全文获取类型
收费全文 | 2408篇 |
免费 | 73篇 |
国内免费 | 10篇 |
专业分类
化学 | 1542篇 |
晶体学 | 69篇 |
力学 | 56篇 |
数学 | 207篇 |
物理学 | 617篇 |
出版年
2023年 | 29篇 |
2022年 | 56篇 |
2021年 | 48篇 |
2020年 | 66篇 |
2019年 | 59篇 |
2018年 | 35篇 |
2017年 | 40篇 |
2016年 | 79篇 |
2015年 | 60篇 |
2014年 | 92篇 |
2013年 | 196篇 |
2012年 | 137篇 |
2011年 | 182篇 |
2010年 | 92篇 |
2009年 | 87篇 |
2008年 | 143篇 |
2007年 | 165篇 |
2006年 | 87篇 |
2005年 | 80篇 |
2004年 | 64篇 |
2003年 | 38篇 |
2002年 | 41篇 |
2001年 | 31篇 |
2000年 | 19篇 |
1999年 | 9篇 |
1998年 | 13篇 |
1997年 | 11篇 |
1996年 | 17篇 |
1995年 | 20篇 |
1994年 | 23篇 |
1993年 | 28篇 |
1992年 | 21篇 |
1991年 | 9篇 |
1990年 | 11篇 |
1989年 | 13篇 |
1988年 | 21篇 |
1987年 | 21篇 |
1986年 | 20篇 |
1985年 | 23篇 |
1984年 | 22篇 |
1983年 | 26篇 |
1982年 | 25篇 |
1981年 | 21篇 |
1980年 | 19篇 |
1979年 | 18篇 |
1978年 | 28篇 |
1977年 | 16篇 |
1976年 | 22篇 |
1974年 | 19篇 |
1973年 | 12篇 |
排序方式: 共有2491条查询结果,搜索用时 15 毫秒
41.
Vicinal proton couplings in cyclopropane have been calculated using Dirac-Van Vleck and variational methods in valence bond formalism. The couplings may be considered as taking place along a path comprising either three or four intervening bonds. Inclusion of all exchange integrals in the appropriate fragment models leads to a satisfactory ratio of Jcis/Jtrans . In this respect valence bond results appear to be superior to those obtained using a molecular orbital approach. The present valence bond results on couplings in cyclopropane are not affected by the choice of models for bonding in cyclopropane, namely the Coulson-Moffitt model or the Walsh model. 相似文献
42.
43.
Condensation of uracil derivatives (1,3,4) with acetone in the presence of triethylamine gave pyranopyrimidines (2,5,6). 相似文献
44.
B. H. Doreswamy Chandra K. Mantelingu M. A. Sridhar M. Mahendra 《Research on Chemical Intermediates》2014,40(8):2659-2665
The isolation of stable carbenes of the Arduengo and Wanzlick type has prompted us to look for stable nitrenium ions of the related structural type 1-Hydroxy-3-methyl benzotriazolium trifluromethane sulfonate. Nitrenium ions are azocations with structure and survive long enough under in vitro conditions to bind with biological nucleophiles, i.e. DNA bases, and play an important role in carcinogenic processes involving the metabolism of aromatic amines. The title compound (C15H16F3N6O5S) was isolated and crystallized in the monoclinic space group Cc with cell parameters a = 19.866(1) Å, b = 13.128(9) Å, c = 7.673(5) Å, and Z = 4. The structure has both intra- and intermolecular hydrogen bonds. 相似文献
45.
Optics has already showed its potency over its electronic complements in case of superfast computing and communication systems. Semiconductor optical amplifier, (SOA) with its several nonlinear properties, plays a very crucial role in the development of high-speed all-optical processor. Multiplexer and demultiplexer are the extremely important element of the processor which takes part in utilizing different actions like encoding, decoding, routing, and the different process of data conversion and generation, etc. In this paper, the authors have proposed a scheme of phase encoded all-optical phase multiplexer using four wave mixing (FWM) property of semiconductor optical amplifier. Thus, the improved tolerance against fiber-nonlinearity and higher receiver sensitivity of phase encoding method with the fast occurring processes like FWM in SOA offers higher speed in this proposed scheme of multiplexing. 相似文献
46.
Jian Zheng Philip A. Barrett Steven J. Pontonio Neil A. Stephenson Preeti Chandra Persefoni Kechagia 《Adsorption》2014,20(1):147-156
High-rate and high-density gas separation adsorbents used in vacuum pressure swing adsorption (VPSA) processes are described. Agglomerated zeolite Li–LSX compositions made using colloidal silica binding agents and having improved nitrogen pore diffusivity compared to like compositions prepared with traditional clay binders, are also described. Preparation methods for the colloidal silica-bound adsorbents are described together with their characterization by mercury (Hg) porosimetry, scanning electron microscopy (SEM) and low dead-volume breakthrough testing, from which the pore diffusivity is obtained. In this article, we show how the location and dispersion of the colloidal silica binding agent within the agglomerated zeolite particle yields pore-architectures that resemble “state-of-the-art” binderless adsorbents. In addition, we use VPSA process simulations to show that the best process performance is achieved by the combination of high-rate and high-density adsorbent properties. 相似文献
47.
This paper reports the extension of earlier work (Dawar and Chandra, 2012) [27] by including the influence of low values of electric field on diffusion limited aggregation (DLA) patterns in polymer electrolyte composites. Subsequently, specified cut-off value of voltage has been determined. Below the cut-off voltage, the growth becomes direction independent (i.e., random) and gives rise to ramified DLA patterns while above the cut-off, growth is governed by diffusion, convection and migration. These three terms (i.e., diffusion, convection and migration) lead to structural transition that varies from dense branched morphology (DBM) to chain-like growth to dendritic growth, i.e., from high field region (A) to constant field region (B) to low field region (C), respectively. The paper further explores the growth under different kinds of electrode geometries (circular and square electrode geometry). A qualitative explanation for fractal growth phenomena at applied voltage based on Nernst–Planck equation has been proposed. 相似文献
48.
Umesh C. Pant A. Dandia Hem Chandra Shweta Goyal Seema Pant 《Phosphorus, sulfur, and silicon and the related elements》2013,188(2):559-571
Abstract Two flavindogenides, 3-(2-chlorobenzylidene)-flavanone and 3-(2,4-dichloro- benzylidene)-flavanone reacted with six 5-substituted-2-aminobenzenethiols, the substituents being fluoro, chloro, bromo, methyl, methoxyl, and ethoxyl, to give respective 12 new compounds, 10-substituted-7-(2-chlorophenyl/2,4-dichlorophenyl)- 6H-6a,7-dihydro-6-phenyl[1]benzopyrano[3,4?c][1,5]benzothiazepines (5a–l) in 60–70% yields. The products were characterized on the basis of microanalytical data for elements and IR, 1 H, and 13 C NMR and mass spectral studies. All the synthesized compounds were evaluated for their antimicrobial activity against the bacteria,Escherichia coli and GFC,and the fungi,Aspergillus niger, Aspergillus flavus,and Curvularia lunata. 相似文献
49.
Moamen S. Refat Sulekh Chandra Monika Tyagi 《Phosphorus, sulfur, and silicon and the related elements》2013,188(1):22-33
Mn(II), Co(II), Ni(II), and Cu(II) complexes have been synthesized with benzil bis(thiosemicarbazone) (L) and characterized by elemental analyses, molar conductance measurements, magnetic susceptibility measurements, thermogravimetric studies, infrared (IR), electronic, and electron paramagnetic resonance (eEPR) spectral studies. The molar conductance measurements of the complexes in DMF correspond to the non-electrolytic nature of the complexes. Thus these complexes may be formulated as [M(L)X2] (where M = Mn(II), Co(II), Ni(II), Cu(II) and X = Cl? and NO3 ?). On the basis of IR, electronic, and EPR spectral studies, an octahedral geometry has been assigned for Mn(II), Co(II), and Ni(II) complexes, whereas a tetragonal geometry for the Cu(II) complexes is presumed. The free ligand and its metal complexes were tested against the phytopathogenic fungi (i.e., Rhizoctonia baticola, Alternaria alternata) in vitro. 相似文献
50.