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   58545篇
  免费   1871篇
  国内免费   68篇
化学   37130篇
晶体学   297篇
力学   1925篇
数学   9167篇
物理学   11965篇
  2023年   414篇
  2022年   406篇
  2021年   630篇
  2020年   814篇
  2019年   798篇
  2018年   1167篇
  2017年   969篇
  2016年   1766篇
  2015年   1636篇
  2014年   1670篇
  2013年   3465篇
  2012年   3919篇
  2011年   4014篇
  2010年   2472篇
  2009年   2034篇
  2008年   3469篇
  2007年   3449篇
  2006年   3124篇
  2005年   2961篇
  2004年   2580篇
  2003年   2117篇
  2002年   1816篇
  2001年   1118篇
  2000年   966篇
  1999年   662篇
  1998年   550篇
  1997年   470篇
  1996年   588篇
  1995年   376篇
  1994年   444篇
  1993年   422篇
  1992年   456篇
  1991年   401篇
  1990年   378篇
  1989年   382篇
  1988年   331篇
  1987年   303篇
  1985年   448篇
  1984年   449篇
  1983年   294篇
  1982年   393篇
  1981年   379篇
  1980年   337篇
  1979年   337篇
  1978年   311篇
  1977年   287篇
  1976年   334篇
  1975年   289篇
  1974年   301篇
  1973年   289篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
1.
The article referenced above was first published online on 30 August 2007 with incorrect pagination; the pagination has now been corrected online and in print. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   
2.
Pentacyclic isoxazolines were obtained by the cycloaddition of benzonitrile oxide to norbornene-azetidinone-fused 3,1-oxazines. The constitutions of two of the isomers obtained, and the configurations and conformations of all products, were determined by means of 1H and 13C NMR spectroscopy and DNOE experiments.  相似文献   
3.
The crystal structure of 1-[3-(3,4-dimethoxyphenyl)-2-propenoyl]pyrrolidine (C15H19NO3) (I) has been determined by X-ray analysis. It crystallizes orthorhombic space group Pbca with a = 24.295(3), b = 15.086(3), c = 7.552(3)A, V = 2768(1)A3, Z = 8, Dcalc = 1.254 g/cm3, mu = (Mo K(alpha)) = 0.87 cm(-1). The title compound has analgesic activity of cycloaliphatic amine part. The molecule is deviated from planar configuration.  相似文献   
4.
Hydrogels of N‐vinylimidazole (VI) and sodium styrenesulfonate (SSS) were synthesized in aqueous solution by radical crosslinking copolymerization with N,N′‐methylene‐bis(acrylamide) as crosslinker. Swelling in several saline solutions was measured for hydrogel samples synthesized with different comonomer concentrations (CT = 10, 25, or 40%) and with SSS mole fractions covering a broad range (fSSS = 0–0.7), while the crosslinker ratio was 2 wt % in all cases. The degree of swelling in aqueous solution with a specific ionic strength (μ), plotted versus the SSS composition of the feed, shows a minimum for any set of samples synthesized with a fixed CT. The dependence of swelling on μ shows both polyelectrolyte (fSSS beyond the minimum) and antipolyelectrolyte behaviors (in the low fSSS limit). It was found that the nonGaussian factor of the crosslinking density and the polymer‐solvent interaction parameter increase with fSSS for any CT. Moreover, in the low fSSS limit, the osmotic swelling pressure is governed not only by the ionic contribution, but also by the polymer‐solvent mixing and, the concentration of mobile counterions inside the gel is not proportional to the net fixed charge but to the addition of cationic and anionic side groups, what discards the formation of ionic pairs. The antipolyelectrolyte effect is interpreted as due to the increasing protonation of VI as μ goes up. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1683–1693, 2007  相似文献   
5.
A palladium‐TPPTS catalyst heterogenized on KF/alumina has been shown to be effective and recyclable for the selective formation of monooctadienylxylopyranosides via the telomerization of butadiene with D ‐xylose. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   
6.
7.
Book reviews     
Ohne Zusammenfassung
Book reviews
  相似文献   
8.
A simple High performance liquid chromatographic (HPLC) method for the specific determination of the molecular weight and concentration of hyaluronic acid (HA) in complex mixtures has been developed. Hyaluronate-binding proteins isolated from bovine cartilage labelled by 125I or fluoresceinisothiocyanate were used as specific markers. The specific binding affinities of the markers were compared and were found to have association constants of 1.6 x 10(7) M-1 and 1.2 x 10(7) M-1 respectively. The HA levels and molecular weight distributions can be easily determined in the range 10-500 ng/mL in complex mixtures by the use of markers, molecular sieving HPLC columns and appropriate detectors. It has been demonstrated clearly that the method is useful for the highly specific determination of the parameters in complex biological samples such as serum and synovial fluids and is recommended for clinical applications.  相似文献   
9.
Continuous mapping of an ozone episode in Paris in June 1999 has been performed using a differential absorption lidar system. The 2D ozone concentration vertical maps recorded over 33 h at the Champ de Mars are compiled in a video clip that gives access to local photochemical dynamics with unprecedented precision. The lidar data are compared over the whole period with point monitors located at 0-, 50-, and 300-m altitudes on the Eiffel Tower. Very good agreement is found when spatial resolution, acquisition time, and required concentration accuracy are optimized. Sensitivity to these parameters for successful intercomparison in urban areas is discussed. Received: 11 February 2002 / Published online: 14 March 2002  相似文献   
10.
About 40 epoxide-amine network polymers with glass transition temperatures ranging from 0 to 232°C were investigated, about one-third being reported for the first time. The glass transition temperature Tgl of the corresponding linear copolymers was first calculated by use of an additivity law whose physical validity was carefully checked. Then the contribution of crosslink mers was determined, and various physical and empirical approaches of the effect of crosslinking on Tg were compared. The results gave evidence in favor of the DiMarzio approach. A predictive relationship based on these considerations is proposed.  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