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   791篇
  免费   4篇
  国内免费   2篇
化学   614篇
晶体学   4篇
力学   16篇
数学   94篇
物理学   69篇
  2023年   3篇
  2022年   13篇
  2021年   5篇
  2020年   5篇
  2019年   6篇
  2018年   4篇
  2017年   6篇
  2016年   6篇
  2015年   7篇
  2014年   10篇
  2013年   30篇
  2012年   48篇
  2011年   67篇
  2010年   39篇
  2009年   27篇
  2008年   70篇
  2007年   57篇
  2006年   57篇
  2005年   56篇
  2004年   41篇
  2003年   42篇
  2002年   21篇
  2001年   10篇
  1999年   5篇
  1998年   6篇
  1997年   14篇
  1996年   11篇
  1995年   7篇
  1994年   7篇
  1993年   6篇
  1992年   10篇
  1991年   3篇
  1990年   4篇
  1989年   2篇
  1988年   5篇
  1987年   5篇
  1986年   3篇
  1985年   9篇
  1984年   10篇
  1983年   8篇
  1982年   6篇
  1981年   5篇
  1980年   5篇
  1979年   10篇
  1978年   4篇
  1977年   3篇
  1976年   5篇
  1975年   6篇
  1974年   2篇
  1973年   2篇
排序方式: 共有797条查询结果,搜索用时 0 毫秒
791.
Carbon black nanopowders were produced using two thermal plasma processes based on DC, respectively ICP plasma torches. Although the produced particles were in the nanometer size range, the values obtained for the surface area of the particles using a Brunauer Emmett Teller technique were very small. This indicated the presence of contaminants in the experimental powders, as confirmed by Raman spectroscopy and Thermogravimetric Analysis. A thermal treatment process was developed in order to extract these volatile compounds, which were then identified using a Gas Chromatography—Mass Spectrometry method. The experimental powders were analyzed using Scanning and Transmission Electron Microscopy, X-Ray Diffraction and Raman Spectroscopy before and after the thermal treatment in order to determine the effect of the heat treatment on the powder structural properties.  相似文献   
792.
Polycyclic structures fused at a central carbon are of great interest due to their appealing conformational features and their structural implications in biological systems. Although progress in the development of synthetic methodologies towards such structures has been impressive, the stereoselective construction of such quaternary stereocentres remains a significant challenge in the total synthesis of natural products. This review highlights the progress in the formation of 1-oxaspiro[4.n]alkan-2-ones (2≤n≤7) with concomitant formation of the quaternary spiro centre.  相似文献   
793.
Properties of thermoplastic modified epoxy network have been studied. The particularity of this work is the use of new thermoplastic epoxies whose structure is close to the final matrix. Blends of thermoset epoxy (Diglycidyl Ether of Bisphenol A/4-4′ methylenebis [3-chloro 2,6-diethylaniline]) with a thermoplastic content from 5 to 40%w have been realised. Initial miscibility in the thermoset precursors shows an UCST behaviour with a maximal value near 130°C for a thermoplastic content of 10%. Due to the presence of tertiary amine and pendant hydroxyl groups on the thermoplastic backbone, epoxy amine reactions are faster than for the neat system but the thermoplastic seems not to have reacted with the thermoset network. The final blends are transparent but toughening increase is rather low.  相似文献   
794.
Aqueous suspensions of nanocrystals can be obtained by acid hydrolysis of native waxy maize starch granules. The disruption of waxy maize starch granules by 2.2N HCl hydrolysis has been followed by scanning and transmission electron microscopy and laser granulometry. The mechanical properties of composite materials made of poly(styrene-co-butyl acrylate) filled with starch nanocrystals were characterized by dynamic mechanical analysis. These nanocrystals appeared to be an interesting reinforcing phase in a thermoplastic matrix.  相似文献   
795.
We present a theoretical study on aluminium triisopropoxide (Al(OiPr)3) using both empirical (Molecular Mechanics, MM, with Dreiding II force field) and quantum-chemical (Austin Model 1, AM1, semiempirical Hartree-Fock) techniques. We determine the most stable geometries for both the tetramer and trimer of aluminium triisopropoxide as well as the thermodynamic characteristics of the equilibrium existing between these two aggregated structures. The theoretical results are compared to experimental data from X-ray diffraction and 27A1 NMR measurements. For the tetramer, it appears that the optimal equilibrium geometries are in good agreement with the experimental X-ray diffraction geometry; another geometry is also obtained with both theoretical approaches, which is slightly less stable but of higher symmetry. On the basis of the most stable configurations for the tetramer and trimer aggregates, the variation of free enthalpy (ΔG) between the two aggregated structures has been estimated. The evolution of the theoretical ΔG values indicates a displacement of equilibrium towards the trimer species with temperature, in good agreement with experimental 1H and 27A1 NMR data. Moreover, the AM1 heats of formation show a gain of 33.9 kcal/mol due to the aggregation of four A1(OiPr)3 instead of three, and thus a better stability of the tetramer. The molecular geometries being well described by the theoretical methods used in this study, we also present a model for the ring-opening polymerization complexes of ε-caprolactone and lactides.  相似文献   
796.
ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.  相似文献   
797.
: In an attempt to obtain a semiconducting poly(paraphenylene) PPP we have looked at the effect of ion implantion on this conjugated polymer (Ref. 1). The properties of the material may be varied according to the implantation parameters (Ref. 2) and the electrical conductivity after ion implantation of PPP remains almost constant for a long period of time.  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