全文获取类型
收费全文 | 25316篇 |
免费 | 3187篇 |
国内免费 | 1541篇 |
专业分类
化学 | 2869篇 |
晶体学 | 195篇 |
力学 | 8069篇 |
综合类 | 328篇 |
数学 | 11567篇 |
物理学 | 7016篇 |
出版年
2024年 | 42篇 |
2023年 | 238篇 |
2022年 | 343篇 |
2021年 | 429篇 |
2020年 | 646篇 |
2019年 | 512篇 |
2018年 | 537篇 |
2017年 | 748篇 |
2016年 | 825篇 |
2015年 | 607篇 |
2014年 | 1114篇 |
2013年 | 1753篇 |
2012年 | 1155篇 |
2011年 | 1297篇 |
2010年 | 1154篇 |
2009年 | 1362篇 |
2008年 | 1395篇 |
2007年 | 1538篇 |
2006年 | 1425篇 |
2005年 | 1412篇 |
2004年 | 1234篇 |
2003年 | 1297篇 |
2002年 | 1134篇 |
2001年 | 1045篇 |
2000年 | 953篇 |
1999年 | 932篇 |
1998年 | 789篇 |
1997年 | 706篇 |
1996年 | 532篇 |
1995年 | 479篇 |
1994年 | 353篇 |
1993年 | 306篇 |
1992年 | 310篇 |
1991年 | 248篇 |
1990年 | 214篇 |
1989年 | 124篇 |
1988年 | 103篇 |
1987年 | 99篇 |
1986年 | 74篇 |
1985年 | 103篇 |
1984年 | 88篇 |
1983年 | 55篇 |
1982年 | 59篇 |
1981年 | 55篇 |
1980年 | 33篇 |
1979年 | 40篇 |
1978年 | 44篇 |
1977年 | 36篇 |
1976年 | 22篇 |
1957年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
131.
A boundary element method for the calculation of noise barrier insertion loss in the presence of atmospheric turbulence 总被引:1,自引:0,他引:1
Yiu Wai Lam 《Applied Acoustics》2004,65(6):583-603
Atmospheric turbulence is an important factor that limits the amount of attenuation a barrier can provide in the outdoor environment. It is therefore important to develop a reliable method to predict its effect on barrier performance. The boundary element method (BEM) has been shown to be a very effective technique for predicting barrier insertion loss in the absence of turbulence. This paper develops a simple and efficient modification of the BEM formulation to predict the insertion loss of a barrier in the presence of atmospheric turbulence. The modification is based on two alternative methods: (1) random realisations of log-amplitude and phase fluctuations of boundary sources and (2) de-correlation of source coherence using the mutual coherence function (MCF). An investigation into the behaviours of these two methods is carried out and simplified forms of the methods developed. Some systematic differences between the predictions from the methods are found. When incorporated into the BEM formulation, the method of random realisations and the method of MCF de-correlation provide predictions that agree well with predictions by the parabolic equation method and by the scattering cross-section method on a variety of thin barrier configurations. 相似文献
132.
In this paper, based on the Lame function and Jacobi elliptic function, the perturbation method is appliedto some nonlinear evolution equations to derive their multi-order solutions. 相似文献
133.
134.
Margherita De RosaMaria Rosaria Acocella Rosaria VillanoAnnunziata Soriente Arrigo Scettri 《Tetrahedron letters》2003,44(32):6087-6090
Initial concentration of enantioenriched or enantiopure catalysts proved to be an important factor for the achievement of a more pronounced amplification of ee's in the Ti(IV)/BINOL-catalyzed aldol reaction of an O-silyldienolate. 相似文献
135.
M. I. Gil' 《Proceedings of the American Mathematical Society》2003,131(12):3737-3746
A linear operator in a separable Hilbert space is called a quasinormal one if it is a sum of a normal operator and a compact one. In the paper, bounds for the spectrum of quasinormal operators are established. In addition, the lower estimate for the spectral radius is derived. Under some restrictions, that estimate improves the well-known results. Applications to integral operators and matrices are discussed. Our results are new even in the finite-dimensional case.
136.
137.
The influence of the parameter of amplitude-phase coupling for nonlinear amplification on the phase-frequency characteristics of an injection laser in the regime of pumping current modulation has been investigated theoretically. The parameter is defined as the coefficient of proportionality between changes in the real and imaginary parts of the refractive index during variation of the density of photons in the cavity. The substantial dependence of the coefficient of phase modulation on the parameter of the amplitude-phase coupling for nonlinear amplification at small modulation frequencies is shown. A technique to determine the indicated parameter based on the frequency dependence of the ratio between the coefficients of phase and amplitude modulation is suggested. 相似文献
138.
N. S. Kazak E. G. Katranji I. A. Utkin A. A. Ryzhevich A. N. Khilo 《Journal of Applied Spectroscopy》2004,71(5):697-701
The phenomenon of self-diffraction of Bessel light beams (BLB) in a nonlinear liquid medium has been studied experimentally and theoretically for the first time. Diffraction maxima which do not correspond to integer orders for an induced periodic structure have been registered. It has been shown that the appearance of these maxima is due to the initial BLB modulation, which can be caused by the departure of the axicon refracting surface from the ideal conical surface, as well as by the imperfection of the form of the Gaussian beam incident on the axicon. 相似文献
139.
本文利用比较原理以及一种特殊变换研究了一类非线性退化方程的混合问题,得到了这类复杂问题解的渐进性质。 相似文献
140.