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   235831篇
  免费   2343篇
  国内免费   805篇
化学   118608篇
晶体学   4078篇
力学   10865篇
综合类   1篇
数学   25913篇
物理学   79514篇
  2020年   1838篇
  2019年   2077篇
  2018年   2742篇
  2017年   2675篇
  2016年   3977篇
  2015年   2451篇
  2014年   4031篇
  2013年   10187篇
  2012年   7648篇
  2011年   9547篇
  2010年   6776篇
  2009年   6720篇
  2008年   8826篇
  2007年   8845篇
  2006年   8397篇
  2005年   7593篇
  2004年   7020篇
  2003年   6203篇
  2002年   6138篇
  2001年   7148篇
  2000年   5452篇
  1999年   4284篇
  1998年   3601篇
  1997年   3511篇
  1996年   3343篇
  1995年   3056篇
  1994年   2990篇
  1993年   3093篇
  1992年   3200篇
  1991年   3372篇
  1990年   3128篇
  1989年   3072篇
  1988年   3112篇
  1987年   3001篇
  1986年   2839篇
  1985年   3844篇
  1984年   4026篇
  1983年   3240篇
  1982年   3438篇
  1981年   3382篇
  1980年   3264篇
  1979年   3380篇
  1978年   3474篇
  1977年   3386篇
  1976年   3485篇
  1975年   3167篇
  1974年   3179篇
  1973年   3225篇
  1972年   2252篇
  1971年   1872篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
Metabolomics is a potential tool for the discovery of new biomarkers in the early diagnosis of diseases. An ultra-fast gas chromatography system equipped to an electronic nose detector (FGC eNose) was used to identify the metabolomic profile of Volatile Organic Compounds (VOCs) in type 2 diabetes (T2D) urine from Mexican population. A cross-sectional, comparative, and clinical study with translational approach was performed. We recruited twenty T2D patients and twenty-one healthy subjects. Urine samples were taken and analyzed by FGC eNose. Eighty-eight compounds were identified through Kovats's indexes. A natural variation of 30% between the metabolites, expressed by study groups, was observed in Principal Component 1 and 2 with a significant difference (p < 0.001). The model, performed through a Canonical Analysis of Principal coordinated (CAP), allowed a correct classification of 84.6% between healthy and T2D patients, with a 15.4% error. The metabolites 2-propenal, 2-propanol, butane- 2,3-dione and 2-methylpropanal, were increased in patients with T2D, and they were strongly correlated with discrimination between clinically healthy people and T2D patients. This study identified metabolites in urine through FGC eNose that can be used as biomarkers in the identification of T2D patients. However, more studies are needed for its implementation in clinical practice.  相似文献   
72.
73.
74.
75.
A uniform dispersion of reactants is necessary to achieve a complete reaction involving multicomponents. In this study, we have examined the role of plasticizer in the reaction of two seemingly unlikely reactants: a highly crystalline hexamethylenetetramine (HMTA) and a strongly hydrogen bonded phenol formaldehyde resin. By combining information from NMR, infrared spectroscopy and differential scanning calorimetry, we were able to determine the role of specific intermolecular interactions necessary for the plasticizer to dissolve the highly crystalline HMTA and to plasticize the phenol formaldehyde resin in this crosslinking reaction. The presence of the plasticizer increased the segmental mobility, disrupted the hydrogen bonded matrix, and freed the hydroxyl units, which further increased the solubility of the HMTA. Both the endothermic and exothermic transitions are accounted for in the calorimetric data obtained. For the first time, it is possible to obtain the effective molar ratio of each component needed to complete the crosslinking reaction efficiently. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1519–1526  相似文献   
76.
The structure and properties of 12Cr1MoV steel irradiated with a zirconium ion beam were studied by optical microscopy, scanning electron microscopy, and micro- and nanoindentation. It is shown that the modification covers the entire cross-section of the irradiated specimens to a depth of 1 mm. The data on irradiation-induced structural changes are used to interpret the changes in mechanical properties of the irradiated specimens under static and cyclic loading. Particular attention is given to analysis of strain estimation by the digital image correlation method.  相似文献   
77.
78.
79.
80.
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