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   3743篇
  免费   163篇
  国内免费   43篇
化学   2739篇
晶体学   17篇
力学   161篇
数学   341篇
物理学   691篇
  2023年   29篇
  2022年   105篇
  2021年   138篇
  2020年   139篇
  2019年   136篇
  2018年   147篇
  2017年   110篇
  2016年   190篇
  2015年   135篇
  2014年   192篇
  2013年   407篇
  2012年   251篇
  2011年   262篇
  2010年   215篇
  2009年   159篇
  2008年   188篇
  2007年   143篇
  2006年   137篇
  2005年   79篇
  2004年   99篇
  2003年   87篇
  2002年   65篇
  2001年   29篇
  2000年   35篇
  1999年   29篇
  1998年   21篇
  1997年   15篇
  1996年   24篇
  1995年   15篇
  1994年   18篇
  1993年   23篇
  1992年   25篇
  1991年   23篇
  1990年   18篇
  1989年   21篇
  1988年   24篇
  1987年   13篇
  1986年   10篇
  1985年   22篇
  1984年   13篇
  1983年   8篇
  1982年   18篇
  1981年   18篇
  1980年   17篇
  1979年   9篇
  1978年   10篇
  1976年   8篇
  1975年   9篇
  1974年   13篇
  1973年   11篇
排序方式: 共有3949条查询结果,搜索用时 15 毫秒
11.
Decisions concerning a project’s expedition, traditionally involved considerations regarding time and cost tradeoff. It was recently suggested that the quality of a project should also be taken into considerations. In this paper, we propose a meta-heuristic solution procedure for the discrete time, cost and quality tradeoff problem. This problem involves the scheduling of project activities in order to minimize the total cost of the project while maximizing the quality of the project and also meeting a given deadline. We apply a so called electromagnetic scatter search to solve this problem. In this process, we initially generate a population of feasible solutions. In so doing, we use frequency memory to well sample the feasible region. A number of these solutions are then selected and improved locally. The improved solutions are then combined to generate new set of solutions. The combination process utilizes attraction–repulsion mechanisms borrowed from the electromagnetism theory. The whole process is stopped when no significant improvement in the set of solutions are observed. The validity of the proposed solution procedure is demonstrated, and its applicability is tested on a randomly generated large and complex problem having 19,900 activities.  相似文献   
12.
Methods for solving the Cauchy problem are suggested. The use nonpolynomial splines, which ensure the required accuracy for a rather wide class of functions. Computational results are presented.  相似文献   
13.
In solid state fermentation, Pleurotus sajor-caju has been found to be able to degrade at least 30% oil palm empty Fruit Bunch (EFB) fibre leaving 70 % useful materials. Conditions under which fermentation carried out were investigated. It was found that, in the temperature range between 25– 28 °C, relative ph between 6–8, moisture between 60–70 % and medium composition of CaCO3: rice bran 2 %: 5 % were the optimum conditions. The results showed in fermented products that, there were substantial reduction in cellulosic component such as Crude Fiber (CF, 18 %); Acid Detergent Fibre (ADF, 45 %), Neutral Detergent Fibre (NDF, 61 %) and Acid Detergent Lignin (ADL, 14 %). However, Crude Protein (CP, 10%) increased resulted from single cell protein enrichment of mycelial microbial mass. The mass reductions of substrate in fermentation process corresponds to the CO2 released during fermentation. Hence, attributable to the decreased in content of CF, ADF, NDF, and ADL. The digestibility study has also been carried out to determine the useful level of this product to ruminant. Aflatoxin content was detected low in both the initial substrates and products. Based on nutritional value and low content of aflatoxin, the product is useful as a source of roughage to ruminant.  相似文献   
14.
Summary Mixed ligand complexes of general formula [UO2L1L2-(H2O)n], where L1 iso-hydroxy-,o-mercapto- oro-aminobenzoic acid and L2 is glycine, L-alanine, or L-valine, have been prepared and characterized. The uranium atom in these complexes is 6- or 8-coordinate. Both acid ligands are bidentate, bonding through the ionized –COO– group and either O, N or S atoms.  相似文献   
15.
Basing on the formalism established by Markovich, we show the completeness of wave operators for the Wigner equation in L2. In the second part, using estimations proved by Castella and Perthame on the one hand, and the Lp→Lq estimations for the Schrödinger group on the other hand, we prove the existence of the wave operators in L2,p spaces. To cite this article: H. Emamirad, P. Rogeon, C. R. Acad. Sci. Paris, Ser. I 334 (2002) 811–816.  相似文献   
16.
Summary The removal of cesium from aqueous waste solution was investigated in a column setup using a relatively coarse SuperLigò644 resin. The bed volume (BV=140) at the onset of breakthrough surpassed the design requirement of 100 BV at 50% breakthrough, and also corresponds to 99.96% cesium removal. Cesium elution with 0.5M HNO3was satisfactory with a peak BV of 2.5. The elution BV for C/C0=0.01 was 10, which is less than the target of 15 BV. The percent of sorbed cesium eluted was 99.88%. Further, the BV of the various solutions used for the supporting process steps was sufficient.  相似文献   
17.
18.
An efficient one-step method for the synthesis of fully substituted thiophenes, from thiomorpholides and α-halo ketones, was developed. A mechanism has also been proposed for the course of reaction.  相似文献   
19.
We investigate some relations concerning the first and the second Beltrami operators corresponding to the fundamental forms I, II, III of a surface in the Euclidean space E3 and we study surfaces which are of finite type in the sense of B.-Y. Chen with respect to the fundamental forms II and III.  相似文献   
20.
The non-linear equations of motion describing the laminar, isothermal and incompressible flow in a rectangular domain bounded by two weakly permeable, moving porous walls, which enable the fluid to enter or exit during successive expansions or contractions, are considered. We apply Lie-group method for determining symmetry reductions of partial differential equations. Lie-group method starts out with a general infinitesimal group of transformations under which given partial differential equations are invariant, then, the determining equations are derived. The determining equations are a set of linear differential equations, the solution of which gives the infinitesimals of the dependent and independent variables. After the group has been determined, a solution to the given partial differential equation may be found from the invariant surface condition such that its solution leads to similarity variables that reduce the number of independent variables in the system. Effect of the permeation Reynolds number Re and the dimensionless wall dilation rate α on self-axial velocity have been studied both analytically and numerically and the results are plotted.  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