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   299271篇
  免费   5297篇
  国内免费   2799篇
化学   143792篇
晶体学   4234篇
力学   15999篇
综合类   195篇
数学   44707篇
物理学   98440篇
  2021年   1918篇
  2020年   2175篇
  2019年   2207篇
  2018年   11230篇
  2017年   12026篇
  2016年   7967篇
  2015年   3648篇
  2014年   4179篇
  2013年   9119篇
  2012年   12122篇
  2011年   21674篇
  2010年   13609篇
  2009年   13605篇
  2008年   18594篇
  2007年   22062篇
  2006年   7805篇
  2005年   13723篇
  2004年   9428篇
  2003年   8653篇
  2002年   6622篇
  2001年   6001篇
  2000年   4823篇
  1999年   3748篇
  1998年   3138篇
  1997年   3065篇
  1996年   2896篇
  1995年   2704篇
  1994年   2666篇
  1993年   2395篇
  1992年   2672篇
  1991年   2797篇
  1990年   2551篇
  1989年   2466篇
  1988年   2427篇
  1987年   2327篇
  1986年   2158篇
  1985年   2718篇
  1984年   2902篇
  1983年   2325篇
  1982年   2529篇
  1981年   2417篇
  1980年   2275篇
  1979年   2491篇
  1978年   2652篇
  1977年   2452篇
  1976年   2525篇
  1975年   2382篇
  1974年   2406篇
  1973年   2460篇
  1972年   1645篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
131.
The investigation of the 1s HFS provides a good possibility for testing QED effects in a combination of a strong electric and magnetic field. Here, we report about the laserspectroscopic measurements of the ground state hyperfine splitting in 207Pb81+. To handle this M1-transition in the infrared optical regime with its long lifetime, we developed a new detection technique using a bunched ion beam. For the observation of fluorescence light, a new mirror system is adapted to the emission characteristics from an ion beam at relativistic velocities. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   
132.
Palladium and molybdenum polycrystalline layers (clusters) have been deposited in a stainless steel UHV system onto a layer of alumina (Al2O3). This layer has been prepared by high temperature oxidation of an aluminium layer. The interaction of this system with nitrogen has been investigated at room temperature by an FEM technique. Under these conditions nitrogen spillover from molybdenum to palladium has been observed.  相似文献   
133.
In this Blackett Lecture, Professor Littlechild, Director General of Electricity Supply in the UK's Office of Electricity Regulation, renews acquaintance with some models he has developed over the years, introduces some models he has recently met, and discusses models that it would seem to be worthwhile developing. He concludes that for all concerned in the electricity industry, success depends on ever improving decision-making, in which OR has a vital role.  相似文献   
134.
NaY zeolite samples loaded with sodium metal by vapor phase deposition have been investigated using129Xe NMR spectroscopy. At low sodium concentration, the129Xe NMR spectrum showed three resonance lines which clearly indicate the existence of distinct domains in the zeolite sample. Such an observation suggests that the diffusion of the xenon atoms into each domain only occurs with respect to the NMR time scale (2.9 ms). As the sodium concentration increases, observation of a single broad line indicate a macroscopic homogenization of the system. The shift of this line is explained in part due to a paramagnetic interaction between the xenon atoms and the unpaired electrons of particles containing an odd number of sodium atoms. The linewidth is due to the distribution of the local magnetic fields partially averaged by the rapid motion of the xenon atoms and to the statistical distribution of the sodium particles in the supercage cavities. The paramagnetic interaction vanishes with the oxidation of the sample leading to a narrowing and a shift of the line to higher magnetic fields.  相似文献   
135.
论述两类岸基光电探测系统;岸基光电跟踪仪和岸基红外监视系统。阐述其典型实例、特点及发展方向。  相似文献   
136.
The first results regarding the formation of a two-dimensional periodic structure of local melting regions on a silicon surface upon pulsed light irradiation are presented. The conditions are established, and the mechanism of the formation of such structures is discussed. Zh. Tekh. Fiz. 67, 97–99 (December 1997)  相似文献   
137.
Experimental and theoretical results are presented on an XeF(C?A) blue-green laser driven by 5-kJ energy. The laser was pumped by a ferrite-induced discharge of 90 cm in length. The output energy of 0.22 J was obtained with a plane-parallel resonator. A program to simulate laser operation has been developed. Numerical results for a wide range of conditions are compared with experiments performed by us and by other authors. It is found that intracavity refractive losses limit laser operation for XeF pressures above 3 torr. The laser efficiency strongly depends on the discharge-to-cavity length ratio. Possible ways to increase the laser power and efficiency are discussed.  相似文献   
138.
Translated from Khimiya Geterotsiklicheskikh Soedinenii. Vol. 30. No. 3, pp. 424–425, March, 1994.  相似文献   
139.
We have measured the PMR spectra of 5, 7-diaryl-1, 4-diazabicyclo[4. ].O]hept-4-enes. We have studied the structure of the 5-(4-nitrophenyl)-7-phenyl derivative by x-ray diffraction. We show that the six-membered heterocycle has the half-chair form. The 6-H and 7-H protons of the aziridine ring are trans, while the 7-aryl is exo relative to the six-membered ring. Introduction of substituents into the aromatic ring has practically no effect on the geometry of the bicycle.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1117–1124, August, 1994.For Communication 11, see [1].  相似文献   
140.
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