全文获取类型
收费全文 | 169028篇 |
免费 | 1677篇 |
国内免费 | 860篇 |
专业分类
化学 | 67083篇 |
晶体学 | 2634篇 |
力学 | 10733篇 |
综合类 | 32篇 |
数学 | 30559篇 |
物理学 | 60524篇 |
出版年
2022年 | 693篇 |
2021年 | 669篇 |
2020年 | 643篇 |
2019年 | 618篇 |
2018年 | 10251篇 |
2017年 | 11086篇 |
2016年 | 5514篇 |
2015年 | 1808篇 |
2014年 | 1375篇 |
2013年 | 2999篇 |
2012年 | 7342篇 |
2011年 | 16702篇 |
2010年 | 10123篇 |
2009年 | 10267篇 |
2008年 | 14353篇 |
2007年 | 17966篇 |
2006年 | 2892篇 |
2005年 | 9828篇 |
2004年 | 5923篇 |
2003年 | 5565篇 |
2002年 | 3291篇 |
2001年 | 3284篇 |
2000年 | 2569篇 |
1999年 | 1697篇 |
1998年 | 1199篇 |
1997年 | 1147篇 |
1996年 | 1141篇 |
1995年 | 949篇 |
1994年 | 839篇 |
1993年 | 747篇 |
1992年 | 1051篇 |
1991年 | 1058篇 |
1990年 | 957篇 |
1989年 | 859篇 |
1988年 | 838篇 |
1987年 | 897篇 |
1986年 | 750篇 |
1985年 | 996篇 |
1984年 | 948篇 |
1983年 | 678篇 |
1982年 | 664篇 |
1981年 | 631篇 |
1980年 | 592篇 |
1979年 | 736篇 |
1978年 | 772篇 |
1977年 | 693篇 |
1976年 | 646篇 |
1975年 | 537篇 |
1974年 | 576篇 |
1973年 | 548篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
N-Acylalkylation of neutral and anionic N-nucleophiles with α-halocarbonyl compounds was investigated by quantum chemical methods
in terms of the density functional theory and by experimental methods for 2,3-dihydroimidazo[2,1-b]quinazolin-1(10)H-5-one, its N-anion, and simpler model structures. High reactivity of these reagents is determined primarily by stabilization
of transition states (TS) by bridge bonds involving halogen or nitrogen atoms rather than by conjugation, as has been commonly
accepted. Bridged TS are formed by both the substitution mechanism S
N
2 and the addition-elimination mechanism. α-Haloalkyl-substituted zwitterions, which are potential intermediates of stepwise
N-acylalkylation of neutral N-nucleophiles, do not exist in the isolated state, but they are rather efficiently stabilized
upon solvation. These zwitterions, as well as analogous O-anions generated from anionic N-nucleophiles, can serve as intermediates
of N-acylalkylation, as was demonstrated by localization of the corresponding TS.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1150–1164, June, 2007. 相似文献
992.
G. N. Kichigin 《Doklady Physics》2003,48(10):565-567
993.
Electron paramagnetic resonance and optical absorption spectra of Cr ions in cadmium phosphate glass
R.V.S.S.N. Ravikumar Ryuichi KomatsuKo. Ikeda A.V. ChandrasekharB.J. Reddy Y.P. ReddyP.S. Rao 《Solid State Communications》2003,126(5):251-253
EPR and optical absorption investigations of chromium doped cadmium phosphate glass are carried out. Crystal field, spin-Hamiltonian and bonding parameters are evaluated. From the results and analyses of the EPR and optical studies, the site symmetry of Cr3+ ion in the glass is ascribed to a distorted octahedron. The bonding parameters suggest the ionic nature of Cr3+ ion with the ligands. 相似文献
994.
Dailly A. Schneider R. Billaud D. Fort Y. Ghanbaja J. 《Journal of nanoparticle research》2003,5(3-4):389-393
A novel chemical reduction method using an activated alkaline hydride (LiH or NaH-t-BuONa) in tetrahydrofuran solvent has been applied to antimony salt reduction. X-ray diffraction and transmission electron microscopy studies have been carried out to characterize the morphology and structure of the materials. Alkali hydride nature influence has been proved. In both cases the process allows to prepare antimony particles in nanometer range from few nanometers to about 20nm which could be used as anodic materials for lithium–ion batteries. With lithium hydride well-crystallized particles inclined to agglomeration were observed whereas finely dispersed amorphous particles were pointing out after activated sodium hydride reduction. 相似文献
995.
V. T. Bublik S. Yu. Matsnev K. D. Shcherbachev M. V. Mezhennyi M. G. Mil’vidskii V. Ya. Reznik 《Physics of the Solid State》2003,45(10):1918-1925
Diffuse x-ray scattering (DXS) is used to study the formation of microdefects (MDs) in heat-treated dislocation-free large-diameter silicon wafers with vacancies. The DXS method is shown to be efficient for investigating MDs in silicon single crystals. Specific defects, such as impurity clouds, are found to form in the silicon wafers during low-temperature annealing at 450°C. These defects are oxygen-rich regions in the solid solution with diffuse coherent interfaces. In the following stages of decomposition of the supersaturated solid solution, oxide precipitates form inside these regions and the impurity clouds disappear. As a result of the decomposition of the supersaturated solid solution of oxygen, interstitial MDs form in the silicon wafers during multistep heat treatment. These MDs lie in the {110} planes and have nonspherical displacement fields. The volume density and size of MDs forming in the silicon wafers at various stages of the decomposition are determined. 相似文献
996.
A. I. Artem’ev D. N. Klochkov K. Oganesyan Yu. V. Rostovtsev M. V. Fedorov 《Laser Physics》2007,17(10):1213-1216
The spatial amplification of a wave in a magnetostatic undulator with noncollinear electron and laser beams is studied in the framework of the dispersion relation for single-frequency and collective regimes. The dependence of the gain on the electron beam width is estimated with regard to the spatial boundedness of the beams. The laser power threshold at which the selection with respect to the transverse velocity is possible is obtained for a free-electron laser without inversion. 相似文献
997.
998.
T. Van Cauteren J. Ryckebusch B. Metsch H. -R. Petry 《The European Physical Journal A - Hadrons and Nuclei》2007,31(4):613-615
We present some of our results for the electromagnetic properties of excited Σ hyperons, computed within the framework of the Bonn constituent-quark model, which is based on the Bethe-Salpeter approach.
The seven parameters entering the model are fitted against the best-known baryon masses. Accordingly, the results for the
form factors and helicity amplitudes are genuine predictions. We compare with the scarce experimental data available and discuss
the processes in which Σ
*'s may play an important role. 相似文献
999.
M. Dubé C. Daneault V. Vuorinen M. Alava M. Rost 《The European Physical Journal B - Condensed Matter and Complex Systems》2007,56(1):15-26
We investigate the structure and dynamics of the interface between
two immiscible liquids in a three-dimensional disordered porous
medium. We apply a phase-field model that includes explicitly
disorder and discuss both spontaneous and forced imbibition.
The structure of the interface is dominated by a length scale
ξ× which arises from liquid conservation. We further show
that disorder in the capillary and permeability act on different
length scales and give rise to different scalings and
structures of the interface properties. We conclude with a
range of applications. 相似文献
1000.
G.G.L. Nashed 《The European Physical Journal C - Particles and Fields》2007,49(3):851-857
An exact charged solution with axial symmetry is obtained in the teleparallel equivalent of general relativity. The associated metric has the structure function G(ξ)=1-ξ2-2mAξ3-q2A2ξ4. The fourth order nature of the structure function can make calculations cumbersome. Using a coordinate transformation we get a tetrad whose metric has the structure function in a factorizable form (1-ξ2)(1+r+Aξ)(1+r-Aξ) with r± as the horizons of Reissner–Nordström space-time. This new form has the advantage that its roots are now trivial to write down. Then, we study the singularities of this space-time. Using another coordinate transformation, we obtain a tetrad field. Its associated metric yields the Reissner–Nordström black hole. In calculating the energy content of this tetrad field using the gravitational energy-momentum, we find that the resulting form depends on the radial coordinate! Using the regularized expression of the gravitational energy-momentum in the teleparallel equivalent of general relativity we get a consistent value for the energy. 相似文献