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61.
62.
Garritt J. Tucker Shreevant Tiwari Jonathan A. Zimmerman David L. McDowell 《Journal of the mechanics and physics of solids》2012,60(3):471-486
Atomistic simulations are employed to investigate the deformation of nanocrystalline copper and the associated strain accommodation mechanisms at 10 K as a function of grain size. Volume-averaged kinematic metrics based on continuum mechanics theory are formulated to analyze the results of molecular dynamics simulations. The metrics rely on both reference and current configurations, along with nearest neighbor lists to estimate nanoscale behavior of atomic deformation fields in nanocrystalline copper. Various deformation mechanisms are activated in the structures, and shown to depend on average grain size of the nanocrystalline structure. Furthermore, grain boundaries, along with dislocation glide, become an important source of strain accommodation as grain size is reduced. It is demonstrated that the metrics capture the contributions of various mechanisms, and provide a sense of the history of atomic regions undergoing both elastic and plastic deformation. The significance of this research is that unique kinematic signatures of the mechanisms are uncovered using certain metrics, and we are able to resolve the contributions of the deformation mechanisms to the overall strain of the structure using Green strain. 相似文献
63.
McDowell NA Knight KS Lightfoot P 《Chemistry (Weinheim an der Bergstrasse, Germany)》2006,12(5):1493-1499
The crystal structure of Aurivillius phase ferroelectric Bi2WO6 has been studied in detail as a function of temperature by using high-resolution powder neutron diffraction. In agreement with an earlier study, a transition from space group P2(1)ab to B2cb occurs at about 660 degrees C. This transition corresponds to the loss of one octahedral tilt mode within the perovskite-like WO4 layer of the structure. A second, reconstructive, phase transition occurs around 960 degrees C, corresponding to the ferroelectric Curie point; in contrast to previous suggestions, the structure of this high-temperature phase contains layers of stoichiometry WO4, with WO6 octahedra sharing edges and corners, and with the fluorite-like Bi2O2 layers remaining essentially unchanged. This structure is closely related to that of the ambient temperature phase of lanthanide-doped derivatives, for example, Bi0.7Yb1.3WO6 recently reported. This phase-transition behaviour is in stark contrast to that of other members of the Aurivillius family, such as SrBi2Ta2O9 and Bi4Ti3O12, which retain the archetypal Aurivillius connectivity at all temperatures. 相似文献
64.
65.
The X-ray photoelectron spectra of some forty-six copper compounds and complexes have been measured. The chemical shifts obtained from accurate determinations of the binding energies have been qualitatively explained on the basis of the Pauling electronegativity concept using the group electronegatives of Huheey for the polyatomic counter anions. The chemical shifts of the copper atoms as well as the atoms in the ligands were found to be dependent not only on the oxidation state but also on the kind and number of ligand atoms. Intense satellite lines were found in the 2p and 2s bands of the cupric compounds; the number and splitting of the satellites were found to be sensitive to the chemical environment. A correlation was found between the satellite splitting and the binding energies and this is explained by a 3d→4s, 4p ‘shake-up’ mechanism. 相似文献
66.
The change in the vibrational anharmonic constant for the linear hydrogen-bonded complexes FHRg, FArHRg, FArHN2, FArHP2 (Rg=Ne, Ar, Kr) was determined at the MP2/6-311++G(3df,3pd) level of theory. It was found that in the vibrationally red-shifted complexes, hydrogen bond formation increases the anharmonic constant while in the vibrationally blue-shifted species (except FArHNe) the anharmonic constant decreases on complexation. 相似文献
67.
Heath H. Powers John E. Hunt David T. Hanson Nate G. McDowell 《Rapid communications in mass spectrometry : RCM》2010,24(3):243-253
High frequency observations of the stable isotopic composition of CO2 effluxes from soil have been sparse due in part to measurement challenges. We have developed an open‐system method that utilizes a flow‐through chamber coupled to a tunable diode laser (TDL) to quantify the rate of soil CO2 efflux and its δ13C and δ18O values (δ13CR and δ18OR, respectively). We tested the method first in the laboratory using an artificial soil test column and then in a semi‐arid woodland. We found that the CO2 efflux rates of 1.2 to 7.3 µmol m?2 s?1 measured by the chamber‐TDL system were similar to measurements made using the chamber and an infrared gas analyzer (IRGA) (R2 = 0.99) and compared well with efflux rates generated from the soil test column (R2 = 0.94). Measured δ13C and δ18O values of CO2 efflux using the chamber‐TDL system at 2 min intervals were not significantly different from source air values across all efflux rates after accounting for diffusive enrichment. Field measurements during drought demonstrated a strong dependency of CO2 efflux and isotopic composition on soil water content. Addition of water to the soil beneath the chamber resulted in average changes of +6.9 µmol m?2 s?1, ?5.0‰, and ?55.0‰ for soil CO2 efflux, δ13CR and δ18OR, respectively. All three variables initiated responses within 2 min of water addition, with peak responses observed within 10 min for isotopes and 20 min for efflux. The observed δ18OR was more enriched than predicted from temperature‐dependent H2O‐CO2 equilibration theory, similar to other recent observations of δ18OR from dry soils (Wingate L, Seibt U, Maseyk K, Ogee J, Almeida P, Yakir D, Pereira JS, Mencuccini M. Global Change Biol. 2008; 14: 2178). The soil chamber coupled with the TDL was found to be an effective method for capturing soil CO2 efflux and its stable isotope composition at high temporal frequency. Published in 2010 by John Wiley & Sons, Ltd. 相似文献
68.
Sean A. C. McDowell 《Journal of computational chemistry》1997,18(13):1664-1667
The change in the electric field at a nucleus in a molecule due to bond stretch is related to the force constant of the stretched bond. The validity of this relationship using approximate wave functions at the SCF and MP2 levels of theory is tested for the diatomic molecules H2, HF, CO, and N2. The effect of basis set variation on H2 is also investigated. © 1997 John Wiley & Sons, Inc. J Comput Chem 18 : 1664–1667, 1997 相似文献
69.
McDowell SA 《The Journal of chemical physics》2004,120(19):9077-9079
Ab initio calculations, using second-order M?ller-Plesset perturbation theory with a 6-311+ +G(2d,2p) basis set, predict the stability of two novel compounds of monofluoroethene, CH2CHF, with the rare-gas atoms Ar and Kr. The dissociation energies to the lowest-energy fragmentation products, CH2CHF + Rg (Rg = Ar,Kr), were computed to be -528 and -449 kJ mol(-1), respectively, at the coupled-cluster singles, doubles, and triples level of theory. Possible transition states (at second-order M?ller-Plesset theory) via a C-Rg-F bending mode for these fragmentation reactions were also located with barrier heights of about 76 and 106 kJ mol(-1), for the Ar- and Kr-containing species, respectively. However, the Ar-containing species may not exist at all as it is less stable than the fragments CH2CH + F + Ar at the higher level of theory and may possibly dissociate via this route. 相似文献
70.
The Hel photoelectron spectrum of the transient species S2O has been measured and the experimentally determined ionization potentials are compared with the results of CNDO/2 molecular orbital calculations using Koopmans' theorem. The assignments are compared with those previously given for the series of related molecules NSF, NSCI, and SO2 by means of a correlation diagram. 相似文献