全文获取类型
收费全文 | 246304篇 |
免费 | 10991篇 |
国内免费 | 1812篇 |
专业分类
化学 | 150958篇 |
晶体学 | 3728篇 |
力学 | 10924篇 |
综合类 | 10篇 |
数学 | 25136篇 |
物理学 | 68351篇 |
出版年
2021年 | 2263篇 |
2020年 | 3434篇 |
2019年 | 5284篇 |
2018年 | 4860篇 |
2017年 | 4485篇 |
2016年 | 8431篇 |
2015年 | 6029篇 |
2014年 | 7894篇 |
2013年 | 13179篇 |
2012年 | 10344篇 |
2011年 | 10871篇 |
2010年 | 9569篇 |
2009年 | 9669篇 |
2008年 | 10587篇 |
2007年 | 9898篇 |
2006年 | 8670篇 |
2005年 | 7998篇 |
2004年 | 7406篇 |
2003年 | 6783篇 |
2002年 | 7427篇 |
2001年 | 7135篇 |
2000年 | 5588篇 |
1999年 | 3607篇 |
1998年 | 2845篇 |
1997年 | 2887篇 |
1996年 | 2678篇 |
1995年 | 2311篇 |
1994年 | 2428篇 |
1993年 | 2350篇 |
1992年 | 2614篇 |
1991年 | 2709篇 |
1990年 | 2609篇 |
1989年 | 2675篇 |
1988年 | 2568篇 |
1987年 | 2590篇 |
1986年 | 2443篇 |
1985年 | 3114篇 |
1984年 | 3171篇 |
1983年 | 2639篇 |
1982年 | 2607篇 |
1981年 | 2567篇 |
1980年 | 2366篇 |
1979年 | 2725篇 |
1978年 | 2701篇 |
1977年 | 2894篇 |
1976年 | 2888篇 |
1975年 | 2668篇 |
1974年 | 2602篇 |
1973年 | 2705篇 |
1972年 | 2083篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
141.
Design and development of a two‐dimensional system based on hydrophilic and reversed‐phase liquid chromatography with on‐line sample treatment for the simultaneous separation of excreted xenobiotics and endogenous metabolites in urine
下载免费PDF全文
![点击此处可从《Biomedical chromatography : BMC》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Diego García‐Gómez Encarnación Rodríguez‐Gonzalo Rita Carabias‐Martínez 《Biomedical chromatography : BMC》2015,29(8):1190-1196
In the present work we describe a two‐dimensional liquid chromatographic system (2D‐LC) with detection by mass spectrometry (MS) for the simultaneous separation of endogenous metabolites of clinical interest and excreted xenobiotics deriving from exposure to toxic compounds. The 2D‐LC system involves two orthogonal chromatographic modes, hydrophilic interaction liquid chromatography (HILIC) to separate polar endogenous metabolites and reversed‐phase (RP) chromatography to separate excreted xenobiotics of low and intermediate polarity. Additionally, the present proposal has the novelty of incorporating an on‐line sample treatment based on the use of restricted access materials (RAMs), which permits the direct injection of urine samples into the system. The work is focused on the instrumental coupling, studying all possible options and attempting to circumvent the problems of solvent incompatibility between the RAM device and the two chromatographic columns, HILIC and RP. The instrumental configuration developed, RAM‐HILIC‐RPLC‐MS/MS, allows the simultaneous assessment of urinary metabolites of clinical interest and excreted compounds derived from exposure to toxic agents with minimal sample manipulation. Thus, it may be of interest in areas such as occupational and environmental toxicology in order to explore the possible relationship between the two types of compounds. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
142.
András Gy. Németh Renáta Szabó Dr. Attila Domján Prof. György M. Keserű Dr. Péter Ábrányi-Balogh 《ChemistryOpen》2021,10(1):16-27
The development of a new three-component chromatography-free reaction of isocyanides, amines and elemental sulfur allowed us the straightforward synthesis of thioureas in water. Considering a large pool of organic and inorganic bases, we first optimized the preparation of aqueous polysulfide solution from elemental sulfur. Using polysulfide solution, we were able to omit the otherwise mandatory chromatography, and to isolate the crystalline products directly from the reaction mixture by a simple filtration, retaining the sulfur in the solution phase. A wide range of thioureas synthesized in this way confirmed the reasonable substrate and functional group tolerance of our protocol. 相似文献
143.
Mocciaro A. Conconi M. S. Rendtorff N. M. Scian A. N. 《Journal of Thermal Analysis and Calorimetry》2021,144(4):1083-1093
Journal of Thermal Analysis and Calorimetry - The effect of monoaluminum phosphate (Al(H2PO4)3) addition in the thermochemical process of a kaolinitic clay was studied and compared with the pure... 相似文献
144.
145.
146.
DFT mechanistic study of the H2‐assisted chain transfer copolymerization of propylene and p‐methylstyrene catalyzed by zirconocene complex
下载免费PDF全文
![点击此处可从《Journal of polymer science. Part A, Polymer chemistry》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Cheng‐Gen Zhang Shu‐Yuan Yu Liaoyun Zhang Huayi Li Zhi‐Xiang Wang 《Journal of polymer science. Part A, Polymer chemistry》2015,53(4):576-585
DFT computations have been performed to investigate the mechanism of H2‐assisted chain transfer strategy to functionalize polypropylene via Zr‐catalyzed copolymerization of propylene and p‐methylstyrene (pMS). The study unveils the following: (i) propylene prefers 1,2‐insertion over 2,1‐insertion both kinetically and thermodynamically, explaining the observed 1,2‐insertion regioselectivity for propylene insertion. (ii) The 2,1‐inserion of pMS is kinetically less favorable but thermodynamically more favorable than 1,2‐insertion. The observation of 2,1‐insertion pMS at the end of polymer chain is due to thermodynamic control and that the barrier difference between the two insertion modes become smaller as the chain length becomes longer. (iii) The pMS insertion results in much higher barriers for subsequent either propylene or pMS insertion, which causes deactivation of the catalytic system. (iv) Small H2 can react with the deactivated [Zr]?pMS?PPn facilely, which displace functionalized pMS?PPn chain and regenerate [Zr]? H active catalyst to continue copolymerization. The effects of counterions are also discussed. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 576–585 相似文献
147.
148.
149.
150.