全文获取类型
收费全文 | 511940篇 |
免费 | 3200篇 |
国内免费 | 937篇 |
专业分类
化学 | 240791篇 |
晶体学 | 7296篇 |
力学 | 28456篇 |
综合类 | 9篇 |
数学 | 79487篇 |
物理学 | 160038篇 |
出版年
2020年 | 4536篇 |
2019年 | 5400篇 |
2018年 | 12534篇 |
2017年 | 12920篇 |
2016年 | 12659篇 |
2015年 | 5521篇 |
2014年 | 9217篇 |
2013年 | 18933篇 |
2012年 | 17539篇 |
2011年 | 25129篇 |
2010年 | 17836篇 |
2009年 | 18113篇 |
2008年 | 23393篇 |
2007年 | 25528篇 |
2006年 | 15579篇 |
2005年 | 16865篇 |
2004年 | 14392篇 |
2003年 | 13853篇 |
2002年 | 12877篇 |
2001年 | 12076篇 |
2000年 | 9855篇 |
1999年 | 7185篇 |
1998年 | 6510篇 |
1997年 | 6420篇 |
1996年 | 6075篇 |
1995年 | 5215篇 |
1994年 | 5334篇 |
1993年 | 5319篇 |
1992年 | 5502篇 |
1991年 | 5890篇 |
1990年 | 5760篇 |
1989年 | 5791篇 |
1988年 | 5510篇 |
1987年 | 5521篇 |
1986年 | 5229篇 |
1985年 | 6373篇 |
1984年 | 6741篇 |
1983年 | 5623篇 |
1982年 | 6018篇 |
1981年 | 5604篇 |
1980年 | 5244篇 |
1979年 | 5867篇 |
1978年 | 6122篇 |
1977年 | 6171篇 |
1976年 | 6250篇 |
1975年 | 5861篇 |
1974年 | 5727篇 |
1973年 | 6047篇 |
1972年 | 4656篇 |
1971年 | 4176篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
961.
Vinay V. ThakurA. Sudalai 《Tetrahedron letters》2003,44(5):989-992
N-Bromoamides catalyze effectively the aziridination of electron-deficient as well as electron-rich olefins using chloramine-T (N-chloro-N-sodio-p-toluenesulfonamide) as a nitrogen source under ambient conditions to afford the corresponding aziridines in good to excellent yields. 相似文献
962.
Thermodynamic fluctuations in systems that are in nonequilibrium steady states are always spatially long ranged, in contrast to fluctuations in thermodynamic equilibrium. In the present paper we consider a fluid subjected to a stationary temperature gradient. Two different physical mechanisms have been identified by which the temperature gradient causes long-ranged fluctuations. One cause is the presence of couplings between fluctuating fields. Secondly, spatial variation of the strength of random forces, resulting from the local version of the fluctuation-dissipation theorem, has also been shown to generate long-ranged fluctuations. We evaluate the contributions to the long-ranged temperature fluctuations due to both mechanisms. While the inhomogeneously correlated Langevin noise does lead to long-ranged fluctuations, in practice, they turn out to be negligible as compared to nonequilibrium temperature fluctuations resulting from the coupling between temperature and velocity fluctuations. 相似文献
963.
Using the DFT/B3LYP method with a 6-31G(d) basis set, the structure, normal vibration frequencies, and the absolute band intensities in the IR spectra of the chlorin molecule and its four symmetric isotopomers have been calculated. Scaling of the force field by the Pulay method in independent and natural coordinates has been carried out. A method for obtaining effective force fields without using experimental data on the fundamental vibration frequencies is proposed. By comparing the vibration modes and constructing special matrices, complete assignment of the fundamental frequencies of porphin and chlorin has been carried out. It has been shown that the majority of porphin macroring vibrations upon pyrrolenine ring hydrogenation are frequency-characteristic and only 12 vibrations change considerably. A frequency correlation with regard for the mode transition between chlorin and all its isotopomers under consideration has been established. Comparative analysis of the force fields of porphin and chlorin in dependent natural coordinates has revealed the unique nonlocal character of the change in force constants of the macroring upon hydrogenation of one pyrrolenine ring. Modeling of the IR spectra of chlorin and its isotopomers has been performed. Assignment and interpretation of the normal vibrations of the molecules under consideration have been carried out. 相似文献
964.
Quasi-wavelets (QWs) are a representation of turbulence consisting of self-similar, eddy-like structures with random orientations and positions in space. They are used in this paper to calculate the scattering, due to turbulent velocity fluctuations, of sound behind noise barriers as a function of the size and spatial location of the eddies. The sound scattering cross-section for QWs of an individual size class (eddy size) is derived and shown to reproduce results for the von Kármán spectrum when the scattered energies from a continuous distribution of QW sizes are combined. A Bragg resonance condition is derived for the eddy size that scatters most strongly for a given acoustic wavenumber and scattering angle. Results for scattering over barriers show that, for typical barrier conditions, most of the scattered energy originates from eddies in the size range of approximately one-half to twice the size of the eddies responsible for maximum scattering. The results also suggest that scattering over the barrier due to eddies with a line of sight to both the source and receiver is generally significant only for frequencies above several kilohertz, for sources and receivers no more than a few meters below the top of the barrier, and for very turbulent atmospheric conditions. 相似文献
965.
A method of solving the problem of sound radiation from a plate with elements of different thickness in its plane is proposed, and the results obtained with this method are presented. It is shown that the main sources of sound radiation are the inhomogeneous flexural fields formed in the elements on both sides of their joint. 相似文献
966.
We examine to what extent the correlation between two quantum observables at a mixed state can be separated into a classical and a quantum term. The nonunique decomposition of quantum mixed states into pure states makes such a separation ambiguous. We outline this fact by a simple example, which also shows that classical and quantum correlations may cancel each other out. 相似文献
967.
In different regimes of exposure of metals to pulsed laser radiation, we show the role played by the liquid-drop phase formed from the target material in a laser-induced erosion plume due to bulk vaporization, in the dynamics of this plume. For some metals, the ranges of power densities of the acting laser radiation, at which the condensed-phase particles influence the passage of laser radiation to the target surface, have been determined experimentally. The mechanism of realization of a low-threshold breakdown has been revealed. 相似文献
968.
Modification of Paving Asphalts with Sulfur 总被引:2,自引:0,他引:2
Syroezhko A. M. Begak O. Yu. Fedorov V. V. Gusarova E. N. 《Russian Journal of Applied Chemistry》2003,76(3):491-496
Sulfur dissolved in oil components of asphalt is a binder, filler, and chemical co-reagent at once. The structure of sulfur-extended asphalt depends on its formula and heating temperature. Road pavement based on sulfur-extended asphalt demonstrates higher durability as compared to that with the conventional binders. 相似文献
969.
Amplitude equations, as well as the effective dispersion and nonlinearity parameters, which define the dynamics of a wave
packet formed by two strongly coupled modes, are derived with allowance for the frequency dependence of the linear mode coupling
coefficient. These equations are used to study the onset of the modulation instability of the two-mode wave packet, soliton-like
pulses, and compression modes. Unlike single-mode systems, the last two effects in optical waveguides may arise for both a
negative and positive disper-sion of the waveguide material. 相似文献
970.
Liang Liao Yi Pang Liming Ding Frank E. Karasz 《Journal of polymer science. Part A, Polymer chemistry》2003,41(20):3149-3158
A soluble cyano‐substituted poly[(1,3‐phenylene vinylene)‐alt‐(1,4‐phenylene vinylene)] derivative ( 9 ) was synthesized and characterized. Comparison between 9 and its model compound ( 10 ) showed that the chromophore in 9 remained to be well defined as a result of a π‐conjugation interruption at adjacent m‐phenylene units. The attachment of a cyano substituent only at the β position of the vinylene allowed the maximum electronic impact of the cyano group on the optical properties of the poly(p‐phenylene vinylene) material. At a low temperature (?108 or ?198 °C), the vibronic structures of 9 and 10 were partially resolved. The absorption and emission spectra of a film of 9 were less temperature‐dependent than those of a film of 10 , indicating that the former had a lower tendency to aggregate. A light‐emitting diode (LED) based on 9 emitted yellow light (λmax ≈ 578 nm) with an external quantum efficiency of 0.03%. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3149–3158, 2003 相似文献