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   44022篇
  免费   883篇
  国内免费   466篇
化学   21341篇
晶体学   379篇
力学   2210篇
综合类   15篇
数学   10793篇
物理学   10633篇
  2023年   131篇
  2022年   268篇
  2021年   744篇
  2020年   674篇
  2019年   733篇
  2018年   1922篇
  2017年   2084篇
  2016年   1682篇
  2015年   1222篇
  2014年   1396篇
  2013年   2308篇
  2012年   4380篇
  2011年   3460篇
  2010年   2601篇
  2009年   2247篇
  2008年   1296篇
  2007年   1252篇
  2006年   1054篇
  2005年   4840篇
  2004年   4167篇
  2003年   2551篇
  2002年   695篇
  2001年   386篇
  2000年   156篇
  1999年   256篇
  1998年   163篇
  1997年   135篇
  1996年   100篇
  1995年   81篇
  1994年   93篇
  1993年   47篇
  1992年   213篇
  1991年   194篇
  1990年   173篇
  1989年   148篇
  1988年   134篇
  1987年   101篇
  1986年   74篇
  1985年   84篇
  1984年   74篇
  1983年   47篇
  1982年   57篇
  1980年   47篇
  1979年   55篇
  1978年   48篇
  1976年   99篇
  1975年   51篇
  1974年   57篇
  1973年   62篇
  1972年   51篇
排序方式: 共有10000条查询结果,搜索用时 328 毫秒
1.
Molecular Diversity - The preparation, characterization and application of hydroxyapatite silica propyl bis aminoethoxy ethane cuprous complex (HASPBAEECC) as a novel hybrid nano-catalyst for...  相似文献   
2.

In the present research, magnesium aluminate spinel was prepared as catalyst support using a novel, facile, and efficient mechanochemical method. The Co-promoted catalysts with 20 wt.% of Ni were fabricated using an impregnation route and the samples were analyzed by the X-ray diffraction (XRD), N2 adsorption/desorption (BET), temperature-programmed reduction and desorption (H2-TPR and O2-TPD), and field emission scanning electron microscopy (FESEM) tests. The results confirmed that all samples have a mesoporous structure with a high specific surface area and the presence of cobalt caused complete CH4 oxidation at low temperatures, and no side reactions were observed. The results indicated that the 3%Co-20%Ni/MgAl2O4 catalyst was the optimal sample among the prepared catalysts, owing to the improvement of reduction features and oxygen mobility. The 50 and 90% of methane conversion was obtained at 530 and 600 °C, respectively. Also, the influence of calcination temperature, GHSV, and feed ratio was determined on the catalytic activity. The obtained outcomes revealed that the calcination temperature has a significant effect on the textural properties and catalytic efficiency. The sample calcined at 700 °C showed the weakest performance, which was related to the sintering of particles at high temperatures. The catalytic stability showed that the 3%Co-20%Ni/MgAl2O4 has acceptable stability during 600 min time of reaction.

Graphical abstract
  相似文献   
3.
Molecular Diversity - A new series of azo chromene dyes were synthesized via a facile cyclocondensation reaction of (E)-1,2-diphenyl-1-diazene and 4-aminocoumarin with 1:2 molar ratio catalyzed by...  相似文献   
4.
The requirement of green and sustainable materials to prepare heterogeneous catalysts has intensified for practical reasons over the past few decades. Carbohydrates are possibly the most plentiful and renewable organic materials in nature with inimitable physiochemical properties, plausible low-cost and large-scale production, and sustainability features could be exploited in the generation of nanostructured heterogeneous catalysts. This review article outlines the organic transformations catalyzed by diverse carbohydrate-based nanostructured catalysts in greener and environmentally friendly processes. Selected examples are highlighted for a variety of organic reactions exploiting the proposed catalysts’ reactivity and reusability, and interactions with the intrinsic nature of the applied carbohydrate supports; advantages and speculated challenges of the introduced catalysts are deliberated as well.  相似文献   
5.
A dispersive solid-phase extraction method based on a new sorbent has been performed on plasma and wastewater samples to determine metoprolol by high-performance liquid chromatography–tandem mass spectrometry. In this study, the analyte was adsorbed from the samples onto microcrystalline cellulose as a green and efficient sorbent and then eluted for use in the determination step. In the mass spectrometer, the analyte was detected in the positive mode and selectivity of the analysis was increased by sequential mass analysis through multiple reaction monitoring. All of the effective parameters in the extraction of metoprolol from plasma and wastewater were optimized. Under optimal conditions the method was linear in the ranges of 1–1,000 and 0.1–1,000 ng/ml in plasma and wastewater samples, respectively. The detection limits of the method were 0.30 and 0.03 ng/ml in plasma and wastewater samples, respectively. The data showed that the method provides low detection limit, wide linear range, good precision and high extraction recovery. Finally several plasma and wastewater samples were successfully analyzed using the method. The use of a small amount of a green and inexpensive sorbent and a low volume of plasma without the need for further pretreatment steps are the main advantages of the method.  相似文献   
6.
7.
8.
Journal of Algebraic Combinatorics - The symbolic powers, in general, are not equal to the ordinary powers. Therefore, one interesting question here is for what classes of ideals ordinary and...  相似文献   
9.
Journal of Sol-Gel Science and Technology - In this paper, a simple method is presented for making a conductive and hydrophobic cotton fabric using a multi-walled carbon nanotube (MWCNT). The...  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