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   869篇
  免费   2篇
  国内免费   11篇
系统科学   21篇
教育与普及   3篇
理论与方法论   7篇
现状及发展   81篇
研究方法   168篇
综合类   572篇
自然研究   30篇
  2021年   4篇
  2020年   5篇
  2019年   1篇
  2018年   7篇
  2017年   6篇
  2016年   9篇
  2015年   8篇
  2014年   3篇
  2013年   19篇
  2012年   85篇
  2011年   118篇
  2010年   29篇
  2009年   9篇
  2008年   82篇
  2007年   88篇
  2006年   77篇
  2005年   76篇
  2004年   63篇
  2003年   54篇
  2002年   67篇
  2001年   5篇
  2000年   6篇
  1999年   3篇
  1998年   2篇
  1997年   3篇
  1996年   2篇
  1995年   2篇
  1994年   4篇
  1993年   2篇
  1992年   4篇
  1991年   1篇
  1990年   3篇
  1987年   1篇
  1986年   2篇
  1978年   1篇
  1977年   8篇
  1976年   5篇
  1975年   3篇
  1974年   5篇
  1972年   1篇
  1970年   1篇
  1968年   1篇
  1967年   2篇
  1966年   1篇
  1965年   3篇
  1954年   1篇
排序方式: 共有882条查询结果,搜索用时 15 毫秒
151.
This paper makes a study of some technical and engineering aspects by using C2+ hydrocarbon separation facility at Guangdong Dapeng LNG (GDLNG) terminal. In the C2+ hydrocarbon extraction process, the cold energy contained in LNG will be utilized. In order to ensure the optimum operating conditions of the terminal and C2+ hydrocarbon extraction facility by optimizing the current operating processes of the terminal, the C2+ hydrocarbon extraction facility construction plan is proposed. We conducted numerous calculations and simulations using such specific analysis software as PRO II. Additionally available flow data are used to verify the cyclic send-out rates from the terminal, thus establishing the current and future projected load factors. This study is intended to make sure that GDLNG can continue to supply gas via the pipeline system safely without interruptions and most significantly solves the effects of flow fluctuations at the terminal gasification send-out facility on the hydrocarbons extraction, ensuring optimum pipeline operations and ensuring safe and effective means for such C2+ hydrocarbons extraction process as well. At the same time, the terminal is also in the optimum operation condition. This is very significant to the terminal safety operation and the energy conservation and emission reduction.  相似文献   
152.
 缺血性心脏病是心力衰竭的常见病因,峰运动VO2已经广泛用于心力衰竭患者心脏储备能力的评估。本研究通过心肺运动试验(Cardiopulmonary Exercise Test,CPET)心脏储备指数、通气效能指数来评价缺血性心脏病和扩张型心肌病的心衰患者运动心肺储备功能变化。选择170例心力衰竭病人(包括100 例缺血性心脏病、70例扩张型心肌病患者)及75例正常人,进行症状限制性CPET。于运动前、运动高峰和运动后,分别测定心率和血压、峰氧耗量(Peak VO2)和VO2与预测最大VO2比例、无氧代谢阀、VE/VCO2斜率(通气效能指数)。结果显示,缺血性心脏病和扩张型心肌病患者左室射血分数相似,血压和静态心率无明显差异, 运动心率明显下降,峰氧摄取量、无氧阈值、VE/VCO2斜率与扩张型心肌病患者相比有显著差别(P<0.05),峰心率明显低于扩张性心肌病。但峰氧耗量/预测峰氧耗量比例无明显差别。45例病人运动心率未达到标准。研究表明,缺血性心脏病患者心肺运动负荷测定的运动高峰VO2、无氧阀值及VE/VCO2斜率提示缺血性心脏病患者运动时通气效能和运动耐力下降比扩张型心肌病患者更为显著。  相似文献   
153.
Clarkson AN  Huang BS  Macisaac SE  Mody I  Carmichael ST 《Nature》2010,468(7321):305-309
Stroke is a leading cause of disability, but no pharmacological therapy is currently available for promoting recovery. The brain region adjacent to stroke damage-the peri-infarct zone-is critical for rehabilitation, as it shows heightened neuroplasticity, allowing sensorimotor functions to re-map from damaged areas. Thus, understanding the neuronal properties constraining this plasticity is important for the development of new treatments. Here we show that after a stroke in mice, tonic neuronal inhibition is increased in the peri-infarct zone. This increased tonic inhibition is mediated by extrasynaptic GABA(A) receptors and is caused by an impairment in GABA (γ-aminobutyric acid) transporter (GAT-3/GAT-4) function. To counteract the heightened inhibition, we administered in vivo a benzodiazepine inverse agonist specific for α5-subunit-containing extrasynaptic GABA(A) receptors at a delay after stroke. This treatment produced an early and sustained recovery of motor function. Genetically lowering the number of α5- or δ-subunit-containing GABA(A) receptors responsible for tonic inhibition also proved beneficial for recovery after stroke, consistent with the therapeutic potential of diminishing extrasynaptic GABA(A) receptor function. Together, our results identify new pharmacological targets and provide the rationale for a novel strategy to promote recovery after stroke and possibly other brain injuries.  相似文献   
154.
155.
面向对象分类分析提取地籍图地块边界   总被引:1,自引:0,他引:1  
提出了一种基于面向对象分类分析的地籍图地块边界提取方法.该方法通过对纸质地籍图的扫描栅格图像进行分割,形成多个待分类的图像对象(区域).结合人工智能专家知识构建模糊逻辑分类器,从形成的多个图像对象中逐层分类提取出地块对象.最后,经矢量化和综合概括处理,最终获得矢量化的地籍图地块边界要素.  相似文献   
156.
The trackway of a swimming theropod (ichnogenus Characichnos) is reported from the Lower Cretaceous Feitianshan Formation of Sichuan, China. These swim tracks help confirm that non-avian theropods were capable of forging moderately deep bodies of water. The trackway occurs on the same surface as a typical walking trackway of a sauropod (ichnogenus Brontopodus). Both occurrences are the first reported from the Cretaceous of Sichuan, and the swim tracks are the first well-preserved example of a Characichnos trackway from China. Additionally, a theropod walking trackway and several ornithopod walking trackways (similar to the ichnogenus Caririchnium) occur in the same horizon. The ornithopod trackways show a parallel orientation, suggesting gregarious behavior of the trackmakers, which may have been iguanodontiforms and/or hadrosauriforms. The co-occurrence of theropod swim tracks and theropod walking tracks suggests a fluctuation of water depth within a distinct time span.  相似文献   
157.
Microscopy is an essential technique for observation on living cells. There is currently great interest in applying scanning probe microscopy to image-living biological cells in their natural environment at the nanometer scale. Scanning ion conductance microscopy is a new form of scanning probe microscopy, which enables non-contact high-resolution imaging of living biological cells. Based on a scanned nanopipette in physiological buffer, the distance feedback control uses the ion current to control the distance between the pipette tip and the sample surface. However, this feedback control has difficulties over slopes on convoluted cell surfaces, which limits its resolution. In this study, we present an improved form of feedback control that removes the contribution of up to the third-order slope from the ion current signal, hence providing a more accurate signal for controlling the distance. We show that this allows faster and lower noise topographic high-resolution imaging.  相似文献   
158.
159.
G-protein-coupled receptors have a major role in transmembrane signalling in most eukaryotes and many are important drug targets. Here we report the 2.7 A resolution crystal structure of a beta(1)-adrenergic receptor in complex with the high-affinity antagonist cyanopindolol. The modified turkey (Meleagris gallopavo) receptor was selected to be in its antagonist conformation and its thermostability improved by earlier limited mutagenesis. The ligand-binding pocket comprises 15 side chains from amino acid residues in 4 transmembrane alpha-helices and extracellular loop 2. This loop defines the entrance of the ligand-binding pocket and is stabilized by two disulphide bonds and a sodium ion. Binding of cyanopindolol to the beta(1)-adrenergic receptor and binding of carazolol to the beta(2)-adrenergic receptor involve similar interactions. A short well-defined helix in cytoplasmic loop 2, not observed in either rhodopsin or the beta(2)-adrenergic receptor, directly interacts by means of a tyrosine with the highly conserved DRY motif at the end of helix 3 that is essential for receptor activation.  相似文献   
160.
Holt LJ  Krutchinsky AN  Morgan DO 《Nature》2008,454(7202):353-357
At the onset of anaphase, sister-chromatid cohesion is dissolved abruptly and irreversibly, ensuring that all chromosome pairs disjoin almost simultaneously. The regulatory mechanisms that generate this switch-like behaviour are unclear. Anaphase is initiated when a ubiquitin ligase, the anaphase-promoting complex (APC), triggers the destruction of securin, thereby allowing separase, a protease, to disrupt sister-chromatid cohesion. Here we demonstrate that the cyclin-dependent kinase 1 (Cdk1)-dependent phosphorylation of securin near its destruction-box motif inhibits securin ubiquitination by the APC. The phosphatase Cdc14 reverses securin phosphorylation, thereby increasing the rate of securin ubiquitination. Because separase is known to activate Cdc14 (refs 5 and 6), our results support the existence of a positive feedback loop that increases the abruptness of anaphase. Consistent with this model, we show that mutations that disrupt securin phosphoregulation decrease the synchrony of chromosome segregation. Our results also suggest that coupling securin degradation with changes in Cdk1 and Cdc14 activities helps coordinate the initiation of sister-chromatid separation with changes in spindle dynamics.  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