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   243960篇
  免费   2429篇
  国内免费   878篇
化学   130899篇
晶体学   3698篇
力学   9935篇
综合类   2篇
数学   27206篇
物理学   75527篇
  2020年   1773篇
  2019年   2044篇
  2018年   2474篇
  2017年   2403篇
  2016年   3825篇
  2015年   2602篇
  2014年   3902篇
  2013年   11189篇
  2012年   8125篇
  2011年   10048篇
  2010年   6594篇
  2009年   6484篇
  2008年   9025篇
  2007年   9186篇
  2006年   8413篇
  2005年   7883篇
  2004年   7031篇
  2003年   6229篇
  2002年   6167篇
  2001年   7225篇
  2000年   5381篇
  1999年   4226篇
  1998年   3566篇
  1997年   3589篇
  1996年   3303篇
  1995年   3152篇
  1994年   2977篇
  1993年   3040篇
  1992年   3328篇
  1991年   3356篇
  1990年   3165篇
  1989年   3124篇
  1988年   3181篇
  1987年   3047篇
  1986年   2942篇
  1985年   4005篇
  1984年   4163篇
  1983年   3415篇
  1982年   3761篇
  1981年   3665篇
  1980年   3534篇
  1979年   3572篇
  1978年   3792篇
  1977年   3629篇
  1976年   3833篇
  1975年   3405篇
  1974年   3534篇
  1973年   3845篇
  1972年   2338篇
  1971年   1794篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
201.
As a leading defence equipment company, the Dynamics Divisionof British Aerospace Defence Company (BAeDef(DD)) has experiencedthe worst effects of post-Coldwar defence cuts, a global recession,and a determination by the U.K. Government to achieve greatervalue for money in defence procurement. The major tenet of thisnew approach to defence prwurement was 'eyes on, hands off',promoting a shift of the management of project risks from theMinistry of Defence (MOD) to industry. Initial industry reactionwas sceptical but bxame increasingly compliant as tenderingrequirements included visibility of contractors' risk assessment.This paper describes the risk-assessment methodology createdby BAeDef(DD) to meet these changed conditions. The human and organizational aspects of these developments onrisk have helped create a pro-active risk-management cultureat BAeDef(DD). Continuous assessment of project risks providesthe impetus for wider thinking beyond risk-taking or risk-aversionpreferences, leading to greater realism in planning. Decisionsto implement risk resolution measures require competence incost-benefit analysis. The conditions to foster this new behaviourare better-informed senior management, with the rejection ofbad news eliminated at all levels, and the laying of a pathto becoming a learning company.  相似文献   
202.
The energies of negative-parity levels based on two-particle states exhibit a nonlinear behavior at low spins versus the core-rotation energy because the alignment process has not yet been completed for them. This behavior of negative-parity levels in the low-spin region is satisfactorily described upon the inclusion of octupole-octupole interactions. This is demonstrated within the rotational model involving the Coriolis mixing of states for the even-even isotopes 162–168Hf.  相似文献   
203.
Ferroelectric composite two-dimensional ferroelectric/aluminum oxide nanostructures were studied. A porous aluminum oxide matrix was used as a template into which a ferroelectric precursor was introduced, followed by annealing. The prepared nanostructures were studied using optical second harmonic generation and micro-Raman scattering.  相似文献   
204.
We study a 7-dimensional brane world scenario with a Ricci-flat 3-brane residing in the core of a composite monopole defect, i.e., a defect composed of a 't Hooft–Polyakov and a global monopole. Admitting a direct interaction between the two bosonic sectors of the theory, we analyse the structure of the space–time in the limits of small, respectively large direct interaction coupling constant. For large direct interaction, the global monopole disappears from the system and leaves behind a negative cosmological constant in the bulk such that gravity-localising solutions are possible without a priori introduction of a bulk cosmological constant.  相似文献   
205.
Intense multi- and single-line laser operation is reported from semiconductively preionised atomic fluorine lasers employing an optimised capacitor transfer excitation circuitry. The performance and spectral characteristics of these lasers are discussed and compared to the characteristics of previously described atomic fluorine lasers. The efficiency of one of these lasers described, at optimum conditions, is reaching a value as high as 0.14%. To the best of our knowledge, this is the highest efficiency reported up date, for an atomic fluorine laser.  相似文献   
206.
207.
208.
209.
MgB2/Fe wires were prepared by electrical self-heating of in situ powder-in-tube wires for the first time at ambient conditions. Characterization of the wires processed at 750 °C, 800 °C and 850 °C for 15 min by XRD, SEM, ϱ–T, susceptibility and JC measurements shows that the MgB2 formed is of high quality particularly with respect to phase purity and transport JC. The method considerably reduces the overall energy consumption vis-à-vis the production cost, simplifies the complexity of the fabrication procedure and is promising for manufacture of high-quality MgB2 superconducting wires. PACS 74.70.Ad; 74.62.Bf; 74.25.Fy; 74.25.Ha; 81.20.Hy  相似文献   
210.
We report on the shape transition from InAs quantum dashes to quantum dots (QDs) on lattice-matched GaInAsP on InP(3 1 1)A substrates. InAs quantum dashes develop during chemical-beam epitaxy of 3.2 monolayers InAs, which transform into round InAs QDs by introducing a growth interruption without arsenic flux after InAs deposition. The shape transition is solely attributed to surface properties, i.e., increase of the surface energy and symmetry under arsenic deficient conditions. The round QD shape is maintained during subsequent GaInAsP overgrowth because the reversed shape transition from dot to dash is kinetically hindered by the decreased ad-atom diffusion under arsenic flux.  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