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   653378篇
  免费   5290篇
  国内免费   1806篇
化学   337709篇
晶体学   9212篇
力学   31425篇
综合类   21篇
数学   85667篇
物理学   196440篇
  2021年   5741篇
  2020年   6336篇
  2019年   7109篇
  2018年   9570篇
  2017年   9658篇
  2016年   13432篇
  2015年   7415篇
  2014年   12238篇
  2013年   28728篇
  2012年   21973篇
  2011年   26253篇
  2010年   19475篇
  2009年   19250篇
  2008年   25041篇
  2007年   24914篇
  2006年   22876篇
  2005年   20567篇
  2004年   18867篇
  2003年   17068篇
  2002年   16943篇
  2001年   17855篇
  2000年   13989篇
  1999年   10769篇
  1998年   9331篇
  1997年   9249篇
  1996年   8660篇
  1995年   7705篇
  1994年   7708篇
  1993年   7532篇
  1992年   7996篇
  1991年   8481篇
  1990年   8127篇
  1989年   8016篇
  1988年   7846篇
  1987年   7615篇
  1986年   7339篇
  1985年   9448篇
  1984年   9881篇
  1983年   8200篇
  1982年   8682篇
  1981年   8104篇
  1980年   7705篇
  1979年   8259篇
  1978年   8680篇
  1977年   8546篇
  1976年   8491篇
  1975年   8073篇
  1974年   7958篇
  1973年   8305篇
  1972年   5959篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
951.
Coupled and decoupled Taylor-Galerkin algorithms are considered for viscoelastic flow and a model problem—transient startup Poiseuille flow in a channel under a fixed pressure gradient. All algorithms reproduce the steady-state solutions and are stable at high elasticity numbers (E). For a fixed mesh, the coupled and decoupled versions (TGC and TGD) give exceptional time-accuracy at low elasticity numbers [to within O(1%) at E = 1] and reasonable accuracy at high elasticity numbers [to within O(10%) at E = 10, 100]. By definition, the decoupled false-transient scheme (TGF), which uses different time scales for velocity and stress time stepping, provides a poor transient history. Where the main requirement is to compute a steady-state algorithm efficiency is crucial. The TGF scheme attains a steady state between six to eight times faster than does the TGC scheme, and the latter is over twice as fast as the TGD form. © 1994 John Wiley & Sons, Inc.  相似文献   
952.
953.
954.
Product integration rules generalizing the Fej?r, Clenshaw-Curtis,and Filippi quadrature rules respectively are derived for integralswith trigonometric and hyperbolic weight factors. The Chebyshevmoments of the weight functions are found to be given by well-conditionedexpressions, in terms of hypergeometric functions 0F1. An a priori error estimator is discussed which is shown bothto avoid wasteful invocation of the integration rule and toincrease significantly the robustness of the automatic quadratureprocedure. Then, specializing to extended Clenshaw-Curtis (ECC) rules,three types of a posteriori error estimates are considered andthe existence of a great risk of their failure is demonstratedby large scale validation tests. An empirical error estimator,superseding them for slowly varying integrands, is found toresult in a spectacular increase in the output reliability. Finally, enhancements in the control of the interval subdivisionstrategy aiming at increasing code robustness is discussed.Comparison with the code DQAWO of QUADPACK, with about a hundredthousand solved integrals, is illustrative of the increasedrobustness and error estimate reliability of our computer codeimplementation of the ECC rules.  相似文献   
955.
956.
In this paper we determine the forcing linearity numbers for semisimple modules over integral domains.  相似文献   
957.
958.
Latent macroscopic defects in silicon are detected by electrical and electron microscope measurements. They lead to anomalous temperature dependence of the Fermi level position and growth in the hole capture coefficient. A level with energy of 0.55 eV measured from the conduction zone controls the recombination process. It is proposed that macroscopic defects develop upon association of oxygen-silicon vacancy complexes. Action of an electron beam leads to reversible changes which increase upon multiple scanning, affecting the value of the diffusion length.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 71–75, April, 1991.  相似文献   
959.
A nonperturbative method is suggested for calculating functional integrals. Its efficiency is demonstrated for the quantum-mechanical anharmonic oscillator. A quantity we are interested in is represented by a series, a finite number of terms of which describes not only the region of small coupling constants but well reproduces the strong coupling limit. The method is formulated only in terms of the Gaussian functional quadratures and diagrams are used of the conventional perturbation theory.  相似文献   
960.
A new design for a single pass X-ray Self-Amplified Spontaneous Emission (SASE) FEL is proposed. The scheme consists of two undulators and an X-ray monochromator located between them. The first stage of the FEL amplifier operates in the SASE linear regime. After the exit of the first undulator the electron bunch is guided through a non-isochronous bypass and the X-ray beam enters the monochromator. The main function of the bypass is to suppress the modulation of the electron beam induced in the first undulator. This is possible because of the finite value of the natural energy spread in the beam. At the entrance to the second undulator the radiation power from the monochromator dominates significantly over the shot noise and the residual electron bunching. As a result the second stage of the FEL amplifier operates in the steady-state regime when the input signal bandwidth is small with respect to that of the FEL amplifier. Integral losses of the radiation power in the monochromator are relatively small because grazing incidence optics can be used. The proposed scheme is illustrated for the example of the 6 nm option SASE FEL at the TESLA Test Facility under construction at DESY. As shown in this paper the spectral bandwidth of such a two-stage SASE FEL (Δλ/λ 5 × 10−5) is close to the limit defined by the finite duration of the radiation pulse. The average brilliance is equal to 7 × 1024 photons/(s × mrad2 × mm2 × 0.1% bandw.) which is by two orders of magnitude higher than the value which could be reached by the conventional SASE FEL. The monochromatization of the radiation is performed at a low level of radiation power (about 500 times less than the saturation level) which allows one to use conventional X-ray optical elements (grazing incidence grating and mirrors) for the monochromator design.  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