全文获取类型
收费全文 | 650篇 |
免费 | 44篇 |
国内免费 | 12篇 |
专业分类
化学 | 529篇 |
晶体学 | 11篇 |
力学 | 17篇 |
数学 | 48篇 |
物理学 | 101篇 |
出版年
2024年 | 2篇 |
2023年 | 6篇 |
2022年 | 30篇 |
2021年 | 28篇 |
2020年 | 21篇 |
2019年 | 27篇 |
2018年 | 25篇 |
2017年 | 17篇 |
2016年 | 43篇 |
2015年 | 26篇 |
2014年 | 22篇 |
2013年 | 77篇 |
2012年 | 42篇 |
2011年 | 52篇 |
2010年 | 34篇 |
2009年 | 22篇 |
2008年 | 24篇 |
2007年 | 24篇 |
2006年 | 19篇 |
2005年 | 15篇 |
2004年 | 13篇 |
2003年 | 16篇 |
2002年 | 19篇 |
2001年 | 7篇 |
2000年 | 7篇 |
1999年 | 6篇 |
1998年 | 3篇 |
1997年 | 4篇 |
1996年 | 4篇 |
1995年 | 2篇 |
1994年 | 5篇 |
1993年 | 2篇 |
1992年 | 4篇 |
1991年 | 6篇 |
1990年 | 2篇 |
1989年 | 8篇 |
1988年 | 6篇 |
1987年 | 3篇 |
1986年 | 2篇 |
1985年 | 5篇 |
1983年 | 5篇 |
1982年 | 3篇 |
1981年 | 2篇 |
1980年 | 4篇 |
1979年 | 2篇 |
1976年 | 1篇 |
1975年 | 2篇 |
1973年 | 2篇 |
1957年 | 1篇 |
1956年 | 2篇 |
排序方式: 共有706条查询结果,搜索用时 46 毫秒
51.
52.
Stuart B. Savage Mohamed Sayed 《Zeitschrift für Angewandte Mathematik und Physik (ZAMP)》1981,32(2):125-143
This paper is concerned with the flow of cohesionless granular materials in conical hoppers. The granules are considered to be coarse such that the effects of interstitial fluid can be neglected. Two analytical approaches based upon the method of integral relations are presented. In the first the detailed variations of shear and normal stresses over the cross-section of the hopper are considered. The second is a very simple analysis which makes use of a mean normal stress averaged over the cross-section. The two analyses yield predictions for the flow rate which are almost identical. After the inclusion of an empirical correction factor to account for non-uniform exit velocity profiles and bulk density reductions near the outlet, the flow rate predictions and their variations with internal and wall friction angles and with hopper wall slope are found to agree well with experimental measurements.
On leave at the Department of Applied Mathematics and Theoretical Physics, University of Cambridge. 相似文献
Résumé Cet article traite de l'écoulement de matières granuleuses sans cohésion dans des trémies coniques. On admet que les granules sont assez gros pour que les effets dûs au fluide intersistiel restent négligeables. 2 méthodes d'approche analytiques basées sur la méthode des relations intégrales sont présentées. La première considère en détail la distribution des contraintes normales et de cisaillement sur la section de la trémie. La seconde méthode est une simple analyse basée sur la moyenne des contraintes normales sur cette section. Les 2 analyses fournissent des prédictions presque identiques sur le taux d'écoulement. En introduisant un facteur de correction empirique pour tenir compte de la non-uniformité de profils de vitesses d'écoulement ainsi que de la réduction de densité à la sortie, l'analyse fournit des taux d'écoulement et leurs variations avec la friction interne, les angles de friction aux parois et l'inclinaison des parois de la trémie. Les résultants obtenus sont en accord avec des mesures expérimentales.
On leave at the Department of Applied Mathematics and Theoretical Physics, University of Cambridge. 相似文献
53.
NOROUZI Parviz GANJALI Mohammad Reza SHAHTAHERI Sayed Jamaleddin DINARVAND Rasoul HAMZEHPOOR Ali 《中国化学》2009,27(4):732-738
A continuous cyclic voltammetric study of methyldopa at gold micro electrode was carried out. The drug in phosphate buffer (pH 2.0) is adsorpted at 400 mV, giving rise to change in the current of well-defined oxidation peak of gold in the flow injection system. The proposed detection method has some of advantages, the greatest one of which are as follows: first, it is no more necessary to remove oxygen from the analyte solution and second, this is a very fast and appropriate technique for determination of the drug compound in a wide variety of chromatographic analysis methods. Signal-to-noise ratio has significantly increased by application of discrete Fast Fourier transform (FFT) method, background subtraction and two-dimensional integration of the electrode response over a selected potential range and time window. Also in this work some parameters such as sweep rate, eluent pH, and accumulation time and potential were optimized. The linear concentration range was of 1.0×10-7—1.0×10-11 mol•L-1 (r=0.9975) with a limit of detection and quantitation 0.004 nmol•L-1 and 0.03 nmol•L-1, respectively. The method has the requisite accuracy, sensitivity, precision and selectivity to assay methyldopa in tablets. The influences of pH of eluent, accumulation potential, sweep rate, and accumulation time on the determination of the methyldopa were considered. 相似文献
54.
Kinetics and Mechanism of Dehydrochlorination of 3‐Chloro‐1,5‐diarylformazans and Their Mass Spectra
In aqueous dioxane containing triethylamine the title 3‐chloroformazans 1 are converted into the corresponding 1,4‐bis(arylazo)‐3,6‐diaryl‐1,2,4,5‐tetrazines 3 via head‐to‐tail dimerization of the intitially formed 1,3‐dipolar ions 2. The kinetics of triethylamine‐catalyzed dehydrochlorination of 1 in 70% dioxane at 27°C and ionic strength of 0.1 were studied. The rate data were linearly correlated with enhanced Hammett substituent constants σx? and an overall ρ value of 0.2 was determined for the variation of the N‐aryl substituent. These results were interpreted in terms of a two‐step mechanism. Also, the mechanisms of the unimolecular fragmentation of 1 and 3 in the mass spectrometer are discussed. 相似文献
55.
Priya Budhani Sayed Aftab Iqbal Suman Malik Mamta Bhattacharya Liviu Mitu 《Journal of Saudi Chemical Society》2010,14(3):281-285
The present paper deals with the synthesis and characterization of Schiff base complexes of pyrazinamide an antitubercular drug. Metals selected for complexation are copper, silver, gold, zinc, mercury, iron and cobalt. The complexes have been suitably synthesized and isolated in pure powdered form. Analytical data agrees with the compositions M(L), M′(L)2 and M″(L)2·2H2O, respectively where M = Ag, M′ = Cu, Au, Zn and Hg and M″ = Fe and Co, ligand metal ratios were also confirmed by monovariation method and Job’s method of continuous variation. Molar conductance values suggest the non ionic nature of the complexes. The tentative structure assigned to the complexes on the basis of stoichiometry and analytical data were further supported by spectral studies viz; IR, NMR, magnetic susceptibility and electronic spectra. A preliminary attempt has also been made to compare the potencies of metal complexes with parent drug. The Cu and Ag complexes are giving encouraging results. Particle size studies further suggest that the drug molecule undergoes reduction in size on complexation. 相似文献
56.
Ramadan El Sayed 《中国化学》2010,28(4):594-600
Benzaldehyde [4‐(4‐bromophenyl)thiazol‐2‐yl]hydrazones 5a – 5d were prepared by reacting the thiosemicarbazones 2a – 2d with 2,4′‐dibromoacetophenone ( 1 ) in absolute ethanol. Acetylation of 5a and 5b with Ac2O/Py at room temperature gave the N‐acetyl derivatives 6a and 6b . 4‐Methyl‐2‐pentanone/cyclopentanone [4‐(4‐bromo‐phenyl)thiazol‐2‐yl]hydrazones ( 8a ) and ( 8b ) were similarly obtained from the reaction of 1 with the thiosemicarbazones 7a and 7b , respectively. Cyclization of D‐galactose thiosemicarbazone ( 9 ) and its tautomers 10 and 11 with 1 afforded an equilibrium mixture of the acyclic 2‐thiazolylhydrazone 12 , together with its respective cyclic galactosyl derivatives 13 and 14 , whose structures were studied by using 1H and 13C NMR spectra. The antimicrobial activity of the synthesized thiazole derivatives was evaluated in vitro by using an agar diffusion technique, and some of these compounds showed potential activity against Candida albicans. 相似文献
57.
Malek T. Maghsoodlou Nourallah Hazeri Sayed M. Habibi-Khorassani Reza Heydari Ghasem Marandi Mojtaba Lashkari Khatereh Bagherpour Zahra Gharechaei 《Monatshefte für Chemie / Chemical Monthly》2010,39(5):351-356
Abstract
Synthesis of five phosphonato esters has been accomplished via reaction between dimethyl acetylenedicarboxylate and triphenyl phosphite in the presence of biological compounds such as theophylline, 4-hydroxypyrimidine, 2H-3,1-benzoxazine-2,4(1H)-dione, 2-chloroaniline, or 3-nitroaniline at ambient temperature. The configuration of the compounds was determined on the basis of coupling constants emerging from the Karplus equation. 相似文献58.
Sayed M. Saleh Rainer Müller Heike S. Mader Axel Duerkop Otto S. Wolfbeis 《Analytical and bioanalytical chemistry》2010,398(4):1615-1623
Fluorescent silica nanoparticles (SiNPs) were prepared by covalent attachment of fluorophores to the amino-modified surface
of SiNPs with a typical diameter of 15 nm. The SiNPs are intended for use in novel kinds of fluorescence resonance energy
transfer (FRET)-based affinity assays at the interface between nanoparticle and sample solution. Various labels were employed
to obtain a complete set of colored SiNPs, with excitation maxima ranging from 337 to 659 nm and emission maxima ranging from
436 nm to the near infrared (710 nm). The nanoparticles were characterized in terms of size and composition using transmission
electron microscopy, thermogravimetry, elemental analysis, and dynamic light scattering. The surface of the fluorescent SiNPs
was biotinylated, and binding of labeled avidin to the surface was studied via FRET in two model cases. In the first, FRET
occurs from the biotinylated fluorescent SiNP (the donor) to the labeled avidin (the acceptor). In the second, FRET occurs
in the other direction. Aside from its use in the biotin–avidin system, such SiNPs also are believed to be generally useful
fluorescent markers in various kinds of FRET assays, not the least because the fluorophore is located on the surface of the
SiNPs (and thus always much closer to the second fluorophore) rather than being doped deep in its interior. 相似文献
59.
Ion-acoustic waves in plasma of warm ions and isothermal electrons using time-fractional KdV equation 下载免费PDF全文
Sayed A. El-Wakil Essam M. Abulwaf Emad K. El-Shewy Abeer A. Mahmoud 《中国物理 B》2011,20(4):40508-040508
The ion-acoustic solitary wave in collisionless unmagnetized plasma consisting of warm ions-fluid and isothermal electrons is studied using the time fractional KdV equation. The reductive perturbation method has been employed to derive the Korteweg-de Vries equation for small but finite amplitude ion-acoustic wave in warm plasma. The Lagrangian of the time fractional KdV equation is used in a similar form to the Lagrangian of the regular KdV equation with fractional derivative for the time differentiation. The variation of the functional of this Lagrangian leads to the Euler-Lagrange equation that gives the time fractional KdV equation. The variational-iteration method is used to solve the derived time fractional KdV equation. The calculations of the solution are carried out for different values of the time fractional order. These calculations show that the time fractional can be used to modulate the electrostatic potential wave instead of adding a higher order dissipation term to the KdV equation. The results of the present investigation may be applicable to some plasma environments,such as the ionosphere plasma. 相似文献
60.
The major objective of this work was the development of a reliable model to describe volumetric properties of ionic liquids
(ILs). In this regard, we have applied the Ihm–Song–Mason equation of state (EOS) to some phosphonium- and imidazolium-based
ILs. Three temperature-dependent parameters in the equation of state have been scaled based on the surface tension and the
liquid density at room temperature. In order to improve the predictive power of the mentioned EOS for ILs, we have proposed
using a simple modification. We have taken 1,228 experimental points to show the reliability of the improved EOS. The comparison
of predicted densities with literature data over a broad range of temperature, 293–472 K, and pressures up to 200 MPa led
to encouraging results. The average absolute deviation of calculated densities from literature values was found to be 0.75%. 相似文献