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   736246篇
  免费   6784篇
  国内免费   2026篇
化学   367959篇
晶体学   10424篇
力学   37365篇
综合类   19篇
数学   105396篇
物理学   223893篇
  2021年   5897篇
  2020年   6478篇
  2019年   7424篇
  2018年   13806篇
  2017年   14091篇
  2016年   15835篇
  2015年   8219篇
  2014年   12895篇
  2013年   30988篇
  2012年   25614篇
  2011年   34701篇
  2010年   24172篇
  2009年   24090篇
  2008年   31659篇
  2007年   33521篇
  2006年   24031篇
  2005年   24648篇
  2004年   21495篇
  2003年   19791篇
  2002年   18615篇
  2001年   19097篇
  2000年   14903篇
  1999年   11493篇
  1998年   9906篇
  1997年   9793篇
  1996年   9260篇
  1995年   8428篇
  1994年   8280篇
  1993年   8123篇
  1992年   8626篇
  1991年   9156篇
  1990年   8760篇
  1989年   8672篇
  1988年   8527篇
  1987年   8254篇
  1986年   7940篇
  1985年   10288篇
  1984年   10767篇
  1983年   9032篇
  1982年   9550篇
  1981年   8989篇
  1980年   8600篇
  1979年   9120篇
  1978年   9568篇
  1977年   9225篇
  1976年   9403篇
  1975年   8800篇
  1974年   8675篇
  1973年   9176篇
  1972年   6435篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
The title compound [systematic name: 8‐chloro‐11‐(piperidin‐4‐yl­idene)‐6,11‐dihydro‐5H‐benzo[4,5]cyclo­hepta­[2,1‐b]pyridine], C19H19ClN2, was crystallized from ethyl acetate. The inter­esting feature of the reported structure is that it does not contain any strong hydrogen bonds, although the mol­ecule contains a secondary NH group, which is a good hydrogen‐bond donor.  相似文献   
992.
993.
994.
995.
Errors in the programs of winding pressure vessels can lead to an asymmetric reinforcement structure, which affects the equilibrium shape of their domes and the tension in the left and right families of filaments. In this paper, equations determining the equilibrium shape of the domes, the tension in filaments, and the shear stresses between layers are obtained. Examples with winding trajectories in the form of geodesics and constant-deviation lines are considered. It is found that, for pressure vessels, in the absence of external torque, the tension levels in filaments of the left and right families differ considerably. It is also revealed that, for actual friction coefficients in winding, the shear stresses taken up by the binder are insignificant. __________ Translated from Mekhanika Kompozitnykh Materialov, Vol. 41, No. 6, pp. 743–752, November–December, 2005.  相似文献   
996.
Mo2Ni3Si/NiSi metal silicide composite coatings with a fine microstructure consisting of Mo2Ni3Si primary dendrites and the interdendritic Mo2Ni3Si/NiSi eutectics were fabricated on austenitic stainless steel AISI 321 by laser cladding process. Small amplitude reciprocating sliding wear resistance of the coatings is evaluated as functions of normal load and slip amplitude and the wear mechanisms were discussed based on worn surface morphology observations. Results showed that the Mo2Ni3Si/NiSi coatings have excellent small amplitude reciprocating sliding wear resistance.  相似文献   
997.
998.
Summary Studies performed by the Brazilian Nuclear Corporation (NUCLEBRAS), in collaboration with the Geological Survey Company of Brazil (CPRM), identified high levels of natural uranium in the districts of Pedra and Venturosa, in the rural region of the state of Pernambuco (PE) - Brazil, where the maximum value found in rocks was 22,000 mg.  相似文献   
999.
Some polythermal sections of the Ni-Bi-Se system have been investigated by DTA, X-ray powder diffraction, microstructural analysis, and microhardness measurements. The liquidus surface has been constructed for this system. The compounds NiBi2Se4 and Ni3Bi2Se2 are formed by dystectic reactions and melt at 1088 and 980 K, respectively. The regions of primary and joint phase crystallization have been determined. A commutation of Bi2Se3-based thermoelements with a metal nickel bus-bar has been proposed.  相似文献   
1000.
Catalytic properties of silica-supported heteropolyacids (HPA) in a mechanical mixture with reduced Me-Ce oxides (Me = Ni, Pd) in n-hexane isomerization are studied. The role of each component of the mixed oxides (Ce and, typically, Ni and Pd) and their optimum content has been illuminated: cerium is not only beneficial for eliminating or preventing coke deposition but is also effective for maintaining the Keggin structure of the highly-organized HPA during the reaction and probably allows a better dispersion of the second metal species. Nickel and palladium, present as Ni0 and Pd0, reinforce the activation of the alkane, which is difficult to obtain by means of a direct attack by an acid, and, thus, enhance noticeably the activity of the catalyst. The best mechanical mixtures are obtained with 30–70 wt % NiCeO-HPW/SiO2 and 50–50 wt % Pd0.1CeO-HPW/SiO2. These mixtures have the highest efficiency for a Ni/(Ni + W) atomic ratio of 0.66 and a Pd/(Pd + W) ratio of 0.40, respectively. Finally, the conversion of n-hexane is in the order HPW > HSiW > HBW, which seems to be consistent with the order of their acid strength as per the literature, but the isomerization selectivity appears to be slightly higher on HSiW. Published in Russian in Kinetika i Kataliz, 2006, Vol. 47, No. 1, pp. 24–28. The text was submitted by the authors in English.  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