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   17940篇
  免费   60篇
  国内免费   55篇
系统科学   283篇
丛书文集   444篇
教育与普及   36篇
理论与方法论   66篇
现状及发展   8375篇
研究方法   761篇
综合类   7888篇
自然研究   202篇
  2012年   231篇
  2011年   463篇
  2010年   115篇
  2009年   100篇
  2008年   291篇
  2007年   368篇
  2006年   305篇
  2005年   320篇
  2004年   289篇
  2003年   344篇
  2002年   276篇
  2001年   616篇
  2000年   616篇
  1999年   352篇
  1993年   93篇
  1992年   340篇
  1991年   253篇
  1990年   300篇
  1989年   274篇
  1988年   266篇
  1987年   282篇
  1986年   284篇
  1985年   344篇
  1984年   248篇
  1983年   219篇
  1982年   208篇
  1981年   246篇
  1980年   264篇
  1979年   590篇
  1978年   475篇
  1977年   483篇
  1976年   366篇
  1975年   382篇
  1974年   593篇
  1973年   470篇
  1972年   422篇
  1971年   516篇
  1970年   675篇
  1969年   591篇
  1968年   512篇
  1967年   541篇
  1966年   449篇
  1965年   337篇
  1964年   95篇
  1959年   203篇
  1958年   296篇
  1957年   193篇
  1956年   172篇
  1955年   167篇
  1954年   159篇
排序方式: 共有10000条查询结果,搜索用时 12 毫秒
141.
IntroductionResearchesonmicromedicaldeviceandmicromedicalrobot,especiallytheinvivoinspectionsystemworkinginalimentarycanalorinbloodvesselwithlittledamageornondamagetohumanbodyhavebeenundertakenandreportedbyexpertsinmanycountries.Thesesystemsusuallyownmanyadvantagessuchas:smallvolume,lowpowerconsumption,littleinterferencetoworkingsurroundings,socangreatlyenlargethescopesofmedicalexaminationandrelievethepatientsofdiscomfortcausedbynormalprocedures.Thecapsuleendoscope“NORIKAsystem”developedby…  相似文献   
142.
143.
Microarray analysis has become a widely used tool for the generation of gene expression data on a genomic scale. Although many significant results have been derived from microarray studies, one limitation has been the lack of standards for presenting and exchanging such data. Here we present a proposal, the Minimum Information About a Microarray Experiment (MIAME), that describes the minimum information required to ensure that microarray data can be easily interpreted and that results derived from its analysis can be independently verified. The ultimate goal of this work is to establish a standard for recording and reporting microarray-based gene expression data, which will in turn facilitate the establishment of databases and public repositories and enable the development of data analysis tools. With respect to MIAME, we concentrate on defining the content and structure of the necessary information rather than the technical format for capturing it.  相似文献   
144.
Nuclear transfer into mouse zygotes   总被引:6,自引:0,他引:6  
  相似文献   
145.
146.
Proteolytic processing of the amyloid precursor protein (APP) generates amyloid beta (Abeta) peptide, which is thought to be causal for the pathology and subsequent cognitive decline in Alzheimer's disease. Cleavage by beta-secretase at the amino terminus of the Abeta peptide sequence, between residues 671 and 672 of APP, leads to the generation and extracellular release of beta-cleaved soluble APP, and a corresponding cell-associated carboxy-terminal fragment. Cleavage of the C-terminal fragment by gamma-secretase(s) leads to the formation of Abeta. The pathogenic mutation K670M671-->N670L671 at the beta-secretase cleavage site in APP, which was discovered in a Swedish family with familial Alzheimer's disease, leads to increased beta-secretase cleavage of the mutant substrate. Here we describe a membrane-bound enzyme activity that cleaves full-length APP at the beta-secretase cleavage site, and find it to be the predominant beta-cleavage activity in human brain. We have purified this enzyme activity to homogeneity from human brain using a new substrate analogue inhibitor of the enzyme activity, and show that the purified enzyme has all the properties predicted for beta-secretase. Cloning and expression of the enzyme reveals that human brain beta-secretase is a new membrane-bound aspartic proteinase.  相似文献   
147.
148.
A gene has been identified that is expressed specifically in stromal cells surrounding invasive breast carcinomas. On the basis of its sequence, the product of this gene, named stromelysin-3, is a new member of the family of metalloproteinase enzymes which degrade the extracellular matrix. The suggestion is that stromelysin-3 is one of the stroma-derived factors that have long been postulated to play an important part in progression of epithelial malignancies.  相似文献   
149.
150.
人们改善环氧树脂基复合材料的脆性通常只改善环氧树脂基体和增强纤维间的界面,提高环氧树脂韧性的另一种常用方法是添加初始相容性良好的热塑性树脂如聚醚酰亚胺,在某一转化率(取决于体系的组成和反应温度)时体系发生相转变,体系最终的形态由相转变速率和环氧树脂反应速度所控制。本文作者研究了两种增强纤维(玻璃纤维和碳纤维)对不同环氧树脂/聚醚酰亚爱体系形态形成的影响,结果发现体系中纤维的存在不会影响基体相分离过程,但会改变体系的最终形态;形态的变化与纤维的品种关系不大;基体中不同组分的粘度是影响复合材料形态形成的关键因素。  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