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991.
We study experimentally the slow growth of a single crack in a fibrous material and observe stepwise growth dynamics. We model the material as a lattice where the crack is pinned by elastic traps and grows due to thermally activated stress fluctuations. In agreement with experimental data we find that the distribution of step sizes follows subcritical point statistics with a power law (exponent 3/2) and a stress-dependent exponential cutoff diverging at the critical rupture threshold. 相似文献
992.
Mauricio Cataldo Sergio del Campo Alberto García 《General Relativity and Gravitation》2001,33(7):1245-1255
We propose an approach for constructing spatial slices of (3+1) spacetimes with cosmological constant but without a matter content, which yields (2+1) vacuum with solutions. The reduction mechanism from (3+1) to (2+1) gravity is supported on a criterion in which the Weyl tensor components are required to vanish together with a dimensional reduction via an appropriate foliation. By using an adequate reduction mechanism from the Plebaski–Carter[A] solution in (3+1) gravity, the (2+1) BTZ solution can be obtained. 相似文献
993.
Carbazole ( 1 ) undergoes electrophilic aromatic substitution with various iodinating reagents. Although, 3‐iodocarbazole ( 1b ) and 3,6‐diiodocarbazole ( 1d ) obtained by iodination of carbazole were isolated and characterized sometime ago, 1‐iodocarbazole ( 1a ), 1,6‐diiodocarbazole ( 1c ) and 1,3,6‐triiodocarbazole ( 1e ) had never been isolated from the reaction mixture. The preparation and subsequent isolation and characterization of 1a, 1b, 1c, 1d and 1e are reported (mp, tr, Rf, 1H‐nmr, 13C‐nmr and ms). As iodinating reagents, NaIO4/I2 and NaIO4/KI mixtures in (i) ethanol doped with catalytical amount of sulfuric acid and in (ii) acetic acid, and N‐odosuccinimide and N‐iodosuccinimide‐silica gel in dichloromethane and in chloroform have been used and their uses have been compared. The iodination reaction of different carbazole derivatives such as 2‐acetoxycarbazole ( 2 ), 3‐bromocarbazole (3) and 3‐nitrocarbazole ( 4 ) was also studied and the corresponding iododerivatives, 2a, 2b, 2c, 3a, 3b, 4a and 4b , are described for the first time. Semiempirical PM3 calculations have been performed in order to predict reactivity of carbazole ( 1 ), substituted carbazoles (2‐4) and iodocarbazoles ( 1a‐1e, 2a‐2c, 3a‐3b, 4a and 4b ) (Scheme 1). Theoretical and experimental results are discussed briefly. 相似文献
994.
Sergio Mantegani Gabriella Traquandi Mario Varasi 《Journal of heterocyclic chemistry》2001,38(3):759-763
As a part of a search for novel biological active ergoline derivatives, the indole ring present in the ergoline skeleton (indole[4,3‐f,g]quinoline) was converted into different heterocyclic ring systems such as quinazoline 2 , benzofurane 3 and benzoxazole 4 . Due to the paramount importance of chirality to attain biological activity, natural dihydrolysergic acid 1 was chosen as starting material and a synthetic pathway conservative in term of chirality was followed. 相似文献
995.
In this work, we discuss the scattering theory of local, relativistic quantum fields with indefinite metric. Since the results
of Haag–Ruelle theory do not carry over to the case of indefinite metric [4], we propose an axiomatic framework for the construction
of in- and out-states, such that the LSZ asymptotic condition can be derived from the assumptions. The central mathematical
object for this construction is the collection of mixed vacuum expectation values of local, in- and out-fields, called the
“form factor functional”, which is required to fulfill a Hilbert space structure condition. Given a scattering matrix with
polynomial transfer functions, we then construct interpolating, local, relativistic quantum fields with indefinite metric,
which fit into the given scattering framework.
Received: 13 September 1999/ Accepted: 1 August 2000 相似文献
996.
The modeling of the elastoplastic behaviour of single crystals with infinite latent hardening leads to a nonconvex energy density, whose minimization produces fine structures. The computation of the quasiconvex envelope of the energy density is faced in this case with huge numerical difficulties caused by the clusters of local minima. By exploiting the structure of the problem, we present a fast and efficient numerical relaxation algorithm as alternative to global optimization techniques usually adopted in literature which are computationally more expensive. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
997.
SergioAmat SoniaBusquier VicenteF.Candela 《计算数学(英文版)》2004,22(1):1-10
An alternative to the classical extrapolations is proposed. The stability and the accuracy are studied. The new extrapolation behaves better than the classical ones when there are problems of stability. This technique will be useful in those problems where the region of stability is very small and it forces to work with too fine scales. 相似文献
998.
999.
Gilberto Casillas Juan Pedro Palomares-Báez José Luis Rodríguez-López Junhang Luo Arturo Ponce Rodrigo Esparza 《哲学杂志》2013,93(35):4437-4453
There is strong interest in studying changes in mechanical properties with reducing grain size. The rational is that consequent dislocation glide cannot be sustained, resulting in an increase in material strength. However, this comes with the cost of a reduction in ductility. It has been shown that coherent twin boundaries in nanostructured Cu improve the ductility to 14% [Lu et al., Science 324 (2009) p. 349]. In this paper, we report for the first time the compression of individual nanoparticles using an in situ force probing holder in the transmission electron microscope. Four types of nanoparticles were tested, three with twin boundaries (decahedra, icosahedra and a single twin) and one free of defects (octahedral). Our results indicate the yield strength of the twinned nanoparticles is between 0.5 and 2.0 GPa. The total malleability for the twinned particles range from 80 to 100%. In addition, experimental results were reproduced by MD simulations of the compression phenomena and suggest that the outstanding mechanical properties are related with partial dislocation multiplication at twin boundaries. 相似文献
1000.
Methoxycarbonylation of olefins catalyzed by palladium(II) complexes containing naphthyl(diphenyl)phosphine ligands 下载免费PDF全文
Santiago Zolezzi Sergio A. Moya Gonzalo Valdebenito Gabriel Abarca Jose Parada Pedro Aguirre 《应用有机金属化学》2014,28(5):364-371
Palladium(II) complexes containing phosphine donor ligands derived from naphthyl(diphenyl)phosphine were synthesized and characterized by NMR and elemental analysis. The complexes were studied as catalyst precursors in the methoxycarbonylation reaction of several aromatic and aliphatic olefins under mild conditions. The catalysts reported high chemoselectivities (over 96%) and regioselectivities between 44% and 93% for different olefins. The best results were obtained over a styrene substrate with 97% of conversion after 6 h of reaction, with high regioselectivity (93%). Kinetic studies permitted the determination of the rate law (v = k [substrate]1.21±0.02 [catalyst]0.94±0.11 [acid]0.52±0.03 [MeOH]0.53±0.05 [CO]0.65±0.03) for methoxycarbonylation of styrene. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献