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   49943篇
  免费   3113篇
  国内免费   3432篇
化学   29436篇
晶体学   615篇
力学   1507篇
综合类   343篇
数学   7926篇
物理学   16661篇
  2023年   311篇
  2022年   648篇
  2021年   720篇
  2020年   929篇
  2019年   719篇
  2018年   919篇
  2017年   971篇
  2016年   1460篇
  2015年   1362篇
  2014年   1655篇
  2013年   3055篇
  2012年   2500篇
  2011年   2992篇
  2010年   1963篇
  2009年   1862篇
  2008年   3002篇
  2007年   2813篇
  2006年   2552篇
  2005年   2130篇
  2004年   1707篇
  2003年   1449篇
  2002年   1378篇
  2001年   1206篇
  2000年   1190篇
  1999年   737篇
  1998年   567篇
  1997年   531篇
  1996年   649篇
  1995年   532篇
  1994年   607篇
  1993年   661篇
  1992年   555篇
  1991年   414篇
  1990年   444篇
  1989年   364篇
  1988年   378篇
  1987年   360篇
  1986年   332篇
  1985年   497篇
  1984年   486篇
  1983年   451篇
  1982年   456篇
  1981年   470篇
  1980年   423篇
  1979年   388篇
  1978年   423篇
  1977年   391篇
  1976年   338篇
  1975年   327篇
  1973年   315篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
91.
92.
Gamma rays of199Au obtained after double neutron capture in197Au were measured at the ILL high flux reactor. A level scheme up to 1770 keV excitation energy is established. The result is compared with IBFM and Boson-Fermion-Symmetry calculations.  相似文献   
93.
94.
The aerodynamic behaviour of a model ski jumper is investigated experimentally at full-scale Reynolds numbers and computationally applying a standard RANS code. In particular we focus on the influence of different postures on aerodynamic forces in a wide range of angles of attack. The experimental results proved to be in good agreement with full-scale measurements with athletes in much larger wind tunnels, and form a reliable basis for further predictions of the effects of position changes on the performance. The comparison of CFD results with the experiments shows poor agreement, but enables a clear outline of simulation potentials and limits when accurate predictions of effects from small variations are required.  相似文献   
95.
96.
97.
Poly(D ,L ‐lactide) and poly(D ,L ‐lactide‐co‐glycolide) with various composition and with one methacrylate and one carboxylate end group were synthesized and grafted onto poly(vinyl alcohol) (PVA) via the carboxylate group. The graft copolymers were crosslinked via the methacrylate groups using a free radical initiator. The polymer networks were characterized by means of NMR and studied qualitatively by means of IR spectroscopy. The influence of the glycolide content in the polyester grafts and of the number of ester units in the grafts on thermal properties and swellability were studied as well. The high swellability in water is characteristic of all hydrogels. Differential scanning calorimetry (DSC) showed a single glass transition temperature that occurs in the range between 51 and 69 °C. Thermogravimetric analysis (TGA) of the networks showed the main loss in weight in the temperature range between 290 and 370 °C. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4536–4544, 2007  相似文献   
98.
Cobalamin-dependent methionine synthase plays a crucial role in folate metabolism and such would appear to be an excellent target for rational antifolate drug design. However, to date, no anticancer agents directed at this enzyme are available, but the enzyme is efficiently and specifically inhibited by N2O and this has proven invaluable for evaluating the biochemical consequence of enzyme inhibition and for mechanistic studies. [1,2] 2,5-Diamino-4-oxo-6-(3-butenyl) pyrimidine, a key intermediate in synthetic inhibitors of methionine synthase, was first synthesized using γbutenyl-β-ketoester and guanidine carbonate (Scheme 1). [3]  相似文献   
99.
The ability of 1,3-dimethyl-5-acetyl-barbituric acid (DAB) to react with primary amines in a chemoselective fashion was applied to the synthesis of a selectively protected spermidine.  相似文献   
100.
Triterpenoid Constituents of Huperzia miyoshiana   总被引:1,自引:0,他引:1  
Thirteen triterpenoids, including three new ones, miyoshianois A (1), B (2) and C (3), were isolated from Huperzia miyoshiana. The structures of these new compounds were established as 3-O-dihydroferuloyltohogenol (1), 16-oxo-3β,21β-dihydrox-y-serrat-14-en-24-ferulate (2) and 16-oxo-3a, 21β-dihydroxy-serrat-14-en-24-ferulate (3), respectively, on the basis of their spectroscopic analysis.  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