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排序方式: 共有162条查询结果,搜索用时 15 毫秒
71.
72.
Nima Khosravi Hamid Reza Sepangi Babak Vakili 《General Relativity and Gravitation》2010,42(5):1081-1102
We employ the familiar canonical quantization procedure in a given cosmological setting to argue that it is equivalent to
and results in the same physical picture if one considers the deformation of the phase-space instead. To show this we use
a probabilistic evolutionary process to make the solutions of these different approaches comparable. Specific model theories
are used to show that the independent solutions of the resulting Wheeler–DeWitt equation are equivalent to solutions of the
deformation method with different signs for the deformation parameter. We also argued that since the Wheeler–DeWitt equation
is a direct consequence of diffeomorphism invariance, this equivalence is only true provided that the deformation of phase-space
does not break such an invariance. 相似文献
73.
74.
Hakimeh Sharafi Mahya Abdoli Hamidreza Hajfarajollah Nima Samie Leila Alidoust Habib Abbasi Jamshid Fooladi Hossein Shahbani Zahiri Kambiz Akbari Noghabi 《Applied biochemistry and biotechnology》2014,173(5):1236-1249
A biosurfactant-producing thermophile was isolated from the Kahrizak landfill of Tehran and identified as a bacterium belonging to the genus Aneurinibacillus. A thermostable lipopeptide-type biosurfactant was purified from the culture medium of this bacterium and showed stability in the temperature range of 20–90 °C and pH range of 5–10. The produced biosurfactant could reduce the surface tension of water from 72 to 43 mN/m with a CMC of 1.21 mg/mL. The strain growing at a temperature of 45 °C produces a substantial amount of 5 g/L of biosurfactant in the medium supplemented with sunflower oil as the sole carbon source. Response surface methodology was employed to optimize the biosurfactant production using sunflower oil, sodium nitrate, and yeast extract as variables. The optimization resulted in 6.75 g/L biosurfactant production, i.e., 35 % improved as compared to the unoptimized condition. Thin-layer chromatography, FTIR spectroscopy, 1H-NMR spectroscopy, and biochemical composition analysis confirmed the lipopeptide structure of the biosurfactant. 相似文献
75.
Masoud Rahman Dr. Fariba Tajabadi Leyla Shooshtari Dr. Nima Taghavinia 《Chemphyschem》2011,12(5):966-973
Hollow structures show both light scattering and light trapping, which makes them promising for dye‐sensitized solar cell (DSSC) applications. In this work, nanoparticulate hollow TiO2 fibers are prepared by layer‐by‐layer (LbL) self‐assembly deposition of TiO2 nanoparticles on natural cellulose fibers as template, followed by thermal removal of the template. The effect of LbL parameters such as the type and molecular weight of polyelectrolyte, number of dip cycles, and the TiO2 dispersion (amorphous or crystalline sol) are investigated. LbL deposition with weak polyelectrolytes (polyethylenimine, PEI) gives greater nanoparticle deposition yield compared to strong polyelectrolytes (poly(diallyldimethylammonium chloride), PDDA). Decreasing the molecular weight of the polyelectrolyte results in more deposition of nanoparticles in each dip cycle with narrower pore size distribution. Fibers prepared by the deposition of crystalline TiO2 nanoparticles show higher surface area and higher pore volume than amorphous nanoparticles. Scattering coefficients and backscattering properties of fibers are investigated and compared with those of commercial P25 nanoparticles. Composite P25–fiber films are electrophoretically deposited and employed as the photoanode in DSSC. Photoelectrochemical measurements showed an increase of around 50 % in conversion efficiency. By employing the intensity‐modulated photovoltage and photocurrent spectroscopy methods, it is shown that the performance improvement due to addition of fibers is mostly due to the increase in light‐harvesting efficiency. The high surface area due to the nanoparticulate structure and strong light harvesting due to the hollow structure make these fibers promising scatterers in DSSCs. 相似文献
76.
Nima Khosravi 《General Relativity and Gravitation》2011,43(5):1417-1426
It is shown that the vacuum state in the presence of Lorentz violation can be followed by a universe filled with particles
at late times similar to the current status of the universe. In this model a modification in dispersion relation (Lorentz
violation) appears representing the regime of quantum gravity which has been dominant in the early universe. The existence
of the particles can be interpreted as an evidence for quantum effects of gravity at early times. It is concluded that the
present observable particles have a geometrical origin due to the well-known correspondence between geometry and gravity. 相似文献
77.
Ataollah Asgharian Davood D. Ganji Soheil Soleimani Sina Asgharian Nima Sedaghaty zade Behzad Mohammadi 《Mathematical Methods in the Applied Sciences》2011,34(18):2209-2217
An analytical study is performed on heat and mass transfer in MHD‐free convection from a moving permeable vertical surface and the results are compared with previous works on this phenomenon to test the validity. The coupled equations of boundary layer are transformed from their non‐linear form to ordinary form using similarity transformation and then are solved by a newly developed method, homotopy analysis method. Having different base functions, homotopy analysis method provides us with great freedom in choosing the solution of a nonlinear problem. Solving the boundry layer equations, the effects of different parameters such as magnetic field strength parameter (M), Prandtl number (Pr), Schmidt number (Sc), buoyancy ratio and suction/blowing parameter (fw) on velocity, temperature, and concentration profiles are taken into consideration. Obtained results show that increment of magnetic field strength parameter (M) leads to decrease in velocity profile. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
78.
Shahram Mohammadnejad Nima Jouyandeh Abkenar Ali Bahrami 《Optical and Quantum Electronics》2013,45(8):873-884
In this paper, the effect of doping concentration and layer thickness on the performance of tunnel junctions (TJs) is studied. We investigate the behavior of single, double and triple layer structures of TJs. Triple layer structure shows better performance in comparison with the other structures and can reach the higher tunneling current besides lower voltage drop. Also, the behavior of the triple layer TJ with different doping concentration profiles is studied. We propose a new normal distribution profile for doping concentration in multilayer TJs which shows better performance in comparison with the linear and graded doping concentration profiles. The higher $\upalpha $ parameters in normal distribution enhance the device performance with increasing the smoothness of doping variations in the center and edge of the TJ. Finally, we examine different thicknesses of triple layer TJ in order to achieve the optimum structure. 相似文献
79.
Transport in Porous Media - We present numerical simulations of post-Darcy flow in thin porous medium: one consisting of staggered arrangements of circular cylinders and one random distribution of... 相似文献
80.
Nima Tofighi Murat Ozbulut James J. Feng Mehmet Yildiz 《Theoretical and Computational Fluid Dynamics》2016,30(5):469-483
Manipulation of the Rayleigh-Taylor instability using an external electric field has been the subject of many studies. However, most of these studies are focused on early stages of the evolution. In this work, the long-term evolution of the instability is investigated, focusing on the forces acting on the interface between the two fluids. To this end, numerical simulations are carried out at various electric permittivity and conductivity ratios as well as electric field intensities using Smoothed Particle Hydrodynamics method. The electric field is applied in parallel to gravity to maintain unstable evolution. The results show that increasing top-to-bottom permittivity ratio increases the rising velocity of the bubble while hindering the spike descent. The opposite trend is observed for increasing top-to-bottom conductivity ratio. These effects are amplified at larger electric field intensities, resulting in narrower structures as the response to the excitation is non-uniform along the interface. 相似文献