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   2407篇
  免费   78篇
  国内免费   9篇
化学   1548篇
晶体学   17篇
力学   45篇
数学   452篇
物理学   432篇
  2023年   16篇
  2022年   27篇
  2021年   49篇
  2020年   61篇
  2019年   53篇
  2018年   26篇
  2017年   33篇
  2016年   77篇
  2015年   65篇
  2014年   74篇
  2013年   127篇
  2012年   160篇
  2011年   153篇
  2010年   86篇
  2009年   87篇
  2008年   136篇
  2007年   153篇
  2006年   135篇
  2005年   118篇
  2004年   102篇
  2003年   92篇
  2002年   88篇
  2001年   48篇
  2000年   35篇
  1999年   33篇
  1998年   26篇
  1997年   30篇
  1996年   24篇
  1995年   21篇
  1994年   29篇
  1993年   26篇
  1992年   27篇
  1991年   18篇
  1989年   8篇
  1988年   8篇
  1987年   9篇
  1986年   10篇
  1985年   17篇
  1984年   12篇
  1983年   16篇
  1982年   15篇
  1981年   13篇
  1980年   12篇
  1979年   8篇
  1978年   17篇
  1977年   16篇
  1976年   13篇
  1975年   17篇
  1974年   16篇
  1973年   7篇
排序方式: 共有2494条查询结果,搜索用时 15 毫秒
81.
Single crystals of K0.30MoO3 and Rb0.30MoO3 were synthesized by electrolytic reduction of MoO3/ A2MoO4 melts. The crystal structures were refined from X-ray diffraction data (3265 and 1280 independent reflections, respectively). The finalR andwR factors were 0.037 and 0.047 for the K bronze and 0.031 and 0.033 for the Rb bronze. The lattice parameters of the body-centered cells used in the present refinements were: K0.30Mo03,a = 16.2311(7),b = 7.5502(4),c = 9.8614(4)A?,β = 94.895(4)o; Rb0.30MoO3,a = 16.361(3),b = 7.555(1),c = 10.094(2)A?,β = 93.87(5)o. The 4d electron distribution over the 20 Mo sites [4Mo(1), 8Mo(2), 8Mo(3)] of the unit cell are 10, 45, and 45% for K0.30Mo03 and 14, 43, and 43% for Rb0.30MoO3, respectively. In both cases about 90% of the 4d electrons are situated on those sites which contribute to the electrical conductivity. The variations of the lattice parameters versus temperature are reported. The thermal linear-expansion coefficient is highly anisotropic. The structural dimensionality depends upon the sublattice under consideration. The K, Mo, and O sublattices are mono-, two-, and three-dimensional, respectively. The relationship between the structural dimensionality of K0.30MoO3 and the physical properties is discussed.  相似文献   
82.
Although the chemistry of elusive tricyanomethane (cyanoform) has been studied during a period of more than 150 years, this compound has very rarely been utilized in the synthesis or modification of heterocycles. Three-membered heterocycles, such as epoxides, thiirane, aziridines, or 2H-azirines, are now treated with tricyanomethane, which is generated in situ by heating azidomethylidene-malonodinitrile in tetrahydrofuran at 45 °C or by adding sulfuric acid to potassium tricyanomethanide. This leads to ring expansion with formation of 2-(dicyanomethylidene)oxazolidine derivatives or creation of the corresponding thiazolidine, imidazolidine, or imidazoline compounds and opens up a new access to these push–pull-substituted olefinic products. The regio- and stereochemistry of the ring-enlargement processes are discussed, and the proposed reaction mechanisms were confirmed by using 15N-labeled substrates. It turns out that different mechanisms are operating; however, tricyanomethanide is always acting as a nitrogen-centered nucleophile, which is quite unusual if compared to other reactions of this species.  相似文献   
83.
Experimental Fourier-transform infrared spectra and DFT calculated infrared spectra are compared to investigate the effect of adsorbed nitrogen on the OH-stretch band complex of water clusters. Using a collisional cooling experiment, pure as well as partially and completely N(2)-covered water clusters consisting of 20-200 water molecules have been generated in thermal equilibrium in the aerosol phase within the temperature range of 5-80 K. Computational IR-spectra simulations have been performed for discrete pure and N(2)-covered water clusters including 10, 15, 20, and 30 water molecules. The adsorbed N(2) molecules especially affect the three-coordinated water molecules at the cluster surface which could be observed as a blue shift of the companion O-H band at 2900 cm(-1) and a red shift of the dangling O-H band at 3700 cm(-1) by about 20 cm(-1) in both cases. The most striking effect of the N(2) adsorbate is an intensity increase of the dangling O-H band by a factor of 3-5. Furthermore, the onset temperature of nitrogen adsorption at the water cluster surface was experimentally found to be roughly 30 K for cluster sizes of about 100 water molecules. Experimental and computational results are in good agreement. The presented results are based on and support the work of V. Buch, J. P. Devlin, and co-workers (e.g., J. Phys. Chem. B, 1997; J. Phys. Chem. A, 2003; Int. Rev. Phys. Chem., 2004).  相似文献   
84.
85.
86.
Electrocatalytic hydrogen gas production is considered a potential pathway towards carbon-neutral energy sources. However, the development of this technology is hindered by the lack of efficient, cost-effective, and environmentally benign catalysts. In this study, a main-group-element-based electrocatalyst, SbSalen , is reported to catalyze the hydrogen evolution reaction (HER) in an aqueous medium. The heterogenized molecular system achieved a Faradaic efficiency of 100 % at −1.4 V vs. NHE with a maximum current density of −30.7 mA/cm2. X-ray photoelectron spectroscopy of the catalyst-bound working electrode before and after electrolysis confirmed the molecular stability during catalysis. The turnover frequency was calculated as 43.4 s−1 using redox-peak integration. The kinetic and mechanistic aspects of the electrocatalytic reaction were further examined by computational methods. This study provides mechanistic insights into main-group-element electrocatalysts for heterogeneous small-molecule conversion.  相似文献   
87.
Journal of Solution Chemistry - Liquid–liquid equilibrium (LLE) data and phase diagrams for new aqueous two-phase systems (ATPSs) containing 1-butyl-3-methylimidazolium chloride...  相似文献   
88.
Journal of Thermal Analysis and Calorimetry - Elephant grass (Pennisetum purpureum Schum.) is one such resource with a great amount of carbohydrate and lignin polymers that together represent high...  相似文献   
89.
Reversed phase and size-exclusion chromatography methods are commonly used for protein separations, although they are based on distinctly different principles. Reversed phase methods yield hydrophobicity-based (loosely-termed) separation of proteins on porous supports, but tend to be limited to proteins with modest molecular weights based on mass transfer limitations. Alternatively, size-exclusion provides complementary benefits in the separation of higher mass proteins based on entropic, not enthalpic, processes, but tend to yield limited peak capacities. In this study, microbore columns packed with a novel trilobal polypropylene capillary-channeled polymer fiber were used in a reversed phase modality for the separation of polypeptides and proteins of molecular weights ranging from 1.4 to 660 kDa. Chromatographic parameters including gradient times, flow rates, and trifluoroacetic acid concentrations in the mobile phase were optimized to maximize resolution and throughput. Following optimization, the performance of the trilobal fiber column was compared to two commercial-sourced columns, a superficially porous C4-derivatized silica and size exclusion, both of which are sold specifically for protein separations and operated according to the manufacturer-specified conditions. In comparison to the commercial columns, the fiber-based column yielded better separation performance across the entirety of the suite, at much lower cost and shorter separation times.  相似文献   
90.
A sensitive method based on differential pulse polarography is described for the determination of elemental sulfur in jet fuels. Its sensitivity makes it suitable for following small changes in the sulfur content of jet fuel during storage. The supporting electrolyte is 0.19 M ammonium acetate/0.088 M acetic acid in 1:1 toluene/methanol. In this medium, the peak potential is ?0.56 V vs. Ag/AgCl. Calibration is linear from 2 to 30 mg l?1. The limit of detection is 0.1 mg l?1. Accuracy is better than 5%. Interferences from oxygen, hydrogen sulfide, thiols, organic sulfides and disulfides, organic peroxides and fuel additives are shown to be of very minor significance.  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