全文获取类型
收费全文 | 1544篇 |
免费 | 257篇 |
国内免费 | 61篇 |
专业分类
化学 | 402篇 |
晶体学 | 96篇 |
力学 | 89篇 |
综合类 | 6篇 |
数学 | 21篇 |
物理学 | 939篇 |
无线电 | 309篇 |
出版年
2023年 | 13篇 |
2022年 | 27篇 |
2021年 | 19篇 |
2020年 | 20篇 |
2019年 | 19篇 |
2018年 | 19篇 |
2017年 | 34篇 |
2016年 | 46篇 |
2015年 | 16篇 |
2014年 | 23篇 |
2013年 | 145篇 |
2012年 | 53篇 |
2011年 | 67篇 |
2010年 | 70篇 |
2009年 | 85篇 |
2008年 | 56篇 |
2007年 | 93篇 |
2006年 | 88篇 |
2005年 | 88篇 |
2004年 | 105篇 |
2003年 | 100篇 |
2002年 | 48篇 |
2001年 | 50篇 |
2000年 | 77篇 |
1999年 | 55篇 |
1998年 | 46篇 |
1997年 | 65篇 |
1996年 | 43篇 |
1995年 | 42篇 |
1994年 | 52篇 |
1993年 | 48篇 |
1992年 | 54篇 |
1991年 | 18篇 |
1990年 | 16篇 |
1989年 | 11篇 |
1988年 | 6篇 |
1987年 | 7篇 |
1986年 | 4篇 |
1985年 | 2篇 |
1984年 | 7篇 |
1983年 | 4篇 |
1982年 | 3篇 |
1981年 | 4篇 |
1980年 | 2篇 |
1979年 | 2篇 |
1977年 | 4篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1973年 | 1篇 |
1972年 | 1篇 |
排序方式: 共有1862条查询结果,搜索用时 15 毫秒
81.
Dr. Takaaki Mitsuhashi Dr. Lena Barra Zachary Powers Volga Kojasoy Andrea Cheng Feng Yang Dr. Yoshimasa Taniguchi Dr. Takashi Kikuchi Prof. Dr. Makoto Fujita Prof. Dr. Dean J. Tantillo Prof. Dr. John A. Porco Jr Prof. Dr. Ikuro Abe 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(52):23980-23989
Fungal meroterpenoids are a diverse group of hybrid natural products with impressive structural complexity and high potential as drug candidates. In this work, we evaluate the promiscuity of the early structure diversity-generating step in fungal meroterpenoid biosynthetic pathways: the multibond-forming polyene cyclizations catalyzed by the yet poorly understood family of fungal meroterpenoid cyclases. In total, 12 unnatural meroterpenoids were accessed chemoenzymatically using synthetic substrates. Their complex structures were determined by 2D NMR studies as well as crystalline-sponge-based X-ray diffraction analyses. The results obtained revealed a high degree of enzyme promiscuity and experimental results which together with quantum chemical calculations provided a deeper insight into the catalytic activity of this new family of non-canonical, terpene cyclases. The knowledge obtained paves the way to design and engineer artificial pathways towards second generation meroterpenoids with valuable bioactivities based on combinatorial biosynthetic strategies. 相似文献
82.
Determining how electrode structure governs the performance of an electrocatalyst requires techniques capable of probing structure at the atomic scale, often in situ and operando. In recent years, there have been numerous advances in the main experimental techniques for determining the structure of the electrochemical interface. In situ/operando synchrotron surface x-ray diffraction measurements are key to investigate the atomic structure of the electrode surfaces as well as understand the structure-reactivity relations in electrocatalysis. Here we discuss some recent improvements that have taken place in surface x-ray diffraction and how we expect them to lead to an enhanced understanding of electrocatalysis. 相似文献
83.
Hongyun Zhang Wen Li Xiaokai Zhang Fahong Miao Taohai Li Haihua Liu 《Surface and interface analysis : SIA》2013,45(5):919-929
A selection of suitable microstructures is critical to fabrication and properties of superhydrophobic surfaces (SHS). In this study, we introduce a three‐dimensional droplet model to thermodynamically analyze the superhydrophobic properties for the purpose of determining the second step of a two‐step microstructure suitable for the SHS based on the common models within the reach of the existing macro‐machining technology. It is found that a sinusoidal microstructure is the most suitable, followed by a cone frustum and a prism in the composite wetting state, as well as the transition from hydrophilic to hydrophobic depends basically on the solid fraction rather than non‐determinative surface microscopic topography. The predictions of the model are found in quite good agreement with the experimental observations. This study will facilitate fabrication of the SHS on how to select the suitable morphology. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
84.
In this paper,large-sized sapphire (230×210 mm,27.5 kg) was grown by SAPMAC method (sapphire growth technique with micro-pulling and shoulder-expanding at the cooled center). Dislocation peculiarity in large sapphire boule (0001) basal plane was investigated by chemical etching,scanning electron microscopy and X-ray topography method. The triangular dislocation etch pit measured is 7.6×101~8.0×102 cm-2,in which relative high-density dislocations were generated at both initial and final stages of crystal growth. The analysis of single-crystal X-ray topography shows that there are no apparent sub-grain boundaries; the dislocation lines are isolated and straight. Finally,the origins of low-density dislocation in sapphire crystal are discussed by numerical analysis method. 相似文献
85.
86.
B. A. Salakhutdinov D. N. Dalimov T. F. Aripov I. I. Tukfatullina R. Kh. Ziyatdinova A. Zh. Dzhuraev F. G. Kamaev L. Yu. Izotova B. T. Ibragimov I. Mavridis P. Giastas 《Chemistry of Natural Compounds》2002,38(3):249-256
Derivatives of 3-O-acetyl-18-H-glycyrrhetic acid were synthesized. Their structures and membrane activities were studied. 相似文献
87.
Steven J. Langford Marcia A.-P. Lee Katherine J. Macfarlane Josephine A. Weigold 《Journal of inclusion phenomena and macrocyclic chemistry》2001,41(1-4):135-139
A series of complexes of the type [Sn(TTP)L2] have been prepared by the condensation of [Sn(TTP)OH2] (TTP = meso-tetratolylporphyrin) with a range of substituted phenols. The resulting complexes were characterised using 1H NMR and single crystal X-ray diffraction techniques. In each case, the condensation of the phenols with the Sn(IV)porphyrin in CDCl3 solution is slow (h) but essentially quantitative. The slow kinetics of the formation of the diaxial phenolate complexes allows for the identification, by 1H NMR spectroscopy, of three independent complexes within this process, namely an outer-sphere (H-bonded) complex as well as two independent phenolate complexes. The rate of condensation is in the order phenol 4-methoxyphenol > 4-nitrophenol and suggests a steric rather than pKa dependency. 相似文献
88.
89.
影响分子沉积膜纳米摩擦特性的几个因素 总被引:5,自引:2,他引:5
利用原子力显微镜探讨了表面电荷及分子端基对分子沉积膜纳米摩擦特性的影响,并考察了表面形貌在不同扫描方向和法向高度上对摩擦力的影响.结果表明:对Si3N4针尖而言,表面净电荷对摩擦特性有一定的影响,不同类型的表面电荷对摩擦力和摩擦系数的影响不同,正电荷影响相对较大;在较小载荷和粘附力的条件下,针尖在表面上滑动时所受的摩擦作用同分子端基有关;单层CuTsPc分子沉积膜表面形貌的取向对摩擦力影响不大,分子沉积膜的表面高度同摩擦力,即时测量值并不存在对应关系,摩擦力受表面形貌的影响较小. 相似文献
90.
为了更好地理解塑性成形滑动接触界面的摩擦行为,构建了一种新型的摩擦试验装置,运用表面纹理化技术制备了两类表面形貌的1050铝材试件,在不同的接触压力和滑动速度条件下进行一系列拉伸摩擦试验.对试验前后试件三维表面形貌进行了测量;提取真实接触面积比、封闭空体面积比和开放空体面积比等三维表面参数,来描述试件表面形貌的变化.试验发现:摩擦系数随名义接触压力和滑动速度增加而逐渐减小;试件初始表面形貌对摩擦有明显的影响;试件表面形貌和参数随接触条件出现了规律性变化.基于机械流变模型的分析表明:随着试件表面形貌变形,不同的机理决定界面摩擦行为,摩擦系数对名义接触压力和滑动速度的依赖性可分别归因于微观塑性流体动压润滑效应和入口区流体动压牵引效应. 相似文献