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   298篇
  免费   27篇
  国内免费   31篇
化学   212篇
晶体学   1篇
力学   15篇
数学   50篇
物理学   78篇
  2023年   2篇
  2022年   9篇
  2021年   8篇
  2020年   10篇
  2019年   8篇
  2018年   10篇
  2017年   9篇
  2016年   11篇
  2015年   7篇
  2014年   12篇
  2013年   23篇
  2012年   21篇
  2011年   30篇
  2010年   11篇
  2009年   17篇
  2008年   22篇
  2007年   22篇
  2006年   14篇
  2005年   12篇
  2004年   20篇
  2003年   16篇
  2002年   17篇
  2001年   12篇
  2000年   8篇
  1999年   8篇
  1998年   2篇
  1997年   2篇
  1996年   3篇
  1994年   1篇
  1993年   2篇
  1991年   1篇
  1990年   1篇
  1989年   1篇
  1987年   1篇
  1985年   1篇
  1979年   2篇
排序方式: 共有356条查询结果,搜索用时 0 毫秒
301.
为了实现光学元件精密检测平台定位误差的自适应补偿,以保证在不同的检测环境中平台能够自行保持高精度,提出了基于检测环境监测和支持向量回归机的定位误差自适应补偿方法。首先,以多组检测环境中温度、湿度和气压的具体测量值作为训练数据,利用支持向量回归机建立定位误差最大值的预测模型,进行最大值预测。然后,将最大值同温度、湿度、气压等环境因素和位置信息一起作为训练数据,迭代使用支持向量回归机,建立任意位置定位误差预测模型。最后,将预测到的定位误差值传入检测平台控制器中进行补偿。应用雷尼绍激光干涉仪,温度、湿度和气压传感器等仪器设备,在光学元件精密检测平台上进行了具体实验。实验结果表明该技术切实可行,预测数据与实测数据差值绝对值的平均值为0.88 m,Pearson相关系数的平方为0.99,自适应补偿后平均定位误差由43 m降为1.4 m。  相似文献   
302.
Tumor progression is intimately associated with the vasculature, as tumor proliferation induces angiogenesis and tumor cells metastasize to distant organs via blood vessels. However, whether tumor invasion is associated with blood vessels remains unknown. As glioblastoma (GBM) is featured by aggressive invasion and vascular abnormalities, we characterized the onset of vascular remodeling in the diffuse tumor infiltrating zone by establishing new spontaneous GBM models with robust invasion capacity. Normal brain vessels underwent a gradual transition to severely impaired tumor vessels at the GBM periphery over several days. Increasing vasodilation from the tumor periphery to the tumor core was also found in human GBM. The levels of vascular endothelial growth factor (VEGF) and VEGF receptor 2 (VEGFR2) showed a spatial correlation with the extent of vascular abnormalities spanning the tumor-invading zone. Blockade of VEGFR2 suppressed vascular remodeling at the tumor periphery, confirming the role of VEGF-VEGFR2 signaling in the invasion-associated vascular transition. As angiopoietin-2 (ANGPT2) was expressed in only a portion of the central tumor vessels, we developed a ligand-independent tunica interna endothelial cell kinase 2 (Tie2)-activating antibody that can result in Tie2 phosphorylation in vivo. This agonistic anti-Tie2 antibody effectively normalized the vasculature in both the tumor periphery and tumor center, similar to the effects of VEGFR2 blockade. Mechanistically, this antibody-based Tie2 activation induced VE-PTP-mediated VEGFR2 dephosphorylation in vivo. Thus, our study reveals that the normal-to-tumor vascular transition is spatiotemporally associated with GBM invasion and may be controlled by Tie2 activation via a novel mechanism of action.Subject terms: Tumour angiogenesis, CNS cancer, Targeted therapies  相似文献   
303.
那日  冯璐  杨体强 《物理》2002,31(11):752-754
将“电场生物学效应”科研工作中取得的部分成果引入大学物理实验教学,探讨了在交叉学科中培养学生创新意识和能力,实现物理实验教学现代化的新途径。  相似文献   
304.
The third part of the paper is devoted to an investigation of the so-called helicon plasma sources. These are RF devices operating with a relatively weak external magnetic field, which is, at the same time, strong enough for the resonant electron gyrofrequency to be much higher than the industrial frequency (ω=8.52×107 s−1). As in [1, 2], a study is made of elongated cylindrical plasma sources in a longitudinal (directed along the cylinder axis) magnetic field and planar disk-shaped plasma sources in a transverse (perpendicular to the plane of the disk) magnetic field. A comparison of the present results with the results that were obtained in [3] without using the helicon approximation leads to the conclusion that elongated helicon sources hold great promise for plasma technologies.  相似文献   
305.
Reaction of polymer-supported α-selenoaldehydes with Grignard reagents afforded polymer-bound β-hydroxyalkyl selenides, which treated with thionyl chloride and triethylamine leading to (E)-1, 2-disubstituted ethenes in good yield.  相似文献   
306.
Cyclocondensation of polystyrene‐supported α‐selenopropionic acid with amidoximes in the presence of 1‐(3‐dimethylaminopropyl)‐3‐ethylcarbodiimide hydrochloride (EDC) followed by oxidative deselenation efficiently afforded 5‐vinyl 1,2,4‐oxadiazoles in good yield and purity with a facile work‐up procedure.  相似文献   
307.
An S6-symmetric hexanuclear gallium metalladiazamacrocycle, [Ga(III)(6)L2(6)S6] with a -(lamda delta)(lamda delta)-chiral sequence and an S-symmetric hexadecanuclear manganese metalladiazamacrocycle, [Mn(III)(16)L2(16)S16] with a -(lamda lamda delta delta)(lamda lamda delta delta)-chiral sequence were prepared using the same N2-trans-cinnamoyl-2-hydroxy-3-naphthoylhydrazide (H3L2) as a bridging pentadentate ligand between the metal centers for the formation of a macrocyclic system.  相似文献   
308.
Gap junction channels formed with connexins directly link to the cytoplasm of adjacent cells and have been implicated in intercellular signaling. Connexin 37 (Cx37) is expressed in the gas-exchange region of the lung. Recently, Cx37 has been reported to be involved in the pathogenesis of inflammatory disease. However, no data are available on the role of Cx37 in allergic airway inflammatory disease. In the present study, we used a murine model of ovalbumin (OVA)- induced allergic airway disease and primary murine epithelial cells to examine the change of Cx37 in allergic airway disease. These mice develop the following typical pathophysiological features of asthma: airway hyperresponsiveness, airway inflammation, and increased IL-4, IL-5, IL-13, intercellular adhesion molecule-1, vascular cell adhesion molecule-1, eotaxin, and RANTES levels in lungs. Cx37 protein and mRNA expression were decreased in OVA-induced allergic airway disease. Immunoreactive Cx37 localized in epithelial layers around the bronchioles in control mice, which dramatically disappeared in allergen-induced asthmatic lungs. Moreover, the levels of Cx37 protein in lung tissues showed significantly negative correlations with airway inflammation, airway responsiveness, and levels of Th2 cytokines in lungs. These findings indicate that change of Cx37 may be associated with the asthma phenotype.  相似文献   
309.
静电干燥原理及应用   总被引:24,自引:0,他引:24  
梁运章  那日  白亚乡  李玉峰 《物理》2000,29(1):39-41
简述了国内外物料干燥的技术水平,提出了一个全新原理的新型干燥技术-静电干燥,介绍其技术原理、技术特点,并对应用开发作了展望。  相似文献   
310.
Meeting the challenge : The zinc/amino alcohol catalyzed enantioselective addition of terminal alkynes to aldehydes is effective with both phenylacetylene and methyl propiolate, leading to chiral secondary propargyl alcohols with very high enantioselectivity (see scheme).

  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