全文获取类型
收费全文 | 297854篇 |
免费 | 5896篇 |
国内免费 | 3737篇 |
专业分类
化学 | 162142篇 |
晶体学 | 4352篇 |
力学 | 13494篇 |
综合类 | 253篇 |
数学 | 32878篇 |
物理学 | 94368篇 |
出版年
2021年 | 2435篇 |
2020年 | 2729篇 |
2019年 | 2852篇 |
2018年 | 4282篇 |
2017年 | 4387篇 |
2016年 | 5540篇 |
2015年 | 3984篇 |
2014年 | 5433篇 |
2013年 | 12479篇 |
2012年 | 11661篇 |
2011年 | 13634篇 |
2010年 | 9664篇 |
2009年 | 9455篇 |
2008年 | 12087篇 |
2007年 | 11916篇 |
2006年 | 11223篇 |
2005年 | 12867篇 |
2004年 | 11653篇 |
2003年 | 9432篇 |
2002年 | 8071篇 |
2001年 | 9610篇 |
2000年 | 7235篇 |
1999年 | 5307篇 |
1998年 | 4227篇 |
1997年 | 4143篇 |
1996年 | 4011篇 |
1995年 | 3515篇 |
1994年 | 3497篇 |
1993年 | 3310篇 |
1992年 | 3806篇 |
1991年 | 3853篇 |
1990年 | 3579篇 |
1989年 | 3379篇 |
1988年 | 3299篇 |
1987年 | 3276篇 |
1986年 | 3187篇 |
1985年 | 4165篇 |
1984年 | 4148篇 |
1983年 | 3352篇 |
1982年 | 3562篇 |
1981年 | 3312篇 |
1980年 | 3067篇 |
1979年 | 3369篇 |
1978年 | 3595篇 |
1977年 | 3653篇 |
1976年 | 3579篇 |
1975年 | 3230篇 |
1974年 | 3391篇 |
1973年 | 3369篇 |
1972年 | 2532篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
41.
Y. Wardi 《Journal of Optimization Theory and Applications》1989,61(3):473-485
A stochastic algorithm for finding stationary points of real-valued functions defined on a Euclidean space is analyzed. It is based on the Robbins-Monro stochastic approximation procedure. Gradient evaluations are done by means of Monte Carlo simulations. At each iteratex
i
, one sample point is drawn from an underlying probability space, based on which the gradient is approximated. The descent direction is against the approximation of the gradient, and the stepsize is 1/i. It is shown that, under broad conditions, w.p.1 if the sequence of iteratesx
1,x
2,...generated by the algorithm is bounded, then all of its accumulation points are stationary. 相似文献
42.
T. Kumita Y. Kamiya M. Babzien I. Ben-Zvi K. Kusche I. V. Pavlishin I. V. Pogorelsky D. P. Siddons V. Yakimenko T. Hirose T. Omori J. Urakawa K. Yokoya D. Cline F. Zhou 《Laser Physics》2006,16(2):267-271
Thomson scattering of high-power laser and electron beams is a good test of electrodynamics in the high-field region. We demonstrated production of high-intensity X-rays in the head-on collision of a CO2 laser and 60-MeV electron beams at Brookhaven National Laboratory, Accelerator Test Facility. The energy of an X-ray photon was limited at 6.5 keV in the linear (lowest order) Thomson scattering, but the nonlinear (higher order) process produces higher energy X-rays. We measured the angular distribution of the high-energy X-rays and confirmed that it agrees with theoretical predictions. 相似文献
43.
The direct regioselective C–H arylation of 3-methoxythiophene and 3,4-ethylenedioxythiophene (EDOT) was performed successfully under ‘Heck-type’ experimental conditions. This novel synthetic methodology has been used to prepare in a more simple way a series of oligothiophenes interesting for the electronic industry to build new synthetic organic materials. 相似文献
44.
(S)-2-[2-(p-Tolylsulfinyl)phenyl]acetaldehyde reacts with different O-silylated ketenethioacetals in the presence of Yb(OTf)3 yielding β-hydroxythioesters in high yields and diastereoselectivities. The obtained compounds were readily transformed into β-hydroxyacids and their corresponding diols. These Mukaiyama aldol reactions are a direct evidence of the ability of the sulfinyl group to control 1,5- and 1,6-asymmetric induction processes. 相似文献
45.
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
46.
An overview of theoretical and experimental studies of the coherent type-B e
+
e-pair production by photons in aligned crystals performed at Nuclear Physics Institute at Tomsk Polytechnic University is given. 相似文献
47.
48.
49.
We analyze diffusion from a periodic array of hemispherical droplets through a membrane. We find that the multiple sources do not interact strongly, even when the droplets are closely spaced, so that the flux through the membrane appears nearly additive. 相似文献
50.