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   24030篇
  免费   1259篇
  国内免费   3157篇
化学   17417篇
晶体学   140篇
力学   856篇
综合类   63篇
数学   3516篇
物理学   6454篇
  2023年   117篇
  2022年   79篇
  2021年   371篇
  2020年   296篇
  2019年   330篇
  2018年   271篇
  2017年   266篇
  2016年   627篇
  2015年   551篇
  2014年   587篇
  2013年   1320篇
  2012年   1436篇
  2011年   2661篇
  2010年   1459篇
  2009年   1569篇
  2008年   1139篇
  2007年   1118篇
  2006年   1039篇
  2005年   1313篇
  2004年   2366篇
  2003年   1513篇
  2002年   1306篇
  2001年   866篇
  2000年   463篇
  1999年   418篇
  1998年   448篇
  1997年   347篇
  1996年   288篇
  1995年   276篇
  1994年   206篇
  1993年   559篇
  1992年   598篇
  1991年   379篇
  1990年   392篇
  1989年   368篇
  1988年   135篇
  1987年   65篇
  1986年   91篇
  1985年   111篇
  1984年   108篇
  1983年   77篇
  1982年   67篇
  1981年   60篇
  1980年   51篇
  1979年   34篇
  1978年   30篇
  1977年   38篇
  1976年   28篇
  1975年   25篇
  1974年   27篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
In this article, a way to employ the diffusion approximation to model interplay between TCP and UDP flows is presented. In order to control traffic congestion, an environment of IP routers applying AQM (Active Queue Management) algorithms has been introduced. Furthermore, the impact of the fractional controller PIγ and its parameters on the transport protocols is investigated. The controller has been elaborated in accordance with the control theory. The TCP and UDP flows are transmitted simultaneously and are mutually independent. Only the TCP is controlled by the AQM algorithm. Our diffusion model allows a single TCP or UDP flow to start or end at any time, which distinguishes it from those previously described in the literature.  相似文献   
2.
Archiv der Mathematik - In this note, we introduce a criterion to detect vanishing of essential algebras of a Green biset functor by means of morphisms. We introduce the inflation morphism and...  相似文献   
3.
CRANAD-2 is a fluorogenic curcumin derivative used for near-infrared detection and imaging in vivo of amyloid aggregates, which are involved in neurodegenerative diseases. We explore the performance of CRANAD-2 in two super-resolution imaging techniques, namely stimulated emission depletion (STED) and single-molecule localization microscopy (SMLM), with markedly different fluorophore requirements. By conveniently adapting the concentration of CRANAD-2, which transiently binds to amyloid fibrils, we show that it performs well in both techniques, achieving a resolution in the range of 45–55 nm. Correlation of SMLM with atomic force microscopy (AFM) validates the resolution of fine features in the reconstructed super-resolved image. The good performance and versatility of CRANAD-2 provides a powerful tool for near-infrared nanoscopic imaging of amyloids in vitro and in vivo.  相似文献   
4.
Central European Journal of Operations Research - This paper presents a discrete event simulation model to support decision-making for the short-term planning of hospital resource needs, especially...  相似文献   
5.
Algebras and Representation Theory - Let A and B be two Morita equivalent finite dimensional associative algebras over a field ��. It is well known that Hochschild cohomology is...  相似文献   
6.
Journal of Solid State Electrochemistry - The addition of nanocrystalline titanium dioxide (P90) to a cathode of Li/S cell enhances its voltammetric charge capacity by 19%, from which only a small...  相似文献   
7.
8.
A new family of distorted ribbon-shaped nanographenes was designed, synthesized, and their optical and electrochemical properties were evaluated, pointing out an unprecedented correlation between their structural characteristics and the two-photon absorption (TPA) responses and electrochemical band gaps. Three nanographene ribbons have been prepared: a seven-membered-ring-containing nanographene presenting a tropone moiety at the edge, its full-carbon analogue, and a purely hexagonal one. We have found that the TPA cross-sections and the electrochemical band gaps of the seven-membered-ring-containing compounds are higher and lower, respectively, than those of the fully hexagonal polycyclic aromatic hydrocarbon (PAH). Interestingly, the inclusion of additional curvature has a positive effect in terms of non-linear optical properties of those ribbons.  相似文献   
9.
The redox activity of molecular phenazine catalysts has been previously exploited for aerobic oxidative amine homo- and cross-coupling reactions. In this contribution, we have extended the reaction scope of this novel type of organocatalyst and used them in the cross-dehydrogenative aza-Henry coupling of isoquinolines with nitromethane under aerobic conditions. Additionally, we have designed and prepared a novel porous organic polymer by cross-linking of tetrakis(4-bromophenyl)silane and dihydrophenazine through Pd-catalyzed Buchwald-Hartwig cross-coupling. This new type of heterogeneous catalyst, apart from being robust and easily reusable, also showed outstanding catalytic activities and improved selectivity compared to its molecular counterpart. A plausible reaction mechanism was proposed based on spectroscopic and kinetic measurements.  相似文献   
10.
As part of previous studies, we introduced a new type of basis function named Simplified Box Orbitals (SBOs) that belong to a class of spatially restricted functions which allow the zero differential overlap (ZDO) approximation to be applied with complete accuracy. The original SBOs and their Gaussian expansions SBO-3G form a minimal basis set, which was compared to the standard Slater-type orbital basis set (STO-3G). In the present paper, we have developed the SBO basis functions at double-zeta (DZ) level, and we have assessed the option of expanding the SBO-DZ as a combination of Gaussian functions. Finally, we have determined the quality of the new basis set by comparing the molecular properties calculated with SBO-nG with those achieved with some standard basis sets.  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