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   403818篇
  免费   3408篇
  国内免费   1081篇
化学   212158篇
晶体学   6574篇
力学   18071篇
综合类   12篇
数学   47130篇
物理学   124362篇
  2020年   3206篇
  2019年   3574篇
  2018年   4693篇
  2017年   4744篇
  2016年   7107篇
  2015年   4218篇
  2014年   6825篇
  2013年   17107篇
  2012年   12712篇
  2011年   15375篇
  2010年   11131篇
  2009年   10879篇
  2008年   14455篇
  2007年   14549篇
  2006年   13708篇
  2005年   12373篇
  2004年   11425篇
  2003年   10375篇
  2002年   10179篇
  2001年   11368篇
  2000年   8707篇
  1999年   6731篇
  1998年   5803篇
  1997年   5707篇
  1996年   5370篇
  1995年   5043篇
  1994年   5135篇
  1993年   4814篇
  1992年   5468篇
  1991年   5446篇
  1990年   5318篇
  1989年   5156篇
  1988年   5205篇
  1987年   5098篇
  1986年   4870篇
  1985年   6414篇
  1984年   6703篇
  1983年   5781篇
  1982年   5925篇
  1981年   5739篇
  1980年   5671篇
  1979年   5773篇
  1978年   6115篇
  1977年   5985篇
  1976年   6090篇
  1975年   5708篇
  1974年   5735篇
  1973年   6004篇
  1972年   4169篇
  1971年   3434篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
We show that an electron confined to a single finite parabolic quantum well in crossed electric and magnetic fields can behave as a double quantum well system. The magnetic field is parallel to the heterostructure layers and the electric field is perpendicular to those. For a suitable choice of both fields and quantum well width, the electron can be confined to a double quantum well effective potential that is very similar to the electronic potential model for diatomic molecules. The double quantum well spectrum is calculated using a numerical algorithm based on semiclassical methods. A physical interpretation of this quantum system is given based on the analogy to the electrons bound to diatomic molecules.  相似文献   
62.
It is shown that the nonstable character of nonlinear interaction of a system of waves similar to that resulting from double stimulated Brillouin scattering is retained upon the oblique incidence of a powerful electromagnetic wave on a layer of supercritical plasma. In this case we have nonlinear back reflection from a layer of supercritical plasma. The threshold intensity of the pump wave is greater than that in the case of subcritical plasma for TE polarization waves but can markedly decrease for waves polarized in the incidence plane. Instability is possible only for a layer of finite thickness.Radiophysical Research Institute, Nizhny Novgorod. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 38, Nos. 3–4, pp. 298–303, March–April, 1995.  相似文献   
63.
Flowing and static gas-phase samples of HNO3 in O2 and N2 were analyzed by long-path ultraviolet/visible (UV/VIS) spectroscopy to reveal the presence of both NO2 and NO3, the concentrations of which were calculated using differential absorption cross sections. NO2 is produced predominantly by the heterogeneous decomposition of HNO3, whereas NO3 is generated in the gas phase by the thermal decomposition of N2O5, a product of the self-disproportionation of liquid HNO3. © 1993 John Wiley & Sons, Inc.  相似文献   
64.
On the basis of periodic Ateb functions, in the resonance and nonresonance cases, we construct the asymptotic approximation of one-frequency solutions of a boundary-value problem for a nonlinear nonautonomous equation.  相似文献   
65.
Modified Stern-Volmer equation is obeyed by bovine serum albumin (BSA)-iodide system showing selective quenching of tryptophanyl fluorescence of BSA. The fraction of accessible protein fluorescence is 0.56 and the effective Stern-Volmer constant is 290 M-1 at pH 7.4 in 0.005 M phosphate buffer at 25°C. Collisional quenching is operative both in the BSA -I−1 system and the model system, tryptophan-I−1. It is supported by the observed relationship between the ratio of quenching rate constants (k q ) and diffusion coefficients and alsok q with bulk viscosity.  相似文献   
66.
Chemical interactions at the phase boundaries of materials applied for the solid oxide fuel cell (SOFC) have been studied by EPMA. The chemical reactivity at the interface of Lay-xSrxMnO3/ZrO2-Y2O3 is dependent on the stoichiometry (y) and the Sr content (x) of the perovskite. Typical reaction products (zirconates) and a diffusion zone in the ZrO2–Y2O3 have been observed. The extension of cation release (Mn) is related to the increasing chemical activity of Mn oxide in the perovskite by the Sr substitution for La. The wettability of the metal/oxide interface in the anode cermet (Ni/ZrO2–Y2O3) has been found to be influenced by chemical reactions resulting from the applied reducing atmosphere with high carbon activity. The disintegration of ZrO2–Y2O3 in contact with molten Ni or Ni-Ti and Ni-Cr alloys leads to the redeposition of Y2O3-enriched oxides and also to Zr-rich intermetallic compounds and eutectics.  相似文献   
67.
Modeling and numerical simulations of the convective flows induced by the vibration of the monocrystal during crystal growth have been performed for two configurations simulating the Cz and FZ methods. This permitted to emphasize the role of different vibrational mechanisms in the formation of the average flows. It is shown that an appropriate combination of these mechanisms can be used to counteract the usual convective flows (buoyancy- and/or thermocapillary-driven) inherent to crystal growth processes from the liquid phase. While vibrational convection is rather complex due to these identified mechanisms, the new modeling used in the present paper opens up very promising perspectives to efficiently control heat and mass transfer during real industrial applications of crystal growth from the liquid phase.  相似文献   
68.
69.
70.
A sufficient Hartman-Ismagilov type condition for the essential self-adjointness of a one-parameter family of unbounded operators that arise in the solution of a Cauchy problem for the wave equation is established. An analog of this result is stated for unbounded integral operators.Translated from Ukrainskii Matematicheskii Zhurnal, Vol. 44, No. 7, pp. 940–948, July, 1992.  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