首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   390844篇
  免费   2269篇
  国内免费   554篇
化学   182486篇
晶体学   5847篇
力学   22400篇
综合类   9篇
数学   56081篇
物理学   126844篇
  2020年   4162篇
  2019年   5055篇
  2018年   7449篇
  2017年   7610篇
  2016年   9532篇
  2015年   4430篇
  2014年   8437篇
  2013年   15856篇
  2012年   12902篇
  2011年   15131篇
  2010年   12616篇
  2009年   12807篇
  2008年   14597篇
  2007年   14381篇
  2006年   12887篇
  2005年   11210篇
  2004年   10843篇
  2003年   10027篇
  2002年   10339篇
  2001年   10480篇
  2000年   8124篇
  1999年   6191篇
  1998年   5690篇
  1997年   5481篇
  1996年   5011篇
  1995年   4469篇
  1994年   4507篇
  1993年   4508篇
  1992年   4587篇
  1991年   5076篇
  1990年   5000篇
  1989年   5048篇
  1988年   4695篇
  1987年   4773篇
  1986年   4461篇
  1985年   5353篇
  1984年   5549篇
  1983年   4726篇
  1982年   4972篇
  1981年   4633篇
  1980年   4286篇
  1979年   4924篇
  1978年   5103篇
  1977年   5299篇
  1976年   5450篇
  1975年   5058篇
  1974年   4888篇
  1973年   5135篇
  1972年   4305篇
  1971年   3944篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
The design of new solid-state proton-conducting materials is a great challenge for chemistry and materials science. Herein, a new anionic porphyrinylphosphonate-based MOF ( IPCE-1Ni ), which involves dimethylammonium (DMA) cations for charge compensation, is reported. As a result of its unique structure, IPCE-1Ni exhibits one of the highest value of the proton conductivity among reported proton-conducting MOF materials based on porphyrins (1.55×10−3 S cm−1 at 75 °C and 80 % relative humidity).  相似文献   
2.
3.
4.
5.
6.
7.
8.
The aim of this work was to determine the parameters that have decisive roles in microwave-assisted reactions and to develop a model, using computational chemistry, to predict a priori the type of reactions that can be improved under microwaves. For this purpose, a computational study was carried out on a variety of reactions, which have been reported to be improved under microwave irradiation. This comprises six types of reactions. The outcomes obtained in this study indicate that the most influential parameters are activation energy, enthalpy, and the polarity of all the species that participate. In addition to this, in most cases, slower reacting systems observe a much greater improvement under microwave irradiation. Furthermore, for these reactions, the presence of a polar component in the reaction (solvent, reagent, susceptor, etc.) is necessary for strong coupling with the electromagnetic radiation. We also quantified that an activation energy of 20–30 kcal mol−1 and a polarity (μ) between 7–20 D of the species involved in the process is required to obtain significant improvements under microwave irradiation.  相似文献   
9.
Seven new copper(II) complexes of type [Cu(A)(L)]?H2O (A = sparfloxacin, ciprofloxacin, levofloxacin, gatifloxacin, pefloxacin, ofloxacin, norfloxacin; L = 5‐[(3‐chlorophenyl)diazenyl]‐4‐hydroxy‐1,3‐thiazole‐2(3H)‐thione) were synthesized and characterized using elemental and thermogravimetric analyses, and electronic, electron paramagnetic resonance (EPR), Fourier transform infrared and liquid chromatography–mass spectroscopies. Tetrahedral geometry around copper is assigned in all complexes using EPR and electronic spectral analyses. All complexes were investigated for their interaction with herring sperm DNA utilizing absorption titration (Kb = 1.27–3.13 × 105 M?1) and hydrodynamic volume measurement studies. The studies suggest the classical intercalative mode of DNA binding. The cleavage reaction on pUC19 DNA was monitored by agarose gel electrophoresis. The results indicate that the Cu(II) complexes can more effectively promote the cleavage of plasmid DNA. The superoxide dismutase mimic activity of the complexes was evaluated by nitroblue tetrazolium assay, and the complexes catalysed the dismutation of superoxide at pH = 7.8 with IC50 values in the range 0.597–0.900 μM. The complexes were screened for their in vitro antibacterial activity against five pathogenic bacteria. All the complexes are good cytotoxic agents and show LC50 values ranging from 5.559 to 11.912 µg ml?1. All newly synthesized Cu(II) complexes were also evaluated for their in vitro antimalarial activity against Plasmodium falciparum strain (IC50 = 0.62–2.0 µg ml?1). Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   
10.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号