全文获取类型
收费全文 | 659933篇 |
免费 | 6974篇 |
国内免费 | 1990篇 |
专业分类
化学 | 342856篇 |
晶体学 | 9926篇 |
力学 | 30542篇 |
综合类 | 20篇 |
数学 | 81699篇 |
物理学 | 203854篇 |
出版年
2021年 | 5798篇 |
2020年 | 6336篇 |
2019年 | 6932篇 |
2018年 | 9134篇 |
2017年 | 9187篇 |
2016年 | 13525篇 |
2015年 | 8296篇 |
2014年 | 13123篇 |
2013年 | 31093篇 |
2012年 | 23782篇 |
2011年 | 29008篇 |
2010年 | 20839篇 |
2009年 | 20638篇 |
2008年 | 26405篇 |
2007年 | 26296篇 |
2006年 | 24236篇 |
2005年 | 21403篇 |
2004年 | 20033篇 |
2003年 | 17657篇 |
2002年 | 17665篇 |
2001年 | 19983篇 |
2000年 | 15150篇 |
1999年 | 11916篇 |
1998年 | 9959篇 |
1997年 | 9610篇 |
1996年 | 9176篇 |
1995年 | 8099篇 |
1994年 | 8081篇 |
1993年 | 7788篇 |
1992年 | 8582篇 |
1991年 | 8816篇 |
1990年 | 8438篇 |
1989年 | 8190篇 |
1988年 | 8041篇 |
1987年 | 8011篇 |
1986年 | 7582篇 |
1985年 | 9789篇 |
1984年 | 9984篇 |
1983年 | 8088篇 |
1982年 | 8253篇 |
1981年 | 8016篇 |
1980年 | 7590篇 |
1979年 | 8219篇 |
1978年 | 8332篇 |
1977年 | 8224篇 |
1976年 | 8150篇 |
1975年 | 7668篇 |
1974年 | 7569篇 |
1973年 | 7552篇 |
1972年 | 5376篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
81.
In developing travel demand models it is generally assumed that the base-year data used in developing the parameters, as well as the forecasted data to be used as independent variables for the design year, are of acceptable quality. The purpose of this paper is to present the application of error propagation theory in assesing the predictive quality of one type of travel demand forecasting model (multinomial logit models) and to demonstrate how error considerations can be used as a tool for identifying the optimal model. The general conclusions of this study are that: (1) it is indeed possible to quantify errors in dependent variables in logit models as a consequence of errors in independent variables; and (2) error consideration can be used as a tool for identifying the optimal model from a set of candidate models. Further research is recommended to develop better insights into the phenomenon of error propagation so that the consideration of errors can be a factor in decisions on model selection. 相似文献
82.
A. M. Vinogradov 《Acta Appl Math》1989,15(1-2):3-21
The main notions and results which are necessary for finding higher symmetries and conservation laws for general systems of partial differential equations are given. These constitute the starting point for the subsequent papers of this volume. Some problems are also discussed. 相似文献
83.
M. R. Booty 《Studies in Applied Mathematics》1989,80(3):203-228
A model equation describing the configuration of a simple plasma maintained by external radiation is studied. A branch of steady solutions of the equation was found by Eckhaus et al. to terminate at a finite critical value of the power of the external source, and this is attributed to the discontinuous nature of a nonlinear term in the governing equation. On introducing a small parameter to render the term continuous, a second branch of solutions is constructed in a neighborhood of the termination point of the original branch. This suggests that the termination point is formed as the limit of a subcritical fold in the surface of the steady solution branch. 相似文献
84.
85.
T. Kumita Y. Kamiya M. Babzien I. Ben-Zvi K. Kusche I. V. Pavlishin I. V. Pogorelsky D. P. Siddons V. Yakimenko T. Hirose T. Omori J. Urakawa K. Yokoya D. Cline F. Zhou 《Laser Physics》2006,16(2):267-271
Thomson scattering of high-power laser and electron beams is a good test of electrodynamics in the high-field region. We demonstrated production of high-intensity X-rays in the head-on collision of a CO2 laser and 60-MeV electron beams at Brookhaven National Laboratory, Accelerator Test Facility. The energy of an X-ray photon was limited at 6.5 keV in the linear (lowest order) Thomson scattering, but the nonlinear (higher order) process produces higher energy X-rays. We measured the angular distribution of the high-energy X-rays and confirmed that it agrees with theoretical predictions. 相似文献
86.
Jun-tao Chang Joerg Evers Marian O. Scully M. Suhail Zubairy 《量子光学学报》2006,12(B08):77-77
We propose a scheme to obtain the distance of two identical atoms placed inside the standing wave field by monitoring the collective resonance fluorescence spectrum emitted by the two particles. We find three different parameter ranges, depending on the distance of the atoms as compared to the transition wavelength. For large interparticle distances, dipole-dipole coupling is negligible, and the main system evolution arises from the interaction with the standing wave field. In the small-distance limit, the dynamics is dominated by the dipole-dipole interaction. Finally, in the intermediate region, a rich interplay of the various couplings arises, which however is lifted for strong driving laser fields. The present measurement procedure allows us to distinguish the three cases. In each of the cases, we show how to determine the distance of the two particles and their respective positions relative to the nodes of the standing wave field with fractional-wavelength precision. 相似文献
87.
M. Rovinsky 《Selecta Mathematica, New Series》2006,11(3-4):491-522
88.
An algorithm is proposed which allows one to construct higher symmetries of arbitrary order for some special classes of hyperbolic
systems possessing integrals. The Pohlmeyer-Lund-Regge system and the open two-dimensional Toda lattices are shown to belong
to the class of systems where our algorithm is applicable.
__________
Translated from Fundamental’naya i Prikladnaya Matematika (Fundamental and Applied Mathematics), Vol. 10, No. 1, Geometry
of Integrable Models, 2004. 相似文献
89.
90.
Non-stoichiometric ternary chalcogenides (Zn,Fe)S were prepared in the film form by pyrolytic spray deposition technique, using air/nitrogen as the carrier gas. The precursor solution comprised of ZnCl2, FeCl2 and thiourea. The depositions were carried out under optimum conditions of experimental parameters viz. carrier gas (air/nitrogen) flow rate, concentration of precursor constituents, nozzle substrate distance and temperature of quartz substrate. The deposited thin films were later sintered in argon at 1073 K for 120 min.The structural, compositional and optical properties of the sintered thin films were studied. X-ray diffraction studies of the thin films indicated the presence of (Zn,Fe)S solid solution with prominent cubic sphalerite phase while surface morphology as determined by scanning electron microscopy (SEM) revealed a granular structure.The chemical composition of the resulting thin films as analyzed by energy dispersive X-ray analysis (EDAX) reflected the composition of the precursor solutions from which the depositions were carried out with Fe at% values ranging from 0.4 up to 33.SEM micrographs of thin films reveal that the grain sizes of the thin films prepared using air as carrier gas and N2 as carrier gas are in the vicinity of 300 and 150 nm, respectively.The diffuse transmittance measurements for thin films, as a function of wavelength reveal the dependence of direct optical band gap on Fe content and type of phase. 相似文献