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141.
Back Cover: Metal‐Directed Self‐Assembly of a Polyoxometalate‐Based Molecular Triangle: Using Powerful Analytical Tools to Probe the Chemical Structure of Complex Supramolecular Assemblies (Chem. Eur. J. 52/2015) 下载免费PDF全文
142.
Acyl‐Chain Elongation Drives Ketosynthase Substrate Selectivity in trans‐Acyltransferase Polyketide Synthases 下载免费PDF全文
Matthew Jenner Dr. José Pedro Afonso Hannah R. Bailey Sarah Frank Dr. Annette Kampa Prof. Dr. Jörn Piel Dr. Neil J. Oldham 《Angewandte Chemie (International ed. in English)》2015,54(6):1817-1821
Type I modular polyketide synthases (PKSs), which are responsible for the biosynthesis of many biologically active agents, possess a ketosynthase (KS) domain within each module to catalyze chain elongation. Acylation of the KS active site Cys residue is followed by transfer to malonyl‐ACP to yield an extended β‐ketoacyl chain (ACP=acyl carrier protein). To date, the precise contribution of KS selectivity in controlling product fidelity has been unclear. Six KS domains from trans‐acyltransferase (trans‐AT) PKSs were subjected to a mass spectrometry based elongation assay, and higher substrate selectivity was identified for the elongating step than in preceding acylation. A close correspondence between the observed KS selectivity and that predicted by phylogenetic analysis was seen. These findings provide insights into the mechanism of KS selectivity in this important group of PKSs, can serve as guidance for engineering, and show that targeted mutagenesis can be used to expand the repertoire of acceptable substrates. 相似文献
143.
Jessica L��pez-Darias Ver��nica Pino Juan H. Ayala Ana M. Afonso 《Mikrochimica acta》2011,174(3-4):213-222
We have evaluated an in-situ ionic liquid-dispersive liquid-liquid microextraction procedure for the determination of six endocrine disrupting phenols in seawaters and industrial effluents using HPLC. The optimized method requires 38???L of the water-soluble ionic liquid 1-butyl-3-methylimidazolium chloride, and 5?mL of seawater or industrial effluent. After appropriate work-up, a drop (~10???L) of an ionic liquid is formed that contains the analytes of interest. It is diluted with acetonitrile and injected into the HPLC system. This procedure is accomplished without heating or cooling the solutions. The method is characterized by (a) average relative recoveries of 90.2%, (b) enrichment factors ranging from 140 to 989, and (c) precisions (expressed as relative standard deviations) of less than 11% when using a spiking level of 10?ng?mL?1. The limits of detection range from 0.8?ng?mL?1 for 4-cumylphenol to 4.8?ng?mL?1 for bisphenol-A. Figure
Scheme of the in situ IL-DLIME procedure to determine endocrine disrupting phenols in environmental waters. 相似文献
144.
The content of ten phenolic compounds present in four different biomass smoke materials: rock rose (Cistus monpelienisis), prickly pear (Opuntia ficus indica), pine needles (Pinus canariensis), and almonds skin (Prunus dulcis), have been evaluated. The sampling method mainly consisted of a trap alkaline solution to solubilize the phenols, and was optimized by an experimental design. Average sampling efficiencies of 78.1% and an average precision value of 10.6% (as relative standard deviation, RSD), were obtained for the selected group of phenols. The trapped phenolates were further analyzed by a headspace-single drop microextraction (HS-SDME) procedure, in combination with high-performance liquid chromatography (HPLC) with UV detection. The optimum variables for the HS-SDME method were: 1-decanol as extractant solvent, 3.5 μL of microdrop volume, 2 mL of sample volume, a pH value of 2, saturation of NaCl, an extraction temperature of 60 °C, and an extraction time of 25 min. The optimized HS-SDME method presented detection limits ranging from 0.35 to 5.8 μg mL−1, RSD values ranging from 0.7 to 7.4%, and an average relative recovery (RR) of 99.8% and an average standard deviation of 5.2. The average content of phenolic compounds in the biomass materials studied were 70, 161, 206 and 252 mg kg−1 of biomass for prickly pear, almonds skin, rock rose, and pine needles, respectively. The main components of the smokes were vanillin, phenol and methoxyphenols, in all smoking materials studied. 相似文献
145.
Afonso ML Silva RA Matos M Viana AS Montemor MF Almeida M 《Langmuir : the ACS journal of surfaces and colloids》2012,28(10):4883-4888
The first stages of the electrocrystallization of (Per)(2)[Au(mnt)(2)] salt from dichloromethane on gold, platinum, and highly orientated pyrolytic graphite (HOPG) were investigated by cyclic voltammetry, atomic force microscopy, and X-ray photoelectron spectroscopy in order to understand the determinant factors for nucleation and crystal growth. The crystal growth occurs from adsorbed films of dithiolate on gold or platinum and of perylene on HOPG, after homogeneous nucleation, and it is controlled by the low diffusion of the species toward the growing surface. 相似文献
146.
Mateus de Freitas Virgílio Pereira José Manoel Balthazar Davi Antônio dos Santos Angelo Marcelo Tusset Davi Ferreira de Castro Igor Afonso Acampora Prado 《Nonlinear dynamics》2017,87(3):1653-1666
This paper presents a polynomial chaos-based framework for designing optimal linear feedback control laws for nonlinear systems with stochastic parametric uncertainty. The spectral decomposition of the original stochastic dynamical model in an orthogonal polynomial basis, prescribed by the Wiener–Askey scheme, provides a deterministic model from which the optimal linear control law is designed. Optimality of the proposed control law is proved by solving the Hamilton–Jacobi–Bellman equation, and asymptotic stability of the controlled nonlinear systems is guaranteed in the Lyapunov sense. We are especially interested in synchronization of chaotic systems. For this reason, the control strategy is applied in the trajectory tracking of periodic orbits for the Duffing oscillator and the Rössler system with uncertain stochastic parameters and initial conditions. The results are verified with Monte Carlo simulations. 相似文献
147.
A.S. Gouveia-Neto L.A. Bueno A.C.M. Afonso J.F. Nascimento E.B. Costa Y. Messaddeq S.J.L. Ribeiro 《Journal of Non》2008,354(2-9):509-514
Energy-transfer excited upconversion luminescence in Ho3+/Yb3+- and Tb3+/Yb3+-codoped PbGeO3–PbF2–CdF2 glass and glass–ceramic under infrared excitation is investigated. In Ho3+/Yb3+-codoped samples, green (545 nm), red (652 nm), and near-infrared (754 nm) upconversion emission corresponding to the 5S2(5F4) → 5I8, 5F5 → 5I8, and 5S2(5F4) → 5I7 transitions, respectively, was observed. Blue (490 nm) emission assigned to the 5F2,3 → 5I8 transition was also detected. In the Tb3+/Yb3+-codoped system, bright UV–visible emission around 384, 415, 438, 473–490, 545, 587, and 623 nm, identified as due to the 5D3(5G6) → 7FJ(J = 6, 5, 4) and 5D4 → 7FJ(J = 6, 5, 4, 3) transitions, was measured. The comparison of the upconversion process in glass ceramic and its glassy precursor revealed that the former samples present much higher upconversion efficiencies. The dependence of the upconversion emission upon pump power, and doping contents was also examined. The results indicated that successive energy-transfer between ytterbium and holmium ions and cooperative energy-transfer between ytterbium and terbium ions followed by excited-state absorption are the dominant upconversion excitation mechanisms herein involved. The viability of using the samples for three-dimensional solid-state color displays is also discussed. 相似文献
148.
Eurico J. S. Cabrita António G. O. Santos Carlos A. M. Afonso 《Phosphorus, sulfur, and silicon and the related elements》2013,188(1)
Abstract The imide-amide rearrangement of cyclic phosphorirnidates 1 leads to the formation of phosphoramidates 3, either thermally (> 180°C) or by the action of a Lewis acid (BF3 OEt2, 60°C)1. In an attempt to develop a synthetic pathway to β aminoalcohols based on this rearrangement, we have thoroughly studied the influence of the structure of the reactants on the Lewis acid catalysed rearrangement. This study brought some insight into the mechanism of this particular reaction, namely the identification of a new polymeric structure 2. not yet described, which is converted into 3 by thermolysis2. The formation of 2 at rt was followed by 3 P NMR. In the case of R′? Ph, the reactivity observed for R2N was: i-Pr2N > Pyrrolidine > n-Bu2N For R2=i-Pr2, the observed R′ group effect was: Bn > 4-NO2Bn > Ph. No reaction was observed for R′ ? Ts, n-Oct. In the case of the groups R′ ? Bn and R2=i Pr2 the Lewis acid effect was also studied: BF3OEt2, BF3MeOr-Bu, Sc(OTf)3 or CITi(Oi-Pr)3 or Me2SnCl2. 相似文献
149.
150.
F. Vega R. Serna J. Solís C. N. Afonso 《Applied Physics A: Materials Science & Processing》1994,58(4):417-421
Laser-induced oxidation of single-crystalline antimony and polycrystalline Sb films is studied. The samples are irradiated in an oxygen atmosphere (1.2 bar) by means of UltraViolet (UV) laser pulses while reflectivity measurements are used to monitor the oxidation process in real time. It is shown that there is an optical coupling between the growing oxide layer and the bulk material underneath which produces a dynamical change of the optical properties and leads to a non-constant growth rate. Depending on the laser energy density used, there is a critical oxide thickness above which a material-loss process starts, limiting the ultimate growth of the oxide layer. 相似文献