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21.
G. Petõ G. Molnár L. Dózsa Z.E. Horváth Zs.J. Horváth E. Zsoldos C.A. Dimitriadis L. Papadimitriou 《Applied Physics A: Materials Science & Processing》2005,81(5):975-980
Epitaxial and polycrystalline orthorhombic GdSi2 films were grown on Si(100) substrates by solid phase reaction between Si and Gd films at different thicknesses of the Gd film . The most important property of these GdSi2/Si interfaces was defect formation. This was investigated by studying the properties of the Schottky barriers by means of current voltage and capacitance–voltage characteristics, deep level transient spectroscopy by double crystal X-ray diffractometry, and transmission electron microscopy. Epitaxial growth of the silicide layer ensured a relatively low interface defect density (about 1010 cm-2), while the non-epitaxial growth induced defects of a much higher density (about 1012 cm-2). The defects generated during the silicide formation are located within a depth of about 10 nm from the GdSi2/Si interface. PACS 68.55.-a; 73.40.-c; 81.15-zThis revised version was published in March 2005. References 10-17 were added in the online version (pdf file). 相似文献
22.
Constantina P. Hadjiantoniou-Maroulis Vassiliki Ikonomou Evi Parisopoulou 《Journal of heterocyclic chemistry》1996,33(3):655-658
The synthesis and lead tetraacetate oxidation of the title mono- and bis-arylhydrazones to the corresponding 5-acyl-4-aryl(alkyl)-2-aryl-1,2,3-triazole 1-oxides is described. 相似文献
23.
A novel medium access control protocol for optical local networks based on data request ordering 总被引:1,自引:0,他引:1
The basic aim of the scheduling algorithms in wavelength division multiplexing (WDM) single-hop optical networks is to construct a short schedule, in order to support quick communication between the nodes of the network. A design of medium access control (MAC) protocol without collisions is introduced for scheduling variable-length data packets based on a broadcast and select architecture. The system includes only data channels and the coordination of the transmissions is achieved via control packets, functioning before the beginning of actual transmissions. The proposed scheme adopts a prediction mechanism in order to eliminate the possible delay introduced by the scheduling computation between the control and data phases of each cycle of transmission. The two common scheduling strategies suggested try to reorder the service sequence of the nodes, by prioritizing the nodes with long data packets, compared with the nodes with short data packets. The final schedule, formed by a scheduling matrix, seems to be much shorter and leads to an increase, in terms of channel utilization. The extensive simulation results show that the novel scheduling techniques offer more free time space for schedule and allow more data to travel on the medium for the same amount of time. Also, the mean packet delay on the queues is reduced and the relation throughput-delay is much better. 相似文献
24.
For a permutation σ of the integers from 1 to n, let ?(σ) be the smallest number of prefix reversals that will transform σ to the identity permutation, and let ?(n) be the largest such ?(σ) for all σ in (the symmetric group) Sn. We show that , and that for n a multiple of 16. If, furthermore, each integer is required to participate in an even number of reversed prefixes, the corresponding function g(n) is shown to obey . 相似文献
25.
John K. Gallos Vassiliki C. Sarli Zoe S. Massen Anastassia C. Varvogli Constantina Z. Papadoyanni Sofia D. Papaspyrou Nicolaos G. Argyropoulos 《Tetrahedron》2005,61(3):565-574
A new method for the synthesis of racemic non-proteinogenic α-amino acids has been developed, which involves (i) hetero-Diels-Alder addition of ethyl 2-nitrosoacrylate to electron rich alkenes such as enol ethers, enamines and allylsilanes, (ii) NaCNBH3 reduction of the CN bond in the oxazines thus generated, the stereochemistry of the products being controlled by epimerisation of the thermodynamically less stable isomer to the more stable one, (iii) protection of the N-H group as N-Boc and (iv) finally, N-O bond cleavage of both free and protected products to give proline or bis-homoserine derivatives, respectively. An example with concomitant reduction of the carboxylate group, resulting in the formation of the respective amino alcohol is reported. Applying this methodology to a homochiral enol ether, the protected parent d-proline was prepared in enantiomerically pure form, whereas the asymmetric synthesis of the respective bis-homoserine was unsuccessful. 相似文献
26.
LC-NMR coupling technology: recent advancements and applications in natural products analysis 总被引:4,自引:0,他引:4
Exarchou V Krucker M van Beek TA Vervoort J Gerothanassis IP Albert K 《Magnetic resonance in chemistry : MRC》2005,43(9):681-687
An overview of recent advances in nuclear magnetic resonance (NMR) coupled with separation technologies and their application in natural product analysis is given and discussed. The different modes of LC-NMR operation are described, as well as how technical improvements assist in establishing LC-NMR as an important tool in the analysis of plant-derived compounds. On-flow, stopped-flow and loop-storage procedures are mentioned, together with the new LC-SPE-NMR configuration. The implementation of mass spectrometry in LC-NMR is also useful on account of the molecular weight and fragmentation information that it provides, especially when new plant species are studied. Cryogenic technology and capillary LC-NMR are the other important recent developments. Since the plant kingdom is endless in producing potential drug candidates, development and optimization of LC-NMR techniques convert the study of natural products to a less-time-consuming task, speeding up identification. 相似文献
27.
Vassiliki Katsika-Tsigourakou Efthimios S. Skordas 《Central European Journal of Physics》2010,8(6):900-904
Various experimental techniques have revealed that the predominant intrinsic point defects in BaF2 are anion Frenkel defects. Their formation, enthalpy and entropy, as well as the corresponding parameters for the fluorine
vacancy and fluorine interstitial motion have been determined. In addition, low temperature dielectric relaxation measurements
in BaF2doped with uranium leads to the parameters τ0, E in the Arrhenius relation τ = τ0 exp(E/k
B
T) for the relaxation time τ. For the relaxation peak associated with a single tetravalent uranium, the migration entropy deduced
from the pre-exponential factor τ0 is smaller than the anion Frenkel defect formation entropy by almost two orders of magnitude. We show that, despite their
great variation, the defect entropies and enthalpies are interconnected through a model based on anharmonic properties of
the bulk material which have been recently studied by employing density-functional theory and density-functional perturbation
theory. 相似文献
28.
We show that the traveling salesman problem with triangle inequality cannot be approximated with a ratio better than
when the edge lengths are allowed to be asymmetric and
when the edge lengths are symmetric, unless P=NP. The best previous lower bounds were
and
respectively. The reduction is from H?stad’s maximum satisfiability of linear equations modulo 2, and is nonconstructive.
* Supported in part by NSF ITR Grant CCR-0121555.
† Supported by NSF award CCR-0307536 and a Sloan foundation fellowship. 相似文献
29.
30.
Natural protective amyloids 总被引:1,自引:0,他引:1
Amyloidoses are a group of diseases including neurodegenerative diseases like Alzheimer's disease and also type II diabetes, spongiform encephalopathies and many others, believed to be caused by protein aggregation and subsequent amyloid fibril formation. However, occasionally, living organisms exploit amyloid fibril formation, a property inherent into amino acid sequences, and perform specific physiological functions from amyloids, in differing biological contexts. Some of these functional amyloids are natural protective amyloids. Here, we review recent evidence on silkmoth chorion protein synthetic peptide-analogues that documents the function of silkmoth chorion, the major component of the eggshell, a structure with extraordinary physiological and mechanical properties, as a natural protective amyloid. Also, we briefly discuss the reported function of other natural, protective amyloids like fish chorion, the protein Pmel17 which forms amyloid fibrils that act as templates and accelerate the covalent polymerization of reactive small molecules into melanin, the hydrophobins and the antifreeze protein from winter flounder. Molecular self-assembly is becoming an increasingly popular route to new supramolecular structures and molecular materials and the inspiration for such structures is commonly derived from self-assembling systems in biology. Therefore, a careful examination of these studies may set the basis for the exploration of new routes for the formation of novel biocompatible polymeric structures with exceptional physico-chemical properties, for potentially new biomedical and industrial applications. 相似文献