全文获取类型
收费全文 | 47089篇 |
免费 | 2765篇 |
国内免费 | 6826篇 |
专业分类
化学 | 42251篇 |
晶体学 | 1560篇 |
力学 | 375篇 |
综合类 | 315篇 |
数学 | 4208篇 |
物理学 | 7971篇 |
出版年
2024年 | 52篇 |
2023年 | 383篇 |
2022年 | 1241篇 |
2021年 | 1155篇 |
2020年 | 1052篇 |
2019年 | 1977篇 |
2018年 | 1055篇 |
2017年 | 1965篇 |
2016年 | 1438篇 |
2015年 | 1323篇 |
2014年 | 1760篇 |
2013年 | 3753篇 |
2012年 | 2753篇 |
2011年 | 2973篇 |
2010年 | 2534篇 |
2009年 | 2827篇 |
2008年 | 2990篇 |
2007年 | 3084篇 |
2006年 | 2903篇 |
2005年 | 2548篇 |
2004年 | 2541篇 |
2003年 | 2089篇 |
2002年 | 1772篇 |
2001年 | 1416篇 |
2000年 | 1366篇 |
1999年 | 1061篇 |
1998年 | 982篇 |
1997年 | 821篇 |
1996年 | 702篇 |
1995年 | 701篇 |
1994年 | 552篇 |
1993年 | 491篇 |
1992年 | 481篇 |
1991年 | 350篇 |
1990年 | 201篇 |
1989年 | 218篇 |
1988年 | 187篇 |
1987年 | 123篇 |
1986年 | 93篇 |
1985年 | 109篇 |
1984年 | 117篇 |
1983年 | 38篇 |
1982年 | 75篇 |
1981年 | 96篇 |
1980年 | 71篇 |
1979年 | 80篇 |
1978年 | 43篇 |
1977年 | 38篇 |
1976年 | 25篇 |
1973年 | 19篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
3.
Shougo Higashi 《Surface science》2006,600(3):591-597
We have studied the individual adsorption of Mn and Bi, and their coadsorption on Cu(0 0 1) by low-energy electron diffraction (LEED). For Mn, we have determined the c(2 × 2) structure formed at 300 K, whose structure had been determined by several methods. We reconfirmed by a tensor LEED analysis that it is a substitutional structure and that a previously reported large corrugation (0.30 Å) between substitutional Mn and remaining surface Cu atoms coincides perfectly with the present value. In the individual adsorption of Bi, we have found a c(4 × 2) structure, which is formed by cooling below ∼250 K a surface prepared by Bi deposition of ∼0.25 ML coverage at 300 K where streaky half-order LEED spots appear. The c(4 × 2) structure has been determined by the tensor LEED analysis at 130 K and it is a substitutional structure. In the coadsorption, we found a c(6 × 4) structure, which has been determined by the tensor LEED analysis. It is very similar to the previously determined structure of the c(6 × 4) formed by coadsorption of Mg and Bi, and embedded MnBi4 clusters are arranged in the top Cu layer instead of MgBi4. Large lateral displacements of Bi atoms in the c(6 × 4)-(Mn + Bi) suggest that the Mn atoms undergo the size-enhancement caused by their large magnetic moment. 相似文献
4.
Debendra K. Mohapatra Dhananjoy Mondal Mukund S. Chorghade Mukund K. Gurjar 《Tetrahedron letters》2006,47(52):9215-9219
The non-proteinogenic amino acid 3-hydroxy-4-methylproline (HMP) is an active constituent of some potent antimicrobials including echinocandins, nostopeptins, pneumocandins, sporiofungin and mulundocandins. A synthesis has been achieved in 10 steps with 29% overall yield; the Evans’ aldol reaction using Crimmins’ modified method was pivotal to the success of the strategy. 相似文献
5.
6.
主要介绍了一个引理,这个引理奠定了K4-同胚图K4(α,1,1,δ,ε,η)色性研究的基础。 相似文献
7.
本文首先将文[1]中的BLD映射推广为弱(L1,L2)-BLD映射,并证明了如下正则性结果:存在两个可积指数 P1=P1(n,L1,L2)<n<q1=q1(n,L1,L2),使得对任意弱(L1,L2)-BLD映射f∈(Ω,Rn),都有f∈(Ω,Rn),即f为(L1,L2)-BLD映射. 相似文献
8.
P. R. Latthe P. S. Shinge Bharati V. Badami P. B. Patil S. N. Holihosur 《Journal of Chemical Sciences》2006,118(3):249-256
3-[4-(Azidocarbonyl)]phenylsydnone (2) obtained from 3-(4-hydrazinocarbonyl) phenylsydnone (1) on Curtius rearrangement with alcohols, water and amines afforded the corresponding carbamates (3a-h), 4,4′-(sydnone-3-yl) diphenyl urea (4) and 4-(heterocyclyl)phenyl ureas (5a-l). Compounds (5a-l) on one-pot ring conversion yielded the 1,3,4-oxadiazolin-2-one derivatives (6a-l), which on reaction with N2H4 gave the 4-amino-1,2,4-triazolin-3-ones (7a-l). All these compounds exhibited moderate antimicrobial activity against the few microbes tested. The carbamates have been
found to be more toxic against fourth instar larvae ofAedes aegypti, in particular, then-butyl derivative (3e). 相似文献
9.
G.P. Romanelli D. Bennardi D.M. Ruiz G. Baronetti H.J. Thomas 《Tetrahedron letters》2004,45(48):8935-8939
Substituted coumarins are synthesized from phenols and β-ketoesters by the Pechmann reaction, using a Wells-Dawson heteropolyacid (H6P2W18O62·24H2O) as catalyst by a solvent-free procedure. This one requires low reaction times, 130 °C temperature and as little as 1 mol % of Wells-Dawson acid, obtaining good to excellent yields of coumarins. The catalyst showed to be reusable with no differences in the yields. The results are compared with those of the reactions performed in toluene solution. The presented synthetic procedure is a convenient, clean and fast alternative for synthesizing 4-substituted coumarins (17 examples). 相似文献
10.
Rhodium catalysts have been prepared on palygorskite and montmorillonite (clay) supports by reduction with hydrogen (1 atmosphere) at room temperature of a cationic organometallic rhodium compound anchored to the support. The activity of these catalysts for the hydrogenation of liquid-phase 1-hexene remains constant with increase of prehydrogenation time and with re-use for several runs. No rhodium leaching is observed. 相似文献