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   4640篇
  免费   162篇
  国内免费   25篇
化学   3178篇
晶体学   46篇
力学   169篇
数学   445篇
物理学   989篇
  2024年   20篇
  2023年   35篇
  2022年   268篇
  2021年   214篇
  2020年   154篇
  2019年   145篇
  2018年   175篇
  2017年   131篇
  2016年   172篇
  2015年   170篇
  2014年   180篇
  2013年   338篇
  2012年   266篇
  2011年   319篇
  2010年   174篇
  2009年   161篇
  2008年   166篇
  2007年   200篇
  2006年   145篇
  2005年   163篇
  2004年   98篇
  2003年   99篇
  2002年   86篇
  2001年   83篇
  2000年   61篇
  1999年   60篇
  1998年   31篇
  1997年   29篇
  1996年   46篇
  1995年   44篇
  1994年   29篇
  1993年   37篇
  1992年   36篇
  1991年   42篇
  1990年   25篇
  1989年   21篇
  1988年   31篇
  1987年   30篇
  1986年   30篇
  1985年   38篇
  1984年   23篇
  1983年   23篇
  1982年   22篇
  1981年   11篇
  1980年   29篇
  1979年   23篇
  1978年   27篇
  1977年   27篇
  1976年   18篇
  1968年   10篇
排序方式: 共有4827条查询结果,搜索用时 15 毫秒
991.
A new method based upon data driven tool, principal component analysis (PCA), for fingerprint enhancement is proposed in this paper. PCA is a very useful statistical technique that has found application in many different fields like image compression, face recognition and is commonly used for finding patterns in data of high dimension. In the proposed method, the input image is first decomposed into directional images using decimation free Directional Filter Bank (DDFB). Then these directional images are normalized. A data driven technique PCA is applied to these normalized directional fingerprint images, which gives the PCA filtered images. These are basically directional images. Then these directional images are reconstructed into one image which is the enhanced one. Simulation results are included illustrating the capability of the proposed method.  相似文献   
992.
13C nmr chemical shifts are used to evaluate successive protonations of the aminopolycarboxylate systems diethylenetriaminepentaacetic acid (DTPA) and triethylenetetraminehexaacetic acid (TTHA).  相似文献   
993.
994.
995.
996.
The structural and magnetic properties of cubic perovskites, PrCoO3 and NdCoO3, are studied using the full potential linearized augmented plane wave (FP-LAPW) method within the frame work of density functional theory (DFT). The structural parameters are also investigated by analytical techniques. The calculated structural parameters are consistent with the experimental results. The strong hybridization of the O-2p, Co-3d and Pr/Nd-4 f states around the Fermi level reveals that these compounds are metallic. It is also found that the origin of ferromagnetism in these compounds is double-exchange interaction between Co-3d states via O-2p states (Co-O-Co).  相似文献   
997.
The recurrence tracking microscope for probing nanostructures on a surface is based on the quantum recurrence phenomenon. We report that condensed atoms bouncing off on the atomic mirror, connected to a cantilever, modify the quantum recurrences. The times at which the recurrences occur depend on the initial energy of the bouncing condensates above the atomic mirror, which change with the density of condensed atoms.  相似文献   
998.
999.
An unusual oleanane triterpene(1) was isolated from aerial parts of Rhododendron campanulatum.The compound had shown prominent antibacterial and immunomodulatory activities.The structure of the compound was determined by spectroscopic data including 1D and 2D NMR spectral analysis.  相似文献   
1000.
Herein we report our studies carried out on the interaction between IMP and gelatin in aqueous medium at 25 °C using conductimetry, surface tensiometry and circular dichroism (CD) techniques. Both surface tensiometry and conductimetry results indicate that the drug interacts with the gelatin in a surfactant-like manner, i.e., both critical aggregation (cac) and polymer saturation points (psp) were observed. The interaction starts with the formation of a highly surface-active complex as revealed by the lowering of surface tension on the addition of drug to the macromolecule. The decrease in cac on increasing gelatin concentration is an indication of the strong interaction between gelatin and IMP. However, at low concentration of gelatin the interaction was not much strong as exposed by surface tension study, i.e., the cac was not very clear (as with higher gelatin concentrations). As usual, the psp increased on increasing the gelatin concentration and was always higher than the critical micelle concentration of the drug in pure aqueous medium. Using CD measurements the influence of IMP on the secondary structure of gelatin in aqueous solutions was also investigated. CD studies (performed at very low drug concentrations) illustrated that the random coil content of gelatin increases with increasing drug concentration. Free energies of aggregation (ΔGagg) and micellization (ΔGmic) were computed with the help of degrees of micelle ionization obtained from the specific conductivity – [IMP] plots.  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