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951.
Woon‐Seop Choi Michael G. Mikhael Anne B. Padias H. K. Hall 《Journal of polymer science. Part A, Polymer chemistry》1999,37(11):1709-1716
The spontaneous copolymerization of 4‐vinylpyridine (4‐VP) activated with lithium perchlorate (LiClO4) with various electron rich monomers (p‐methoxystyrene, MeOSt; p‐methylstyrene, MeSt; styrene, St) was investigated in various solvent systems at 75°C. Increasing the LiClO4 concentration and the nucleophilicity of the electron rich monomer increased the copolymer yields. Both 1H‐NMR and elemental analysis confirmed the almost 1:1 copolymer structure for VP/MeOSt system which possessed high molecular weight and narrow polydispersity (PDI). Compared to 4‐VP activated with zinc chloride, LiClO4 systems showed slightly lower yields and much narrower PDI. We also investigated the spontaneous copolymerization of 4‐VP activated with various protic acids in the reaction with various electron rich comonomers. However, generally protic salt forms showed less solubility in organic solvents and showed low molecular weight polymer products with low yields. The proposed initiation mechanism exhibits the formation of a σ‐bond between the β‐carbons of the two donor‐acceptor monomers, creating the 1,4‐tetramethylene biradical intermediate initiating the copolymerization. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1709–1716, 1999 相似文献
952.
953.
Mark Eashoo Leonard J. Buckley Anne K. St. Clair 《Journal of Polymer Science.Polymer Physics》1997,35(1):173-185
Control of the internal morphology of wet-spun fibers from a fluorinated polyimide has been achieved by varying the rate of polymer coagulation through adjustments in nonsolvent/solvent miscibility and precipitation strength of the coagulation bath. Filament internal morphologies ranged from very porous or sponge-like to fully solid. Intermediate structures included fibers containing a spongy core with a nonporous skin, sponge-like fibers containing large voids, and a relatively solid material containing randomly spaced small voids. The cross-sectional shape of the fiber is dependent upon the coagulation process as well as the volume contraction of the initial extrudate. Drawn fibers (3×) retained the original asspun cross-sectional shape and also lost porosity. Mechanical properties of poly(6FDA-4BDAF) fibers have an inverse relationship to filament porosity. Maximum modulus and break strength for drawn fibers is approximately 6 CPa and 200 MPa, respectively. Asspun mechanical properties were dependent upon the processing conditions and have moduli between 0.4–3.0 Gpa and break strengths of 10–160 MPa. A dielectric constant of 2.50 for nonporous films was measured over a frequency range between 1.0 MHz to 1.8 GHz, showing little dispersion. © 1997 John Wiley & Sons, Inc. 相似文献
954.
Bndicte Picquet Sbastien Heroux Abderraouf Chebbi Jean-Franois Doussin Rgine Durand-Jolibois Anne Monod Hlne Loirat Patrick Carlier 《国际化学动力学杂志》1998,30(11):839-847
Some relative rate experiments have been carried out at room temperature and at atmospheric pressure. This concerns the OH-oxidation of some oxygenated volatile organic compounds including methanol (k1), ethanol (k2), MTBE (k3), ethyl acetate (k4), n-propyl acetate (k5), isopropyl acetate (k6), n-butyl acetate (k7), isobutyl acetate (k8), and t-butyl acetate (k9). The experiments were performed in a Teflon-film bag smog chamber. The rate constants obtained are (in cm3 molecule−1 s−1): k1=(0.90±0.08)×10−12; k2=(3.88±0.11)×10−12; k3=(2.98±0.06)×10−12; k4=(1.73±0.20)×10−12; k5=(3.56±0.15)×10−12; k6=(3.97±0.18)×10−12; k7=(5.78±0.15)×10−12; k8=(6.77±0.30)×10−12; and k9=(0.56±0.11)×10−12. The agreement between the obtained rate constants and some previously published data has allowed for most of the studied compounds to point out a coherent group of values and to suggest recommended values. Atmospheric implications are also discussed. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 839–847, 1998 相似文献
955.
956.
Mary E. Rampey Carrie E. Halkyard Angela R. Williams April J. Angel Douglas R. Hurst Jessica D. Townsend Anne E. Finefrock Charles F. Beam Shannon L. Studer-Martinez 《Photochemistry and photobiology》1999,70(2):176-183
Abstract. The carbomethoxyhydrazone of 2'-hydroxyacetophenone was trilithiated with excess lithium diisopropylamide and C-acylated with a variety of benzoate esters followed by acid cyclization of the intermediates to 2-(5-aryl-l-car-bomethoxy-lHpyrazol-3-yl)phenols [3-(2-hydroxyphen-yl)-lH-pyrazoles]. The products were characterized by Fourier transform-IR, l H NMR, 13 C NMR, UV-visible absorption and fluorescence. All the derivatives in n -heptane have an absorption maximum at ˜304 nm and an extremely weak (φr = 10 4 ) fluorescence with maxima in the range of 335–460 nm. The broad range of fluorescence maxima and fluorescence quantum yields is attributed to varying contributions of charge transfer that are dependent on both the identity of the substituent and solvent polarity. A phenomenally large Stokes-shifted fluorescence maximum at 620 nm was observed for 2-(l-car-bomethoxy-5-[4-dimethylaminophenyl]-lff-pyrazol-3-y1)phenol in n-heptane and attributed to excited-state intramolecular proton transfer. As a result, competitive excited-state proton/charge transfer properties have been observed in the pyrazoles studied, of which the spectral properties can be fine tuned by substituent as well as solvent effects. 相似文献
957.
Anne M. Sauer Frank R. Fronczek Betty C. R. Zhu William E. Crowe Gregg Henderson Roger A. Laine 《Acta Crystallographica. Section C, Structural Chemistry》2003,59(5):o254-o256
Nootkatone, or (4R,4aS,6R)‐4,4a,5,6,7,8‐hexahydro‐4,4a‐dimethyl‐6‐(1‐methylethenyl)naphthalen‐2(3H)‐one, C15H22O, a sesquiterpene with strong repellent properties against Formosan subterranean termites and other insects, has the valencene skeleton. The dibromo derivative (1S,3R,4S,4aS,6R,8aR)‐1,3‐dibromo‐6‐isopropyl‐4,4a‐dimethyl‐1,2,3,4,5,6,7,8‐octahydronaphthalen‐2‐one, C15H24Br2O, has two independent molecules in the asymmetric unit, which differ in the rotation of the isopropyl group with respect to the main skeleton. The C—Br distances are in the range 1.950 (4)–1.960 (4) Å. Both independent molecules form zigzag chains, with very short intermolecular carbonyl–carbonyl interactions, having the perpendicular motif and O⋯C distances of 2.886 (6) and 2.898 (6) Å. These chains are flanked by intermolecular Br⋯Br interactions of distances in the range 4.067 (1)–4.218 (1) Å. The absolute configuration of the dibromo derivative was determined, from which that of nootkatone was inferred. 相似文献
958.
959.
960.
Raphaël Tripier Marcel Hollenstein Mourad Elhabiri Anne‐Sophie Chauvin Gaël Zucchi Claude Piguet Jean‐ClaudeG. Bünzli 《Helvetica chimica acta》2002,85(7):1915-1929
Bis(2‐{6‐(diethylcarbamoyl)‐4‐[(4‐isothiocyanatophenyl)ethynyl]pyridin‐2′‐yl}‐1‐ethylbenzimidazol‐5‐yl)methane ( L G ) reacts with trivalent lanthanide ions in acetonitrile to yield triple‐stranded dimetallic helicates [Ln2( L G )3]6+. 1H‐NMR Data point to the helicates being the only species formed under stoichiometric conditions and having a time‐averaged D3 symmetry on the NMR time scale. The photophysical properties of L G and its helicates are discussed with respect to the closely related ligands L B , L E , and their complexes, two ligands devoid of the isothiocyanatophenylethynyl substituent. The quantum yield of the ligand fluorescence is three times smaller compared to L E , while that of the EuIII‐centered luminescence (1.1%) is three times larger. On the other hand, the luminescence of TbIII is not sensitized by L G . This is explained in terms of energy differences between the singlet and triplet states on one hand, and between the 0‐phonon transition of the triplet state and the excited metal ion states on the other. This work demonstrates that bulky substituents in the 4‐position of the pyridine ring do not prevent the formation of triple‐stranded helicates, opening the way for luminescent probes that can easily be coupled to biological materials. 相似文献