全文获取类型
收费全文 | 475篇 |
免费 | 41篇 |
国内免费 | 8篇 |
专业分类
化学 | 373篇 |
力学 | 19篇 |
数学 | 66篇 |
物理学 | 66篇 |
出版年
2023年 | 1篇 |
2022年 | 6篇 |
2021年 | 10篇 |
2020年 | 19篇 |
2019年 | 11篇 |
2018年 | 35篇 |
2017年 | 28篇 |
2016年 | 33篇 |
2015年 | 26篇 |
2014年 | 37篇 |
2013年 | 51篇 |
2012年 | 39篇 |
2011年 | 53篇 |
2010年 | 35篇 |
2009年 | 23篇 |
2008年 | 20篇 |
2007年 | 23篇 |
2006年 | 15篇 |
2005年 | 18篇 |
2004年 | 13篇 |
2003年 | 6篇 |
2002年 | 8篇 |
2001年 | 3篇 |
2000年 | 3篇 |
1999年 | 3篇 |
1998年 | 1篇 |
1993年 | 1篇 |
1991年 | 1篇 |
1982年 | 1篇 |
1979年 | 1篇 |
排序方式: 共有524条查询结果,搜索用时 15 毫秒
41.
<正>A new practical method for the synthesis of unsymmetrical ureas was achieved by reaction of phenylurea with primary and secondary amines under neutral and mild condition in very good yields.The reaction took place in refluxing dioxane and does not require any catalyst or additives. 相似文献
42.
Bis(p-aminophenoxy)diphenylsilane (BPS), bis(m-aminophenoxy)diphenylsilane (BMS) and bis(5-amino-1-naphthoxy)diphenylsilane (BAS) as three silane-diamines were prepared by the reactions of 4-aminophenol, 3-aminophenol, and 5-amino-1-naphthol respectively, with dichlorodiphenylsilane in the presence of triethylamine. The related silane-containing polyimides were prepared by two-step polycondensation reactions of these diamines with three different aromatic dianhydrides. All the polymers were characterized and their physical and thermal properties were studied. The polymers showed high thermal stability while their solubility was greatly increased in polar aprotic solvents. Wide angle X-ray diffraction showed that all the polyimides were almost amorphous. Also their dielectric constants were decreased due to the incorporation of softening and low-polarizing siloxane units into the polymer backbone. 相似文献
43.
Majid Moghadam Shahram Tangestaninejad Valiollah MirkhaniIraj Mohammadpoor-Baltork Shadab Gharaati 《Polyhedron》2010
Tin(IV)tetraphenylporphyrinato tetrafluoroborate, [SnIV(TPP)(BF4)2], was used as an efficient catalyst for trimethylsilylation of alcohols and phenols with hexamethyldisilazane (HMDS). High-valent [SnIV(TPP)(BF4)2] catalyzes trimethylsilylation of primary, secondary and tertiary alcohols as well as phenols, and the corresponding TMS-ethers were obtained in high yields and short reaction times at room temperature. While, under the same reaction conditions [SnIV(TPP)Cl2] is less efficient to catalyze these reactions. One important feature of this catalyst is its ability in the chemoselective silylation of primary alcohols in the presence of secondary and tertiary alcohols and phenols. The catalyst was reused several times without loss of its catalytic activity. 相似文献
44.
Majid Moghadam Shahram TangestaninejadValiollah Mirkhani Iraj Mohammadpoor-BaltorkMostafa Khajehzadeh Farshid KosariMehdi Araghi 《Polyhedron》2010
High-valent tin(IV)octabromotetraphenylporphyrinato trifluoromethanesulfonate, [SnIV(Br8TPP)(OTf)2], was used for selective methoxymethylation of alcohols and phenols with formaldehyde dimethyl acetal (FDMA) at room temperature. Different primary, secondary and tertiary alcohols as well as phenols were converted to their corresponding methoxymethyl ethers with FMDA in the presence of an electron deficient tin(IV) porphyrin. The catalyst was reused several times without significant loss of its activity. 相似文献
45.
Attenborough K Bashir I Taherzadeh S 《The Journal of the Acoustical Society of America》2011,129(5):2806-2819
Many models for the acoustical properties of rigid-porous media require knowledge of parameter values that are not available for outdoor ground surfaces. The relationship used between tortuosity and porosity for stacked spheres results in five characteristic impedance models that require not more than two adjustable parameters. These models and hard-backed-layer versions are considered further through numerical fitting of 42 short range level difference spectra measured over various ground surfaces. For all but eight sites, slit-pore, phenomenological and variable porosity models yield lower fitting errors than those given by the widely used one-parameter semi-empirical model. Data for 12 of 26 grassland sites and for three beech wood sites are fitted better by hard-backed-layer models. Parameter values obtained by fitting slit-pore and phenomenological models to data for relatively low flow resistivity grounds, such as forest floors, porous asphalt, and gravel, are consistent with values that have been obtained non-acoustically. Three impedance models yield reasonable fits to a narrow band excess attenuation spectrum measured at short range over railway ballast but, if extended reaction is taken into account, the hard-backed-layer version of the slit-pore model gives the most reasonable parameter values. 相似文献
46.
Soleyman Hosseinzadeh Shahryar Pashaei Younes Saadat Zahra Khodaparast Roshan Javanifar Masoud Mohamadpour Alireza Hosseinzadeh 《Polymer Science Series B》2017,59(5):544-550
Cross-linked polystyrene (PS) particles having red blood corpuscle (RBC)-like shape were synthesized by one-pot dispersion polymerization of styrene with ethanol/water mixture and ethylene glycol dimethacrylate (EGDMA) as the reaction medium and cross-linker, respectively. Monitoring of the reaction showed that RBC-like shape forms due to asymmetric shrinkage of a cross-linked network during the phase separation. In addition, three dimensional phase diagram was generated based on the yielded data that showed that the formation of such unique shape extremely depends on the polarity of the medium and injection time of the cross-linker. In situ synthesis of RBC-like particles, as promising biomaterials in targeted drug delivery and a model for the understanding of the cell behavior, via such fast and high solid content approach makes it to be conducive to subsequent scale up, i.e. potential commercial adoption. 相似文献
47.
Morteza BararjanianSaeedeh Hosseinzadeh Saeed Balalaie Hamid Reza BijanzadehEckardt Wolf 《Tetrahedron letters》2011,52(26):3329-3332
An efficient palladium-catalyzed protocol for the stereoselective synthesis of 3-(aminoarylmethylene)-oxindoles has been developed. In this approach, Ugi-4-component reaction adducts were used as starting materials for carbopalladative cyclization-Buchwald reaction sequences. 相似文献
48.
G.R. Jahanshahloo F. Hosseinzadeh Lotfi M. Zohrehbandian 《Applied mathematics and computation》2005,170(2):985-993
Data envelopment analysis (DEA) is basically a linear programming based technique used for measuring the relative performance of organizational units, referred to as decision-making units (DMUs), where the presence of multiple inputs and outputs makes comparisons difficult. The ability of identifying frontier DMUs prior to the DEA calculation is of extreme importance to an effective and efficient DEA computation. In this paper, a method for identifying the efficient frontier is introduced. Then, the efficiency score and returns to scale (RTS) characteristic of DMUs will be produced by means of the equation of efficient frontier. 相似文献
49.
In this paper a genetic algorithm for solving a class of project scheduling problems, called Resource Investment Problem, is presented. Tardiness of project is permitted with defined penalty. Elements of algorithm such as chromosome structure, unfitness function, crossover, mutation, immigration and local search operations are explained. 相似文献
50.
M. A. Zanjanchi K. Tabatabaeian F. Hosseinzadeh 《Russian Journal of Coordination Chemistry》2005,31(8):585-587
Diffuse reflectance spectroscopy (DRS) can be regarded as a solid state technique contribution to monitoring the process of encapsulation of transition metal complexes into cavities of zeolites. By employing DRS, it was revealed that the effectiveness of a method suggested for higher encapsulation of a tris(bipyridine) cobalt(II) complex into zeolite Y can easily be manifested. In addition to that, simple diffuse reflectance spectra give information about a relative amount of two spin states of the cobalt bipyridine complexes and their alteration upon the method used.__________From Koordinatsionnaya Khimiya, Vol. 31, No. 8, 2005, pp. 618–620.Original English Text Copyright © 2005 by Zanjanchi, Tabatabaeian, Hosseinzadeh.This text was submitted by the authors in English. 相似文献