全文获取类型
收费全文 | 11164篇 |
免费 | 2022篇 |
国内免费 | 1317篇 |
专业分类
化学 | 8198篇 |
晶体学 | 69篇 |
力学 | 688篇 |
综合类 | 82篇 |
数学 | 1091篇 |
物理学 | 4375篇 |
出版年
2024年 | 46篇 |
2023年 | 286篇 |
2022年 | 395篇 |
2021年 | 439篇 |
2020年 | 506篇 |
2019年 | 449篇 |
2018年 | 381篇 |
2017年 | 311篇 |
2016年 | 561篇 |
2015年 | 486篇 |
2014年 | 579篇 |
2013年 | 836篇 |
2012年 | 1040篇 |
2011年 | 1026篇 |
2010年 | 666篇 |
2009年 | 661篇 |
2008年 | 757篇 |
2007年 | 660篇 |
2006年 | 578篇 |
2005年 | 494篇 |
2004年 | 382篇 |
2003年 | 340篇 |
2002年 | 279篇 |
2001年 | 220篇 |
2000年 | 224篇 |
1999年 | 267篇 |
1998年 | 218篇 |
1997年 | 200篇 |
1996年 | 222篇 |
1995年 | 181篇 |
1994年 | 146篇 |
1993年 | 128篇 |
1992年 | 111篇 |
1991年 | 77篇 |
1990年 | 86篇 |
1989年 | 59篇 |
1988年 | 49篇 |
1987年 | 46篇 |
1986年 | 30篇 |
1985年 | 28篇 |
1984年 | 14篇 |
1983年 | 9篇 |
1982年 | 13篇 |
1981年 | 8篇 |
1980年 | 5篇 |
1979年 | 3篇 |
1957年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
971.
Continuous-time portfolio selection with liability: Mean–variance model and stochastic LQ approach 总被引:1,自引:1,他引:0
In this paper we formulate a continuous-time mean–variance portfolio selection model with multiple risky assets and one liability in an incomplete market. The risky assets’ prices are governed by geometric Brownian motions while the liability evolves according to a Brownian motion with drift. The correlations between the risky assets and the liability are considered. The objective is to maximize the expected terminal wealth while minimizing the variance of the terminal wealth. We derive explicitly the optimal dynamic strategy and the mean–variance efficient frontier in closed forms by using the general stochastic linear-quadratic (LQ) control technique. Several special cases are discussed and a numerical example is also given. 相似文献
972.
The electronic structure and properties of polypyrrole (p-Pyr) based substituting by the group of pyrazine (Pyz) and their
model compounds were studied by the density functional theory (DFT) at the B3LYP level with 6-31G* basis set. The bond length,
the topological analyses and nucleus-independent chemical shifts (NICS) were analyzed and correlated with the electronic properties.
The bond length of all compounds is reduced with the increase in the degree of polymerization, and the band gap of these compounds
is decreased. The change of NICS shows that the conjugation degree in central section of the polymeric axis is stronger than
that in outer section, and the structure of central part was close to quinoid structure in polymers. The theoretical results
suggest that the band gap of p-Pyz (polymer of Pyz) (0.37 eV) is much smaller than the band gap of polypyrrole (1.84 eV).
The narrow band gap, large HOMO and LUMO bandwidths and small effective masses make p-Pyz have a remarkable elevation of the
conductivity, so it may be considered as a very good candidate for conducting material. 相似文献
973.
A novel reactive polyorganosiloxane containing nitro groups was first prepared by acid-catalyzed equilibration polymerization. The monomer, 3-(4-nitrophenoxy)propylmethyldichlorosilane (NPPMS), was synthesized by hydrosilylation reaction with dicyclopentadienylplatinum(II) chloride (Cp2PtC12) catalyst in high yield. The synthesized polynitroorganosiloxane, which has potential application as a precursor for synthesizing advanced functional polymers, was characterized by FTIR, 1H-NMR, 13C-NMR, 29Si-NMR, vapor pressure osmometry (VPO) and gel permeation chromatography (GPC). 相似文献
974.
谢帕莎 《广东微量元素科学》1999,6(7):24
总结了抢救急性心肌梗塞猝死的体会,包括:及早确诊及有效的心肺脑复苏、合理使用复苏药物和警惕老年性心梗的不典型性。 相似文献
975.
976.
977.
An alternative approach to synthesize pedamide,a key building block of pederin was described.Iodine-induced asymmetric heterocyclization was used as the key step to construct the skeleton,a tetrahydropyran ring with three chiral centers.Brown's asymmetric allylation and Lewis acid-mediated allylation were investigated to introduce chains and chiral alcohols.Sharpless dihydroxylation decorated the side chain.And high optically pure target was obtained by removing the epimers formed in these reactions on c... 相似文献
978.
A new approach to synthesize tolunitriles is reported. Tolunitriles can be prepared by selective ammoxidation of methylbenzyl
chlorides prepared by chloromethylation of toluene. The total yields can reach 83% and the selectivity of tolunitriles is
almost 100%. This approach provides a new path for preparing alkylbenzonitriles and other aromatic nitriles. 相似文献
979.
Electromagnetically induced phase grating controlled by spontaneous emission is studied. The results show that the diffraction efficiency of phase grating is strikingly enhanced due to the existence of spontaneously generated coherence (SGC), and the diffraction efficiency of 35% can be obtained in optimal SGC parameter case. Furthermore, at large SGC parameter, the high efficiency of phase grating can be maintained by the application of weak coupled and relatively strong probe fields, and choosing the proper length of atomic sample. At the same time, in such an atomic system, the SGC effect depends on incoherent pump. 相似文献
980.