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   9211篇
  免费   536篇
  国内免费   676篇
化学   4680篇
晶体学   81篇
力学   2474篇
综合类   33篇
数学   939篇
物理学   2216篇
  2024年   42篇
  2023年   483篇
  2022年   194篇
  2021年   215篇
  2020年   270篇
  2019年   237篇
  2018年   214篇
  2017年   250篇
  2016年   332篇
  2015年   252篇
  2014年   395篇
  2013年   530篇
  2012年   476篇
  2011年   523篇
  2010年   449篇
  2009年   539篇
  2008年   465篇
  2007年   515篇
  2006年   432篇
  2005年   474篇
  2004年   363篇
  2003年   304篇
  2002年   284篇
  2001年   250篇
  2000年   193篇
  1999年   175篇
  1998年   160篇
  1997年   165篇
  1996年   125篇
  1995年   149篇
  1994年   112篇
  1993年   113篇
  1992年   73篇
  1991年   91篇
  1990年   72篇
  1989年   58篇
  1988年   95篇
  1987年   63篇
  1986年   63篇
  1985年   44篇
  1984年   37篇
  1983年   17篇
  1982年   49篇
  1981年   22篇
  1980年   10篇
  1979年   15篇
  1978年   12篇
  1976年   6篇
  1974年   3篇
  1971年   4篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
131.
The synthesis and electronic properties of new linear organic π-conjugated systems incorporating phosphole rings are described. Well defined α,α′-(phosphole-thiophene) oligomers possess low HOMO-LUMO gaps and their optical and electrochemical properties can be tuned via chemical modifications of the P-atoms. The physical properties of these compounds make them valuable materials for OLED’s. The coordination ability of phosphole-based dipoles has been exploited for the synthesis of efficient multipolar NLO-phores. Lastly, phospholes have been used for the synthesis of assemblies exhibiting through-bond interaction between two π-systems via P-P σ-skeletons.  相似文献   
132.
根据CR传输线模型和QR电路之间的关系,建立了拟合其初值的计算方法,借助Z-View软件,可求得各元件精确值.根据电容(Ci)和电阻(Ri)随特征频率(f*)的分布,推导了元件相对增量与恒相位角元件(Q)指数参数n的关系. 结果表明, 当n小于0.5时,Ci比Ri增加得更快,从新的角度说明了n的物理意义及其和界面脱层之间的关系.作为应用实例,拟合了不同特征的电化学阻抗谱,分析了有机涂层/金属腐蚀体系阻抗变化的具体过程,区分了点蚀和脱层因素对阻抗谱的影响,从高阻抗体系同时得到了与不同空隙率有关的涂层电容和电阻值,并根据涂层体系的不均匀特征探讨了模型结构的物理意义.  相似文献   
133.
Raman spectra of intramolecular vibration mode for each guest species in the methane + tetrafluoromethane (CF4) mixed-gas hydrate crystal have been measured at 291.1 K. Both of pure guest species generate the structure-I hydrate in the present pressure ranges. Isothermal phase-equilibrium curve exhibits two discontinuous points around the equilibrium methane compositions (water-free) in the gas phase of 0.3 and 0.8. At the above points, the Raman spectra of both guest molecules have been drastically changed. One of the most important findings is that the crystal of methane + tetrafluoromethane mixed-gas hydrate shows the structural phase-transition (from the structure-I to the structure-II and back to the structure-I) caused by composition changes.  相似文献   
134.
炭气凝胶及其有机气凝胶前驱体的研究进展   总被引:7,自引:0,他引:7  
介绍了炭气凝胶及其有机气凝胶前驱体的发展概况,着重总结了制备工艺条件(如催化剂及其浓度、反应物总浓度、反应物配比、溶剂、反应温度及时间、超临界干燥工艺条件和炭化工艺条件等)对有机气凝胶及其相应炭气凝胶中孔网络结构的影响,综述了中孔网络结构改性研究的进展.  相似文献   
135.
Optical methods are appropriate tools to detect organic micro-pollutants in fluids. A new technique is introduced which uses the decay of interaction processes like fluorescence and elastically scattered radiation by a fluid. Principally two different parameters are determined:

1. (i) the decay-time of the conventional interaction τC, which occurs at relatively short path-lengths of the incidence beam in the fluid, and

2. (ii) the decay-time τMP of the multi-path-saturation interaction originating at long path-lengths, e.g. in multi-path-reflection cuvettes, where the incidence beam is fully absorbed by the fluid.

A relation between the decay-time and the absorption coefficient of a fluid is theoretically derived. A simple preliminary experiment is performed considering distilled water polluted with non-fluorescent azobenzene and fluorescent quinine-sulphate. A nitrogen laser has been used to generate the fluorescence and scattering signals. The reciprocal value of the difference between the decay-time of the multi-path and conventional signals, 1/(τMP − τC), yields the total absorption coefficient directly. In comparison to the conventional absorption technique the decay-time method is characterized by a higher sensitivity.  相似文献   

136.
Imide-siloxane block copolymer/silica hybrid membranes with covalent bonds were prepared via sol–gel reaction. The structural informations of these hybrid membranes were obtained by using Fourier transform-infrared spectrometry (FT-IR), 29Si nuclear magnetic resonance (29Si NMR), XPS and thermogravimetric analysis (TGA). The gas separation properties of the hybrid membranes were also investigated in terms of organosiloxane (PDMS) or silica content at various temperatures. In the hybrids, the addition of PDMS phase increased the permeabilities of gases such as He, CO2, O2, and N2, indicating that the gas transport occurred mainly through rubbery organic matrix. Meanwhile, the PDMS phase contributed the decreased gas selectivities to nitrogen but the reduction in selectivities was very small in comparison with other siloxane containing polymeric membranes. This might be due to the restriction of chain mobility by the existence of inorganic component such as silica network in the hybrids. Additionally, the increase of silica content in these hybrid membranes considerably retarded the falling-off of gas selectivity at elevated temperature. The increase of silica content in hybrid membranes resulted in well-formed silica networks and hence these inorganic components restricted the plasticization of organic matrix by the thermal segmental motion of organic components, leading to preventing the large decrease of the gas selectivity.  相似文献   
137.
Cyclisation of the title compounds leads to mixtures of the expected 4H-cyclopental[b]pyrrol-4-ones (1), the corresponding 6-ones (3) by a single rearrangement, and the cyclopenta[c]pyrrol-4-ones (4) by a double rearrangement, the proportions depending upon the substituents.The 1H NMR spectra of 2-methyl-4H-cyctopenta[b]thiophen-6-one (3f) shows 6J long range CH3CH2 coupling, but this is absent in the corresponding pyrroles (3c,d). The 13C spectra of 1 and 3 cannot be interpreted on the basis of substituent chemical shifts in pyrroles and thiophens, and are clearly -CH = CH-X (X = NMe, NPh, S) bridged derivatives of cyclopent-2-enone.  相似文献   
138.
The harmonic force constants, vibrational frequencies and integrated intensity ratios of CH2, H2O, CH2O, C2H2, CO2, HCN, CH3, CH4, and C2H4 have been calculated using the MINDO—FORCES program and the Pulay method for the calculation of the molecular force constants. The results obtained are in general quite satisfactory when compared with available literature values. The results are, however, not as satisfactory in case of molecules containing heteroatoms, due to the neglect of some dipolar repulsion integrals for the heteroatoms by the MINDO/3 method. Calculated integrated intensities for CH3 and C2H4 agree well with experimental results. The calculated integrated intensities for other molecules are obtained for the first time and no comparison with published data is therefore possible.Part of the M.Sc. Thesis of K. H. A. 1978.  相似文献   
139.
有机分子与聚电解质静电吸附成膜特性研究   总被引:1,自引:0,他引:1  
选取多种有机分子及聚电解质,采用静电吸附自组装法制备了聚电解质,聚电解质、聚电解质,有机分子、有机分子,有机分子的复合薄膜,讨论了这些体系的静电吸附成膜特性及其成膜机理.  相似文献   
140.
色谱技术在中药指纹图谱研究中的应用   总被引:18,自引:1,他引:18  
刘朝燊  王冬梅  白洁  杨得坡 《色谱》2003,21(6):572-576
 介绍了气相色谱、高效液相色谱、超临界流体色谱和高速逆流色谱等几种色谱技术在中药指纹图谱研究中的应用现状,以及使用模糊聚类分析和人工神经网络分析指纹图谱对中药进行模式识别和质量鉴定的方法。  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