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981.
The formation of ions following the termination of power in a pulsed glow discharge ion source is investigated. The populations of ionized species containing sputtered atoms M+, M 2 1 :, and MAr+ are observed to maximize after the termination of discharge power. Collisions involving sputtered atoms and metastable argon atoms, Penning and associative ionization, are considered to be responsible for the formation of ions in the discharge afterpeak time regime. The domination of these ion formation processes during the afterpeak time regime is supported by the results from investigations of discharge operating parameters, metastable argon atom quenching, and ion kinetic energy distributions. 相似文献
982.
Steven C. Beu Michael W. Senko John P. Quinn Fred W. McLafferty 《Journal of the American Society for Mass Spectrometry》1993,4(2):190-192
Initial results from a Fourier-transform mass spectrometer with a 6.2 Tesla magnet using electrospray ionization show substantial improvements in resolution, mass accuracy, mass range, signal/noise, and tandem mass spectromehy capabilities compared to our earlier 2.8 T instrument that demonstrated the first unit resolution mass spectra of molecules as large as myoglobin (17 kDa). The new instrument exhibits greater than 106 and 105 resolving power for 8.6 and 29 kDa, respectively, proteins. Using an internal standard, the mass measuring error for myoglobin is less than 1 ppm. Nozzle-skimmer dissociation during electrospray of carbonic anhydrase (29 kDa) has yielded 38 fragment ions for which both mass and charge are identifiable; of these, 21 have been assigned to expected oligopeptide fragments. 相似文献
983.
Aaron Balog Christina Harris Kenneth Savin Xiu-Guo Zhang Ting-Chao Chou Samuel J. Danishefsky 《Angewandte Chemie (International ed. in English)》1998,37(19):2675-2678
The stabilization of the transition state through a favorable interaction between the double bond of 1 and the carbonyl group of 2 appears to be responsible for the high diastereoface selectivity of the aldol reaction. This key step in the highly concise total synthesis of epothilone B is followed by a Suzuki coupling to introduce the thiazole domain, a Noyori reduction to control the stereochemistry at C3, and a final macrolactonization (see reaction scheme). X=protecting group. 相似文献
984.
Marcus Frey Steven G. Harris Jeremy M. Holmes David A. Nation Simon Parsons Peter A. Tasker Simon J. Teat Richard E. P. Winpenny 《Angewandte Chemie (International ed. in English)》1998,37(23):3245-3248
Simple organic molecules can have many functions. The active ingredient in the corrosion inhibitor 3-(4-methylbenzoyl)propionate works because it addresses the metal sites of a surface through carboxylate groups, forms hydrogen bonds with surface hydroxide groups (see picture), and provides excellent surface coverage through efficient packing of substituted aromatic groups. 相似文献
985.
Hiroshi Saito Xiangdong Wu J. Milton Harris Allan S. Hoffman 《Macromolecular rapid communications》1997,18(7):547-550
Graft copolymers of poly(ethylene glycol) (PEG) on a chitosan backbone (PEG-g-chitosan) have been synthesized and their aqueous solution properties were investigated. At pH 6.5 the graft copolymers are 100% soluble, while chitosan phase separates from solution at those conditions. These interesting graft copolymers may be especially suitable as carriers for delivery of anionic drugs, such as proteins, glycosaminoglycans, and DNA plasmids or oligonucleotides. 相似文献
986.
987.
Results are reported of an unsteady Reynolds‐averaged Navier–Stokes (RANS) method for simulation of the boundary layer and wake and wave field for a surface ship advancing in regular head waves, but restrained from body motions. Second‐order finite differences are used for both spatial and temporal discretization and a Poisson equation projection method is used for velocity–pressure coupling. The exact kinematic free‐surface boundary condition is solved for the free‐surface elevation using a body‐fitted/free‐surface conforming grid updated in each time step. The simulations are for the model problem of a Wigley hull advancing in calm water and in regular head waves. Verification and validation procedures are followed, which include careful consideration of both simulation and experimental uncertainties. The steady flow results are comparable to other steady RANS methods in predicting resistance, boundary layer and wake, and free‐surface effects. The unsteady flow results cover a wide range of Froude number, wavelength, and amplitude for which first harmonic amplitude and phase force and moment experimental data are available for validation along with frequency domain, linear potential flow results for comparisons. The present results, which include the effects of turbulent flow and non‐linear interactions, are in good agreement with the data and overall show better capability than the potential flow results. The physics of the unsteady boundary layer and wake and wave field response are explained with regard to frequency of encounter and seakeeping theory. The results of the present study suggest applicability for additional complexities such as practical ship geometry, ship motion, and maneuvering in arbitrary ambient waves. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献
988.
989.
L. C. Pan und D. Harris 《Fresenius' Journal of Analytical Chemistry》1933,95(7-8):291-292
Ohne Zusammenfassung 相似文献
990.
Carbon-13 CP/MAS spectra have been obtained for seven polymorphic and solvated forms of cortisone acetate. For signals in the high-frequency region, the spinning sideband manifolds of the spectra recorded under slow-spinning conditions have been analysed to obtain the shielding tensor components for the five resonances at the highest frequency. These show characteristic values for given types of carbon and have enabled assignments of the C5 and C22 signals to be made with some certainty, providing firm evidence of cross-over between the shifts for these carbons between different polymorphs. Assignments are suggested more tentatively for the resonances from C3, C11 and C20. Comparison of chemical shifts for those forms with published X-ray structures enables conclusions to be drawn regarding hydrogen bonding in the remaining forms. Hydrogen bonding induces a high-frequency change in the isotropic chemical shift of approximately 3 ppm. 相似文献