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   85597篇
  免费   1744篇
  国内免费   1304篇
化学   29816篇
晶体学   852篇
力学   7085篇
综合类   78篇
数学   32677篇
物理学   18137篇
  2023年   185篇
  2022年   198篇
  2021年   268篇
  2020年   418篇
  2019年   454篇
  2018年   10702篇
  2017年   10509篇
  2016年   6434篇
  2015年   1255篇
  2014年   807篇
  2013年   956篇
  2012年   4572篇
  2011年   11236篇
  2010年   6177篇
  2009年   6486篇
  2008年   7105篇
  2007年   9225篇
  2006年   624篇
  2005年   1612篇
  2004年   1744篇
  2003年   2169篇
  2002年   1246篇
  2001年   473篇
  2000年   418篇
  1999年   320篇
  1998年   295篇
  1997年   218篇
  1996年   263篇
  1995年   166篇
  1994年   128篇
  1993年   145篇
  1992年   100篇
  1991年   109篇
  1990年   87篇
  1989年   91篇
  1988年   79篇
  1987年   67篇
  1986年   65篇
  1985年   51篇
  1984年   47篇
  1983年   41篇
  1982年   44篇
  1981年   40篇
  1980年   48篇
  1979年   45篇
  1914年   45篇
  1913年   40篇
  1912年   40篇
  1909年   41篇
  1908年   40篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
利用微环谐振腔阵列进行光码分多址编解码过程中,微环谐振腔反射谱的自由频谱宽度(FSR)范围制约该系统用户容量的提升.本文提出了一种新型的基于游标效应的串联哑铃型微环谐振腔光编解码器.利用Matlab建立了半径分别为40μm-30μm-40μm的哑铃型微环谐振腔光编解码器模型.详细分析了光反射谱伪模抑制与耦合系数的关系,研究了耦合系数、码片速率对串联哑铃型微环谐振腔光编解码器性能的影响.结果表明,与半径分别为40μm-40μm-40μm的传统串联微环谐振腔编解码器相比,哑铃型微腔编解码器FSR值扩大了4倍.理想情况下,用户容量可呈指数增长.同时,互相关峰值比(P/W)与自相关峰值旁瓣比(P/C)分别提高了约33%和8%.  相似文献   
2.
Li  Han  Jintao  Feng  Wang  Zhen  Jia  Yan  Li  Peng  Yao  Cuixia  Qu  Zongjin 《Journal of fluorescence》2022,32(5):1825-1832

The development of fluorescent probes for nitroreductase (NTR) has received intense attention because of its biological significance and wide application. In this work, a novel fluorescent probe for the detection of NTR in aqueous solution was designed and synthesized on a 1,8-naphthalimide scaffold. In the presence of NTR and nicotinamide adenine dinucleotide (NADH) under physiological conditions, the probe was converted into a 4-hydroxy-1,8-naphthalimide derivative and exhibited a sharp fluorescence enhancement at 550 nm, with a high selectivity for NTR over various analytes. The detection limit for NTR was determined to be 9.8 ng/ml by this probe. Due to its low signal background, this probe showed?>?70-fold fluorescence enhancement. Theoretical calculations revealed that the reason for the fluorescence quenching of this probe is the photoinduced electron transfer (PET) from both the nitrobenzene and morpholine groups to the naphthalimide fluorophore.

  相似文献   
3.
窦双团  付利平  贾楠  王天放 《应用光学》2022,43(6):1165-1174
基于微通道板电子倍增电荷分割型阳极的成像探测器常用于行星大气、气辉等微弱信号探测。针对探测器读出电路增益不相等导致探测器成像产生畸变的问题,利用理论分析结合MATLAB仿真揭示了不同畸变图像的形成机制,在该基础上提出了一种探测器读出电路增益补偿方法减小探测器成像畸变。通过MATLAB仿真和实验测试结果表明该文提出的方法能够有效减小由于读出电路增益不相等导致的探测器成像畸变。  相似文献   
4.
Juglandis Mandshuricae Cortex is the bark of Juglans mandshurica Maxim., which has been used as a folk medicine plant in China and India. In this study, an ultra-high performance liquid chromatography–quadrupole/orbitrap high-resolution mass spectrometry method was developed to clarify and quantify the chemical profiling of Juglandis Mandshuricae Cortex rapidly. A total of 113 compounds were characterized. Among them, seven flavonoids were simultaneously quantified in 15 min, including myricetin, myricetrin, taxifolin, kaempferol, quercetin, quercitrin, and naringenin. The method was validated for accuracy, precision, and the limits of detection and quantification. All calibration curves showed a good linear relationship (r > 0.9990) within test ranges. The intra- and inter-day relative standard deviations were less than 2.16%. Accuracy validation showed that the recovery was between 95.6 and 101.3% with relative standard deviation values below 2.85%. The validated method was successfully applied to determine the contents of seven flavones in Juglandis Mandshuricae Cortex from seven sources and the contents of these places were calculated respectively. This method provides a theoretical basis for further developing the medicinal value of Juglandis Mandshuricae Cortex.  相似文献   
5.
含顺二酚的分子,如糖类、糖苷和糖蛋白等,在糖组学、代谢组学和蛋白质组学等不同领域都有着至关重要的作用。但是这类样品分子通常存在于非常低丰度的环境中且与许多干扰化合物共存,给临床和生物学上的分离检测带来了巨大的困难。因此,开发建立对顺二醇类化合物简单高效的分离方法具有重要意义。分子印迹技术,是基于模拟生物体内抗原与抗体相互作用原理而发展起来的一种新兴技术,近年来因其特定的分子识别能力以及材料的稳定性和重复性而引起了广泛关注,也在顺二醇类化合物的分离方面得到了许多应用。本综述总结了在顺二醇类化合物的分子印迹方面的最新进展,并对其未来发展的方向和前景进行了讨论。  相似文献   
6.
研究生助教培养是高校化学本科生实验教学的重要组成部分,对提高实验课程的教学水平具有重要的推动作用。为解决助教培训时间仓促、定位不清、内容不连续、工具性倾向明显,以及在助教聘用、考核、激励、教学质量评估等方面规范性和系统性不足等问题,仪器分析实验室以《GB/T 19028–2018质量管理人员参与和能力指南》为理论依据,探索提高研究生助教深度参与实验教学的方法。连续三年追踪助教培养改进效果反馈数据表明,实验室采取的一系列改进措施有效提升了研究生助教的教学能力,促进了教学质量的提高,实现助教管理的改革创新与“过程增值”。  相似文献   
7.
Feng  Zhanxia  Chang  Zongyu  Deng  Chao  Zhao  Lin  Chen  Jia  Zhang  Jiakun  Zheng  Zhongqiang 《Nonlinear dynamics》2022,108(3):2007-2022
Nonlinear Dynamics - Wave glider is an unmanned surface vehicle that can directly convert wave energy into forward propulsion and fulfill long-term marine monitoring. A previous study suggested...  相似文献   
8.
Anderson localization is a famous wave phenomenon that describes the absence of diffusion of waves in a disordered medium. Here we generalize the landscape theory of Anderson localization to general elliptic operators and complex boundary conditions using a probabilistic approach, and further investigate some mathematical aspects of Anderson localization that are rarely discussed before. First, we observe that under the Neumann boundary condition, the low energy quantum states are localized on the boundary of the domain with high probability. We provide a detailed explanation of this phenomenon using the concept of extended subregions and obtain an analytical expression of this probability in the one-dimensional case. Second, we find that the quantum states may be localized in multiple different subregions with high probability in the one-dimensional case and we derive an explicit expression of this probability for various boundary conditions. Finally, we examine a bifurcation phenomenon of the localization subregion as the strength of disorder varies. The critical threshold of bifurcation is analytically computed based on a toy model and the dependence of the critical threshold on model parameters is analyzed.  相似文献   
9.
The electrochemical CO2 reduction (CO2RR) is a sustainable approach to mitigate the increased CO2 emissions and simultaneously produce value-added chemicals and fuels. Metal-nitrogen-carbon (M-N-C) based single-atom catalysts (SACs) have emerged as promising electrocatalysts for CO2RR with high activity, selectivity, and stability. To design efficient SACs for CO2RR, the key influence factors need to be understood. Here, we summarize recent achievements on M-N-C SACs for CO2RR and highlight the significance of the key constituting factors, metal sites, the coordination environment, and the substrates, for achieving high CO2RR performance. The perspective views and guidelines are provided for the future direction of developing M-N-C SACs as CO2RR catalysts.  相似文献   
10.
The spin polarization of carbon nanomaterials is crucial to design spintronic devices. In this paper, the first-principles is used to study the electronic properties of two defect asymmetric structures, Cap-(9, 0)-Def [6, 6] and Cap-(9, 0)-Def [5, 6]. We found that the ground state of Cap-(9, 0)-Def [6, 6] is sextet and the ground state of Cap-(9, 0)-Def [5, 6] is quartet, and the former has a lower energy. In addition, compared with Cap-(9, 0) CNTs, the C adatom on C30 causes spin polarization phenomenon and Cap-(9, 0)-Def [6, 6] has more spin electrons than Cap-(9, 0)-Def [5, 6] structure. Moreover, different adsorb defects reveal different electron accumulation. This finding shows that spin polarization of the asymmetric structure can be adjusted by introducing adatom defects.  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