全文获取类型
收费全文 | 3278篇 |
免费 | 97篇 |
国内免费 | 22篇 |
专业分类
化学 | 2061篇 |
晶体学 | 11篇 |
力学 | 145篇 |
数学 | 495篇 |
物理学 | 685篇 |
出版年
2024年 | 4篇 |
2023年 | 35篇 |
2022年 | 78篇 |
2021年 | 114篇 |
2020年 | 100篇 |
2019年 | 80篇 |
2018年 | 58篇 |
2017年 | 45篇 |
2016年 | 129篇 |
2015年 | 91篇 |
2014年 | 134篇 |
2013年 | 168篇 |
2012年 | 222篇 |
2011年 | 252篇 |
2010年 | 142篇 |
2009年 | 123篇 |
2008年 | 199篇 |
2007年 | 193篇 |
2006年 | 207篇 |
2005年 | 191篇 |
2004年 | 146篇 |
2003年 | 127篇 |
2002年 | 126篇 |
2001年 | 49篇 |
2000年 | 23篇 |
1999年 | 29篇 |
1998年 | 32篇 |
1997年 | 38篇 |
1996年 | 36篇 |
1995年 | 21篇 |
1994年 | 22篇 |
1993年 | 17篇 |
1992年 | 7篇 |
1991年 | 15篇 |
1990年 | 9篇 |
1989年 | 9篇 |
1988年 | 12篇 |
1987年 | 9篇 |
1986年 | 11篇 |
1985年 | 9篇 |
1984年 | 10篇 |
1983年 | 9篇 |
1982年 | 12篇 |
1981年 | 14篇 |
1980年 | 7篇 |
1979年 | 10篇 |
1978年 | 5篇 |
1977年 | 4篇 |
1976年 | 2篇 |
1975年 | 9篇 |
排序方式: 共有3397条查询结果,搜索用时 31 毫秒
21.
Anna Rizzolo Patrizia Lombardi Maristella Vanoli Stefano Polesello 《Journal of separation science》1995,18(5):309-314
Capillary GC/sensory analysis was used to judge if dynamic headspace on sliced pulp and on intact fruit, and solvent extraction could collect the “character impact” and the “contributory” aroma compounds in peaches. Capillary GC/sensory data showed that the headspace techniques selectively recovered the “contributory” volatile compounds, which are strictly related to the characteristic odor of the various peach cultivars, whereas solvent extraction better quantified the “character impact” compounds (lactones). 相似文献
22.
Franco Cecconi Andrew Duthie Pedro Gili Stefano Midollini 《Journal of solid state chemistry》2004,177(3):786-792
By reaction of Zn(CH3COO)2 with p,p′-diphenylmethylenediphosphinic acid in water a new inorganic-organic polymeric hybrid of formula [Zn(CH2(P(Ph)O2)2)] has been synthesized and completely characterized. The X-ray analysis established that the structure consists of 2D-layered polymeric array, the 2D-sheets being built up through strong covalent linkages between the zinc metal and the oxygen donors of the phenylphosphinate ligand. The 2D-layers, which are featuring a mesh-net fashion, present voids of various dimensionality, up to 24-membered rings. The organic parts of the hybrid ligand, namely the phenyl rings, are shielding the inorganic skeleton of the layers, preventing the propagation of the polymer in the third dimension. No water molecules are present in the lattice, both of coordination and crystallization. Crystal data are: monoclinic, P21/c, a=11.840(2), b=9.646(9), c=12.516(5) Å, β=95.03(2), V=1423.9(15) Å3, Z=4. The solid material has been characterized by 31P MAS NMR spectroscopy and thermogravimetric analysis. 相似文献
23.
Lucarini M Franchi P Pedulli GF Gentilini C Polizzi S Pengo P Scrimin P Pasquato L 《Journal of the American Chemical Society》2005,127(47):16384-16385
ESR spectroscopy has been used to study the interaction of para-pentylbenzyl hydroxyalkyl nitroxide with the monolayer of water-soluble protected gold clusters having a core diameter ranging from 1.6 to 5.3 nm. The solubilization of the nitroxide probe in the more hydrophobic environment of the monolayer strongly depends on the size of the gold core. In particular, the partition equilibrium constant increases as the nanoparticle diameter decreases. These results have been attributed to the different packing of the chains in the monolayer resulting from the different radius of curvature of the investigated nanoparticles. This represents, to the best of our knowledge, the first report demonstrating that the core size of metallic nanoparticles affects the solvating properties of the protective organic monolayer. 相似文献
24.
Polymer-supported O-alkylisoureas were prepared by reaction of an alcohol with a polymer-supported carbodiimide under copper(II) catalysis. These reagents were used to transform carboxylic acids into the corresponding methyl, benzyl, allyl, and p-nitrobenzyl esters in a highly chemoselective manner in high yields and in very high purity after simple resin filtration and solvent evaporation. The reactions could be carried out using both conventional or microwave heating, with reaction times as short as 3-5 min in the latter case, without compromising yield, purity, or chemoselectivity. Unfortunately, the corresponding solid-supported tert-butyl isoureas could not be prepared. 相似文献
25.
Gabriele Proietti Stefano Corsano Enzo Castagnino 《Journal of heterocyclic chemistry》1981,18(2):415-416
The synthesis of cis- and trans-2-methyl-3-phenyl-1,4-benzodioxans 4 and 5 , from 1-phenyl-2-[2-(hydroxy)-phenoxy]propanols threo 2 and erythro 3 are described. 相似文献
26.
Stefano Vecchio Alessia Catalani Vanessa Rossi Mauro Tomassetti 《Thermochimica Acta》2004,420(1-2):99-104
The thermal behaviour of acetanilide (Ac) and two of its analogues, namely the para-ethoxyacetanilide (p-Eto Ac) and the para-bromoacetanilide (p-Br Ac), which are used as analgesics in the pharmaceutical industry was studied with a simultaneous TG/DSC unit. The examined analgesics showed two endothermic DSC peaks due to melting and vaporization. By combining the experimental TG data with the corresponding reference vapour pressure data obtained with the Antoine equation the plot of P versus v was derived. From the slope of this equation the constant k-value was determined for Ac. Then, using the same k-value the vapour pressures of p-Eto Ac and p-Br Ac were determined in the same temperature range. The vaporization enthalpies for all the studied compounds were obtained from different methods and a very good agreement was found. Vaporization follows a zero-order kinetics. The activation energy of vaporization (Evap) was calculated from the dynamic TG experiments, using the Arrhenius equation. 相似文献
27.
采用改进颗粒床模型的CFD方法模拟了实验室规模冷模装置内鼓泡床的流体流动时空特性。模拟结果表明表观气速是影响气固动态特征和压力波动的主要因素之一:随表观气速的增大,气泡数目增加,气泡体积增大,压力波动增强;气速越高时均压降越大;在内循环鼓泡流化床内固体颗粒呈“单室”流型。上述与实验观察相吻合的模拟结果将有助于放大和设计商业化的内循环流化床生物质气化炉。 相似文献
28.
Di Pietro C Serroni S Campagna S Gandolfi MT Ballardini R Fanni S Browne WR Vos JG 《Inorganic chemistry》2002,41(11):2871-2878
The synthesis and characterization of two dinuclear ruthenium polypyridyl complexes based on the bridging ligands 5,5'-bis(pyridin-2' '-yl)-3,3'-bis(1H-1,2,4-triazole) and 5,5'-bis(pyrazin-2' '-yl)-3,3'-bis(1H-1,2,4-triazole) and of their mononuclear precursors are reported. The dinuclear compounds have been prepared by a Ni(0) catalyzed coupling of a mononuclear ruthenium(II) polypyridyl complex containing a brominated triazole moiety. Electrochemical and photophysical studies indicate that, in these dinuclear complexes, the protonation state of the bridge may be used to tune the intercomponent interaction between the two metal centers and that these species act as proton driven three-way molecular switches that can be read by electrochemical or luminescence techniques. 相似文献
29.
Boseggia E Gatos M Lucatello L Mancin F Moro S Palumbo M Sissi C Tecilla P Tonellato U Zagotto G 《Journal of the American Chemical Society》2004,126(14):4543-4549
A series of cis-cis-triaminocyclohexane Zn(II) complex-anthraquinone intercalator conjugates, designed in such a way to allow their easy synthesis and modification, have been investigated as hydrolytic cleaving agents for plasmid DNA. The ligand structure comprises a triaminocyclohexane platform linked by means of alkyl spacers of different length (from C(4) to C(8)) to the anthraquinone group which may intercalate the DNA. At a concentration of 5 microM, the complex of the derivative with a C(8) alkyl spacer induces the hydrolytic stand scission of supercoiled DNA with a rate of 4.6 x 10(-6) s(-1) at pH 7 and 37 degrees C. The conjugation of the metal complex with the anthraquinone group leads to a 15-fold increase of the cleavage efficiency when compared with the anthraquinone lacking Zn-triaminocyclohexane complex. The straightforward synthetic procedure employed, allowing a systematic change of the spacer length, made possible to gain more insight on the role of the intercalating group in determining the reactivity of the systems. Comparison of the reactivity of the different complexes shows a remarkable increase of the DNA cleaving efficiency with the length of the spacer. In the case of too-short spacers, the advantages due to the increased DNA affinity are canceled due to the incorrect positioning of the reactive group, thus leading to cleavage inhibition. 相似文献
30.
Concetta?De?Stefano Antonio?Gianguzza Daniela?Piazzese Silvio?SammartanoEmail author 《Analytical and bioanalytical chemistry》2003,375(7):956-967
The binding ability of diethylene triamine pentaacetate (dtpa(5-)) and triethylene tetraamine hexaacetate (ttha(6-)) ligands towards major components, H(+), Na(+), Mg(2+) and Ca(2+), of natural waters was studied in both single and mixed ionic media at different ionic strengths and at T=25 degrees C. Some measurements, performed in Mg(2+)-Ca(2+) mixtures, allowed us to find the formation of new mixed species MgCa(dtpa), MgCa(ttha) and MgCaH(ttha), here reported for the first time. All the complexes formed in the various systems were characterized in terms of both stoichiometry and stability, and an attempt was made to find general rules for the stability of mixed metal complexes in comparison with that of simple species. To obtain quantitative information on the complexing ability of dtpa and ttha in seawater, measurements have been carried out in artificial seawater ionic medium (Na(+), K(+), Ca(2+), Mg(2+), Cl(-) and SO(4)(2-)). Calculations, performed by considering the salt mixture as single salt BA, allowed us to find some quite stable B(i)H(j)L species. Under the natural seawater conditions [S(salinity)=35], we found for the most important species logbeta( B(dtpa))=9.64 and. Literature data comparison is also reported. 相似文献