全文获取类型
收费全文 | 712篇 |
免费 | 21篇 |
国内免费 | 3篇 |
专业分类
化学 | 570篇 |
晶体学 | 1篇 |
力学 | 6篇 |
数学 | 97篇 |
物理学 | 62篇 |
出版年
2023年 | 8篇 |
2022年 | 29篇 |
2021年 | 42篇 |
2020年 | 38篇 |
2019年 | 20篇 |
2018年 | 18篇 |
2017年 | 14篇 |
2016年 | 28篇 |
2015年 | 26篇 |
2014年 | 34篇 |
2013年 | 43篇 |
2012年 | 42篇 |
2011年 | 52篇 |
2010年 | 28篇 |
2009年 | 40篇 |
2008年 | 53篇 |
2007年 | 26篇 |
2006年 | 32篇 |
2005年 | 24篇 |
2004年 | 22篇 |
2003年 | 14篇 |
2002年 | 19篇 |
2001年 | 4篇 |
2000年 | 8篇 |
1999年 | 7篇 |
1997年 | 4篇 |
1996年 | 3篇 |
1995年 | 2篇 |
1994年 | 4篇 |
1993年 | 5篇 |
1992年 | 7篇 |
1991年 | 1篇 |
1990年 | 3篇 |
1989年 | 2篇 |
1988年 | 3篇 |
1987年 | 1篇 |
1986年 | 4篇 |
1985年 | 2篇 |
1984年 | 2篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1981年 | 2篇 |
1980年 | 1篇 |
1978年 | 4篇 |
1977年 | 5篇 |
1975年 | 1篇 |
1974年 | 3篇 |
1968年 | 1篇 |
1967年 | 1篇 |
1937年 | 1篇 |
排序方式: 共有736条查询结果,搜索用时 15 毫秒
131.
Monteiro MJ Bazito FF Siqueira LJ Ribeiro MC Torresi RM 《The journal of physical chemistry. B》2008,112(7):2102-2109
Lithium salt solutions of Li(CF3SO2)2N, LiTFSI, in a room-temperature ionic liquid (RTIL), 1-butyl-2,3-dimethyl-imidazolium cation, BMMI, and the (CF3SO2)2N(-), bis(trifluoromethanesulfonyl)imide anion, [BMMI][TFSI], were prepared in different concentrations. Thermal properties, density, viscosity, ionic conductivity, and self-diffusion coefficients were determined at different temperatures for pure [BMMI][TFSI] and the lithium solutions. Raman spectroscopy measurements and computer simulations were also carried out in order to understand the microscopic origin of the observed changes in transport coefficients. Slopes of Walden plots for conductivity and fluidity, and the ratio between the actual conductivity and the Nernst-Einstein estimate for conductivity, decrease with increasing LiTFSI content. All of these studies indicated the formation of aggregates of different chemical nature, as it is corroborated by the Raman spectra. In addition, molecular dynamics (MD) simulations showed that the coordination of Li+ by oxygen atoms of TFSI anions changes with Li+ concentration producing a remarkable change of the RTIL structure with a concomitant reduction of diffusion coefficients of all species in the solutions. 相似文献
132.
Roberta Bernini Enrico Mincione Fernanda Crisante Maurizio Barontini Giancarlo Fabrizi 《Tetrahedron letters》2007,48(39):7000-7003
The efficiency of dimethyl carbonate (DMC) as chemoselective carbomethoxylating agent of the alcoholic chain of phenols has been investigated. In the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) or sulfuric acid as catalysts, new carbomethoxylated phenolic compounds were obtained in quantitative yields. A new efficient derivatization of the aliphatic alcoholic chain of the precious natural hydroxytyrosol is described, which increases the lipophilicity of the hydroxytyrosol. The antioxidant activity of this new carboxymethylated hydroxytyrosol 8 has been investigated using DPPH radical scavenging test. The results showed that this new compound has an antioxidant activity similar to hydroxytyrosol. 相似文献
133.
Gonzalez MF Giménez X Gonzalez-Aguilar J Bofill JM 《The journal of physical chemistry. A》2007,111(41):10226-10233
A method for obtaining quantum trajectories from a discrete-variable representation computation of the quantum potential is presented. The method exploits the linearity of the Schr?dinger equation, deals smoothly with the quantum potential singularities, and readily performs the time propagation up to fairly large total elapsed times. A one-dimensional test of the general n-dimensional formulation is included. 相似文献
134.
Milhazes N Martins P Uriarte E Garrido J Calheiros R Marques MP Borges F 《Analytica chimica acta》2007,596(2):231-241
A complete physicochemical characterisation of MDMA and its synthetic precursors MDA, 3,4-methylenedioxybenzaldehyde (piperonal) and 3,4-methylenedioxy-β-methyl-β-nitrostyrene was carried out through voltammetric assays and Raman spectroscopy combined with theoretical (DFT) calculations. The former provided important analytical redox data, concluding that the oxidative mechanism of the N-demethylation of MDMA involves the removal of an electron from the amino-nitrogen atom, leading to the formation of a primary amine and an aldehyde. The vibrational spectroscopic experiments enable to afford a rapid and reliable detection of this type of compounds, since they yield characteristic spectral patterns that lead to an unequivocal identification.Moreover, the rational synthesis of the drug of abuse 3,4-methylenedioxymethamphetamine (MDMA or “ecstasy”) from one of its most relevant precursors 3,4-methylene-dioxyamphetamine (MDA), is reported. In addition, several approaches for the N-methylation of MDA, a limiting synthetic step, were attempted and the overall yields compared. 相似文献
135.
Summary: In this paper, films were prepared from soy protein and corn starch in different proportions and thermal stability and kinetic parameters were determined through degradation reactions in an inert atmosphere. Solid residues and decomposition products were identified by infrared spectroscopy. Films from corn starch were less thermally stable than soy protein films. The films containing both components had lower thermal stabilities when compared to those of the pure biopolymers. The mechanism of starch thermal degradation seems to occur in a single step, which can be confirmed by the constant E-values during the thermal degradation reaction. For the pure protein and its mixtures an increase in the activation energy was observed during the reaction. Solid residues for protein at different temperatures showed mainly bands related to CO stretching, angular deformation of N H and C H groups. For starch, absorptions related to free and bound O H, CO stretching of CO2, CO and carbonyl compounds were observed. For the 50/50 mixture bands related to soy protein and corn starch were observed. The gaseous products for soy protein showed absorptions related to CO2, CO, CO, NH3 and C H stretching. For pure starch absorptions related to O H stretching from alcohol, CO from CO2, CO and carbonyl compounds. The 50/50 mixture had the same characteristics as pure soy protein and corn starch. 相似文献
136.
Steffen Gebhardt Stefan Bihler Manfred Schubert‐Zsilavecz Sergio Riva Daniela Monti Laura Falcone Bruno Danieli 《Helvetica chimica acta》2002,85(7):1943-1959
A series of specific derivatives of the complex protopanaxadiol glycoside ginsenoside Rb1 ( 1 ) were prepared by catalysis of two unrelated enzymes: the β‐1,4‐galactosyltransferase from bovine colostrum (GalT) and the lipase B from Candida antarctica (Novozym ® 435). Both of the enzymes showed the expected regioselectivity towards specific glucose OH groups (i.e., OH C(4) for GalT and preferentially the primary OH C(6) for Novozym ® 435), accompanied by a nonpredictable ‘site selectivity' for the gentiobiose disaccharide unit linked at C(20) of the dammarane skeleton. The galactosylated products 1a – e and the acetylated products 1f – h were isolated by HPLC and fully characterized by extensive MS and NMR analysis. 相似文献
137.
Jony Javorski Eckert Fabio Mazzariol Santiciolli Elvis Bertoti Eduardo dos Santos Costa Fernanda Cristina Corrêa Ludmila Corrêa de Alkmin e Silva 《基于设计的结构力学与机械力学》2018,46(2):238-253
In the present work, the Adaptive-Weight Genetic Algorithm was employed in order to determine the gear shifting strategies that allow an automobile to work in the best compromise among fuel consumption, engine emissions, and vehicle performance. For the assessment of each of the three objective functions, a simulation model based on engine data and on the well-established equations of the longitudinal dynamics was developed. The driving cycle chosen for the calculations was the FTP-75, which takes into account both cold and hot starts, meaning that the transient operation during the warm-up of the catalyst is also considered. 相似文献
138.
Eloi Feitosa Fernanda Rosa Alves Elisabete M. S. Castanheira M. Elisabete C. D. Real Oliveira 《Colloid and polymer science》2009,287(5):591-599
In the millimolar concentration domain (typically 1 mM), dioctadecyldimethylammonium bromide and chloride (DODAX, X representing
Br− or Cl− counterions) molecules assemble in water as large unilamellar vesicles. Differential-scanning calorimetry (DSC) is a suitable
technique to obtain the melting temperature (T
m) characteristic of surfactant bilayers, while fluorescence spectroscopy detects formation of surfactant aggregates, like
bilayers. These two techniques were combined to investigate the assembly of DODAX molecules at micromolar concentrations,
from 10 to 100 μM. At 1 mM surfactant, T
m ≈ 45 °C and 49 °C, respectively, for DODAB and DODAC. DSC and fluorescence of Nile Red were used to show the formation of
DODAX aggregates, at the surfactant concentration as low as 10 μM, whose T
m decreases monotonically with increasing DODAX concentration to attain the value for the ordinary vesicles. The data indicate
that these aggregates are organized as bilayer-like structures. 相似文献
139.
Stenger Fernanda Cristina Cechinel-Filho Valdir Meyre-Silva Christiane Bresolin Tania Mari Belle Rodrigues Clvis Antonio 《Chromatographia》2009,69(2):183-187
Chitosan was modified with lauroyl chloride to prepare a solid support for the application of matrix solid-phase dispersion (MSPD) for the extraction of flavonoids from the medicinal plant Aleurites moluccana. The laurylchitosan prepared by the homogenous and heterogeneous method, was characterized using analytical methods such as potentiometric titration and infrared spectroscopy. The hydrophobic properties were studied by naphthalene incorporation. The MSPD performance was evaluated by extraction of swertisin and 2″-O-rhamnosylswertisin from the leaves of A. moluccana. 相似文献
140.
Fátima Duarte Freire C. Flávio Soares Aragão T. Flávio Accioly de Lima e Moura Fernanda Nervo Raffin 《Journal of Thermal Analysis and Calorimetry》2009,97(1):355-357
Chlorpropamide ((4-chloro-N-(propylamino)-carbonyl)-benzenesulfonamide) belongs to compounds having sulfonylurea group and is widely used as an oral
antidiabetic agent. In this work differential scanning calorimetry (DSC) was used during pre-formulation of chlorpropamide
tablets to determine the drug-excipients compatibility. The DSC curves of chlorpropamide and binary mixtures with excipients
(sodium croscarmellose, sodium lauryl sulfate, microcrystalline cellulose, magnesium stearate and calcium carbonate) showed
that chlorpropamide exhibited interaction with magnesium stearate and sodium lauryl sulfate. The binary mixtures of chlorpropamide–magnesium
stearate presented a single endothermic process at 96–108 °C and chlorpropamide–sodium lauryl sulfate showed a wide endotherm
at 99–120 °C. 相似文献