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   677757篇
  免费   5077篇
  国内免费   1651篇
化学   332221篇
晶体学   9731篇
力学   36246篇
综合类   27篇
数学   98445篇
物理学   207815篇
  2021年   5831篇
  2020年   6288篇
  2019年   7173篇
  2018年   13616篇
  2017年   13885篇
  2016年   15558篇
  2015年   7599篇
  2014年   12390篇
  2013年   27994篇
  2012年   23813篇
  2011年   31982篇
  2010年   22895篇
  2009年   23015篇
  2008年   29557篇
  2007年   31317篇
  2006年   22001篇
  2005年   22266篇
  2004年   19686篇
  2003年   18319篇
  2002年   17179篇
  2001年   17607篇
  2000年   13700篇
  1999年   10470篇
  1998年   8998篇
  1997年   8911篇
  1996年   8472篇
  1995年   7552篇
  1994年   7617篇
  1993年   7336篇
  1992年   7889篇
  1991年   8285篇
  1990年   8000篇
  1989年   7921篇
  1988年   7726篇
  1987年   7456篇
  1986年   7183篇
  1985年   9188篇
  1984年   9617篇
  1983年   8035篇
  1982年   8269篇
  1981年   7826篇
  1980年   7403篇
  1979年   8054篇
  1978年   8279篇
  1977年   8188篇
  1976年   8189篇
  1975年   7824篇
  1974年   7612篇
  1973年   8014篇
  1972年   5867篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
21.
We investigate theoretically the effects of optical absorption and saturation of the non-linear index of refraction on the bistability of non-linear distributed feedback structures (NLDFBs). By assuming that the Kerr non-linearity saturates in an exponential fashion, we obtain for the first time closed-form expressions for the so-called self-phase and cross-phase modulation terms. Our investigation shows that both absorption and, in particular, saturation significantly affect the bistable properties of this structure and in many cases eliminate this response completely. In some cases, however, saturation alters the NLDFB's transfer characteristics in a potentially useful manner. We find that weak levels of saturation may increase the contrast ratio between the intensities of the high and low bistable states. At increasing levels of saturation, where bistability is no longer observed, we find regimes where the NLDFB structure could possibly be used for optical amplification. This revised version was published online in November 2006 with corrections to the Cover Date.  相似文献   
22.
Terrestrial basalt rocks considered to be a good analogues of the Martian regolith were studied by using the Mössbauer spectroscopy and X-ray diffraction. The weathering of basalts was followed by the changes of the Fe2+/Fe3+ ratio. The basalt rock undergoes alteration from primary mineral composition (olivine+pyroxene+plagioclase) through smectite to kaolinite clay with oxides and hydroxides with the increase of the Fe2+/Fe3+ ratio.  相似文献   
23.
The interaction of isotactic polypropylene with ethylene propylene diene terpolymer in their blends has been investigated by use of differential scanning calorimetry, dynamic mechanical analysis, wide- and small-angle x-ray scattering, and by investigating the nucleation and kinetics of crystallization of the iPP component under the polarization microscope. It is found, that the dispersion of the EPDM component in the iPP matrix is dependent on blend composition and is maximal at 10% EPDM content. An interface layer between the two components is formed by migration of iPP molecules into the EPDM phase. A model for this interface is proposed.  相似文献   
24.
An optimal control model of exhaustible resources is used to clarify the long run relationship between mineral rent and depletion cost at the industry level. A standard first order condition of the time rate of change of rents is reformulated to reveal that rent data may be used to help forecast the rise in extraction costs resulting from resource depletion. This application of the theory of exhaustible resources is illustrated using historical mineral industry rent and extraction cost data. A forecast of U.S. coal extraction costs, following the method proposed in this paper, suggests that future rates of extraction cost increases will be similar to rates experienced in the past.  相似文献   
25.
We report on a cw mode-locked non-critically phase matched KTP optical parametric oscillator synchronously pumped by a picosecond Ti:Sapphire laser. High average signal output power of up to 950 mW over a large tuning range has been achieved. For this OPO the influence of resonator-length detuning on the output power, pulse duration and spectral bandwidth has been investigated. The measured data are in good agreement with the results of a numerical simulation using a split-step Fourier method which considers the group-velocity mismatch, the group-velocity dispersion and the self-phase modulation. The numerical simulation also describes the measured strong pump depletion and its influence on the OPO output and efficiency.  相似文献   
26.
27.
The Thoms effect observed with water-soluble cationic copolymer of acrylamide was studied in relation to the concentration, molecular weight, and molecular weight polydispersity of the polymeric additive, with its composition and composition polydispersity being constant.  相似文献   
28.
An explicit way for producing invariants for 6-valent graphs with rigid vertices within the framework of Kauffman's approach to graph invariants is presented. These invariants can be used to detect the chirality of a 6-valent graph with rigid vertices. A relevant example is considered. Bibliography: 19 titles. Translated fromZapiski Nauchnykh Seminarov POMI, Vol. 223, 1995, pp. 251–262. Translated by A. M. Nikitin  相似文献   
29.
30.
We consider the method of normal forms, the Bogolyubov averaging method, and the method of asymptotic decomposition proposed by Yu. A. Mitropol’skii and the author of this paper. Under certain assumptions about group-theoretic properties of a system of zero approximation, the results obtained by the method of asymptotic decomposition coincide with the results obtained by the method of normal forms or the Bogolyubov averaging method. We develop a new algorithm of asymptotic decomposition by a part of the variables and its partial case — the algorithm of averaging on a compact Lie group. For the first time, it became possible to consider asymptotic expansions of solutions of differential equations on noncommutative compact groups.  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