全文获取类型
收费全文 | 48943篇 |
免费 | 1969篇 |
国内免费 | 21篇 |
专业分类
化学 | 31561篇 |
晶体学 | 238篇 |
力学 | 1575篇 |
数学 | 7517篇 |
物理学 | 10042篇 |
出版年
2023年 | 377篇 |
2022年 | 298篇 |
2021年 | 508篇 |
2020年 | 681篇 |
2019年 | 641篇 |
2018年 | 1016篇 |
2017年 | 828篇 |
2016年 | 1531篇 |
2015年 | 1404篇 |
2014年 | 1407篇 |
2013年 | 2907篇 |
2012年 | 3403篇 |
2011年 | 3473篇 |
2010年 | 2090篇 |
2009年 | 1703篇 |
2008年 | 2978篇 |
2007年 | 2998篇 |
2006年 | 2629篇 |
2005年 | 2558篇 |
2004年 | 2202篇 |
2003年 | 1792篇 |
2002年 | 1550篇 |
2001年 | 914篇 |
2000年 | 772篇 |
1999年 | 545篇 |
1998年 | 413篇 |
1997年 | 377篇 |
1996年 | 471篇 |
1995年 | 297篇 |
1994年 | 359篇 |
1993年 | 352篇 |
1992年 | 379篇 |
1991年 | 327篇 |
1990年 | 314篇 |
1989年 | 308篇 |
1988年 | 248篇 |
1987年 | 253篇 |
1986年 | 222篇 |
1985年 | 380篇 |
1984年 | 347篇 |
1983年 | 236篇 |
1982年 | 327篇 |
1981年 | 313篇 |
1980年 | 274篇 |
1979年 | 268篇 |
1978年 | 249篇 |
1976年 | 263篇 |
1975年 | 240篇 |
1974年 | 247篇 |
1973年 | 238篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
201.
Horst W. Hamacher Martine Labbé Stefan Nickel Anders J.V. Skriver 《Annals of Operations Research》2002,110(1-4):33-53
Locating a facility is often modeled as either the maxisum or the minisum problem, reflecting whether the facility is undesirable (obnoxious) or desirable. But many facilities are both desirable and undesirable at the same time, e.g., an airport. This can be modeled as a multicriteria network location problem, where some of the sum-objectives are maximized (push effect) and some of the sum-objectives are minimized (pull effect).We present a polynomial time algorithm for this model along with some basic theoretical results, and generalize the results also to incorporate maximin and minimax objectives. In fact, the method works for any piecewise linear objective functions. Finally, we present some computational results. 相似文献
202.
We study the vibration of an elastic thin shell which is pre-constrained by a large displacement with a small deformation. In this first Note we prove the solutions exist and we investigate both the interior regularity and the boundary regularity which is known to be important in the shape differentiation of hyperbolic equations. To cite this article: J. Cagnol, J.-P. Zolésio, C. R. Acad. Sci. Paris, Ser. I 334 (2002) 161–166 相似文献
203.
204.
A. S. Parkins C. W. Gardiner M. L. Stéyn-Ross 《Zeitschrift für Physik B Condensed Matter》1991,83(3):413-424
We examine the effect of dissipation on coherent quantum tunnelling between the two lowest levels of a double-well system using an adjoint equation approach developed previously in the treatment of quantum-optical problems. Dissipation is modelled by a linear coupling to a bath of harmonic oscillators. The high frequency portion of the bath is adiabatically eliminated using a cumulant expansion technique, which generates frequency renormalisation terms. Making a low temperature assumption, an approximate two-level model is developed in the form of three coupled stochastic differential equations. With a decorrelation approximation, the two-level equations can be solved to yield familiar results. However, the adjoint equation also permits the use of direct stochastic simulation as a means of solution, and simulations are carried out for a range of parameters. A comparison with the decorrelation approximation is made. 相似文献
205.
Principles and recent developments in ultrasound contrast agents 总被引:6,自引:1,他引:5
The behaviour of gas bubbles and gas encapsulated spheres as echographic contrast agents is reviewed. Compared with rigid spheres, gas bubbles are superior scattering agents and they offer a number of useful properties which can be exploited in a variety of ways. The analysis of their velocity of sound, back-scatter intensity, second harmonic emission and resonant frequency opens up new perspectives in the development of contrast agents for echocardiographic research with potential clinical applications. 相似文献
206.
In this paper we present photoionization cross sections of the Cu and Al dimers and tetramers. The local spin density method is applied to calculate the electronic structure and the ground state potential. The cross sections are calculated using the continuum multiple scattering method, and a basic analysis of the photoionization process is given within the independent particle picture. 相似文献
207.
L Soltés 《Biomedical chromatography : BMC》1992,6(1):43-49
The paper presents results and experience obtained in the isolation of a number of basic polar/non-polar solutes (drugs, their metabolites and structural derivatives) from biological samples by using the separation minicolumns Separcol SI and Separcol SI C18 L/T. The aim was to provide insight into some simple procedures yielding optimal selectivity/specificity of the isolation of the studied solutes either by proper selection of the sorbent or by applying sophisticated elution schemes which use to advantage different properties of the eluents/solvents. 相似文献
208.
A classical Fermi accelerator model (FAM) is known to show chaotic behavior. The FAM is defined by a free particle bouncing elastically from two rigid walls, one fixed and the other oscillating periodically in time. The central aim of this paper is to connect the quantum and the classical solutions to the FAM in the semiclassical limit. This goal is accomplished using a finite inverted parametric oscillator (FIPO), confined to a box withfixed walls, as an alternative representation of the FAM. In the FIPO representation, an explicit correspondence between classical and quantum limits is accomplished using a Husimi representation of the quasienergy eigenfunctions. 相似文献
209.
Herlin-Boime N. Vicens J. Dufour C. Ténégal F. Reynaud C. Rizk R. 《Journal of nanoparticle research》2004,6(1):63-70
Small SiC nanoparticles (10 nm diameter) have been grown in a flow reactor by CO2 laser pyrolysis from a C2H2 and SiH4 mixture. The laser radiation is strongly absorbed by SiH4 vibration. The energy is transferred to the reactive medium and leads to the dissociation of molecules and the subsequent growth of the nanoparticles. The reaction happens with a flame. The purpose of the experiments reported in this paper is to limit the size of the growing particles to the nanometric scale for which specific properties are expected to appear. Therefore the effects of experimental parameters on the structure and chemical composition of nanoparticles have been investigated. For a given reactive mixture and gas velocity, the flame temperature is governed by the laser power. In this study, the temperature was varied from 875°C to 1100°C. The chemical analysis of the products indicate that their composition is a function of the temperature. For the same C/Si atomic ratio in the gaseous phase, the C/Si ratio in the powder increases from 0.7 at 875°C up to 1.02 at 1100°C, indicating a growth mechanism limited by C2H2 dissociation. As expected, X-ray diffraction has shown an improved crystallisation with increasing temperature. Transmission electron microscopy observations have revealed the formation of 10 nm grains for all values of laser power (or flame temperature). These grains appear amorphous at low temperature, whereas they contain an increasing number of nanocrystals (2 nm diameter) when the temperature increases. These results pave the way to a better control of the structure and chemical composition of laser synthesised SiC nanoparticles in the 10 nm range. 相似文献
210.
József Kaizer József Pap Gábor Speier Marius Réglier Michel Giorgi 《Transition Metal Chemistry》2004,29(6):630-633
The preparation, spectroscopic properties, and crystal structure of [Cu(ind)(mco)] [ind is the anion of 1,3-bis(2-pyridylimino)isoindoline and mco is the anion of 3-hydroxy-(4H)-benzopyran-4-one] are reported. The complex exhibits a distorted square pyramidal geometry. The oxygenation of the title compound results in oxidative cleavage of the heterocyclic ring to give the (acetylsalicylato)copper(II) complex [Cu(asp)(ind)] (asp is the anion of acetylsalicylic acid) and CO as primary products, which reaction resembles the enzymatic reaction of quercetinase. 相似文献