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   700篇
  免费   83篇
  国内免费   99篇
化学   494篇
晶体学   2篇
力学   59篇
综合类   9篇
数学   108篇
物理学   210篇
  2022年   7篇
  2021年   6篇
  2020年   12篇
  2019年   15篇
  2018年   21篇
  2017年   9篇
  2016年   20篇
  2015年   23篇
  2014年   20篇
  2013年   38篇
  2012年   28篇
  2011年   38篇
  2010年   43篇
  2009年   27篇
  2008年   48篇
  2007年   39篇
  2006年   30篇
  2005年   39篇
  2004年   28篇
  2003年   26篇
  2002年   26篇
  2001年   27篇
  2000年   22篇
  1999年   25篇
  1998年   10篇
  1997年   6篇
  1996年   15篇
  1995年   15篇
  1993年   9篇
  1992年   4篇
  1990年   10篇
  1989年   9篇
  1988年   8篇
  1987年   8篇
  1985年   4篇
  1984年   5篇
  1983年   5篇
  1982年   9篇
  1981年   4篇
  1980年   5篇
  1979年   9篇
  1978年   10篇
  1977年   7篇
  1976年   11篇
  1975年   9篇
  1973年   8篇
  1972年   7篇
  1971年   9篇
  1970年   4篇
  1909年   4篇
排序方式: 共有882条查询结果,搜索用时 218 毫秒
871.
i-PP/m-EPR reactor alloy were prepared through ethylene/propylene slurry copolymerization catalyzed by metallocene(rac-Et(Ind)_2ZrCl_2)supported on porous iPP particles.Polar monomer(dihydromyrcene alcohol)treated with triethyaluminum was added in the preparation of porous iPP particles to introduce hydroxyl groups and thus enhance the ability for chemically supporting the metallocene catalyst.The effects of MAO/Zr ratio and monomer composition in feed on the reaction activity and property of polymer wer...  相似文献   
872.
873.
Laxiflorin J, isolated from the leaves of Isodon eriocalyx var. laxiflora, showed significant inhibitory activity toward T‐24 cells. A series of laxiflorin J derivatives were synthesized and their in vitro activity was evaluated against BEL‐7402, A‐549, HT‐29, HL‐60, MOLT‐4 tumor cell lines, with IC50 values ranging from 1.3 to 42.2 μg mL?1.  相似文献   
874.
Cyclopentane-1,2-dione bis(tert-butyldimethylsilyl) enol ether readily undergoes organocatalytic reactions with α,β-unsaturated aldehydes resulting in Mukaiyama–Michael adducts which, after reduction of the aldehyde group and deprotection result in chiral 1,2-diketones (in mono-enolic form) in good yields (up to 66%) and with high stereoselectivity (up to 94% ee).  相似文献   
875.
A variant of the level set method and applications to image segmentation   总被引:6,自引:0,他引:6  
In this paper we propose a variant of the level set formulation for identifying curves separating regions into different phases. In classical level set approaches, the sign of level set functions are utilized to identify up to phases. The novelty in our approach is to introduce a piecewise constant level set function and use each constant value to represent a unique phase. If phases should be identified, the level set function must approach predetermined constants. We just need one level set function to represent unique phases, and this gains in storage capacity. Further, the reinitializing procedure requested in classical level set methods is superfluous using our approach. The minimization functional for our approach is locally convex and differentiable and thus avoids some of the problems with the nondifferentiability of the Delta and Heaviside functions. Numerical examples are given, and we also compare our method with related approaches.

  相似文献   

876.
An experimental and theoretical study of the absolute value of the one-bond spin-spin coupling constant |(1)J(Si,H)| in SiH(n)Cl(4-n) (n = 0-4) dissolved in THF-d(8) is presented. We found |(1)J(Si,H)| to increase with an increasing number of chlorine substituents, and the quantitative changes were found to differ from the values previously reported for the same compounds dissolved in cyclohexane-d(12). We also report on the variations in |(1)J(Si,H)| as a function of temperature, which we found to be linearly temperature dependent for the chlorine-substituted silanes and temperature independent for SiH(4). Furthermore, the temperature dependence of |(1)J(Si,H)| varied between the different chlorosilanes. Solvent-solute interactions were studied by quantum chemical DFT calculations. The variations in chloro-silane bond lengths upon adduct formation and the different adduct interaction energies may explain the temperature dependences of the coupling constants.  相似文献   
877.
对多壁碳纳米管/高密度聚乙烯(MWNTs/HDPE)复合材料的导电性和动态流变行为进行了研究.发现复合材料的复数粘度η*随MWNTs含量φ的增大而增大.当φ>3wt%时,η*发生突变,在低ω区域表现为非牛顿流体行为,出现强烈的剪切变稀现象.将其称为流变渗流现象,对应的填料含量即渗流阈值φc.在动态储能模量(G′)、损耗模量(G″)与频率(ω)关系曲线上,随φ增加出现“第二平台”,第二平台的出现表明MWNTs与MWNTs之间、MWNTs与聚合物之间存在相互缠结形成网络的结构.同时发现,在tanδ~ω曲线上的低ω区出现凹谷.认为这是由于MWNTs长链结构在低ω时伸长/收缩,MWNTs与MWNTs相互接触形成了次级网络造成的.经过不同时间热处理后的ω扫描以及动态间扫描的结果证实了这种结构的存在.研究结果表明复合材料的流变渗流阈值与电渗流阈值相一致(均在3%~5%之间),动态流变行为与导电性存在一定的相关性.  相似文献   
878.
N-(取代噻唑-2-基)-菊酰胺类化合物的合成及生物活性研究   总被引:1,自引:0,他引:1  
为了寻求新颖的氨基噻唑类先导化合物,以含各种取代基的2-氨基噻唑在其氨基位置与拟除虫菊酯类农药的活性基团——菊酸采用亚结构对接的方法合成了35个N-(取代噻唑-2-基)-菊酰胺类化合物,经1HNMR,ESI-MS和元素分析对化合物的结构进行了表征.初步生物活性测定表明,化合物3d,6a,6d,4a,4b,4c和4e在600mg/L下对小菜蛾具有100%的杀虫活性,化合物3d和7c在50mg/L下对多种常见病原菌具有显著的杀菌活性,全部化合物没有明显的除草活性.  相似文献   
879.
Designing superacids: A computational study of protonated boratabenzenes and the gas‐phase acidity of their conjugate acids is presented. Conjugate acids of boratabenzenes substituted with CN or CF3 groups (see figure) are highly acidic species; the protonated hexacyanoboratabenzene and hexakis(trifluoromethyl)boratabenzene have computational gas‐phase acidities of 250.5 and 276.8 kcal mol?1, respectively.

  相似文献   

880.
Variance function estimation in multivariate nonparametric regression is considered and the minimax rate of convergence is established in the iid Gaussian case. Our work uses the approach that generalizes the one used in [A. Munk, Bissantz, T. Wagner, G. Freitag, On difference based variance estimation in nonparametric regression when the covariate is high dimensional, J. R. Stat. Soc. B 67 (Part 1) (2005) 19-41] for the constant variance case. As is the case when the number of dimensions d=1, and very much contrary to standard thinking, it is often not desirable to base the estimator of the variance function on the residuals from an optimal estimator of the mean. Instead it is desirable to use estimators of the mean with minimal bias. Another important conclusion is that the first order difference based estimator that achieves minimax rate of convergence in the one-dimensional case does not do the same in the high dimensional case. Instead, the optimal order of differences depends on the number of dimensions.  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