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101.
As previously reported for for solventless reactions, gold nanoparticles supported on ceria are also excellent general heterogeneous catalysts for the aerobic oxidations of alcohols in organic solvents. Among organic solvents it was found that toluene is a convenient one. A systematic study on the influence of the particle size and gold content on the support has established that the activity correlates linearly with the total number of external gold atoms, and with the surface coverage of the support. Amongst catalysts with different supports, but similar gold particle size and content, gold on ceria exhibits the highest activity. By means of a kinetic study (influence of sigma+ parameter, kinetic isotopic effect, temperature, alcohol concentration and oxygen pressure) a mechanistic proposal consisting of the formation of metal-alcoholate, beta-hydride shift from carbon to metal and M--H oxidation has been proposed that explains all experimental results. 相似文献
102.
The application of a general synthetic approach to prepare molecular chains is reported. It is based on a step-by-step method each consisting first in a Pd-catalyzed reaction between ArI and HXAr′Br (Ar=aryl, Ar′=arylene) to give ArXAr′Br followed by a Cu-catalyzed replacement of Br by I to give ArXAr′I that can be reacted with HXAr′Br in the following step. The application of this method is here illustrated to prepare phenylene sulfide oligomers (X=S). Starting from RC6H4I-4 (R=H, MeO, NO2, NH2) and HSC6H4Br-x (x=2, 4) it is possible to grow chains in one direction to give X(C6H4S-m)nC6H4R-4 (n=1, X=Br, m=4, R=H, MeO, NO2, NH2, SMe and m=2, R=H, MeO, NO2; n=1, X=I, m=2 or 4, R=H, MeO, NO2; n=2, X=Br, m=2 or 4, R=H, MeO, NO2; n=2, X=I, m=4, R=MeO, NO2; n=3, X=Br, m=4, R=MeO, NO2; n=3, X=I, m=4, R=NO2 and n=4, X=Br or I, m=4, R=NO2). From HSC6H4Br-x and IC6H4I-4 the chains can grow in two directions to give X(C6H4S-4)nC6H4X-4 (n=2 or 4, X=Br or I), 2-XC6H4(SC6H4-4)nSC6H4X-2 (n=3 or 5, X=Br). Using diiodomesitylene the dithioethers C6HMe3-2,4,6-(SC6H4X-4)2-1,3 (X=Br, I) have been prepared. The series of sulfoxides X(C6H4S(O)-4)nC6H4R-4 (X=Br, n=1, R=MeO, n=3, R=NO2, n=4, R=Br; X=R=I, n=2) has been obtained from the corresponding thioethers and PhICl2. 相似文献
103.
García-Lopera R Codoñer A Bañó MC Abad C Campos A 《Journal of chromatographic science》2005,43(5):226-234
The characterization of polymers by size-exclusion chromatography basically consists of the determination of the weight-average molar mass (Mw), number-average molar mass (Mn), and polydispersity index (I). An accurate estimation of these magnitudes requires the use of a reliable and trusted calibration curve. Three procedures for building up a calibration curve are analyzed in this work. The first is the classical universal calibration (UC), based on the elution of tetrahydrofuran-polystyrene in a system as reference. The second is based on the proper calibration curve made with standards of the sample under study. However, two main drawbacks arise when using these methodologies: the nonfulfilment of the UC when secondary mechanisms, other than pure size-exclusion, are present in the separation process; and the lack of a broad set of narrow standards of the sample under analysis in the second procedure. In order to circumvent these difficulties, a third, recently-proposed approach based on fractal considerations is applied. The accuracy and reliability of this method is proven through the calculation of the deviations observed in the estimation of the Mw values for polymer samples in different solvent-gel chromatographic systems. Whereas the classical UC shows a mean deviation of approximately 80% relative to the values given by the manufacturer, the fractal calibration yields a mean deviation of approximately 16%, similar to that obtained from the proper calibration. Moreover, the fractal procedure only needs one polymeric sample to generate the calibration curve. 相似文献
104.
105.
Concejero M Ramos L Jiménez B Gómara B Abad E Rivera J González MJ 《Journal of chromatography. A》2001,917(1-2):227-237
Feasibility of several sorbents, Amoco PX-21, Carbosphere, Carbopack B and C and 2-(1-pyrenyl) ethyldimethylsilylated silica gel (PYE), for the fractionation of polychlorinated biphenyls, polychlorinated dibenzo-p-dioxins and furans was investigated. Selection was based on their suitability for quantitative isolation of the target compounds with special attention for the most toxic mono- and non-ortho-CBs to prevent coelution with other congeners during the final determination by gas chromatography. Cost-effectiveness in terms of solvent and time consumption as well as feasibility for routine analysis and automation were considered additional merits of the methods compared. Final evaluation of the procedures providing the best results was done by comparison of the results obtained from the analysis of real-life samples. The results showed that, among the sorbents tested, Carbopack B and PYE were the most suitable for routine analysis. In particular, these sorbents allowed a more reliable determination of the toxic congeners and, consequently, of the toxic equivalents of 2,3,7,8-tetrachlorodibenzo-p-dioxin content in environmental samples. 相似文献
106.
Abad A Moreno MJ Pelegrí R Martínez MI Sáez A Gamón M Montoya A 《Journal of chromatography. A》1999,833(1):3-12
Carbaryl, carbofuran and methiocarb are three of the most important N-methylcarbamate pesticides. In the present work, the application of laboratory-developed monoclonal antibody-based enzyme-linked immunosorbent assays (ELISAs) to the determination of these compounds in fruits and vegetables is described. Cucumbers and strawberries were spiked with the three carbamates at 10, 50 and 200 ppb. After extraction and clean-up, samples were analyzed by immunoassay and by HPLC with post-column derivatization and fluorescence detection (US Environmental Protection Agency Method 531.1). Results obtained by ELISA correlated well with those obtained by HPLC, both in terms of accuracy and precision. Recoveries were in the 60-90% range by ELISA and in the 50-90% range by HPLC, depending on the particular combination of commodity, pesticide, and fortification level under consideration. ELISAs were also applied to the analysis of non-purified sample extracts with excellent results: recoveries close to 100% were obtained, while maintaining similar precision values. This approach avoids the use of solid-phase extraction columns, saves time, and considerably increases the sample throughput. Results clearly indicate that the developed immunoassays may be suitable for the quantitative and reliable determination of carbaryl, carbofuran and methiocarb in fruits and vegetables even without including clean-up steps. These considerations make these ELISAs very useful analytical tools for monitoring and regulatory programs, without the need of complex and expensive instrumentation. 相似文献
107.
108.
Abad S Nolis P Gispert JD Spengler J Albericio F Rojas S Herance JR 《Chemical communications (Cambridge, England)》2012,48(49):6118-6120
Fast cysteine labelling of peptides promoted by an adjacent arginine has been observed with a standard labelling agent specific for amines, N-succinimidyl 4-[(18)F]fluorobenzoate. 相似文献
109.
Gold nanoparticles supported on nanocrystalline ceria has been found to be more active and chemoselective than palladium and gold(core)-palladium(shell) nanoparticles for the aerobic oxidation of allylic alcohols. 相似文献
110.
Péres VF Saffi J Melecchi MI Abad FC Martinez MM Oliveira EC Jacques RA Caramão EB 《Journal of chromatography. A》2006,1105(1-2):148-153
Piperaceae family is original from tropical regions and it shows more than 700 species around the world. Piper gaudichaudianum Kunth is the specie more abundant in Brazil, occurring from Northeast to South Brazil. In this paper, it was investigated the influence of some experimental parameters on the pressurized liquid extraction (PLE) of P. gaudichaudianum Kunth leaves, using petroleum ether as extractor solvent. The optimization of the main variables involved in the PLE process (extraction temperature and time) has been done by response surface methodology (RSM) using, as responses, the extraction yield and the chromatographic profile (GC/MS) of the extracts. The optimized procedure employed 3 g of ground leaves, 10 min of extraction and one cycle of extraction at 85 degrees C. The major compounds present in the petroleum ether extracts were: palmitic acid, stearic acid and nerolidol. The results presented in this work show the possibility of using a fast and easy process to recover compounds from P. gaudichaudianum Kunth. 相似文献