全文获取类型
收费全文 | 26628篇 |
免费 | 3705篇 |
国内免费 | 2666篇 |
专业分类
化学 | 19831篇 |
晶体学 | 267篇 |
力学 | 1225篇 |
综合类 | 214篇 |
数学 | 3086篇 |
物理学 | 8376篇 |
出版年
2024年 | 45篇 |
2023年 | 453篇 |
2022年 | 763篇 |
2021年 | 751篇 |
2020年 | 961篇 |
2019年 | 1023篇 |
2018年 | 765篇 |
2017年 | 689篇 |
2016年 | 1158篇 |
2015年 | 1167篇 |
2014年 | 1323篇 |
2013年 | 1735篇 |
2012年 | 2166篇 |
2011年 | 2277篇 |
2010年 | 1558篇 |
2009年 | 1506篇 |
2008年 | 1711篇 |
2007年 | 1571篇 |
2006年 | 1364篇 |
2005年 | 1226篇 |
2004年 | 1110篇 |
2003年 | 1024篇 |
2002年 | 1083篇 |
2001年 | 942篇 |
2000年 | 644篇 |
1999年 | 533篇 |
1998年 | 354篇 |
1997年 | 307篇 |
1996年 | 315篇 |
1995年 | 259篇 |
1994年 | 234篇 |
1993年 | 197篇 |
1992年 | 200篇 |
1991年 | 185篇 |
1990年 | 181篇 |
1989年 | 122篇 |
1988年 | 125篇 |
1987年 | 82篇 |
1986年 | 95篇 |
1985年 | 117篇 |
1984年 | 87篇 |
1983年 | 76篇 |
1982年 | 51篇 |
1981年 | 48篇 |
1980年 | 48篇 |
1979年 | 36篇 |
1977年 | 37篇 |
1976年 | 37篇 |
1974年 | 48篇 |
1973年 | 44篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
81.
非负矩阵与有向图的谱半径 总被引:2,自引:0,他引:2
本文给出非负矩阵的谱半径的上界、下界,由此给出有向图的谱半径的界. 相似文献
82.
张(1884—1950),植物学家和生物学教育家.中国近代植物学的先驱、现代实验性植物生理学和植物生态学的奠基人.最先自编我国第一套植物学、植物形态学、植物解剖学、种子植物分类学、植物生态学和植物生理学教科书.长期从事植物学科的教学和研究工作,为我国的生物科学和教育事业的建立和发展,贡献了自己的一切知识和力量. 相似文献
83.
J.S. Tsay H.W. Chang Y.L. Chiou K.T. Huang W.Y. Chan Y.D. Yao 《Journal of magnetism and magnetic materials》2006
Influences of oxygen exposure on the magnetic properties of Co/Ge(1 1 1) ultrathin films have been investigated by surface magneto-optic Kerr effect technique. As the oxygen exposure increases on Co/Ge(1 1 1) films, their magnetic properties could be modified. As an example for 15 ML Co/Ge(1 1 1) films, the coercivity increases from 730 to 920 Oe and the remanence Kerr intensity is reduced for 500 Langmuir (L) of oxygen exposure. Corresponding compositions analyzed by Auger electron spectroscopy measurement shows that the amount of oxygen on the surface layers increases with increasing the oxygen exposure time. Oxygen distributes on the topmost layers of the film. The adsorbed oxygen influences the electronic density of states of Co and results in the changes of the magnetic properties. Besides, the appearance of O/Co/Ge interface could modify the stress anisotropy, and as a result the coercivity of ultrathin Co/Ge(1 1 1) film is enhanced. 相似文献
84.
85.
For approximate wave functions, we prove the theorem that there is a one‐to‐one correspondence between the constraints of normalization and of the Fermi–Coulomb and Coulomb hole charge sum rules at each electron position. This correspondence is surprising in light of the fact that normalization depends on the probability of finding an electron at some position. In contrast, the Fermi–Coulomb hole sum rule depends on the probability of two electrons staying apart because of correlations due to the Pauli exclusion principle and Coulomb repulsion, while the Coulomb hole sum rule depends on Coulomb repulsion. We demonstrate the theorem for the ground state of the He atom by the use of two different approximate wave functions that are functionals rather than functions. The first of these wave function functionals is constructed to satisfy the constraint of normalization, and the second that of the Coulomb hole sum rule for each electron position. Each is then shown to satisfy the other corresponding sum rule. The significance of the theorem for the construction of approximate “exchange‐correlation” and “correlation” energy functionals of density functional theory is also discussed. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 相似文献
86.
87.
The new phenylpropanoid diglycoside ligusinenoside A ( 1 ), and the two new 8,4′‐oxyneolignan(‘8‐O‐4′‐neolignan’) diglycosides ligusinenosides B ( 2 ) and C ( 3 ), together with nine known compounds, were isolated from the rhizomes of Ligusticum sinensis Oliv. The structures of 1 – 3 were elucidated on the basis of spectroscopic analyses. 相似文献
88.
89.
Ju Yeon Woo Eun Hee Kim Tae Hoon Yoon Jae Chang Kim Byung Kyu Kim 《Liquid crystals》2007,34(9):1115-1120
Chiral dopants were added to the formulation of holographic polymer-dispersed liquid crystals and the effects studied in terms of grating formation dynamics, morphology, diffraction efficiency, contrast ratio and electro-optical properties of the films. A gradual increase of real-time diffraction efficiency, decrease of droplet size and increase of diffraction efficiency of the composite film were obtained with the addition and increasing content of chiral dopant, due to the increased viscosity of the liquid crystal (LC) doped with the chiral dopant leading to decreased droplet coalescence. The contrast ratio decreased with increasing content of chiral dopant due to the difficult orientation of LC molecules caused by the formation of a helical structure. Addition of a small amount of the chiral dopant increased the driving voltage slightly, whereas the decay time is decreased significantly as a result of the high twisting of the helical structure. 相似文献
90.
应用七级目测法对浙江中部地区27个夏熟作物田样点的杂草进行了优势等级调查,将所得数据转换成重要值,以杂草在27个样点中的重要值为运算指标,应用主成分分析法和图论聚类中的最小生成树法,对24种杂草的生态学相似性进行了比较,指出了长期使用单一的除草剂导致农田杂草种群迅速更迭的原因:杂草对除草剂的敏感性差异及不同杂草间的生态学特性的差异。 相似文献