全文获取类型
收费全文 | 47261篇 |
免费 | 4072篇 |
国内免费 | 6866篇 |
专业分类
化学 | 35156篇 |
晶体学 | 585篇 |
力学 | 1559篇 |
综合类 | 773篇 |
数学 | 7059篇 |
物理学 | 13067篇 |
出版年
2023年 | 548篇 |
2022年 | 844篇 |
2021年 | 1815篇 |
2020年 | 1517篇 |
2019年 | 1383篇 |
2018年 | 1108篇 |
2017年 | 1253篇 |
2016年 | 1622篇 |
2015年 | 1609篇 |
2014年 | 2076篇 |
2013年 | 3536篇 |
2012年 | 2532篇 |
2011年 | 2793篇 |
2010年 | 2581篇 |
2009年 | 3042篇 |
2008年 | 3199篇 |
2007年 | 3403篇 |
2006年 | 2780篇 |
2005年 | 1919篇 |
2004年 | 1808篇 |
2003年 | 1760篇 |
2002年 | 1468篇 |
2001年 | 1394篇 |
2000年 | 1102篇 |
1999年 | 837篇 |
1998年 | 811篇 |
1997年 | 674篇 |
1996年 | 707篇 |
1995年 | 649篇 |
1994年 | 655篇 |
1993年 | 625篇 |
1992年 | 623篇 |
1991年 | 401篇 |
1990年 | 342篇 |
1989年 | 267篇 |
1988年 | 288篇 |
1987年 | 238篇 |
1986年 | 218篇 |
1985年 | 345篇 |
1984年 | 256篇 |
1983年 | 156篇 |
1982年 | 303篇 |
1981年 | 476篇 |
1980年 | 430篇 |
1979年 | 471篇 |
1978年 | 369篇 |
1977年 | 280篇 |
1976年 | 238篇 |
1974年 | 75篇 |
1973年 | 156篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
71.
72.
Prof. Ian J. S. Fairlamb 《Angewandte Chemie (International ed. in English)》2015,54(36):10415-10427
This Minireview highlights the redox and non‐innocent behavior of NOx ligands (x=1, 2, or 3) in selected Pd‐mediated processes, for example, alkene and aromatic oxidation processes. A focus is placed on mechanistic understanding and linking recent transformations, such as C? H bond activation/functionalization and Wacker oxidation, with previous work on the functionalization of aromatics and alkenes by PdII salts. 相似文献
73.
Substituted Tetraaza‐ and Hexaazahexacenes and their N,N′‐Dihydro Derivatives: Syntheses,Properties, and Structures 下载免费PDF全文
Jens U. Engelhart Dr. Benjamin D. Lindner Manuel Schaffroth David Schrempp Olena Tverskoy Prof. Uwe H. F. Bunz 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(22):8121-8129
The palladium‐catalyzed coupling of a substituted o‐diaminoanthracene and a substituted o‐diaminophenazine to substituted 2,3‐dichloroquinoxalines furnishes 10 differently substituted N,N′‐dihydrotetraaza‐ or ‐hexaazahexacenes with the quinoxaline group of the azaacenes carrying fluorine, chlorine, or nitro groups. The N,N′‐dihydrotetraazahexacenes with hydrogen, chlorine, and fluorine subtituents are oxidized to azaacenes, whereas only the parent N,N′‐dihydrohexaazahexacenes, with hydrogen substituents, are oxidized by MnO2. The resultant azaacenes are characterized by their optical and spectroscopic data. In addition, single‐crystal X‐ray structures have been obtained for the parent tetraazahexacenes and their difluoro‐substituted derivatives. The di‐ and tetrachloro derivatives of the N,N′‐dihydrohexaazahexacene have also been structurally characterized. 相似文献
74.
价廉易得的L-亮氨酸先以苄基同时保护氨基及羧基得(S)-2-(二苄胺基)-4-甲基戊酸苄酯,进而在碱性条件下与乙腈发生亲核取代反应得(S)-4-(二苄胺基)-6-甲基-3-氧代庚腈,再经硼氢化钠选择性还原羰基得(3S,4S)-4-(二苄胺基)-3-羟基-6-甲基庚腈,用双氧水氧化得(3S,4S)-4-(二苄胺基)-3-羟基-6-甲基庚酸,最后在Pd(OH)2/C-H2作用下脱掉苄基得到(3S,4S)-4-氨基-3-羟基-6-甲基庚酸,即(3S,4S)-statine。整个合成路线总产率为33.6%。 相似文献
75.
Size controlled synthesis of ornithine‐based polypeptides by ring‐opening polymerization of α‐(amino acid)‐N‐carboxyanhydrides 下载免费PDF全文
76.
Ag nanoparticle (NP)‐decorated MIL‐125(Ti) microspheres (Ag@MIL‐125(Ti)) were firstly fabricated via a facile hydrothermal and following photo‐reduction method. The photocatalysts were characterized using X‐ray diffraction, scanning and transmission electron microscopies, X‐ray photoelectron spectroscopy and UV–visible diffuse reflectance spectroscopy. The characterization results indicated that Ag NPs were dispersed on the surface of MIL‐125(Ti) microspheres, and the Ag NPs had a uniform diameter of about 40 nm. The composites exhibited excellent visible‐light absorption, due to the modification with the Ag NPs. The photocatalytic activity for the visible‐light‐promoted degradation of Rhodamine B was improved through the optimization of the amount of Ag loaded as a co‐catalyst, this amount being determined as 3 wt%. Additionally, studies performed using radical scavengers indicated that O2? and e? served as the main reactive species. The catalyst can be reused at least five times without significant loss of its catalytic activity. Furthermore, a photocatalytic mechanism for degradation of organics over Ag@MIL‐125(Ti) is also proposed. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
77.
78.
In this work, a continuum model is presented for size and orientation dependent thermal buckling and post-buckling of anisotropic nanoplates considering surface and bulk residual stresses. The model with von-Karman nonlinear strains and material cubic anisotropy of single crystals contains two parameters that reflect the orientation effects. Using Ritz method, closed form solutions are given for buckling temperature and post-buckling deflections. Regarding self-instability states of nanoplates and their recovering at higher temperatures, an experiment is discussed based on low pressurized membranes to verify the predictions. For simply supported nanoplates, the size effects are lowest when they are aligned in [100] direction. When the edges get clamped, the orientation dependence is ignorable and the behavior becomes symmetric about [510] axis. The surface residual stress makes drastic increase in buckling temperature of thinner nanoplates for which a minimum thickness is pointed to stay far from material softening at higher temperatures. Deflection of [100]-oriented buckled nanoplates is higher than [110] ones but this reverses at higher temperatures. The results for long nanoplates show that the buckling mode numbers are changed by orientation which is verified by FEM. 相似文献
79.
Scalable Synthesis of Piperazines Enabled by Visible‐Light Irradiation and Aluminum Organometallics 下载免费PDF全文
Dr. Samuel Suárez‐Pantiga Kilian Colas Dr. Magnus J. Johansson Dr. Abraham Mendoza 《Angewandte Chemie (International ed. in English)》2015,54(47):14094-14098
The development of more active C? H oxidation catalysts has inspired a rapid, scalable, and stereoselective assembly of multifunctional piperazines through a [3+3] coupling of azomethine ylides. A combination of visible‐light irradiation and aluminum organometallics is essential to promote this transformation, which introduces visible‐light photochemistry of main‐group organometallics and sets the basis for new and promising catalysts. 相似文献
80.
Intermolecular Stetter reaction of aromatic aldehydes with (E)-(2-nitrovinyl)cyclohexane catalyzed by thiazolium A has been developed. The reaction rate and efficiency are profoundly impacted by the presence of thiourea B. The reaction affords moderate to good yields of the Stetter product. Some factors influencing yield were discussed. 相似文献