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91.
Bianchi type V perfect fluid cosmological models are investigated with cosmological term varying with time. Using a generation technique (Camci et al., 2001), it is shown that Einsteins field equations are solvable for any arbitrary cosmic scale function. Solutions for particular forms of cosmic scale functions are also obtained. The cosmological constant is found to be a decreasing function of time, which is supported by results from recent type Ia supernovae observations. Some physical aspects of the models are also discussed. 相似文献
92.
Novel efficient mesoporous solid acid catalyst UDCaT-4: dehydration of 2-propanol and alkylation of mesitylene 总被引:1,自引:0,他引:1
A novel mesoporous solid acid catalyst named UDCaT-4 was synthesized by incorporating superacidic centers of persulfated alumina and zirconia into highly ordered and well-defined hexagonal mesoporous silica. The catalyst is well characterized, and its properties are compared with those of bulk persulfated alumina and zirconia (PAZ) by FTIR spectroscopy, X-ray diffraction, Brunauer-Emmett-Teller surface area, pore size analysis, scanning electron microscopy, energy dispersive X-ray spectroscopy, and ammonia temperature-programmed desorption. UDCaT-4 is more acidic than PAZ. The dehydration processes of 2-propanol, diisopropyl ether, and n-propanol were studied independently including a mixture of n-propanol and 2-propanol to throw light on kinetics and mechanism. For alkylation of mesitylene with 2-propanol, UDCaT-4 exhibits superior catalytic activity in comparison with PAZ and also it shows remarkable stability toward coke formation. Kinetic interpretations of the observed rate data are presented for all reactions, and mechanistic models are developed. The results are novel. 相似文献
93.
Majeti BK Singh RS Yadav SK Bathula SR Ramakrishna S Diwan PV Madhavendra SS Chaudhuri A 《Chemistry & biology》2004,11(4):427-437
Herein, we report enhanced intravenous mouse lung transfection using novel cyclic-head-group analogs of usually open-head cationic transfection lipids. Design and synthesis of the new cyclic-head lipid N,N-di-n-tetradecyl-3,4-dihydroxy-pyrrolidinium chloride (lipid 1) and its higher alkyl-chain analogs (lipids 2-4) and relative in vitro and in vivo gene transfer efficacies of cyclic-head lipids 1-4 to their corresponding open-head analogs [lipid 5, namely N,N-di-n-tetradecyl-N,N-(2-hydroxyethyl)ammonium chloride and its higher alkyl-chain analogs, lipids 6-8] have been described. In stark contrast to comparable in vitro transfection efficacies of both the cyclic- and open-head lipids, lipids 1-4 with cyclic heads were found to be significantly more efficient (by 5- to 11-fold) in transfecting mouse lung than their corresponding open-head analogs (5-8) upon intravenous administration. The cyclic-head lipid 3 with di-stearyl hydrophobic tail was found to be the most promising for future applications. 相似文献
94.
Nishizawa M Yadav VK Skwarczynski M Takao H Imagawa H Sugihara T 《Organic letters》2003,5(10):1609-1611
[reaction: see text] Hg(OTf)(2) exhibits remarkable catalytic activity for the hydroxylative cyclization of 1,6-enynes. The present procedure should involve a sequence of mercuration of a terminal alkyne, carbocyclization, hydration, and protodemercuration that regenerates the catalyst. 相似文献
95.
96.
The Friedel-Crafts acylation of 2-methoxynaphthol is generally carried out by using highly polluting acids such as HF, AlCl(3), BF(3) which are used in more than stoichiometric quantities and are neutralized at the end creating large quantities of waste, corrosion problems and hazard. This paper discusses the use of acid treated clays such as K-10 and Filtrol-24 and cation exchange resins such as Amberlyst-36, Amberlyst-15 and Indion-130 as catalysts at 25 degrees C which have different acid strengths and pore size distributions. The activities were in the following order:Amberlyst-36>Indion-130>Amberlyst-15>K-10clay>Filtrol-24.In the presence of ultrasound, the activities were found to increase by more than a factor of 3 in the case of large porous resins than the clays. The selectivity to 1-acyl-2-methoxynaphthalene was found to remain the same. Utrasonics did not promote isomerisation or direct conversion to 6-acyl-2-methoxynaphthalene. 相似文献
97.
Anirudh Pradhan Purnima Pandey Kanti Jotania Mahesh Kumar Yadav 《International Journal of Theoretical Physics》2007,46(11):2774-2787
Plane symmetric viscous fluid cosmological models of the universe with a variable cosmological term are investigated. The
viscosity coefficient of bulk viscous fluid is assumed to be a power function of mass density whereas the coefficient of shear
viscosity is to be proportional to rate of expansion in the model. We have also obtained a special model in which the shear
viscosity is assumed to be zero. The cosmological constant Λ is found to be a decreasing function of time and a positive which is supported by results from recent supernovae Ia observations.
Some physical and geometric properties of the models are also discussed. 相似文献
98.
S. Ghosh Nishchhal Yadav 《The European Physical Journal B - Condensed Matter and Complex Systems》2007,59(2):173-178
Using the hydrodynamical model and following the coupled mode approach, detailed analytical investigation of stimulated Brillouin
scattering is performed in an electrostrictive semiconductor. The total induced current density including diffusion current
density and the effective Brillouin susceptibility are obtained under off-resonant laser irradiation. The analysis deals with
the qualitative behaviour of the Brillouin gain and transmitted intensity with respect to excess doping concentration and
magnetic field. Efforts are directed towards optimizing the doping level and magnetic field to achieve maximum Brillouin gain
at pump intensities far below the optical damage threshold level. It is found that by immersing a moderately doped semiconductor
in a sufficiently strong magnetic field in transverse direction, one can achieve resonant enhancement of Brillouin gain provided
the generated acoustic mode lies in the dispersionless regime. 相似文献
99.
Magnetoelectric (ME) nanocomposites containing Ni0.75Co0.25Fe2O4-BiFeO3 phases were prepared by citrate sol-gel process. X-ray diffraction (XRD) analysis showed phase formation of xNi0.75Co0.25Fe2O4-(1−x)BiFeO3 (x=0.1, 0.2, 0.3 and 0.4) composites on heating at 700 °C. Transmission electron microscopy revealed the formation of powders of nano order size and the crystal size was found to vary from 30 to 85 nm. Dispersion in dielectric constant (ε) and dielectric loss (tan δ) in the low-frequency range have been observed. It is seen that nanocomposites exhibit strong magnetic properties and a large ME effect. On increasing Ni0.75Co0.25Fe2O4 contents in the nanocomposites, the saturation magnetization (MS) and coercivity (HC) increased after annealing at 700 °C. The large ME output in the nanocomposites exhibits strong dependence on magnetic bias and magnetic field frequency. The large value of ME output can be attributed to small grain size of ferrite phase of nanocomposite being prepared by citrate precursor process. 相似文献
100.
Polycrystalline sample with (Na0.5Bi0.5)ZrO3 (NBZ) stoichiometry was prepared using a high-temperature solid-state reaction technique. X-ray diffraction (XRD) analyses indicate the formation of a single-phase perovskite-type orthorhombic structure. AC impedance plot is used as tool to analyse the electrical behaviour of the sample as a function of temperature at different frequency. The AC impedance studies revealed the presence of grain boundary effect and evidence of a negative temperature coefficient of resistance (NTCR) character. Pseudo Cole-Cole and complex electric modulus analyses indicated non-Debye-type dielectric relaxation. The AC conductivity obeys the universal power law. The pair approximation type correlated barrier hopping (CBH) model explains the universal behaviour of the s exponent. The apparent activation energy to the conduction process and minimum hopping distance are discussed. 相似文献