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   14305篇
  免费   2761篇
  国内免费   2204篇
化学   10132篇
晶体学   231篇
力学   900篇
综合类   161篇
数学   1599篇
物理学   6247篇
  2024年   56篇
  2023年   285篇
  2022年   520篇
  2021年   585篇
  2020年   695篇
  2019年   643篇
  2018年   584篇
  2017年   540篇
  2016年   788篇
  2015年   787篇
  2014年   926篇
  2013年   1148篇
  2012年   1465篇
  2011年   1416篇
  2010年   1031篇
  2009年   1019篇
  2008年   1085篇
  2007年   917篇
  2006年   775篇
  2005年   648篇
  2004年   508篇
  2003年   399篇
  2002年   396篇
  2001年   333篇
  2000年   272篇
  1999年   233篇
  1998年   163篇
  1997年   143篇
  1996年   140篇
  1995年   107篇
  1994年   112篇
  1993年   76篇
  1992年   82篇
  1991年   73篇
  1990年   76篇
  1989年   48篇
  1988年   35篇
  1987年   23篇
  1986年   25篇
  1985年   26篇
  1984年   22篇
  1983年   16篇
  1982年   12篇
  1981年   12篇
  1977年   2篇
  1974年   5篇
  1972年   2篇
  1966年   2篇
  1959年   2篇
  1957年   2篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
891.
Single-mode low-loss optical fibers for long-wave infrared transmission   总被引:1,自引:0,他引:1  
Yang Z  Luo T  Jiang S  Geng J  Lucas P 《Optics letters》2010,35(20):3360-3362
In this Letter, we report single-mode fibers made of chalcogenide glasses with low loss in the 5-12μm range. Glasses from the Ge-As-Te-Se system were optimized to prevent nucleation and to exhibit low density of charge carriers. Single-mode fibers were obtained through the rod-in-tube method by substituting 2% Te/Se between the core and cladding glasses. The resulting single-mode fibers had a core diameter of 30μm and exhibited losses of ~6 dB/m at 10.6μm, and as low as 3-4dB/m in the 6-10μm range.  相似文献   
892.
基于目标在回路的三路光纤传输激光相干合成实验   总被引:1,自引:0,他引:1       下载免费PDF全文
耿超  李新阳  张小军  饶长辉 《物理学报》2012,61(3):34204-034204
实验验证了一种利用目标回光在远场实现相干合成的技术——目标在回路技术.建立了目标在回路相干合成的物理模型,利用可同时调整光程和光束倾斜的自适应光纤光源准直器,同时补偿合成光束间的平移和倾斜相差,实现了基于目标在回路的三路光纤传输激光间相干合成,获得了接近理想情况的合成效果.文章为在实际大气环境中实现多路相干组束的长距离传输提供了参考.  相似文献   
893.
We systematically studied the influence of magnetic field on zero-line modes (ZLMs) in graphene and demonstrated the physical origin of their enhanced robustness by employing nonequilibrium Green’s functions and the Landauer–Büttiker formula. We found that a perpendicular magnetic field can separate the wavefunctions of the counter-propagating kink states into opposite directions. Specifically, the separation vanishes at the charge neutrality point and increases as the Fermi level deviates from the charge neutrality point and can reach a magnitude comparable to the wavefunction spread at a moderate field strength. Such spatial separation of oppositely propagating ZLMs effectively suppresses backscattering and is more significant under zigzag boundary condition than under armchair boundary condition. Moreover, the presence of magnetic field enlarges the bulk gap and suppresses the bound states, thereby further reducing the scattering. These mechanisms effectively increase the mean free paths of the ZLMs to approximately 1 μm in the presence of a disorder.  相似文献   
894.
人类免疫缺陷病毒(HIV)是一种严重威胁生命的病毒,感染艾滋病毒患者一般经历四个阶段:i)艾滋病毒阴性的窗口期(W);ii)阳性的无症状潜伏期(E);iii)有症状期(Ⅰ);以及iv)移除阶段(A).为深入研究艾滋病传播过程,建立SWEIA艾滋病毒传染模型,定义基本再生数,分析无病与地方病平衡点的存在性和局部稳定性,根据2004至2015年中国艾滋病患者数据,采用遗传算法对SWEIA模型中参数进行估计.通过对基本再生数敏感性分析以及模型数值随参数不同而产生的变化,揭示艾滋病窗口期的接触率是影响艾滋病流行的主要原因之一.  相似文献   
895.
Two‐dimensional (2D) PtSe2 shows the most prominent layer‐dependent electrical properties among various 2D materials and high catalytic activity for hydrogen evolution reaction (HER), and therefore, it is an ideal material for exploring the structure–activity correlations in 2D systems. Here, starting with the synthesis of single‐crystalline 2D PtSe2 with a controlled number of layers and probing the HER catalytic activity of individual flakes in micro electrochemical cells, we investigated the layer‐dependent HER catalytic activity of 2D PtSe2 from both theoretical and experimental perspectives. We clearly demonstrated how the number of layers affects the number of active sites, the electronic structures, and electrical properties of 2D PtSe2 flakes and thus alters their catalytic performance for HER. Our results also highlight the importance of efficient electron transfer in achieving optimum activity for ultrathin electrocatalysts. Our studies greatly enrich our understanding of the structure–activity correlations for 2D catalysts and provide new insight for the design and synthesis of ultrathin catalysts with high activity.  相似文献   
896.
An unprecedented stereoselective synthesis of trisubstituted vinylboronates is reported to proceed by direct borylation of lithium ketone enolates under transition‐metal‐free conditions. The stereospecific C?O borylation of lithium enolates was triggered by a carbonyl‐induced 1,3‐metalate rearrangement via a C‐bound boron enolate. DFT calculations and control experiments revealed that the stereoselectivity is controlled by sterics. A variety of stereospecific trisubstituted vinylboronates, together with several tetrasubstituted vinylboronates, were conveniently synthesized with the newly developed methodology. Based on the transformation of stereospecific vinylboronate, a single isomer of Dienestrol was efficiently obtained.  相似文献   
897.
Two new eudesmane derivatives, 1α,6β,9β-trihydroxy-eudesm-3-ene-1-O-β-d -glucopyranoside ( 1 ) and 1α,6β,9β-trihydroxy-eudesm-3-ene-1-(6-cinnamoyl)-O-β-d -glucopyranoside ( 2 ) were discovered from Merremia yunnanensis. The structures were elucidated by analysis of their spectroscopic data including HR-ESI-MS, 1D, and 2D NMR. It should be noted that this is the first report about structure elucidation and NMR assignment of compounds from Myunnanensis.  相似文献   
898.
应用局部耦合模理论,对非对称臂宽带光纤耦合器熔锥的形成及耦合发生过程作了较为精确的分析,计算得出了这类耦合器的熔锥特性曲线及光谱特性曲线,本文理论的进一步简化可得到一个特例,它与其它文献所用理论一致  相似文献   
899.
Sincethedevelopmentofmethodslformassproductionoffullerenes,therehasbeengTeatinterestinit-Manyresearchgroups'havepaidgreatattentiontothefiJllerene-dopedpolymersinceWang'discoveredinl992thatfullerene-dopedPVKhasanapparentlyincreasingphotoconductivity.WehavereportedthechendcalattachmentofCcocagestothecarbazolegroupsonpoly(epoxypropylcarbazole)(PEPC)chainsbyFriedel-Craftstypereaction'.Here,theopticalpropertiesoffullerenatedPEPCwereinvestigated,whichwillshowitsgoodphotoconductivepropeFtiesbe…  相似文献   
900.
Polymers are widely recognized to entail random conformations in good solvent governed by the need for achieving the highest entropy to reach thermodynamic equilibrium. In this context, it remains grand challenging to directly arrange them into ordered conformation as building blocks for further self-assembling into hierarchal structures. Herein, we report a simple yet viable strategy to progressively assemble rationally designed azobenzene-containing alternating copolymer(i.e., poly(binaphthyls...  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