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   992篇
  免费   27篇
  国内免费   6篇
化学   514篇
晶体学   2篇
力学   72篇
数学   225篇
物理学   212篇
  2023年   8篇
  2022年   21篇
  2021年   17篇
  2020年   20篇
  2019年   24篇
  2018年   19篇
  2017年   21篇
  2016年   53篇
  2015年   26篇
  2014年   33篇
  2013年   52篇
  2012年   68篇
  2011年   87篇
  2010年   49篇
  2009年   43篇
  2008年   60篇
  2007年   69篇
  2006年   45篇
  2005年   41篇
  2004年   40篇
  2003年   42篇
  2002年   24篇
  2001年   12篇
  2000年   10篇
  1999年   8篇
  1998年   4篇
  1997年   8篇
  1996年   10篇
  1995年   12篇
  1994年   11篇
  1993年   8篇
  1992年   6篇
  1991年   8篇
  1990年   4篇
  1989年   5篇
  1988年   3篇
  1987年   3篇
  1986年   8篇
  1985年   6篇
  1984年   4篇
  1983年   3篇
  1982年   2篇
  1981年   4篇
  1980年   3篇
  1979年   2篇
  1978年   4篇
  1977年   6篇
  1974年   3篇
  1972年   2篇
  1957年   2篇
排序方式: 共有1025条查询结果,搜索用时 46 毫秒
111.
Hydrazinophthalazine and dihydrazinophthalazine were found to be good organic reagents for the determination of vanadium. They develope a stable straw-yellow colour in 5% sodium acetate at pH 5.5. Colorimetric and spcctrophotometric determinations are easily carried out. Beer's law is obeyed reasonably well. Interferences with other metals or ions are recorded and the advantages of the described method with reference to others commonly used are pointed out.  相似文献   
112.
In the present work are reported the results of a monitoring on a vast scale, carried out through evaluation of opportune chemical parameters, of the pollution state of the agricultural-industrial settlement of the Fucino Plain. The parameters took into consideration have been the presence of wide consumption pesticides and of ionic species as Cl-, NO2-, NO3-, NH4+, the quantification of the dissolved oxygen, of the temperature, of the conductivity, of pH and eH. Collected data are used for chemometric elaboration. The water systems examined, by means of drawing campaigns carried out at regular intervals in winter, spring, summer, and autumnal seasons, are represented by superficial waters constituting a network of irrigation canals fed by stratum and meteoric waters. In this work are reported the results relative to the drawing campaigns.  相似文献   
113.
114.
Two new organic reagents for the determination of ferric iron, hydrazinophthalazine and dihydrazinophthalazine, are proposed. They yield a red-violet colour which is stabilized with 10% sodium acetate at pu 11.2. Good results are obtainable in colorimetric and spectrophoto metric determinations, especially with hydrazinophthalazine. Beer's law being obeyed. Interferences with other ions or metals arc recorded and the advantages of these reagents over others are pointed out.  相似文献   
115.
In this note we supply an elementary proof of the following well-known theorem of R. Stanley: the -vectors of Gorenstein algebras of codimension 3 are SI-sequences, i.e. are symmetric and the first difference of their first half is an -sequence.

  相似文献   

116.
We performed density functional theory calculations of the atomic and electronic structure of a dense monolayer of phenyl-terminated alkyl chains chemisorbed onto the (100) Si surface. Different adsorption sites were characterized for both the pristine and (2 x 1) reconstructed surface. A strong effect on the ordering and alignment of the molecular energy levels with respect to the Fermi level of silicon is observed, consequent to intermolecular screening in the monolayer and of the appearance of surface localized states, as a function of the different bonding arrangements. Some possible consequences of these findings are discussed in the framework of the experimental synthesis of such monolayers as molecular current rectifiers in silicon-integrated nanoscale electronics.  相似文献   
117.
The Rayleigh diffraction bound sets the minimum separation for two point objects to be distinguishable in a conventional imaging system. We demonstrate sub-Rayleigh resolution by scanning a focused beam--in an arbitrary, object-covering pattern that is unknown to the imager--and using N-photon photodetection implemented with a single-photon avalanche detector array. Experiments show resolution improvement by a factor ~(N-N(max))(?) beyond the Rayleigh bound, where N(max) is the maximum average detected photon number in the image, in good agreement with theory.  相似文献   
118.
We are concerned with a general abstract equation that allows to handle various degenerate first and second order differential equations in Banach spaces. We indicate sufficient conditions for existence and uniqueness of a solution. Periodic conditions are assumed to improve previous approaches on the abstract problem to work on \((-\infty ,\infty )\). Related inverse problems are discussed, too. All general results are applied to some systems of partial differential equations. Inverse problems for degenerate evolution integro-differential equations might be described, too.  相似文献   
119.
The Yule–Simon distribution is usually employed in the analysis of frequency data. As the Bayesian literature, so far, has ignored this distribution, here we show the derivation of two objective priors for the parameter of the Yule–Simon distribution. In particular, we discuss the Jeffreys prior and a loss-based prior, which has recently appeared in the literature. We illustrate the performance of the derived priors through a simulation study and the analysis of real datasets.  相似文献   
120.
This paper is concerned with minimization and maximization problems of eigenvalues. The principal eigenvalue of a differential operator is minimized or maximized over a set which is formed by intersecting a rearrangement class with an affine subspace of finite co-dimension. A solution represents an optimal design of a 2-dimensional composite membrane Ω, fixed at the boundary, built out of two different materials, where certain prescribed regions (patches) in Ω are occupied by both materials. We prove existence results, and present some features of optimal solutions. The special case of one patch is treated in detail.  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