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   841篇
  免费   23篇
  国内免费   4篇
化学   626篇
晶体学   4篇
力学   12篇
数学   141篇
物理学   85篇
  2023年   5篇
  2022年   18篇
  2021年   25篇
  2020年   13篇
  2019年   20篇
  2018年   14篇
  2017年   15篇
  2016年   21篇
  2015年   27篇
  2014年   34篇
  2013年   52篇
  2012年   53篇
  2011年   64篇
  2010年   37篇
  2009年   40篇
  2008年   65篇
  2007年   45篇
  2006年   50篇
  2005年   40篇
  2004年   41篇
  2003年   32篇
  2002年   26篇
  2001年   6篇
  2000年   6篇
  1999年   6篇
  1998年   8篇
  1997年   13篇
  1996年   8篇
  1995年   7篇
  1994年   6篇
  1993年   7篇
  1992年   6篇
  1991年   6篇
  1990年   4篇
  1988年   4篇
  1987年   3篇
  1986年   2篇
  1985年   6篇
  1984年   7篇
  1983年   4篇
  1982年   6篇
  1980年   2篇
  1979年   2篇
  1977年   4篇
  1976年   1篇
  1975年   1篇
  1974年   1篇
  1972年   1篇
  1970年   1篇
  1959年   1篇
排序方式: 共有868条查询结果,搜索用时 62 毫秒
31.
In the last two decades protection against electrostatic hazards became a very important topic. The increase in the range of possible faults fast automated systems and complex fault analysis is required.The tools of artificial intelligence and expert systems have been applied successfully on this field and this paper aims to take a step further. While giving some insight to the currently used tools, another AI method, the 'support vector machines' are introduced in this paper. Besides a brief review on SVMs they are introduced to the SCOUT system, a novel approach to electrostatic hazard management.  相似文献   
32.
33.
A novel optically active 5,5-dioxophenothiazine-1,9 bis(thiourea) containing glucopyranosyl groups was synthesized and its enantiomeric recognition properties were examined towards the enantiomers of tetrabutylammonium salts of chiral α-hydroxy and N-protected α-amino acids using UV–vis spectroscopy.  相似文献   
34.
We give a comment to Theorem 1.1 published in our paper “Ring elements as sums of units” [Cent. Eur. J. Math., 2009, 7(3), 395–399].  相似文献   
35.
36.
The process of casting in sand moulds is used for a large volume of castings manufactured in the world. Internal channels and complex geometries of these products are formed by the placement of cores within the mould cavity. Resin-bound sand mixtures are essential ingredients in engine component manufacturing. In this study, a state-of-the-art application of Fourier thermal analysis in foundry technologies is presented. Investigation of decomposition phenomena of resin-bound moulding materials during casting production is a brand new area to use the potential of thermal sciences. Temperature measurements in test samples of standard types of moulding mixtures were performed. The registered cooling curves were processed by a numerical iteration algorithm to determine the amount of heat absorbed during degradation of the moulding material. Thermogravimetric analysis (TG) and differential thermal analysis (DTA) of sand mixtures were carried out to compare the results of the Fourier thermal analysis with TG and DTA curves.  相似文献   
37.
38.
The electronic spectra of neutral PaO and PaO2 and their mono- (PaO+, PaO2 +) and dications (PaO2+, PaO2 2+) were studied by performing multiconfigurational quantum chemical calculations at the CASSCF/CASPT2 level of theory taking into account spin–orbit coupling. Including the protactinium 7s, 6d, and 5f orbitals as well as selected orbitals of oxygen in the active space, the vertical excitation energies at the ground-state geometries have been computed up to ca. 36,000 cm?1. The gas-phase electronic spectra at 298 and 3,000 K were evaluated on the basis of the computed oscillator strengths.  相似文献   
39.
The synthesis of 3-deoxyoripavine (7) was realized as a novel and promising intermediate towards the synthesis of the important class of dopaminergic and/or serotonergic 10-deoxyaporphines and the special pharmacological tool µ opioid antagonist cyprodime. Generally, the preparation of these valuable biologically active compounds was achieved in remarkable yields.   相似文献   
40.
Abstract

Different reaction conditions were investigated for the preparation of benzyl 2-acetamido-3,6-di-O-benzyl-2-deoxy-β-D-glucopyranoside (5). Compound 5 on reaction with 2,3,4,6-tetra-O-acetyl-α-D-galactopyranosyl bromide afforded the 4-O-substituted 2-acetamido-2-deoxy-β-D-glucopyranosyl derivative which, on O-deacetylation, gave benzyl 2-acetamido-3,6-di-O-benzyl-2-deoxy-4-O-β-D-galactopyranosyl-β-D-glucopyranoside (8). The trimethylsilyl (Me3Si) derivative of 8, on treatment with pyridineacetic anhydride-acetic acid for 2 days, gave the disaccharide derivative having an O-acetyl group selectively introduced at the primary position and Me3Si groups at the secondary positions. The latter groups were readily cleaved by treatment with aqueous acetic acid in methanol to afford benzyl 2-acetamido-4-O-(6-O-acetyl-β-D-galactopyranosyl)-3,6-di-O-benzyl-2-deoxy-β-D-glucopyranoside, which on isopropylidenation gave the desired, key intermediate benzyl 2-acetamido-4-O-(6-O-acetyl-3,4-O-isopropylidene-β-D-galactopyranosyl)-3,6-di-O-benzyl-2-deoxy-β-D-glucopyranoside (12). Reaction of 12 with 2,3,4-tri-O-benzyl-α-L-fucopyranosyl bromide under catalysis by bromide ion afforded the trisaccharlde derivative from which the title trisaccharide was obtained by systematic removal of the protective groups. The structures of the final trisaccharide and of various intermediates were established by 1H and 13C NMR spectroscopy.  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