全文获取类型
收费全文 | 103篇 |
免费 | 3篇 |
国内免费 | 1篇 |
专业分类
化学 | 57篇 |
力学 | 7篇 |
数学 | 19篇 |
物理学 | 24篇 |
出版年
2023年 | 1篇 |
2022年 | 1篇 |
2021年 | 3篇 |
2020年 | 4篇 |
2019年 | 6篇 |
2018年 | 7篇 |
2017年 | 5篇 |
2016年 | 9篇 |
2015年 | 3篇 |
2014年 | 5篇 |
2013年 | 3篇 |
2012年 | 8篇 |
2011年 | 7篇 |
2010年 | 9篇 |
2008年 | 6篇 |
2007年 | 7篇 |
2006年 | 6篇 |
2005年 | 5篇 |
2003年 | 1篇 |
2002年 | 3篇 |
2000年 | 2篇 |
1999年 | 2篇 |
1997年 | 1篇 |
1993年 | 1篇 |
1980年 | 1篇 |
1974年 | 1篇 |
排序方式: 共有107条查询结果,搜索用时 31 毫秒
51.
Erkan Ertürk 《Tetrahedron》2008,64(32):7555-7560
The cobalt(II) porphyrin complex (CoTPP) was found to be an efficient catalyst for the Wittig type olefination of acyl phosphonates with ethyl diazoacetate (EDA) in the presence of triphenylphosphine (Ph3P). By using this one pot methodology under mild conditions, densely functionalized vinyl phosphonates were obtained in high yields and high E/Z selectivities in relatively short reaction times. A rather broad substrate spectrum and steric influence on the reaction rate were observed. 相似文献
52.
The physiologically essential oxidation of sulfite to sulfate is catalyzed by the molybdoheme enzyme, sulfite oxidase. Deficiencies of this enzyme in human patients lead to severe neurological symptoms, which usually result in death in early childhood. Up to date eleven missense mutations in the gene encoding sulfite oxidase have been identified from sulfite oxidase deficient patients. The structural characterization of these mutants is now possible after the chicken sulfite oxidase gene has been synthesized chemically and due to the high homology to the human enzyme it provides a good model of human sulfite oxidase. This review focuses on the possible effects of the sulfite oxidase deficiency causing mutations based on our new structures of recombinant chicken sulfite oxidase. 相似文献
53.
Mark M. Meerschaert Erkan Nane P. Vellaisamy 《Journal of Mathematical Analysis and Applications》2011,379(1):216-228
Fractional derivatives can be used to model time delays in a diffusion process. When the order of the fractional derivative is distributed over the unit interval, it is useful for modeling a mixture of delay sources. This paper provides explicit strong solutions and stochastic analogues for distributed-order time-fractional diffusion equations on bounded domains, with Dirichlet boundary conditions. 相似文献
54.
We introduce a class of isotropic time dependent random fields on the non-homogeneous sphere which is represented by a time-changed spherical Brownian motion of order ν∈(0,1]. We can capture some anisotropies in Cosmology with this model. This process is a time-changed rotational diffusion (TRD) or the stochastic solution to the equation involving the spherical Laplace operator and a time-fractional derivative of order ν. TRD is a diffusion on the non-homogeneous sphere and therefore, the spherical coordinates given by TRD represent the coordinates of a non-homogeneous sphere by means of which an isotropic random field is indexed. The time dependent random fields we present in this work are therefore realized through composition and can be viewed as isotropic random field on randomly varying sphere. 相似文献
55.
Erkan Biber Güng?r Gündüz Bora Mavis Uner Colak 《Applied Physics A: Materials Science & Processing》2010,99(2):477-487
The impact strength of Nylon 6 can be further improved by blending it with ethylene-butyl acrylate-maleic anhydride elastomer.
The blending is achieved in solution phase. Due to incompatibility of Nylon 6 and the elastomer, a special mixture of solvents
is used to dissolve both components. The solution is electrospun, and the effects of the process parameters on the expected
radii of nanofibers are investigated. The effects of process parameters such as polymer concentration in solution, electrical
field, diameter of the syringe needle, feed rate, and collector geometry on nanofibers were investigated. Statistical analysis
is carried out using the Johnson SB distribution. A relation is proposed to relate the effect of the process parameters feed rate, electrical voltage, and tip
to collector distance on the expected diameter of fibers. It is found that concentration and electrical field have a profound
effect on the diameter of fibers compared to those of the syringe diameter and feed rate. 相似文献
56.
In this paper, a novel robust watermarking technique using particle swarm optimization and k-nearest neighbor algorithm is introduced to protect the intellectual property rights of color images in the spatial domain. In the embedding process, the color image is separated into non-overlapping blocks and each bit of the binary watermark is embedded into the individual blocks. Then, in order to extract the embedded watermark, features are obtained from watermark embedded blocks using the symmetric cross-shape kernel. These features are used to generate two centroids belonging to each binary (1 and 0) value of the watermark implementing particle swarm optimization. Subsequently, the embedded watermark is extracted by evaluating these centroids utilizing k-nearest neighbor algorithm. According to the test results, embedded watermark is extracted successfully even if the watermarked image is exposed to various image processing attacks. 相似文献
57.
Zeid A. Alothman Erkan Yilmaz Mohamed Habila Azza Shabaka Mustafa Soylak 《Mikrochimica acta》2013,180(7-8):669-674
We describe the application of temperature-controlled ionic liquid based microextraction (TC-IL-ME) of lead(II) ion. The method does not require the use of an organic solvent or a ligand. Rather, the IL is directly added to the aqueous sample containing Pb(II) in a centrifuge tube, and the mixture is heated to 80 °C for 4 min. After cooling at 0 °C, the solution turns cludy due to the formation of fine droplets of the IL containing Pb(II). The IL is separated by centrifugation, acidified, and directly submitted to FAAS by microinjection. The effects of pH value, volume of IL, extraction time, temperature, sample volume and matrix were optimized to result in a preconcentration factor of 30, a detection limit of 5.8 μg L?1, and a limit of quantification of 19.3 μg L?1. The method was validated by analyzing a certified reference material (NCSZC81002B; hair). A recovery test performed with spiked samples gave values between 102 % and 105 %. The method was also used to determine Pb(II) in hair samples. Figure
We describe the application of temperature-controlled ionic liquid based microextraction (TC-IL-ME) of lead(II) ion. The effects of pH value, volume of IL, extraction time, temperature, sample volume and matrix were optimized. 相似文献
58.
A practical synthesis of novel α-dibromoalkyl- and trimethylsilylmethyl-aminoboranes and derivatives
Söyleyici HC Fırıncı E Eyduran F Akbulat F Sahin Y 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2011,78(3):1139-1142
Addition of LDA to a mixture of trimethylborate and dibromomethane in THF at a temperature of -78°C leads to the formation of dibromomethyllithium and its capture by borate ester. ClB(OMe)(2) converts the resulting borate salt to dimethoxy(dibromomethyl)borane 2. N,N-Dimethylamino(methoxy)(dibromomethyl)borane 3 and N,N-bis(dimethylamino)(dibromomethyl)borane 4 were prepared by an amination reaction between N,N-dimethylaminotrimethylsilane and dimethoxy(dibromomethyl)borane 2. To obtain dichlorotrimethylsilylmethylborane 7 not containing the α-halomethyl group, N,N-bis(dimethylamino)(trimethylsilylmethyl)borane 5 was first obtained from the reaction of ClB(NMe(2))(2) with an organolithium reagent. Dimethoxy(trimethylsilylmethyl)borane 6 was then prepared by methoxylation of compound 5. Finally, compound 7 was prepared by chlorination of 6 using BCl(3). The chemical structures of these compounds were determined using (13)C, (1)H, (11)B NMR and GC/MS/MS techniques. 相似文献
59.
60.
The dispersion equation for the axisymmetric modes of viscothermal acoustic wave propagation in uniform hard-walled circular ducts containing a quiescent perfect gas is classical. This has been extended to cover the non-axisymmetric modes and real fluids in contemporary studies. The fundamental axisymmetric mode has been the subject of a large number of studies proposing approximate solutions and the characteristics of the propagation constants for narrow and wide ducts with or without mean flow is well understood. In contrast, there are only few publications on the higher order modes and the current knowledge about their propagation characteristics is rather poor. On the other hand, there is a void of papers in the literature on the effect of the mean flow on the quiescent modes of propagation. The present paper aims to contribute to the filling of these gaps to some extent. The classical theory is re-considered with a view to cover all modes of acoustic propagation in circular ducts carrying a real fluid moving axially with a uniform subsonic velocity. The analysis reveals a new branch of propagation constants for the axisymmetric modes, which appears to have escaped attention hitherto. The solution of the governing wave equation is expressed in a modal transfer matrix form in frequency domain and numerical results are presented to show the effects over wide ranges of frequency, viscosity and mean flow parameters on the propagation constants. The theoretical formulation allows for the duct walls to have finite impedance, but no numerical results are presented for lined ducts or ducts carrying a sheared mean flow. 相似文献