全文获取类型
收费全文 | 731篇 |
免费 | 25篇 |
国内免费 | 11篇 |
专业分类
化学 | 497篇 |
力学 | 15篇 |
数学 | 112篇 |
物理学 | 143篇 |
出版年
2023年 | 3篇 |
2022年 | 2篇 |
2021年 | 16篇 |
2020年 | 18篇 |
2019年 | 11篇 |
2018年 | 7篇 |
2017年 | 9篇 |
2016年 | 14篇 |
2015年 | 16篇 |
2014年 | 27篇 |
2013年 | 48篇 |
2012年 | 47篇 |
2011年 | 45篇 |
2010年 | 22篇 |
2009年 | 30篇 |
2008年 | 28篇 |
2007年 | 49篇 |
2006年 | 32篇 |
2005年 | 42篇 |
2004年 | 44篇 |
2003年 | 35篇 |
2002年 | 29篇 |
2001年 | 23篇 |
2000年 | 16篇 |
1999年 | 15篇 |
1998年 | 10篇 |
1997年 | 11篇 |
1996年 | 4篇 |
1995年 | 3篇 |
1994年 | 8篇 |
1993年 | 6篇 |
1992年 | 10篇 |
1991年 | 7篇 |
1990年 | 15篇 |
1989年 | 3篇 |
1988年 | 14篇 |
1987年 | 8篇 |
1986年 | 11篇 |
1985年 | 4篇 |
1984年 | 8篇 |
1983年 | 2篇 |
1982年 | 3篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1976年 | 3篇 |
1973年 | 1篇 |
1968年 | 1篇 |
1908年 | 1篇 |
排序方式: 共有767条查询结果,搜索用时 359 毫秒
721.
Electrochemical hydride generation as a sample-introduction technique in atomic spectrometry: fundamentals,interferences, and applications 总被引:1,自引:0,他引:1
Electrochemical hydride generation (EC-HG) has been proposed as a valid alternative to chemical generation as a sample-introduction
technique in atomic spectrometry. In this review fundamental aspects of the technique are revised, including designs of electrolytic
cells, mechanisms of the generation process, and interferences caused by the presence of different species. Special attention
is paid to the role of the configuration of the cathodes and their materials on the efficiency of hydride generation and on
interferences from concomitant species. An overview of the application of EC-HG to the analysis of real samples is also given. 相似文献
722.
Alves CN Martí S Castillo R Andrés J Moliner V Tuñón I Silla E 《Chemistry (Weinheim an der Bergstrasse, Germany)》2007,13(27):7715-7724
Human immunodeficiency virus type-1 integrase (HIV-1 IN) is an essential enzyme for effective viral replication. Diketo acids such as L-731,988 and S-1360 are potent and selective inhibitors of HIV-1 IN. In this study, we used molecular dynamics simulations, within the hybrid quantum mechanics/molecular mechanics (QM/MM) approach, to determine the protein-ligand interaction energy between HIV-1 IN and L-731,988 and 10 of its derivatives and analogues. This hybrid methodology has the advantage that it includes quantum effects such as ligand polarisation upon binding, which can be very important when highly polarisable groups are embedded in anisotropic environments, as for example in metal-containing active sites. Furthermore, an energy decomposition analysis was performed to determine the contributions of individual residues to the enzyme-inhibitor interactions on averaged structures obtained from rather extensive conformational sampling. Analysis of the results reveals first that there is a correlation between protein-ligand interaction energy and experimental strand transfer into human chromosomes and secondly that the Asn-155, Lys-156 and Lys-159 residues and the Mg(2+) ion are crucial to anti-HIV IN activity. These results may explain the available experimental data. 相似文献
723.
Castillo MR Fousse L Fraile JM García JI Mayoral JA 《Chemistry (Weinheim an der Bergstrasse, Germany)》2007,13(1):287-291
Supported ionic-liquid films (SILF) of nanometric thickness containing bis(oxazoline)-copper complexes can be used as recoverable catalysts for enantioselective cyclopropanation reactions. When a thin film of ionic liquid is supported on a clay, the system behaves as a nearly two-dimensional nanoreactor in which the restrictions in rotational mobility and the close proximity to the surface support produce variations in the stereo- and enantioselectivities, leading to a complete reversal of the overall selectivity of the reaction. As an example, the cyclopropanation of styrene changes from a preference for the (1S,2S)-trans isomer in bulk solution to the (1R,2S)-cis isomer in the SILF. This variation is strongly dependent on both the thickness of the film and the nature of the support. Only layered solids with negative charges in the layers (clays) give rise to this type of behaviour, showing that the formation of ion pairs may be a decisive factor. This unexpected effect also shows the existence of differences between bulk and supported ionic liquids. 相似文献
724.
A rapid and easy method has been proposed, optimized and evaluated for quantitative determination at trace level of a representative group of non-polar pesticides in fat samples. The method includes n-hexane-saturated acetonitrile extraction, fat precipitation by cooling pre clean-up followed by dispersive solid-phase extraction (d-SPE) based on QuEChERS procedure clean-up. Determination was performed by gas chromatography?Cmass spectrometry (GC-MS) in selected ion monitoring (SIM) mode. Efficiency of the d-SPE clean-up step was evaluated by comparison with fat oxidation treatment and gel permeation chromatography. Different combinations of d-SPE extraction reagents and sample amounts were tested in order to minimize matrix co-extractives and interferences. Best recoveries were obtained with 1200?mg of MgSO4, 400?mg of end-capped C18, 400?mg of PSA and 1?g of sample amount. SIM method, matrix effect, precision, and accuracy were evaluated with spiked pork fat samples for 38 representative pesticides. Results of this study showed that this technique is applicable in routine analysis for its application into monitoring programs. It simplifies time-consuming clean-up steps and allows a satisfactory long-term chromatographic performance. 相似文献
725.
726.
We present an experimental and computational study of the reaction of aryl substituted benzyl 1-alkynyl sulfides with potassium alkoxide in acetonitrile, which produces 2-aryl 2,3-dihydrothiophenes in poor to good yields. The cyclization is most efficient with electron withdrawing groups on the aromatic ring. Evidence indicates there is rapid exchange of protons and tautomerism of the alkynyl unit prior to cyclization. Theoretical calculations were also conducted to help rationalize the base induced 5-endo cyclization of benzyl 1-propynyl sulfide (1a). The potential energy surface was calculated for the formation of 2,3-dihydrothiophene in a reaction of benzyl 1-propynyl sulfide (1a) with potassium methoxide. Geometries were optimized with CAM-B3LYP/6-311+G(d,p) in acetonitrile with the CPCM solvent model. It is significant that the benzyl propa-1,2-dien-1-yl sulfane (6) possessed a lower benzylic proton affinity than the benzyl prop-2-yn-1-yl sulfane (8) thus favoring the base induced reaction of the former. From benzyl(propa-1,2-dien-1-yl sulfane (6), 2,3-dihydrothiophene can be formed via a conjugate base that undergoes 5-endo-trig cyclization followed by a protonation step. 相似文献
727.
G.E. Buono-Core A.H. KlahnC. Castillo M.J. BustamanteE. Muñoz G. CabelloB. Chornik 《Polyhedron》2011,30(1):201-206
In this paper non-stoichiometric tungsten oxide thin films have been successfully prepared by direct UV irradiation of bis-β-diketonate dioxotungsten(VI) precursor complexes spin-coated Si(1 0 0) substrates. Photodeposited films were characterized by Fourier transform-infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) and the surface morphology examined by Atomic Force Microscopy (AFM). The results of XRD analysis showed that the as-photodeposited WO3−x films are amorphous and have a rougher surface than thermally treated films. Post-annealing of the films in air at 500 °C transforms the sub-oxides to a monoclinic WO3 phase. 相似文献
728.
J. del Castillo A. C. Yanes J. Méndez-Ramos J. J. Velázquez V. D. Rodríguez 《Journal of Sol-Gel Science and Technology》2011,60(2):170-176
Sol–gel derived glass–ceramics containing CeF3 nanocrystals have been developed for the first time, to the best of our knowledge, by adequate heat treatments of precursor
bulk glasses with composition 95SiO2–5CeF3 doped with 0.1 Eu3+ or 0.1 Sm3+ and co-doped with 0.3 Yb3+ and 0.1 Er3+ ions (in mol%). X-Ray Diffraction and High Resolution Transmission Electron Microscopy confirm the precipitation of CeF3 nanocrystals. Moreover, this structural analysis is completed using Eu3+ and Sm3+ as probe ions of the different local environments for rare-earth ions in the nano-structured glass–ceramics. Luminescence
measurements led us to discern the final environments for the ions, revealing the partition of a large fraction of these ions
into like-crystalline environment of the precipitated CeF3 nanocrystals. Near infrared emission at 1.5 μm was observed after excitation at 980 nm in Yb3+–Er3+ co-doped samples for potential applications in telecommunications. 相似文献
729.
A confirmatory and sensitive procedure has been developed for the determination of 40 currently used pesticides (CUPs) in airborne particulate matter (PM 10) at trace level. The proposed method includes extraction of PM 10-bound pesticides by microwave-assisted extraction (MAE) followed by a gel permeation chromatography (GPC) clean-up and determination by GC-MS/MS. The injection mode and the main parameters in MS/MS were optimized. The matrix effect was also evaluated. Recoveries ranged from 70 to 120% except for pyrimethanil and pirimicarb. The limit of quantification (LOQ) ranged from 1.32 to 39.47 pg m(-3), when air volumes of 760 m(3) were collected. The method was applied to 38 samples collected from a rural station belonging to the atmospheric monitoring network of the Regional Valencia Government (Spain) during April-June 2010. Eighteen out of 40 pesticides investigated were found in at least one sample (bifenthrin, chlorothalonil, chlorpyriphos-e, chlorpyriphos-m, clorpropham, diazinon, dicofol, diphenylamine, fipronil, fludioxonil, folpet, malathion, metalaxyl, penconazole, quinoxyfen, triadimefon, trifluralin, and vinclozoline), with concentrations ranging from 1.32 to 625.80 pg m(-3). 相似文献
730.
Raúl Castillo Pérez Vladislav V. Kravchenko Rabindranath Reséndiz Vázquez 《Mathematical Methods in the Applied Sciences》2011,34(4):455-468
We propose a method for solving boundary value and eigenvalue problems for the elliptic operator D = div p grad + qin the plane using pseudoanalytic function theory and in particular pseudoanalytic formal powers. Under certain conditions on the coefficients p and q with the aid of pseudoanalytic function theory a complete system of null solutions of the operator can be constructed following a simple algorithm consisting in recursive integration. This system of solutions is used for solving boundary value and spectral problems for the operator D in bounded simply connected domains. We study theoretical and numerical aspects of the method. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献