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41.
The crystal structures of Bi2.5Na0.5Ta2O9 and Bi2.5Nam-1.5NbmO3m+3 (m=3,4) have been investigated by the Rietveld analysis of their neutron powder diffraction patterns (λ=1.470 Å). These compounds belong to the Aurivillius phase family and are built up by (Bi2O2)2+ fluorite layers and (Am-1BmO3m+1)2- (m=2-4) pseudo-perovskite slabs. Bi2.5Na0.5Ta2O9 (m=2) and Bi2.5Na2.5Nb4O15 (m=4) crystallize in the orthorhombic space group A21am, Z=4, with lattice constants of a=5.4763(4), b=5.4478(4), c=24.9710 (15) and a=5.5095(5), b=5.4783(5), c=40.553(3) Å, respectively. Bi2.5Na1.5Nb3O12 (m=3) has been refined in the orthorhombic space group B2cb, Z=4, with the unit-cell parameters a=5.5024(7), b=5.4622(7), and c=32.735(4) Å. In comparison with its isostructural Nb analogue, the structure of Bi2.5Na0.5Ta2O9 is less distorted and bond valence sum calculations indicate that the Ta-O bonds are somewhat stronger than the Nb-O bonds. The cell parameters a and b increase with increasing m for the compounds Bi2.5Nam-1.5NbmO3m+3 (m=2-4), causing a greater strain in the structure. Electron microscopy studies verify that the intergrowth of mixed perovskite layers, caused by stacking faults, also increases with increasing m. 相似文献
42.
Jimmy Weterings Cristianne J. F. Rijcken Harald Veldhuis Tommi Meulemans Darya Hadavi Matt Timmers Maarten Honing Hans Ippel Rob M. J. Liskamp 《Chemical science》2020,11(33):9011
We describe the development of TMTH-SulfoxImine (TMTHSI) as a superior click reagent. This reagent combines a great reactivity, with small size and low hydrophobicity and compares outstandingly with existing click reagents. TMTHSI can be conveniently functionalized with a variety of linkers allowing attachment of a diversity of small molecules and (peptide, nucleic acid) biologics.TMTHSI was developed as new reagent for strain-promoted azide–alkyne cycloaddition reactions, enabling connection of a diversity of small to large molecular constructs. 相似文献
43.
M. S. Johansen P. Alstrom J. Borg M. T. Levinsen 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,11(4):665-676
Significant statistical bias in LDA measurements and how to adequately deal with it is a subtle problem when dealing with
turbulent flows. In order to attempt a clarification we have performed measurements on a non-standard “grid experiment” where
a clear bias effect is found. We have investigated the effect of several corrective measures and find that best results, in
the sense of having the first moment converge to zero, are obtained when using the time between events as statistical weights.
The corrected time series have been used to check for extended self-similarity (ESS). Even though no scaling regime is seen
for the third moment and the flow certainly is neither isotropic nor homogeneous, perfect ESS scaling based on the absolute
third moment is observed up to the twelfth moment, extending into a time domain regime where the Taylor hypothesis of frozen
turbulence is obviously violated. Reversing the argument this indicates that the correction scheme needed can be experimentally
decided on using the criterion stated above and especially so if ESS is to be expected. Finally we have used the corrected
data to quantify the deviations from Gaussian behavior of the velocity difference probability density function for a weakly
turbulent flow. Through comparison with results on the Gaussian-Lorentzian distribution we find that the even part of the
experimental distribution can be reproduced quite well by a single-parameter family of distributions with second moment equal
unity.
Received 5 August 1998 and Received in final form 21 December 1998 相似文献
44.
Constitutive models for the linear viscoelasticity of polymers are presented for the relation between the relaxation modulus and the molecular weight distribution (MWD). We also compute the MWD from a simulated relaxation modulus curve by first obtaining the rheologically effective distribution (RED) as a function of time, and converting this into the MWD by melt calibration; that is, the relation between timescale and the molecular weight. This procedure has similarities with the widely used universal calibration with solved polymers. The main principles of our technique are applied here to familiar relaxation modulus data, for which we present two models: (1) an analytical model derived from control theory, which is known capable of modelling partially observed system and (2) a practical characteristic model for obtaining usable results. No relaxation time or spectrum procedures are used to model the process of linear viscoelastic relaxation. The use of relative calculations and melt calibration dispenses with the need to know the real chain structures such as branching and entangled chain dynamics, and the model remains useful for future investigations of polymer chain structures and dynamics, such as using tube theory. 相似文献
45.
The first atomistic simulations of the CO-induced lifting of the Pt(100)-hex reconstruction have been performed. During this phase transformation the surface changes back to bulk-terminated Pt(100)-(1 x 1), whereby the surface atom density decreases by approximately 20%. The simulations reveal a mechanism collective in nature, indicating that restructuring proceeds through ejection of chains of Pt atoms. These chains explain the anisotropy as seen in scanning tunneling microscopy experiments. The restructuring rate depends nonlinearly on the CO coverage, but the absence of local clustering of CO excludes an explanation in terms of elementary reaction kinetics as proposed previously. 相似文献
46.
Wang JG Li WX Borg M Gustafson J Mikkelsen A Pedersen TM Lundgren E Weissenrieder J Klikovits J Schmid M Hammer B Andersen JN 《Physical review letters》2005,95(25):256102
Using core-level spectroscopy and density functional theory we show that a one-dimensional (1D) oxide structure forms at the steps of the Pt(332) surface after exposure. The 1D oxide is found to be stable in an oxygen pressure range, where bulk oxides are only metastable, and is therefore argued to be a precursor to the Pt oxidation. As an example of the consequences of such a precursor exclusively present at the steps, we investigate the reaction of CO with oxygen covered Pt(332). Albeit more strongly bound, the oxidic oxygen is found to react more easily with CO than oxygen chemisorbed on the Pt terraces. 相似文献
47.
The cross sections for excitation of the members of the ground state (g.s.) band 0?(g.s.), 2+ (1.37 MeV) and 4+ (4.12 MeV) and the γ-band 2+γ (4.24 MeV), 3γ+(5.24 MeV) and 4γ+(6.01 MeV) in 24Mg have been measured in inelastic α-scattering at Eα = 120 MeV. The excitation of these states are found to be well described by a coupled-channel calculation (CCBA) performed in the framework of the asymmetric rotational model. Two sets of parameters are found to give excellent fits to the data, but in both a direct coupling between the ground state and the 4+ state is found necessary. 相似文献
48.
49.
Using the OSIRIS on-line isotope separator facility, the decays of 130Sn and 130, 132Sb have been studied. On the basis of singles γ and γ-γ coincidence Ge(Li) spectra and conversion electron Si(Li) measurements, level schemes for 130Sb, 130Te and 132Te have been constructed. The corresponding half-lives have been measured using multiscaling technique. The 3.8 min ground state of 130Sn populates only positive parity states in the πν?3 nucleus 130Sb: the energetically lowest 5+ state with the configuration assignment; the state at 70.0 keV; the 2+ state at 262.5 keV; the (0, 1)+ state at 697.2 keV; the 3+ state at 813.1 keV and the 1+ state at 1042.3 keV excitation energy. A 1.7 min isomeric state in 130Sn, with the tentative spin assignment (7?), populates several odd parity levels in 130Sb. These arise from the and/or configurations and are located 84.7 keV (6?), 144.9 keV (7?), 688.5 keV and 1044.0 keV above the 40 min 8? β- decaying state. No transitions between odd and even parity states have been observed.The most important excited states in 130Te found in the β? decay of the 6.6 min 130Sb 5+ state are: 839.4 keV, 2+; 1632.8 keV, 4+; 1815.1 keV, 6+; 2100.8 keV, 5?.Levels in the π2ν?2 nucleus 132Te were observed in the β? decays of the 2.8 min 132Sb (4+) and the 4.2 min 132Sb (8?) states. Unique spin and parity assignments have been given to the following states: 973.9 keV, 2+; 1670.7 keV, 4+; 1774.1 keV, 6+; 1924.7 keV, 7?; 2053.0 keV, 5?. 相似文献
50.
D. Denegri Y. Pons R. Barloutaud A. Borg C. Louedec M. Spiro K. Paler T.P. Shah S.N. Tovey B. Chaurand B. Drevillon G. Labrosse R. Lestienne R.A. Salmeron 《Nuclear Physics B》1975,91(1):54-68
We have studied the reaction K-p → K-π+π-π+π-p at 14.3 GeV/c to search for evidence of the double dissociation process . In the channel π1-π2-Δ++ (1236) there is evidence for simultaneous production of low-mass enhancements in the and Δ++(1236)π2- subsystems which correspond to the and decay modes. In this particular final state the double fragmentation system is produced with a cross section of the order of a few microbarns. Our data are consistent with the factorizable pomeron exchange model of double diffractive dissociation. 相似文献