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   25482篇
  免费   4008篇
  国内免费   2375篇
化学   17765篇
晶体学   310篇
力学   1540篇
综合类   109篇
数学   3022篇
物理学   9119篇
  2024年   38篇
  2023年   495篇
  2022年   660篇
  2021年   819篇
  2020年   1005篇
  2019年   984篇
  2018年   804篇
  2017年   716篇
  2016年   1181篇
  2015年   1113篇
  2014年   1312篇
  2013年   1777篇
  2012年   2376篇
  2011年   2436篇
  2010年   1587篇
  2009年   1515篇
  2008年   1674篇
  2007年   1522篇
  2006年   1379篇
  2005年   1180篇
  2004年   865篇
  2003年   663篇
  2002年   627篇
  2001年   463篇
  2000年   448篇
  1999年   519篇
  1998年   436篇
  1997年   445篇
  1996年   458篇
  1995年   369篇
  1994年   312篇
  1993年   240篇
  1992年   243篇
  1991年   208篇
  1990年   175篇
  1989年   145篇
  1988年   102篇
  1987年   111篇
  1986年   88篇
  1985年   88篇
  1984年   56篇
  1983年   45篇
  1982年   42篇
  1981年   23篇
  1980年   14篇
  1979年   12篇
  1977年   10篇
  1976年   14篇
  1975年   17篇
  1974年   12篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
981.
Beam emittance plays an important role in any accelerator, and it is a main parameter to judge the performance of an accelerator. Emittance optimization is an indispensable part in conditioning and operation of the facility. For a laser-driven high brightness injector, different time structure of the laser pulse has different effects on transverse emittance. In order to compare Gaussian and flat-top laser pulse, systematic simulations of 500 pC have been done. From the simulation results, one can see that flat-top pulse laser will yield smaller minimal transverse beam size and transverse beam emittance than Gaussian pulse laser.  相似文献   
982.
The sorption equilibria of carbon dioxide on three types of silica gel (SG) with different pore size distributions in the presence of water were studied experimentally using a volumetric method at 275?K with pressures from 0 to 3.7?MPa. Both the pore size distribution of the silica gel and the quantity of pre-sorbed water impact the formation of the CO2 hydrates. For wet silicon gel A(SG-A) with water loading ratio of 0.75, the highest CO2 sorption was about 2.5?mmol of CO2 per gram of dry sorbent at 275?K. Similarly, the highest sorption was about 2.7?mmol for wet SG-B with R w =0.81. However, CO2 hydrate did not form on the wet surface of SG-C due to its large pore sizes.  相似文献   
983.
984.
985.
Cauchy’s theorem for analytic functions on complex numbers is extended to analytic functions on the quaternions. For this purpose, we carefully define the notions of differentiation and integration on two or three dimensional manifolds embedded in the quaternions.  相似文献   
986.
987.
988.
989.
990.
Three new metal coordination complexes, namely [Co(BPY)2(H2O)2](BPY)(BS)2(H2O)4 ( 1 ), [Co(BPY)2(H2O)4](ABS)2(H2O)2 ( 2 ) and [Co(BPY)(H2O)4](MBS)2 ( 3 ) (BPY = 4,4′‐bipyridine, BS = phenylsulfonic acid, ABS = p‐aminobenzenesulfonic acid, MBS = p‐methylbenzenesulfonic acid), were obtained under hydrothermal conditions. Complexes 1 , 2 , 3 were structurally characterized using single‐crystal X‐ray diffraction and infrared spectroscopy. All of them display low‐dimensional motifs: complex 1 displays a two‐dimensional structure; and complexes 2 and 3 exhibit a one‐dimensional tape structure. Through strong intermolecular hydrogen bonding interactions and weak packing interactions, all of them further stack to generate a three‐dimensional supramolecular architecture. Catalysts 1 , 2 , 3 were involved in the green synthesis of a variety of 3,4‐dihydropyrimidin‐2(1H)‐ones under solvent‐free conditions through Biginelli reactions. The corresponding catalytic product was obtained in quantitative yields (99%) under eco‐friendly synthesis conditions for the variety of reactions. Catalysts 1 , 2 , 3 exhibit excellent efficiency for the desired product, and their catalytic performance shows the following order: 2  >  1  ≈  3 , which can be ascribed to the hydrophobic interactions of different phenylsulfonate groups. The catalytic performance for the Biginelli reaction is not only dependent on the selected solvents, but also inversely proportional to the polarities of the solvents. Copyright © 2016 John Wiley & Sons, Ltd.  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