全文获取类型
收费全文 | 80557篇 |
免费 | 412篇 |
国内免费 | 385篇 |
专业分类
化学 | 25875篇 |
晶体学 | 805篇 |
力学 | 6813篇 |
数学 | 32308篇 |
物理学 | 15553篇 |
出版年
2022年 | 68篇 |
2021年 | 113篇 |
2020年 | 99篇 |
2019年 | 67篇 |
2018年 | 10465篇 |
2017年 | 10279篇 |
2016年 | 6172篇 |
2015年 | 937篇 |
2014年 | 372篇 |
2013年 | 462篇 |
2012年 | 3949篇 |
2011年 | 10687篇 |
2010年 | 5743篇 |
2009年 | 6120篇 |
2008年 | 6746篇 |
2007年 | 8911篇 |
2006年 | 383篇 |
2005年 | 1436篇 |
2004年 | 1668篇 |
2003年 | 2068篇 |
2002年 | 1088篇 |
2001年 | 283篇 |
2000年 | 305篇 |
1999年 | 165篇 |
1998年 | 204篇 |
1997年 | 168篇 |
1996年 | 221篇 |
1995年 | 133篇 |
1994年 | 93篇 |
1993年 | 104篇 |
1992年 | 69篇 |
1991年 | 67篇 |
1990年 | 59篇 |
1989年 | 67篇 |
1988年 | 70篇 |
1987年 | 65篇 |
1986年 | 64篇 |
1985年 | 61篇 |
1984年 | 60篇 |
1983年 | 51篇 |
1982年 | 54篇 |
1981年 | 51篇 |
1980年 | 56篇 |
1979年 | 55篇 |
1978年 | 38篇 |
1914年 | 47篇 |
1913年 | 40篇 |
1912年 | 40篇 |
1909年 | 41篇 |
1908年 | 40篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
41.
Attila R. Imre W. Alexander Van Hook Bong Ho Chang Young Chan Bae 《Monatshefte für Chemie / Chemical Monthly》2003,134(12):1529-1539
Summary. Upper critical solution temperatures (UCSTs) for liquid–liquid demixing in a set of mixtures of linear alkanes (pentane (N
1=5) to pentacontane (N
1=50)) with an oligostyrene (1241amu, N
2=12) are reported. We find strong correlation between the Hildebrand solubility parameters of the alkanes and the UCST. Correlations are developed which enable predictions concerning the miscibility of mixtures of compounds with longer chains. 相似文献
42.
Alessandro Agneni 《Meccanica》1992,26(4):221-228
This paper deals with some aliasing effects in the time domain that can lead to unacceptable misestimations of modal parameters. When a frequency response function of a vibrating system is sampled and inverse Fourier transformed, the resulting impulse response is given by an infinite geometric series, the single term of which is the impulse response itself shifted in time. For this reason, some modal parameters, if estimated in the time domain, are biased; in particular, while the damping factor and the natural frequency are not influenced by the aliasing phenomenon, the magnitude and phase of the residue can be highly biased. Corrective terms are theoretically evaluated and their efficiency is shown in numerical simulations.
Sommario In questo articolo e' presentato un metodo per correggere gli errori che si compiono nella stima di alcuni parametri modali, quando essi vengono ricavati nel dominio del tempo. Infatti se la funzione di risposta in frequenza e' ottenuta con eccitazioni particolari—quali ad esempio lo pseudo-random, lo stepped-sine a passo costante o lo sweep in frequenza-la risposta impulsiva, ottenuta per mezzo dell'antitrasformata discreta di Fourier, risulta periodica, con periodo pari all'inverso della spaziatura tra le righe spettrali. Cio' comporta un errore nella stima dell'ampiezza e della fase del residuo, mentre nessuna conseguenza si ha sulla frequenza naturale e sul fattore di smorzamento. L'errore sulle stime risulta tanto piu' grande, quanto meno smorzata e' la risposta impulsiva all'interno dell'intervallo di osservazione.Partendo dall'inviluppo e dalla fase istantanea dei segnali complessi, ottenibili per mezzo della trasformata di Hilbert, si sono ricavati i termini correttivi sia per il modulo, che per la fase del residuo. La validita' delle correzioni e' mostrata con esempi numerici.相似文献
43.
Robert W. Ashford Peter Connard Robert Daniel 《The Journal of the Operational Research Society》1992,43(5):519-531
The time-consuming process of solving large-scale Mixed Integer Programming problems using the branch-and-bound technique can be speeded up by introducing a degree of parallelism into the basic algorithm. This paper describes the development and implementation of a parallel branch-and-bound algorithm created by adapting a commercial MIP solver. Inherent in the design of this software are certain ad hoc methods, the use of which are necessary in the effective solution of real problems. The extent to which these ad hoc methods can successfully be transferred to a parallel environment, in this case an array of at most nine transputers, is discussed. Computational results on a variety of real integer programming problems are reported. 相似文献
44.
Jorge Pavez Maritza Páez Armelle Ringuedé Fethi Bedioui José H. Zagal 《Journal of Solid State Electrochemistry》2005,9(1):21-29
We studied the electrocatalytic activity of cobalt tetra-aminophthalocyanine (CoTAPc) for the reduction of molecular oxygen (O2) on adsorbed monomeric and on electropolymerized films of different thicknesses on glassy carbon (GC) electrode. The polymeric films, denoted poly-CoTAPc, were first characterized by electrochemical impedance spectroscopy and it appears that the types of phenomena revealed to be occurring depend less on the film thickness in basic than in acid media. For O2 reduction, the results showed that poly-CoTAPc is more active than the monomeric CoTAPc adsorbed on GC. Indeed, rotating ring-disk electrode data showed that polymeric CoTAPc promotes the four-electron reduction of O2 to water in parallel to a two-electron reduction to give peroxide. On monomeric and thin films of poly-CoTAPc, a two-electron reduction mechanism predominates. In basic media the activity increases very slightly with thickness, whereas in acid media this increase is more pronounced. This parallels the observed behavior revealed by electrochemical impedance spectroscopy. 相似文献
45.
Sorption of uranium by non-living water hyacinth roots 总被引:1,自引:0,他引:1
S. Shawky M. Abdel Geleel Ashraf Aly 《Journal of Radioanalytical and Nuclear Chemistry》2005,265(1):81-84
Summary Many studies have shown that water hyacinth (Eichhornia crassipes) roots can be used to accumulate high concentrations of organic as well as inorganic pollutants. They are currently used to remediate aquatic environments and aqueous solutions. In the present study, sorption of uranium from aqueous solutions by using dried roots of water hyacinth has been investigated. The sorption of uranium was examined as a function of initial concentration, pH, weight of roots and contact time. Five different concentrations 20, 40, 60, 80, and 100 μg . ml-1 were used. Sorption proves to be very rapid and depend on pH, weight of roots and concentration of uranium. Maximum sorption capacity of water hyacinth roots was 64,000 U6+ μg/g. The sorption of uranium by water hyacinth roots follows a Langmuir isotherm. 相似文献
46.
Burley GA Fowler PW Soncini A Sandall JP Taylor R 《Chemical communications (Cambridge, England)》2003,(24):3042-3043
Ipsocentric current-density maps for a fluorofullerene derivative, C60F15H3, modelling the addition pattern of the experimentally characterised C60F15[CBr(CO2Et)2]3 which contains an [18]trans-annulene system, reveal a diamagnetic ring current dominated by the contribution of the four HOMO electrons, as in a classical (4n + 2) aromatic annulene. 相似文献
47.
L. F. Brum Malta Marta Eloisa Medeiros 《Journal of Thermal Analysis and Calorimetry》2005,81(1):149-152
Summary This work reports the room-temperature stabilization of the Bi4V2-xFexIIO11-1.5x γ ‘ phase, a promising ionic conductive material that finds application in solid oxide fuel cell and oxygen sensor devices. The Fe(II) cation proved to be a better stabilizer than Fe(III), which was previously used, since a lower substitution degree of V5+ is needed for the former. Powder X-ray diffraction, Fourier-transform infrared spectroscopy and differential scanning calorimetry were used in these experiments. 相似文献
48.
Hans König 《Analytical and bioanalytical chemistry》1973,266(2):119-124
The separation of a mixture of 22 bactericides has been achieved by gas chromatography on columns with silicone rubber W-982 as stationary phase with temperatures between 100° and 300°C. The unchanged compounds as well as their silylation products have been used. The latter are more conveniently used especially for the quantitative determination. To be able to calculate the retention indices after Kovats gas chromatography has been performed isothermally at 180°C for the more volatile compounds and at 250°C for all other bactericides.The retention indices obtained under these conditions are tabulated together with the limits of detection. 相似文献
49.
Alessandro Lezzi Sandra Cobianco Arnaldo Roggero 《Journal of polymer science. Part A, Polymer chemistry》1994,32(10):1877-1883
A series of chelating resins, derived from a macroreticular styrene-divinylbenzene (2%) copolymer beads grafted with various poly(ethylene glycols) HO? (? CH2? CH2? O? )n? H(n = 0, 4, 9, 13) and containing thiol groups as chelating functions, have been synthesized in a three-step reaction sequence. The structure of the functionalized resins was confirmed by IR spectrophotometry, elemental analysis, and differential scanning calorimetry. The complexation behavior of these thiol resins was investigated towards Hg(II), Cu(II), and Pb(II) ions in aqueous solution by a batch equilibration technique. The influence of pH on adsorption capacity was also examined. The adsorption values for metal ions' intake followed the order Hg(II) > Cu(II) > Pb(II). The affinity of these polymers towards Hg(II) ions was so high that the total mercury level in the liquid decreased from 20 ppm to below 10 ppb after 2 h of treatment. Polymers can be regenerated by washing with a solution of hydrochloric acid (6N) and 10% by weight of an aqueous solution of thiourea. © 1994 John Wiley & Sons, Inc. 相似文献
50.
(2S)- and (2R)-2-Amino-4-bromobutanoic acid were prepared starting from N-Boc-glutamic acid α tert-butyl ester. The double tert-butyl protection was necessary to prevent a partial racemisation during Barton’s radical decarboxylation used to transform the γ-carboxylic group into a bromide. This bromide reacted with different nitrogen, oxygen and sulphur nucleophiles to give nonnatural amino acids characterised by basic or heterocyclic side chains. The title compound was also used to prepare a conformationally constrained peptidomimetic. 相似文献