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51.
Particulates of platinum group elements (Pt, Rh, Pd, Ir) emitted in automotive catalyst exhaust were measured down to the pg/m(3) level. Samples were taken from a standard type three-way catalyst equipped gasoline engine, running on a computer controlled dynamometer. Particulates in catalyzed car exhaust were sampled fractionated according to size by using the cascade impactor with separate targets mounted underneath each nozzle. The targets, small flat discs made of pure graphite, were subsequently analyzed by ETV-ICP-MS without any preparatory steps. 相似文献
52.
Arisaka K Auerbach LB Axelrod S Belz J Biery KA Buchholz P Chapman MD Cousins RD Diwan MV Eckhause M Ginkel JF Guss C Hancock AD Heinson AP Highland VL Hoffmann GW Horvath J Irwin GM Joyce D Kaarsberg T Kane JR Kenney CJ Kettell SH Kinnison WW Knibbe P Konigsberg J Kuang Y Lang K Lee DM Margulies J Mathiazhagan C McFarlane WK McKee RJ Melese P Milner EC Molzon WR Ouimette DA Riley PJ Ritchie JL Rubin P Sanders GH Schwartz AJ Sivertz M Slater WE Urheim J Vulcan WF Wagner DL Welsh RE Whyley RJ 《Physical review letters》1993,71(24):3910-3913
53.
H. Hoffmann C. Thunig D. Miller 《Colloids and surfaces. A, Physicochemical and engineering aspects》2002,210(2-3):147-158
We have studied the phase behavior of N-methyl-N-alkanoylglucamin (GA) in water and the influence of various additives on the phases of GA. We find that GA forms a large L1-phase that extends up to 60 wt.% of surfactant. The viscosity in this phase increases with increasing concentration and decreasing temperature. When solutions are cooled down below the Krafft-temperature (21–28 °C) Tk the samples transform into clear gels that are stable for several months. The transformation of the gel to the L1-phase proceeds in two separate steps. Acid–base titration experiments show that the commercially produced GA is not a pure well defined compound but contains about 10% of an anionic surfactant, most likely dodecanoic acid. Solutions of GA can be continuously mixed with the anionic surfactant sodiumdodecylsulfate (SDS) or sodiumdodecylethoxysulfate (SDES) to clear and low viscous phases. Solutions of GA mixed with increasing amount of cationic surfactant tetradecyltrimethylamoniumbromide (TTABr) transform first into two phase systems and then again into low viscous single phases. The influence of several chemically different co-surfactants like n-alcohols, octanoic acid, oleic acid, 2-ethylhexylmonoglyceride (EHMG) and oleylmonoglyceride on the phase behavior of phases with 5% GA has been studied. With increasing mole fraction of the co-surfactants the well-known sequence of phases is observed, namely a L1-phase, a two phase region L1/L- and a L-phase. However, the properties of the L-phase for the different systems are very different. For samples with octanol the L-phase is an optically isotropic, transparent, highly viscoelastic gel. Without shear the gel like phase shows no birefringence. FF-TEM micrographs show that it consists of rather monodisperse, unilamellar vesicles with a diameter of about 500 Å. The L-phases for the other co-surfactants are turbid and have a rather low viscosity. They also contain vesicles but with a rather broad size distribution ranging from 200 to 1000 Å. For the same co-surfactant/surfactant ratio the various systems differ also in their conductivity. For some systems the conductivity is only about 20% lower than in the corresponding L1-phase while in other systems the difference is more than a factor two. These results are an indication that small uni-lamellar vesicles contain more ionic groups at the outside than on the inside of the bilayer and that some systems are composed of uni-lamellar vesicles while others are composed of multilamellar vesicles (onions). 相似文献
54.
E. Hoffmann 《Fresenius' Journal of Analytical Chemistry》1962,185(5):372-376
Summary The use of chloranilic acid salts is recommended for the micro determination of aluminium salts, sulphides, thiosulphates, cyanides and fluorides. The methods are based on the reaction of an insoluble chloranilate with the ion to be determined to form an insoluble compound of this ion and an equivalent of liberated chloranilic acid which can be measured. The procedures are simple and rapid.
Zusammenfassung Zur Mikrobestimmung von Aluminiumsalzen, Sulfiden, Thiosulfaten, Cyaniden und Fluoriden wird die Verwendung von Salzen der Chloranilsäure empfohlen. Die Methoden beruhen auf der Reaktion unlöslicher Chloranilate mit dem zu bestimmenden Ion unter Bildung einer unlöslichen Verbindung dieses Ions und Freisetzung einer äquivalenten Menge Chloranilsäure, die gemessen wird. Die Bestimmungen lassen sich einfach und schnell durchführen.相似文献
55.
Most reported examples of transition metal-mediated pericyclic reactions have been shown to proceed by nonpericyclic mechanisms. However, the conversion of Fe(CO)(4)-cyclobutenes to Fe(CO)(3)-butadienes and CO is thought to proceed through a mechanism involving an electrocyclic ring-opening step. We have examined this and possible alternative mechanisms for the parent Fe(CO)(4)-cyclobutene complex using hybrid HF-DFT calculations. In this system there is an inherent preference for a pathway that avoids metal-templated electrocyclic ring-opening, but the pericyclic pathway is predicted to dominate for more elaborate substrates and under certain experimental conditions. 相似文献
56.
Prässler F Hoffmann V Schumann J Wetzig K 《Analytical and bioanalytical chemistry》1996,355(7-8):840-846
An algorithm is presented as a concept for the quantification in direct current and radiofrequency glow discharge (GD) modes for GD optical emission spectroscopy. The algorithm is divided into excitation and sputtering part and thus it is possible to distinguish between the different excitation processes and to consider equivalent sputtering crater formations in both modes. Intensity-time profiles are affected corresponding to the method by several effects. One important effect is that sputtering occurs at a single time in different depths because of curved crater bottoms, this is usually called crater effect. The main purpose is to introduce an iterative deconvolution technique which for the quantification numerically takes into account the curved sputtering crater bottom. Input data for the deconvolution technique are the calibrated mass-time profile, the partial densities of the sample constituents and the measured final shape of the sputtering crater. Using a relatively simple model for ion sputtering the deconvolution technique improves iteratively the calculated layer structure by means of information on crater formation. The mathematical handling is illustrated for the quantification of a depth profile of a multilayer sample of ten 100 nm layers. The resulting concentration-depth profile reflects excellently the real elemental distribution of the multilayer system. 相似文献
57.
Regulation of polar cap formation in the life cycle of Escherichia coli 总被引:48,自引:0,他引:48
58.
Stephan Weinbruch Michael Wentzel Manfred Kluckner Peter Hoffmann Hugo M. Ortner 《Mikrochimica acta》1997,125(1-4):137-141
In this paper procedures for the characterization of individual aerosol particles by element mapping in the electron microprobe are presented. The number, size and qualitative chemical composition of particles is derived from a combination of secondary or backscattered electron images and element distribution maps. Accuracy of the size distribution and reliability of the qualitative analysis procedure were checked with silicate samples. In order to obtain a semi-quantitative estimate of the chemical composition of individual particles the count rates taken from element distribution maps are corrected for matrix and geometric effects using particle ZAF procedures.Dedicated to Professor Dr. rer. nat. Dr. h.c. Hubertus Nickel on the occasion of his 65th birthday 相似文献
59.
By means of model calculations it could be shown for an irreversible surface reaction of 1st order that the determination of the activation energy of the desorption of the reactant or, respectively, of the surface reaction is possible by application of the method of variation of the heating rate to the desorption curve of the reactant, according to circumstances whether the ratio of the activation energy of the surface reaction and of the desorption of the reactant is greater or smaller than one.The possibilities of the kinetic evaluation are applied to the isomerization of cyclopropane on a NaX-zeolite catalyst. The resulting heat of adsorption of cyclopropane and the activation energy of the reaction agree well with the values of literature obtained by isothermal measurements in a pulse reactor.
Zusammenfassung Mit Modellrechnungen für eine irreversible Oberflächenreaktion 1. Ordnung konnte gezeigt werden, daß durch Anwendung der Methode der Variation der Heizgeschwindigkeit auf die Desorptionskurve des Ausgangsstoffes die näherungsweise Bestimmung der Aktivierungsenergie der Desorption des Ausgangsstoffes bzw. der Oberflächenreaktion möglich ist, je nachdem ob das Verhältnis der Aktivierungsenergien für die Oberflächenreaktion und der Desorption des Ausgangsstoffes größer oder kleiner als eins ist.Die Möglichkeiten der kinetischen Auswertung werden auf die isomerisierung von Cyclopropan an einem NaX-Zeoliten angewendet. Die erhaltene Adsorptionswärme für Cyclopropan und die Aktivierungsenergie für die Reaktion stimmen gut mit Literatur-werten von isothermen Messungen im Impulsreaktor überein.相似文献
60.