全文获取类型
收费全文 | 625362篇 |
免费 | 5675篇 |
国内免费 | 1796篇 |
专业分类
化学 | 311859篇 |
晶体学 | 8882篇 |
力学 | 32350篇 |
综合类 | 20篇 |
数学 | 96633篇 |
物理学 | 183089篇 |
出版年
2021年 | 4871篇 |
2020年 | 5314篇 |
2019年 | 5937篇 |
2018年 | 17585篇 |
2017年 | 17410篇 |
2016年 | 17005篇 |
2015年 | 7448篇 |
2014年 | 11062篇 |
2013年 | 25000篇 |
2012年 | 22516篇 |
2011年 | 32877篇 |
2010年 | 22177篇 |
2009年 | 22591篇 |
2008年 | 27699篇 |
2007年 | 29405篇 |
2006年 | 20052篇 |
2005年 | 18884篇 |
2004年 | 17919篇 |
2003年 | 16505篇 |
2002年 | 15595篇 |
2001年 | 16809篇 |
2000年 | 12751篇 |
1999年 | 9918篇 |
1998年 | 8438篇 |
1997年 | 8056篇 |
1996年 | 7792篇 |
1995年 | 7001篇 |
1994年 | 6872篇 |
1993年 | 6718篇 |
1992年 | 7249篇 |
1991年 | 7597篇 |
1990年 | 7240篇 |
1989年 | 7129篇 |
1988年 | 7026篇 |
1987年 | 6920篇 |
1986年 | 6571篇 |
1985年 | 8454篇 |
1984年 | 8538篇 |
1983年 | 6899篇 |
1982年 | 7060篇 |
1981年 | 6948篇 |
1980年 | 6439篇 |
1979年 | 7031篇 |
1978年 | 7260篇 |
1977年 | 7237篇 |
1976年 | 7248篇 |
1975年 | 6678篇 |
1974年 | 6642篇 |
1973年 | 6782篇 |
1972年 | 4958篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
141.
Pascher W. Den Besten J. H. Caprioli D. Leijtens X. Smit M. van Dijk R. 《Optical and Quantum Electronics》2003,35(4-5):453-464
Based on a rigorous vectorial analysis, a fast travelling-wave Mach–Zehnder modulator is modelled and designed. The cross-section of the semiconductor layer stack and the lossy electrodes are carefully modelled using the method of lines in order to investigate propagation characteristics, velocity and losses. This yields an accurate microwave and optical field distribution to explain the behaviour of the component. In order to enhance the modulation efficiency, design curves are derived and the cross-sectional dimensions for minimum microwave loss are determined. The loss of the optimized modulator agrees very well with small-signal measurements up to 40 GHz and HFSS simulations. The layerstack of the fabricated device is suitable for integration with InP multi-wavelength lasers. 相似文献
142.
M. V. Zyubin I. A. Rudnev V. A. Kashurnikov 《Journal of Experimental and Theoretical Physics》2003,96(6):1065-1077
A system of Abrikosov vortices in a quasi-two-dimensional HTSC plate is considered for various periodic lattices of pinning centers. The magnetization and equilibrium configurations of the vortex density for various values of external magnetic field and temperature are calculated using the Monte Carlo method. It is found that the interaction of the vortex system with the periodic lattice of pinning centers leads to the formation of various ordered vortex states through which the vortex system passes upon an increase or a decrease in the magnetic field. It is shown that ordered vortex states, as well as magnetic field screening processes, are responsible for the emergence of clearly manifested peaks on the magnetization curves. Extended pinning centers and the effect of multiple trapping of vortices on the behavior of magnetization are considered. Melting and crystallization of the vortex system under the periodic pinning conditions are investigated. It is found that the vortex system can crystallize upon heating in the case of periodic pinning. 相似文献
143.
The results of a numerical study are described in which the interactions of a primary shock wave with a secondary diaphragm
in expansion tubes are taken into account. The developing wave pattern in the interacting process of the shock with a secondary
diaphragm are visualized by many kinds of figures (e.g., the time-distance diagrams of the wave phenomena on the axis, the
acoustic impedance contours, and the time histories of the pitot pressure on the axis), and the influences of the shape and
rupture process of the diaphragm on the quality of the test gas are explored. 相似文献
144.
J. M. Martínez‐Burgos R. Benavente E. Prez M. L. Cerrada 《Journal of Polymer Science.Polymer Physics》2003,41(11):1244-1255
Several composites were prepared on the basis of an ethylene homopolymer and different copolymers of ethylene and 1‐hexene, synthesized with a metallocene catalyst, as matrices and a content of a 5 wt % of short glass fiber. The effect of the fiber incorporation on the structure and mechanical and viscoelastic behaviors was analyzed for the different samples. The glass fibers induced a slightly higher crystallinity, and the crystallite morphology significantly changed (long spacings and crystal orientation). The incorporation of fibers did not reinforce the different matrices under study at this low content; consequently, the mechanical parameters, such as Young's modulus, yielding stress, and microhardness, were lower in the composites as compared with those values found in the neat polyolefins. The location and apparent activation energies of distinct relaxation processes are also discussed. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1244–1255, 2003 相似文献
145.
M. D. Srinivas 《Pramana》2003,60(6):1137-1152
We derive an optimal bound on the sum of entropic uncertainties of two or more observables when they are sequentially measured
on the same ensemble of systems. This optimal bound is shown to be greater than or equal to the bounds derived in the literature
on the sum of entropie uncertainties of two observables which are measured on distinct but identically prepared ensembles
of systems. In the case of a two-dimensional Hilbert space, the optimum bound for successive measurements of two-spin components,
is seen to be strictly greater than the optimal bound for the case when they are measured on distinct ensembles, except when
the spin components are mutually parallel or perpendicular 相似文献
146.
We perform a comparative analysis of two mechanisms for the formation of a synthetic aperture radar (SAR) image of the ocean, which are related to orbital velocities on the surface. It is shown that the mechanism of fluctuations of the effective density of scatterers, which is commonly accepted at present as the physical basis for the SAR diagnostics of roughness, actually works only in the case of a sufficiently flat swell. In the presence of wind roughness, the dominant role is played by another mechanism, namely, fluctuations of the number of surface elements whose images are randomly shifted and superimposed in the image plane due to orbital velocities. In the case of a developed wind roughness propagated in the azimuthal direction, the power contributions of two above-mentioned mechanisms to the SAR imaging of the ocean differ by about two orders of magnitude. 相似文献
147.
In order to reduce infant mortality in the municipality of Rio de Janeiro it is desired to encourage all mothers-to-be to attend the appropriate health care facilities. A 3-level hierarchical prenatal–neonatal health care system is described together with a basic model for optimising accessibility to facilities. A genetic algorithm to solve the basic model is developed and some numerical experience reported. Extensions to the basic model and planned future research are briefly indicated. 相似文献
148.
Howard Anton and Robert C. Busby, John Wiley & Sons, Inc., 2002. 相似文献
149.
M. Ohsaki 《Archive of Applied Mechanics (Ingenieur Archiv)》2003,73(3-4):241-251
Summary Characteristics of optimal solutions under nonlinear buckling constraints are investigated by using a bar-spring model. It is demonstrated that optimization under buckling constraints of a symmetric system often leads to a structure with hill-top branching, where a limit point and bifurcation points coincide. A general formulation is derived for imperfection sensitivity of the critical load factor corresponding to a hill-top branching point. It is shown that the critical load is not imperfection-sensitive even for the case where an asymmetric bifurcation point exists at the limit point. 相似文献
150.
Florin Ambro 《manuscripta mathematica》2003,111(1):43-49
We prove a precise inversion of adjunction formula for the log variety (ℂ
d
+1,X), where X is a non-degenerate hypersurface. As a corollary, the minimal log discrepancies of non-degenerate normal hypersurface singularities
are bounded by dimension.
Received: 17 September 2002 / Revised version: 22 November 2002
Published online: 14 February 2003
Current address: DPMMS, CMS, University of Cambridge, Wilberforce Road, Cambridge CB3 0WB, England. e-mail: f.ambro@dpmms.cam.ac.uk
Mathematics Subject Classification (2000): Primary 14B05; Secondary 14M25, 52B20 相似文献