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   105519篇
  免费   3647篇
  国内免费   5327篇
系统科学   4300篇
丛书文集   3562篇
教育与普及   2922篇
理论与方法论   418篇
现状及发展   552篇
研究方法   256篇
综合类   102461篇
自然研究   22篇
  2024年   421篇
  2023年   1008篇
  2022年   1809篇
  2021年   1927篇
  2020年   1479篇
  2019年   763篇
  2018年   1079篇
  2017年   1364篇
  2016年   1493篇
  2015年   2914篇
  2014年   4114篇
  2013年   4066篇
  2012年   4612篇
  2011年   5256篇
  2010年   5167篇
  2009年   5621篇
  2008年   6332篇
  2007年   6214篇
  2006年   5053篇
  2005年   4803篇
  2004年   3596篇
  2003年   3613篇
  2002年   3941篇
  2001年   3889篇
  2000年   3311篇
  1999年   4434篇
  1998年   3491篇
  1997年   3581篇
  1996年   3260篇
  1995年   2710篇
  1994年   2502篇
  1993年   2105篇
  1992年   1809篇
  1991年   1724篇
  1990年   1403篇
  1989年   1320篇
  1988年   1032篇
  1987年   638篇
  1986年   375篇
  1985年   115篇
  1984年   30篇
  1983年   7篇
  1982年   11篇
  1981年   7篇
  1980年   7篇
  1979年   9篇
  1977年   7篇
  1974年   9篇
  1971年   9篇
  1955年   8篇
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
821.
The vibrational energy transfer from highly vibrationally excited CO to H2O molecules is studied by time-resolved Fourier transform infrared emission spectroscopy (TR FTIR). Following the 193 nm laser photolysis of CHBr3 and O2 the secondary reactions generate CO(v). The infrared emission of CO(v → v−1) is detected by TR FTIR. The excitation of H2O molecules is not observed. By the method of the spectral simulation and the differential technique, 8 rate constants for CO(v)/H2O system are obtained: (1.7 ±0.1), (3.4 ±0.2), (6.2 ±0.4), (8.0 ±1.0), (9.0 ±2.0), (12 ±3), (16 ±4) and (18 ±7) (1013cm3 · molecule-1· s-1). At least two reasons lead to the efficient energy transfer. One is the contributions of the rotational energy to the vibational energy defect and the other is the result of the complex collision. With the SSH andab initio calculations, the quenching mechanism of CO(v) by H2O is suggested.  相似文献   
822.
A type of recurring relations on syndrome series is presnted. After some important theorems are established, an algorithm for computing a minimal polynomial set is presented. Finally an algorithm for determining the unknown syndromes with incorporating α majority scheme is presented.  相似文献   
823.
The chemical composition of sedimentary rocks is mainly controlled by source rocks. Through examining chemical composition of terrigenous rocks, the tectonic evolution of a sedimentary basin can be modeled. The Turpan Basin belongs to a continental sedimentary basin and its source rocks are derived from the upper continental crust.  相似文献   
824.
825.
Study of forming mechanism of "chimney-effect" (CE) has important significance for basic theory and applications of non-seismic geophysical and geochemical methods locating oil/gas reservoirs. The theoretical basis of comprehensive evaluation of mechanism of CE using principles of magnetism, geochemistry and mineralogy has been reviewed, with the problems to be solved: (ⅰ) study for the relationship between process of oil/gas migration and geochemical field; (ⅱ) analysis of genesis of magnetic, geochemical and mineralogical anomaly; (ⅲ) interpretation of surface soil magnetism and geochemical anomaly combined with seismic data.  相似文献   
826.
A new calculation method was discussed to treat the Hückel Hamiltonian of any fullerene with a certain point group symmetry. This method was applied to calculating the electronic energy levels of icosahedral fullerenes C\-n(I\-h: n=60h 2, 0相似文献   
827.
The shear-wave splitting was analyzed on the deep seismic sounding profile crossing the Dabieshan area. The direction difference between Sg and SmS fast wave polarization indicates that the modem regional tectonic stress field in the Dabieshan area exists only in uppcr crust, while the crack alignment caused by the Triassic collision between the North China and the Yangtze blocks had been preserved in mid-lower curst.  相似文献   
828.
Having reviewed the major classification systems proposed by various scholars across the world, it is found thatindica andjaponica underO. sativa L. are two major directions thoroughly differentiated from the Asian cultivated rice, forming the framework of the classification structure. A system withindica andjaponica as the only two subspecies is therefore reiterated. There are various ways to determine the indica-japonica identity of hybrid rice, but the “combined morphological trait index” (CMT index) method is more efficient and easier to handle, although the isozyme analysis, molecular marker analysis and grain quality assay methods are also feasible.  相似文献   
829.
This is a report of carbon isotope anomaly in marbles associated with the UHP eclogites from the Dabie Mountains. The results place constraints on the degree of crust-mantle interaction during the UHP metamorphism.  相似文献   
830.
In order to ascertain the mechanism of interaction between carbide and metallic catalyst and formation of diamond under high pressure and high temperature, and find a new method to synthesize diamond with special properties, it is necessary to investigate the reaction behavior of different carbides and metallic solvent_catalysts under high pressure and high temperature. A system of Cr-3C-2 powder and Ni 70Mn 25Co 5 alloy in weight ratio of 1∶6 was treated under 6 0 GPa and 1 500℃ for 20, 30 or 60 min respectively. X_ray diffraction of the samples indicated the Cr 3C 2 decomposed partially after high pressure and temperature treatment, and Cr 7C 3, Cr and diamond formed respectively. There was not any trace of graphite in the samples. The result suggested that the separated carbon atoms could form diamond directly without conversion process of graphite into diamond. The observation of SEI, WDX and EDX also showed that diamond crystals were synthesized in the system, which have perfect surfaces and shapes, with the average grain size of about 40 μm. The properties of the crystals are being investigated.  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