized examples were obtained with L = tetrahydrofuran (THF). To explore variation in L, displacement reactions between {[(Me(3)Si)(2)N](2)(THF)Y}(2)(μ-η(2):η(2)-N(2)), 1, and benzonitrile, pyridine (py), 4-dimethylaminopyridine (DMAP), triphenylphosphine oxide, and trimethylamine N-oxide were investigated. THF is displaced by all of these ligands to form {[(Me(3)Si)(2)N](2)(L)Y}(2)(μ-η(2):η(2)-N(2)) complexes (L = PhCN, 2; py, 3; DMAP, 4; Ph(3)PO, 5; Me(3)NO, 6) that were fully characterized by analytical, spectroscopic, density functional theory, and X-ray crystallographic methods. The crystal structures of the Y(2)N(2) cores in 2-5 are similar to that in 1 with N-N bond distances between 1.255(3) ? and 1.274(3) ?, but X-ray analysis of the N-N distance in 6 shows it to be shorter: 1.198(3) ?. 相似文献
189.
Dr. Bingguang Zhang Dr. Yunjing Li Dr. Rui Liu Dr. Timothy M. Pritchett Dr. Alexander Azenkeng Dr. Angel Ugrinov Dr. Joy E. Haley Zhongjing Li Dr. Mark R. Hoffmann Dr. Wenfang Sun 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(15):4593-4606
A platinum complex with the 6‐(7‐benzothiazol‐2′‐yl‐9,9‐diethyl‐9H‐fluoren‐2‐yl)‐2,2′‐bipyridinyl ligand ( 1 ) was synthesized and the crystal structure was determined. UV/Vis absorption, emission, and transient difference absorption of 1 were systematically investigated. DFT calculations were carried out on 1 to characterize the electronic ground state and aid in the understanding of the nature of low‐lying excited electronic states. Complex 1 exhibits intense structured 1π–π* absorption at λabs<440 nm, and a broad, moderate 1M LCT/1LLCT transition at 440–520 nm in CH2Cl2 solution. A structured 3π–π*/3M LCT emission at about 590 nm was observed at room temperature and at 77 K. Complex 1 exhibits both singlet and triplet excited‐state absorption from 450 nm to 750 nm, which are tentatively attributed to the 1π–π* and 3π–π* excited states of the 6‐(7‐benzothiazol‐2′‐yl‐9,9‐diethyl‐9H‐fluoren‐2‐yl)‐2,2′‐bipyridine ligand, respectively. Z‐scan experiments were conducted by using ns and ps pulses at 532 nm, and ps pulses at a variety of visible and near‐IR wavelengths. The experimental data were fitted by a five‐level model by using the excited‐state parameters obtained from the photophysical study to deduce the effective singlet and triplet excited‐state absorption cross sections in the visible spectral region and the effective two‐photon absorption cross sections in the near‐IR region. Our results demonstrate that 1 possesses large ratios of excited‐state absorption cross sections relative to that of the ground‐state in the visible spectral region; this results in a remarkable degree of reverse saturable absorption from 1 in CH2Cl2 solution illuminated by ns laser pulses at 532 nm. The two‐photon absorption cross sections in the near‐IR region for 1 are among the largest values reported for platinum complexes. Therefore, 1 is an excellent, broadband, nonlinear absorbing material that exhibits strong reverse saturable absorption in the visible spectral region and large two‐photon‐assisted excited‐state absorption in the near‐IR region. 相似文献
190.
Shyamapada Shit Joy Chakraborty Brajagopal Samanta Alexandra M. Z. Slawin Volker Gramlich Samiran Mitra 《Structural chemistry》2009,20(4):633-642
Three new coordination complexes, 2{[Co(L1)2]ClO4} · 0.5CH3OH (1), [Mn(L2)2] (2), and [Cu(HL2)(L2)]ClO4 · 2H2O (3) have been synthesized from two tridentate N,N,O-donor hydrazone ligands HL1, 2-acetylpyridine-salicyloylhydrazone, and HL2, 2-benzoylpyridine-salicyloylhydrazone, respectively and thoroughly characterized by elemental analysis, FT-IR, UV–Vis, electrochemical,
and room temperature magnetic susceptibility measurements. Structures of the complexes have been unequivocally established
by single crystal X-ray diffraction technique. Structural analysis reveals that 1 consists of two chemically similar but crystallographically independent cationic [Co(L1)2]+ units and 2 consists of a neutral [Mn(L2)2] molecule while 3 consists of a cationic [Cu(HL2)(L2)]+ unit. Metal ions display distorted octahedral geometry in 1 and 2 while in 3 it shows a distorted square pyramidal geometry. Ligand conformations around the metal ions are stabilized by the presence
of intra-ligand hydrogen bonding in all the complexes. Structure of 3 reveals that a perchlorate ion linked to the complex by hydrogen bonding via a water molecule. 相似文献