全文获取类型
收费全文 | 794516篇 |
免费 | 7636篇 |
国内免费 | 2235篇 |
专业分类
化学 | 411698篇 |
晶体学 | 11300篇 |
力学 | 38371篇 |
综合类 | 26篇 |
数学 | 105216篇 |
物理学 | 237776篇 |
出版年
2021年 | 7050篇 |
2020年 | 7667篇 |
2019年 | 8575篇 |
2018年 | 11193篇 |
2017年 | 11161篇 |
2016年 | 16317篇 |
2015年 | 9583篇 |
2014年 | 15297篇 |
2013年 | 35808篇 |
2012年 | 27620篇 |
2011年 | 33345篇 |
2010年 | 24297篇 |
2009年 | 24062篇 |
2008年 | 31260篇 |
2007年 | 30995篇 |
2006年 | 28638篇 |
2005年 | 25612篇 |
2004年 | 23741篇 |
2003年 | 21151篇 |
2002年 | 20935篇 |
2001年 | 22660篇 |
2000年 | 17507篇 |
1999年 | 13675篇 |
1998年 | 11574篇 |
1997年 | 11280篇 |
1996年 | 10802篇 |
1995年 | 9775篇 |
1994年 | 9692篇 |
1993年 | 9361篇 |
1992年 | 10126篇 |
1991年 | 10484篇 |
1990年 | 10149篇 |
1989年 | 9815篇 |
1988年 | 9678篇 |
1987年 | 9517篇 |
1986年 | 9042篇 |
1985年 | 11691篇 |
1984年 | 12087篇 |
1983年 | 9923篇 |
1982年 | 10302篇 |
1981年 | 9897篇 |
1980年 | 9298篇 |
1979年 | 9996篇 |
1978年 | 10300篇 |
1977年 | 10177篇 |
1976年 | 10066篇 |
1975年 | 9547篇 |
1974年 | 9417篇 |
1973年 | 9590篇 |
1972年 | 6914篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
951.
952.
953.
954.
This research extends previous work with dynamic models to manage groundwater quality by using the consumptive nitrate use rate instead of the nitrate application rate. The analysis indicates that misspecification results in overestimation of economic benefits, and supra-optimum nitrogen fertilizer application rates and groundwater nitrate stocks at a steady state. 相似文献
955.
We characterize orbifolds in terms of their sheaves, and show that orbifolds correspond exactly to a specific class of smooth groupoids. As an application, we construct fibered products of orbifolds and prove a change-of-base formula for sheaf cohomology. 相似文献
956.
This is the second in a two-part series of articles in which we analyze a system similar in structure to the well-known Zakharov equations from weak plasma turbulence theory, but with a nonlinear conservation equation allowing finite time shock formation. In this article we analyze the incompressible limit in which the shock speed is large compared to the underlying group velocity of the dispersive wave (a situation typically encountered in applications). After presenting some exact solutions of the full system, a multiscale perturbation method is used to resolve several basic wave interactions. The analysis breaks down into two categories: the nonlinear limit and the linear limit, corresponding to the form of the equations when the group velocity to shock speed ratio, denoted by ε, is zero. The former case is an integrable limit in which the model reduces to the cubic nonlinear Schrödinger equation governing the dispersive wave envelope. We focus on the interaction of a “fast” shock wave and a single hump soliton. In the latter case, the ε=0 problem reduces to the linear Schrödinger equation, and the focus is on a fast shock interacting with a dispersive wave whose amplitude is cusped and exponentially decaying. To motivate the time scales and structure of the shock-dispersive wave interactions at lowest orders, we first analyze a simpler system of ordinary differential equations structurally similar to the original system. Then we return to the fully coupled partial differential equations and develop a multiscale asymptotic method to derive the effective leading-order shock equations and the leading-order modulation equations governing the phase and amplitude of the dispersive wave envelope. The leading-order interaction equations admit a fairly complete analysis based on characteristic methods. Conditions are derived in which: (a) the shock passes through the soliton, (b) the shock is completely blocked by the soliton, or (c) the shock reverses direction. In the linear limit, a phenomenon is described in which the dispersive wave induces the formation of a second, transient shock front in the rapidly moving hyperbolic wave. In all cases, we can characterize the long-time dynamics of the shock. The influence of the shock on the dispersive wave is manifested, to leading order, in the generalized frequency of the dispersive wave: the fast-time part of the frequency is the shock wave itself. Hence, the frequency undergoes a sudden jump across the shock layer.In the last section, a sequence of numerical experiments depicting some of the interesting interactions predicted by the analysis is performed on the leading-order shock equations. 相似文献
957.
958.
Fixed-frequency and threshold photoelectron spectra have been recorded for ionization from the S ls shells in SF6, CS2 and COS, the Si 1s shells in SiH4 and SiCl4 and the Cl 1s shell in SiCl4 using synchrotron radiation. Fixed-frequency spectra generally showed a single strong ionization feature with associated weak satellite structure due to excited ionized states. Threshold spectra closely resembled X-ray absorption spectra but with an additional feature due to direct ionization. In cases where resonant process enhanced the NEXAFS spectrum direct ionization was not observed. 相似文献
959.
M. Trunk J. Popp I. Hartmann M. Lankers E. Urlaub W. Kiefer 《Fresenius' Journal of Analytical Chemistry》1996,355(3-4):354-356
In situ measurements of gas-liquid surface reactions of single aerosol microdroplets are presented. By means of optical levitation in combination with elastic (Mie) and inelastic (Raman) light scattering it is possible to get information on the chemistry of e.g. acid/base reactions as well as the physical behavior of single microparticles. 相似文献
960.
V. M. Ogenko A. V. Fesenko N. D. Konovalova V. I. Stepanenko 《Reaction Kinetics and Catalysis Letters》1993,50(1-2):169-173
It has been established that CO oxidation with excess O2 on Ni/Al2O3 forms an inhomogeneous NiO contact surface that smoothes critical effects in the sharp reactivity change. 相似文献