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   180372篇
  免费   804篇
  国内免费   267篇
化学   90217篇
晶体学   2638篇
力学   9389篇
综合类   7篇
数学   20972篇
物理学   58220篇
  2020年   1864篇
  2019年   2304篇
  2018年   3402篇
  2017年   3529篇
  2016年   4175篇
  2015年   1872篇
  2014年   3606篇
  2013年   7070篇
  2012年   5572篇
  2011年   6645篇
  2010年   5535篇
  2009年   5649篇
  2008年   6404篇
  2007年   6372篇
  2006年   5657篇
  2005年   5032篇
  2004年   4866篇
  2003年   4393篇
  2002年   4549篇
  2001年   5011篇
  2000年   3702篇
  1999年   2834篇
  1998年   2612篇
  1997年   2467篇
  1996年   2265篇
  1995年   2057篇
  1994年   2016篇
  1993年   2047篇
  1992年   2086篇
  1991年   2383篇
  1990年   2316篇
  1989年   2386篇
  1988年   2267篇
  1987年   2296篇
  1986年   2127篇
  1985年   2657篇
  1984年   2671篇
  1983年   2320篇
  1982年   2375篇
  1981年   2281篇
  1980年   2078篇
  1979年   2396篇
  1978年   2551篇
  1977年   2648篇
  1976年   2729篇
  1975年   2536篇
  1974年   2469篇
  1973年   2636篇
  1972年   2154篇
  1971年   1976篇
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
41.
42.
The contribution of electrons moving at large angles to the barrier junction plane to the tunnel current is calculated. This contribution turns out to be small only if the Fermi energy of the electrons equals several electron volts. Otherwise, specifically, when the Fermi energy is no higher than 1–2 eV, this contribution dominates in high and thin potential barriers. It is found that the tunnel magnetic resistance in ferromagnet-insulator-ferromagnet contacts correlates with this contribution. It is this correlation that is responsible for a decrease in this contribution as the potential barriers get lower and thicker.  相似文献   
43.
An infinite row of periodically spaced, identical rigid circularcylinders is excited by an acoustic line source which is parallelto the generators of the cylinders. A method for calculatingthe scattered field accurately and efficiently is presented.When the cylinders are sufficiently close together, Rayleigh–Blochsurface waves that propagate energy to infinity along the arrayare excited. An expression is derived which enables the amplitudesof these surface waves to be computed without requiring thesolution to the full scattering problem.  相似文献   
44.
We theoretically investigated the mass dependence of the sympathetic cooling rate of gas-phase ions trapped in a linear radio-frequency-quadrupole ion trap. Using an a priori molecular dynamical calculation, tracing numerically with Newtonian equations of motion, we found that ions with a mass greater than 0.54±0.04 times that of the laser-cooled ions are sympathetically cooled; otherwise, they are heated. To understand the mass dependence obtained using the molecular-dynamical calculation, we made a heat-exchange model of sympathetic cooling, which shows that the factor of 0.54±0.04 is a consequence of absence of micro-motion along the axis of the linear ion trap. Received: 10 December 2001 / Revised version: 28 January 2002 / Published online: 14 March 2002  相似文献   
45.
The statistical characteristics of conditional time before ruin of an insurance company, including the distribution function of conditional time and its moments are examined.  相似文献   
46.
47.
48.
49.
A theory that predicts the effect of the counterion size on the swelling and collapse of a weakly charged polyelectrolyte gel was developed. In addition to excluded-volume interactions between monomer units of the gel, the theory involves the counterion-monomer unit and counterion-counterion interactions in terms of the virial approximation. The character of interactions between different units in the system varies from repulsion to attraction depending on the type of solvent, counterion, and dielectric permittivity of the solvent. For solvents with a low permittivity, the effect of condensation of counterions resulting in the formation of ion pairs is taken into account.  相似文献   
50.
The known plant growth regulator, Al'den [1-(3,7-dimethyloctyl)-1-allylpiperidinium bromide], and also 1-(3,7-dimethyloctyl)-1-methylpiperidinium iodide were synthesized by treatment with allyl bromide and methyl iodide, respectively, of exhaustively hydrogenated 1-nerylpiperidine which was obtained by anionic telomerization of isoprene with piperidine. Various quaternary ammonium salts having a terpene sub- stituent with unnatural dimethyloctane skeleton, which effectively stimulate florification of Aster Chinensis L., were prepared by the action of alkyl halides on telomerization products derived from isoprene and piperidine or morpholine in the presence of palladium complexes.  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