全文获取类型
收费全文 | 117120篇 |
免费 | 23654篇 |
国内免费 | 5462篇 |
专业分类
化学 | 97819篇 |
晶体学 | 804篇 |
力学 | 6025篇 |
综合类 | 297篇 |
数学 | 16101篇 |
物理学 | 25190篇 |
出版年
2024年 | 569篇 |
2023年 | 5125篇 |
2022年 | 2961篇 |
2021年 | 4263篇 |
2020年 | 6559篇 |
2019年 | 4194篇 |
2018年 | 4966篇 |
2017年 | 3341篇 |
2016年 | 8533篇 |
2015年 | 8326篇 |
2014年 | 8165篇 |
2013年 | 9142篇 |
2012年 | 9997篇 |
2011年 | 7054篇 |
2010年 | 7913篇 |
2009年 | 7558篇 |
2008年 | 4435篇 |
2007年 | 3772篇 |
2006年 | 2953篇 |
2005年 | 6235篇 |
2004年 | 5371篇 |
2003年 | 3452篇 |
2002年 | 1798篇 |
2001年 | 1340篇 |
2000年 | 939篇 |
1999年 | 1004篇 |
1998年 | 783篇 |
1997年 | 764篇 |
1996年 | 748篇 |
1995年 | 738篇 |
1994年 | 576篇 |
1993年 | 633篇 |
1992年 | 613篇 |
1991年 | 512篇 |
1990年 | 424篇 |
1989年 | 360篇 |
1988年 | 350篇 |
1987年 | 266篇 |
1979年 | 247篇 |
1978年 | 249篇 |
1977年 | 354篇 |
1976年 | 475篇 |
1975年 | 517篇 |
1974年 | 544篇 |
1973年 | 372篇 |
1972年 | 495篇 |
1971年 | 441篇 |
1970年 | 659篇 |
1969年 | 497篇 |
1968年 | 537篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
91.
A lithium(I) coordination polymer has been formed from LiClO4 and the 2,2′‐bipyrimidine (bpym) ligand in which each square pyramidal lithium(I) atom is coordinated in the basal plane by four nitrogen donor atoms derived from two bpym ligands and one water molecule at the apical position. These are connected into a layer structure via hydrogen‐bonding interactions involving the perchlorate anions. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
92.
93.
Akira Yanagisawa Prof. Dr. Takahiro Suzuki Takanori Koide Shogo Okitsu Takayoshi Arai Prof. Dr. 《化学:亚洲杂志》2008,3(10):1793-1800
A Barbier‐type regioselective propargylation of aldehydes and ketones with (3‐bromobut‐1‐ynyl)trimethylsilane has been achieved using reactive barium as a low‐valent metal in THF. Especially in the case of ketones, the corresponding homopropargylic alcohols form almost exclusively. In the reaction of α,β‐unsaturated carbonyl compounds, only 1,2‐adducts have been observed. This method is also applicable to propargylation of imines, and the corresponding homopropargylic amines are obtained regiospecifically in good yields with diastereomeric ratios of up to 87:13. 相似文献
94.
Silica@copper (SiO2@Cu) core–shell nanoparticles were synthesized and well characterized by XRD, TEM, AFM, XPS, UV/Vis, TGA–MS, and ICP–AES techniques. The synthesized SiO2@Cu core–shell nanoparticles were employed as catalysts for the conjugate addition of amines to α,β‐unsaturated compounds in water to obtain β‐amino carbonyl compounds in excellent yields in shorter reaction times. Furthermore, the catalyst works well for hetero‐Michael addition reactions of heteroatom nucleophiles such as thiols to α,β‐unsaturated compounds. As the reaction is performed in water, it allows for easy recycling of the catalyst with consistent activity. 相似文献
95.
96.
You‐Liang Zhao Chuan‐Fu Chen Fu Xi 《Journal of polymer science. Part A, Polymer chemistry》2003,41(14):2156-2165
The atom transfer radical copolymerization of N‐substituted maleimides such as N‐phenylmaleimide (PhMI), N‐cyclohexylmaleimide (ChMI), and N‐butylmaleimide (NBMI) with styrene initiated with dendritic polyarylether 2‐bromoisobutyrates in an ionic liquid, 1‐butyl‐3‐methylimidazolium hexafluorophosphate ([bmim][PF6]), at room temperature and anisole at 110 °C was investigated. The dendritic‐linear block copolymers obtained in ionic liquid possessed well‐defined molecular weight and low polydispersity (1.05 < Mw/Mn < 1.32) and could be used as a macroinitiator for chain‐extension polymerization, suggesting the living nature of the reaction system. The ionic liquids containing catalyst could be recycled in the atom transfer radical polymerization systems without further treatment. Compared with polymerization conducted in anisole, the polymerization in ionic liquid had a stronger tendency for alternation. The tendency for alternation decreased in the order PhMI > NBMI > ChMI in [bmim][PF6] and PhMI > ChMI > NBMI in anisole. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2156–2165, 2003 相似文献
97.
98.
In this article, we present results of an empirical study with 500 German students of grades 7 and 8. The study focussed on students' mathematics achievement and their interest in mathematics as well as on the relation between these two constructs. In particular, the results show that the development of an individual student's achievement between grade 7 and grade 8 depends on the achievement level of the specific classroom and therefore on the specific mathematics instruction Interest in mathematics could be regarded a predictor for mathematics achievement Moreover, our findings suggest that the students show hardly any fear of mathematics independent of their achievement level. 相似文献
99.
有限循环群的Fuzzy子群的等价类数 总被引:4,自引:2,他引:2
有限循环群G的F子群可以有无数个.但是.若当两个F子群的水平集构成的集合相等就称其等价的话,那么其等价类数是有限的。通过研究群的合成群列、商群列以及数的因数列和极大因数列找出了有限循环群的极大F子群和F子群的等价类数的求解公式.并给出二者之间的关系式. 相似文献
100.
A novel super-hydrophobic stearic acid (STA) film with a water contact angle of 166o was prepared by chemical adsorption on aluminum wafer coated with polyethyleneimine (PEI) film. The micro-tribological behavior
of the super-hydrophobic STA monolayer was compared with that of the polished and PEI-coated Al surfaces. The effect of relative
humidity on the adhesion and friction was investigated as well. It was found that the STA monolayer showed decreased friction,
while the adhesive force was greatly decreased by increasing the surface roughness of the Al wafer to reduce the contact area
between the atomic force microscope (AFM) tip and the sample surface to be tested. Thus the friction and adhesion of the Al
wafer was effectively decreased by generating the STA monolayer, which indicated that it could be feasible and rational to
prepare a surface with good adhesion resistance and lubricity by properly controlling the surface morphology and the chemical
composition. Both the adhesion and friction decreased as the relative humidity was lowered from 65% to 10%, though the decrease
extent became insignificant for the STA monolayer.
The project supported by the National Natural Science Foundation of China (50375151, 50323007, 10225209) and the Chinese Academy
of Sciences (KJCX-SW-L2) 相似文献