全文获取类型
收费全文 | 2356篇 |
免费 | 592篇 |
国内免费 | 218篇 |
专业分类
化学 | 878篇 |
晶体学 | 53篇 |
力学 | 118篇 |
综合类 | 27篇 |
数学 | 477篇 |
物理学 | 1613篇 |
出版年
2024年 | 4篇 |
2023年 | 24篇 |
2022年 | 60篇 |
2021年 | 70篇 |
2020年 | 78篇 |
2019年 | 46篇 |
2018年 | 63篇 |
2017年 | 97篇 |
2016年 | 87篇 |
2015年 | 81篇 |
2014年 | 154篇 |
2013年 | 179篇 |
2012年 | 157篇 |
2011年 | 201篇 |
2010年 | 180篇 |
2009年 | 170篇 |
2008年 | 184篇 |
2007年 | 206篇 |
2006年 | 176篇 |
2005年 | 130篇 |
2004年 | 119篇 |
2003年 | 124篇 |
2002年 | 94篇 |
2001年 | 75篇 |
2000年 | 63篇 |
1999年 | 49篇 |
1998年 | 50篇 |
1997年 | 54篇 |
1996年 | 31篇 |
1995年 | 24篇 |
1994年 | 18篇 |
1993年 | 14篇 |
1992年 | 20篇 |
1991年 | 10篇 |
1990年 | 7篇 |
1989年 | 15篇 |
1988年 | 3篇 |
1987年 | 4篇 |
1986年 | 10篇 |
1985年 | 2篇 |
1984年 | 5篇 |
1983年 | 2篇 |
1982年 | 4篇 |
1981年 | 2篇 |
1980年 | 4篇 |
1979年 | 8篇 |
1976年 | 1篇 |
1974年 | 1篇 |
1973年 | 3篇 |
1959年 | 1篇 |
排序方式: 共有3166条查询结果,搜索用时 546 毫秒
131.
Abil E. Aliev Alex J. Sinclair Shen Zhou John D. Wilden Stephen Caddick Dimitri M. Kullmann Dmitri A. Rusakov 《Journal of Physical Organic Chemistry》2009,22(6):607-612
While developing a synthesis towards tagged dizocilpine (MK‐801) analogues, we observed highly restricted inversion of a nitrogen centre in a number hydroxylamines obtained as key intermediates. These compounds are shown to possess some of the structural elements which are expected to significantly hinder the nitrogen inversion, potentially leading to hydroxylamines with a chiral nitrogen centre. Free energy barriers (ΔG≠) of the nitrogen inversion were estimated to be ca. 22 kcal mol?1 at temperatures near 420 K using variable temperature NMR measurements in DMSO‐d6. Further density functional studies of a number model systems were undertaken in order to better rationalize the measured inversion barriers and follow the role of various structural factors in raising the barrier height of the nitrogen inversion process. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
132.
Vadim V. Ilyushin Zbigniew Kisiel Heinrich Mäder 《Journal of Molecular Spectroscopy》2010,259(1):26-12194
A new program is described for fitting rotation-torsion energy levels in molecules like toluene, in which the frame (C6H5) has C2v symmetry and the methyl top has C3v symmetry, i.e., for molecules where the internal rotation barrier is expanded in cos6nα, where α is the internal rotation angle and n = 1,2,…. The program is based on the theoretical framework developed by Sørensen and Pedersen in their application of the Longuet-Higgins permutation-inversion group G12 to the microwave spectrum of CH3NO2. It is specifically designed for sixfold barrier molecules, and allows the user to select almost any symmetry-allowed torsion-rotation term for inclusion in the fitting Hamiltonian. This program leads to a very successful fit of transitions in the microwave spectrum of toluene characterized by J ? 30, Ka ? 12, and by the free-rotor quantum number ∣m∣ ? 3. In these fits we included both published and rather extensive unpublished new measurements, for which fits using other torsion-rotation programs have not been very successful. The fit presented here uses 28 parameters to give an overall standard deviation of 7.4 kHz for 372 line frequencies, and results in a much improved value for the sixfold barrier for toluene, V6 = 13.832068(3) cal mol−1. 相似文献
133.
In relation to a metallic nanowire in the presence of an electrochemical potential, the sensitivity to this potential of the nanowire width is evaluated by defining and calculating a suitable parameter that can be regarded as a normalized gradient, this gradient being the derivative of the above-mentioned width with respect to the electrochemical potential. 相似文献
134.
135.
This article considers a Markov-dependent risk model with a constant dividend barrier. A system of integro-differential equations with boundary conditions satisfied by the expected discounted penalty function, with given initial environment state, is derived and solved. Explicit formulas for the discounted penalty function are obtained when the initial surplus is zero or when all the claim amount distributions are from rational family. In two state model, numerical illustrations with exponential claim amounts are given. 相似文献
136.
Vladislav S. Olkhovsky Vittoria Petrillo Jacek Jakiel Wiesław Kantor 《Central European Journal of Physics》2008,6(1):122-127
The particle tunneling through a 3-D rectangular potential barrier has been studied. The simplest model for multiple internal
reflections has been assumed. The explicit expression for all the transmission and reflection probability amplitudes have
been derived, as well as the tunneling and reflection phase times.
相似文献
137.
Performance of gain-switched all-solid-state quasi-continuous-wave tunable Ti:sapphire laser system 下载免费PDF全文
We have made a gain-switched all-solid-state quasi-continuous-wave (QCW) tunable Ti:sapphire laser system, which is pumped by a 532 nm intracavity frequency-doubled Nd:YAG laser. Based on the theory of gain-switching and the study on the influencing factors of the output pulse width, an effective method for obtaining high power and narrow pulse width output is proposed. Through deliberately designing the pump source and the resonator of the Ti:sapphire laser, when the repetition rate is 6 kHz and the length of the cavity is 220 mm, at an incident pump power of 22 W, the tunable Ti:sapphire laser from 700 to 950nm can be achieved. It has a maximum average output power of 5.6W at 800nm and the pulse width of 13.2 ns, giving an optical conversion efficiency of 25.5% from the 532 mn pump laser to the Ti:sapphire laser. 相似文献
138.
139.
140.
S. P. Sidorov 《Journal of Approximation Theory》2002,118(2):188-201
In this paper, we will show that Lagrange interpolatory polynomials are optimal for solving some approximation theory problems concerning the finding of linear widths.In particular, we will show that
, where
n is a set of the linear operators with finite rank n+1 defined on
−1,1], and where
n+1 denotes the set of polynomials p=∑i=0n+1aixi of degreen+1 such that an+11. The infimum is achieved for Lagrange interpolatory polynomial for nodes
. 相似文献
Full-size image