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201.
Uniform distribution of projection data for improved reconstruction quality of 4D EPR imaging 总被引:2,自引:2,他引:0
Ahmad R Vikram DS Clymer B Potter LC Deng Y Srinivasan P Zweier JL Kuppusamy P 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2007,187(2):277-287
In continuous wave (CW) electron paramagnetic resonance imaging (EPRI), high quality of reconstruction in a limited acquisition time is a high priority. It has been shown for the case of 3D EPRI, that a uniform distribution of the projection data generally enhances reconstruction quality. In this work, we have suggested two data acquisition techniques for which the gradient orientations are more evenly distributed over the 4D acquisition space as compared to the existing methods. The first sampling technique is based on equal solid angle partitioning of 4D space, while the second technique is based on Fekete points estimation in 4D to generate a more uniform distribution of data. After acquisition, filtered backprojection (FBP) is applied to carry out the reconstruction in a single stage. The single-stage reconstruction improves the spatial resolution by eliminating the necessity of data interpolation in multi-stage reconstructions. For the proposed data distributions, the simulations and experimental results indicate a higher fidelity to the true object configuration. Using the uniform distribution, we expect about 50% reduction in the acquisition time over the traditional method of equal linear angle acquisition. 相似文献
202.
Boyd BJ Rizwan SB Dong YD Hook S Rades T 《Langmuir : the ACS journal of surfaces and colloids》2007,23(25):12461-12464
Attempts to understand the complex 3D morphology of non-lamellar liquid-crystalline nanostructured particles, formed by the dispersion of a reversed hexagonal phase (hexosomes) and bicontinuous cubic phase (cubosomes) in water, have been limited by the lack of suitable 3D imaging techniques. Using cryo-field emission scanning electron microscopy, we show that whereas the structure of cubosomes generally reflects that anticipated from modeling approaches, hexosomes, which were previously proposed to be flat hexagonal prisms, in fact often possess a "spinning-top-like" structure, which is likely to influence their interactions with surfaces. 相似文献
203.
Hud Wahab Guangyuan Xu Christine Jansing Markus Gilbert Marc F. Tesch Jianyong Jin Hans‐Christoph Mertins Heiko Timmers 《X射线光谱测定》2015,44(6):468-473
In graphene oxide, the graphite lattice is intercalated with oxygen groups that bond to carbon atoms. These groups have a bearing on the possibility of using graphene oxide as a precursor to make graphene. The nature of carbon bonds in graphene oxide has been characterized with soft x‐ray reflection spectroscopy across the carbon K‐edge. Results distinguish graphene oxide synthesized with Hummers' method from that made using a method suggested by Tour. The observed spectra are consistent with those from near‐edge x‐ray absorption fine structure (NEXAFS) measurements. In particular, the expected carbon K‐edge resonances associated with excitations into molecular π*‐ and σ*‐states of C? C bonds can be identified. Importantly, the greater oxidation efficiency of the method by Tour may be the reason for the observation of additional resonances that have been assigned to carbon bonding with molecular groups containing oxygen. The additional resonances have been interpreted as the excitations of carbon 1 s electrons into the carbonyl π*(C?O) orbital in the molecular group –COOH and into the hydroxyl π*(C? OH) orbital, respectively. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
204.
The effect of UV radiation on the thermal degradation of cellulose triacetate (CTA) has been investigated. Simultaneous thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) have been performed on CTA samples of 0.25 mm thickness. These samples were exposed to different energy fluences of UV in the range 2.3–113 kJ/cm2. The specific heat capacity, Cp, has been evaluated for unexposed and exposed CTA samples using DSC method. The results indicate that the transition temperatures, onset temperatures of evaporation, specific heat capacity and the thermal activation energy of decomposition, Ea are affected by the UV energy fluence owing to the simultaneous processes of degradation and crosslinking. 相似文献
205.
A novel layerwise C0-type higher order shear deformation theory (layerwise C0-type HSDT) for the analysis of laminated composite and sandwich plates is proposed. A C0-type HSDT is used in each lamina layer and the continuity of in-plane displacements and transverse shear stresses at inner-laminar layer is consolidated. The present layerwise theory retains only seven variables without increasing the number of variables when the number of lamina layers are intensified. The shear stresses through the plate thickness derived from the constitutive equation of the present theory have the same shape as those calculated from the equilibrium equation. In addition, the artificial constraints are added in the principle of virtual displacements (PVD) and are certainly fulfilled through a penalty approach. In this paper, two C0-continuity numerical methods, such as the Finite Element Method (FEM) and Bézier isogeometric element (BIEM) are utilized to solve a discrete system of equations derived from the PVD. Several numerical examples with various geometries, aspect ratios, stiffness ratios, and boundary conditions are investigated and compared with the 3D elasticity solution, the analytical, as well as, numerical solutions based on various plate theories. 相似文献
206.
M. Asghar F. Iqbal S. Faraz V. Jokubavicius Q. Wahab M. Syväjärvi 《Physica B: Condensed Matter》2012,407(15):3041-3043
In this study deep level transient spectroscopy has been performed on boron–nitrogen co-doped 6H-SiC epilayers exhibiting p-type conductivity with free carrier concentration (NA–ND)∼3×1017 cm−3. We observed a hole H1 majority carrier and an electron E1 minority carrier traps in the device having activation energies Ev+0.24 eV, Ec −0.41 eV, respectively. The capture cross-section and trap concentration of H1 and E1 levels were found to be (5×10−19 cm2, 2×1015 cm−3) and (1.6×10−16 cm2, 3×1015 cm−3), respectively. Owing to the background involvement of aluminum in growth reactor and comparison of the obtained data with the literature, the H1 defect was identified as aluminum acceptor. A reasonable justification has been given to correlate the E1 defect to a nitrogen donor. 相似文献
207.
Nodal integral methods (NIMs) have been developed and successfully used to numerically solve several problems in science and engineering. The fact that accurate solutions can be obtained on relatively coarse mesh sizes, makes NIMs a powerful numerical scheme to solve partial differential equations. However, transverse integration procedure, a step required in the NIMs, limits its applications to brick‐like cells, and thus hinders its application to complex geometries. To fully exploit the potential of this powerful approach, abovementioned limitation is relaxed in this work by first using algebraic transformation to map the arbitrarily shaped quadrilaterals, used to mesh the arbitrarily shaped domain, into rectangles. The governing equations are also transformed. The transformed equations are then solved using the standard NIM. The scheme is developed for the Poisson equation as well as for the time‐dependent convection–diffusion equation. The approach developed here is validated by solving several benchmark problems. Results show that the NIM coupled with an algebraic transformation retains the coarse mesh properties of the original NIM. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
208.
Despite extensive studies on the flexibility of manufacturing systems over the last two decades, a unified measurement approach has not been developed. To this end, we integrate two domains of machine flexibility models from the literature: operational capability-based machine flexibility and time and cost-based machine flexibility, and propose a generic model to measure machine flexibility with consideration of uncertainties in the system. Furthermore, in our approach we include part characteristics such as processing time and processing cost, the number of operations that a machine can perform, and uncertainties in demand and machine-part assignment. The resulting framework to measure machine flexibility is a two-stage model: a super efficiency Data Envelopment Analysis Model and a flexibility model. The results show that the marginal system machine flexibility does not always increase as the number of operations that a machine can perform increases, and the system machine flexibility depends on the demand uncertainty. 相似文献
209.
Mohammad A. Wahab Il Kim Chang‐Sik Ha 《Journal of polymer science. Part A, Polymer chemistry》2004,42(20):5189-5199
Hybrid nanocomposite films of poly(vinylsilsesquioxane) (PVSSQ) and polyimide (PI) (PI/PVSSQ) were prepared via sol‐gel process from triethoxyvinylsilane (VSSQ) and thermal imidization from 3,3′,4,4′‐biphenyltetracarboxylic dianhydride (BPDA)‐p‐phenylene diamine (PDA) polyamic acid (BPDA‐PDA PAA). We investigated the microstructure; interfacial interaction; and optical, thermal, dielectric, and mechanical properties of the hybrid films. The phase morphologies and degree of surface roughness were evaluated by scanning electron microscope (SEM) and atomic force microscope (AFM), respectively. It was found that the surface topography was influenced by the composition of PVSSQ. Hydrogen bonding interactions between polyimide (PI) matrix and PVSSQ domains were proved with FT‐IR spectroscopy. The transparency of the hybrid films was found to be dependent on the PVSSQ content. Incorporating of the PVSSQ in the hybrid composites increased the glass transition temperature of PI. Dielectric constants of the hybrid films were in the range of 2.37–3.59. Properties of the PI films were also significantly enhanced by adding 5–30 wt % of PVSSQ. For comparison, we also prepared the hybrid composites of PI and mixtures of VSSQ and tetraethoxysilane (TEOS) and the PI/silica hybrid composite containing 30 wt % of silica obtained from TEOS. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5189–5199, 2004 相似文献
210.
Tristan Chidley Islam Jameel Shafa Rizwan Philippe A. Peixoto Laurent Pouysgu Stphane Quideau W. Scott Hopkins Graham K. Murphy 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(47):17115-17121
A facile and highly chemoselective synthesis of doubly activated cyclopropanes is reported where mixtures of alkenes and β‐dicarbonyl‐derived iodonium ylides are irradiated with light from blue LEDs. This metal‐free synthesis gives cyclopropanes in yields up to 96 %, is operative with cyclic and acyclic ylides, and proceeds with a variety of electronically‐diverse alkenes. Computational analysis explains the high selectivity observed, which derives from exclusive HOMO to LUMO excitation, instead of free carbene generation. The procedure is operationally simple, uses no photocatalyst, and provides access in one step to important building blocks for complex molecule synthesis. 相似文献