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211.
Liu C Walter D Neuhauser D Baer R 《Journal of the American Chemical Society》2003,125(46):13936-13937
Crown ethers have the remarkable property of recognizing and binding specific metal cations in complex mixtures. We propose to combine molecular recognition with molecular electric conductance. The question we address is: can the event of binding a cation be sensed by a change in conductance? Specifically, we study a short molecular wire (MW) containing a crown-6 molecule connected via sulfur atoms to two gold atomic wires acting as metallic leads. Upon binding a cation, the density of states of the system is only slightly affected. This reflects the fact that the cation binding is largely electrostatic in nature and is accompanied by little electronic reorganization. Yet, the cationic binding does significantly lower conductance. We also identify strong interference affecting the conductance. A striking feature is the insensitivity of conductance to the type of ligand with the exception of the proton. 相似文献
212.
The graphical unitary group approach has been applied in an efficient implementation of a general multireference configuration interaction (MRCI) method for use with small active molecular orbital spaces in a semiempirical framework. Gradients can be computed analytically for molecular orbitals from a closed-shell or a half-electron open-shell Hartree-Fock calculation. CPU times for single point energy and gradient calculations are reported. The code allows MRCI geometry optimizations of large molecules, as illustrated for the singlet ground state and the four lowest triplet states of fullerene C(76). 相似文献
213.
Wan H Holmén AG Wang Y Lindberg W Englund M Någård MB Thompson RA 《Rapid communications in mass spectrometry : RCM》2003,17(23):2639-2648
A high-throughput pKa screening method based on pressure-assisted capillary electrophoresis (CE) and mass spectrometry (MS) is presented. Effects of buffer type and ionic strength on sensitivity and pKa values were investigated. Influence of dimethyl sulfoxide (DMSO) concentration present in the sample on effective mobility measurement was examined. A series of ten volatile buffers, covering a pH range from 2.5 to 10.5 with the same ionic strength, was employed. The application of volatile background electrolytes resulted in significant signal increase as compared with commonly used non-volatile phosphate buffers. In general, the CE/MS system provided a ten-fold higher sensitivity than conventional UV detection. The newly developed CE/MS method offers high-throughput capacity by pooling a number of compounds into a single sample. Simultaneous measurement of more than 50 compounds was readily achieved in less than 150 min. The measured pKa values are consistent with the published data obtained from the CE/UV method and are also in good agreement with data generated by other methods. Other advantages of using CE/MS for pKa screening are illustrated with typical examples, including poorly soluble compounds and non-UV-absorbing compounds. 相似文献
214.
Prashanth W. Menezes Carsten Walter Jan Niklas Hausmann Rodrigo Beltrn‐Suito Christopher Schlesiger Sebastian Praetz Valeriy Yu. Verchenko Andrei V. Shevelkov Matthias Driess 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(46):16722-16727
Erstmals wurden, in einen intermetallischen Vorläuferansatz, durch In‐situ‐Elektrokonversion von Mangangallid (MnGa4) hochleistungsfähige und langzeitstabile MnOx‐basierte Elektrokatalysatoren für die Wasseroxidation in alkalischem Medium hergestellt. Überraschend führt seine Elektrokorrosion, unter gleichzeitigem Verlust von Ga, gleichzeitig zu drei kristallinen Typen von MnOx‐Mineralien mit verschiedenen Strukturen und induzierten Defekten: Birnessit δ‐MnO2, Feitknechtit β‐MnOOH und Hausmannit α‐Mn3O4. Das Vorkommen und die intrinsische Stabilität von aktiven MnIII/MnIV‐Zentren in den drei gebildeten MnOx‐Phasen erklärt die hervorragende Effizienz und Stabilität des Systems für die elektrokatalytische Wasseroxidation. Nach der elektrophoretischen Abscheidung des MnGa4‐Vorläufers auf elektrisch leitfähigem Nickelschaum wurde ein niedriges Überpotential von 291 mV bei der Stromdichte von 10 mA cm?2 erreicht, das praktisch den Überpotentialen von edelmetallbasierten Katalysatoren entspricht und für mehr als fünf Tage beständig ist. 相似文献
215.
Kristina O. Kvashnina Anna Yu. Romanchuk Ivan Pidchenko Lucia Amidani Evgeny Gerber Alexander Trigub Andre Rossberg Stephan Weiss Karin Popa Olaf Walter Roberto Caciuffo Andreas C. Scheinost Sergei M. Butorin Stepan N. Kalmykov 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(49):18044-18044
216.
Larissa Canilha Walter Carvalho Maria das Graças de Almeida Felipe João Batista de Almeida e Silva Marco Giulietti 《Applied biochemistry and biotechnology》2010,161(1-8):84-92
The objective of this study was to evaluate the ethanol production from the sugars contained in the sugarcane bagasse hemicellulosic hydrolysate with the yeast Pichia stipitis DSM 3651. The fermentations were carried out in 250-mL Erlenmeyers with 100 mL of medium incubated at 200 rpm and 30 °C for 120 h. The medium was composed by raw (non-detoxified) hydrolysate or by hydrolysates detoxified by pH alteration followed by active charcoal adsorption or by adsorption into ion-exchange resins, all of them supplemented with yeast extract (3 g/L), malt extract (3 g/L), and peptone (5 g/L). The initial concentration of cells was 3 g/L. According to the results, the detoxification procedures removed inhibitory compounds from the hemicellulosic hydrolysate and, thus, improved the bioconversion of the sugars into ethanol. The fermentation using the non-detoxified hydrolysate led to 4.9 g/L ethanol in 120 h, with a yield of 0.20 g/g and a productivity of 0.04 g L?1 h?1. The detoxification by pH alteration and active charcoal adsorption led to 6.1 g/L ethanol in 48 h, with a yield of 0.30 g/g and a productivity of 0.13 g L?1 h?1. The detoxification by adsorption into ion-exchange resins, in turn, provided 7.5 g/L ethanol in 48 h, with a yield of 0.30 g/g and a productivity of 0.16 g L?1 h?1. 相似文献
217.
Michael Brieseck Walter Lengauer Birgit Gneiß Karin Wagner Stephanie Wagner 《Mikrochimica acta》2010,168(3-4):309-316
Measurement of the grain-size distribution of sintered hardmetals is indispensable in research and production. In this study we present a method consisting of etching hardmetals in order to remove the cobalt binder and to analyse the remaining tungsten-cobalt (WC) crystallites on the basis of scanning electron microscopy images subjected to a PC-based image analysis by use of MATLAB®. Sample preparation is easy and quick to perform, i.e. it does not require metallographic polishing. It consists of either deep etching of the cobalt binder of cut and optionally hand-ground surfaces or etching of fracture surfaces. Two different computational routines and one combination thereof are described. Two hardmetal grades with average WC grain sizes slightly larger as well as smaller than 200 nm were investigated. The results of the computer algorithms are compared to data obtained by manual linear intercept analysis. The analysis has good potential to avoid time-consuming metallographic preparation and pre-treatment for grain-size analysis, which is especially true for hardmetals with extremely fine grain size. 相似文献
218.
Florian?Nettesheim Claus?B.?Müller Ulf?Olsson Walter?RichteringEmail author 《Colloid and polymer science》2004,282(8):918-926
We report on the shear induced transition from the L3- to the L-phase studied by means of flow birefringence using the system pentaethyleneglycol monododecylether (C12E5), decane, water. The dependence of the critical shear rate, at which the transition from the isotropic state to the anisotropic takes place, on membrane volume fraction was studied in temperature ramp experiments at different constant shear rates and in isothermal shear ramp experiments. These results are compared with relaxation experiments from the shear aligned state back to the isotropic. For all these experiments power law exponents in the membrane volume fraction between 1.6 and 2.8 were found, which are rather low compared to values of current theories. The values found for the inverse critical shear rate and the decay times from the relaxation experiments differ by four orders of magnitude. 相似文献
219.
Stefan Furegati Walter Ganci Fabrizio Gorla Urs Ringeisen Peter Rüedi 《Helvetica chimica acta》2004,87(10):2629-2661
Phosphorylation of suitable piperidine precursors yielded a series of novel decalin‐type O,N,P‐heterocycles. The title compounds, P(3)‐axially and P(3)‐equatorially X‐substituted, cis‐ and trans‐configurated 2,4‐dioxa‐7‐aza‐, 2,4‐dioxa‐8‐aza‐, and 2,4‐dioxa‐9‐aza‐3‐phosphabicyclo[4.4.0]decane 3‐oxides (X=Cl, F, 4‐nitrophenoxy, and 2,4‐dinitrophenoxy), are configuratively fixed and conformationally constrained P‐analogues of acetylcholine and as such represent acetylcholine (7‐aza and 9‐aza isomers) or γ‐homo‐acetylcholine mimetics (8‐aza isomers). Being irreversible inhibitors of acetylcholinesterase (AChE), the compounds are considered to be suitable probes for the investigation of the stereochemical course of the inhibition reaction by 31P‐NMR spectroscopy. Moreover, the design of these mimetics will enable studies of molecular interactions with AChE, in particular, the recognition conformation of acetylcholine. 相似文献
220.
Chemical Bonding and Pressure‐Induced Change of the Electron Configuration of Ytterbium in β‐YbAgGa2
Ulrich Schwarz Marcus Schmidt Roman Gumeniuk Walter Schnelle Michael Hanfland Konstantin Klementiev Yuri Grin 《无机化学与普通化学杂志》2004,630(1):122-126
Single‐phase polycrystalline samples of the intermetallic compound β‐YbAgGa2 were synthesized by inductive heating and subsequent annealing for eight weeks at 670 K. Magnetic properties were characterized by susceptibility measurements and indicated intermediate valence of ytterbium at ambient pressure. Angle‐dispersive X‐ray powder diffraction data of orthorhombic β‐YbAgGa2 indicate stability of the phase in the investigated pressure range from 0.1 MPa (ambient pressure) to 19 GPa. The pressure‐induced volume decrease is accompanied by an increase of the effective valence from 2.17 at ambient conditions to 2.71 at 16 GPa as evaluated by X‐ray absorption spectroscopy at the Yb LIII threshold. Analysis of the chemical bonding in β‐YbAgGa2 by integrating the electron density of the polyanion in basins as defined by the electron localization function results in an electron count Yb2.7+[(Ag0.2—)(Ga1(3b)1.0—)(Ga2(4b)1.5—)]. This finding is close to the expected values calculated by means of the Zintl rules and fits well the results of magnetic susceptibility measurements and XAS investigations. 相似文献