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   204008篇
  免费   1965篇
  国内免费   591篇
化学   102410篇
晶体学   3340篇
力学   9288篇
综合类   5篇
数学   22510篇
物理学   69011篇
  2021年   1655篇
  2020年   1802篇
  2019年   1999篇
  2018年   2700篇
  2017年   2607篇
  2016年   3766篇
  2015年   2309篇
  2014年   3617篇
  2013年   9087篇
  2012年   7242篇
  2011年   8734篇
  2010年   6040篇
  2009年   5909篇
  2008年   7914篇
  2007年   8081篇
  2006年   7725篇
  2005年   6894篇
  2004年   6428篇
  2003年   5653篇
  2002年   5497篇
  2001年   6430篇
  2000年   4936篇
  1999年   3797篇
  1998年   3039篇
  1997年   3048篇
  1996年   2792篇
  1995年   2618篇
  1994年   2463篇
  1993年   2340篇
  1992年   2693篇
  1991年   2674篇
  1990年   2602篇
  1989年   2400篇
  1988年   2450篇
  1987年   2450篇
  1986年   2280篇
  1985年   2962篇
  1984年   3220篇
  1983年   2631篇
  1982年   2803篇
  1981年   2772篇
  1980年   2638篇
  1979年   2729篇
  1978年   2825篇
  1977年   2819篇
  1976年   2887篇
  1975年   2672篇
  1974年   2646篇
  1973年   2778篇
  1972年   1763篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Cheng  W. H.  Wang  W. H.  Yang  Y. D.  Kuang  J. H. 《Optical and Quantum Electronics》1997,29(10):939-951
A finite-element method (FEM) analysis has been carried out on the thermally-induced stresses generated when stainless steel covers are laser-welded to Au-coated Kovar TO headers during the packaging of semiconductor lasers. In particular, the effect of varying the Au coating thickness of these stresses was investigated. Maximum stresses were found to be reduced as the Au thickness increased. This effect is attributed to the reduction in thermal gradient in the weld region provided by the increased thermal conduction of thicker Au. The result suggests that adequate Au coating thickness is important for ensuring good reliability of opto-electronic packages having laser-welded caps. This revised version was published online in June 2006 with corrections to the Cover Date.  相似文献   
992.
In this work, on the basis of a semiempirical model of bonding forces that allows for three-particle interaction and uses the pseudopotential method, the mechanical and electrical properties of barium chalcogenides are studied at hydrostatic pressure. Equations of state are formulated from hydrostatic compression, parameters of the type B1–B2 structural phase transitions are calculated, the change in the energy band spectrum under the influence of the pressures is studied, and an estimate is made of the critical amount of compression that induces a dielectric-metal phase transition. The results obtained are in agreement with the available data.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 44–52, November, 1991.  相似文献   
993.
As acknowledged in almost all monographs on the fracture of composite materials, one of the major fracture mechanism in unidirectional fibrous composites under uniaxial compression along the reinforcing elements is the stability loss of the material structure (the structural instability). According to this mechanism, theoretical investigations of the fracture along the fibres are reduced to those of the stability loss in the material structure, and the value of external critical forces is accepted as the value of failure forces. At present, numerous theoretical investigations have been carried out in this field with the use of the three-dimensional linearized theory of stability in the framework of the piecewise-homogeneous body model. However, in all the investigations it is assumed that the matrix and the fibre material are isotropic. It is evident that in many cases it is necessary to take into account the anisotropy of the matrix material when investigating the stability loss of fibres. In view of the above, in the framework of the piecewise-homogeneous body model using the three-dimensional linearized theory of stability, the present paper considers the stability loss of the fibre in the anisotropic (transversally isotropic) matrix. The effect of the properties of the matrix material on the critical values of the external loading is examined.Submitted to the 10th International Conference of Mechanics of Composite Materials (Riga, April 20–23, 1998).Published in Mekhanika Kompozitnykh Materialov, Vol. 33, No. 5, pp. 603–611. September–October, 1997.  相似文献   
994.
We have investigated ion-beam-enhanced diffusion of Au in undoped and B doped amorphous Si. The diffusion coefficients depend linearly on ion flux and exibit an Arrhenius-like temperature dependence with an activation energy of 0.37 eV in the temperature range 200–350° C. Moreover the diffusivity is enhanced by a factor of 5 by B-doping at a concentration of 1×1020 atoms/cm3. A similar enhancement is observed in thermal diffusion of Au which has an activation energy of 1.5 eV. On the basis of these results a model for the ion-beam-enhanced diffusion of Au is proposed where the high density of defects present in amorphous Si act as traps for the fast moving interstitial Au atoms. The effectiveness of this trapping process can be changed by the high concentration of mobile defects generated by the beam and also by a change in the charge state of the traps induced by the presence of B.  相似文献   
995.
996.
997.
We report the discovery of 57 new fir laser lines from13CD3OH molecule optically pumped by a waveguide CO2 laser of 300 MHz tunability. For all lines, precise frequency offset measurements between the CO2 line center and the center of the absorbing13CD3OH line were performed using the transferred Lamb-Dip technique. We have also measured directly the frequency of seven FIR laser lines by heterodyning with already known laser lines. We present a complete list of all known laser lines (134) and frequency measurements (24) for this molecule.Work supported by CNPq, FAPESP, FAEP-Brasil, and CNR-Italia  相似文献   
998.
Using a scanning tunneling microscope, quantum dots with diameters of order 25 Å have been fabricated on the cleaved InSb (110) surface in UHV. Both In and Sb atoms were clearly resolved on the cleaved surface.  相似文献   
999.
The structure of the nematic-isotropic interfacial layer is studied theoretically for systems formed by rod-like and persistent macromolecules. It is shown that the width of interfacial layer is normally of the order of the straight part of a molecule. This allows us to use the approach which describes intermolecular interactions phenomenologically (i.e. it allows us to consider all interactions), at the same time this approach describes molecular flexibility microscopically (i.e. it allows us to study the effects of flexibility correctly). It was found, that non-monotonic gradient profiles in the surface layer of the order parameter or of the concentration of molecules as a function of the coordinate perpendicular to the interface are possible. For example, a thin layer with abnormal ordering of molecules along the surface may exist near the interface for some systems.  相似文献   
1000.
Kinetics of oxidation of indigo carmine (IC) by sodium hypohalites, NaOX (X ? Cl or Br), in alkaline buffer of pH 9–11 has been studied spectrophotometrically at λ = 610 nm. The experimental rate law obtained is ?d[IC]/dt = k[OX?][IC][OH?]x where x <1. Variation of ionic strength or dielectric constant of the medium had no effect on the reaction rate while the addition of halide ions slightly retarded the rate. A most plausible mechanism proposed on the basis of experimental results involves the formation of isatin sulphonate which undergoes further oxidation to anthranilate. Activation parameters have been evaluated.  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