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   62262篇
  免费   263篇
  国内免费   552篇
系统科学   1243篇
丛书文集   684篇
教育与普及   296篇
理论与方法论   522篇
现状及发展   34685篇
研究方法   1015篇
综合类   22441篇
自然研究   2191篇
  2013年   800篇
  2012年   662篇
  2011年   2745篇
  2009年   620篇
  2008年   894篇
  2007年   1019篇
  2006年   1055篇
  2005年   1243篇
  2004年   2304篇
  2003年   1971篇
  2002年   1652篇
  2001年   1607篇
  2000年   1171篇
  1999年   1037篇
  1998年   651篇
  1997年   774篇
  1996年   533篇
  1994年   685篇
  1993年   687篇
  1992年   999篇
  1991年   859篇
  1990年   954篇
  1989年   771篇
  1988年   735篇
  1987年   749篇
  1986年   801篇
  1985年   996篇
  1984年   844篇
  1983年   731篇
  1982年   827篇
  1981年   882篇
  1980年   983篇
  1979年   1651篇
  1978年   1440篇
  1977年   1477篇
  1976年   1225篇
  1975年   1207篇
  1974年   1286篇
  1973年   1406篇
  1972年   1474篇
  1971年   1548篇
  1970年   1733篇
  1969年   1549篇
  1968年   1462篇
  1967年   1369篇
  1966年   1141篇
  1965年   900篇
  1959年   546篇
  1958年   740篇
  1957年   598篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
901.
Three gene families that rearrange during the somatic development of T cells have been identified in the murine genome. Two of these gene families (alpha and beta) encode subunits of the antigen-specific T-cell receptor and are also present in the human genome. The third gene family, designated here as the gamma-chain gene family, is rearranged in murine cytolytic T cells but not in most helper T cells. Here we present evidence that the human genome also contains gamma-chain genes that undergo somatic rearrangement in leukaemia-derived T cells. Murine gamma-chain genes appear to be encoded in gene segments that are analogous to the immunoglobulin gene variable, constant and joining segments. There are two closely related constant-region gene segments in the human genome. One of the constant-region genes is deleted in all three T-cell leukaemias that we have studied. The two constant-region gamma-chain genes reside on the short arm of chromosome 7 (7p15); this region is involved in chromosomal rearrangements identified in T cells from individuals with the immunodeficiency syndrome ataxia telangiectasia and observed only rarely in routine cytogenetic analyses of normal individuals. This region is also a secondary site of beta-chain gene hybridization.  相似文献   
902.
电力系统稳定器(PSS)和可控串联补偿器(TCSC)被广泛用于电力系统的稳定控制,多个控制器之间的交互作用会影响彼此的控制效果.本文介绍了生物免疫系统中不同类型的抗体协调合作,共同清除抗原的协调机理.提出了基于免疫网络理论的协调控制方法,用于在线协调控制多个阻尼控制器,抑制电力系统的功率振荡.以装有多个PSS和TCSC的系统为例,对多个控制器进行在线协调阻尼控制.仿真结果表明该方法能够缩短抑制振荡所需的时间,具有有效性和鲁棒性.  相似文献   
903.
Bone marrow cells regenerate infarcted myocardium   总被引:455,自引:0,他引:455  
Myocardial infarction leads to loss of tissue and impairment of cardiac performance. The remaining myocytes are unable to reconstitute the necrotic tissue, and the post-infarcted heart deteriorates with time. Injury to a target organ is sensed by distant stem cells, which migrate to the site of damage and undergo alternate stem cell differentiation; these events promote structural and functional repair. This high degree of stem cell plasticity prompted us to test whether dead myocardium could be restored by transplanting bone marrow cells in infarcted mice. We sorted lineage-negative (Lin-) bone marrow cells from transgenic mice expressing enhanced green fluorescent protein by fluorescence-activated cell sorting on the basis of c-kit expression. Shortly after coronary ligation, Lin- c-kitPOS cells were injected in the contracting wall bordering the infarct. Here we report that newly formed myocardium occupied 68% of the infarcted portion of the ventricle 9 days after transplanting the bone marrow cells. The developing tissue comprised proliferating myocytes and vascular structures. Our studies indicate that locally delivered bone marrow cells can generate de novo myocardium, ameliorating the outcome of coronary artery disease.  相似文献   
904.
运用沉积学及层序地层学原理 ,分析了河流类型及其形成发育特征 ,借鉴国内外有关冲积河流环境下的层序地层模式 ,特别是近海冲积环境下层序地层模式 ,并以济阳坳陷新近系为例 ,建立了更具普遍意义的冲积河流相层序地层模式 ,探讨了不同体系域的特征及其与河流类型之间的关系。研究结果表明 ,冲积河流相地层的体系域划分具有 4分性特征 ,即主要发育有低水位体系域、扩张体系域、高水位体系域 ,当基准面缓慢下降时 ,还可发育收缩体系域。根据济阳坳陷新近系沉积特征 ,指出了各体系域对应的沉积体系及相类型。其中 ,低水位体系域时期以侵蚀下切、冲积扇辫状河沉积为主 ;扩张体系域时期是由辫状河向曲流河过渡 ;高水位体系域时期以曲流河为主 ;收缩体系域时期是由曲流河向辫状河过渡。  相似文献   
905.
Genome sequencing in microfabricated high-density picolitre reactors   总被引:21,自引:0,他引:21  
The proliferation of large-scale DNA-sequencing projects in recent years has driven a search for alternative methods to reduce time and cost. Here we describe a scalable, highly parallel sequencing system with raw throughput significantly greater than that of state-of-the-art capillary electrophoresis instruments. The apparatus uses a novel fibre-optic slide of individual wells and is able to sequence 25 million bases, at 99% or better accuracy, in one four-hour run. To achieve an approximately 100-fold increase in throughput over current Sanger sequencing technology, we have developed an emulsion method for DNA amplification and an instrument for sequencing by synthesis using a pyrosequencing protocol optimized for solid support and picolitre-scale volumes. Here we show the utility, throughput, accuracy and robustness of this system by shotgun sequencing and de novo assembly of the Mycoplasma genitalium genome with 96% coverage at 99.96% accuracy in one run of the machine.  相似文献   
906.
提出用有限元计算应力模拟光弹性等差条纹图像的方法,介绍了由计算应力计算图像显示点应力的算法.通过主应力差与光弹性等差条纹的关系获得了模拟条纹图.模拟条纹图的灰度值可由数字图像卡快速全场显示,实现了计算应力的光弹性应力等差条纹图的模拟.所提出的新思路为计算结果与实验结果进行全场定量比较提供了新的途径  相似文献   
907.
鱼尼丁受体类杀虫剂的研究进展   总被引:2,自引:0,他引:2  
介绍了鱼尼丁受体类新型杀虫剂的创制现状、作用机制、生物活性、毒性及合成方法,由于此类杀虫剂的结构独特、作用方式新颖、对鳞翅目害虫效果好、杀虫广谱,该类药剂对各种益虫和天敌安全,对现用的杀虫剂无交互抗性,具有良好的发展前景.  相似文献   
908.
热敏电阻伏安特性自动测试系统   总被引:6,自引:0,他引:6  
利用PC机通过RS-232C标准串行接口同智能万用表组成自动测试系统,通过并行打印接口控制外围接口电路,实现电压程序控制及通道自动转换,并实现了对热敏电阻伏安特性曲线的自动测试和自动数据处理。该系统能同时对10只热敏元件进行测试,各个通道具有单元单独击穿自动保护电路,程序VISUAL C++6.0语言编写。  相似文献   
909.
The demography of a population of Yarrow's spiny lizard, Sceloporus jarrovii , was examined from 2004 to 2006 in the canyon Las Piedras Encimadas, located in Gomez Palacio, Durango, México. Lizards were studied using a mark-recapture technique. Reproduction in females occurred between November and May, coinciding with dry conditions. Reproductive activity was highest (percent of females with vitellogenic follicles or embryos) in the middle of the dry season (November and December). Thirteen percent of females reached sexual maturity at an average age of 8.5 months. The population structure was similar in spring and fall, but not in summer. A notable feature of summer, coinciding with the wet season, was the greater number of hatchlings and juveniles. The overall sex ratio did not differ from 1:1. The density of adults varied from 12 to 62 animals ? 0.5 ha –1 . Temperate and arid-adapted populations of S. jarrovii exhibited broad similarity in timing of the reproductive season, whereas factors such as density, growth, age at sexual maturity, and survivorship differed.  相似文献   
910.
We measured predation on 120 artificial Sage Grouse ( Centrarcus urophasianus ) nests in montane sagebrush grassland in northern Utah. We examined nests in areas that had been chained and seeded 25 years previously (treated areas) and in areas that were untreated. Predation rates of artificial nests were higher in areas of untreated sagebrush, even though these areas had greater sagebrush cover, taller shrubs, and greater horizontal plant cover. These results differ from those previously hypothesized for treated sagebrush habitat and may reflect a greater abundance of other potential prey species, especially lagomorphs, in untreated areas that attracted greater densities of predators. In addition, over 80% of nests were depredated by mammals, which hunt using olfaction and are less likely than avian predators to be affected by nest cover. We conclude that, after treated sagebrush has recovered to some degree, predation rates of Sage Grouse nests may be lower in treated sagebrush. Consequently, factors other than nest predation (e.g., winter food, thermal cover, insects, perennial forb abundance) may be more important reasons for preserving mature sagebrush stands for Sage Grouse.  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