全文获取类型
收费全文 | 930篇 |
免费 | 48篇 |
国内免费 | 23篇 |
专业分类
化学 | 841篇 |
晶体学 | 4篇 |
力学 | 20篇 |
数学 | 59篇 |
物理学 | 77篇 |
出版年
2024年 | 4篇 |
2023年 | 7篇 |
2022年 | 29篇 |
2021年 | 37篇 |
2020年 | 64篇 |
2019年 | 55篇 |
2018年 | 80篇 |
2017年 | 39篇 |
2016年 | 73篇 |
2015年 | 54篇 |
2014年 | 71篇 |
2013年 | 125篇 |
2012年 | 67篇 |
2011年 | 73篇 |
2010年 | 52篇 |
2009年 | 47篇 |
2008年 | 43篇 |
2007年 | 29篇 |
2006年 | 9篇 |
2005年 | 13篇 |
2004年 | 6篇 |
2003年 | 7篇 |
2002年 | 6篇 |
2001年 | 3篇 |
1999年 | 1篇 |
1994年 | 1篇 |
1991年 | 1篇 |
1986年 | 1篇 |
1985年 | 3篇 |
1976年 | 1篇 |
排序方式: 共有1001条查询结果,搜索用时 17 毫秒
911.
Shahriar Sajjadi Fatemeh Jahanzad Michael Yianneskis 《Colloids and surfaces. A, Physicochemical and engineering aspects》2004,240(1-3):149-155
This paper deals with the phase behaviour of model abnormal emulsions of cyclohexane/water/polyethoxylated surfactant in the vicinity of the locus of transitional inversion (optimum formulation). Abnormal emulsions are formed under dynamic conditions if the phase containing the soluble surfactant becomes the dispersed phase. Phase maps have been suggested in the literature that define the boundaries of emulsion morphologies. On these maps, only one transformation can be observed for any formulation scan. Furthermore, the morphology of emulsions in the vicinity of the locus of transitional inversion is rather vague. One might assume that by changing the HLB of the surfactant used in an abnormal emulsion to favour the continuous phase, that abnormal emulsion will gradually transform to a normal emulsion of the same type. A new experimental procedure was adopted in which emulsification was started with abnormal emulsions. To transform the abnormal emulsions to the normal emulsion of the same type, they were exposed to variations in temperature, the surfactant HLB, or water–oil phase ratio. As the optimum formulation was approached, the abnormal emulsions became so unstable that could not exist anymore in the un-favoured morphology and inverted to the normal emulsion of the opposite type. Further variation in the formulation along the route led to a transitional inversion to the normal emulsion of the original type. The result indicates an important finding that the transformation of abnormal emulsions to normal ones of the same type occurred via two successive inversions of catastrophic and transitional nature. It appears that the boundaries of catastrophic inversion correspond to the emulsions with the finest drop size. A modification to the phase behaviour maps, to include the locus of catastrophic phase inversion in the vicinity of the transitional inversion, was thus suggested. 相似文献
912.
The solvent-free acylation of alcohols and phenols with acyl chlorides using ZnO as a catalyst is described. The remarkable selectivity under mild and neutral conditions, and recyclability of the catalyst, are advantages. 相似文献
913.
Joseph J. Pesek Fatemeh Niyati-Shirkhodaee Mohammad Kashefi 《Journal of heterocyclic chemistry》1985,22(5):1379-1380
The pH-rate profile for the decomposition of 2,2,5,5-tetramethyl-4-thiazolidinecarboxylic acid is used to identify the presence of a Schiff base intermediate. Activation parameters for the reaction are reported at various pH's and in particular the negative values of entropy support the presence of a carbinolamine in the rate-determining step. 相似文献
914.
Françoise Barrière Jean-Claude Barrière Derek H.R. Barton Jeanine Cleophax Alice Gateau-Olesker Stephan D. Géro Fatemeh Tadj 《Tetrahedron letters》1985,26(26):3119-3120
The 1-cyano-1-thiophenylcyclopentone derivative , obtained from (R,R)-(+)-tartaric acid, has been converted into a number of derivatives including the important -(3S,4S)-dibenzyloxycyclopentanone in 45% overall yield. 相似文献
915.
高效和可重复使用的Zr-MCM-41催化剂催化Strecker反应合成氨基腈类化合物(英文) 总被引:1,自引:0,他引:1
The synthesis and characterization of Zr-MCM-41 nanoreactors and their catalytic activity in the synthesis of new amino nitrile derivatives by the Strecker reaction in high yields and in short reaction times is reported. 相似文献
916.
Arash Ghorbani-Choghamarani Maryam Hajjami Hamid Goudarziafshar Mohsen Nikoorazm Shadpour Mallakpour Fatemeh Sadeghizadeh Gouhar Azadi 《Monatshefte für Chemie / Chemical Monthly》2009,12(4):607-610
Abstract
A simple and efficient catalytic oxidation of urazoles and a bis-urazole to the corresponding triazolinediones by treatment with Al(NO3)3.9H2O in the presence of a catalytic amount of silica sulfuric acid is described. A good range of urazole derivatives was selectively oxidized in CH2Cl2 at room temperature in good to excellent yields. 相似文献917.
Hossien A. Oskooie Majid M. Heravi Akbar Sadnia Fatemeh Jannati Farahnaz K. Behbahani 《Monatshefte für Chemie / Chemical Monthly》2008,139(1):27-29
Summary. The multicomponent Strecker reaction using trimethylsilyl cyanide was performed at room temperature and α-aminonitriles were prepared in excellent yields
in the presence of a catalytic amount of H2SO4 adsorbed on silica gel. 相似文献
918.
The formation constants of the species formed in the systems H+ + dimethyltin(IV) + 5′‐IMP and 5′‐UMP, H+ + 5′‐IMP and H+ + 5′‐UMP have been determined in aqueous solution in the pH range 1.5–9.5 at constant temperature (25 °C) and constant ionic strength (0.1 mol dm−3 NaClO4), using spectrophotometric and potentiometric techniques. 1H and 31P NMR investigations in aqueous solution confirmed the species formation. The precipitated complexes of IMP and UMP by Me2Sn(IV)2+ at low pH values were characterized by elemental analysis and FTIR spectroscopy methods, the bonding sites of the ligands were determined and ruled out purine and pyrimidine moieties (N‐7 and N‐1 in IMP and N‐3 in UMP, respectively) while a bidentated coordination of the phosphate group is concluded in both cases. Finally, the experiments revealed the existence of complexes with trigonal bipyramidal structures that is in agreement with similar systems resulted previously. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
919.
Parya?Siahcheshm Fatemeh?GoharpeyEmail author Reza?FoudaziView authors OrcID profile 《Rheologica Acta》2018,57(11):729-743
We studied the influence of nanoparticles with different surface modifications on the interfacial tension and relaxation of model polymer blend after cessation of different strains. The droplet retraction experiments were carried out on a model system composed of polydimethylsiloxane (PDMS) as the suspending fluid and polyisobutylene (PIB) as droplet at room temperature in the presence of hydrophobic and hydrophilic nanosilica. Different weight fractions of particles were dispersed in the PIB droplet before forming a dispersed droplet by using a microsyringe in shear cell. We found that applied strain, nanoparticle concentration and their thermodynamically preferred localization affect both nominal interfacial tension and droplet retraction process. By addition of nanoparticles at a concentration as low as 0.2%wt, the nominal interfacial tension decreases from 3.12?±?0.15 mN/m for neat PIB-PDMS interface depending on the surface characteristics of nanosilica. Hydrophilic nanosilica has the most effect on nominal interfacial tension and decreases it as low as 0.2?±?0.21 mN/m at 1 wt.% loading under a strain of 7. The results show that the retraction process in this system is mainly controlled by interfacial phenomena rather than bulk rheological properties. Additionally, the shape evolution of droplets changes and the retraction rate slows down in the presence of nanoparticles. 相似文献
920.
Fatemeh Ahangari 《理论物理通讯》2018,69(5):477-505
Problems of thermodynamic phase transition originate inherently in solidification, combustion and various other significant fields. If the transition region among two locally stable phases is adequately narrow, the dynamics can be modeled by an interface motion. This paper is devoted to exhaustive analysis of the invariant solutions for a modified Kuramoto-Sivashinsky equation in two spatial and one temporal dimensions is presented. This nonlinear partial differential equation asymptotically characterizes near planar interfaces, which are marginally long-wave unstable. For this purpose, by applying the classical symmetry method for this model the classical symmetry operators are attained.Moreover, the structure of the Lie algebra of symmetries is discussed and the optimal system of subalgebras, which yields the preliminary classification of group invariant solutions is constructed. Mainly, the Lie invariants corresponding to the infinitesimal symmetry generators as well as associated similarity reduced equations are also pointed out. Furthermore,the nonclassical symmetries of this nonlinear PDE are also comprehensively investigated. 相似文献