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   113605篇
  免费   30628篇
  国内免费   19294篇
化学   73923篇
晶体学   1456篇
力学   7666篇
综合类   259篇
数学   13431篇
物理学   66792篇
  2024年   1120篇
  2023年   1685篇
  2022年   2369篇
  2021年   2622篇
  2020年   2989篇
  2019年   3465篇
  2018年   2932篇
  2017年   3318篇
  2016年   4139篇
  2015年   4197篇
  2014年   4501篇
  2013年   6807篇
  2012年   7632篇
  2011年   9106篇
  2010年   11311篇
  2009年   11386篇
  2008年   5479篇
  2007年   4799篇
  2006年   4303篇
  2005年   4290篇
  2004年   4808篇
  2003年   3875篇
  2002年   3607篇
  2001年   3754篇
  2000年   2814篇
  1999年   3191篇
  1998年   2702篇
  1997年   2491篇
  1996年   2793篇
  1995年   3142篇
  1994年   3130篇
  1993年   3125篇
  1992年   2699篇
  1991年   2324篇
  1990年   1971篇
  1989年   1999篇
  1988年   1938篇
  1987年   1221篇
  1986年   1302篇
  1985年   932篇
  1984年   1007篇
  1982年   922篇
  1981年   761篇
  1980年   779篇
  1979年   530篇
  1978年   533篇
  1977年   633篇
  1976年   1042篇
  1973年   438篇
  1972年   532篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
41.
We described the design and synthesis of a colorimetric and ratiometric fluorescent probe based on a conjugated π-electron system that displayed differential fluorescence responses towards cyanide and sulfide and demonstrated its utility in intracellular ion imaging and logic gate.  相似文献   
42.
Amino acids are firstly employed in transition-metal free heterocyclization reaction of 1,3-diynes in the presence of K3PO4 and DMSO at 120 °C. This method produces 2,3,6-trisubstituted pyridines with up to 86% yield. The –CO2H group on the amino acids is crucial for this heterocyclization reaction. The mechanism of such a heterocyclization reaction is discussed, as well.  相似文献   
43.
He  Yanfang  Niu  Xiangheng  Shi  Libo  Zhao  Hongli  Li  Xin  Zhang  Wenchi  Pan  Jianming  Zhang  Xifeng  Yan  Yongsheng  Lan  Minbo 《Mikrochimica acta》2017,184(7):2181-2189
Microchimica Acta - The authors describe a photometric method for the determination of free cholesterol based on the oxidation of cholesterol by the catalytic action of the enzyme...  相似文献   
44.
The effect of combined filtration efficiency on the performance of anthracite filtration?micro electrolysis?sand filtration (AMS) was investigated. Impact of different operating parameters, such as iron?carbon ratio of micro electrolytic units, filtration velocity of AMS, were studied. It was found that when iron?carbon ratio was 6: 4, the AMS’s average turbidity removal rate was 96.75% at the filtration velocity of 3 m h?1. The results showed that when the filtration rate was 3?9 m h?1 and iron?carbon ratio was 6: 4, the turbidity removal efficiency was over 94%, and the turbidity of the effluent was less than 1 NTU in effective filtration cycle. The effective filtration cycle can last for more than 5 h or longer. Meanwhile, the removal rate of UV254 was above 33%, and the concentration of iron ions in the effluent is less than 0.15 mg L?1. Turbidity and iron indicators have reached the national drinking water standards. It was also found the mechanism of iron?carbon micro electrolytic enhanced filtration by infrared spectroscopy and scanning electron microscopy. And It was also speculated the reasons for the reaction passivation.  相似文献   
45.
46.
Vizing conjectured that every edge chromatic critical graph contains a 2-factor. Believing that stronger properties hold for this class of graphs, Luo and Zhao (2013) showed that every edge chromatic critical graph of order n with maximum degree at least 6n7 is Hamiltonian. Furthermore, Luo et al. (2016) proved that every edge chromatic critical graph of order n with maximum degree at least 4n5 is Hamiltonian. In this paper, we prove that every edge chromatic critical graph of order n with maximum degree at least 3n4 is Hamiltonian. Our approach is inspired by the recent development of Kierstead path and Tashkinov tree techniques for multigraphs.  相似文献   
47.
An arc uv of a digraph D is called universal if uv and w are in a common cycle for any vertex w of D. In this paper we characterize local tournaments whose every arc is universal.  相似文献   
48.
Developing a green and sustainable method to upgrade biogas wastes into high value-added products is attracting more and more public attention. The application of solid residues as a performance enhancer in the manufacture of biofilms is a prospective way to replace conventional plastic based on fossil fuel. In this work, solid digestates from the anaerobic digestion of agricultural wastes, such as straw, cattle and chicken manures, were pretreated by an ultrasonic thermo-alkaline treatment to remove the nonfunctional compositions and then incorporated in plasticized starch paste to prepare mulching biofilms by the solution casting method. The results indicated that solid digestate particles dispersed homogenously in the starch matrix and gradually aggregated under the action of a hydrogen bond, leading to a transformation of the composites to a high crystalline structure. Consequently, the composite biofilm showed a higher tensile strength, elastic modulus, glass transition temperature and degradation temperature compared to the pure starch-based film. The light, water and GHG (greenhouse gas) barrier properties of the biofilm were also reinforced by the addition of solid digestates, performing well in sustaining the soil quality and minimizing N2O or CH4 emissions. As such, recycling solid digestates into a biodegradable plastic substitute not only creates a new business opportunity by producing high-performance biofilms but also reduces the environmental risk caused by biogas waste and plastics pollution.  相似文献   
49.
Rutin (R) and quercetin (Q) are two widespread dietary flavonoids. Previous studies regarding the plasma cholesterol-lowering activity of R and Q generated inconsistent results. The present study was therefore carried out to investigate the effects of R and Q on cholesterol metabolism in both HepG2 cells and hypercholesterolemia hamsters. Results from HepG2 cell experiments demonstrate that both R and Q decreased cholesterol at doses of 5 and 10 µM. R and Q up-regulated both the mRNA and protein expression of sterol regulatory element binding protein 2 (SREBP2), low-density lipoprotein receptor (LDLR), and liver X receptor alpha (LXRα). The immunofluorescence study revealed that R and Q increased the LDLR expression, while only Q improved LDL-C uptake in HepG2 cells. Results from hypercholesterolemia hamsters fed diets containing R (5.5 g/kg diet) and Q (2.5 g/kg diet) for 8 weeks demonstrate that both R and Q had no effect on plasma total cholesterol. In the liver, only Q reduced cholesterol significantly. The discrepancy between the in vitro and in vivo studies was probably due to a poor bioavailability of flavonoids in the intestine. It was therefore concluded that R and Q were effective in reducing cholesterol in HepG2 cells in vitro, whereas in vivo, the oral administration of the two flavonoids had little effect on plasma cholesterol in hamsters.  相似文献   
50.
Wang  Danbo  Guo  Guangming  Chen  Renzeng  Gong  Yuzhen  Sun  Lishui  Zhao  Yingjie 《中国科学:化学(英文版)》2021,64(9):1510-1514
Two charge-transfer single crystals induced by donor-acceptor interactions were obtained. The presence of different solvent molecules led to 1D(linear) or 2D self-assembly modes. More interestingly, the 2 D organic framework, a layer-by-layer 2D organic framework single crystal, showed a stronger charge-transfer effect than the 1D cocrystal.  相似文献   
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