全文获取类型
收费全文 | 9116篇 |
免费 | 311篇 |
国内免费 | 75篇 |
专业分类
化学 | 6669篇 |
晶体学 | 41篇 |
力学 | 205篇 |
数学 | 1379篇 |
物理学 | 1208篇 |
出版年
2023年 | 33篇 |
2022年 | 47篇 |
2021年 | 110篇 |
2020年 | 145篇 |
2019年 | 142篇 |
2018年 | 102篇 |
2017年 | 73篇 |
2016年 | 221篇 |
2015年 | 189篇 |
2014年 | 222篇 |
2013年 | 517篇 |
2012年 | 544篇 |
2011年 | 687篇 |
2010年 | 310篇 |
2009年 | 240篇 |
2008年 | 520篇 |
2007年 | 605篇 |
2006年 | 628篇 |
2005年 | 503篇 |
2004年 | 460篇 |
2003年 | 395篇 |
2002年 | 364篇 |
2001年 | 135篇 |
2000年 | 90篇 |
1999年 | 86篇 |
1998年 | 81篇 |
1997年 | 107篇 |
1996年 | 113篇 |
1995年 | 85篇 |
1994年 | 77篇 |
1993年 | 91篇 |
1992年 | 75篇 |
1991年 | 57篇 |
1990年 | 84篇 |
1989年 | 46篇 |
1988年 | 63篇 |
1987年 | 61篇 |
1986年 | 74篇 |
1985年 | 107篇 |
1984年 | 104篇 |
1983年 | 70篇 |
1982年 | 96篇 |
1981年 | 94篇 |
1980年 | 96篇 |
1979年 | 68篇 |
1978年 | 85篇 |
1977年 | 65篇 |
1976年 | 59篇 |
1975年 | 53篇 |
1973年 | 59篇 |
排序方式: 共有9502条查询结果,搜索用时 0 毫秒
161.
Redl FX Black CT Papaefthymiou GC Sandstrom RL Yin M Zeng H Murray CB O'Brien SP 《Journal of the American Chemical Society》2004,126(44):14583-14599
We have investigated the structural, magnetic, and electronic properties of nonstoichiometric iron oxide nanocrystals prepared by decomposition of iron(II) and iron(0) precursors in the presence of organic solvents and capping groups. The highly uniform, crystalline, and monodisperse nanocrystals that were produced enabled a full structural and compositional survey by electron microscopy and X-ray diffraction. The complex and metastable behavior of nonstoichiometric iron oxide (wüstite) at the nanoscale was studied by a combination of Mossbauer spectroscopy and magnetic characterization. Deposition from hydrocarbon solvents with subsequent self-assembly of iron oxide nanocrystals into superlattices allowed the preparation of continuous thin films suitable for electronic transport measurements. 相似文献
162.
Hassan Rabaâ Bernd Engels Thomas Hupp A. Stephen K. Hashmi 《International journal of quantum chemistry》2007,107(2):359-365
A general scheme for the endo‐ and exo‐cyclization of furan reactivity with [L ‐AuIII, IClx] with (x = 3, 1 and L ‐acetylene and vinylidene) complexes is investigated using density functional theory (DFT) code. Two conceivable mechanisms via a [4 + 2] Diels–Alder process or carbene complex are analyzed. According to the activation energy values of the gold (III and I) catalyst, the first mechanism, which implies the Diels–Alder reaction of AuIII, is thermodynamically favored and gives more evidence of the intramolecular addition of the furan with the alkynes. The second mechanism, presumably assisted by the spontaneous formation of the exo‐vinylidene complexes and intermediates of gold (III, I) by forming the carbene complex, is kinetically favored. Additionally, we compare our results with other structures with intramolecular additions that exhibit the quasi‐similarity of gold analogue structures. Differences in activation energies are observed, according to the functional used. Finally, we probe the solvent effects, which decrease the energy barrier in the path. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 相似文献
163.
164.
The hetero-Diels-Alder reaction between aldehydes, 2, and bis-1,1-dimethoxy-3-trimethyl-1-siloxy-1,3-butadiene, 1, produces 2-methoxy-5,6-dihydro-γ-pyrones, 4, which are subsequently hydrolyzed to afford substituted 3-oxo-δ-lactones, 5, in high yields, providing a new and efficient approach to these synthetically useful intermediates. 相似文献
165.
Summary We describe an approach to protein structure comparison designed to detect distantly related proteins of similar fold, where the procedure must be sufficiently flexible to take into account the elasticity of protein folds without losing specificity. Protein structures are represented as a series of secondary structure elements, where for each element a local environment describes its relations with the elements that surround it. Secondary structures are then aligned by comparing their features and local environments. The procedure is illustrated with searches of a database of 468 protein structures in order to identify proteins of similar topology to porcine pepsin, porphobilinogen deaminase and serum amyloid P-component. In all cases the searches correctly identify protein structures of similar fold as the search proteins. Multiple cross-comparisons of protein structures allow the clustering of proteins of similar fold. This is exemplified with a clustering of /- and -class protein structures. We discuss applications of the comparison and clustering of three-dimensional protein structures to comparative modelling and structure-based protein design. 相似文献
166.
Two partial ordersP andQ on a setX arecomplementary (written asPQ) if they share no ordered pairs (except for loops) but the transitive closure of the union is all possible ordered pairs. For each positive integern we form a graph Pos
n
consisting of all nonempty partial orders on {1, ,n} with edges denoting complementation. We investigate here properties of the graphs Pos
n
. In particular, we show:
| The diameter of Pos n is 5 for alln>2 (and hence Pos n is connected for alln); | |
| With probability 1, the distance between two members of Pos n is 2; | |
| The graphs Pos n are universal (i.e. every graph occurs as an induced subgraph of some Pos n ); | |
|
The maximal size (n) of an independent set of Pos
n
satisfies the asymptotic formula
|
设为首页 | 免责声明 | 关于勤云 | 加入收藏 |
Copyright©北京勤云科技发展有限公司 京ICP备09084417号 |