全文获取类型
收费全文 | 50242篇 |
免费 | 6653篇 |
国内免费 | 10516篇 |
专业分类
化学 | 49640篇 |
晶体学 | 669篇 |
力学 | 1887篇 |
综合类 | 358篇 |
数学 | 1730篇 |
物理学 | 13127篇 |
出版年
2024年 | 95篇 |
2023年 | 651篇 |
2022年 | 1072篇 |
2021年 | 1653篇 |
2020年 | 2443篇 |
2019年 | 1912篇 |
2018年 | 1867篇 |
2017年 | 2023篇 |
2016年 | 2331篇 |
2015年 | 2258篇 |
2014年 | 2975篇 |
2013年 | 4650篇 |
2012年 | 3077篇 |
2011年 | 3532篇 |
2010年 | 2746篇 |
2009年 | 3141篇 |
2008年 | 3236篇 |
2007年 | 3532篇 |
2006年 | 3251篇 |
2005年 | 3009篇 |
2004年 | 2703篇 |
2003年 | 2455篇 |
2002年 | 1751篇 |
2001年 | 1393篇 |
2000年 | 1443篇 |
1999年 | 1207篇 |
1998年 | 1006篇 |
1997年 | 941篇 |
1996年 | 787篇 |
1995年 | 707篇 |
1994年 | 622篇 |
1993年 | 492篇 |
1992年 | 470篇 |
1991年 | 337篇 |
1990年 | 240篇 |
1989年 | 228篇 |
1988年 | 183篇 |
1987年 | 124篇 |
1986年 | 99篇 |
1985年 | 131篇 |
1984年 | 91篇 |
1983年 | 48篇 |
1982年 | 79篇 |
1981年 | 62篇 |
1980年 | 48篇 |
1979年 | 45篇 |
1977年 | 38篇 |
1976年 | 50篇 |
1974年 | 42篇 |
1973年 | 37篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
181.
A highly stereoselective synthesis of the side Chain enantiomers of brassinolide and dolicholide through an aldol reaction of 2-(4-methoxy benzyloxyl) proplonylaldehyde (5) with the anion of 1- ( tert -butyldimethylsiloxy )-3-methyl-2-butanone ( 6 ) is described. 相似文献
182.
The N-carboethoxy precursor to (±)-tecomanine has been prepared in 11 steps from 2-methyl-1-buten-3-yne. The key step, Pauson-Khand cyclization of a methylated 5-aza-6-nonen-1-yne succeeds, but only in low yield, a consequence of the dialkyl substitution about the azaenyne framework. Nevertheless, the overall sequence to that point is one of the more efficient to be described. 相似文献
183.
构造了第孙中禹种强度不等的非对称三态叠加多模叠加态光场|ψ1(ABC)〉q.利用多模压缩态理论研究了态|ψ1(ABC)〉q第一正交分量高次和压缩.结果发现:①当构成态|ψ1(ABC)〉q的三个多模相干态光场的强度不相等时,在一定条件下,态|ψ1(ABC)〉q的第一正交分量可出现任意幂次的高次和压缩.②当上述的三个多模相干态光场强度相等时,态|ψ1(ABC)〉q的第一正交分量的高次和压缩现象消失.在这种情况下,态|ψ1(ABC)〉q的第一正交分量恒处于NH最小测不准态. 相似文献
184.
本文研究了催化反应非线性奇摄动边值问题.利用微分不等式理论和方法,得到了问题的解的任意次近似渐近估计. 相似文献
185.
This work is concerned with the maximum principle for optimal control problem governed by magnetohydrodynamic equations, which describe the motion of a viscous incompressible conducting fluid in a magnetic field and consist of a subtle coupling of the Navier-Stokes equation of viscous incompressible fluid flow and the Maxwell equation of electromagnetic field. An integral type state constraint is considered. 相似文献
186.
O. N. Lyubenko I. G. Ermolenko L. D. Patsenker Krasovitskii I. N. Tur 《Chemistry of Heterocyclic Compounds》2003,39(4):511-519
The behavior of dimethylamino-substituted 7H-benzo[de]pyrazolo[5,1-a]isoquinoline-7-ones, synthesized for the first time, under conditions of the Vilsmeier-Haack reaction. It has been shown that, on heating with POCl3 and DMF, they are converted by electrophilic substitution at the position ortho to the dimethylamino group, followed by cyclization of the iminium adduct to a quinazolinium salt. When an acetyl group is present, the Arnold reaction, leading to the formation of a chloroaryl, accompanies the heterocyclization. The rates and proportion of the reaction products depend on the position of the dimethyl groups relative to the pyrazole ring. 相似文献
187.
To find an effective extraction and removal method for palladium(II), which is one of the main fission products from an acidic nuclear spent fuel solution, the extraction behavior of palladium(II) from a nitric acid medium by an acidic chelating extractant, 1-phenyl-3-methyl-4-trifluoroacetylpyrazolone-5-one (HPMTP) and a tertiary amine of high molecular weight, tri-n-octylamine (TOA), has been studied by spectrophotometry. A noticeable antagonistic extraction effect was observed in the extraction system under the given conditions. To understand this phenomenon, a preliminary investigation was performed to explain the mechanism of this reaction. According to the theory of corresponding solutions (TCS), the association reaction between HPMTP and TOA is proposed in the organic phase. An associated species, HPMTP·TOA, formed through hydrogen bonding in a chloroform medium might be the main reason why an antagonistic extraction effect occurred. The association constant between HPMTP and TOA was calculated to be 2.86±0.05. 相似文献
188.
Based on the newly constructed two mutually conjugate 3-mode entangled states of continuum variablesin three-mode Fock space we introduce entangled fractional Fourier transform (EFFT) for the tripartite entangled staterepresentations, which are not a direct product of three 1-dimensional FFTs. The eigenmodes of EFFT are obtained,which is different from the usual Hermite polynomials. The EFFT of the three-mode squeezed state is derived. 相似文献
189.
Piotr Winiarek Juan Carlos Q. Colmenares Elbieta Fedoryska 《Reaction Kinetics and Catalysis Letters》2004,81(2):333-339
Transformations of diphenylmethane were investigated in a flow system in the presence of alumina and silica-alumina modified
with boron trifluoride at atmospheric pressure and elevated temperatures. Hydrogen transfer reaction was observed on addition
of tetralin to the substrate stream.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
190.
M. B. Litvinova 《Journal of Applied Spectroscopy》2004,71(1):143-146
It has been established that undoped gallium crystals exhibit anti-Stokes radiation with energy corresponding to the interband recombination of these crystals. Its appearance is most probably determined by EL2 defects, and its intensity depends on the product of the cross sections for photoionization of electrons and the holes from these defects. 相似文献