全文获取类型
收费全文 | 34852篇 |
免费 | 1760篇 |
国内免费 | 688篇 |
专业分类
化学 | 23279篇 |
晶体学 | 345篇 |
力学 | 1345篇 |
综合类 | 47篇 |
数学 | 3272篇 |
物理学 | 9012篇 |
出版年
2023年 | 267篇 |
2022年 | 611篇 |
2021年 | 751篇 |
2020年 | 681篇 |
2019年 | 704篇 |
2018年 | 586篇 |
2017年 | 566篇 |
2016年 | 1078篇 |
2015年 | 892篇 |
2014年 | 1167篇 |
2013年 | 2022篇 |
2012年 | 2566篇 |
2011年 | 2785篇 |
2010年 | 1782篇 |
2009年 | 1595篇 |
2008年 | 2283篇 |
2007年 | 2124篇 |
2006年 | 1966篇 |
2005年 | 1789篇 |
2004年 | 1563篇 |
2003年 | 1225篇 |
2002年 | 1199篇 |
2001年 | 823篇 |
2000年 | 706篇 |
1999年 | 468篇 |
1998年 | 361篇 |
1997年 | 384篇 |
1996年 | 414篇 |
1995年 | 328篇 |
1994年 | 325篇 |
1993年 | 330篇 |
1992年 | 315篇 |
1991年 | 258篇 |
1990年 | 180篇 |
1989年 | 164篇 |
1988年 | 163篇 |
1987年 | 139篇 |
1986年 | 110篇 |
1985年 | 184篇 |
1984年 | 128篇 |
1983年 | 106篇 |
1982年 | 133篇 |
1981年 | 93篇 |
1980年 | 84篇 |
1978年 | 82篇 |
1977年 | 89篇 |
1976年 | 96篇 |
1975年 | 104篇 |
1974年 | 82篇 |
1973年 | 106篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
921.
Rotationally resolved infrared emission spectra of HCl(v=1-3) in the reaction of Cl+CH3SH, initiated with radiation from a laser at 308 nm, are detected with a step-scan Fourier-transform spectrometer. Observed rotational temperature of HCl(v=1-3) decreases with duration of reaction due to collisional quenching; a short extrapolation to time zero based on data in the range 0.25-4.25 micros yields a nascent rotational temperature of 1150+/-80 K. The rotational energy averaged for HCl(v=1-3) is 8.2+/-0.9 kJ mol(-1), yielding a fraction of available energy going into rotation of HCl, fr=0.10+/-0.01, nearly identical to that of the reaction Cl+H(2)S. Observed temporal profiles of the vibrational population of HCl(v=1-3) are fitted with a kinetic model of formation and quenching of HCl(v=1-3) to yield a branching ratio (68+/-5):(25+/-4):(7+/-1) for formation of HCl(v=1):(v=2):(v=3) from the title reaction and its thermal rate coefficient k(2a)=(2.9+/-0.7)x10(-10) cm(3) molecule(-1) s(-1). Considering possible estimates of the vibrational population of HCl(v=0) based on various surprisal analyses, we report an average vibrational energy 36+/-6 kJ mol(-1) for HCl. The fraction of available energy going into vibration of HCl is f(v)=0.45+/-0.08, significantly greater than a value fv=0.33+/-0.06 determined previously for Cl+H2S. Reaction dynamics of Cl+H(2)S and Cl+CH3SH are compared; the adduct CH3S(Cl)H is likely more transitory than the adduct H(2)SCl. 相似文献
922.
923.
Physical and electrochemical characterizations of microwave-assisted polyol preparation of carbon-supported PtRu nanoparticles 总被引:9,自引:0,他引:9
Liu Z Lee JY Chen W Han M Gan LM 《Langmuir : the ACS journal of surfaces and colloids》2004,20(1):181-187
PtRu nanoparticles supported on Vulcan XC-72 carbon and carbon nanotubes were prepared by a microwave-assisted polyol process. The catalysts were characterized by transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy (XPS). The PtRu nanoparticles, which were uniformly dispersed on carbon, were 2-6 nm in diameter. All PtRu/C catalysts prepared as such displayed the characteristic diffraction peaks of a Pt face-centered cubic structure, excepting that the 2theta values were shifted to slightly higher values. XPS analysis revealed that the catalysts contained mostly Pt(0) and Ru(0), with traces of Pt(II), Pt(IV), and Ru(IV). The electro-oxidation of methanol was studied by cyclic voltammetry, linear sweep voltammetry, and chronoamperometry. It was found that both PtRu/C catalysts had high and more durable electrocatalytic activities for methanol oxidation than a comparative Pt/C catalyst. Preliminary data from a direct methanol fuel cell single stack test cell using the Vulcan-carbon-supported PtRu alloy as the anode catalyst showed high power density. 相似文献
924.
Zhang Y Salaita K Lim JH Lee KB Mirkin CA 《Langmuir : the ACS journal of surfaces and colloids》2004,20(3):962-968
This paper describes a simple and convenient strategy for reducing the dimensions of organic micro-and nanostructures on metal surfaces. By varying electrochemical desorption conditions, features patterned by dip-pen nanolithography or micro contact printing and made of linear alkanethiols or selenols can be gradually desorbed in a controlled fashion. The process is referred to as electrochemical whittling because the adsorbate desorption is initiated at the exterior of the feature and moves inward as a function of time. The whittling process and adsorbate desorption were studied as a function of substrate morphology, adsorbate head and tail groups, and electrolyte solvent and salt. Importantly, one can independently address different nanostructures made of different adsorbates and effect their miniaturization based upon ajudicious selection of adsorbate, applied potential, and supporting electrolyte. Some of the physical and chemical origins of these observations have been elucidated. 相似文献
925.
Chiral Lewis acid-catalyzed asymmetric Baylis-Hillman reactions 总被引:1,自引:0,他引:1
An effective chiral Lewis acid-catalyzed asymmetric Baylis-Hillman reaction is described. Good to high enantioselectivities were obtained using 3 mol % chiral catalyst. Novel camphor-derived dimerized ligands were prepared from the condensation of (+)-ketopinic acid with the corresponding diamines and hydrazine under acidic conditions. When alpha-naphthyl acrylate was used as a Michael acceptor, the reaction is complete within 20 min with high stereoselectivity and in reasonable chemical yields. 相似文献
926.
In the present work, corrosion resistance of surface-coated galvanized steel was quantitatively determined by an analysis
of the alternating current (AC) impedance spectra measured on the salt-spray-tested specimen. To evaluate the corrosion resistance
of the surface-coated galvanized steel, AC impedance spectroscopy was performed on the salt-spray-tested specimen previously
exposed to salt-sprayed corrosive environment. From the analysis of the impedance spectra, the area fraction transient of
white rust θ
2(t) was theoretically derived from the equivalent circuit equation by using two fitting parameters. The values of the two fitting
parameters were determined by fitting the empirical transient equations to the area fraction of the resin coating layer and
to the total resistance obtained from the impedance spectra measured, respectively. From the analyses of θ
2(t) for four kinds of surface-coated galvanized steels with various resin coating layers, it is indicated that as the values
of the two fitting parameters decrease in the order of CP, GI, OD and OM (commercial trade names) specimens, the corrosion
resistance increases in that order as well. Furthermore, from the quantitative comparison of the two fitting parameters with
the polarization resistance of the upper resin coating layer R
p determined from the potentiodynamic polarization curve, it is suggested that the two fitting parameters decrease in value
as well with increasing R
p. 相似文献
927.
The effect of anodic oxidation on high-strength polyacrylonitrile-based carbon fibers has been studied in terms of fiber surface energetics and fracture toughness of the composites. According to contact angle measurements based on the wicking rate of a test liquid, anodic oxidation leads to an increase in surface free energy, mainly due to the increase of its specific (or polar) component. For the carbon-fiber-reinforced epoxy resin matrix system, a direct linear relationship is shown between the specific component and the critical stress intensity factor measured by the single edge notched beam fracture toughness test. From a surface-energetic point of view, the anodic treatment may be suitable for carbon fibers incorporated in a polar organic matrix, resulting in an increased specific component of the surface free energy. Good wetting plays an important role in improving the degree of adhesion at interfaces between fibers and matrices of the resulting composites. Copyright 2000 Academic Press. 相似文献
928.
Wen-Feng Liaw Chia-Huei Lai Ming-Hsi Chiang Cheng-Kang Hsieh Gcne-Hsiang Lee Shie-Ming Peng 《中国化学会会志》1993,40(5):437-444
Diphenyldichalcogenides (PhE)2 (E = Te, Se) react with Fe(0)-phenylchalcogenolate [PPN] [PhEFe(CO)4] to yield the products of oxidative addition, Fe(II)-mixed-phenylchalcogenolate fac- [PPN][Fe(CO)3(TePh)n(ScPh)3-n] (n = 1, 2). Reactions of [PPN][REFe(CO)4] (E=Se, R=Me; E=S, R=Et) and diphenyldichalcogenides yielded ligand-exchange products [PPN][PhEFe(CO)4] (E=Te, Se, S). The compounds [Fe(CO)3(TePh)(ScPh)2]? (l) and [Fe(CO)3(TePh)2 (2) crystallize in the isomorphous monoclinic space group C2/e, with a = 32.035(8), b = 11.708(6), c = 28.909(6) Å, Z = 8, R = 0.048, and Rw = 0.044 (1); with a = 32.089(5), b= 11.745(2), c = 28.990(8) Å, Z = 8, R = 0.048, and Rw = 0.048 (2). The complexes 1 and 2 crystallize as discrete cations of PPN+ and anions of [Fe(CO)3(TcPh)u(ScPh)3-n] (n=1, 2), and one half solvent molecule THF. The geometry around Fe(II) is a distorted octahedron with three carbonyl groups and three phenylchalcogenolate ligands occupying facial positions. 相似文献
929.
930.
We have produced nanophase metal clusters, (Fe)n, (Cr)n, (Mo)n and (W)n, by multiphoton decomposition of the corresponding metal carbonyls with a 10.6 μm CO2 laser in the presence of Ar and SF6. The size distribution was narrow and the average diameter was 6, 3.5, 2 and 1 nm for Fe, Cr, Mo and W clusters, respectively. The structure was found to be bcc for both Fe and Cr clusters, fcc for Mo clusters, and amorphous for W clusters (note that all the bulk metals have bcc structure). Considering the cluster sizes (9630, 1870, 230 and 30 for Fe, Cr, Mo and W clusters, respectively) estimated from their average diameters, it is likely that there exists a structural transition from fcc to bulk bcc with increasing cluster size in these metal clusters. 相似文献