全文获取类型
收费全文 | 82677篇 |
免费 | 6546篇 |
国内免费 | 6790篇 |
专业分类
化学 | 30478篇 |
晶体学 | 299篇 |
力学 | 7321篇 |
综合类 | 1300篇 |
数学 | 31741篇 |
物理学 | 24874篇 |
出版年
2023年 | 668篇 |
2022年 | 1211篇 |
2021年 | 1502篇 |
2020年 | 1662篇 |
2019年 | 1916篇 |
2018年 | 1518篇 |
2017年 | 1838篇 |
2016年 | 2255篇 |
2015年 | 2008篇 |
2014年 | 2953篇 |
2013年 | 5488篇 |
2012年 | 3319篇 |
2011年 | 3734篇 |
2010年 | 3270篇 |
2009年 | 4559篇 |
2008年 | 5168篇 |
2007年 | 5339篇 |
2006年 | 4876篇 |
2005年 | 4093篇 |
2004年 | 3721篇 |
2003年 | 3810篇 |
2002年 | 3473篇 |
2001年 | 2919篇 |
2000年 | 2854篇 |
1999年 | 2554篇 |
1998年 | 2394篇 |
1997年 | 1939篇 |
1996年 | 1725篇 |
1995年 | 1546篇 |
1994年 | 1438篇 |
1993年 | 1233篇 |
1992年 | 1184篇 |
1991年 | 903篇 |
1990年 | 760篇 |
1989年 | 682篇 |
1988年 | 640篇 |
1987年 | 494篇 |
1986年 | 424篇 |
1985年 | 522篇 |
1984年 | 494篇 |
1983年 | 231篇 |
1982年 | 376篇 |
1981年 | 444篇 |
1980年 | 338篇 |
1979年 | 318篇 |
1978年 | 248篇 |
1977年 | 237篇 |
1976年 | 179篇 |
1974年 | 118篇 |
1973年 | 115篇 |
排序方式: 共有10000条查询结果,搜索用时 234 毫秒
81.
(S)-2-[2-(p-Tolylsulfinyl)phenyl]acetaldehyde reacts with different O-silylated ketenethioacetals in the presence of Yb(OTf)3 yielding β-hydroxythioesters in high yields and diastereoselectivities. The obtained compounds were readily transformed into β-hydroxyacids and their corresponding diols. These Mukaiyama aldol reactions are a direct evidence of the ability of the sulfinyl group to control 1,5- and 1,6-asymmetric induction processes. 相似文献
82.
In this paper, we propose and demonstrate a new and original model for theoretical calculation and experimental measurement of the noise power spectral density (NPSD) in phase-modulated optical links. The phase modulation is operated in the RF frequency range by an external phase modulator. The NPSD is derived for the first time in interferometric systems, by considering all effects such as the 1/f FM noise of the laser and white noise applied to light from a 1550 nm DFB laser, with phase modulation. The results show for the first time the influence of the phase modulation index, modulation frequency and interferometric delay in the phase-to-intensity noise conversion. The experimental and simulation results of conversion of FM-noise to intensity noise in an optical link by considering the external RF phase modulation are shown with good agreement. 相似文献
83.
This paper considers the problem of building a set of hybrid abstractions for affine systems in order to compute over approximations of the reachable space. Each abstraction is based on a decomposition of the continuous state space that is defined by hyperplanes generated by linear combinations of two vectors. The choice of these vectors is based on consideration of the dynamics of the system and uses, for example, the left eigenvectors of the matrix that defines these dynamics. We show that the reachability calculus can then be performed on a combination of such abstractions and how its accuracy depends on the choice of hyperplanes that define the decomposition. 相似文献
84.
We inquire into the time evolution of quantum systems associated with pseudo-or quasi-Hermitian Hamiltonians. We obtain, in
the pseudo-Hermitian case, a generalized Liouville-von Neumann equation for closed systems. We show that quantum systems with
quasi-Hermitian Hamiltonians admit the proper interpretation in terms of open quantum system and derive a generalized Lindblad-Kossakowski
equation. Finally, we extend such formalism to the study of decaying systems.
Partially supported by PRIN “Sintesi”. 相似文献
85.
We consider the problem of maximizing a linear fractional function on the Pareto efficient frontier of two other linear fractional functions. We present a finite pivoting-type algorithm that solves the maximization problem while computing simultaneously the efficient frontier. Application to multistage efficiency analysis is discussed. An example demonstrating the computational procedure is included. 相似文献
86.
Franz H. Asthalter T. Dommach M. Ehnes A. Messel K. Sergueev I. 《Hyperfine Interactions》2002,141(1-4):131-134
The present status of the new nuclear resonance beamline PETRA 1 at HASYLAB, DESY, Hamburg is described. Besides an overview
of the experimental setup some examples of recent experiments are given. Those cover the main applications, i.e., inelastic
scattering from iron alloys and quasielastic scattering from glass-forming liquids.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
87.
We obtain a solution of the DGLAP equation for the gluon at low x first by expanding the gluon in a Taylor series and then using the method of characteristics. We test its validity by comparing
it with that of Glück, Reya and Vogt. The convergence criteria of the approximation used are also discussed. We also calculate
εF
2(x,Q)2/ε In Q
2 using its approximate relations with the gluon distribution at low x. The predictions are then compared with the HERA data. 相似文献
88.
溴酸钾-亚甲蓝催化光度法测定硝酸盐和亚硝酸盐 总被引:16,自引:1,他引:15
用镉柱将NO-3 还原为NO-2 ,利用在磷酸介质中NO-2 对溴酸钾氧化亚甲蓝这一褪色反应具有强烈的催化作用而建立了同时测定痕量硝酸盐和亚硝酸盐的新方法。本方法简便、快速、灵敏 ,对亚硝酸盐和硝酸盐测定的线性范围分别为 0~ 2 0 0 μg·L-1和 0 0 0 3~ 310 μg·L-1,用于测定各种水样中痕量NO-2 和NO-3 的含量均获得满意结果 相似文献
89.
Q. Sun Q. Wang X.G. Gong V. Kumar Y. Kawazoe 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2002,18(1):77-81
We report results of the atomic and electronic structures of Al7C cluster using ab initio molecular dynamics with ultrasoft pseudopotentials and generalized gradient approximation. The lowest energy structure is
found to be the one in which carbon atom occupies an interstitial position in Al7 cluster. The electronic structure shows that the recent observation [Chem. Phys. Lett. 316, 31 (2000)] of magic behavior of Al7C- cluster is due to a large highest occupied and lowest unoccupied molecular orbital (HOMO-LUMO) gap which makes Al7C- chemically inert. These results have further led us to the finding of a new neutral magic cluster Al7N which has the same number of valence electrons as in Al7C- and a large HOMO-LUMO gap of 1.99 eV. Further, calculations have been carried out on (Al7N)2 to study interaction between magic clusters.
Received 28 July 2001 相似文献
90.
S.-Y. Sheu D.-Y. Yang H.L. Selzle E.W. Schlag 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2002,20(3):557-563
Charge transport is one important example of signal transduction in a protein which is responsible for action at a distance,
and is a fundamental process in biochemical action. A model is presented in which electronic effects interact with motional
processes to combine into a bifunctional model. This model is investigated with new detailed molecular dynamics calculations
and successfully explains such action at a distance.
Received 1st February 2002 / Received in final form 26 May 2002 Published online 13 September 2002 相似文献