全文获取类型
收费全文 | 85440篇 |
免费 | 10929篇 |
国内免费 | 8602篇 |
专业分类
化学 | 39081篇 |
晶体学 | 808篇 |
力学 | 3588篇 |
综合类 | 476篇 |
数学 | 6483篇 |
物理学 | 22285篇 |
综合类 | 32250篇 |
出版年
2024年 | 347篇 |
2023年 | 1468篇 |
2022年 | 2499篇 |
2021年 | 2674篇 |
2020年 | 2714篇 |
2019年 | 2378篇 |
2018年 | 2257篇 |
2017年 | 2189篇 |
2016年 | 3109篇 |
2015年 | 3449篇 |
2014年 | 4436篇 |
2013年 | 5305篇 |
2012年 | 6269篇 |
2011年 | 6515篇 |
2010年 | 4856篇 |
2009年 | 4970篇 |
2008年 | 5214篇 |
2007年 | 4933篇 |
2006年 | 4588篇 |
2005年 | 3793篇 |
2004年 | 3053篇 |
2003年 | 2472篇 |
2002年 | 2561篇 |
2001年 | 2269篇 |
2000年 | 2170篇 |
1999年 | 2513篇 |
1998年 | 2070篇 |
1997年 | 2022篇 |
1996年 | 1927篇 |
1995年 | 1618篇 |
1994年 | 1448篇 |
1993年 | 1182篇 |
1992年 | 1115篇 |
1991年 | 946篇 |
1990年 | 874篇 |
1989年 | 727篇 |
1988年 | 676篇 |
1987年 | 459篇 |
1986年 | 291篇 |
1985年 | 222篇 |
1984年 | 135篇 |
1983年 | 78篇 |
1982年 | 65篇 |
1981年 | 46篇 |
1980年 | 24篇 |
1979年 | 15篇 |
1959年 | 3篇 |
1957年 | 8篇 |
1955年 | 5篇 |
1930年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
61.
We extend the matrix version of Cochran's statistical theorem to outer inverses of a matrix. As applications, we investigate the Wishartness and independence of matrix quadratic forms for Kronecker product covariance structures. 相似文献
62.
In this paper, the transformation processes of two types of bis(8-hydroxyquinoline)zinc: Znq2 dihydrate and anhydrous (Znq2)4 were investigated by X-ray diffraction (XRD), infrared spectra (IR), scanning electron microscope (SEM), differential scanning calorimetry (DSC) and thermogravimetry (TG). The effects of crystal structure on optical properties of bis(8-hydroxyquinoiline)zinc were analyzed. Znq2 dihydrate can be transformed into anhydrous (Znq2)4 during heating under vacuum. Reversal transformation occurs by the interaction between chloroform and (Znq2)4. But (Znq2)4 was partially transformed into Znq2 dihydrate by the interaction between ethanol and (Znq2)4. The different molecular structure results in different crystal stacking and electronic structure, thereby affect its optical properties. 相似文献
63.
A Promising MoO_x-based Catalyst for n-Heptane Isomerization 总被引:1,自引:0,他引:1
Song Hai CHAI Xin Ping WANG* Ying Jun WANG Tian Xi CAI State Key Laboratory of Fine Chemicals Dalian University of Technology Dalian 《中国化学快报》2003,14(7)
The increasing demand for higher-octane gasoline and the regulations limiting the amount of aromatics in the fuel motivate the interest in catalytic isomerization of n-alkanes. In the last ten years, transition metal oxides or oxycarbides based on molybdenum or tungstate have attracted much attention due to their high activity and isomerization selectivity compared to the conventional bifunctional supported platinum catalyst and high resistance to sulphur and nitrogen catalyst poisons1-5. Ma… 相似文献
64.
65.
物理模型教学与创新能力的培养 总被引:1,自引:0,他引:1
现代教学以能力培养为重点.学生获得研究问题、探索问题的能力是十分重要的,物理模型的教学过程对培养学生研究问题、探索问题、发展创造思维、培养辩证唯物主义思想方法都极为有利,因此,必须重视物理模型教学. 相似文献
66.
针对目前电磁场数值处理中计算繁杂慢速和耗费资源过多的问题,本文提出了一种基于神经网络小波理论进行计算的新方法。文中着重介绍了利用小波变换神经网络的基本理论,结合电磁场数值计算的基本方法,推导变换进而得到一个通用公式和处理方法。在该方法中,神经网络学习内容少和数据处理简单,因此该方法具有计算速度快,占用资源少的优点,并有用硬件实现的前景。 相似文献
67.
Wang Qing-Lun Zhao Bin Liao Dai-Zheng Yan Shi-Ping Cheng Peng Jiang Zong-Hui 《Transition Metal Chemistry》2003,28(3):326-330
The copper(II) complex Na2[Cu(pba)] · 6H2O has been synthesized and its structure determined. It consists of a novel two-dimensional network of [Cu(pba)]2– anions connected to each other through the sodium ion as a bridge. Spectroscopic and magnetic properties have been measured and a spin distribution calculation has been carried out with the GAUSSIAN-94. Theoretical calculations indicate the rather larger spin density on nitrogen atoms should be responsible for the satellite lines appearing beside the main hyperfine coupling signs of copper(II). The antiferromagnetic interaction may be due to the intermolecular interaction and/or different copper(II) ions through the Na+ bridge in the same layer. 相似文献
68.
A new family of photonucleases, naphthalimide-thiazoles was synthesized and evaluated. These compounds intercalated into DNA efficiently and damaged DNA photochemically at concentrations as low as 5 μM. Mechanistic experiment suggests that a novel naphthalimide-thiazole radical produced via an excited triple state might be involved in the DNA photodamage. Different activity may arise from the impact of substituents at 2-phenyl ring of thiazole on the electron population of excited triple state according to AM1 semi-empirical calculation. 相似文献
69.
H.L. Xu A. Persson S. Svanberg 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2003,23(2):233-236
New radiative lifetime measurements based on time-resolved laser-induced fluorescence techniques are reported for 18 even-parity
levels belonging to the 4f5d26p and 4f
25d
2 configurations of Ce I and 6 even-parity levels belonging to the 5d26s, 4f5d6p, and 4f6s6p configurations of Ce II. Free neutral and singly ionized cerium atoms were produced by laser ablation. The Ce I and Ce II
levels range in energy from 26 545 to 29 102 cm-1, and 42 573 to 48 152 cm-1, respectively.
Received 25 September 2002 Published online 4 March 2003 相似文献
70.
揭示了城市技术创新系统和城乡统筹发展的内涵,并在技术创新、增长极和区域经济等理论框架下阐述了城市技术创新与城乡统筹发展的关系,进而提出城市技术创新促进城乡统筹的路径。 相似文献