全文获取类型
收费全文 | 1045篇 |
免费 | 40篇 |
国内免费 | 9篇 |
专业分类
化学 | 584篇 |
晶体学 | 1篇 |
力学 | 18篇 |
数学 | 133篇 |
物理学 | 240篇 |
无线电 | 118篇 |
出版年
2021年 | 12篇 |
2020年 | 12篇 |
2019年 | 15篇 |
2018年 | 12篇 |
2016年 | 21篇 |
2015年 | 16篇 |
2014年 | 26篇 |
2013年 | 45篇 |
2012年 | 51篇 |
2011年 | 47篇 |
2010年 | 31篇 |
2009年 | 23篇 |
2008年 | 64篇 |
2007年 | 52篇 |
2006年 | 36篇 |
2005年 | 48篇 |
2004年 | 48篇 |
2003年 | 31篇 |
2002年 | 52篇 |
2001年 | 25篇 |
2000年 | 25篇 |
1999年 | 19篇 |
1998年 | 21篇 |
1997年 | 18篇 |
1996年 | 14篇 |
1995年 | 13篇 |
1994年 | 14篇 |
1993年 | 12篇 |
1992年 | 12篇 |
1991年 | 11篇 |
1990年 | 7篇 |
1989年 | 13篇 |
1988年 | 10篇 |
1987年 | 12篇 |
1986年 | 14篇 |
1985年 | 20篇 |
1984年 | 8篇 |
1983年 | 12篇 |
1982年 | 16篇 |
1981年 | 20篇 |
1980年 | 12篇 |
1979年 | 12篇 |
1978年 | 16篇 |
1977年 | 12篇 |
1976年 | 16篇 |
1975年 | 15篇 |
1974年 | 9篇 |
1973年 | 10篇 |
1972年 | 5篇 |
1971年 | 5篇 |
排序方式: 共有1094条查询结果,搜索用时 359 毫秒
31.
Jelica Strauch James McDonald Bogdan E. Chapman Philip W. Kuchel Brian S. Hawkett G. Evan Roberts Matthew P. Tonge Robert G. Gilbert 《Journal of polymer science. Part A, Polymer chemistry》2003,41(16):2491-2501
The diffusion coefficients are reported of rubbery ternary systems consisting of the polymer, its monomer analogue (i.e., the saturated equivalent of the monomer), and trace quantities of oligomers (dimer, trimer, tetramer and hexamer) for 2‐hydroxyethyl methacrylate (HEMA). These have been obtained with pulsed‐field‐gradient NMR spectroscopy with a polymer weight fraction (fp) of 0 ≤ fp ≤ 0.4. The oligomers are macromonomers synthesized with a cobalt catalytic chain‐transfer agent. The diffusion coefficients are about an order of magnitude smaller than those for monomers such as methyl methacrylate; this effect is ascribed to hydrogen bonding in HEMA. The diffusion coefficient Di of an i‐meric oligomer has been fitted with moderate accuracy by an empirical universal scaling relation, Di(fp)/D1(fp) ≈ i, previously found to provide an adequate fit to corresponding data for styrene and for methyl and butyl methacrylates. The approximate empirical scaling relation seems to hold for a remarkably wide range of types of monomer/polymer systems. These results are of use in modeling rates and molecular weight distributions in free‐radical polymerization, particularly for termination (which is chain‐length‐dependent and is controlled by the diffusion coefficient of chains of the low degrees of polymerization studied here). © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2491–2501, 2003 相似文献
32.
J.R. McDonald 《Chemical physics》1976,13(4):339-347
The fluorescence emission spectrum and analysis of NSF vapor is presented. Single vibronic level excitation near the S1 origin gives rise to a 10 μs radiative decay. The fluorescence lifetime for excitation of levels with ? 4500 cm?1 excess vibrational energy becomes controlled by a unimolecular radiationless process which is likely photodissociation; the dependence of this radiationless rate on energy and vibrational mode is investigated. The perturbations resulting from coupling of zero-order S1 states with other vibronic levels which control the excited state dynamics of SO2 are apparently not operative for NSF. Attempts are made to rationalize the grossly different dynamic behavior of the S1 levels of these two otherwise very similar systems. 相似文献
33.
34.
35.
2-(1-Azulyl)ethyl tosylate methylene scrambling in the gas phase. Similarities to solution chemistry
R. Graham Cooks Richard N. McDonald James R. Curtis Herbert E. Petty 《Journal of mass spectrometry : JMS》1971,5(7):785-788
The methylene groups in 2-(1-azulyl)ethyl tosylate are largely scrambled prior to formation of fragment ions in the mass spectrometer. This process also occurs in the 3-nitro analog, where it is dependent on sample residence time in the source and independent of electron energy. It could not be observed with the isomeric 6-substituted tosylates, the 1-substituted acetates or alcohols, or with 2-(p-anisyl)ethyl tosylate. Scrambling occurs in the neutral molecule prior to ionization and a mechanism involving absorption onto the metal surface and formation of a complexed ethylenazulonium ion is suggested. 相似文献
36.
Verkerk U Fujita M Dzwiniel TL McDonald R Stryker JM 《Journal of the American Chemical Society》2002,124(34):9988-9989
A topologically unique, conformationally constrained tetradentate ligand system for polymetallic coordination chemistry has been developed: tetrakis(2-hydroxyphenyl)ethene (1a) and substituted derivatives. The design exploits the planarity of the tetraphenylethylene core to impart rigidity to the roughly square oxygen binding array, while maintaining a degree of conformational mobility associated with rotation about the aryl-ethylene carbon-carbon bonds. Tetrakis(2-hydroxyphenyl)ethene derivatives are designed to promote multiple metal bridging over chelating coordination modes. The ligand is synthesized from anisole or 4-tert-butylanisole in four steps via the 2,2'-dimethoxybenzophenone hydrazones 4a,b. The sterically hindered ortho-substituted tetraphenylethylene core is produced in high yield by acid-catalyzed decomposition of the corresponding diaryl diazomethane prepared in situ by oxidation of the hydrazone using nickel peroxide. Deprotection of the methyl ethers using boron tribromide gives tetrakis(2-hydroxyphenyl)ethene (1a), characterized by X-ray crystallography, and tetrakis(5-tert-butyl-2-hydroxyphenyl)ethene (1b). Sterically isolating substituents in the 3-position can be installed via Claisen rearrangement/hydrogenation, providing tetrakis(3-n-propyl-2-hydroxyphenyl)ethene (6) efficiently. To illustrate potential applications of this unprecedented ligand class, two coordination complexes are reported, including tetrakis(2-diethylaluminoxyphenyl)ethene (8), a structurally robust eight-membered-ring aluminum/oxygen crown complex characterized both in solution and in the solid state. 相似文献
37.
Antunes MA Ferrence GM Domingos A McDonald R Burns CJ Takats J Marques N 《Inorganic chemistry》2004,43(21):6640-6643
Reaction of [(Tp(Me)2)(2)UI] with KNR(2) (R = C(6)H(5), SiMe(3)) in tetrahydrofuran (THF) afforded the monomeric trivalent actinide amide complexes [(Tp(Me)2)(2)U[N(C(6)H(5))(2)]], 1, and [(Tp(Me)2)(2)U[N(SiMe(3))(2)]], 2. The complexes have been fully characterized by spectroscopic methods and their structures were confirmed by X-ray crystallographic studies. In the solid state 1 and 2 exhibit distorted pentagonal bipyramidal geometries. The U-NR(2) bond lengths in both complexes are the same but in complex 2 the greater steric demands of the N(SiMe(3))(2) ligand led to elongated U-N(pz) bonds, especially those opposite the amido ligand. 相似文献
38.
Internal pressurization testing at room temperature was performed to evaluate the effect of flame-spraying aluminum-12silicon on the leakage behavior of fibre-reinforced composite tubes fabricated by filament winding, and covered with garnet sand. A group of specimens were completely coated with flame-sprayed metal particles and another set remained uncoated. t-tests were conducted at a significance level of α = 0.05 to analyze the effect of the coating on the leakage behavior of the specimens. Ultraviolet imaging was used to illustrate a characteristic leakage pattern of the composite tubes. Coated specimens exhibited slightly higher mean leakage and burst pressures than the uncoated specimens. The t-test results provided strong statistical evidence that the flame spraying process employed did not damage the filament wound tubes covered with garnet sand. 相似文献
39.
Patrick McDonald 《Potential Analysis》2013,38(4):1365-1372
We establish comparison theorems involving exit time moments for Brownian motion and eigenvalues for the Dirichlet problem for domains in a complete Riemannian manifold. These theorems are closely related to isoperimetric properties of the ambient manifold. 相似文献
40.