全文获取类型
收费全文 | 4139篇 |
免费 | 141篇 |
国内免费 | 27篇 |
专业分类
化学 | 2154篇 |
晶体学 | 21篇 |
力学 | 89篇 |
数学 | 484篇 |
物理学 | 989篇 |
无线电 | 570篇 |
出版年
2022年 | 42篇 |
2021年 | 61篇 |
2020年 | 67篇 |
2019年 | 56篇 |
2018年 | 33篇 |
2017年 | 49篇 |
2016年 | 102篇 |
2015年 | 77篇 |
2014年 | 87篇 |
2013年 | 155篇 |
2012年 | 191篇 |
2011年 | 209篇 |
2010年 | 161篇 |
2009年 | 127篇 |
2008年 | 201篇 |
2007年 | 217篇 |
2006年 | 207篇 |
2005年 | 207篇 |
2004年 | 175篇 |
2003年 | 132篇 |
2002年 | 158篇 |
2001年 | 81篇 |
2000年 | 84篇 |
1999年 | 44篇 |
1998年 | 39篇 |
1997年 | 47篇 |
1996年 | 64篇 |
1995年 | 44篇 |
1994年 | 66篇 |
1993年 | 79篇 |
1992年 | 63篇 |
1991年 | 49篇 |
1990年 | 63篇 |
1989年 | 40篇 |
1988年 | 47篇 |
1987年 | 49篇 |
1986年 | 44篇 |
1985年 | 73篇 |
1984年 | 55篇 |
1983年 | 44篇 |
1982年 | 47篇 |
1981年 | 38篇 |
1980年 | 34篇 |
1979年 | 57篇 |
1978年 | 40篇 |
1977年 | 36篇 |
1976年 | 32篇 |
1975年 | 33篇 |
1974年 | 39篇 |
1973年 | 31篇 |
排序方式: 共有4307条查询结果,搜索用时 15 毫秒
171.
Nicole Frankenberg Christine Hager-Braun Ute Feiler Markus Fuhrmann Hans Rogl Nikolaus Schneebauer Nathan Nelson Günter Hauska 《Photochemistry and photobiology》1996,64(1):14-19
Membranes of Chlorobium tepidum contain about 35, 45 and2–10 molecules of menaquinone-7, chlorobium quinone (1′-oxo-menaquinone-7) and of the polar menaquinone (probably 1′-OH-menaquinone-7) per reaction center, respectively. None of these quinones was retained during the isolation of P840-reaction centers beyond the detection limit of about 0.2 quinones per reaction center, neither in the core complex nor in functionally intact reaction center preparations. The latter is shown to catalyze the formation of an electrochemical proton gradient in the presence of ascorbate and phenazinium methosulfate, when it is incorporated into lipid vesicles. 相似文献
172.
Evan R. Moore Andrew E. Bourne Todd J. Hoppe Philip J. Abode Steven R. Boone Gordon H. Purser 《国际化学动力学杂志》2004,36(10):554-564
The mechanism by which an excess of iron(II) ion reacts with aqueous chlorine dioxide to produce iron(III) ion and chloride ion has been determined. The reaction proceeds via the formation of chlorite ion, which in turn reacts with additional iron(II) to produce the observed products. The first step of the process, the reduction of chlorine dioxide to chlorite ion, is fast compared to the subsequent reduction of chlorite by iron(II). The overall stoichiometry is The rate is independent of pH over the range from 3.5 to 7.5, but the reaction is assisted by the presence of acetate ion. Thus the rate law is given by At an ionic strength of 2.0 M and at 25°C, ku = (3.9 ± 0.1) × 103 L mol?1 s?1 and kc = (6 ± 1) × 104 L mol?1 s?1. The formation constant for the acetatoiron(II) complex, Kf, at an ionic strength of 2.0 M and 25°C was found to be (4.8 ± 0.8) × 10?2 L mol?1. The activation parameters for the reaction were determined and compared to those for iron(II) ion reacting directly with chlorite ion. At 0.1 M ionic strength, the activation parameters for the two reactions were found to be identical within experimental error. The values of ΔH? and ΔS? are 64 ± 3 kJ mol?1 and + 40 ± 10 J K?1 mol?1 respectively. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 554–565, 2004 相似文献
173.
Aaron R. Todd 《Annali di Matematica Pura ed Applicata》1981,128(1):317-323
Summary A new class of locally convex linear topological spaces, the (db)-spaees, recently defined by Robertson, Tweddle and Yeomans, interpolates the classes of Baire-like and unordered Baire-like spaces. Saxon and Narayanaswami have given convex metrizable spaces that distinguish among these classes. This paper gives a new characterization of (db)-spaces from which is extracted the class of b-spaces. This class interpolates the classes of -spaces and -spaces of Lehner. Let C(X) be the space of all real-valued continuous functions on the completely regular Sausdorf space X, supplied with the topology of uniform convergence on compact sets of X. It is shown that C(X) is a b-space if and only if it is an -space. A characterization of X for which C(X) is a (db)-space is unknown. Other open questions are stated in the paper. 相似文献
174.
It is argued that preparation of a quantum state characterized by density operator not commuting with a superselection operatorQ does not by itself constitute an instance of superselection rule violation. It would, however, be an instance of state restriction violation. It is held that superselection rule violation is only possible with simultaneous observable and state restriction violations. It is shown that it is a priori conceivable to subdivide an ensemble whose satisfies[, Q] = 0 into subensembles whose density operators violate the state restrictions. The dynamics of the subdivision process is not considered. 相似文献
175.
Peter G. Nelson 《Foundations of Chemistry》2013,15(2):199-208
The basis of the Periodic Table is discussed. Electronic configuration recurs in only 21 out of the 32 groups. A better basis is derived by considering the highest classical valency (v) exhibited by an element and a new measure, the highest valency in carbonyl compounds (v*). This leads to a table based on the number of outer electrons possessed by an atom (N) and the number of electrons required for it to achieve an inert (noble) gas configuration (N*). Periodicity of these is nearly complete. The new basis helps to settle the question of the best form of table and related issues. 相似文献
176.
Many complex unsteady mechanisms are thought to facilitate the high efficiency and agility commonly observed in small biological flyers. One of these, the flexible clap and fling maneuver, has not been extensively studied; an experimental characterization is the focus of this work. The clap–fling mechanism is approximated with a single flexible membrane flapping wing, replacing the symmetry plane between two wings with a splitter plate simulating the pair wing. This produces a complex vibro-impact aeroelastic problem, the deformation resulting from which is measured with a high-speed visual image correlation system. A low-dimensional representation of the ensuing large data set is obtained with proper orthogonal decomposition. The POD modes, and the relative importance of each, can help elucidate crucial mechanisms and relationships within the flapping system, and are computed for various membrane wing structures and flapping frequencies, with or without the presence of the splitter plate. 相似文献
177.
Tryptanthrin is a natural product with numerous important pharmacological properties. Tryptanthrin and its analogs are commonly prepared by condensation of isatoic anhydride and isatin. In this Letter we investigate the formation of tryptanthrin derivatives upon Oxone-induced oxidative dimerization of indole-3-carbaldehydes. 相似文献
178.
D. B. Shustov A. A. Lebedev S. P. Lebedev D. K. Nelson A. A. Sitnikova M. V. Zamoryanskaya 《Semiconductors》2013,47(9):1267-1270
n-3C-SiC/n-6H-SiC heterostructures grown by vacuum sublimation on CREE commercial 6H-SiC substrates are studied. Transmission electron microscopy (TEM) demonstrated that a transitional layer of varying thickness, composed of a mixture of 3C- and 6H-SiC polytypes, is formed on the substrate. A 3C polytype layer was obtained on the interlayer. Cathodoluminescence study of the surface of the film demonstrated that defects in the form of inclusions of another phase (6H-polytype), stacking faults, and twin boundaries (separating domains of cubic modification, grown in various orientations) are found on the surface and in the surface layer with a thickness on the order of 100 μm. Varying the growth conditions changes the concentration of various types of defects. 相似文献
179.
Daniel A. Nelson Gustaaf B. Jacobs David A. Kopriva 《Theoretical and Computational Fluid Dynamics》2016,30(4):363-385
The effect of curved-boundary representation on the physics of the separated flow over a NACA 65(1)-412 airfoil is thoroughly investigated. A method is presented to approximate curved boundaries with a high-order discontinuous-Galerkin spectral element method for the solution of the Navier–Stokes equations. Multiblock quadrilateral element meshes are constructed with the grid generation software GridPro. The boundary of a NACA 65(1)-412 airfoil, defined by a cubic natural spline, is piecewise-approximated by isoparametric polynomial interpolants that represent the edges of boundary-fitted elements. Direct numerical simulation of the airfoil is performed on a coarse mesh and fine mesh with polynomial orders ranging from four to twelve. The accuracy of the curve fitting is investigated by comparing the flows computed on curved-sided meshes with those given by straight-sided meshes. Straight-sided meshes yield irregular wakes, whereas curved-sided meshes produce a regular Karman street wake. Straight-sided meshes also produce lower lift and higher viscous drag as compared with curved-sided meshes. When the mesh is refined by reducing the sizes of the elements, the lift decrease and viscous drag increase are less pronounced. The differences in the aerodynamic performance between the straight-sided meshes and the curved-sided meshes are concluded to be the result of artificial surface roughness introduced by the piecewise-linear boundary approximation provided by the straight-sided meshes. 相似文献
180.