全文获取类型
收费全文 | 190807篇 |
免费 | 10001篇 |
国内免费 | 12842篇 |
专业分类
化学 | 35208篇 |
晶体学 | 395篇 |
力学 | 9425篇 |
综合类 | 1408篇 |
数学 | 32104篇 |
物理学 | 30427篇 |
综合类 | 104683篇 |
出版年
2024年 | 316篇 |
2023年 | 1248篇 |
2022年 | 2151篇 |
2021年 | 2535篇 |
2020年 | 2867篇 |
2019年 | 2990篇 |
2018年 | 2692篇 |
2017年 | 3296篇 |
2016年 | 3687篇 |
2015年 | 4305篇 |
2014年 | 7382篇 |
2013年 | 9253篇 |
2012年 | 9005篇 |
2011年 | 10353篇 |
2010年 | 8549篇 |
2009年 | 10449篇 |
2008年 | 11183篇 |
2007年 | 13124篇 |
2006年 | 12372篇 |
2005年 | 11466篇 |
2004年 | 9767篇 |
2003年 | 9322篇 |
2002年 | 8184篇 |
2001年 | 6947篇 |
2000年 | 6315篇 |
1999年 | 5305篇 |
1998年 | 4611篇 |
1997年 | 4012篇 |
1996年 | 3673篇 |
1995年 | 3337篇 |
1994年 | 3113篇 |
1993年 | 2757篇 |
1992年 | 2599篇 |
1991年 | 2271篇 |
1990年 | 2070篇 |
1989年 | 1956篇 |
1988年 | 1653篇 |
1987年 | 1099篇 |
1986年 | 799篇 |
1985年 | 646篇 |
1984年 | 576篇 |
1983年 | 286篇 |
1982年 | 454篇 |
1981年 | 502篇 |
1980年 | 389篇 |
1979年 | 379篇 |
1978年 | 294篇 |
1977年 | 276篇 |
1976年 | 220篇 |
1973年 | 161篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
61.
基于时变Copula模型,获得预测方差,确定单个基金收益率序列的边缘分布.利用常见的静态Copula和时变Copula模型对基金收益率序列间两两相依关系进行建模并进行对比分析.应用研究表明,基于MCMC方法的时变Copula模型能更有效地度量基金收益率序列的风险. 相似文献
62.
63.
We give a sheaf theoretic interpretation of Potts models with external magnetic field, in terms of constructible sheaves and their Euler characteristics. We show that the polynomial countability question for the hypersurfaces defined by the vanishing of the partition function is affected by changes in the magnetic field: elementary examples suffice to see non-polynomially countable cases that become polynomially countable after a perturbation of the magnetic field. The same recursive formula for the Grothendieck classes, under edge-doubling operations, holds as in the case without magnetic field, but the closed formulae for specific examples like banana graphs differ in the presence of magnetic field. We give examples of computation of the Euler characteristic with compact support, for the set of real zeros, and find a similar exponential growth with the size of the graph. This can be viewed as a measure of topological and algorithmic complexity. We also consider the computational complexity question for evaluations of the polynomial, and show both tractable and NP-hard examples, using dynamic programming. 相似文献
64.
Jiatian Chen Le Yang Ying Li Qinghua Hou Lanlan Li Peng Jin 《International journal of quantum chemistry》2019,119(16):e25961
Inorganic planar ring-shape molecules with 4n + 2 π electrons are always the focus of experimental synthesis and theoretical research due to their potential aromaticity and stability. In this work, the whole series of five-membered heterocycle monoanions X nY5-n− (X, Y = group 15 elements; n = 1-4) were thoroughly investigated by means of density functional theory calculations. They all have large formation energies and HOMO-LUMO gap energies, suggesting the potential thermodynamic and kinetic stability. Their aromaticities are comparable to that of typical aromatic hydrocarbons. Their thermal stabilities were firmly established by the ab initio molecular dynamics simulations. As most of them are predicted for the first time, their various spectra were simulated for experimental characterization. Furthermore, we demonstrate that these five-membered cyclic anions can be employed as η5-ligand to construct novel all-inorganic metallocenes, which may serve as the building blocks of low-dimensional nanomaterials. 相似文献
65.
66.
67.
《Physics letters. A》2020,384(35):126904
Exploring the evolution of cooperation has been one of the most challenging topics and attracted increasing attention in various realms. Hereby, we institute an analytic model where the relationship between the common benefits and contributions is the typical saturation nonlinearity. We investigate the evolutionary dynamics of cooperation with and without consensual incentives respectively. The population always evolves to full defection when the incentives are absent. Cooperation, however, can be promoted by consensual incentives when the degree of cooperators' preferences for incentives is great. There exists an intermediate amount of resources at which the positive effect of consensual incentives on cooperation is minimized. Furthermore, there is a U-shape relationship between the degree of cooperators' preference for an incentive and the evolution of cooperation. A greater basin of full cooperation can be induced when the cooperators' preference for reward is more pronounced than that for punishment. 相似文献
68.
Norbornene derivatives from a metal‐free,strain‐promoted cycloaddition reaction: New building blocks for ring‐opening metathesis polymerization reactions
下载免费PDF全文
![点击此处可从《Journal of polymer science. Part A, Polymer chemistry》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Christopher E. Hobbs Binhong Lin Thomas Malinski 《Journal of polymer science. Part A, Polymer chemistry》2015,53(20):2357-2362
The preparation of new ring opening metathesis polymerization (ROMP) monomers using a 1,3‐dipolar cycloaddition between aryl azides and norbornadiene is described. Various norbornenetriazolines, obtained through a solvent‐and catalyst‐free reaction, can subsequently be incorporated into polymer backbones through ROMP reactions. Furthermore, thermal decomposition of the triazoline moiety can allow for further polymer functionalization. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 2357–2362 相似文献
69.
Alkynes cycloaddition reactions are powerful tools for constructing cyclic molecules with optimal atom efficiency, but these reactions cannot proceed at ambient temperature without transition-metal catalysts. In this work, a heterobimetallic complex featuring an Nb–Fe triple bond, Nb(iPrNPMe2)3Fe–PMe3, has been evaluated as the potential catalyst for acetylene cycloaddition, using density functional theory. The calculated results show that the singlet-state (i.e. ground-state) Nb(iPrNPMe2)3Fe–PMe3 can be applied to benzene synthesis, but is not suitable for cyclobutadiene. Benzene can be obtained easily at room temperature and is the unique product on the singlet potential surface. The irradiation of infrared-red light can drive the excitation of singlet Nb(iPrNPMe2)3Fe–PMe3 to its triplet state. Both benzene and cyclobutadiene can be formed on the triplet reaction potential surface due to their low energy barriers. Therefore, Nb(iPrNPMe2)3Fe–PMe3 is a potential high reactivity heterobimetallic catalyst for the cyclotrimerization of alkynes. In the reaction process, the catalytic active site of Nb(iPrNPMe2)3Fe–PMe3 moves from niobium to iron. 相似文献
70.
综述了创新、创新监测与评价的研究进展,辨析了国家创新能力、创新型国家和科技强国的内涵特征,分析了中国创新能力在世界上的相对位置,探讨了建设科技强国的经验及启示。 相似文献