全文获取类型
收费全文 | 235篇 |
免费 | 7篇 |
国内免费 | 23篇 |
专业分类
化学 | 149篇 |
晶体学 | 3篇 |
力学 | 17篇 |
数学 | 31篇 |
物理学 | 65篇 |
出版年
2024年 | 1篇 |
2022年 | 6篇 |
2021年 | 5篇 |
2020年 | 7篇 |
2019年 | 12篇 |
2018年 | 9篇 |
2017年 | 14篇 |
2016年 | 23篇 |
2015年 | 10篇 |
2014年 | 16篇 |
2013年 | 26篇 |
2012年 | 21篇 |
2011年 | 18篇 |
2010年 | 7篇 |
2009年 | 4篇 |
2008年 | 16篇 |
2007年 | 11篇 |
2006年 | 10篇 |
2005年 | 9篇 |
2004年 | 6篇 |
2002年 | 6篇 |
2001年 | 2篇 |
2000年 | 2篇 |
1999年 | 1篇 |
1997年 | 1篇 |
1996年 | 3篇 |
1994年 | 3篇 |
1993年 | 3篇 |
1992年 | 1篇 |
1991年 | 3篇 |
1989年 | 3篇 |
1988年 | 2篇 |
1987年 | 1篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1972年 | 1篇 |
排序方式: 共有265条查询结果,搜索用时 31 毫秒
91.
Niobium-promoted Fe/CNTs catalysts were prepared using a wet impregnation method.Samples were characterized by nitrogen adsorption,H2-TPR,TPD,XRD and TEM.The Fischer-Tropsch Synthesis(FTS) was carried out in a fixed-bed microreactor at 220 ℃,1 atm and H2/CO=2 for 5 h.Addition of niobium into Fe/CNTs increased the dispersion,decreased the average size of iron oxide nanoparticles and the catalyst reducibility.Niobium-promoted Fe catalyst resulted in appreciable increase in the selectivity of C5+ hydrocarbons and suppressed methane formation.These effects were more pronounced for the 0.04%Nb/Fe/CNTs catalyst,compared to those observed from other niobium compositions.The 0.04%Nb/Fe/CNTs catalyst enhanced the C5+ hydrocarbons selectivity by a factor of 67.5% and reduced the methane selectivity by a factor of 59.2%. 相似文献
92.
V. Goodarzi S. ALI Monemian F. Maleki M. Torabi Angaji 《Journal of Macromolecular Science: Physics》2013,52(3):472-484
In the preparation of nanocomposites, there is competition between the dispersion of nanoparticles and the formation of agglomerates. In this study, radical copolymerization of ethyl acrylate and methyl methacrylate initiated by 2,2‐azobis (isobutyro) nitrile (AIBN) was performed, in the presence of titanium oxide (TiO2) nanoparticles modified in a new approach; a good dispersion of the nanoparticles in the unsaturated polyester (UP) matrix was obtained. The TiO2 nanoparticles were exposed to 3‐(methacryloxy) propyl trimethoxy silane as the coupling agent. The presence of coupling agent‐grafted TiO2 nanoparticles in the copolymerization process resulted in the formation of a polymeric layer on the surface of the TiO2 nanoparticles (doubly modified‐TiO2). The grafting of coupling agent molecules and consequently copolymer macromolecular chains onto the surface of TiO2 nanoparticles was investigated using Fourier transform infrared (FTIR) analysis. It found that the formation of an acrylate layer on the surface of nanoparticles was successful. Then, unsaturated polyester (UP)/TiO2 nanocomposites were prepared. The morphology was studied using transmission electron microscopy (TEM). Mechanical properties and ultraviolet visible (UV/VIS) spectroscopy of various samples, including the doubly modified‐TiO2 nanoparticles, with different nanoparticle inclusions and the unmodified‐TiO2 nanoparticles, were also investigated. The results showed the doubly modified‐TiO2 nanoparticles, compared to those of unmodified‐TiO2, had better nanoparticle dispersion causing improvement in the mechanical properties and UV shielding. 相似文献
93.
We compute fractal dimension and permutation entropy for healthy and people who have experienced heart failure. Our result shows that permutation entropy is a suitable approach as well as detrend fluctuation analysis (DFA). The result of DFA shows that the fractal dimensions for healthy and heart failure are different as well as the permutation entropy result. The fluctuation value for permutation entropy for an individual who has experienced heart failure is bigger than for a healthy person. There is some specific change in the interbeat signal of a person who has experienced heart failure, but there is not previous trend for a healthy person. 相似文献
94.
FAHRETTIN GOGTAS HALIL OTURAK NIYAZI BULUT SINAN AKPINAR ALI KÖKCE 《Molecular physics》2013,111(12):1901-1909
The state-to-state and state-to-all reaction probabilities have been calculated for three-dimensional reactive and inelastic scattering of Ne + H+ 2 (v = 0, j = 0, 1, 2) at zero total angular momentum. The time-dependent Schrödinger equation is solved by means of Fourier grid and discrete variable representation techniques. The inelastic and reactive scattering probabilities for a broad range of energies are calculated. Although the system has an open reactive channel, the results show that a high fraction of the flux is reflected back due to endoergicity and potential barriers leading to inelastic scattering. 相似文献
95.
Mohammad Rahmani 《代数通讯》2013,41(11):4361-4372
The purpose of this paper is to calculate all the character tables of Hecke algebras associated with finite Chevalley groups of exceptional type and their maximal parabolic subgroups when they are commutative. In the case when the groups are of classical type, the character values of Hecke algebras are expressed by using the q-Krawtchouk polynomials and the q-Hahn polynomials (See [10] and [15]). On the other hand, the character tables of commutative Hecke algebras associated with exceptional Weyl groups and their maximal parabolic subgroups are given in [12]. In §1, we discuss the structure of Hecke algebras and in §2, we calculate all the character tables of these commutative Hecke algebras associated with finite Chevalley groups of exceptional type. Although some of them are well known, we include them for completeness 相似文献
96.
This paper presents the exact, explicit solution for the transient motion of a circular plate surface bonded by two piezoelectric layers, based on Kirchhoff plate model. The distribution of eclectic potential along the thickness direction is simulated by a quadratic function so that the Maxwell static electricity equation is satisfied. The piezoelectric layers are electrically grounded over the edge and electrodes at the two surfaces of the piezoelectric layers are shortly connected. The differential equations of motion are solved for simply supported and clamped boundary conditions. The solutions are expressed by elementary Bessel functions and obtained via exact inverse Laplace transform. 相似文献
97.
Donya Rahmani Reza Ramezanian Parviz Fattahi Mahdi Heydari 《Applied Mathematical Modelling》2013,37(20-21):8957-8971
Production planning (PP) is one of the most important issues carried out in manufacturing environments which seeks efficient planning, scheduling and coordination of all production activities that optimizes the company’s objectives. In this paper, we studied a two-stage real world capacitated production system with lead time and setup decisions in which some parameters such as production costs and customer demand are uncertain. A robust optimization model is developed to formulate the problem in which minimization of the total costs including the setup costs, production costs, labor costs, inventory costs, and workforce changing costs is considered as performance measure. The robust approach is used to reduce the effects of fluctuations of the uncertain parameters with regards to all the possible future scenarios. A mixed-integer programming (MIP) model is developed to formulate the related robust production planning problem. In fact the robust proposed model is presented to generate an initial robust schedule. The performance of this schedule could be improved against of any possible occurrences of uncertain parameters. A case from an Iran refrigerator factory is studied and the characteristics of factory and its products are discussed. The computational results display the robustness and effectiveness of the model and highlight the importance of using robust optimization approach in generating more robust production plans in the uncertain environments. The tradeoff between solution robustness and model robustness is also analyzed. 相似文献
98.
Ali AHMAD Kashif ALI Martin BAA Petr KOVR Andrea SEMANIOV-FENOVíKOV 《数学学报(英文版)》2012,28(9):1865-1874
Let G = (V, E) be a finite, simple and undirected graph with p vertices and q edges. An (a, d)-vertex-antimagic total labeling of G is a bijection f from V (G) ∪ E(G) onto the set of consecutive integers 1, 2, . . . , p + q, such that the vertex-weights form an arithmetic progression with the initial term a and difference d, where the vertex-weight of x is the sum of the value f (x) assigned to the vertex x together with all values f (xy) assigned to edges xy incident to x. Such labeling is called super if the smallest possible labels appear on the vertices. In this paper, we study the properties of such labelings and examine their existence for 2r-regular graphs when the difference d is 0, 1, . . . , r + 1. 相似文献
99.
M.?Rahmani B.?Lukiyanchuk T.?Tahmasebi Y.?Lin T.?Y.?F.?Liew M.?H.?HongEmail author 《Applied Physics A: Materials Science & Processing》2012,107(1):23-30
Arrays of planar symmetric coupled oligomers support higher sensitivity optical response than uncoupled plasmonic systems.
In this work, the transition from isolated to collective optical modes in plasmonic oligomers, such as pentamers and quadrumers,
is investigated via experimental characterization and simulation with good agreement. The designed and fabricated metallic
oligomers consist of a single central disk and outer ring-like disks in nanoscales. It is shown that while the far-field spectral
responses of oligomers are polarization-independent, due to the structure symmetry, the spatial localization of near-field
energy in nanogaps can be polarization-controlled. This localization is established at a normal-incident light of a single
source rather than co-illumination by two light sources accompanied by different incident angles or phase shift. It can overcome
the spatial restrictions of conventional optics. The influence of the nano-disk sizes and gaps among them on the intensity
and shape of the localized near-field energy in pentamers and quadrumers is also studied. 相似文献
100.
B Fakrach A Rahmani H Chadli K Sbai M Benhamou M Bentaleb JL Bantignies JL Sauvajol 《J Phys Condens Matter》2012,24(33):335304
In this work, the infrared active modes are computed for homogeneous bundles of single-walled boron nitride nanotubes (BBNNTs), using the so-called spectral moments method. The dependence of the wavenumber on these modes in terms of diameters, lengths, and numbers of tubes, is investigated. To this end, use is made of a Lennard-Jones potential for describing the van der Waals interactions between tubes in a bundle. We find that, for a finite homogeneous bundle, additional modes appear as a specific signature. Finally, these results are useful for the interpretation of the experimental infrared spectra of BBNNTs. 相似文献