全文获取类型
收费全文 | 4276篇 |
免费 | 177篇 |
国内免费 | 32篇 |
专业分类
化学 | 3220篇 |
晶体学 | 24篇 |
力学 | 116篇 |
数学 | 527篇 |
物理学 | 598篇 |
出版年
2023年 | 38篇 |
2022年 | 56篇 |
2021年 | 79篇 |
2020年 | 100篇 |
2019年 | 102篇 |
2018年 | 71篇 |
2017年 | 57篇 |
2016年 | 138篇 |
2015年 | 121篇 |
2014年 | 152篇 |
2013年 | 228篇 |
2012年 | 299篇 |
2011年 | 366篇 |
2010年 | 163篇 |
2009年 | 131篇 |
2008年 | 323篇 |
2007年 | 316篇 |
2006年 | 306篇 |
2005年 | 253篇 |
2004年 | 218篇 |
2003年 | 192篇 |
2002年 | 155篇 |
2001年 | 34篇 |
2000年 | 34篇 |
1999年 | 37篇 |
1998年 | 36篇 |
1997年 | 41篇 |
1996年 | 56篇 |
1995年 | 31篇 |
1994年 | 28篇 |
1993年 | 24篇 |
1992年 | 21篇 |
1991年 | 25篇 |
1990年 | 25篇 |
1989年 | 13篇 |
1988年 | 12篇 |
1987年 | 12篇 |
1986年 | 8篇 |
1985年 | 22篇 |
1984年 | 27篇 |
1983年 | 11篇 |
1982年 | 14篇 |
1981年 | 13篇 |
1980年 | 18篇 |
1979年 | 10篇 |
1978年 | 16篇 |
1977年 | 12篇 |
1976年 | 13篇 |
1975年 | 8篇 |
1974年 | 10篇 |
排序方式: 共有4485条查询结果,搜索用时 15 毫秒
51.
Kevin J. Sheehy Lorraine M. Bateman Niko T. Flosbach Martin Breugst Peter A. Byrne 《Chemical science》2020,11(35):9630
The preferred site of alkylation of diazine N-oxides by representative hard and soft alkylating agents was established conclusively using the 1H–15N HMBC NMR technique in combination with other NMR spectroscopic methods. Alkylation of pyrazine N-oxides (1 and 2) occurs preferentially on nitrogen regardless of the alkylating agent employed, while O-methylation of pyrimidine N-oxide (3) is favoured in its reaction with MeOTf. As these outcomes cannot be explained in the context of the hard/soft acid/base (HSAB) principle, we have instead turned to Marcus theory to rationalise these results. Marcus intrinsic barriers (ΔG‡0) and ΔrG° values were calculated at the DLPNO-CCSD(T)/def2-TZVPPD/SMD//M06-2X-D3/6-311+G(d,p)/SMD level of theory for methylation reactions of 1 and 3 by MeI and MeOTf, and used to derive Gibbs energies of activation (ΔG‡) for the processes of N- and O-methylation, respectively. These values, as well as those derived directly from the DFT calculations, closely reproduce the observed experimental N- vs. O-alkylation selectivities for methylation reactions of 1 and 3, indicating that Marcus theory can be used in a semi-quantitative manner to understand how the activation barriers for these reactions are constructed. It was found that N-alkylation of 1 is favoured due to the dominant contribution of ΔrG° to the activation barrier in this case, while O-alkylation of 3 is favoured due to the dominant contribution of the intrinsic barrier (ΔG‡0) for this process. These results are of profound significance in understanding the outcomes of reactions of ambident reactants in general.Marcus theory enables rationalisation and quantification of selectivities in reactions of ambident nucleophiles for which the HSAB principle cannot operate. 相似文献
52.
Turney K Drake TJ Smith JE Tan W Harrison WW 《Rapid communications in mass spectrometry : RCM》2004,18(20):2367-2374
Nanoparticles for the extraction of peptides and subsequent analysis using atmospheric pressure matrix-assisted laser desorption/ionization (APMALDI) have been evaluated. The atmospheric pressure source allows for particles to be directly introduced in the liquid matrix, minimizing sample loss and analysis time. Described in this work are two sample preparation procedures for liquid APMALDI analysis: a C18 functionalized silica nanoparticle for hydrophobic extractions, and an aptamer functionalized magnetite core nanoparticle for rapid, affinity extractions. The C18 particles provide a non-selective support for rapid profiling applications, while the aptamer particles are directed towards reducing the complexity in biological samples. The aptamer functionalized particles provide a more selective analyte-nanoparticle interaction whereby the tertiary structure of the analyte becomes more critical to the extraction. In both cases, the liquid APMALDI matrix provides a support for ionization, and acts as the releasing agent for the analyte-particle interaction. Additionally, analyte enrichment was possible due to the large surface-to-volume ratio of the particles. The experiments conducted with functionalized nanoparticles, in an atmospheric pressure liquid matrix, present a basis for further methodologies and utilities of silica nanoparticles to be developed. 相似文献
53.
Rozing G van de Goor T Yin H Killeen K Glatz B Kraiczek K Lauer HH 《Journal of separation science》2004,27(17-18):1391-1401
A systematic investigation of the influence of the perimeter shape of open and particle packed fused silica capillaries on chromatographic properties such as resistance to flow and dispersion of solutes propelled through these channels has been conducted. Verifications of these uncommon experiments with existing theoretical treatments are presented and the insights transferred to a novel polymer chip design with integrated facilities for complex separations. A comparison of the chromatographic performance of a real life proteomics sample on this chip with a capillary column of "similar" dimensions is presented. 相似文献
54.
Designs, Codes and Cryptography - Douglas (J Comb Theory 6(4):323–339, 1969) proved that for odd spread k and dimension $$d={\frac{1}{2}(3k+3)}$$ all maximum length (d, k) circuit... 相似文献
55.
Agwu Anthony Harris Phillip James Kevin Kannan Siddarth Li Huixi 《The Ramanujan Journal》2022,58(1):75-120
The Ramanujan Journal - For an elliptic curve $$E/{\mathbb {Q}}$$ , let $$a_p$$ denote the trace of its Frobenius endomorphism over $${\mathbb {F}}_p$$ , where p is a prime of good reduction for E.... 相似文献
56.
Kevin McLeod Albert Milani 《NoDEA : Nonlinear Differential Equations and Applications》1996,3(1):79-114
We prove that the quasilinear parabolic initial-boundary value problem (1.1) below is globally well-posed in a class of high order Sobolev solutions, and that these solutions possess compact, regular attractors ast+. 相似文献
57.
Stability of rarefaction waves in viscous media 总被引:3,自引:0,他引:3
We study the time-asymptotic behavior of weak rarefaction waves of systems of conservation laws describing one-dimensional viscous media, with strictly hyperbolic flux functions. Our main result is to show that solutions of perturbed rarefaction data converge to an approximate, Burgers rarefaction wave, for initial perturbations w
0 with small mass and localized as w
0(x)=
The proof proceeds by iteration of a pointwise ansatz for the error, using integral representations of its various components, based on Green's functions. We estimate the Green's functions by careful use of the Hopf-Cole transformation, combined with a refined parametrix method. As a consequence of our method, we also obtain rates of decay and detailed pointwise estimates for the error.This pointwise method has been used successfully in studying stability of shock and constant-state solutions. New features in the rarefaction case are time-varying coefficients in the linearized equations and error waves of unbounded mass
(log (t)). These diffusion waves have amplitude
(t
-1/2logt) in linear degenerate transversal fields and
(t
-1/2logt) in genuinely nonlinear transversal fields, a distinction which is critical in the stability proof. 相似文献
58.
Darensbourg DJ Maynard EL Holtcamp MW Klausmeyer KK Reibenspies JH 《Inorganic chemistry》1996,35(9):2682-2684
59.
60.
Dissolved aluminium in seawater and freshwater is determined by cathodic stripping voltammetry (c.s.v.) preceded by adsorptive collection of complex ions with 1,2-dihydroxyanthraquinone-3-sulphonic acid (DASA) on the hanging mercury drop electrode. Complexation of aluminium by DASA is rapid and no waiting period or heating of the sample is required. Optimal conditions are a DASA concentration of 10?5 M, a solution pH of 7.1–7.3 and an adsorption potential of ?0.9 V; the c.s.v. scan is done in the differential-pulse mode. The limit of detection is 1 nM aluminium for an adsorption time of 45 s. The total time needed, including 50min deaeration and a standard addition, is 10–15 min per sample. No serious interferences were found; u.v. irradiation is recommended for samples containing high levels of organic materials. 相似文献