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   31920篇
  免费   145篇
  国内免费   281篇
化学   12893篇
晶体学   339篇
力学   1428篇
综合类   12篇
数学   9034篇
物理学   8640篇
  2020年   54篇
  2018年   1207篇
  2017年   1450篇
  2016年   749篇
  2015年   600篇
  2014年   515篇
  2013年   793篇
  2012年   3220篇
  2011年   2421篇
  2010年   1876篇
  2009年   1611篇
  2008年   690篇
  2007年   761篇
  2006年   758篇
  2005年   4585篇
  2004年   4064篇
  2003年   2437篇
  2002年   642篇
  2001年   382篇
  2000年   171篇
  1999年   214篇
  1998年   148篇
  1997年   103篇
  1996年   93篇
  1995年   82篇
  1994年   87篇
  1993年   73篇
  1992年   242篇
  1991年   195篇
  1990年   173篇
  1989年   141篇
  1988年   137篇
  1987年   98篇
  1986年   77篇
  1985年   93篇
  1984年   94篇
  1983年   57篇
  1982年   62篇
  1981年   69篇
  1980年   53篇
  1979年   77篇
  1978年   63篇
  1977年   46篇
  1976年   107篇
  1975年   61篇
  1974年   72篇
  1973年   70篇
  1972年   50篇
  1969年   46篇
  1968年   45篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
Morphological characterization of the organic-inorganic vesicular nanohybrids, Cerasomes, was performed in aqueous media from two aspects. Firstly, a novel Cerasome-forming lipid having two triethoxysilyl groups in the head moiety was synthesized and the physical property of the Cerasome was investigated. While the morphological stability of the Cerasomes, as evaluated from the vesicular collapse behavior against a micelle-forming nonionic surfactant, Triton-X 100, was extremely higher than that of the conventional phospholipid liposome, the stabilities were comparable to each other for the Cerasomes derived from the dual- and single-head lipids. On the other hand, the surface property of the Cerasome formed with the dual-head lipid more closely resembled the colloidal silica particles rather than that derived from the single-head lipid, as suggested by zeta-potential measurements. Secondly, the effect of the media pH on the morphological stability of the Cerasome formed with the single-head lipid was evaluated and appeared as a time difference in obtaining the morphological stability of the Cerasome. These morphological characteristics of the Cerasomes could be mainly owing to the development of the siloxane network on the vesicular surface.  相似文献   
22.
An improved high-performance liquid-chromatographic (HPLC) assay for the simultaneous determination of serotonin (5-HT) and 5-hydroxyindole-3-acetic acid (5-HIAA) in human urine is reported. Following the automated precolumn derivatization of urinary 5-HT and 5-HIAA with benzylamine, the derivatives are separated by isocratic elution on a reversed-phase C18 semi-microbore column, and are fluorometrically detected at an excitation wavelength of 345 nm and an emission wavelength of 480 nm. The detection limits of 5-HT and 5-HIAA are 7 and 5 nmol/l in urine (0.7 and 0.5 fmol/20-microl injection). The proposed automatic method permits a highly selective and sensitive determination of 5-HT and 5-HIAA in human urine without any sample purification.  相似文献   
23.
Two novel polymerizable dyes with anthraquinone residue have been prepared, and their structures were characterized by FTIR, ^1HNMR and EA.  相似文献   
24.
25.
In this paper, we hall discuss the projective flatness of complex Finsler metrics by investigating the geometry of projective bundles associated with a holomorphic vector bundle.  相似文献   
26.
Paramagnetic pyrroline and 1,2,3,6-tetrahydropyridine derivatives of BODIPY and their diamagnetic analogs have been synthesized and characterized as novel redox double sensor and cation sensitive reagents.  相似文献   
27.
28.
We prove that RANDOM EDGE, the simplex algorithm that always chooses a random improving edge to proceed on, can take a mildly exponential number of steps in the model of abstract objective functions (introduced by Williamson Hoke [Completely unimodal numberings of a simple polytope, Discrete Appl. Math. 20 (1988) 69-81.] and by Kalai [A simple way to tell a simple polytope from its graph, J. Combin. Theory Ser. A 49(2) (1988) 381-383.] under different names). We define an abstract objective function on the n-dimensional cube for which the algorithm, started at a random vertex, needs at least exp(const·n1/3) steps with high probability. The best previous lower bound was quadratic. So in order for RANDOM EDGE to succeed in polynomial time, geometry must help.  相似文献   
29.
This paper establishes a link between a generalized matrix Matsumoto-Yor (MY) property and the Wishart distribution. This link highlights certain conditional independence properties within blocks of the Wishart and leads to a new characterization of the Wishart distribution similar to the one recently obtained by Geiger and Heckerman but involving independences for only three pairs of block partitionings of the random matrix.In the process, we obtain two other main results. The first one is an extension of the MY independence property to random matrices of different dimensions. The second result is its converse. It extends previous characterizations of the matrix generalized inverse Gaussian and Wishart seen as a couple of distributions.We present two proofs for the generalized MY property. The first proof relies on a new version of Herz's identity for Bessel functions of matrix arguments. The second proof uses a representation of the MY property through the structure of the Wishart.  相似文献   
30.
Some existence results are obtained for periodic solutions of nonautonomous second-order differential inclusions systems with p-Laplacian.  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