全文获取类型
收费全文 | 4855篇 |
免费 | 235篇 |
国内免费 | 10篇 |
专业分类
化学 | 3562篇 |
晶体学 | 18篇 |
力学 | 99篇 |
数学 | 749篇 |
物理学 | 672篇 |
出版年
2024年 | 5篇 |
2023年 | 30篇 |
2022年 | 46篇 |
2021年 | 70篇 |
2020年 | 79篇 |
2019年 | 81篇 |
2018年 | 93篇 |
2017年 | 76篇 |
2016年 | 169篇 |
2015年 | 151篇 |
2014年 | 164篇 |
2013年 | 301篇 |
2012年 | 351篇 |
2011年 | 413篇 |
2010年 | 220篇 |
2009年 | 203篇 |
2008年 | 321篇 |
2007年 | 315篇 |
2006年 | 258篇 |
2005年 | 252篇 |
2004年 | 233篇 |
2003年 | 184篇 |
2002年 | 158篇 |
2001年 | 111篇 |
2000年 | 120篇 |
1999年 | 91篇 |
1998年 | 62篇 |
1997年 | 25篇 |
1996年 | 55篇 |
1995年 | 36篇 |
1994年 | 33篇 |
1993年 | 36篇 |
1992年 | 36篇 |
1991年 | 40篇 |
1990年 | 31篇 |
1989年 | 18篇 |
1988年 | 16篇 |
1987年 | 21篇 |
1986年 | 21篇 |
1985年 | 23篇 |
1984年 | 25篇 |
1983年 | 9篇 |
1982年 | 24篇 |
1981年 | 14篇 |
1980年 | 6篇 |
1979年 | 9篇 |
1978年 | 10篇 |
1977年 | 13篇 |
1976年 | 11篇 |
1975年 | 9篇 |
排序方式: 共有5100条查询结果,搜索用时 20 毫秒
951.
Fernández-Navarro JJ Ruiz-Ángel MJ García-Álvarez-Coque MC 《Journal of separation science》2012,35(10-11):1303-1309
The chromatographic behavior of seven tricyclic antidepressants (amitryptiline, clomipramine, doxepin, imipramine, maprotiline, nortryptiline, and trimipramine) was examined with micellar mobile phases containing the nonionic surfactant Brij-35. Acetonitrile-water mixtures were also used for comparison purposes. Tricyclic antidepressants are moderately polar basic drugs, which are positively charged in the usual working pH. This gives rise to a strong association with the alkyl chains and residual ionized silanols in silica-based stationary phases, which is translated in a high consumption of organic solvent to get appropriate retention times. Brij-35 modifies the surface of the stationary phases creating a neutral bilayer that masks silanols and reduces the polarity. Consequently, the retention times are decreased. A simple chromatographic procedure for the control of tricyclic antidepressants in pharmaceutical formulations was developed, using 0.02 M Brij-35 at pH 3 and UV detection. Satisfactory recoveries were achieved, with intra- and inter-day relative standard deviations usually below 1 and 2%, respectively. The preparation of the samples was simple and only required solubilization and filtration steps previous to injection. The proposed procedure has the advantage of not using an organic solvent in the mobile phase, and the biodegradable character of Brij-35. This makes an example of "green" liquid chromatographic analysis. 相似文献
952.
Iparraguirre A Navarro P Prieto A Rodil R Olivares M Fernández LÁ Zuloaga O 《Analytical and bioanalytical chemistry》2012,402(9):2897-2907
Membrane-assisted solvent extraction coupled to large volume injection in a programmable temperature vaporisation injector
using gas chromatography–mass spectrometry analysis was optimised for the simultaneous determination of a variety of endocrine
disrupting compounds in environmental water samples (estuarine, river and wastewater). Among the analytes studied, certain
hormones, alkylphenols and bisphenol A were included. The nature of membranes, extraction solvent, extraction temperature,
solvent volume, extraction time, ionic strength and methanol addition were evaluated during the optimisation of the extraction.
Matrix effects during the extraction step were studied in different environmental water samples: estuarine water, river water
and wastewater (influent and effluent). Strong matrix effects were observed for most of the compounds in influent and effluent
samples. Different approaches were studied in order to correct or minimise matrix effects, which included the use of deuterated
analogues, matrix-matched calibration, standard addition calibration, dilution of the sample and clean-up of the extract using
solid-phase extraction (SPE). The use of deuterated analogues corrected satisfactorily matrix effect for estuarine and effluent
samples for most of the compounds. However, in the case of influent samples, standard addition calibration and dilution of
the sample were the best approaches. The SPE clean-up provided similar recoveries to those obtained after correction with
the corresponding deuterated analogue but better chromatographic signal was obtained in the case of effluent samples. Method
detection limits in the 5–54 ng L−1 range and precision, calculated as relative standard deviation, in the 2–25% range were obtained. 相似文献
953.
J. J. Baeza-Baeza Y. Dávila J. J. Fernández-Navarro M. C. García-álvarez-Coque 《Analytical and bioanalytical chemistry》2012,404(10):2973-2984
Two approaches are proposed to measure the effect of different experimental factors (such as the modifier concentration and temperature) on the elution strength and peak shape in reversed-phase liquid chromatography, which quantify the percentage change in the retention factor and peak width (referred to the weakest conditions) per unit change in the experimental factor. The approaches were applied to the separation of a set of flavonoids with aqueous micellar mobile phases of the surfactant Brij-35 (polyoxyethylene(23)dodecanol), in comparison with acetonitrile–water mixtures, using an Eclipse XDB-C18 column. The particular interaction of each flavonoid with the oxyethylene chains of Brij-35 molecules (adsorbed on the stationary phase or forming micelles) changed the elution window, distribution of chromatographic peaks and partitioning kinetics, depending on the hydroxyl substitution in the aromatic rings of flavonoids. At 25?°C, peak shape with Brij-35 mobile phases was significantly poorer with regard to acetonitrile–water mixtures. At increasing temperature, the efficiency of Brij-35 increased, approaching at 80?°C the values obtained at equilibrium conditions, already reached with acetonitrile at 25?°C. 相似文献
954.
955.
Beatriz Martín‐García Prof. M. Mercedes Velázquez Dr. Francesco Rossella Prof. Vittorio Bellani Prof. Enrique Diez Prof. Jose Luis García Fierro Jose Antonio Pérez‐Hernández Juan Hernández‐Toro Sergi Claramunt Prof. Albert Cirera 《Chemphyschem》2012,13(16):3682-3690
Films of a few layers in thickness of reduced graphite oxide (RGO) sheets functionalized by the zwitterionic surfactant N‐dodecyl‐N,N‐dimethyl‐3‐ammonio‐1‐propanesulfonate (DDPS) are obtained by using the Langmuir–Blodgett method. The quality of the RGO sheets is checked by analyzing the degrees of reduction and defect repair by means of X‐ray photoelectron spectroscopy, atomic force microscopy (AFM), field‐emission scanning electron microscopy (SEM), micro‐Raman spectroscopy, and electrical conductivity measurements. A modified Hummers method is used to obtain highly oxidized graphite oxide (GO) together with a centrifugation‐based method to improve the quality of GO. The GO samples are reduced by hydrazine or vitamin C. Functionalization of RGO with the zwitterionic surfactant improves the degrees of reduction and defect repair of the two reducing agents and significantly increases the electrical conductivity of paperlike films compared with those prepared from unfunctionalized RGO. 相似文献
956.
Vidal-Iglesias FJ Arán-Ais RM Solla-Gullón J Garnier E Herrero E Aldaz A Feliu JM 《Physical chemistry chemical physics : PCCP》2012,14(29):10258-10265
The electrocatalytic properties of palladium nanocubes towards the electrochemical oxidation of formic acid were studied in H(2)SO(4) and HClO(4) solutions and compared with those of spherical Pd nanoparticles. The spherical and cubic Pd nanoparticles were characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The intrinsic electrocatalytic properties of both nanoparticles were shown to be strongly dependent on the amount of metal deposited on the gold substrate. Thus, to properly compare the activity of both systems (spheres and nanocubes), the amount of sample has to be optimized to avoid problems due to a lower diffusion flux of reactants in the internal parts of the catalyst layer resulting in a lower apparent activity. Under the optimized conditions, the activity of the spheres and nanocubes was very similar between 0.1 and 0.35 V. From this potential value, the activity of the Pd nanocubes was remarkably higher. This enhanced electrocatalytic activity was attributed to the prevalence of Pd(100) facets in agreement with previous studies with Pd single crystal electrodes. The effect of HSO(4)(-)/SO(4)(2-) desorption-adsorption was also evaluated. The activity found in HClO(4) was significantly higher than that obtained in H(2)SO(4) in the whole potential range. 相似文献
957.
Cabeza JA García-Álvarez P Pérez-Carreño E Pruneda V 《Dalton transactions (Cambridge, England : 2003)》2012,41(15):4313-4315
The C-alkyl groups of C-alkylpyrazinium-derived ligands have been selectively deprotonated by K[N(SiMe(3))(2)], through charge-controlled processes, to give neutral products that contain C-alkylidenepyrazine-derived ligands. 相似文献
958.
The thermal isomerization reaction converting previtamin D to vitamin D is an intramolecular [1,7]-sigmatropic hydrogen shift with antarafacial stereochemistry. We have studied the dynamics of this reaction by means of the variational transition-state theory with multidimensional corrections for tunneling in both gas-phase and n-hexane environments. Two issues that may have important effects on the dynamics were analyzed in depth, i.e., the conformations of previtamin D and the quantum effects associated with the hydrogen-transfer reaction. Of the large number of conformers of previtamin D that were located, there are 16 that have the right disposition to react. The transition-state structures associated with these reaction paths are very close in energy, so all of them should be taken into account for an accurate calculation of both the thermal rate constants and the kinetic isotope effects. This issue is particularly important because the contribution of each of the reaction paths to the total thermal rate constant is quite sensitive to the environment. The dynamics results confirm that tunneling plays an important role and that model systems that were considered previously to study the hydrogen shift reaction cannot mimic the complexity introduced by the flexibility of the rings of previtamin D. Finally, the characterization of the conformers of both previtamin D and vitamin D allowed the calculation of the thermal equilibrium constants of the isomerization process. 相似文献
959.
Three different strategies for cucurbit[8]uril immobilization on a glassy carbon electrode have been assayed. The electrochemical properties of the resulting modified electrodes in solutions containing neutral, positively and negatively charged potential cucurbit[8]uril guests were investigated by cyclic voltammetry and electrochemical impedance spectroscopy. The comparison of the electrochemical behaviour exhibited by the unmodified electrodes against various probes, with respect to that of each modified electrode, resulted in an appropriate method to choose among different strategies for the development of electrochemical sensors. These sensors are based on the incorporation of the cucurbit[8]uril molecular selection properties that depend on the chemical characteristics of the potential analytes. Furthermore, atomic force microscopy was employed for the characterization of the different surfaces developed. 相似文献
960.
Teresa de Haro Prof. Dr. Enrique Gómez‐Bengoa Dr. Riccardo Cribiú Dr. Xiaogen Huang Prof. Dr. Cristina Nevado 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(22):6811-6824
The late transition metal catalyzed rearrangement of propargyl acetates offers an interesting platform for the development of synthetically useful transformations. We have recently shown that gold complexes can catalyze a highly selective tandem 1,2‐/1,2‐bis‐acetoxy migration in 1,4‐bis‐propargyl acetates to form 2,3‐bis‐acetoxy‐1,3‐dienes. In this way, (1Z,3Z)‐ or (1Z,3E)‐ and (1E,3Z)‐1,3‐dienes could be obtained in a stereocontrolled manner depending on the electronic and steric features of the ancillary ligand bound to gold and the substituents at the propargylic positions. In this work, we report an experimental study on the scope of this transformation, plus a detailed theoretical examination of the reaction mechanism, which has revealed the key features responsible for the reaction stereoselectivity. Synthetic applications towards the one‐pot synthesis of quinoxaline heterocycles and tandem Diels–Alder processes have also been devised. 相似文献