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   4402篇
  免费   597篇
  国内免费   434篇
化学   3248篇
晶体学   49篇
力学   249篇
综合类   37篇
数学   486篇
物理学   1364篇
  2023年   68篇
  2022年   70篇
  2021年   128篇
  2020年   148篇
  2019年   169篇
  2018年   148篇
  2017年   125篇
  2016年   213篇
  2015年   180篇
  2014年   224篇
  2013年   322篇
  2012年   321篇
  2011年   363篇
  2010年   248篇
  2009年   243篇
  2008年   274篇
  2007年   248篇
  2006年   252篇
  2005年   178篇
  2004年   170篇
  2003年   144篇
  2002年   163篇
  2001年   124篇
  2000年   95篇
  1999年   99篇
  1998年   86篇
  1997年   72篇
  1996年   48篇
  1995年   70篇
  1994年   61篇
  1993年   49篇
  1992年   51篇
  1991年   31篇
  1990年   36篇
  1989年   15篇
  1988年   17篇
  1987年   11篇
  1986年   25篇
  1985年   24篇
  1984年   17篇
  1983年   14篇
  1982年   12篇
  1981年   12篇
  1980年   6篇
  1979年   10篇
  1978年   4篇
  1977年   4篇
  1974年   7篇
  1973年   7篇
  1969年   4篇
排序方式: 共有5433条查询结果,搜索用时 281 毫秒
21.
The polarization of a D-shaped fiber is modulated after immersing it in magnetic fluid(MF)and applying a magnetic field.Theoretical analysis predicts that magneto-optical dichroism of MF plays a key role in light polarization modulation.During light polarization modulation,the evanescent wave polarized parallel to the magnetic field has greater loss than its orthogonal component.Light polarization of a D-shaped fiber with a wide polished surface can be modulated easily.High concentration MF and a large magnetic field all have great ability to modulate light polarization.  相似文献   
22.
正Hydrogen has been considering as a promising renewable energy source and a viable alternative for fossil-fuels in the future [1,2]. Hydrogen oxidation reaction (HOR) and its reverse reaction are two significant reactions for applying hydrogen as electrochemical energy. On the other hand,platinum is believed to be the best electrocatalyst for HOR,although several drawbacks including high cost, scarcity,low CO tolerance, and depressed catalytic activities in alkaline environments, hinder the development of hydrogen application [1]. Hence, it is highly desired to develop Pt-free HOR catalysts with high activity, high stability, low cost, and good resistance to CO poisoning [3].  相似文献   
23.
Baicalin is extracted from a traditional Chinese herb, Scutellaria baicalensis. In this study, the anticancer activity and underlying mechanisms of baicalin towards human osteosarcoma cell (HOS) were investigated. Baicalin could inhibit HOS cell proliferation in a concentration-dependent manner. Mitochondrial membrane potential decreased obviously after treated with different concentration of baicalin by flow cytometry assay and revealed that baicalin triggered a significant generation of reactive oxygen species (ROS). Western blotting assay further revealed that baicalin-induced cell apoptosis by suppressing Bcl-2 level, then activating caspase-9 and caspase-3. In vivo experiment, baicalin significantly suppressed tumour growth in female BALB/C nude mice bearing HOS tumours. In addition, baicalin did show toxicity to treated animal by comparing the body weight increase and mortality. In general, the present results demonstrated that baicalin-induced apoptosis in human osteosarcoma cell via a ROS-mediated mitochondrial pathway. The paper indicated that baicalin is a promising candidate for the treatment of HOS.  相似文献   
24.
High-throughput metabolic analysis is of significance in diagnostics, while tedious sample pretreatment has largely hindered its clinic application. Herein, we designed FeOOH@ZIF-8 composites with enhanced ionization efficiency and size-exclusion effect for laser desorption/ionization mass spectrometry (LDI-MS)-based metabolic diagnosis of gynecological cancers. The FeOOH@ZIF-8-assisted LDI-MS achieved rapid, sensitive, and selective metabolic fingerprints of the native serum without any enrichment or purification. Further analysis of extracted serum metabolic fingerprints successfully discriminated patients with gynecological cancers (GCs) from healthy controls and also differentiated three major subtypes of GCs. Given the low cost, high-throughput, and easy operation, our approach brings a new dimension to disease analysis and classification.  相似文献   
25.
The self-assembly of inorganic nanoparticles into well-ordered structures in the absence of solvents is generally hindered by van der Waals forces, leading to random aggregates between them. To address the problem, we functionalized rigid rare-earth (RE) nanoparticles with a layer of flexible polymers by electrostatic complexation. Consequently, an ordered and solvent-free liquid crystal (LC) state of RE nanoparticles was realized. The RE nanomaterials including nanospheres, nanorods, nanodiscs, microprisms, and nanowires all show a typical nematic LC phase with one-dimensional orientational order, while their microstructures strongly depend on the particles’ shape and size. Interestingly, the solvent-free thermotropic LCs possess an extremely wide temperature range from −40 °C to 200 °C. The intrinsic ordering and fluidity endow anisotropic luminescence properties in the system of shearing-aligned RE LCs, offering potential applications in anisotropic optical micro-devices.  相似文献   
26.
This work demonstrates photoredox vicinal dichlorination of alkenes, based on the homolysis of CuCl2 in response to irradiation with visible light. This catalysis proceeds via a ligand to metal charge transfer process and provides an exciting opportunity for the synthesis of 1,2-dichloride compounds using an inexpensive, low-molecular-weight chlorine source. This new process exhibits a wide substrate scope, excellent functional group tolerance, extraordinarily mild conditions and does not require external ligands. Mechanistic studies show that the ready formation of chlorine atom radicals is responsible for the facile formation of C−Cl bonds in this synthetic process.  相似文献   
27.
Developing efficient and recyclable heterogeneous catalysts for organic reactions in water is important for the sustainable development of chemical industry. In this work, Pd nanoparticles supported on DABCO-functionalized porous organic polymer was successfully prepared through an easy copolymerization and successive immobilization method. Characterization results indicated that the prepared catalyst featured big surface area, hierarchical porous structure, and excellent surface amphiphilicity. We demonstrated the use of this amphiphilic catalyst in two case reactions, i.e. the aqueous hydrodechlorination and Suzuki-Miyaura coupling reactions. Under mild reaction conditions, the catalyst showed high catalytic activities for the two reactions. In addition, the catalyst could be easily recovered and reused for several times. Also, no obvious Pd leaching and aggregation of Pd nanoparticles occurred up during the consecutive reactions.  相似文献   
28.
Two new macrocyclic dibenzotetraaza[14]annulene (DBTAA) compounds with indolenine ( 5 ) and pyridoindolenine ( 6 ) moieties were synthesized and characterized by spectroscopy. Both DBTAAs exhibit strong UV-Vis absorption properties in the Soret band region. The theoretical second-order nonlinear optical property, electric dipole moment (μ), dispersion-free dipole polarizability (α) and first hyper-polarizability values were calculated by density functional theory and time dependent density functional theory. The ab-initio quantum mechanical calculation by time-dependent Hartree-Fock method was utilized to investigate the dynamic dipole polarizabilities, dynamic second-order, static, and dynamic third-order (γ) hyper-polarizabilities of the DBTAAs. The configuration interaction technique of all doubly occupied molecular orbitals possesses theoretically defined single-photon absorption (OPA) specifications for the examined structures. The computed maximum OPA wavelengths on both macrocyclic compounds coincide with the preceding measurement outcomes.  相似文献   
29.
The successful commercialization of promising silicon-based anode materials has been hampered by their poor cycling stability caused by the huge volume change. Integration of the carbon matrix with silicon-based (C/Si-based) anode materials has been demonstrated to be a powerful solution to achieve satisfactory electrochemical performance. This minireview aims to outline recent developments on C/Si-based composites, with the emphasis on the importance of carbon distribution at multiple scales. In addition, the forms of the carbon framework (carbon sources and doping of heteroatoms) have been summarized. Particularly, a novel C/Si-based hybrid with carbon distributed at the atomic scale has been highlighted.  相似文献   
30.
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