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   586844篇
  免费   70519篇
  国内免费   180篇
  2018年   6103篇
  2017年   5531篇
  2016年   7963篇
  2015年   11018篇
  2014年   12587篇
  2013年   17563篇
  2012年   19762篇
  2011年   19358篇
  2010年   12638篇
  2009年   11558篇
  2008年   16641篇
  2007年   17243篇
  2006年   16041篇
  2005年   15223篇
  2004年   15013篇
  2003年   14360篇
  2002年   13711篇
  2001年   23878篇
  2000年   24407篇
  1999年   19507篇
  1998年   7097篇
  1997年   7387篇
  1996年   6969篇
  1995年   6896篇
  1994年   6816篇
  1993年   6709篇
  1992年   16735篇
  1991年   16523篇
  1990年   16161篇
  1989年   15735篇
  1988年   14726篇
  1987年   14147篇
  1986年   13227篇
  1985年   13373篇
  1984年   11289篇
  1983年   9909篇
  1982年   7680篇
  1981年   7174篇
  1980年   6732篇
  1979年   11159篇
  1978年   8704篇
  1977年   7982篇
  1976年   7558篇
  1975年   8441篇
  1974年   8741篇
  1973年   8594篇
  1972年   8105篇
  1971年   7052篇
  1970年   6193篇
  1969年   5868篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
Distribution of matter in the discharge channel formed upon a nanosecond electrical explosion of a single wire in air and vacuum was studied experimentally. Simultaneous use of optical, UV, and X-ray diagnostics made it possible to distinguish qualitatively different regions of the discharge channel, such as the current-carrying layers and the region occupied by a weakly conducting cold plasma. Several series of experiments with 25-μm-diameter 12-mm-long wires made of different materials were performed. The charging voltage and the current amplitude were varied in the ranges of U 0 = 10–20 kV and I max ∼ 5–10 kA, respectively. Explosion regimes with a current pause and with and without current interruption, as well as with wire preheating in air and vacuum, were studied. Shadow and schlieren images of the discharge channel were obtained using optical probing at the second harmonic of a YAG: Nd+3 laser (λ = 0.532 μm, τ ∼ 10 ns). In the experiments carried out in vacuum, X-ray images of the discharge channel were also obtained using an X-pinch as a point source of probing radiation and UV images were recorded using a four-frame MCP camera.  相似文献   
62.
63.
64.
During fermantation studies on the production of anthracycline antibiotics by Streptomyces C5, it was observed that among the intermediate metabolism enzymes tested, only phosphoenolpyruvate carboxylase (PEPCase; EC 4.1.1.31) increased significantly in specific activity during stationary phase. The specific activity of the Streptomyces C5 PEPCase increased ca. 3-fold during antibiotic production phase from the logarithmic phase levels. To characterize the regulation of the enzyme further, the Streptomyces C5 PEPCase was purified 150-fold from crude extracts. Acetyl-CoA and Mg2+ were shown to be required for PEPCase activity. The activity of the partially purified PEPCase was stimulated slightly by fructose 1,6-bisphosphate and AMP, and was inhibited severely by oxaloacetate, aspartate, malate, succinate, ATP, citrate, and CoASH.  相似文献   
65.
Anatomical and neurophysiological findings have demonstrated neuronal connections between the diencephalic habenular nuclei and brain stem serotonergic raphe nuclei. Therefore we examined some neurochemical consequences of habenular lesions. Sixteen hours and one week after bilateral lesions serotonin metabolism (as reflected by concentrations of its metabolite, 5-hydroxyindoleacetic acid) was significantly increased in the dorsal but not the median raphe nuclei. Unilateral lesions produced a proportionally smaller augmentation. Motron locomotor activity was enhanced during the light and dark illumination phases in lesioned animals but only attained statistical significance during the day.  相似文献   
66.
When cotyledonary tissue of G., barbadense cotton are treated with the mutagen ethyl methanesulfonate and then germinated, an enhanced, unscheduled DNA synthesis response is observed, along with a concomitant increase in the thymidine triphosphate precursor pool size. The implications of these results are discussed in this paper.  相似文献   
67.
68.
Growth of fathead minnow yolk sac larvae was characterized from changes in dry weight and total content and concentrations of RNA, DNA and protein in fish exposed to a sublethal level of HCN (58 micrograms/l) and in age matched controls. Cyanide toxicosis occurred within 24 hr of exposure as evidenced by significant reductions in protein and RNA content and RNA/DNA ratio of larvae. After 96 hr exposure to HCN, larvae exhibited the same growth rate and protein synthetic rate (RNA/DNA) as control fish. HCN toxicosis and recovery is rapid and at least partial tolerance to HCN develops within 96 hr of exposure in larval fathead minnows.  相似文献   
69.
70.
G R Otten  M R Loken 《Cytometry》1982,3(3):182-187
Forward angle light scattering of two different wavelengths by cells in a flow cytometer was used to investigate physical differences between lymphocytes of different lineage, functional subclass and developmental stage. Correlation of the ultraviolet (UV: 351 nm and 364 nm) and 488 nm light scattering signals produced by lymphoid cells demonstrated that the two signals were not equivalent and that they placed different emphasis on the physical parameters characterizing lymphocytes. Both small T and B lymphocytes from peripheral lymphoid tissues and mitogenically activated large T and B lymphocyte blasts were discriminated by both wavelengths. Differences between the Lyt-2 negative and Lyt-2 positive T lymphocyte subsets were also apparent. Two color light scattering could also discriminate between immature thymocytes and mature peripheral T cells and between small bone marrow cells and mature peripheral B cells. In bone marrow an increase in UV light scattering coincided with the appearance of cell surface immunoglobulin on small cells. These data establish that two color light scattering is a sensitive probe for distinguishing cells of apparently similar morphology and that it can be used to study the physical changes that occur during lymphoid cell differentiation.  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