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   3153篇
  免费   88篇
  国内免费   13篇
系统科学   115篇
丛书文集   3篇
教育与普及   2篇
理论与方法论   26篇
现状及发展   1004篇
研究方法   32篇
综合类   2061篇
自然研究   11篇
  2024年   17篇
  2023年   27篇
  2022年   9篇
  2020年   12篇
  2017年   9篇
  2016年   28篇
  2015年   55篇
  2014年   27篇
  2013年   23篇
  2012年   146篇
  2011年   160篇
  2008年   8篇
  2007年   10篇
  2006年   14篇
  2005年   199篇
  2004年   504篇
  2003年   466篇
  2002年   141篇
  2001年   75篇
  2000年   138篇
  1999年   75篇
  1992年   37篇
  1991年   25篇
  1990年   35篇
  1989年   34篇
  1988年   32篇
  1987年   16篇
  1986年   26篇
  1985年   35篇
  1984年   26篇
  1983年   24篇
  1982年   15篇
  1981年   11篇
  1980年   16篇
  1979年   61篇
  1978年   39篇
  1977年   30篇
  1976年   29篇
  1975年   38篇
  1974年   56篇
  1973年   49篇
  1972年   50篇
  1971年   66篇
  1970年   54篇
  1969年   68篇
  1968年   59篇
  1967年   41篇
  1966年   55篇
  1965年   41篇
  1964年   11篇
排序方式: 共有3254条查询结果,搜索用时 171 毫秒
231.
The influence of polymer addition on microstructure, performance and mechanical properties of lightweight aggregate concrete was investigated. It was found that the addition of polymer improved the performance and mechanical properties of lightweight aggregate concrete. It was ascertained that the modification of microstructural uniformity and densification with the addition of polymer is responsible for the enhancement of mechanical properties.With respect to compressive strength and bending strength, the lightweight aggregate concrete added with 13% ethylene-acetate ethylene interpolymer ( EVA ) exhibits preferred mechanical properties.  相似文献   
232.
0 IntroductionTriamcinoloneAcetonide,9 fluoro 11β,2 1 dihydroxy 16α,17[(1 methylethylidene)bis (oxy) ] pregn 1,4 diene 3,2 0 dione,Mris 4 34.4 8(abr.TA ) .Itbelongstoaclassofadrenalcortexhormonedrug ,whosefunctionsareaffectionofsugarmetabolism ,anti inflammationandresistancehypersusceptibilityetc.TAcanbesuitabletorheumatoidarthritisanddermatosissuchashypersusceptibilityandneuropathicdermatitis .ThefunctionsofTAisstrongerandlongerthantriamcinolone[1 ,2 ] .ThecommonmethodsofdetectingTAm…  相似文献   
233.
The present study was undertaken to investigate the effect of oxidized low density lipoprotein (oxLDL) on the expression of maerophage matrix metalloproteinase-12 (MMP-12), and the possible mechanisms. Activation of extracellular signal-regulated kinase 1/2 (ERK1/2) was detected by Western blot analysis.Enzymatic activity of MMP-12 was determined by β-casein zymography. RT-PCR analysis was used to measure the mRNA expression level of MMP-12. OxLDL-stimulated macrophages produced increased casein-degrading activities and oxLDL also significantly increased the mRNA level of MMP-12 in a dose-dependent manner.OxLDL stimulated the phosphorylation of ERK1/2 in macrophages. The use of the specific inhibitor indicated that the ERK1/2 signaling pathway was required for the induction of MMP-12.These data demonstrated that oxLDL induced MMP-12 expression in macrophages through an ERK1/2-dependent pathway.  相似文献   
234.
Xylanisthemajorcomponentofplantcellwallsandthemostabundantpentosaninnature.Xylancontainsβ 1,4 linkedd xylosebackboneswitharabinose,4 O methyl d glucuronicacid,andaceticacidsubstituents[1 ] .Thexylan degradingen zymesincludeendoxylanase(1,4 β d xylanxylanohydrolase;EC3.2 .1.8)and β xylosidase (1,4 β d xylanxylanohydrolase;EC 3.2 .1.37) .xylanasedegradesthexylanbackboneintoxylo bioseandxylooligosaccharides,whileβ xylosidasereleasesxylosylresiduesfromthenonreducingendsofxylooligosaccha…  相似文献   
235.
0 IntroductionIntheprocessofR&Ditem ,thereisusuallycompetition .ManyfirmswanttoinvestR&Dtoobtainapatent.Lury[1 ] gaveanequilibriuminvestmentundertheconditionofone timeinvestmentandcontinuousinvestmentwhenthepayoffwasafixedprize.Ref.[2 ]analyzedtheeffectofinformationsharinginR&D .TherearemanyliteraturesaboutanalysisofR&DlevelswhenfirmscompeteinoutputmarketafterR&Dsucceeds[3 ,4] .Additionally ,todecideinvestmentlevel,therearemanyliteraturesaboutR&Ditem’spricing .MondherBellalah[5] ,K .…  相似文献   
236.
Respective roles of short-and long-range interactions in protein folding   总被引:2,自引:0,他引:2  
A new method was presented to discuss the respective roles of short- and long-range interactions in protein folding. It‘s based on an off-lattice model, which is also being called as toy model. Simulated annealing algorithm was used to search its native conformation. When it is applied to analysis proteins lagt and laho, we find that helical segment catmot fold into native conformation without the influence of long-range interactions. That‘s to say that long-range interactions are the main determinants in protein folding.  相似文献   
237.
A tetranuclear zinc benzoate Zr4O(C6H5CO2)6 was synthesized and characterized by X-ray single crystal determination. It crystallizes in cubic, space group Ia-3d. Its crystal cell is very large, α=4.10063(18)nm, V=68.953(5)nm^3 and Z=48. The structure is composed of discrete Zr4O(C6H5CO2)6 molecules. In each molecule, four zinc atoms are held together by a central oxygen atom, which results in the formation of a regular tetrahedron. All benzoate ligands coordinate to zinc atoms in a bidentate bridging mode. Each zinc atom is in a slightly distorted tetrahedral geometry, coordinated by three benzoate oxygen atoms and the central oxygen atom. The intermolecular interactions result in the formation of a three-dimensional supramolecular framework, with non-intersecting parallel channels.  相似文献   
238.
A new algorithm called spatially aware routing algorithm with enhanced learning (SAREL) is proposed to guarantee the rationality of route selecting in inter-vehicle communication scenario. Firstly, the traffic model is discussed and set up by using Poisson distribution. Then we analyze the process of traffic evaluation with enhanced learning, and exploit movement estimation to assist state memorization. The improvement of algorithm is provided at last compared with our previous work. Simulation results show that SAREL algorithm could achieve better performance in packet delivery ratio, especially when network connection ratio is average.  相似文献   
239.
In this paper, an improved algorithm, named STC-I. is proposed for Chinese Web page clustering based on Chinese language characteristics, which adopts a new unit choice principle and a novel suffix tree construction policy. The experimental results show that the new algorithm keeps advantages of STC, and is better than STC in precision and speed when they are used to cluster Chinese Web page.  相似文献   
240.
Twenty Yongzheng copper coins were fast dat-ing by X-ray energy dispersive spectrometer with scanning electron microscope (SEM-EDS) according to official ingredient. The 20 coins were classified in two groups according to the results. Group 1 represents the coins minted before AD 1 728 and group 2 represents the coins minted after AD 1 728. At concentration range (0.00%-5.13%), the distribution of lead is not homogeneous in the coins, and the largest diameter of lead inclusion is less than 15 μm. The official fineness degrade is supported.  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