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   411242篇
  免费   4302篇
  国内免费   1097篇
化学   204308篇
晶体学   5920篇
力学   19435篇
综合类   7篇
数学   59493篇
物理学   127478篇
  2020年   1983篇
  2019年   1956篇
  2018年   10974篇
  2017年   11853篇
  2016年   7907篇
  2015年   4153篇
  2014年   4550篇
  2013年   14101篇
  2012年   15026篇
  2011年   25631篇
  2010年   15521篇
  2009年   15444篇
  2008年   22727篇
  2007年   26290篇
  2006年   12389篇
  2005年   18266篇
  2004年   13570篇
  2003年   12170篇
  2002年   10044篇
  2001年   10258篇
  2000年   7980篇
  1999年   6151篇
  1998年   4935篇
  1997年   4751篇
  1996年   4915篇
  1995年   4451篇
  1994年   4191篇
  1993年   4004篇
  1992年   4492篇
  1991年   4356篇
  1990年   4071篇
  1989年   3897篇
  1988年   4199篇
  1987年   3861篇
  1986年   3736篇
  1985年   5395篇
  1984年   5484篇
  1983年   4449篇
  1982年   4824篇
  1981年   4866篇
  1980年   4621篇
  1979年   4751篇
  1978年   4763篇
  1977年   4670篇
  1976年   4651篇
  1975年   4553篇
  1974年   4399篇
  1973年   4603篇
  1972年   2594篇
  1971年   1896篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
221.
A firm's logistics cost, including shipping and inventory-carrying, is a substantial percentage of its sales. Nevertheless, typical inventory-control methods ignore or insufficiently represent the shipping cost. This paper describes a recursive algorithm that determines the reorder cycle-time that minimizes total logistics cost. It allows for a realistic accounting of shipping cost, which is modelled here as a function of shipping distance and weight. The algorithm uses a relaxation procedure to identify a suitable initial approximation to the optimal order cycle-time and then, through a series of recursive steps, moves to the optimal result. We demonstrate the algorithm with a single item, with a group of items that share a common order cycle, and with multi-items when item demands are random variables. Experience with this algorithm indicates that it converges to the optimal result in a very few steps.  相似文献   
222.
223.
224.
Summary In 1980 Dahmen-DeVore-Scherer introduced a modulus of continuity which turns out to reflect invariance properties of compound cubature rules effectively. Accordingly, sharp error bounds are derived, the existence of relevant counterexamples being a consequence of a quantitative resonance principle, established previously.  相似文献   
225.
The excitation function of the fission probability P E E x) for238U has been measured in the reaction238U(α, α′ f) at 480 MeV bombarding energy. The reaction mechanism of this reaction is discussed for excitation energies belowB nf , the threshold for second chance fission, and aboveB nf up toE x =37 MeV. In comparing with results from fission induced by photons and by particle transfer reactions the (α, α′f) reaction gives too low values for the fission probabilityP f at excitation energies well aboveB nE . The role of the quasi-elastic knock-out process in this reaction is discussed.  相似文献   
226.
227.
J. H. Eberly 《Laser Physics》2006,16(6):921-926
We examine the application of Schmidt mode analysis to pure-state entanglement. Several examples permitting exact analytic calculation of Schmidt eigenvalues and eigenfunctions are included, as well as evaluation of the associated degree of entanglement.  相似文献   
228.
Thomson scattering of high-power laser and electron beams is a good test of electrodynamics in the high-field region. We demonstrated production of high-intensity X-rays in the head-on collision of a CO2 laser and 60-MeV electron beams at Brookhaven National Laboratory, Accelerator Test Facility. The energy of an X-ray photon was limited at 6.5 keV in the linear (lowest order) Thomson scattering, but the nonlinear (higher order) process produces higher energy X-rays. We measured the angular distribution of the high-energy X-rays and confirmed that it agrees with theoretical predictions.  相似文献   
229.
The tape stripping technique is an experimental method frequently used for reconstruction of the in-depth distribution of various topically administered substances within the horny layer of human skin, e.g., compounds contained in sunscreens. Titanium dioxide (TiO2) nanoparticles (25–200 nm in diameter) are one such compound. Optical techniques which apply blue light are found to be suitable for reconstruction. However, the presence of particles affects the light propagation within the skin and therefore causes incorrect determination of strip thickness, leading to an improper reconstructed distribution of nanoparticle concentration revealed from the experimental data. This study evaluates the errors emerging from the use of blue (400 nm) and NIR (800 nm) radiation and finds the use of longer wavelength light more advantageous. Particles of different diameters are considered, and it is revealed that the application of small particles (25–60 nm) results in the lowest rate of error.  相似文献   
230.
The synthesis of rigid spirocyclic bislactams derived from pyroglutamic acid has been established.  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