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   3499篇
  免费   507篇
  国内免费   337篇
化学   2532篇
晶体学   20篇
力学   179篇
综合类   34篇
数学   324篇
物理学   1254篇
  2024年   1篇
  2023年   82篇
  2022年   69篇
  2021年   69篇
  2020年   133篇
  2019年   136篇
  2018年   105篇
  2017年   103篇
  2016年   145篇
  2015年   145篇
  2014年   193篇
  2013年   257篇
  2012年   336篇
  2011年   359篇
  2010年   241篇
  2009年   205篇
  2008年   257篇
  2007年   214篇
  2006年   187篇
  2005年   147篇
  2004年   117篇
  2003年   89篇
  2002年   82篇
  2001年   78篇
  2000年   56篇
  1999年   63篇
  1998年   60篇
  1997年   58篇
  1996年   65篇
  1995年   70篇
  1994年   38篇
  1993年   25篇
  1992年   34篇
  1991年   30篇
  1990年   17篇
  1989年   16篇
  1988年   15篇
  1987年   3篇
  1986年   4篇
  1985年   10篇
  1984年   4篇
  1983年   3篇
  1982年   7篇
  1981年   6篇
  1980年   1篇
  1979年   4篇
  1974年   1篇
  1940年   1篇
  1902年   1篇
  1898年   1篇
排序方式: 共有4343条查询结果,搜索用时 18 毫秒
1.
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   
2.
3.
Qu  Yang  Metrikine  Andrei V. 《Nonlinear dynamics》2021,103(4):3059-3082
Nonlinear Dynamics - This paper is first of the two papers dealing with the nonlinear modelling and investigation of coupled cross-flow and in-line vortex-induced vibrations (VIVs) of flexible...  相似文献   
4.
采用简单的水热法合成了六角柱形NaErF_(4)和NaErF_(4)@NaYF_(4)核壳上转换发光材料,利用扫描电镜(SEM)、X射线粉末衍射(XRD)和荧光光谱(PL)等表征对材料的形貌、结构和上转换发光性能进行了研究。结果表明,纯NaErF_(4)样品为六角柱形,边长和厚度均为1μm左右,样品表面光滑。随着NaYF_(4)壳层的包覆,六角相NaErF_(4)周围出现了大量的立方相NaYF_(4)纳米颗粒,得到了NaErF_(4)@NaYF_(4)核壳结构。荧光光谱表明,通过在六角柱形NaErF_(4)表面包覆NaYF_(4)壳层,可以有效增强上转换发光强度,其中,位于527,543,663 nm处的3个发射峰分别对应于Er^(3+)的^(2)H_(11/2)→^(4)I_(15/2)、^(4)S_(3/2)→^(4)I_(15/2)和^(4)F_(9/2)→^(4)I1_(5/2)能级跃迁。随着壳层中Y∶F比例的增加,立方相NaYF_(4)的晶体衍射峰逐渐增强;同时,对应的NaErF_(4)@NaYF_(4)样品发射光谱中红绿比(R/G)逐渐增大,发光颜色也从绿色、经黄绿色逐渐向黄色过渡,实现了多色发光。  相似文献   
5.
极端工况双矩形腔静压推力轴承动态特性   总被引:1,自引:0,他引:1  
静压推力轴承动态特性受润滑油黏度、油膜厚度和油腔面积等因素影响, 极端工况运行过程中经常承受阶跃载荷或正弦载荷作用, 突加载荷将导致静压推力轴承动态特性改变, 表现为轴承的抗冲击能力和恢复平衡所需时间的变化. 为获得高速重载微间隙极端工况条件下双矩形腔静压推力轴承动态特性, 分别在不同油膜厚度、不同润滑油黏度以及不同油腔尺寸条件下对双矩形腔静压推力轴承的动态性能进行理论分析, 探讨了阶跃载荷作用下润滑油黏度、油膜厚度和油腔面积对轴承动态性能的影响, 揭示了油膜动态变化规律, 探究了正弦载荷作用下双矩形腔静压推力轴承的稳定性. 结果表明: 润滑油黏度、油膜厚度和油腔尺寸变化对其动态性能有很大的影响. 润滑油黏度越大、油膜厚度越小、油腔面积越大突加载荷作用下润滑油膜抵抗冲击的能力越强, 旋转工作台受到突加外力作用下恢复至平衡状态所用时间越短. 双矩形腔静压推力轴承油膜具有较大的阻尼系数, 轴承具有极强的抵抗正弦加载作用的能力   相似文献   
6.
Wang  Cong  Wu  Yancong  Qu  Yao  Zhao  Kun  Xu  Jianhua  Xia  Xinzhao  Wu  Huilu 《Transition Metal Chemistry》2020,45(8):523-529

Three new metal complexes, namely: [Mn(AIDB)Cl2]·DMF (1), [Zn(AIDB)Br2]·CH3OH (2) and [Co(AIDB)Cl2]·CH3OH (3) having a ligand bis(benzimidazol-2-ylmethyl)allylamine (AIDB), have been synthesized in high yields and characterized by elemental analyses, molar conductivities, IR, UV–Vis spectra and single-crystal X-ray diffraction. The structural analysis revealed that all the three complexes 13 have five-coordinated trigonal bipyramid geometry where the degree of distorting is 1>3>2. In vitro antioxidant activity assay demonstrates that the complexes 1 and 3 display high scavenging activity against hydroxyl (OH·) and superoxide (O2−·) radicals.

  相似文献   
7.
Sponges, Neofibularia nolitangere, can regenerate spontaneously after being broken down into small pieces, and the regenerated structure maintains the original appearance and function. Synthetic materials with such capabilities are highly desired but hardly achieved. Presented here is a sponge‐inspired self‐regenerative powder from a double‐network (DN) tough hydrogel. Hydrogels are regenerated from their powder form, by addition of water, with preservation of the original appearance and mechanical properties. The powder‐hydrogel‐powder cycle can be repeated multiple times with little loss in mechanical properties, analogous to the regeneration of sponges. These DN hydrogels can be conveniently stored and easily shaped upon regeneration. This work may have implications in the development of regenerative materials for coatings and adhesives.  相似文献   
8.
Littordial F (1), a novel meroterpenoid with an unique 6/8/9/4-tetracyclic nucleus, was isolated from the leaves of Psidium littorale. The structure of 1 was established by spectroscopic data, and its absolute configuration was confirmed by computational 13C NMR and optical rotation data. Compound 1 exhibited potential cytotoxic activities on A549, B16, and HepG-2 cell lines. A plausible biosynthetic pathway for littordial F was proposed.  相似文献   
9.
The ester bond as a universal linker has recently been applied in gene delivery systems owing to its efficient gene release by electrostatic repulsion after its cleavage. However, the ester bond is nonlabile and is difficult to cleave in cells. This work reports a method in which a secondary amine was introduced to the β-position of the ester bond to generate a hydrogen-bond cyclization (HBC) structure that can make the ester bond hydrolysis ultrafast. A series of molecules comprising ultrasensitive esters that can be activated by H2O2 were synthesized, and it was found that those able to form an HBC structure showed complete ester hydrolysis within 5 h in both water and phosphate-buffered saline solution, which was several times faster than other methods reported. Then, a series of amphiphilic poly(amidoamine) dendrimers were constructed, comprising the ultrasensitive ester groups for gene delivery; it was found that they could effectively release genes under quite a low concentration of H2O2 (<200 μm ) and transport them into the nucleus within 2 h in Hela cells with high safety. Their gene transfection efficiencies were higher than that of PEI25k. The results demonstrated that the hydrogen-bond-induced ultrasensitive esters could be powerfully applied to construct gene delivery systems.  相似文献   
10.
Due to the difficulty in achieving high efficiency and high color purity simultaneously, blue emission is the limiting factor for the performance and stability of OLEDs. Since 2003, we have been working on organic light‐emitting diodes (OLEDs), especially on blue light. After a series of molecular designs, novel strategies have been proposed from different aspects. At first, highly efficient deep blue emission could be achieved through molecular design with highly twisted structure to suppress fluorescence quenching and redshift. Deep blue emitters with high efficiency in solid state, a twisted structure with aggregation induced emission (AIE) characteristics was incorporated to inhibit molecular aggregation, and triplet‐triplet fusion (TTF) and hybridized localized charge transfer (HLCT) were adopted to increase the ratio of triplet exciton used. Secondly, a highly efficient blue OLED could be achieved through improving charge transport. New electron transport materials (ETMs) with wide band gap were developed to control charge transport balance in devices. Thirdly, a highly efficient deep blue emission could be achieved through a mesoscopic structure of out‐coupling layer. A mesoscopic photonic structured organic thin film was fabricated on the top of metal electrode by self‐aggregation in order to improve the light out‐coupling efficiency.  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