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41.
Analysis of a new paramagnetic spin 1 spectrum, labeled Si-S1, indicates that it corresponds to the neutral charge state of the Si-B1 center in an excited spin-triplet state. An extension of this analysis to the Si-P1, -P2, -P3, -P4, and -P5 centers suggest that these centers comprise a family of defects which consist of a string of neighboring vacancies in a single (110) plane. In particular, it appears that the Si-P1 center is an odd ( 3)-vacancy defect; the Si-P2 center is a 2-vacancy, oxygen defect; the Si-P3 center is a 4-vacancy defect; the Si-P4 center is a 3-vacancy, oxygen defect of symmetry lower than C2v ; and the Si-P5 center is a 3-vacancy, oxygen defect having C2v symmetry.  相似文献   
42.
Recently, a large increase in the equilibrium hydrogen pressure has been reported for MG thin films capped with a Pd layer. We show that this increase is due to intermixing of Mg and Pd, as opposed to a strain effect as previously claimed. Transmission electron microscopy and depth profiling x-ray photoemission spectroscopy are used to directly measure interfacial intermixing between Mg and Pd, and we find that intermixing and equilibrium hydrogen pressure both increase with annealing. We present a thermodynamic model of the effect of alloying on equilibrium pressure, and find that the observed equilibrium pressure increase is consistent with the observed thickness of the intermixed region, which is of the order of a few nm. We also show that stress measured during hydrogenation corresponds to a negligible increase in equilibrium pressure.  相似文献   
43.
Abstract

Solutions of 1° and 2° nitroalkanes were thermolyzed in the presence of amines at hydrostatic pressures up to 1.5 GPa, and their rates of decomposition were measured. The probable mechanism of base catalyzed decomposition involves a bimolecular reaction of the aci-nitroalkane with the nitronate ion to yield a carbonyl compound, its corresponding oxime, and nitrite ion as the early rate determining steps. In the case of nitroethane, the products react further to give 3,4,5-trimethylisoxazole as a major product above 0.4 GPa. The effects of pressure on the rates of decomposition of nonacidic nitroalkanes, 2,2-dinitropropane and 2-methyl-2-nitropropane, were measured and used to infer the rate-controlling steps. 2,2-Dini-tropropane appears to undergo rearrangement to a nitrite, followed by homolysis to give acetone, NO and NO2. 2-Methyl-2-nitropropane gives mostly isobutylene and HONO, probably via a 5-membered cyclic transition state.  相似文献   
44.
We present a new multigrid solver that is suitable for the Dirac operator in the presence of disordered gauge fields. The key behind the success of the algorithm is an adaptive projection onto the coarse grids that preserves the near null space. The resulting algorithm has weak dependence on the gauge coupling and exhibits very little critical slowing down in the chiral limit. Results are presented for the Wilson-Dirac operator of the 2D U(1) Schwinger model.  相似文献   
45.
We present an extensive study of a new Monte Carlo acceleration algorithm introduced by Wolff for the Ising model. It differs from the Swendsen-Wang algorithm by growing and flipping single clusters at a random seed. In general, it is more efficient than Swendsen-Wang dynamics ford>2, giving zero critical slowing down in the upper critical dimension. Monte Carlo simulations give dynamical critical exponentsz w=0.33±0.05 and 0.44+0.10 ind=2 and 3, respectively, and numbers consistent withz w=0 ind=4 and mean-field theory. We present scaling arguments which indicate that the Wolff mechanism for decorrelation differs substantially from Swendsen-Wang despite the apparent similarities of the two methods.  相似文献   
46.
Single crystals of yttrium boride YBn with n = 61 ± 3 were grown from the melt. Precision density and lattice parameter measurements indicate a congruent melting point at n = 61.7 and a stoichiometric composition at n = 68. Measurements of elastic constants, acoustic attenuation, electrical resistivity and optical absorption are presented. High resolution transmission electron microscopy reveals a complex crystal structure similar to that found by using X-rays. A comparison of the properties of YBn with those of β-boron show that there are many similarities.  相似文献   
47.
48.
The electric form factor of the neutron was determined from measurements of the d-->(e-->,e'n)p reaction for quasielastic kinematics. Polarized electrons were scattered off a polarized deuterated ammonia (15ND3) target in which the deuteron polarization was perpendicular to the momentum transfer. The scattered electrons were detected in a magnetic spectrometer in coincidence with neutrons in a large solid angle detector. We find G(n)(E)=0.0526+/-0.0033(stat)+/-0.0026(sys) and 0.0454+/-0.0054+/-0.0037 at Q(2)=0.5 and 1.0 (GeV/c)(2), respectively.  相似文献   
49.
The development of peptide beta-hairpins is problematic, because folding depends on the amino acid sequence and changes to the sequence can significantly decrease folding. Robust beta-hairpins that can tolerate such changes are attractive tools for studying interactions involving protein beta-sheets and developing inhibitors of these interactions. This paper introduces a new class of peptide models of protein beta-sheets that addresses the problem of separating folding from the sequence. These model beta-sheets are macrocyclic peptides that fold in water to present a pentapeptide beta-strand along one edge; the other edge contains the tripeptide beta-strand mimic Hao [JACS 2000, 122, 7654] and two additional amino acids. The pentapeptide and Hao-containing peptide strands are connected by two delta-linked ornithine (deltaOrn) turns [JACS 2003, 125, 876]. Each deltaOrn turn contains a free alpha-amino group that permits the linking of individual modules to form divalent beta-sheets. These "cyclic modular beta-sheets" are synthesized by standard solid-phase peptide synthesis of a linear precursor followed by solution-phase cyclization. Eight cyclic modular beta-sheets 1a-1h containing sequences based on beta-amyloid and macrophage inflammatory protein 2 were synthesized and characterized by 1H NMR. Linked cyclic modular beta-sheet 2, which contains two modules of 1b, was also synthesized and characterized. 1H NMR studies show downfield alpha-proton chemical shifts, deltaOrn delta-proton magnetic anisotropy, and NOE cross-peaks that establish all compounds but 1c and 1g to be moderately or well folded into a conformation that resembles a beta-sheet. Pulsed-field gradient NMR diffusion experiments show little or no self-association at low (相似文献   
50.
 Bilirubins with propionic acids at C-8 and C-12 engage in intramolecular hydrogen bonding and are thought to be monomeric in solution, although the latter is unproven. In contrast, their dimethyl esters and etiobilirubin analogs (with the C-8 and C-12 propionic acids replaced by alkyl residues) favor intermolecular hydrogen bonding and are thought to be dimeric in nonpolar solvents. There is little information on the molecularity of the bilirubin dianion in solution. In this work, vapor pressure osmometry studies of chloroform solutions of bilirubins, their dimethyl esters, and etio-analogs clearly indicate that the diacids and dianions are monomeric, whereas the diesters and dialkyls are dimeric. However, the presence of a C-10 gem-dimethyl group causes the ester and the etiobilirubin to become monomeric.  相似文献   
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