全文获取类型
收费全文 | 4002篇 |
免费 | 142篇 |
国内免费 | 5篇 |
专业分类
化学 | 2227篇 |
晶体学 | 27篇 |
力学 | 87篇 |
数学 | 562篇 |
物理学 | 1246篇 |
出版年
2021年 | 47篇 |
2020年 | 47篇 |
2019年 | 44篇 |
2018年 | 83篇 |
2017年 | 84篇 |
2016年 | 162篇 |
2015年 | 142篇 |
2014年 | 180篇 |
2013年 | 346篇 |
2012年 | 156篇 |
2011年 | 150篇 |
2010年 | 126篇 |
2009年 | 88篇 |
2008年 | 109篇 |
2007年 | 82篇 |
2006年 | 126篇 |
2005年 | 108篇 |
2004年 | 105篇 |
2003年 | 75篇 |
2002年 | 73篇 |
2001年 | 48篇 |
2000年 | 66篇 |
1999年 | 43篇 |
1998年 | 48篇 |
1997年 | 56篇 |
1996年 | 49篇 |
1995年 | 50篇 |
1994年 | 43篇 |
1993年 | 51篇 |
1992年 | 37篇 |
1991年 | 44篇 |
1990年 | 47篇 |
1989年 | 30篇 |
1988年 | 55篇 |
1987年 | 45篇 |
1986年 | 37篇 |
1985年 | 65篇 |
1984年 | 61篇 |
1983年 | 43篇 |
1982年 | 36篇 |
1981年 | 39篇 |
1980年 | 44篇 |
1979年 | 42篇 |
1978年 | 41篇 |
1977年 | 57篇 |
1976年 | 52篇 |
1975年 | 45篇 |
1974年 | 58篇 |
1973年 | 40篇 |
1969年 | 29篇 |
排序方式: 共有4149条查询结果,搜索用时 46 毫秒
51.
The application of NMR and MS methods for detection of adulteration of wine,fruit juices,and olive oil. A review 总被引:2,自引:0,他引:2
Ogrinc N Kosir IJ Spangenberg JE Kidric J 《Analytical and bioanalytical chemistry》2003,376(4):424-430
This review covers two important techniques, high resolution nuclear magnetic resonance (NMR) spectroscopy and mass spectrometry (MS), used to characterize food products and detect possible adulteration of wine, fruit juices, and olive oil, all important products of the Mediterranean Basin. Emphasis is placed on the complementary use of SNIF-NMR (site-specific natural isotopic fractionation nuclear magnetic resonance) and IRMS (isotope-ratio mass spectrometry) in association with chemometric methods for detecting the adulteration. 相似文献
52.
J. Mikuski J. Moravec E. Ochab V. Šára 《Journal of Radioanalytical and Nuclear Chemistry》1981,62(1-2):83-90
The structure of the complex Ag·TBDTPP (tetrabutyl S-dithiopyrophosphate) formed in HNO3 medium was studied by IR and NMR methods. The complex is involved in the isomerization of TBDTPP in the presence of Ag+ and NO
3
−
ions. In the IR spectra, we observe the disappearance of P−O−P band and the occurrence of new absorption bands associated
with P−S−P and P=O bonds. The silver ion is coordinated to the thiophosphoryl group of the isomer molecule. The influence
of HNO3 concentration on complex formation is discussed. 相似文献
53.
Large configuration interaction calculations of the proton—proton coupling constant for several geometrical configurations of the ammonia molecules are reported. The analytical expressions for the energy surface and the coupling constant as functions of two cartesian displacement coordinates are fitted to the calculated values. The potential is used for the calculation of the vibrational wavefunctions for 15NH3 and 15ND3 species and the vibrational averaging of the coupling constant is carried out using these functions. Though the value of the coupling constants shows a very strong geometry dependence, the vibrational corrections are found to be small. A possible correlation of the proton—proton coupling constant with an angular parameter in the NH2 group in RNH2 compounds is indicated. 相似文献
54.
Viktor Kettmann Jan Lokaj Viktor Milata Jozef Salo Mohamed Mahmoud Mohamed Hasan 《Acta Crystallographica. Section C, Structural Chemistry》2002,58(6):o365-o366
The title compound, C11H10N3+·Cl?·H2O, belongs to the N1‐methyl‐substituted imidazo[4,5‐f]quinoline family, in which the heterocyclic ring is protonated at the pyridine rather than at the imidazole N atom. The molecule as a whole is almost exactly planar. The molecular structure has been compared with that of the 2‐amino analogue described in the literature, and it was found that the extra amino group of the latter is involved in conjugation with the adjacent double bond, i.e. the conjugation does not extend over the entire heterocyclic system. The cation of the title compound forms a strong hydrogen bond with the Cl? anion and the anions are interconnected by the water solvent molecule. 相似文献
55.
Summary High-molecular-weight sodium hyaluronate isolated from rooster combs was degraded by ultrasonication. The molecular weight of hyaluronate and its polydispersity was determined by gel-permeation chromatography. During 75-min treatment the molecular weight value decreased from 1.39×106 Da to 2.25×105 Da while the polydispersity of the molecular weight increased from 1.29 to 2.36. The reciprocal value of the square of the hyaluronate's molecular weight was linearly proportional to the time of ultrasonication. 相似文献
56.
Summary A high molecular weight sodium hyaluronate isolated from rooster combs was degraded by ultrasonication. High-performance size exclusion chromatography allowed rapid and accurate determination of molecular weight parameters (Mz, Mw, Mn) and distributions. The time dependence of hyaluronan ultrasonication to the molecular characteristics of the polymer was investigated. A non-random nature of the degradation process was demonstrated and the reciprocal Mn value was found to be linearly proportional to the time of ultrasonication. 相似文献
57.
It was found that lead reacts with 18-crown-6 (L) and tungstosilicic acid (H4A) in acidic solutions and a compound with low solubility is formed what was confirmed by radiometric titration methods. Coprecipitation of complexion cation of PbL2+ with similar complexion cations of calcium, strontium and barium was studied. Formation of low soluble salts was utilized for separation of Pb from Ca, Sr and Ba from 1 mol·dm–3 HNO3. The ratio of Pb/Ca, Pb/Sr and Pb/Ba in the precipitation or separation factors Spb/M undoubted depends on the ratio of the stability constants of lead and metals with 18C6 (Pb/M), what can be used for determination of more precise constant stability M. 相似文献
58.
Thermal decomposition of zircon mineral was studied in the presence of sodium and potassium hydroxides and in the presence of a waste mixture of hydroxides containing NaOH and KOH in a weight ratio of 23. The process was controlled in such a way that the decomposition products might contain the minimum possible amount of alkali and could thus be applied for the syntheses of zircon pigments without any further exacting treatment.
Zusammenfassung Es wurde die thermische Zersetzung eines Zirkonminerals in Gegenwart von Natrium- und Kaliumhydroxid-sowie in Gegenwart einer Altlauge mit einem Gehalt an NaOH und KOH im Verhältnis zwei zu drei untersucht. Der Prozeß wird derart gesteuert, daß die Zersetzungsprodukte einen möglichst geringen Anteil an Lauge besitzen und so ohne weitere arbeitsaufwändige Behandlungen zur Synthese von Zirkonpigmenten verwendet werden können.
, 23. , , , - .相似文献
59.
Electrooxidation of sulfide ion catalysed by microcrystals of cobalt phthalocyanine was investigated by cyclic voltammetry in 0.5M KNO3 at pH 9.22. Traces of catalyst were immobilized at the surface of a paraffin-impregnated graphite electrode by the mechanical transfer of its powder. The electro-oxidation of HS– proceeds in two irreversible steps, with the first peak between 0 V and –0.12 V and the second at 0.17 V. The first step is second order in HS– and its product is the adsorbed disulfide, which may further dissociate to give adsorbed sulfur atoms. The reduction of sulfur occurs at –0.1 V. 相似文献
60.
Urška L. Štangar Urh Černigoj Polonca Trebše Ksenija Maver Silvia Gross 《Monatshefte für Chemie / Chemical Monthly》2006,137(5):647-655
Summary. Transparent TiO2 films with a high photodegradation activity towards an azo dye in aqueous solution were prepared by sol–gel processing. Films
on soda–lime glass supports protected with a thin silica barrier layer exhibited better crystallization and monodisperse nanoparticles,
higher absorption of light below 370 nm, and higher photocatalytic activity than those films deposited on bare glass supports
proving the detrimental effect of interdiffused sodium ions on the development of the anatase nanostructure. The effect of
substrate was more pronounced in thinner films (300 nm) than in thicker ones (1200 nm), which were achieved by adding a template
(i.e. Pluronic F127) to the sol. 相似文献