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51.
Wislei?R.?Osório Noé?Cheung José?E.?Spinelli Pedro?R.?Goulart Amauri?GarciaEmail author 《Journal of Solid State Electrochemistry》2007,11(10):1421-1427
Hypoeutectic aluminum–silicon alloys can have significant improvements in mechanical properties by inducing structural modification
in the normally occurring eutectic. The eutectic modification may affect not only the mechanical properties but also the corrosion
resistance of such alloys. It is well known that structural parameters such as grain size and interdendritic spacing can significantly
affect corrosion resistance of alloys. However, to date, few researches have been performed to experimentally evaluate the
effects of an effective modification of eutectic morphology on surface corrosion behavior of Al–Si alloys. In the present
study, modified and unmodified samples of an Al 9 wt.% Si alloy were solidified under similar solidification conditions, and
after metallographic procedures, the corrosion resistance was analyzed by both the electrochemical impedance spectroscopy
technique and the Tafel extrapolation method carried out in a 0.5 M NaCl test solution at 25 °C. The impedance parameters
and corrosion rate were obtained from an equivalent circuit analysis. It was found that the Al-9 wt.% Si alloy casting in
the modified condition tends to have its corrosion resistance decreased when compared to the unmodified alloy. 相似文献
52.
General performance of density functionals 总被引:1,自引:0,他引:1
The density functional theory (DFT) foundations date from the 1920s with the work of Thomas and Fermi, but it was after the work of Hohenberg, Kohn, and Sham in the 1960s, and particularly with the appearance of the B3LYP functional in the early 1990s, that the widespread application of DFT has become a reality. DFT is less computationally demanding than other computational methods with a similar accuracy, being able to include electron correlation in the calculations at a fraction of time of post-Hartree-Fock methodologies. In this review we provide a brief outline of the density functional theory and of the historic development of the field, focusing later on the several types of density functionals currently available, and finishing with a detailed analysis of the performance of DFT across a wide range of chemical properties and system types, reviewed from the most recent benchmarking studies, which encompass several well-established density functionals together with the most recent efforts in the field. Globally, an overall picture of the level of performance of the plethora of currently available density functionals for each chemical property is drawn, with particular attention being dedicated to the relative performance of the popular B3LYP density functional. 相似文献
53.
Shukoor MI Natalio F Tahir MN Ksenofontov V Therese HA Theato P Schröder HC Müller WE Tremel W 《Chemical communications (Cambridge, England)》2007,(44):4677-4679
Polymer coated superparamagnetic gamma-Fe(2)O(3) nanoparticles were derivatized with a synthetic double-stranded RNA [poly(IC)], a known allosteric activator of the latent (2-5)A synthetase, to separate a single 35 kDa protein from a crude extract which cross reacted with antibodies raised against the sponge enzyme. 相似文献
54.
Pereira CC Balula SS Paz FA Valente AA Pillinger M Klinowski J Gonçalves IS 《Inorganic chemistry》2007,46(21):8508-8510
The oxo-bridged dimer [Mo(2)O(4)(mu(2)-O)Cl(2)(pzH)(4)] (1; pzH = pyrazole) exhibits unusually high activity in the liquid-phase catalytic epoxidation of the cyclic olefins cyclooctene and (R)-(+)-limonene under mild conditions and in the absence of additional organic solvents, using tert-butyl hydroperoxide as the oxidant. The complex is stable under the reaction conditions and can be used in further catalytic runs without significant loss of performance. An X-ray crystallographic investigation reveals that 1 has an unprecedented and extremely rare all-cis configuration at each of the MoO(2)-(mu(2)-O)Cl(pzH)(2) cores, which can be understood by considering supramolecular contacts and geometric factors. 相似文献
55.
A. O. Porto G. Goulart Silva W. F. Magalhes 《Journal of Polymer Science.Polymer Physics》1999,37(3):219-226
Positron lifetime measurements were carried out in a series of poly(ethylene oxide)—PEO—of different average molecular weights (M w): 1000, 1500, 6000, 10,000, 300,000, and 4 M. The mean radius (R ) and the mean free volume size (Vf) values were determined using a semiempirical equation that correlates the ortho‐positronium (o‐Ps) lifetime (τ3) and size of holes existing in the amorphous phases. The hole mean radius values determined at room temperature from lifetime spectra were found to be between 2.68 and 2.97 Å, and the hole volumes between 80 and 110 Å3. Free volume size evolution was studied with temperature variation until the melting temperature of the PEO samples. The degree of crystallinity and the melting temperatures were determined by Differential Scanning Calorimetry. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 219–226, 1999 相似文献
56.
Filipe Monteiro-Silva Carla Queirs Andreia Leite María T. Rodríguez María J. Rojo Toms Torroba Rui C. Martins Ana M. G. Silva Maria Rangel 《Molecules (Basel, Switzerland)》2021,26(16)
Functional organic dyes play a key role in many fields, namely in biotechnology and medical diagnosis. Herein, we report two novel 2,3- and 3,4-dihydroxyphenyl substituted rosamines (3 and 4, respectively) that were successfully synthesized through a microwave-assisted protocol. The best reaction yields were obtained for rosamine 4, which also showed the most interesting photophysical properties, specially toward biogenic amines (BAs). Several amines including n- and t-butylamine, cadaverine, and putrescine cause spectral changes of 4, in UV–Vis and fluorescence spectra, which are indicative of their potential application as an effective tool to detect amines in acetonitrile solutions. In the gas phase, the probe response is more expressive for spermine and putrescine. Additionally, we found that methanolic solutions of rosamine 4 and n-butylamine undergo a pink to yellow color change over time, which has been attributed to the formation of a new compound. The latter was isolated and identified as 5 (9−aminopyronin), whose solutions exhibit a remarkable increase in fluorescence intensity together with a shift toward more energetic wavelengths. Other 9-aminopyronins 6a, 6b, 7a, and 7b were obtained from methanolic solutions of 4 with putrescine and cadaverine, demonstrating the potential of this new xanthene entity to react with primary amines. 相似文献
57.
Inês M. V. Silva Fernanda Machado Maria Joo Moreno Cludia Nunes Manuel A. Coimbra Filipe Coreta-Gomes 《Molecules (Basel, Switzerland)》2021,26(15)
Several classes of polysaccharides have been described to have hypocholesterolemic potential, namely cholesterol bioaccessibility and bioavailability. This review will highlight the main mechanisms by which polysaccharides are known to affect cholesterol homeostasis at the intestine, namely the effect (i) of polysaccharide viscosity and its influence on cholesterol bioaccessibility; (ii) on bile salt sequestration and its dependence on the structural diversity of polysaccharides; (iii) of bio-transformations of polysaccharides and bile salts by the gut microbiota. Different quantitative structure–hypocholesterolemic activity relationships have been explored depending on the mechanism involved, and these were based on polysaccharide physicochemical properties, such as sugar composition and ramification degree, linkage type, size/molecular weight, and charge. The information gathered will support the rationalization of polysaccharides’ effect on cholesterol homeostasis and highlight predictive rules towards the development of customized hypocholesterolemic functional food. 相似文献
58.
T. Pinheiro A. Barreiros L. C. Alves M. Neres R. Fleming J. N. Silva P. Filipe R. Silva 《Journal of Radioanalytical and Nuclear Chemistry》2009,281(2):161-164
Skin as a manageable organ can provide direct or indirect information of tissue iron overload resulting from inherited disorders
as hemochromatosis. Patients with hemochromatosis were evaluated at three consecutive phases along the therapy programme.
Nuclear microprobe techniques were used to assess skin iron and Total Reflection X-ray Fluorescence to determine the plasma
iron concentrations. Results showed that iron pools were differently correlated at the three therapy phases. These variations
highlighted the value of skin iron content to assess organ iron deposition and therapy efficacy. Skin iron content can be
used for a better management of patients with iron overload pathologies. 相似文献
59.
An optical fiber biosensor has been developed for the determination of catecholamines (dopamine, norepinephrine and epinephrine) based on the recognition capacity of the enzyme laccase. In this study, a glass tube constituted by a fused silica fiber coated with a film of polystyrene/divinylbenzene resin (PS/DVB) was used for catecholamines separation. Firstly, the analyzer was tested for calibration and its analytical performance for catecholamines detection was compared with a classical analytical method, namely high performance liquid chromatography-electrochemical detector (HPLC-ED). The developed analytical device shows a high potential for catecholamines quantification with a detection limit of 2.1, 2.6 and 3.4 pg mL−1 for dopamine, norepinephrine and epinephrine, respectively. The analytical sensitivity, inferred from the slope of the calibration curves established for a range of concentrations between 5 and 125 pg mL−1, was found to be 0.344, 0.252 and 0.140 dB/pg mL−1 for dopamine, norepinephrine and epinephrine, respectively. Furthermore, catecholamines speciation with the PS/DVB fiber was completely achieved in 3 min. The analytical performance of the reported sensor was also evaluated and found adequate for catecholamines determination in human urine and plasma samples. 相似文献
60.
Phabyanno Rodrigues Lima Paulo Rogério Barbosa de Miranda Adriano Bof de Oliveira Marília Oliveira Fonseca Goulart Lauro Tatsuo Kubota 《Electroanalysis》2009,21(21):2311-2320
The present work explores, for the first time, the electrocatalytic oxidation of ascorbic acid (AscH2) and its determination in the presence of uric acid (UA) on the in situ activated 4‐nitrophthalonitrile modified carbon paste electrode. The kinetic constant κ for the catalytic reaction for the electrocatalytic oxidation of ascorbic acid, evaluated by cyclic voltammetry, chronoamperometry and RDE voltammetry provided values around 106 L mol?1 s?1. The sensor provided a linear response range for AscH2 and UA from 5.0 up to 120.0 μmol L?1 with detection limits of 1.6 μmol L?1and 1.3 μmol L?1, respectively. The sensor was applied for the simultaneous determination of AscH2 and UA in urine samples and the average recoveries for these samples were 99.8 (±3.1)% and 99.9 (±2.1)%, respectively . 相似文献