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   694197篇
  免费   8353篇
  国内免费   2520篇
化学   361514篇
晶体学   9607篇
力学   32645篇
综合类   80篇
数学   93949篇
物理学   207275篇
  2021年   6188篇
  2020年   6947篇
  2019年   7642篇
  2018年   10331篇
  2017年   10395篇
  2016年   14831篇
  2015年   8728篇
  2014年   13444篇
  2013年   31317篇
  2012年   25036篇
  2011年   29502篇
  2010年   21650篇
  2009年   21185篇
  2008年   27684篇
  2007年   27369篇
  2006年   25117篇
  2005年   22720篇
  2004年   20766篇
  2003年   18500篇
  2002年   18267篇
  2001年   19060篇
  2000年   14899篇
  1999年   11424篇
  1998年   9801篇
  1997年   9571篇
  1996年   9192篇
  1995年   8209篇
  1994年   8318篇
  1993年   7971篇
  1992年   8545篇
  1991年   8872篇
  1990年   8483篇
  1989年   8311篇
  1988年   8069篇
  1987年   7777篇
  1986年   7534篇
  1985年   9779篇
  1984年   10112篇
  1983年   8490篇
  1982年   8898篇
  1981年   8359篇
  1980年   7882篇
  1979年   8429篇
  1978年   8800篇
  1977年   8687篇
  1976年   8714篇
  1975年   8236篇
  1974年   8154篇
  1973年   8483篇
  1972年   6257篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
981.
The occurrence of two noninteracting forms of Pd-porphin in the ground electronic state in a wide number of Shpolskii matrices maintained at liquid helium temperature was observed with the help of the finestructure phosphorescence spectra of these systems. The ratio between these forms can be varied in wide limits by choosing a particular matrix, which indicates that the matrix plays a significant role in the conformation formation of the tetrapyrrole macrocycle. Based on comparison both with the resonance Raman spectra and with the results of investigations of metal complexes of porphin by the selective spectroscopy methods, the shortwavelength spectral form has been shown to correspond to the planar molecular structure, whereas the longwavelength form has been attributed to a nonplanar molecular structure with the distortion of the porphin macrocycle of the saddle type. The results obtained indicate that the methods of fine structure spectroscopy make it possible to efficiently study distorted molecular structures stabilized in solid matrices at low temperatures.  相似文献   
982.
Experimental study of low-frequency dynamics of an intracavity frequency-doubled Nd:YAG laser demonstrates the influence of the interaction of orthogonally polarized modes, participating in frequency doubling (type II phase matching), on the stability of the laser output. At a sufficiently low pump rate and low conversion efficiency, the laser shows stable operation with a low noise level at the frequencies of relaxation oscillations. At a high pump power and/or a high conversion efficiency, the laser emission becomes unstable as a result of Hopf bifurcation at the frequencies of relaxation oscillations that are responsible for the anti-phase polarization dynamics of the laser.  相似文献   
983.

A colourless quartz crystal doped with 57Fe3+ was obtained by hydrothermal synthesis in an NH4F solution. The crystal was transformed into violet amethyst by gamma-irradiation. The change in colour was accompanied by changes in the Mössbauer spectrum that can be interpreted as the conversion of trivalent iron into the tetravalent state: Fe3+→Fe4+.

  相似文献   
984.
985.
986.
Porous silicon (PS) exhibits several photoluminescence (PL) bands, whose spectral position and intensity depend strongly on the actual conditions of preparation of PS, its treatment, and subsequent use. The PS PL band peaking at about 1.8 eV and usually assigned to the intrinsic emission of silicon nanocrystals was studied. It was shown that the temperature-induced variation of the PL kinetics in the 80 to 300-K interval follows a complex pattern and depends noticeably on the actual point on the band profile. The temperature behavior of PL decay in the 1.8-eV band is determined by the electron-hole recombination rate within a nanocrystal and the cascade carrier transitions from small to large nanocrystals, with an attendant decrease in energy.  相似文献   
987.
This paper is devoted to the analysis of function spaces modeled on Besov spaces and their applications to non-linear partial differential equations, with emphasis on the incompressible, isotropic Navier-Stokes system and semi-linear heat equations. Specifically, we consider the class, introduced by Hideo Kozono and Masao Yamazaki, of Besov spaces based on Morrey spaces, which we call Besov-Morrey or BM spaces. We obtain equivalent representations in terms of the Weierstrass semigroup and wavelets, and various embeddings in classical spaces. We then establish pseudo-differential and para-differential estimates. Our results cover non-regular and exotic symbols. Although the heat semigroup is not strongly continuous on Morrey spaces, we show that its action defines an equivalent norm. In particular, homogeneous BM spaces belong to a larger class constructed by Grzegorz Karch to analyze scaling in parabolic equations. We compare Karch's results with those of Kozono and Yamazaki and generalize them by obtaining short-time existence and uniqueness of solutions for arbitrary data with subcritical regularity. We exploit pseudo-differential calculus to extend the analysis to compact, smooth, boundaryless, Riemannian manifolds. BM spaces are defined by means of partitions of unity and coordinate patches, and intrinsically in terms of functions of the Laplace operator.

  相似文献   

988.
The contribution to electrical resistance due to scattering of charge carriers by domain walls is analyzed. It is revealed that “unusual” domain walls are created by frustrations in ferromagnet-antiferromagnet multilayer magnetic structures. The thickness of an unusual domain wall is substantially less than that of a usual domain wall. It is shown that scattering of charge carriers by unusual domain walls can contribute significantly to the magnetoresistance of ferromagnet-antiferromagnet multilayer magnetic structures. An analysis of the contribution made by the Levy-Zhang mechanism to the magnetoresistance demonstrates that the initial estimate obtained for this contribution is considerably exaggerated.  相似文献   
989.
Let G be a connected graph with minimum degree at least 3. We prove that there exists an even circuit C in G such that GE(C) is either connected or contains precisely two components one of which is isomorphic to a 1-bond. We further prove sufficient conditions for there to exist an even circuit C in a 2-connected simple graph G such that GE(C) is 2-connected. As a consequence of this, we obtain sufficient conditions for there to exist an even circuit C in a 2-connected graph G for which GE(C) is 2-connected.  相似文献   
990.
Several composites were prepared on the basis of an ethylene homopolymer and different copolymers of ethylene and 1‐hexene, synthesized with a metallocene catalyst, as matrices and a content of a 5 wt % of short glass fiber. The effect of the fiber incorporation on the structure and mechanical and viscoelastic behaviors was analyzed for the different samples. The glass fibers induced a slightly higher crystallinity, and the crystallite morphology significantly changed (long spacings and crystal orientation). The incorporation of fibers did not reinforce the different matrices under study at this low content; consequently, the mechanical parameters, such as Young's modulus, yielding stress, and microhardness, were lower in the composites as compared with those values found in the neat polyolefins. The location and apparent activation energies of distinct relaxation processes are also discussed. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1244–1255, 2003  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