全文获取类型
收费全文 | 3788篇 |
免费 | 216篇 |
国内免费 | 12篇 |
专业分类
化学 | 2794篇 |
晶体学 | 8篇 |
力学 | 141篇 |
数学 | 480篇 |
物理学 | 593篇 |
出版年
2023年 | 34篇 |
2022年 | 54篇 |
2021年 | 80篇 |
2020年 | 87篇 |
2019年 | 85篇 |
2018年 | 72篇 |
2017年 | 58篇 |
2016年 | 126篇 |
2015年 | 110篇 |
2014年 | 144篇 |
2013年 | 178篇 |
2012年 | 301篇 |
2011年 | 342篇 |
2010年 | 151篇 |
2009年 | 146篇 |
2008年 | 283篇 |
2007年 | 246篇 |
2006年 | 242篇 |
2005年 | 236篇 |
2004年 | 198篇 |
2003年 | 117篇 |
2002年 | 134篇 |
2001年 | 37篇 |
2000年 | 28篇 |
1999年 | 31篇 |
1998年 | 15篇 |
1997年 | 28篇 |
1996年 | 32篇 |
1995年 | 16篇 |
1994年 | 24篇 |
1993年 | 23篇 |
1992年 | 26篇 |
1990年 | 18篇 |
1989年 | 13篇 |
1988年 | 12篇 |
1987年 | 8篇 |
1986年 | 11篇 |
1985年 | 23篇 |
1984年 | 21篇 |
1983年 | 15篇 |
1982年 | 18篇 |
1981年 | 22篇 |
1980年 | 14篇 |
1979年 | 18篇 |
1978年 | 18篇 |
1977年 | 19篇 |
1976年 | 14篇 |
1975年 | 16篇 |
1974年 | 16篇 |
1973年 | 8篇 |
排序方式: 共有4016条查询结果,搜索用时 15 毫秒
981.
In an effort to standardize the determination of overpotential values for H(2)-evolving catalysts in non-aqueous solvents and allow a reliable comparison of catalysts prepared and assayed by different groups, we propose to adopt the half-wave potential as reference potential. We provide a simple method for measuring it from usual stationary cyclic voltammograms, and we derive the formulas to which the measured potential should be compared, taking into account the effect of homoconjugation. We also revisit tabulated values of the standard reduction potential of protons in nonaqueous solvents, E(H+/H(2))°. 相似文献
982.
Lo CH Liao KS De Guzman M Rouessac V Wei TC Lee KR Lai JY 《Langmuir : the ACS journal of surfaces and colloids》2010,26(22):17470-17476
In this study, Doppler broadening energy spectroscopy (DBES) combined with slow positron beam was used to discuss the effect of substrate types on the fine structure of a plasma-polymerized SiOCH layer as a function of depth. From the SEM pictures, the SiOCH films formed on different substrates showed hemispherical macrostructures, and the deposition rate was dependent on the mean pore size. It appears that the morphology of the plasma-polymerized SiOCH films was associated with the porosity-related characteristics of the substrate such as the size/shape of pores. As deposited on the MCE-022 substrate (mixed cellulose esters membrane with a mean pore size of 0.22 μm) with a nodular structure, the SiOCH films had pillar-like structures and high gas permeabilities. DBES results showed that the SiOCH films deposited on different substrates were composed of three layers: the SiOCH bulk layer, the transition layer, and the substrate. It was observed that the microstructure of the SiOCH films was affected layer by layer; a higher surface pore size in the substrates induced thicker transition layers with higher microporosities and led to thinner bulk layers having higher S parameter values during the plasma polymerization. It was also observed that the change in O(2)/N(2) selectivity was consistent with the DBES analysis results. The gas separation performance and DBES analysis results agreed with each other. 相似文献
983.
984.
Dr. Vincent Jamier Dr. Lalla A. Ba Prof. Dr. Claus Jacob 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(36):10920-10928
Various human diseases, including different types of cancer, are associated with a disturbed intracellular redox balance and oxidative stress (OS). The past decade has witnessed the emergence of redox‐modulating compounds able to utilize such pre‐existing disturbances in the redox state of sick cells for therapeutic advantage. Selenium‐ and tellurium‐based agents turn the oxidizing redox environment present in certain cancer cells into a lethal cocktail of reactive species that push these cells over a critical redox threshold and ultimately kill them through apoptosis. This kind of toxicity is highly selective: normal, healthy cells remain largely unaffected, since changes to their naturally low levels of oxidizing species produce little effect. To further improve selectivity, multifunctional sensor/effector agents are now required that recognize the biochemical signature of OS in target cells. The synthesis of such compounds provides interesting challenges for chemistry in the future. 相似文献
985.
Andrew G. Tennyson Dr. Robert J. Ono Todd W. Hudnall Dr. Dimitri M. Khramov Dr. Joyce A. V. Er Justin W. Kamplain Dr. Vincent M. Lynch Dr. Jonathan L. Sessler Prof. Christopher W. Bielawski Prof. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(1):304-315
Reaction of bromanil with N,N′‐dimesitylformamidine followed by deprotonation with NaN(SiMe3)2 afforded 1,1′,3,3′‐tetramesitylquinobis(imidazolylidene) ( 1 ), a bis(N‐heterocyclic carbene) (NHC) with two NHC moieties connected by a redox active p‐quinone residue, in 72 % yield of isolated compound. Bimetallic complexes of 1 were prepared by coupling to FcN3 ( 2 ) or FcNCS ( 3 ; Fc=ferrocenyl) or coordination to [M(cod)Cl] ( 4 a or 4 b , where M=Rh or Ir, respectively; cod=1,5‐cyclooctadiene). Treatment of 4 a and 4 b with excess CO(g) afforded the corresponding [M(CO)2Cl] complexes 5 a and 5 b , respectively. Analysis of 2 – 5 by NMR spectroscopy and X‐ray diffraction indicated that the electron‐deficient quinone did not significantly affect the inherent spectral properties or coordination chemistry of the flanking imidazolylidene units, as compared to analogous NHCs. Infrared spectroscopy and cyclic voltammetry revealed that decreasing the electron density at MLn afforded an increase in the stretching energy and a decrease in the reduction potential of the quinone, indicative of metal–quinone electronic interaction. Differential pulse voltammetry and chronoamperometry of the metal‐centered oxidations in 2 – 4 revealed two single, one‐electron peaks. Thus, the metal atoms bound to 1 are oxidized at indistinguishable potentials and do not appear electronically coupled. However, the metal–quinone interaction was used to increase the electron density at coordinated metal atoms. Infrared spectroelectrochemistry revealed that the average νCO values for 5 a and 5 b decreased by 14 and 15 cm?1, respectively, upon reduction of the quinone embedded within 1 . These shifts correspond to 10 and 12 cm?1 decreases in the Tolman electronic parameter of this ditopic ligand. 相似文献
986.
987.
Jaeger A Lecomte VJ Weimerskirch H Richard P Cherel Y 《Rapid communications in mass spectrometry : RCM》2010,24(23):3456-3460
Stable isotopes are increasingly being used to trace wildlife movements. A fundamental prerequisite of animal isotopic tracking is a good knowledge of spatial isotopic variations in the environment. Few accessible reference maps of the isotopic landscape ("isoscapes") are available for marine predators. Here, we validate for the first time an isotopic gradient for higher trophic levels by using a unique combination of a large number of satellite-tracks and subsequent blood plasma isotopic signatures from a wide-ranging oceanic predator. The plasma δ(13)C and δ(15)N values of wandering albatrosses (n = 45) were highly and positively correlated to the Southern Ocean latitudes at which the satellite-tracked individuals foraged. The well-defined latitudinal baseline carbon isoscapes in the Southern Ocean is thus reflected in the tissue of consumers, but with a positive shift due to the cumulative effect of a slight (13)C-enrichment at each trophic level. The data allowed us to estimate the carbon isotopic position of the main oceanic fronts in the area, and thus to delineate robust isoscapes of the main foraging zones for top predators. The plasma δ(13)C and δ(15)N values were positively and linearly correlated, thus suggesting that latitudinal isoscapes also occur for δ(15)N at the base of the food web in oceanic waters of the Southern Ocean. The combination of device deployments with sampling of relevant tissues for isotopic analysis appears to be a powerful tool for investigating consumers' isoscapes at various spatio-temporal scales. 相似文献
988.
Vincent B. F. Mathot 《Journal of Thermal Analysis and Calorimetry》2010,102(2):403-412
In the past 35 years, the emphasis of the activities has been doing and stimulating of fundamental research; managing research
toward improvement and development of polymer systems and industrial applications of polymers; and providing analytical support
by utilization and development of characterization techniques and methods. Throughout, the research activities concerned the
study of molecular structure, crystallization and melting, morphology, and thermal properties of polymeric systems, and to
study and understand the relations between these topics. With respect to Thermal Analysis & Calorimetry (TA&C), quantitative
research has been realized in the field of crystallization of polymers of which a choice of recent work is discussed. 相似文献
989.
Clayden J Hennecke U Vincent MA Hillier IH Helliwell M 《Physical chemistry chemical physics : PCCP》2010,12(45):15056-15064
Poly aromatic ureas and poly aromatic amides are important classes of foldamers-oligomers with well defined conformations. We have explored the origins of the conformational preference of some N,N'-diaryl-N,N'-dimethyl ureas by a combination of NMR spectroscopy and electronic structure calculations using both a recently developed density functional (M06-2X) and a DFT approach (DFT-D) having empirical corrections for dispersive interactions. We have validated the DFT-D approach for structures of this type using high level wavefunction calculations, (CCSD(T)), of the unsubstituted N,N'-diphenyl-N,N'-dimethyl urea. For the N,N'-diaryl-N,N'-dimethyl ureas we have identified a number of 'endo' conformers (i.e. having an E,E geometrical conformation about the two urea C-N bonds), both π- and tert-butyl-stacked, as well as 'exo' structures (having a Z geometrical conformation about at least one of the C-N bonds), and have computed the relative energies of these conformers as well as the barriers for their interconversion. We find that the relative energies of the 'endo' structures closely follow the relative values of the dispersive interactions. The calculations have allowed us to associate different conformers with the various peaks in the NMR spectra, which point to relatively small differences in energy between the conformers. Somewhat larger energy differences are predicted by the two computational approaches, with the M06-2X functional performing the better of the two. It is suggested that the continuum model employed may not be sufficiently accurate to reflect the solvation of the various conformers. 相似文献
990.
Asymmetrical-flow field flow fractionation (AFFF) separates constituents based on hydrodynamic size and is emerging as a powerful
tool for obtaining high-resolution information on the size, molecular weight, composition, and stability of nanoscale particles
in liquid media. We employ a customized AFFF system combining on-line detectors for multi-angle light scattering, dynamic
light scattering, and UV–Vis absorption. Our objective is to develop optimized measurement protocols for the characterization
of gold nanoparticles (GNPs), which are widely utilized in biomedical research and other nanotechnology applications. Experimental
conditions have been optimized by controlling key parameters, including injection volume and solids concentration, mobile
phase composition, membrane type and pore size, and ratio of channel-to-cross-flow rates. Individual citrate-stabilized GNP
components (nominally 10, 20, 30, 40, and 60 nm) and GNPs functionalized with polyethylene glycol were separated from multicomponent
GNP mixtures by AFFF and subsequently characterized. We discuss the effects due to variations in measurement parameters and
GNP surface modification on observed retention, recovery, and peak resolution. 相似文献