全文获取类型
收费全文 | 55617篇 |
免费 | 2760篇 |
国内免费 | 1580篇 |
专业分类
化学 | 29445篇 |
晶体学 | 882篇 |
力学 | 3032篇 |
综合类 | 91篇 |
数学 | 6067篇 |
物理学 | 20440篇 |
出版年
2023年 | 366篇 |
2022年 | 628篇 |
2021年 | 873篇 |
2020年 | 899篇 |
2019年 | 873篇 |
2018年 | 1057篇 |
2017年 | 905篇 |
2016年 | 1379篇 |
2015年 | 1095篇 |
2014年 | 1570篇 |
2013年 | 2804篇 |
2012年 | 2915篇 |
2011年 | 3414篇 |
2010年 | 2496篇 |
2009年 | 2430篇 |
2008年 | 2725篇 |
2007年 | 2614篇 |
2006年 | 2439篇 |
2005年 | 2011篇 |
2004年 | 1796篇 |
2003年 | 1525篇 |
2002年 | 1425篇 |
2001年 | 1941篇 |
2000年 | 1463篇 |
1999年 | 1242篇 |
1998年 | 998篇 |
1997年 | 896篇 |
1996年 | 842篇 |
1995年 | 782篇 |
1994年 | 706篇 |
1993年 | 611篇 |
1992年 | 658篇 |
1991年 | 685篇 |
1990年 | 670篇 |
1989年 | 585篇 |
1988年 | 571篇 |
1987年 | 534篇 |
1986年 | 460篇 |
1985年 | 541篇 |
1984年 | 515篇 |
1983年 | 388篇 |
1982年 | 436篇 |
1981年 | 417篇 |
1980年 | 349篇 |
1979年 | 423篇 |
1978年 | 406篇 |
1977年 | 421篇 |
1976年 | 413篇 |
1975年 | 362篇 |
1973年 | 354篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
62.
L. Tong Y. Tian Q. Y. Wang Y. S. Ling 《International Journal of Infrared and Millimeter Waves》2006,27(9):1307-1314
The paper introduces a special system calibration technology in s-parameters measurement of microwave and millimeter wave
devices. The 8-term errors module is built by analyzing the signals flowing in the measurement system. Then the calibration
technology using non-standard kits is designed on the base. Finally, the experiment using the calibration technology is introduced. 相似文献
63.
Natural populations, whose generations are non-overlapping, can be modelled by difference equations that describe how the populations evolve in discrete time-steps. In the 1970s ecological research detected chaos and other forms of complex dynamics in simple population dynamics models, initiating a new research tradition in ecology. However, in former studies most of the investigations of complex population dynamics were mainly concentrated on single populations instead of higher dimensional ecological systems. This paper reports a recent study on the complicated dynamics occurring in a class of discrete-time models of predator–prey interaction based on age-structure of predator. The complexities include (a) non-unique dynamics, meaning that several attractors coexist; (b) antimonotonicity; (c) basins of attraction (defined as the set of the initial conditions leading to a certain type of an attractor) with fractal properties, consisting of pattern of self-similarity and fractal basin boundaries; (d) intermittency; (e) supertransients; and (f) chaotic attractors. 相似文献
64.
许多实验对用CsI(Tl)闪烁晶体作为探测器来寻找和探测暗物质的可行性进行了研究.本工作利用8MeV单能中子轰击CsI(Tl)晶体探测器来研究Cs核和I核的QuenchingFactor.在数据处理中,运用脉冲形状甄别(PSD)方法来分辨反冲核信号和本底信号.实验结果表明,在7keV到132keV的能区中,Quench ingFactor随着反冲核能量的减少而增加.在探测暗物质的实验中,这一性质对于CsI(Tl)晶体探测器获得较低的能量阈值是很有利的. 相似文献
65.
F. Chen L. Liu Z. Shen G.Q. Xu T.S.A. Hor 《Applied Physics A: Materials Science & Processing》2002,74(2):317-319
Free-standing and supported hydrogenated amorphous carbon films (a:C–H) were prepared upon pyrolysis of the polymer formed
by ethanolamine (EA) and citric acid (CA), under an ambient atmosphere at 300 °C. EA facilitates the formation of the macroscopic
films, while CA is essential for obtaining the a:C–H microstructure, which comprises a mixture of sp2 and sp3 carbon.
Received: 29 May 2001 / Accepted: 17 August 2001 / Published online: 20 December 2001 相似文献
66.
Q. Sun Q. Wang X.G. Gong V. Kumar Y. Kawazoe 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2002,18(1):77-81
We report results of the atomic and electronic structures of Al7C cluster using ab initio molecular dynamics with ultrasoft pseudopotentials and generalized gradient approximation. The lowest energy structure is
found to be the one in which carbon atom occupies an interstitial position in Al7 cluster. The electronic structure shows that the recent observation [Chem. Phys. Lett. 316, 31 (2000)] of magic behavior of Al7C- cluster is due to a large highest occupied and lowest unoccupied molecular orbital (HOMO-LUMO) gap which makes Al7C- chemically inert. These results have further led us to the finding of a new neutral magic cluster Al7N which has the same number of valence electrons as in Al7C- and a large HOMO-LUMO gap of 1.99 eV. Further, calculations have been carried out on (Al7N)2 to study interaction between magic clusters.
Received 28 July 2001 相似文献
67.
This paper is concerned with the implementation and testing of an algorithm for solving constrained least-squares problems. The algorithm is an adaptation to the least-squares case of sequential quadratic programming (SQP) trust-region methods for solving general constrained optimization problems. At each iteration, our local quadratic subproblem includes the use of the Gauss–Newton approximation but also encompasses a structured secant approximation along with tests of when to use this approximation. This method has been tested on a selection of standard problems. The results indicate that, for least-squares problems, the approach taken here is a viable alternative to standard general optimization methods such as the Byrd–Omojokun trust-region method and the Powell damped BFGS line search method. 相似文献
68.
Solitary waves and their bifurcations of KdV like equation with higher order nonlinearity 总被引:3,自引:1,他引:2
We investigate the KdV like equation with higher order nonlinearity ut + a(1 +bun)unux + uxxx = 0with n ≥ 1, a, b ∈ R and α≠ 0. The bifurcations and explicit expressions of solitary wave solutions for theequation are discussed by using the bifurcation method and qualitative theory of dynamical systems. Thebifurcation diagrams, existence and number of the solitary waves are given. 相似文献
69.
The revival of the nondiffracting vortex beam after its interaction with the 2D on axis obstacle is examined. We show that the phase topology and the spatial distribution of the orbital angular momentum of the beam transmitted through the obstacle regenerate to the initial form during further free propagation. We verify that the healing effect appears even if the interaction is accompanied by the exchange of the orbital angular momentum. 相似文献
70.
采用粒子模拟的方法并考虑电子束与电磁波的相互作用,首次直接得到了速调管输出信号的离子噪声图像,阐述了束电子、二次电子、离子、电磁场之间的相互作用的动力学过程. 指出离子噪声所表现出来的相位波动是由电子束速度的波动引起的,电子束速度的变化来源于管内离子数量的变化,离子的数量的变化又与电子束状态变化相互影响,这是离子噪声产生的根本原因. 二次电子对离子噪声产生过程的影响甚微,但是其行为却反映了离子噪声的形成机理. 离子噪声引发的输出信号幅度波动取决于电子束速度和半径的改变,与离子行为密切相关.
关键词:
离子噪声
速调管
粒子模拟
电子束 相似文献