全文获取类型
收费全文 | 16981篇 |
免费 | 1954篇 |
国内免费 | 1430篇 |
专业分类
化学 | 4223篇 |
晶体学 | 141篇 |
力学 | 1763篇 |
综合类 | 392篇 |
数学 | 8491篇 |
物理学 | 5355篇 |
出版年
2024年 | 35篇 |
2023年 | 172篇 |
2022年 | 337篇 |
2021年 | 411篇 |
2020年 | 479篇 |
2019年 | 448篇 |
2018年 | 432篇 |
2017年 | 567篇 |
2016年 | 683篇 |
2015年 | 495篇 |
2014年 | 865篇 |
2013年 | 1463篇 |
2012年 | 816篇 |
2011年 | 1037篇 |
2010年 | 904篇 |
2009年 | 1056篇 |
2008年 | 1069篇 |
2007年 | 1078篇 |
2006年 | 988篇 |
2005年 | 899篇 |
2004年 | 743篇 |
2003年 | 746篇 |
2002年 | 646篇 |
2001年 | 547篇 |
2000年 | 518篇 |
1999年 | 427篇 |
1998年 | 384篇 |
1997年 | 329篇 |
1996年 | 303篇 |
1995年 | 270篇 |
1994年 | 172篇 |
1993年 | 158篇 |
1992年 | 138篇 |
1991年 | 119篇 |
1990年 | 89篇 |
1989年 | 83篇 |
1988年 | 70篇 |
1987年 | 42篇 |
1986年 | 48篇 |
1985年 | 54篇 |
1984年 | 51篇 |
1983年 | 19篇 |
1982年 | 34篇 |
1981年 | 31篇 |
1980年 | 17篇 |
1979年 | 26篇 |
1978年 | 14篇 |
1977年 | 16篇 |
1976年 | 9篇 |
1973年 | 8篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
Based on the beam coherence-polarization (BCP) matrix, the polarization property of coherent and incoherent Gaussian beam combinations is studied in detail. The general expressions for the degree of polarization P of the resulting beam in case of incoherent and coherent combinations are derived. It is shown that P is dependent on the incoherent or coherent combination, propagation distance, separation, azimuth of the polarization plane and numbers of beamlets in general. The irradiance distribution of the resulting beam for the coherent cases depends on the azimuth of the polarization plane of beamlets. However, for the incoherent case it does not. 相似文献
42.
A solution to the phase problem in optics is considered within the context of the analysis of signals changing in time and of the effect of an object under investigation on the probing radiation. The solution is based on the use of a twin-wave interferometer with phase modulation in one of the channels and a spectrometer. The analysis is concerned, in particular, with signals and processes of ultrashort duration. To shift the frequency in the interferometer channel, an electrooptical crystal of ferroelectric perovskite is used as the modulator. Errors of amplitude and phase structures determined are analyzed. 相似文献
43.
Hidetaka Tobita 《Journal of Polymer Science.Polymer Physics》2004,42(14):2801-2812
The matrix formula developed in the context of heterochain theory, M?w = M?wp + WF ( I ? M )?1 S , was applied to describe the molecular weight development during free‐radical multicomponent polymerization. All of the required probabilistic parameters are expressed in terms of the kinetic‐rate constants and the various concentrations associated with them. In free‐radical polymerization, the number of heterochain types, N, needs to be extrapolated to infinity, and such extrapolation is conducted with only three different N values. This matrix formula can be used as a benchmark test if other approximate approaches can give reasonable estimates of the weight‐average molecular weights. The moment equations with the average pseudo‐kinetic‐rate constants for branching and crosslinking reactions may provide poor estimates when the copolymer composition drift during polymerization is very significant. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2801–2812, 2004 相似文献
44.
45.
以三能级V型系统为例研究修饰态布居的选择性激发对无反转激光增益的作用. 当非 相干驱动场的频谱宽度远小于驱动场产生的修饰态能级的间距时,非相干驱动场只将一个修 饰态的布居抽运至激发态. 借助原子的衰减通道,系统中形成单向布居转移通道,从而建立 修饰态布居的选择性激发. 利用修饰态布居的选择性激发,可以摆脱裸态共振无反转激光的 三个限制: (1) 不再要求辅助的低频驱动跃迁比高频激光跃迁具有更高的衰减速率;(2) 显 著降低非相干激发速率的阈值;(3) 无反转激光的线性增益不再反比于相干驱动场的强
关键词:
修饰态布居的选择性激发
无反转激光增益
原子衰减速率
非相干激发阈值速率 相似文献
46.
47.
Introduction In the previous studies on the oxidation reaction,peroxydisulfate was widely used as an oxidizingagent.1-5 One of the advantages of this oxidant lies in itsstability in a wide range of pH values. The reaction be-tween Fe(CN)5L3- (L=N-aromatic heterocyclic li-gands) and S2O8 2- has been proved to proceed throughan outer-sphere electron transfer mechanism.5 For a re-action [(Eq. (1)] to be under an outer-sphere mechanismthe steps involved are the formation of a reactant … 相似文献
48.
In this paper, we prove convergence rates for spherical spline Hermite interpolation on the sphere Sd−1 via an error estimate given in a technical report by Luo and Levesley. The functionals in the Hermite interpolation are either point evaluations of pseudodifferential operators or rotational differential operators, the desirable feature of these operators being that they map polynomials to polynomials. Convergence rates for certain derivatives are given in terms of maximum point separation. 相似文献
49.
In this paper, we study the stability conditions of the MMAP[K]/G[K]/1/LCFS preemptive repeat queue. We introduce an embedded Markov chain of matrix M/G/1 type with a tree structure and identify conditions for the Markov chain to be ergodic. First, we present three conventional methods for the stability problem of the queueing system of interest. These methods are either computationally demanding or do not provide accurate information for system stability. Then we introduce a novel approach that develops two linear programs whose solutions provide sufficient conditions for stability or instability of the queueing system. The new approach is numerically efficient. The advantages and disadvantages of the methods introduced in this paper are analyzed both theoretically and numerically. 相似文献
50.
本文给出了 n阶 r-不可分矩阵的本原指数的上界 ,即任 n阶 r—不可分矩阵 A的本原指数 (A)≤n+(r- ) 2r (1≤ r相似文献