全文获取类型
收费全文 | 415篇 |
免费 | 23篇 |
国内免费 | 11篇 |
专业分类
化学 | 279篇 |
晶体学 | 4篇 |
力学 | 19篇 |
数学 | 59篇 |
物理学 | 84篇 |
综合类 | 4篇 |
出版年
2023年 | 1篇 |
2022年 | 8篇 |
2021年 | 17篇 |
2020年 | 14篇 |
2019年 | 28篇 |
2018年 | 32篇 |
2017年 | 26篇 |
2016年 | 29篇 |
2015年 | 13篇 |
2014年 | 26篇 |
2013年 | 56篇 |
2012年 | 39篇 |
2011年 | 28篇 |
2010年 | 12篇 |
2009年 | 22篇 |
2008年 | 20篇 |
2007年 | 22篇 |
2006年 | 13篇 |
2005年 | 8篇 |
2004年 | 5篇 |
2003年 | 3篇 |
2002年 | 1篇 |
2001年 | 5篇 |
1997年 | 2篇 |
1994年 | 1篇 |
1989年 | 2篇 |
1985年 | 2篇 |
1983年 | 1篇 |
1981年 | 1篇 |
1980年 | 2篇 |
1978年 | 5篇 |
1977年 | 2篇 |
1975年 | 1篇 |
1974年 | 2篇 |
排序方式: 共有449条查询结果,搜索用时 15 毫秒
391.
An efficient and simple approach of the synthesis of some spiro indeno[1,2-b]quinoxalines via a one-pot three-component reaction of 11H-indeno[1,2-b]quinoxalin-11-one, pyrazolone, and malononitrile in the presence of Na2CO3 at 70 °C is reported. This reaction has shown to have high atom economy. 相似文献
392.
Determination of mercury(II) ions in seafood samples after extraction and preconcentration by a novel functionalized magnetic metal–organic framework nanocomposite 下载免费PDF全文
Ebrahim Ghorbani‐Kalhor Rahim Hosseinzadeh‐Khanmiri Jafar Abolhasani Mirzaagha Babazadeh Akbar Hassanpour 《Journal of separation science》2015,38(7):1179-1186
This work describes a novel functionalized magnetic metal–organic framework nanocomposite [(Fe3O4‐2,5‐dimercapto‐1,3,4‐thiadiazole)/metal–organic framework] and its application in the preconcentration of Hg(II) ions. The parameters affecting the preconcentration procedure were optimized by a Box–Behnken design through response surface methodology. Three variables (uptake time, magnetic nanosorbent amount, and pH value) were selected as the main factors affecting the sorption step, while four variables (type, volume, and concentration of the eluent; and elution time) were selected as main factors in the optimization study of the elution step. Following the sorption and elution of analytes, the ions were quantified by cold vapor atomic absorption spectrometry. Under the optimum conditions, the limit of detection was 0.01 ng/mL and all the relative standard deviations were less than 10%. The obtained sorption capacity (in mg/g) of this new sorbent was 124. Ultimately, this nanocomposite was successfully applied to the rapid extraction of trace quantities of Hg(II) ions in seafood samples and satisfactory results were obtained. 相似文献
393.
3-Aminoimidazo[1,2-a]pyridines have been synthesized in good to excellent yields in the presence of the ionic liquid 1-butyl-3-methylimidazolium bromide [bmim]Br, the reaction workup is simple and the ionic liquid can be easily separated from the product and reused. 相似文献
394.
395.
An efficient one-pot three-component procedure has been developed for the preparation of tetrahydrobenzo[b]pyrans, dihydropyrano[3,2-c]chromenes, and pyrano[2,3-d]pyrimidines using nano-SiO2 as catalyst in H2O at 70 °C. This methodology has a number of advantages such as: use of reusable catalyst, easy access, short reaction times, high yields, easy work up and use of non-toxic and green solvent. 相似文献
396.
In this work, the potential energy surfaces for methane, ethane, propane, iso-butane and neo-pentane, obtained from the ab initio calculations via different levels of electron-correlation, were used in the framework of the kinetic theory to calculate the transport collision integrals and their corresponding low-density transport coefficients. The theoretical results are compared with the available experimental data and the effective scaling potential parameters of methane, ethane, propane, iso-butane and neo-pentane along with the kinetic theory collision integrals and higher order correction factors were obtained. Relation between different potentials and kinetic theory collision integrals are discussed and it was shown that the Mason–Monchick approach is a reliable approximation in the calculation of diffusion coefficients and shear viscosities of chain alkanes, whereas the full predictive Boltzmann weighting method is successful only for lighter alkanes, such as methane and ethane. 相似文献
397.
Mohsen Nikpour Mahmoud Gholami Ebrahim Molashahi 《Journal of heterocyclic chemistry》2016,53(5):1457-1460
Some new 3‐(alkylsulfanyl)‐7‐chloro‐1‐phenyl‐1H‐pyridazino [4,3‐e][1,3,4]thiadiazine were synthesized by treatment of the alkyl‐2‐phenylhydrazinecarbodithioates with 4‐bromo‐3,6‐dichloropyridazine in alkaline acetonitrile. Orientation of the reaction has been determined by X‐ray crystallography technique. The chlorine atom on the number 7 position of these products was replaced by secondary amines at reflux condition. 相似文献
398.
Mehdi Roohani Youssef Habibi Ghanbar Ebrahim Alain Dufresne 《European Polymer Journal》2008,44(8):2489-2498
Nanocomposite materials were prepared from copolymers of polyvinyl alcohol and polyvinyl acetate and a colloidal aqueous suspension of cellulose whiskers prepared from cotton linter. The degree of hydrolysis of the matrix was varied in order to vary the hydrophilic character of the polymer matrix and then the degree of interaction between the filler and the matrix. Nanocomposite films were conditioned at various moisture contents, and the dynamic mechanical and thermal properties were characterized using dynamic mechanical analysis and differential scanning calorimetry, respectively. Tensile tests were performed at room temperature to estimate mechanical properties of the films in the non linear range. All the results show that stronger filler/matrix interactions occur for fully hydrolyzed PVA compared to partially hydrolyzed samples. For moist samples, a water accumulation at the interface was evidenced. The reinforcing effect was found to be all the higher as the degree of hydrolysis of the matrix was high. 相似文献
399.
400.
The electrocatalytic reduction of nitrite has been studied by poly(ortho‐toluidine) films modified carbon paste electrode (P‐OT/MCPE). Cyclic voltammetry and chronoamperometry techniques were used to investigate the suitability of poly(ortho‐toluidine) as a mediator for the electrocatalytic nitrite reduction in aqueous solution with various pH. Results showed that pH 0.00 is the most suitable for this purpose. In the optimum pH, the reduction of nitrite occurs at a potential about 600 mV more positive than unmodified carbon paste electrode. The catalytic reaction rate constant, (kh), was calculated 8.68×102 M?1 s?1 by the data of chronoamperometry. The catalytic reduction peak current was linearly dependent on the nitrite concentration and the linearity range obtained was 5.00×10?4 M–1.90×10?2 M. Detection limit has been found to be 3.38×10?4 M (2σ). This method has been successfully employed for quantification of nitrite in real sample. 相似文献