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   489607篇
  免费   7535篇
  国内免费   1470篇
化学   271354篇
晶体学   6576篇
力学   19023篇
综合类   11篇
数学   61251篇
物理学   140397篇
  2021年   3185篇
  2020年   3939篇
  2019年   3838篇
  2018年   4612篇
  2017年   4475篇
  2016年   8093篇
  2015年   6275篇
  2014年   8170篇
  2013年   22677篇
  2012年   17733篇
  2011年   21190篇
  2010年   13760篇
  2009年   13371篇
  2008年   19674篇
  2007年   19471篇
  2006年   18692篇
  2005年   17200篇
  2004年   15304篇
  2003年   13576篇
  2002年   13193篇
  2001年   14969篇
  2000年   11615篇
  1999年   9101篇
  1998年   7068篇
  1997年   6939篇
  1996年   7111篇
  1995年   6414篇
  1994年   6263篇
  1993年   6067篇
  1992年   6875篇
  1991年   6606篇
  1990年   6271篇
  1989年   5985篇
  1988年   6187篇
  1987年   5916篇
  1986年   5602篇
  1985年   7798篇
  1984年   7843篇
  1983年   6448篇
  1982年   6772篇
  1981年   6691篇
  1980年   6383篇
  1979年   6715篇
  1978年   6841篇
  1977年   6755篇
  1976年   6629篇
  1975年   6404篇
  1974年   6249篇
  1973年   6422篇
  1972年   3855篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
992.
One of the shortcomings of R&D evaluation is a lack of emphasis on analytical assessment of the value of an on-going R&D project. This paper addresses the problem concerning the distributive aspect of access to superior knowledge. Decision tree analysis and probability models appear to be appropriate tools for assessing the values of an intermediate result and patent reward of a firm's R&D decision in an environment of perfect information and oligopolistic competition. The assessed values are used as minimum prices acceptable to the firm when the knowledge is disseminated to the public. This paper will attempt to resolve this problem through the determination of the appropriate values of the reservation price of the first-stage invention in terms of the final reward and of a patent reward for which the inventor is willing to apply.  相似文献   
993.
An analytically solvable model for sand avalanches of noninteracting grains of sand, based on the Chapman-Kolmogorov equations, is presented. For a single avalanche, distributions of lifetimes, sizes of overflows and avalanches, and correlation functions are calculated. Some of these are exponentials, some are power laws. Spatially homogeneous distributions of avalanches are also studied. Computer simulations of avalanches of interacting grains of sand are compared to the solutions to the Chapman-Kolmogorov equations. We find that within the range of parameters explored in the simulation, the approximation of noninteracting grains of sand is a good one.  相似文献   
994.
X-ray photoelectron and Auger spectroscopies were used to examine surface bonding and overlayer growth during palladium and copper deposition onto films of fullerene, C60 The results were consistent with metal cluster formation on C60. The observed positive core electron binding energy shifts in small metallic clusters supported on C60 were shown to originate in metal-fullerene interaction accompanied by charge transfer. Palladium-fullerene intermixing at temperatures as low as ss 50°C was observed for small Pd coverages.  相似文献   
995.
We present a pole expansion for the propagators in the framework of normalized quantum electrodynamics and compare it with the more canonical results from S-matrix theory.  相似文献   
996.
997.
A program to optimize the structure of large molecules at the Hartree–Fock level of theory running concurrently on a network of workstations is presented. Problems encountered in obtaining nearly optimal speedup and their solutions are discussed. A simple scheduling algorithm is presented that enables up to 99.5% of the code to run in parallel. © 1993 John Wiley & Sons, Inc.  相似文献   
998.
The technique of Field-Cycled Dynamic Nuclear Polarization (FC-DNP) involves the EPR irradiation of a free radical solution and the subsequent observation of the NMR signal, the experiment being carried out at a range of magnetic field strengths in order to measure the free radical’s EPR spectrum. In this work FC-DNP has been used to study the EPR spectrum of DMPO spin-trapped hydroxyl free radicals at magnetic field strengths between 0.5 mT and 13.0 mT (5–130 Gauss). The low-field EPR spectrum contains six separate EPR lines, in contrast to the well-known X-band spectrum where only four are seen. Knowledge of the spin-adduct’s EPR spectrum will be of use to workers involved in low-field EPR, especially those conducting biological or in-vivo spin-trapping experiments.  相似文献   
999.
The magnetic properties of a TbNi2Si2 single crystal have been investigated experimentally by specific heat, anisotropy of the susceptibility and magnetization, and inelastic neutron scattering, leading to the determination of its fundamental coupling parameters. The lowest crystalline electric field states are two singlets close to each other, that explains the different magnetic structures evidenced in TbNi2Si2: an amplitude modulated structure, close to T N , a simple antiferromagnetic one at low temperature and an antiphase one when an external magnetic field is applied along the c direction.  相似文献   
1000.
Taking advantage of the long 13C T1 values generally encountered in solids, selective saturation and inversion of more than one resonance in 13C CP/MAS experiments can be achieved by sequentially applying several DANTE pulse sequences centered at different transmitter frequency offsets. A new selective saturation pulse sequence is introduced composed of a series of 90 degrees DANTE sequences separated by interrupted decoupling periods during which the selected resonance is destroyed. Applications of this method, including the simplification of the measurement of the principal values of the 13C chemical shift tensor under slow MAS conditions, are described. The determination of the aromaticity of coal using a relatively slow MAS rate is also described.  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