全文获取类型
收费全文 | 2077篇 |
免费 | 71篇 |
国内免费 | 13篇 |
专业分类
化学 | 1505篇 |
晶体学 | 6篇 |
力学 | 69篇 |
数学 | 168篇 |
物理学 | 413篇 |
出版年
2022年 | 11篇 |
2021年 | 20篇 |
2020年 | 23篇 |
2019年 | 30篇 |
2018年 | 22篇 |
2017年 | 22篇 |
2016年 | 56篇 |
2015年 | 47篇 |
2014年 | 40篇 |
2013年 | 151篇 |
2012年 | 105篇 |
2011年 | 152篇 |
2010年 | 78篇 |
2009年 | 56篇 |
2008年 | 116篇 |
2007年 | 105篇 |
2006年 | 106篇 |
2005年 | 108篇 |
2004年 | 70篇 |
2003年 | 74篇 |
2002年 | 76篇 |
2001年 | 53篇 |
2000年 | 39篇 |
1999年 | 31篇 |
1998年 | 32篇 |
1997年 | 21篇 |
1996年 | 26篇 |
1995年 | 23篇 |
1994年 | 27篇 |
1993年 | 30篇 |
1992年 | 19篇 |
1991年 | 27篇 |
1990年 | 25篇 |
1989年 | 20篇 |
1988年 | 20篇 |
1987年 | 20篇 |
1986年 | 12篇 |
1985年 | 16篇 |
1984年 | 16篇 |
1983年 | 19篇 |
1982年 | 17篇 |
1981年 | 17篇 |
1980年 | 20篇 |
1979年 | 16篇 |
1978年 | 19篇 |
1977年 | 14篇 |
1976年 | 18篇 |
1975年 | 13篇 |
1974年 | 12篇 |
1973年 | 18篇 |
排序方式: 共有2161条查询结果,搜索用时 703 毫秒
991.
992.
In this paper, we investigate the convergence of a novel simulation scheme to the target diffusion process. This scheme, the Quasi-EA, is closely related to the Exact Algorithm (EA) for diffusion processes, as it is obtained by neglecting the rejection step in EA. We prove the existence of a myopic coupling between the Quasi-EA and the diffusion. Moreover, an upper bound for the coupling probability is given. Consequently we establish the convergence of the Quasi-EA to the diffusion with respect to the total variation distance. 相似文献
993.
Joseph P. Kennedy 《高分子科学杂志,A辑:纯化学与应用化学》2013,50(5):695-714
Extensive studies on the mechanism of carbocationic polymerization initiation and particularly termination yielded information useful for the synthesis of a variety of new mono- and difunctional (telechelic) polymers. Unraveling aspects of initiation has opened new avenues toward the synthesis of novel cationic graft copolymers. Understanding details of termination has led among other products to cyclopentadiene-ended polyisobutylene and cyclopentadienylated rubbers. The latters crosslink by Diels-Alder addition and produce thermally reversible networks. Combination of controlled initiation and termination yielded telechelic products e. g., α, ω-diene-polyisobutylene. The use of Cl2/BCl3 combinations led to the synthesis of α, ω-dichloropolyisobutylenes. 相似文献
994.
995.
Robert E. Mulvey Alan R. Kennedy J. Hilko Schulte 《Acta Crystallographica. Section C, Structural Chemistry》2001,57(11):1288-1289
The metallation reaction between dibutylmagnesium and 2,6-diisopropyl-N-(trimethylsilyl)aniline gives the unusual monomeric three-coordinate complex (diethyl ether-κO)bis[2,6-diisopropyl-N-(trimethylsilyl)anilido-κN]magnesium(II), [Mg(C15H26NSi)2(C4H10O)] or [Mg{(Me3Si)(2,6-iPr2C6H3)N}2(Et2O)]. This low-coordinate species has a distorted trigonal-planar coordination environment, with an additional short Mg—Cipso contact of 2.799 (2) Å. 相似文献
996.
S. Conti P. I. Porshnev A. Fridman L. A. Kennedy J. M. Grace K. D. Sieber D. R. Freeman K. S. Robinson 《Experimental Thermal and Fluid Science》2001,24(3-4):79-91
A capacitively coupled nitrogen discharge driven at a frequency of 40 kHz was analyzed using a particle-in-cell (PIC) code, electrical probe measurements and optical emission spectra (OES). The configuration studied is used to generate plasmas for surface modification of polymer webs and consists of a pair of coplanar electrodes spaced several centimeters from the web plane and housed in a grounded shield. Both the probe measurements and the simulations indicate the presence of a group of high-energy electrons in concentrations of order 0.1% of the bulk electron concentration. Furthermore, bulk electron temperatures from the simulations are less than 1 eV. The energetic electrons and the low temperature of the bulk electrons are both characteristics of discharges operating in the gamma regime, where secondary electron emission from ion bombardment of the cathode sustains the ionization in the discharge. Because ions can respond to the instantaneous potential at the low-driving frequency used, half of the current at the electrode location is ion current. (In contrast, displacement current from the electron motion dominates at significantly higher driving frequencies.) The energetic electrons can provide a valuable source of N+ ions through dissociative ionization. The formation of the N+ ion was not included in the simulation, but was detected by the OES measurements. The atomic nitrogen ions and neutrals, together with the high-energy electrons, may be responsible for the formation of nitrogen-containing species in the surface region of polymer films treated with nitrogen plasmas using the configuration studied in this work. 相似文献
997.
Jian Li Wang Stewart James Campbell Shane Joseph Kennedy Shi Xue Dou Guang Heng Wu 《Hyperfine Interactions》2012,208(1-3):43-48
The physical properties of DyNi2Mn doped with 57Fe have been investigated by X-ray diffraction, magnetisation (10–300?K) and 57Fe M?ssbauer spectroscopy measurements (5–300?K). DyNi2Mn(57Fe) crystallizes in the MgCu2-type cubic structure (Fd??3m space group). The ordering temperature is found to be TC?=?99(2) K, much higher than those of DyNi2 (~22?K) and DyMn2 (~35?K). Analyses of isothermal M–H curves and the related Arrott plots confirm that the magnetic phase transition at TC is second order. The magnetic entropy change around TC is 4.0?J/kg?K for a magnetic field change of 0?T to 5?T. The spectra above TC exhibit features consistent with quadrupolar effects while below TC the spectra exhibit magnetic hyperfine splitting. The Debye temperature for DyNi2Mn has been determined as θD?=?200(20) K from a fit to the variable temperature isomer shift IS(T). 相似文献
998.
Nia W. Griffiths Mark F. Wyatt Suzanna D. Kean Andrew E. Graham Bridget K. Stein A. Gareth Brenton 《Rapid communications in mass spectrometry : RCM》2010,24(11):1629-1635
A method for the accurate mass measurement of positive radical ions by matrix‐assisted laser desorption/ionisation time‐of‐flight mass spectrometry (MALDI‐TOFMS) is described. Initial use of a conjugated oligomeric calibration material was rejected in favour of a series of meso‐tetraalkyl/tetraalkylaryl‐functionalised porphyrins, from which the two calibrants required for a particular accurate mass measurement were chosen. While all measurements of monoisotopic species were within ±5 ppm, and the method was rigorously validated using chemometrics, mean values of five measurements were used for extra confidence in the generation of potential elemental formulae. Potential difficulties encountered when measuring compounds containing multi‐isotopic elements are discussed, where the monoisotopic peak is no longer the lowest mass peak, and a simple mass‐correction solution can be applied. The method requires no significant expertise to implement, but care and attention is required to obtain valid measurements. The method is operationally simple and will prove useful to the analytical chemistry community. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
999.
Michelle Dickinson Thomas B. Scott Richard A. Crane Olga Riba Robert J. Barnes Gareth M. Hughes 《Journal of nanoparticle research》2010,12(6):2081-2092
In order to increase the longevity of contaminant retention on the particle surface, a method is sought to improve the corrosion resistance of bimetallic iron nickel nanoparticles (INNP) used for the remediation of contaminated water, and thereby extend their industrial lifetime. A multi-disciplinary approach was used to investigate changes induced by vacuum annealing (<5 × 10?8 mbar) at 500 °C on the bulk and surface chemistry of INNP. The particle size was determined to increase significantly as a result of annealing and the thickness of the surface oxide increased by 50%. BET analysis recorded a decrease in INP surface area from 44.88 to 8.08 m2 g?1, consistent with observations from scanning electron microscopy (SEM) and transmission electron microscopy (TEM) which indicated the diffusion bonding of previously discrete particles at points of contact. X-ray diffraction (XRD) confirmed that recrystallisation of the metallic cores had occurred, converting a significant fraction of initially amorphous iron nickel alloy into crystalline FeNi alloy. X-ray photoelectron spectroscopy (XPS) indicated a reduction in the proportion of surface iron oxide and a change in its stoichiometry related to annealing-induced disproportionation. This was also evidenced by an increased proportion of Fe(0) and Ni(0) to Fe- and Ni-oxides, respectively. The data also indicated the concurrent development of boron oxide at the metal surfaces, which accounts for the overall increase measured in surface oxide thickness. The improved core crystallinity and the presence of passivating impurity phases at the INNP surfaces may act to improve the corrosion resistance and reactive lifespan of the vacuum annealed INNP for environmental applications. 相似文献
1000.
Jungmee Kang Gabor Erdodi Joseph P. Kennedy 《Journal of polymer science. Part A, Polymer chemistry》2011,49(11):2461-2467
Because of the presence of extensive H‐bonding in the hard segments, polyureas are processed by solution techniques (e.g., dry spinning) by the use of relatively costly and environmentally unfriendly solvents. Thus, the objective of this research was to render polyureas melt processible, (i.e., to reduce their flow temperature, Tflow) without compromising their excellent mechanical properties. We hypothesized and herein demonstrate that by using conventional chain extenders (CEs) in combination with small amounts of H‐bond acceptor chain extenders (HACEs), the Tflow of polyureas can be significantly reduced from ~230 to ~180 °C, and thus melt processible products with excellent mechanical properties can be obtained. We document the synthesis of conventional polytetramethylene oxide‐based and novel polyisobutylene (PIB)‐based polyureas with Tflows ~ 180 °C and excellent mechanicals by the addition of few percents of commercially available HACEs. Products were characterized by various techniques, including Instron (tensile strengths, elongations), durometer (Shore A Hardness), dynamic mechanical thermal analysis (Tflow), and thermal gravimetric analysis (TGA) (thermal weight loss). According to TGA, a polyurea with Tflow of ~180 °C did not degrade up to ~234 °C in air. A micromorphology for melt processible polyureas is proposed that emphasizes flexibilized hard segments in the presence of HACEs. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011 相似文献