全文获取类型
收费全文 | 614449篇 |
免费 | 6119篇 |
国内免费 | 1442篇 |
专业分类
化学 | 308152篇 |
晶体学 | 8981篇 |
力学 | 30839篇 |
综合类 | 20篇 |
数学 | 84997篇 |
物理学 | 189021篇 |
出版年
2021年 | 4862篇 |
2020年 | 5316篇 |
2019年 | 5943篇 |
2018年 | 13124篇 |
2017年 | 13531篇 |
2016年 | 13914篇 |
2015年 | 7184篇 |
2014年 | 10910篇 |
2013年 | 24866篇 |
2012年 | 21853篇 |
2011年 | 30998篇 |
2010年 | 21462篇 |
2009年 | 21600篇 |
2008年 | 28313篇 |
2007年 | 30342篇 |
2006年 | 20030篇 |
2005年 | 21569篇 |
2004年 | 18522篇 |
2003年 | 16901篇 |
2002年 | 15658篇 |
2001年 | 16734篇 |
2000年 | 12743篇 |
1999年 | 9838篇 |
1998年 | 8324篇 |
1997年 | 7961篇 |
1996年 | 7755篇 |
1995年 | 6976篇 |
1994年 | 6883篇 |
1993年 | 6705篇 |
1992年 | 7261篇 |
1991年 | 7613篇 |
1990年 | 7251篇 |
1989年 | 7147篇 |
1988年 | 7052篇 |
1987年 | 6943篇 |
1986年 | 6591篇 |
1985年 | 8494篇 |
1984年 | 8594篇 |
1983年 | 6949篇 |
1982年 | 7106篇 |
1981年 | 6988篇 |
1980年 | 6476篇 |
1979年 | 7087篇 |
1978年 | 7320篇 |
1977年 | 7248篇 |
1976年 | 7295篇 |
1975年 | 6710篇 |
1974年 | 6679篇 |
1973年 | 6841篇 |
1972年 | 4978篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
71.
We summarize the results of our recent work on Bäcklund transformations (BTs), particularly focusing on the relation between BTs and infinitesimal symmetries. We present a BT for an associated Degasperis–Procesi (aDP) equation and its superposition principle and investigate the solutions generated by applying this BT. Following our general methodology, we use the superposition principle of the BT to generate the infinitesimal symmetries of the aDP equation. 相似文献
72.
Mesoscopic modeling at the pore scale offers great promise in exploring the underlying structure transport performance of flow through porous media. The present work studies the fluid flow subjected to capillarity-induced resonance in porous media characterized by different porous structure and wettability. The effects of porosity and wettability on the displacement behavior of the fluid flow through porous media are discussed. The results are presented in the form of temporal evolution of percentage saturation and displacement of the fluid front through porous media. The present study reveals that the vibration in the form of acoustic excitation could be significant in the mobilization of fluid through the porous media. The dependence of displacement of the fluid on physicochemical parameters like wettability of the surface, frequency along with the porosity is analyzed. It was observed that the mean displacement of the fluid is more in the case of invading fluid with wetting phase where the driving force strength is not so dominant. 相似文献
73.
74.
75.
Dr. Antonio M. Rodríguez Dr. Pilar Prieto Prof. Antonio de la Hoz Prof. Ángel Díaz-Ortiz D. Raúl Martín Prof. José I. García 《ChemistryOpen》2015,4(3):308-317
The aim of this work was to determine the parameters that have decisive roles in microwave-assisted reactions and to develop a model, using computational chemistry, to predict a priori the type of reactions that can be improved under microwaves. For this purpose, a computational study was carried out on a variety of reactions, which have been reported to be improved under microwave irradiation. This comprises six types of reactions. The outcomes obtained in this study indicate that the most influential parameters are activation energy, enthalpy, and the polarity of all the species that participate. In addition to this, in most cases, slower reacting systems observe a much greater improvement under microwave irradiation. Furthermore, for these reactions, the presence of a polar component in the reaction (solvent, reagent, susceptor, etc.) is necessary for strong coupling with the electromagnetic radiation. We also quantified that an activation energy of 20–30 kcal mol−1 and a polarity (μ) between 7–20 D of the species involved in the process is required to obtain significant improvements under microwave irradiation. 相似文献
76.
77.
78.
Antonio R. Montoro Bustos Marta Garcia-Cortes Hector González-Iglesias Jorge Ruiz Encinar José M. Costa-Fernández Miguel Coca-Prados Alfredo Sanz-Medel 《Analytica chimica acta》2015
A generic strategy based on the use of CdSe/ZnS Quantum Dots (QDs) as elemental labels for protein quantification, using immunoassays with elemental mass spectrometry (ICP-MS), detection is presented. In this strategy, streptavidin modified QDs (QDs-SA) are bioconjugated to a biotinylated secondary antibody (b-Ab2). After a multi-technique characterization of the synthesized generic platform (QDs-SA-b-Ab2) it was applied to the sequential quantification of five proteins (transferrin, complement C3, apolipoprotein A1, transthyretin and apolipoprotein A4) at different concentration levels in human serum samples. It is shown how this generic strategy does only require the appropriate unlabeled primary antibody for each protein to be detected. Therefore, it introduces a way out to the need for the cumbersome and specific bioconjugation of the QDs to the corresponding specific recognition antibody for every target analyte (protein). Results obtained were validated with those obtained using UV–vis spectrophotometry and commercial ELISA Kits. 相似文献
79.
Experimental and Theoretical Studies on the Rearrangement of 2‐Oxoazepane α,α‐Amino Acids into 2′‐Oxopiperidine β2,3,3‐Amino Acids: An Example of Intramolecular Catalysis
下载免费PDF全文
![点击此处可从《Chemistry (Weinheim an der Bergstrasse, Germany)》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Dr. Diego Núñez‐Villanueva Dr. M. Ángeles Bonache Laura Lozano Dr. Lourdes Infantes Prof. José Elguero Prof. Ibon Alkorta Prof. M. Teresa García‐López Dr. Rosario González‐Muñiz Dr. Mercedes Martín‐Martínez 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(6):2489-2500
Enantiopure β‐amino acids represent interesting scaffolds for peptidomimetics, foldamers and bioactive compounds. However, the synthesis of highly substituted analogues is still a major challenge. Herein, we describe the spontaneous rearrangement of 4‐carboxy‐2‐oxoazepane α,α‐amino acids to lead to 2′‐oxopiperidine‐containing β2,3,3‐amino acids, upon basic or acid hydrolysis of the 2‐oxoazepane α,α‐amino acid ester. Under acidic conditions, a totally stereoselective synthetic route has been developed. The reordering process involved the spontaneous breakdown of an amide bond, which typically requires strong conditions, and the formation of a new bond leading to the six‐membered heterocycle. A quantum mechanical study was carried out to obtain insight into the remarkable ease of this rearrangement, which occurs at room temperature, either in solution or upon storage of the 4‐carboxylic acid substituted 2‐oxoazepane derivatives. This theoretical study suggests that the rearrangement process occurs through a concerted mechanism, in which the energy of the transition states can be lowered by the participation of a catalytic water molecule. Interestingly, it also suggested a role for the carboxylic acid at position 4 of the 2‐oxoazepane ring, which facilitates this rearrangement, participating directly in the intramolecular catalysis. 相似文献
80.