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901.
Marcos A. Scapin Vera L. R. Salvador Marycel E. B. Cotrim Maria A. F. Pires Ivone M. Sato 《Journal of Radioanalytical and Nuclear Chemistry》2011,287(3):807-811
Uranium silicide (U3Si2), 20% 235U enriched powder, is an intermetallic compound used as nuclear fuel material, which is the state-of-the-art among nuclear fuel materials used in modern research reactors. It is produced by IPEN and used as nuclear fuel of the IEA-R1 reactor (IPEN/CNEN, São Paulo, Brazil); U3Si2 has 92.3 wt% Utotal and 7.7 wt% Si. The qualification of this material requires chemical and physical tests such as Si and Utotal content, isotope ratio, impurities, density, specific surface area and particle size determination. The Si and Utotal were determined by gravimetric and volumetric procedures. Usually, these classical methods require a long time for analyses and are expensive. The objective of this study was to establish a fast and efficient analytical method to meet ISO/IEC 17025:2005 requirements in the Si and Utotal determination. The X-ray fluorescence techniques (XRF) were chosen to allow a direct and non-destructive analysis, what is the main advantage compared to other instrumental techniques, since previous chemical treatments are not necessary. In this study, the performance of the wavelength dispersive (WDXRF) and energy dispersive (EDXRF) X-ray fluorescence techniques was evaluated. Furthermore, two different sample preparation procedures, plain powdered and pressed powdered, were evaluated. Statistical tools were used to evaluate the results and a comparison between these results and the conventional methods was done. 相似文献
902.
Marcos N. Eberlin 《Angewandte Chemie (International ed. in English)》2011,50(14):3116-3116
903.
Silvia Hernández‐Ainsa Joaquín Barberá Mercedes Marcos José Luis Serrano 《Journal of polymer science. Part A, Polymer chemistry》2011,49(1):278-285
A series of codendrimers constituted by the first five generations of poly(propylene imine) (PPI) ionically substituted with 2H,2H,3H,3H‐perfluoroundecanoic acid (af) and undecanoic acid (ac) in a fixed ratio (1:3) (af:ac) have been synthesized and their supramolecular organization has been investigated both in the liquid crystalline state and on a surface. Incompatibility generated by combining fluorinated and perhydrogenated chains in the same molecule causes a peculiar mesomorphic behavior depending on the generation of the codendrimers. Thus smectic A, frustrated smectic A or rectangular columnar mesophases have been detected. Despite their different liquid crystalline organization, they are capable to self‐assemble in homogeneous molecular length layers on a surface, namely mica as revealed by atomic force microscopy (AFM) experiments. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 278–285, 2011 相似文献
904.
J. Fuentes J. Portelles A. Pérez M. D. Durruthy-Rodríguez C. Ostos O. Raymond J. Heiras M. P. Cruz J. M. Siqueiros 《Applied Physics A: Materials Science & Processing》2012,108(3):733-738
We report the results of studies of rectifying behavior, positive magneoresistance (MR), the charge-transport mechanism and the effect of an electric field on a ZnO (n)/La0.5Pr0.2Sr0.3MnO3 (LPSMO) (p)/SrNb0.002Ti0.998O3 (SNTO) (n) heterostructure comprising two p–n junctions fabricated using the pulsed laser deposition technique. The heterostructure exhibits rectifying behavior over a wide temperature and field range having hysteresis in I–V behavior (forward bias) due to the tunneling of charge carriers. It is also observed that, depending on the nature of the electric field bias to n-type ZnO and SNTO, the junction resistance becomes modified, which has been explained on the basis of spin injection in the heterostructure. The observation of unconventional positive MR at room temperature has been justified on the basis of interface effects and the reduction in barrier height obtained using fitting of the I–V data by a thermionic emission model. 相似文献
905.
906.
Dr. Alberto Feliciano Juan L. Vázquez Luis J. Benítez-Puebla Iván Velazco-Cabral Dr. David Cruz Cruz Dr. Francisco Delgado Dr. Miguel A. Vázquez 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(32):8233-8251
The structure of Fischer carbene complexes (FCCs) is electron deficient. If bearing an α,β-unsaturated system, it can generate a wide variety of compounds by undergoing many different transformations, including higher-order cycloadditions. The latter are described as pericyclic reactions in which more than six electrons participate. These reactions have been employed in various areas of organic synthesis, resulting in highly selective compounds with a broad range of scaffolds. The first studies on higher-order cycloadditions involving FCCs frequently yielded competing byproducts. Many groups have attempted to increase selectivity by exploring distinct reaction conditions, reagents and co-catalysts (e. g., metal-mediated cycloadditions). The present review is the first to focus exclusively on using higher-order cycloadditions involving FCCs to synthesize carbocycles and heterocycles. Based on two decades of reports, an analysis is made of the main aspects of the mechanisms proposed for higher-order cycloadditions and the structural diversity obtained by the substituent effect. 相似文献
907.
Abderrahman Bouhamidi Mohammed Bellalij Rentsen Enkhbat Khalid Jbilou Marcos Raydan 《Journal of Optimization Theory and Applications》2018,176(1):163-177
We consider the problem of optimizing the ratio of two convex functions over a closed and convex set in the space of matrices. This problem appears in several applications and can be classified as a double-convex fractional programming problem. In general, the objective function is nonconvex but, nevertheless, the problem has some special features. Taking advantage of these features, a conditional gradient method is proposed and analyzed, which is suitable for matrix problems. The proposed scheme is applied to two different specific problems, including the well-known trace ratio optimization problem which arises in many engineering and data processing applications. Preliminary numerical experiments are presented to illustrate the properties of the proposed scheme. 相似文献
908.
Fernando Augusto Pedersen Voll Andreia Fatima Zanette Vladimir Ferreira Cabral Claudio Dariva Rodrigo Octavio Mendon?a Alves De Souza Lucio Cardozo Filho Marcos Lúcio Corazza 《Applied biochemistry and biotechnology》2012,168(5):1121-1142
This work reports the experimental data and kinetic modeling of diacylglycerol (DAG) production from palm oil using a commercial immobilized lipase (Lipozyme RM IM) in a solvent-free medium. The experiments were performed in batch mode, at 55?°C and 400?rpm, and the effects of enzyme concentration (0.68?C2.04?wt% related to the mass of substrates), initial water concentration (5?C15?wt% related to the mass of oil), and reaction time were evaluated. A novel kinetic model is presented based on the ordered-sequential bi?Cbi mechanism considering hydrolysis and esterification steps, in which a correlation between water-in-oil solubility and surfactant molecules concentration in the oil allowed the model to describe the induction period in the beginning of the hydrolysis reaction. Moreover, mass transfer limitations related to the enzyme concentration in the system were also taken into account. The proposed model presented a very satisfactory agreement with the experimental data, thus allowing a better understanding of the reaction kinetics. The best conditions obtained for the product (partially hydrolyzed palm oil) in terms of DAG yield (35.91?wt%) were 2.87?wt% enzyme/substrate, 2.10?wt% water/oil, and 72?h of reaction. 相似文献
909.
This project focuses on the wake behind a two‐dimensional blunt‐trailing‐edged body. The body has an elliptical nose followed by a straight section that ends in a blunt base. For this geometry, the boundary layer separation points are defined and fixed. This permits a better assessment of the vital influence of the boundary layers upon the wake. It is hoped that the study of this special geometry will contribute to a better understanding of the wake flow. The main aim of this article is to study the topology of the wake by means of direct numerical simulation (DNS). The idea is to mimic some experimental work techniques. In an experiment, by means of proper synchronization, one is able to arrive at the mean‐in‐the‐phase fields. In the present work, similar data are obtained through the use of numerical tools. The topology of the wake flow in terms of the movement of the coherent structures, fields of Reynolds stresses, position of saddles and centers, is obtained for certain values of the body's aspect ratio. A comparison with the circular cylinder experimental data is also provided. An important result is the finding of some specific stress distributions around the main structures. The stress level lines are a hint of where the energy transfer of the mean flow to the turbulence is more intense. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献