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101.
Paul T. Allen Lars Andersson Alvaro Restuccia 《Communications in Mathematical Physics》2011,301(2):383-410
In this paper we consider the classical initial value problem for the bosonic membrane in light cone gauge. A Hamiltonian
reduction gives a system with one constraint, the area preserving constraint. The Hamiltonian evolution equations corresponding
to this system, however, fail to be hyperbolic. Making use of the area preserving constraint, an equivalent system of evolution
equations is found, which is hyperbolic and has a well-posed initial value problem. We are thus able to solve the initial
value problem for the Hamiltonian evolution equations by means of this equivalent system. We furthermore obtain a blowup criterion
for the membrane evolution equations, and show, making use of the constraint, that one may achieve improved regularity estimates. 相似文献
102.
We show that H/D exchange between H(2)O and D(2)O in ultrathin ice films adsorbed on Cu(100) does not occur through autoionization at temperatures below 140 K. The exchange is, however, facile if a proton deficiency is induced in the ice films by having small amounts of OH preadsorbed on the copper surface. The system was studied using surface infrared vibrational spectroscopy with the aid of density functional theory calculations. 相似文献
103.
104.
Rahul B. Kawthekar Sai Kumar Chakka Vivian Francis Pher G. Andersson Hendrik G. Kruger Glenn E.M. Maguire Thavendran Govender 《Tetrahedron: Asymmetry》2010,21(7):846-852
A novel family of eleven new tetrahydroisoquinoline (TIQ)–oxazoline intermediates and five corresponding copper(II) catalysts has been developed and applied to the catalytic asymmetric Henry reaction of various aldehydes with nitromethane to provide β-hydroxy nitroalkanes in high conversion (>99%). This paper describes the synthesis of the TIQ compounds from l-dihydroxyphenylalanine (l-DOPA) as the starting material. The chiral ligands were complexed in situ with various transition metals such as Cu, Sc, Co, Zn, Ni and Mn and tested as a chiral catalyst for the Henry reaction. The reaction was optimized in terms of the metal, counter ion, solvent, temperature and over a range of substrates. The corresponding catalyst with copper(II) acetate and 2-propanol as the solvent provides the best enantioselectivities (up to 77% ee) of the corresponding nitroalcohol for 4-chlorobenzaldehyde. 相似文献
105.
Henry Holmstrand Maria Unger Daniel Carrizo Per Andersson Örjan Gustafsson 《Rapid communications in mass spectrometry : RCM》2010,24(14):2135-2142
The bromine isotope composition is potentially diagnostic in both degradation monitoring and source apportionment of organobromines in the environment. A method for compound‐specific bromine isotope analysis (δ81Br) based on gas chromatography multiple collector inductively coupled plasma mass spectrometry (GC/ICPMS) was developed for common brominated diaromatic compounds. Brominated diphenyl ethers (BDEs) in Bromkal 70‐5DE, a technical flame‐retardant mixture containing mainly BDEs #47, #99 and #100, were used as test substances, with standard bracketing for the samples achieved through co‐injected monobromobenzene (MBB) with a known δ81Br of ?0.39‰ vs. Standard Mean Ocean Bromine (SMOBr). Three different heated transfer lines were constructed and tested to achieve efficient conduction of the BDEs from the gas chromatograph to the ICPMS instrument. The MBB was analyzed with a precision of 0.4‰ (1 s, n = 18). The precision for BDEs was 1.4–1.8‰ (1 s, n = 10–12 depending on the congener). The lower precision for the BDEs than for MBB may reflect the heat required to prevent condensation of the analytes in ICP torch assembly. The use of an internal standard of similar chemical structure to the analytes alleviates this problem, as illustrated by a difference of 0.3 ± 0.7‰ (1 s, n = 6) between the δ81Br values of co‐injected methoxy BDE‐47 and BDE‐47 extracted from whale blubber. Improvements in precision and accuracy may be achieved by the use of a more efficient heating of the torch assembly in conjunction with a set of internal standards that match the target compounds. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
106.
Byron K. Peters Sai Kumar Chakka Tricia Naicker Glenn E.M. Maguire Hendrik G. Kruger Pher G. Andersson Thavendran Govender 《Tetrahedron: Asymmetry》2010,21(6):679-687
The use of the tetrahydroisoquinoline scaffold is well documented in biologically active compounds. However, reports of the utilisation of tetrahydroisoquinoline compounds in asymmetric catalysis are limited. The synthesis of novel diamine ligands possessing the tetrahydroisoquinoline (tetrahydroisoquinoline) backbone and evaluation of their activity in the asymmetric transfer hydrogenation of acetophenone are presented. The diamine ligands in conjunction with i-PrOH as the hydrogen source and [RhCl2(Cp1)]2 as the metal precursor proved to be the most effective of the tetrahydroisoquinoline derivatives for this catalytic system. Water was found to have a profound influence on the enantioselectivity of the reaction. Optimisation of the amount water, i-PrOH and catalytic loading content rendered the best result of 70% enantioselectivity for the (S)-1-phenylethanol isomer product. 相似文献
107.
Summary The efficiency of the extraction methods with acetone and with ethyl acetate are compared with regard to the multi-pesticide residue determination in fruits and vegetables. The ethyl acetate-method gave higher recoveries for polar pesticides and seems somewhat easier, quicker and cheaper in handling but some co-extractives in the GC-extract are observed. In general, both methods gave acceptable and equivalent recoveries for the pesticides tested. 相似文献
108.
A. Andersson 《Journal of sound and vibration》2003,260(2):195-212
A method to determine the dynamic load between two rotating elastic helical gears is presented. The stiffness of the gear teeth is calculated using the finite element method and includes the contribution from the elliptic distributed tooth load. To make sure that the new incoming contacts which are the main excitation source are properly simulated, the necessary deformation of the gears is determined by using the true geometry and positions of the gears for every time step of the dynamic calculation. This allows the contact to be positioned outside the plane of action. A numerical example is presented with figures that show the behaviour of the dynamic transmission error as well as the variation of the contact pressure due to the dynamic load for different rotational speeds. 相似文献
109.
Numerical simulation and laser-based imaging of mixture formation, ignition, and soot formation in a diesel spray 总被引:2,自引:0,他引:2
S. Vogel C. Hasse J. Gronki S. Andersson N. Peters J. Wolfrum C. Schulz 《Proceedings of the Combustion Institute》2005,30(2):2029-2036
Laser-based imaging of fuel vapor distribution, ignition, and soot formation in diesel sprays was carried out in a high-pressure, high-temperature spray chamber under conditions that correspond to temperature and pressure in a diesel engine. Rayleigh scattering and laser-induced incandescence are used to image fuel density and soot volume fraction. The experimental results provide data for comparison with numerical simulations. An interactive cross-sectionally averaged spray model based on Eulerian transport equations was used for the simulation of the spray, and the turbulence-chemistry interaction was modeled with the representative interactive flamelet (RIF) concept. The flamelet calculation is coupled to the Kiva3V computational fluid dynamics (CFD) code using the scalar dissipation rate and pressure as an input to the RIF-code. The flamelet code computes the instationary flamelet profiles for every time step. These profiles were integrated over mixture fraction space using a prescribed β-PDF to obtain mean values, which are passed back to the CFD-code. Thereby, the temperature and the relevant species in each CFD-cell were obtained. The fuel distribution, the average ignition delay as well as the location of ignition are well predicted by the simulation. Furthermore, simulations show that the experimentally observed injection-to-injection variations in ignition delay are due to temperature inhomogeneities. Experimental and simulated spatial soot and fuel vapor density distributions are compared during and after second stage ignition. 相似文献
110.
2 +? O2→H2O on polycrystalline Pt foils has been studied by detection of desorbing OH radicals using the Resonance Enhanced Multiphoton
Ionization, REMPI, technique. The measurements were performed at catalyst temperatures of 1000–1400 K and a total pressure
below 10-4 mbar. The studies of OH desorption by REMPI were achieved using a two-photon excitation D2Σ-–X2Π (1–0), followed by one-photon ionization. The ions were detected in a time-of-flight mass spectrometer, TOF-MS, in order
to avoid interference from non-resonantly ionized molecules. By applying TOF-MS, a simultaneous non-resonant ionization and
detection of H2, O2 and H2O was achieved. Recorded REMPI spectra were compared with spectra simulated using known molecular constants. The kinetics
of the reaction derived from the measurements were compared with what was obtained in earlier LIF detection of OH, performed
at higher total reactant pressure using the A–X transition. REMPI TOF-MS is shown to be a complement to LIF for reaction studies
below 5×10-4 mbar total pressure, where LIF is too insensitive for quantification. The reaction kinetics was found to be in agreement
with a theoretic model and previous LIF studies.
Received: 8 March 1996/Revised version: 4 October 1996 相似文献