全文获取类型
收费全文 | 21995篇 |
免费 | 1077篇 |
国内免费 | 1459篇 |
专业分类
化学 | 16242篇 |
晶体学 | 232篇 |
力学 | 540篇 |
综合类 | 12篇 |
数学 | 3327篇 |
物理学 | 4178篇 |
出版年
2023年 | 147篇 |
2022年 | 271篇 |
2021年 | 534篇 |
2020年 | 408篇 |
2019年 | 456篇 |
2018年 | 475篇 |
2017年 | 451篇 |
2016年 | 853篇 |
2015年 | 693篇 |
2014年 | 824篇 |
2013年 | 1869篇 |
2012年 | 1679篇 |
2011年 | 1814篇 |
2010年 | 1134篇 |
2009年 | 1276篇 |
2008年 | 1340篇 |
2007年 | 1277篇 |
2006年 | 1046篇 |
2005年 | 1016篇 |
2004年 | 1277篇 |
2003年 | 920篇 |
2002年 | 845篇 |
2001年 | 435篇 |
2000年 | 351篇 |
1999年 | 267篇 |
1998年 | 248篇 |
1997年 | 218篇 |
1996年 | 230篇 |
1995年 | 186篇 |
1994年 | 139篇 |
1993年 | 162篇 |
1992年 | 134篇 |
1991年 | 92篇 |
1990年 | 100篇 |
1989年 | 87篇 |
1988年 | 81篇 |
1987年 | 73篇 |
1986年 | 82篇 |
1985年 | 121篇 |
1984年 | 121篇 |
1983年 | 88篇 |
1982年 | 80篇 |
1981年 | 88篇 |
1980年 | 68篇 |
1979年 | 50篇 |
1978年 | 39篇 |
1977年 | 46篇 |
1976年 | 44篇 |
1975年 | 33篇 |
1974年 | 46篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
141.
We discuss role of partially gravitating scalar fields, scalar fields whose energy–momentum tensors vanish for a subset of dimensions, in dynamical compactification of a given set of dimensions. We show that the resulting spacetime exhibits a factorizable geometry consisting of usual four-dimensional spacetime with full Poincaré invariance times a manifold of extra dimensions whose size and shape are determined by the scalar field dynamics. Depending on the strength of its coupling to the curvature scalar, the vacuum expectation value (VEV) of the scalar field may or may not vanish. When its VEV is zero the higher-dimensional spacetime is completely flat and there is no compactification effect at all. On the other hand, when its VEV is nonzero the extra dimensions get spontaneously compactified. The compactification process is such that a bulk cosmological constant is utilized for curving the extra dimensions. 相似文献
142.
The preparation of polyhydroxylated 6-oxa-nor-tropane glycomimetics structurally related to the glycosidase inhibitor family of the calystegines is reported. The synthetic strategy involves the furanose→piperidine rearrangement of 5-deoxy-5-ureido-l-idose precursors, followed by intramolecular glycosylation involving the primary hydroxyl group. Inversion of the configuration at C-3 in the resulting 6-oxa-(+)-calystegine B2 analogue allows accessing the elusive 3-epi-6-oxa-(+)-calystegine B2 skeleton. Acid-catalyzed opening of the nor-tropane bicycle was observed, however, which could be avoided by careful neutralization of the reaction mixture. The inhibition results suggest that (+)-calystegine B2 derivatives and the corresponding C-3 epimers can be seen as glucomimetics and galactomimetics, respectively, pointing to a 1-azasugar mode of action for this family of alkaloids. 相似文献
143.
144.
XPS spectra of YIG, YIG:Bi, YIG:Ca and YIG:Co(Ge, Ca) epitaxial garnet films were measured at room temperature. The positions of the main iron, yttrium, oxygen, bismuth, cobalt and lead spectral lines were determined and their changes with the surface treatments were investigated. It was found that divalent and trivalent states of iron and cobalt ions can be resolved, the chemical shifts being in oposite directions.The authors would like to thank Dr. J. imová for careful EPMA analysis of the samples. 相似文献
145.
This paper proves that any set of n points in the plane contains two points such that any circle through those two points encloses at least
points of the set. The main ingredients used in the proof of this result are edge counting formulas for k-order Voronoi diagrams and a lower bound on the minimum number of semispaces of size at most k.Work on this paper by the first author has been supported by Amoco Fnd. Fac. Dev. Comput. Sci. 1-6-44862 and by the National Science Foundation under Grant CCR-8714565, by the second author has been partially supported by the Digital Equipment Corporation, by the fourth author has been partially supported by the Office of Naval Research under Grant N00014-86K-0416. 相似文献
146.
Alberto García Nora Bretón Isidore Hauser 《International Journal of Theoretical Physics》1988,27(5):635-640
It is shown that any stationary axisymmetric solution to the vacuum field equations of the Jordan-Brans-Dicke (JBD) theory may be obtained from a composition of any stationary axisymmetric vacuum Einstein spacetime with the Weyl class of metrics. Thus, generating solution techniques can be used to obtain any stationary axisymmetric JBD vacuum solution. In this manner, C. B. G. McIntosh's results concerning this topic are improved upon. 相似文献
147.
I. B. Plećaš A. D. Perić S. D. Glodić A. M. Kostadinović 《Journal of Radioanalytical and Nuclear Chemistry》1992,166(5):373-381
Results are presented of a series of experimental tests performed to determine the influence of matrix characteristics on the leaching mechanism of copper aluminium oxychloride immobilized into cement matrices. The objective of this research was to investigate the leaching mechanism of copper as a constituent of copper aluminium oxychloride (CAOX). 相似文献
148.
K. Kriŝtiaková O. Ŝauŝa J. Kriŝtiak Ŝ. Jánoŝ 《Zeitschrift für Physik B Condensed Matter》1989,75(2):197-199
We have measured the ac susceptibility of a wire with a Nb core (1.27 mm diam.) and a Cu cladding (0.37 mm thickness) atT50 K andB0.1 mG. Due to its proximity to Nb, the Cu becomes fully superconducting. From the data we find a breakdown fieldH
b
=1.2 (mG) and a coherence length =2.2T
–1/2 (m) for the Cu, as well as a field penetration depth -34T
1/2 (m) at the Cu/Nb interface. 相似文献
149.
150.