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51.
2,2′‐Dipyridyl‐3,3′‐dipyridyl,5,5′‐dipyridyl‐diselenides have been synthesized by a convenient method employing non‐cryogenic conditions. Various bromopyridines (2‐Bromopyridine, 2,5‐dibromopyridines and 2,3,5‐Tribromopyridines) undergo selective monobromine–magnesium exchange to yield the corresponding pyridyl magnesium chlorides at room temperature upon treatment with iPrMgCl. The resulting pyridyl magnesium chloride is quenched with elemental selenium, which upon further oxidation affords the above diselenides in good yields. The compounds prepared using this methodology have been characterized by elemental analysis, IR, NMR (1H, 13C, 77Se) and mass spectral analysis. The molecular structure of 2,2′,5,5′‐Tetrabromo‐3,3′‐dipyridyldiselenide has been established by single‐crystal X‐ray diffraction analysis. It exists as a dimeric form due to the non‐bonding interactions between the selenium of one pyridine moiety and the hydrogen of the other. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
52.
RF SQUID behaviour has been observed at 77 K in YBaCuO thick films prepared by screen printing technique. A hole shunted with
a microbridge type of geometry is patterned manually for observing rf SQUID behaviour. Flux noise spectrum is also studied
and it is found to depend on the quality of the film. The spectral density of the flux noise in the white noise region is
1.7×10−3 Φ0/√Hz at 77 K. 相似文献
53.
Glasses with composition xGeO2.(0.30−x)M2O.0.70B2O3 (M=Li, K) containing 2.0 mol% of V2O5 have been prepared in the range 0.00≤x≤0.15 by normal melt quenching method. Electron paramagnetic resonance (EPR), optical transmission and absorption spectra and dc conductivity of these glasses have been studied. Spin Hamiltonian parameters (SHPs) of VO2+ ions, dipolar hyperfine coupling parameter, P, Fermi contact interaction parameter, K and molecular orbital coefficients (α2 and γ2) have been calculated. In GeO2·Li2O·B2O3 glasses there is no change in the tetragonality of the V4+O6 complex and the size of 3dxy orbit also remains unchanged with increase in GeO2 content. In GeO2·K2O·B2O3 glasses, there is an increase in the tetragonality of the V4+O6 complex and the 3dxy orbit expands with increase in GeO2 content. Values of the theoretical optical basicity, Λth, have also been reported. Optical band gap decreases with increase in GeO2 content. The dc conductivity of these glasses decreases and the activation energy increases with increase in GeO2:M2O ratio. 相似文献
54.
55.
We consider the time evolution of a class of stochastic systems of finite size with polynomial nearest neighbor transition rates. We obtain analytical expressions for the first passage time (FPT) and its moments. We show that the mean FPT, averaged over a uniform initial distribution, shows a simple asymptotoc behavior with the system size and the parameters of the transition rates. 相似文献
56.
57.
H. L. Arora 《Zeitschrift für Angewandte Mathematik und Physik (ZAMP)》1977,28(4):625-632
The whirling motion of a fixed-free viscoelastic shaft, whose model is based on the spring and dashpot (Kelvin element), is considered. We develop a precise determination of the system's stability by first obtaining its eigenvalues. It is shown that asymptotic stability exists as long as the angular speed of the shaft is below a certain critical value. On the other hand, the degree of stability first increases as the speed is increased but eventually decreases again.
Zusammenfassung Es wird die Wirbelbewegung einer einseitig eingespannten Welle untersucht, deren Modell dem Kelvin-Element entspricht. Zur Beurteilung der Stabilität werden zunächst die Eigenwerte ermittelt. Dann wird gezeigt, dass asymptotische Stabilität besteht, solange die Winkelgeschwindigkeit unter einem kritischen Wert liegt. Der Stabilitätsgrad nimmt zuerst mit der Winkelgeschwindigkeit zu und schliesslich wieder ab.相似文献
58.
CM Thaker S Rayaprol Krushna Mavani DS Rana MS Sahasrabudhe SI Patil DG Kuberkar 《Pramana》2002,58(5-6):1035-1039
The effect of simultaneous substitution of a fluctuating cation and a divalent cation in LaMnO3 perovskite modifies the properties of the material to exhibit large valence colossal magnetoresistance (CMR) effect. A good
example of these properties is (La1−2x
Pr
x
Ca
x
)MnO3 (LPCMO) type CMR material. In this communication it is reported that, with the increase in x (for x=0.1, 0.15, 0.2), the T
c varies between 100 and 120 K with improvisation in metal-insulator transition. Interestingly, resistance increases with x from few hundred ohms to few kilo ohms with corresponding decrease in the unit cell volume. The results of the studies using
X-ray diffraction (XRD), electrical resistivity, magnetoresistance and ac susceptibility measurements on LPCMO samples for
understanding the structural, transport and magnetic properties are discussed in detail. 相似文献
59.
R. K. Mohanty Samir Karaa Urvashi Arora 《Numerical Methods for Partial Differential Equations》2007,23(3):640-651
This article develops a new two‐level three‐point implicit finite difference scheme of order 2 in time and 4 in space based on arithmetic average discretization for the solution of nonlinear parabolic equation ε uxx = f(x, t, u, ux, ut), 0 < x < 1, t > 0 subject to appropriate initial and Dirichlet boundary conditions, where ε > 0 is a small positive constant. We also propose a new explicit difference scheme of order 2 in time and 4 in space for the estimates of (?u/?x). The main objective is the proposed formulas are directly applicable to both singular and nonsingular problems. We do not require any fictitious points outside the solution region and any special technique to handle the singular problems. Stability analysis of a model problem is discussed. Numerical results are provided to validate the usefulness of the proposed formulas. © 2006 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq, 2007 相似文献
60.
Akhilesh K. Arora R. Kesavamoorthy A.K. Sood G. Venkatraraman R. Krishnaswamy Debendranath Sahoo 《Journal of Physics and Chemistry of Solids》1984,45(1):69-77
The formation and dissolution behaviour of precipitates in NaCl : Pb2+ single crystals is studied by light scattering and ultramicroscopy. Measurements of Rayleigh ratio together with ultramicroscopic observations during isochronal annealing indicate the presence of two types of precipitates in as-grown crystals. Isochronal annealing after suitable thermal and thermomechanical treatments shows that precipitates of first type (I) which form and dissolve at lower temperatures, are due to homogeneous nucleation. Precipitates of the second category (II) which form and dissolve at higher temperatures are aligned along crystallographic directions and have their origin in stress-assisted processes. Isothermal annealing has been used to study the dissolution kinetics of the precipitated fraction, and it is found that the kinetics is of first order. The enthalpy of solution is determined from a study of the temperature dependence of the kinetic rate constant, in conjunction with available information on the migration energy of Pb2+ in NaCl. The enthalpy so deduced is in good agreement with the value as usually obtained from the concentration dependence of the dissolution temperature. 相似文献