全文获取类型
收费全文 | 980篇 |
免费 | 34篇 |
国内免费 | 5篇 |
专业分类
化学 | 750篇 |
晶体学 | 4篇 |
力学 | 22篇 |
数学 | 91篇 |
物理学 | 152篇 |
出版年
2023年 | 6篇 |
2022年 | 16篇 |
2021年 | 35篇 |
2020年 | 28篇 |
2019年 | 27篇 |
2018年 | 19篇 |
2017年 | 17篇 |
2016年 | 34篇 |
2015年 | 25篇 |
2014年 | 39篇 |
2013年 | 50篇 |
2012年 | 59篇 |
2011年 | 69篇 |
2010年 | 36篇 |
2009年 | 43篇 |
2008年 | 51篇 |
2007年 | 43篇 |
2006年 | 51篇 |
2005年 | 39篇 |
2004年 | 31篇 |
2003年 | 30篇 |
2002年 | 25篇 |
2001年 | 12篇 |
2000年 | 16篇 |
1999年 | 12篇 |
1998年 | 10篇 |
1997年 | 11篇 |
1996年 | 11篇 |
1994年 | 7篇 |
1992年 | 13篇 |
1990年 | 6篇 |
1986年 | 5篇 |
1985年 | 5篇 |
1980年 | 3篇 |
1978年 | 13篇 |
1977年 | 6篇 |
1976年 | 7篇 |
1975年 | 5篇 |
1973年 | 5篇 |
1971年 | 5篇 |
1970年 | 4篇 |
1968年 | 6篇 |
1966年 | 8篇 |
1964年 | 5篇 |
1962年 | 4篇 |
1960年 | 3篇 |
1956年 | 5篇 |
1955年 | 4篇 |
1948年 | 5篇 |
1943年 | 3篇 |
排序方式: 共有1019条查询结果,搜索用时 18 毫秒
101.
Interaction of lectins withYersinia pestis strains 总被引:1,自引:0,他引:1
Maria S. M. Cavalcanti Alzira M. P. Almeida Luana C. B. B. Coelho 《Applied biochemistry and biotechnology》1990,26(2):125-131
The ability of lectins to interact with Yersinia pestis strains isolated from rodent fleas and human biological fluids, obtained from different geographic areas, was examined. Lectins of Canavalia ensiformis, Ulex europaeus, Phaseolus vulgaris, and Triticum vulgaris, as well as a new autochthonous lectin of Swartzia pickellii of undefined specificity, were used. Most of the Y. pestis strains did not agglutinate with U. europaeus or C. ensiformis lectin. However, P. vulgaris lectin agglutinated suspensions of all the bacillus strains used. Fifteen of the 19 strains tested positive for assays using S. pickellii lectin. It is believed this is the first report of Y. pestis strain agglutination by lectins. A similar agglutination pattern was obtained for lectins with specificity for oligosaccharides containing N-acetylglucosamine and S. pickellii lectin, which did bind to the affinity matrix chitin, a polysaccharide of N-acetylglucosamine. The use of bacterial strains and commercial lectins of defined specificity may be an approach to providing evidence about the lectin binding sites of undefined monosaccharide specificity. 相似文献
102.
Luiz Augusto N. Barroso Márcia H.C. Fampa Rafael Kelman Mario V.F. Pereira Priscila Lino 《Annals of Operations Research》2002,117(1-4):247-270
The objective of this work is to investigate market power issues in bid-based hydrothermal scheduling. Initially, market power was simulated with a single stage Cournot–Nash equilibrium model. In this static model the equilibrium was calculated analytically. It was shown that the total production of N strategic agents is smaller than the least-cost solution by a factor of (N/(N+1)). Market power analysis for multiple stages was then carried through a stochastic dynamic programming scheme, where the decision in each stage and state is the Cournot–Nash equilibrium of a multi-agent game. Case studies with data taken from the Brazilian system are presented. 相似文献
103.
This paper considers the inversion of experimental field data collected with light receiving systems designed to meet operational requirements. Such operational requirements include system deployment in free drifting configurations and a limited number of acoustic receivers. A well-known consequence of a reduced spatial coverage is a poor sampling of the vertical structure of the acoustic field, leading to a severe ill-conditioning of the inverse problem and data to model cost function with a massive sidelobe structure having many local extrema. This causes difficulties to meta-heuristic global search methods, such as genetic algorithms, to converge to the true model parameters. In order to cope with this difficulty, broadband high-resolution processors are proposed for their ability to significantly attenuate sidelobes, as a contribution for improving convergence. A comparative study on simulated data shows that high-resolution methods did not outperform the conventional Bartlett processor for pinpointing the true environmental parameter when using exhaustive search. However, when a meta-heuristic technique is applied for exploring a large multidimensional search space, high-resolution methods clearly improved convergence, therefore reducing the inherent uncertainty on the final estimate. These findings are supported by the results obtained on experimental field data obtained during the Maritime Rapid Environmental Assessment 2003 sea trial. 相似文献
104.
“Green Electrodes” Modified with Au Nanoparticles Synthesized in Glycerol,as Electrochemical Nitrite Sensor
下载免费PDF全文
![点击此处可从《Electroanalysis》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Dino Gobelli N. Mariano Correa M. Fátima Barroso Fernando Moyano Patricia G. Molina 《Electroanalysis》2015,27(8):1883-1891
A new environmentally friendly Au nanoparticles (Au NPs) synthesis in glycerol by using ultraviolet irradiation and without extra‐added stabilizers is described. The synthesis proposed in this work may impact on the non‐polluting production of noble nanoparticles with simple chemicals normally found in standard laboratories. These Au NPs were used to modify a carbon paste electrode (CPE) without having to separate them from the reaction medium. This green electrode was used as an electrochemical sensor for the nitrite detection in water. At the optimum conditions the green sensor presented a linear response in the 2.0×10?7–1.5×10?5 M concentration range, a good detection sensitivity (0.268 A L mol?1), and a low detection limit of 2.0×10?7 M of nitrite. The proposed modified green CPE was used to determine nitrite in tap water samples. 相似文献
105.
This paper describes the automation of a fully electrochemical system for preconcentration, cleanup, separation and detection, comprising the hyphenation of a thin layer electrochemical flow cell with CE coupled with contactless conductivity detection (CE-C?D). Traces of heavy metal ions were extracted from the pulsed-flowing sample and accumulated on a glassy carbon working electrode by electroreduction for some minutes. Anodic stripping of the accumulated metals was synchronized with hydrodynamic injection into the capillary. The effect of the angle of the slant polished tip of the CE capillary and its orientation against the working electrode in the electrochemical preconcentration (EPC) flow cell and of the accumulation time were studied, aiming at maximum CE-C?D signal enhancement. After 6 min of EPC, enhancement factors close to 50 times were obtained for thallium, lead, cadmium and copper ions, and about 16 for zinc ions. Limits of detection below 25 nmol/L were estimated for all target analytes but zinc. A second separation dimension was added to the CE separation capabilities by staircase scanning of the potentiostatic deposition and/or stripping potentials of metal ions, as implemented with the EPC-CE-C?D flow system. A matrix exchange between the deposition and stripping steps, highly valuable for sample cleanup, can be straightforwardly programmed with the multi-pumping flow management system. The automated simultaneous determination of the traces of five accumulable heavy metals together with four non-accumulated alkaline and alkaline earth metals in a single run was demonstrated, to highlight the potentiality of the system. 相似文献
106.
Baki B. Sadi Chunsheng Li Gary H. Kramer Cassandra L. Johnson Queenie Ko Edward P. C. Lai 《Journal of Radioanalytical and Nuclear Chemistry》2011,290(2):415-425
A new radioanalytical method was developed for rapid determination of 226Ra in drinking water samples. The method is based on extraction and preconcentration of 226Ra from a water sample to an organic solvent using a dispersive liquid-liquid microextraction (DLLME) technique followed by
radiometric measurement using liquid scintillation counting. In DLLME for 226Ra, a mixture of an organic extractant (toluene doped with dibenzo-21-crown-7 and 2-theonyltrifluoroacetone) and a disperser
solvent (acetonitrile) is rapidly injected into the water sample resulting in the formation of an emulsion. Within the emulsion,
226Ra reacts with dibenzo-21-crown-7 and 2-theonyltrifluoroacetone and partitions into the fine droplets of toluene. The water/toluene
phases were separated by addition of acetonitrile as a de-emulsifier solvent. The toluene phase containing 226Ra was then measured by liquid scintillation counting. Several parameters were studied to optimize the extraction efficiency
of 226Ra, including water immiscible organic solvent, disperser and de-emulsifier solvent type and their volume, chelating ligands
for 226Ra and their concentrations, inorganic salt additive and its concentration, and equilibrium pH. With the optimized DLLME conditions,
the accuracy (expressed as relative bias, B
r
) and method repeatability (expressed as relative precision, S
B
) were determined by spiking 226Ra at the maximum acceptable concentration level (0.5 Bq L−1) according to the Guidelines for Canadian Drinking Water Quality. Accuracy and repeatability were found to be less than −5%
(B
r
) and less than 6% (S
B
), respectively, for both tap water and bottled natural spring water samples. The minimum detectable activity and sample turnaround
time for determination of 226Ra was 33 mBq L−1 and less than 3 h, respectively. The DLLME technique is selective for extraction of 226Ra from its decay progenies. 相似文献
107.
Andrade CA de Oliveira HP Oliveira MD Correia MT Coelho LC de Melo CP 《Colloids and surfaces. B, Biointerfaces》2011,88(1):100-107
This work is dedicated to the investigation of the prevailing molecular interactions between Cratylia mollis (Cramoll) and Concanavalin A (Con A) lectins and ionic (sodium dodecylsulfate, SDS) and non-ionic (Triton X-100, TX-100) surfactants, where we have used electrical impedance spectroscopy to map the dielectric characteristics of mixed lectin/surfactant solutions. The disorder induced in the lectin conformation is proportional to the extent of the access of the surfactant to the fluorophore present in the protein, resulting in its progressive unfolding. The complete unfolding of the lectin is associated to the formation of micelles in the core of the protein, each one of them containing a large number of detergent molecules, and therefore the process can be accompanied by measuring the electrical response of the binary surfactant/lectin system. For instance, the change in the real part of the impedance as a function of the relative concentration of the surfactant in the binary solution exhibits a breaking in its linear behavior that can be taken as indicative of a qualitative change in the environment surrounding the protein residue. We consider these results strong evidence in favor of using impedance spectroscopy methods for the analysis of protein-surfactant associations and for the characterization of the interactions that must prevail when the protein unfolds as the relative surfactant concentration is increased in aqueous solutions of these binary systems. 相似文献
108.
Trubetskaya OE Shaloiko LA Demin DV Marchenkov VV Proskuryakov II Coelho C Trubetskoj OA 《Analytica chimica acta》2011,(2):263-268
Polyacrylamide gel electrophoresis of chernozem soil humic acids (HAs) followed by observation under UV (312 nm) excitation light reveals new low molecular weight (MW) fluorescent fractions. Ultrafiltration of HAs sample in 7 M urea on a membrane of low nominal MW retention (NMWR, 5 kDa) was repetitively used for separation of fluorescent and non-fluorescent species. Thirty ultrafiltrates and the final retentate R were obtained. Fluorescence maxima of separate ultrafiltrates were different and non-monotonously changed in the range of 475–505 nm. Fluorescence maxima of less than 490 nm were detected only in the four first utrafiltrates. For further physical–chemical analyses all utrafiltrates were combined into a fraction called UF < 5 (NMW < 5 kDa). Retentate R demonstrated very weak fluorescence under 270 nm excitation, while fluorescence intensity of UF < 5 was about six times higher than of the bulk HAs. Fraction UF < 5 was further ultrafiltrated on membranes of MNWR 3 kDa and 1 kDa, yielding three subfractions UF3-5, UF1-3 and UF < 1 with NMW 3–5 kDa, 1–3 kDa and <1 kDa, respectively. The validation of the UF procedure was performed by size exclusion chromatography on Sephadex G-25 column. The fluorescence maxima were found to be at 505, 488 and 465 nm for UF3-5, UF1-3 and UF < 1, respectively, with increasing of fluorescence intensity from UF3-5 to UF1-3 to UF < 1 fraction. EPR analysis showed that the amount of free radicals was the largest in retentate R and drastically decreased in fluorescent ultrafiltrates. The results demonstrate that more than one fluorophore is present in chernozem soil HAs complex. 相似文献
109.
The detailed syntheses of complexes 1-4, Re(O)(X)(DAP) (X = Me, 1; Cl, 2; I, 3; OTf (triflate), 4) incorporating the diamido pyridine (DAP) ancillary ligand (2,6-bis((mesitylamino)methyl)pyridine) are described and shown to be effective catalysts for oxygen atom transfer (OAT) reactions of PyO to PPh(3). The catalytic activities are as follows: 4≈3 > 2 > 1. The observed electronic trend is consistent with the turnover limiting reduction of the proposed Re(VII) dioxo intermediate, Re(O)(2)(X)(DAP), during the catalytic cycle. The catalytic activity of complexes 1-3 was compared to previously published diamido amine (DAAm) oxorhenium complexes of the type Re(O)(X)(DAAm) (X = Me, 5; Cl, 6; I, 7 and DAAm = N,N-bis(2-arylaminoethyl)methylamine) which exhibit hydrolytic degradation during the catalytic reaction. Complexes 1-3 displayed higher turnover frequencies compared to 5-7. This higher catalytic activity was attributed to the more rigid DAP ligand backbone, which makes the complexes less susceptible to decomposition. However, another decomposition pathway was proposed for this catalytic system due to the observation of Re(O)(3)((MesNCH(2))(MesNCH)NC(5)H(3)) 8 in which one arm of the DAP ligand is oxidized. 相似文献
110.
A hybrid shuffled complex evolution approach based on differential evolution for unconstrained optimization 总被引:1,自引:0,他引:1
Viviana Cocco Mariani Luiz Guilherme Justi Luvizotto Leandro dos Santos Coelho 《Applied mathematics and computation》2011,217(12):5822-5829
Numerous optimization methods have been proposed for the solution of the unconstrained optimization problems, such as mathematical programming methods, stochastic global optimization approaches, and metaheuristics. In this paper, a metaheuristic algorithm called Modified Shuffled Complex Evolution (MSCE) is proposed, where an adaptation of the Downhill Simplex search strategy combined with the differential evolution method is proposed. The efficiency of the new method is analyzed in terms of the mean performance and computational time, in comparison with the genetic algorithm using floating-point representation (GAF) and the classical shuffled complex evolution (SCE-UA) algorithm using six benchmark optimization functions. Simulation results and the comparisons with SCE-UA and GAF indicate that the MSCE improves the search performance on the five benchmark functions of six tested functions. 相似文献