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   256052篇
  免费   3399篇
  国内免费   1000篇
化学   138944篇
晶体学   3732篇
力学   10201篇
综合类   3篇
数学   29883篇
物理学   77688篇
  2021年   1860篇
  2020年   2147篇
  2019年   2487篇
  2018年   2816篇
  2017年   2670篇
  2016年   4368篇
  2015年   3107篇
  2014年   4399篇
  2013年   11990篇
  2012年   9031篇
  2011年   11132篇
  2010年   7184篇
  2009年   6994篇
  2008年   9792篇
  2007年   9873篇
  2006年   9112篇
  2005年   8534篇
  2004年   7538篇
  2003年   6630篇
  2002年   6500篇
  2001年   7411篇
  2000年   5516篇
  1999年   4380篇
  1998年   3692篇
  1997年   3727篇
  1996年   3435篇
  1995年   3263篇
  1994年   3052篇
  1993年   3092篇
  1992年   3371篇
  1991年   3399篇
  1990年   3203篇
  1989年   3137篇
  1988年   3201篇
  1987年   3072篇
  1986年   2965篇
  1985年   4047篇
  1984年   4199篇
  1983年   3429篇
  1982年   3805篇
  1981年   3686篇
  1980年   3551篇
  1979年   3584篇
  1978年   3803篇
  1977年   3637篇
  1976年   3848篇
  1975年   3413篇
  1974年   3543篇
  1973年   3840篇
  1972年   2335篇
排序方式: 共有10000条查询结果,搜索用时 187 毫秒
851.
A Monte-Carlo model is described which has been developed for calculation of multiply scattered LIDAR returns. Results are shown for the common problem selected by the MUSCLE (MUltiple SCattering LIDAR Experiments) group for intercomparison, which represents a typical ground-based cloud-sensing scenario. This is contrasted with returns from the same cloud sensed by a space-based LIDAR, where multiple-scattering effects are much greater. The magnitude of multiple-scattering effects is seen to be largely determined by the optical depth across the receiver field of view at the cloud.  相似文献   
852.
The behaviour of La-containing strontium zirconate based ceramics is revised. La additions contribute to improve the sinterability of these materials, but transport properties are also affected. Sr1−xLaxZrO3+δ show some unusual changes in transport properties when samples are exposed to H2-containing atmospheres and then reoxidized. Nevertheless, the transport properties of (Sr,La)(Zr,Y)O3−δ, are still mainly dependent on Y additions, at least for cases when the fraction of Y exceeds the fraction of La. In addition, lanthanum causes significant changes in lattice parameters. A combination of conductivity measurements in different atmospheres, ion blocking experiments, and emf measurements has been used to demonstrate that electron hole conductivity increases with temperature, and the protonic contribution drops. The transport properties at temperatures lower than about 800 °C might be affected by structural changes. Paper presented at the 2nd Euroconference on Solid State Ionics, Funchal, Madeira, Portugal, Sept. 10–16, 1995  相似文献   
853.
854.
The embedded atom method (EAM) was used to determine the equilibrium shapes of small platinum and palladium clusters (N=5–60 atoms). The stability of various polyhedral symmetries was performed at 0K. A search for the presence of any other lower energy structures was also carried out. Pt clusters reconstructed to lower energy clusters even at the magic numbers. Pd also reconstructed to lower energy structures except at the magic number sizes where the icosahedron was found to be the most stable.  相似文献   
855.
856.
A one-dimensional multispecies model of a helium filled AC plasma display cell is described. The model includes a continuity equation for each species. Poisson's equation and a circuit equation. The fill gas is helium for which the cross sections for ionization and excitation for the various atomic states are well known. The reaction rates for the different species were parametrized as functions of E/N using a zero-dimensional spatial and two velocity multispecies Boltzmann kinetic code. Calculations are performed using address and sustain voltages operating at a frequency of 50 kHz to model the pulse behavior of a dielectric barrier discharge which occurs over a time scale on the order of 200 ns or less. The code simulates multipulse behavior and shows reasonably good agreement with experimental data. The simulations show that the circuit filters out the discharge dynamics and that the pulse width of the discharge current depends on the circuit RC time constant. Using a 123 V sustain voltage, the calculated discharge current is 0.4 A with a full width at half maximum (FWHM) of 28 ns. Experimental values using a 120 V sustain voltage are 0.32 A discharge current and 42 ns FWHM  相似文献   
857.
We study the stochastic dynamics of deposition-evaporation cooperative processes of dimers, trimers, etc., in two- and higher-dimensional lattices. The dimer system in bipartite lattices allows for an exact solution of dynamic correlations and scaling functions by means of a quantum spin equivalence. Autocorrelations exhibit a diffusive asymptotic kinetics and crossovers of different dynamic regimes in highly anisotropic lattices. Monte Carlo simulations combined with finite-size scaling arguments support the validity of the diffusive picture in more general situations. Steady-state coverages and diffusion constants are obtained using mean-field approaches, spin wave calculations, and random walk analyses in nearly jammed configurations.  相似文献   
858.
859.
Starting from a general hamiltonian system with superstable pairwise potential, we construct a stochastic dynamics by adding a noise term which exchanges the momenta of nearby particles. We prolve that, in the scaling limit, the time conserved quantities, energy, momenta and density, satisfy the Euler equation of conservation laws up to a fixed timet provided that the Euler equation has a smooth solution with a given initial data up to timet. The strength of the noise term is chosen to be very small (but nonvanishing) so that it disappears in the scaling limit.Research partially supported by U.S. National Science Foundation grants DMS 89001682, DMS 920-1222 and a grant from ARO, DAAL03-92-G-0317Research partially supported by U.S. National Science Foundation grants DMS-9101196, DMS-9100383, and PHY-9019433-A01, Sloan Foundation Fellowship and David and Lucile Packard Foundation Fellowship  相似文献   
860.
Interpolation formulas for higher-order derivatives which guarantee the invariance of the results under changes of coordinates are obtained for functions of a vector argument.Translated from Matematicheskie Metody i Fiziko-Mekhanicheskie Polya, No. 25, pp. 22–25, 1987.  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