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21.
S. Pandey K. A. Fletcher W. E. Acree Jr C. Fetzer 《Fresenius' Journal of Analytical Chemistry》1998,360(6):669-674
Applicability of the nitromethane selective quenching rule for discriminating between alternant versus nonalternant polycyclic
aromatic hydrocarbons (PAHs) is examined for 20 representative PAH solutes dissolved in micellar sodium dodecylsulfate (SDS)
+ cetyltrimethylammonium bromide (CTAB), SDS + dodecyltrimethylammonium bromide (DTAB), SDS + Brij-35, and SDS + sodium octanoate
(SO) mixed surfactant solvent media. Experimental results show that nitromethane quenched fluorescence of all 8 alternant
PAHs studied in the four different solvent systems. Unexpected quenching behavior was observed, however, in the case of nonalternant
PAHs. Nitromethane quenched fluorescence emission of nonalternant PAHs dissolved in the SDS + SO solvent media, which is contrary
to the selective quenching rule. In the case of the mixed anionic + cationic surfactant solvent media, nitromethane quenching
selectivity was restored at concentration ratios of approximately 4 : 1 (anionic:cationic) or less.
Received: 22 May 1997 / Revised: 29 September 1997 / Accepted: 3 October 1997 相似文献
22.
A. N. Fletcher 《Applied physics. B, Lasers and optics》1985,37(1):31-34
Reducing the diameter of the flashlamp was found to increase the percent improvement in output of a solid-state laser caused by using luminescent coolants. However, this change in diameter also reduced the effectiveness of the flashlamp in producing long (150 s) laser pulses so that an overall decrease in laser output was observed. In the formation of short (40 s) laser pulses, on the other hand, larger absolute values were obtained using the smaller diameter flashlamp. 相似文献
23.
L. A. Currie J. D. Kessler R. A. Fletcher J. E. Dibb 《Journal of Radioanalytical and Nuclear Chemistry》2005,263(2):399-411
Summary Results are given from the NIST component of a pilot (“winter-over”) study of seasonal patterns of natural and anthropogenic species in air and snow transported to Summit, Greenland. Central to this research is the quantitative apportionment of fossil and biomass particulate carbon, based on advanced (micromolar) 14C accelerator mass spectrometry (AMS) applied to remote snow samples containing as little as 9 µg C/kg. The measurements were made practicable through stringent attention to the nature and sources of the isotopic-chemical blank, resulting in a blank reduction from »5 µg C to <0.5 µg C. An important result of this work is the first evidence of a seasonal pattern in biomass-C particles in Greenland snow. Although 14C AMS data serve to resolve fossil and biomass carbon quantitatively, a deeper understanding of the aerosol sources and character demands a multidisciplinary approach. This is illustrated with “multi-spectrometric” macro- and micro-analytical data for two cases involving substantial incursions of biomass aerosol to the Summit, Greenland snow. 相似文献
24.
W. Russell Bowman Anthony J. Fletcher Peter J. Lovell Elena Hernández López Graeme B.S. Potts 《Tetrahedron》2007,63(1):191-203
Amides have been successfully used as precursors of imidoyl radicals for radical cyclisation. The amides have been converted to imidoyl selanides via reaction with phosgene to yield imidoyl chlorides followed by reaction with potassium phenylselanide. Imidoyl selanides were reacted with tributyltin hydride (Bu3SnH) as the radical mediator with triethylborane or AIBN as initiators to yield imidoyl radicals for cyclisation reactions. Imidoyl radicals have been cyclised onto alkenes to yield 2,3-substituted-indoles and -quinolines and also onto pyrroles and indoles to give bi- and tricyclic heteroarenes. 相似文献
25.
Tam Nga Chiu Fletcher Stephen P. Vogels Christopher M. Westcott Stephen A. Decken Andreas 《Transition Metal Chemistry》2003,28(1):103-109
Dioxomolybdenum(VI) complexes derived from ethyl maltol (2-ethyl-3-hydroxy-4-pyrone) and 1-alkyl-2-ethyl-3-hydroxy-4-pyridin-4(1H)-ones have been prepared and characterized using physical methods including 1H- and 13C-n.m.r. spectroscopy, i.r., elemental analysis, and X-ray diffraction for the pyrone and 3-methylpyridine pyridinone derivatives. The octahedral complexes have the general formula cis-MoO2L2 where L = the deprotonated pyrone or pyridinone ligand. 相似文献
26.
Fletcher MT Wood BJ Brereton IM Stok JE De Voss JJ Kitching W 《Journal of the American Chemical Society》2002,124(26):7666-7667
The origins of the oxygen atoms in 1,7-dioxaspiro[5.5]undecane (1) and hydroxyspiroacetal (2) from Bactrocera cacuminata, and in 2,8-dimethyl-1,7-dioxaspiro[5.5]undecane (3) and hydroxyspiroacetal (4) from B. cucumis, have been investigated by incorporation studies from both [(18)O(2)]-dioxygen and [(18)O]-water. Combined GC-MS examination and high-field NMR analysis have demonstrated that all oxygen atoms in 1 and 2 from B. cacuminata are dioxygen derived, but in contrast, the spiroacetals 3 and 4 from B. cucumis incorporate one ring oxygen from water and one ring oxygen (and the hydroxyl oxygen in 4) from [(18)O(2)]-dioxygen. These results reveal not only the generality of monoxygenase mediation of spiroacetal formation in Bactrocera sp., but also an unexpected complexity in their biosynthesis. A general paradigm accommodating these and other observations is presented. 相似文献
27.
Tarus J Agbaria RA Morris K Mwongela S Numan A Simuli L Fletcher KA Warner IM 《Langmuir : the ACS journal of surfaces and colloids》2004,20(16):6887-6895
Poly(sodium undecenoyl-L-leucinate) (poly-L-SUL) was fractionated by the use of different molecular weight cutoff (MWCO) filters to narrow the polydispersity of the macromolecular sizes of the polymeric surfactant. The resulting polymeric surfactant fractions were characterized by the use of three techniques: (1) pulsed field gradient nuclear magnetic resonance (PFG-NMR) was used to determine the hydrodynamic radii, (2) analytical ultracentrifugation (AUC) was used to determine the molecular weights, and (3) steady-state fluorescence was used to determine the polarity of the nonfractionated and fractionated polymeric surfactants. From the data acquired from PFG-NMR, AUC, and fluorescence, it was noted that the hydrodynamic radii and molecular weight of the fractionated poly-L-SUL increased, while the polarity decreased with the increase in the size of the MWCO filter. However, a similarity in physical properties was observed between the nonfractionated and 10-30K fractionated poly-L-SUL except for the hydrodynamic radius and diffusion coefficients. The influence of different macromolecular sizes of poly-L-SUL on the chiral separation of phenylthiohydantion (PTH)-amino acids and coumarinic derivatives, as test analytes, was elucidated by the use of micellar electrokinetic chromatography (MEKC). The size of polymeric surfactants as a prerequisite for chiral separation was demonstrated by comparing the separation properties of fractionated versus nonfractionated polymeric surfactants. Fractionated poly-L-SUL resulted in enhanced resolution and separation efficiency of the test analytes as compared to the case of the nonfractionated poly-L-SUL. This observation indicates that minimizing polydispersity of polymeric surfactants may be important for some chiral separation applications. 相似文献
28.
Pulse radiolysis of solutions of alkali metals in methylamine and ethylamine shows the formation of three distinct species; the solvated electron e?s, the alkali metal anion M? and a species considered to be a metal—electron pair with stoichiometry M. Hitherto no kinetic or conclusive optical evidence has been obtained for the species M in amine solutions. The three species coexist in equilibrium according to the equation c?s + M+ ? M + e? ? M? with the corresponding rate constants markedly dependent on the solvent and nature of the alkali metal. 相似文献
29.
Premeiotic and meiotic whole testes from grasshoppers were compared for the presence of meiosis associated proteins using one- and two-dimensional sodium dodecyl sulphate-polyacrylamide gel electrophoresis. One-dimensional sodium dodecyl sulphate-polyacrylamide gels detected differences between premeiotic and meiotic samples but two-dimensional gels gave more precise results. Isoelectric focusing revealed only one meiosis-associated protein, while nonequilibrium pH gradient electrophoresis detected five more. It is not known whether these proteins relate to the nuclear aspects of meiosis, or associated cellular changes. These proteins have been electrophoretically purified and monoclonal antibodies are being prepared. 相似文献
30.
There is considerable interest in hydrogen adsorption on carbon nanotubes and porous carbons as a method of storage for transport and related energy applications. This investigation has involved a systematic investigation of the role of functional groups and porous structure characteristics in determining the hydrogen adsorption characteristics of porous carbons. Suites of carbons were prepared with a wide range of nitrogen and oxygen contents and types of functional groups to investigate their effect on hydrogen adsorption. The porous structures of the carbons were characterized by nitrogen (77 K) and carbon dioxide (273 K) adsorption methods. Hydrogen adsorption isotherms were studied at 77 K and pressure up to 100 kPa. All the isotherms were Type I in the IUPAC classification scheme. Hydrogen isobars indicated that the adsorption of hydrogen is very temperature dependent with little or no hydrogen adsorption above 195 K. The isosteric enthalpies of adsorption at zero surface coverage were obtained using a virial equation, while the values at various surface coverages were obtained from the van't Hoff isochore. The values were in the range 3.9-5.2 kJ mol(-1) for the carbons studied. The thermodynamics of the adsorption process are discussed in relation to temperature limitations for hydrogen storage applications. The maximum amounts of hydrogen adsorbed correlated with the micropore volume obtained from extrapolation of the Dubinin-Radushkevich equation for carbon dioxide adsorption. Functional groups have a small detrimental effect on hydrogen adsorption, and this is related to decreased adsorbate-adsorbent and increased adsorbate-adsorbate interactions. 相似文献