全文获取类型
收费全文 | 37665篇 |
免费 | 4650篇 |
国内免费 | 4122篇 |
专业分类
化学 | 15426篇 |
晶体学 | 167篇 |
力学 | 3112篇 |
综合类 | 603篇 |
数学 | 14194篇 |
物理学 | 12935篇 |
出版年
2024年 | 71篇 |
2023年 | 349篇 |
2022年 | 753篇 |
2021年 | 878篇 |
2020年 | 1046篇 |
2019年 | 938篇 |
2018年 | 870篇 |
2017年 | 1099篇 |
2016年 | 1327篇 |
2015年 | 1238篇 |
2014年 | 1961篇 |
2013年 | 2670篇 |
2012年 | 2110篇 |
2011年 | 2593篇 |
2010年 | 2185篇 |
2009年 | 2529篇 |
2008年 | 2591篇 |
2007年 | 2645篇 |
2006年 | 2463篇 |
2005年 | 2212篇 |
2004年 | 1897篇 |
2003年 | 1625篇 |
2002年 | 1481篇 |
2001年 | 1187篇 |
2000年 | 1099篇 |
1999年 | 937篇 |
1998年 | 830篇 |
1997年 | 779篇 |
1996年 | 621篇 |
1995年 | 535篇 |
1994年 | 432篇 |
1993年 | 352篇 |
1992年 | 311篇 |
1991年 | 250篇 |
1990年 | 244篇 |
1989年 | 163篇 |
1988年 | 162篇 |
1987年 | 117篇 |
1986年 | 107篇 |
1985年 | 117篇 |
1984年 | 104篇 |
1983年 | 60篇 |
1982年 | 100篇 |
1981年 | 77篇 |
1980年 | 58篇 |
1979年 | 50篇 |
1978年 | 39篇 |
1977年 | 35篇 |
1976年 | 42篇 |
1973年 | 25篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
ABSTRACTQM(UB3LYP)/MM(AMBER) calculations were performed for the locations of the transition structure (TS) of the oxygen–oxygen (O–O) bond formation in the S4 state of the oxygen-evolving complex (OEC) of photosystem II (PSII). The natural orbital (NO) analysis of the broken-symmetry (BS) solutions was also performed to elucidate the nature of the chemical bonds at TS on the basis of several chemical indices defined by the occupation numbers of NO. The computational results revealed a concerted bond switching (CBS) mechanism for the oxygen–oxygen bond formation coupled with the one-electron transfer (OET) for water oxidation in OEC of PSII. The orbital interaction between the σ-HOMO of the Mn(IV)4–O(5) bond and the π*-LUMO of the Mn(V)1=O(6) bond plays an important role for the concerted O–O bond formation for water oxidation in the CaMn4O6 cluster of OEC of PSII. One electron transfer (OET) from the π-HOMO of the Mn(V)1=O(6) bond to the σ*-LUMO of the Mn(IV)4–O(5) bond occurs for the formation of electron transfer diradical, where the generated anion radical [Mn(IV)4–O(5)]-? part is relaxed to the ?Mn(III)4?…?O(5)- structure and the cation radical [O(6)=Mn(V)1]+ ? part is relaxed to the +O(6)–Mn(IV)1? structure because of the charge-spin separation for the electron-and hole-doped Mn–oxo bonds. Therefore, the local spins are responsible for the one-electron reductions of Mn(IV)4->Mn(III)4 and Mn(V)1->Mn(IV)1. On the other hand, the O(5)- and O(6)+ sites generated undergo the O–O bond formation in the CaMn4O6 cluster. The Ca(II) ion in the cubane- skeleton of the CaMn4O6 cluster assists the above orbital interactions by the lowering of the orbital energy levels of π*-LUMO of Mn(V)1=O(6) and σ*-LUMO of Mn(IV)4–O(5), indicating an important role of its Lewis acidity. Present CBS mechanism for the O–O bond formation coupled with one electron reductions of the high-valent Mn ions is different from the conventional radical coupling (RC) and acid-base (AB) mechanisms for water oxidation in artificial and native photosynthesis systems. The proton-coupled electron transfer (PC-OET) mechanism for the O–O bond formation is also touched in relation to the CBS-OET mechanism. 相似文献
3.
4.
5.
本文利用统计计量学方法对山西省农民家庭消费水平和消费结构的变化及原因进行了实证分析。建立了山西省农民人均消费的消费函数和山西省农民总消费支出与食品、衣着、住房、日用品、燃料及文化生活服务支出的关系模型,并通过模型分析解释了农民的消费结构,也为决策者提供一些有效的参考资料。 相似文献
6.
A.-S. F. Obada M. Darwish H. H. Salah 《International Journal of Theoretical Physics》2002,41(9):1755-1768
In this paper a new state called odd-excited binomial state (OEBS) is introduced. It interpolates between the odd number state and the odd-excited coherent state. We discuss some statistical properties, such as the Glauber second-order correlation function and squeezing phenomenon (normal and amplitude-squared squeezing) for this state. The quasiprobability distribution functions (Husimi Q-function and Wigner W-function) are also examined. 相似文献
7.
Gang Wang Xiulin Zhu Zhenping Cheng Jian Zhu 《Journal of polymer science. Part A, Polymer chemistry》2006,44(9):2912-2921
A series of (di)picolinic acids and their derivates are investigated as novel complexing tridentate or bidentate ligands in the iron‐mediated reverse atom transfer radical polymerization of methyl methacrylate in N,N‐dimethylformamide at 100 °C with 2,2′‐azobisisobutyrontrile as an initiator. The polymerization rates and polydispersity indices (1.32–1.8) of the resulting polymers are dependent on the structures of the ligands employed. Different iron complexes may be involved in iron‐mediated reverse atom transfer radical polymerization, depending on the type of acid used. 1H NMR spectroscopy has been used to study the structure of the resulting polymers. Chain‐extension reactions have been performed to further confirm the living nature of this catalytic system. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2912–2921, 2006 相似文献
8.
Necessary conditions for the Gâteaux differentiability of the distance function to a set are considered. A series of characterizing results is obtained. 相似文献
9.
Eva Lesanska 《Applications of Mathematics》2002,47(5):411-426
The problem is to determine nonsensitiveness regions for threshold ellipsoids within a regular mixed linear model. 相似文献
10.
B. Dubrulle F. Hersant 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,26(3):379-386
We generalize an analogy between rotating and stratified shear flows. This analogy is summarized in Table 1. We use this analogy
in the unstable case (centrifugally unstable flow vs. convection) to compute the torque in Taylor-Couette configuration, as a function of the Reynolds number. At low Reynolds
numbers, when most of the dissipation comes from the mean flow, we predict that the non-dimensional torque G = T/ν2
L, where L is the cylinder length, scales with Reynolds number R and gap width η, G = 1.46η3/2(1 - η)-7/4
R
3/2. At larger Reynolds number, velocity fluctuations become non-negligible in the dissipation. In these regimes, there is no
exact power law dependence the torque versus Reynolds. Instead, we obtain logarithmic corrections to the classical ultra-hard (exponent 2) regimes: G = 0.50
. These predictions are found to be in excellent agreement with avail-able experimental data. Predictions for scaling of velocity
fluctuations are also provided.
Received 7 June 2001 and Received in final form 7 December 2001 相似文献