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1H and 19F spin-lattice relaxation times in polycrystalline diammonium hexafluorozirconate have been measured in the temperature range
of 10–400 K to elucidate the molecular motion of both cation and anion. Interesting features such as translational diffusion
at higher temperatures, molecular reorientational motion of both cation and anion groups at intermediate temperatures and
quantum rotational tunneling of the ammonium group at lower temperatures have been observed. Nuclear magnetic resonance (NMR)
relaxation time results correlate well with the NMR second moment and conductivity studies reported earlier. 相似文献
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Mallikarjunaiah KJ Paramita KC Ramesh KP Damle R 《Solid state nuclear magnetic resonance》2007,32(1):11-15
1H NMR spin-lattice relaxation time measurements have been carried out in [(CH3)4N]2SeO4 in the temperature range 389-6.6 K to understand the possible phase transitions, internal motions and quantum rotational tunneling. A broad T1 minimum observed around 280 K is attributed to the simultaneous motions of CH3 and (CH3)4N groups. Magnetization recovery is found to be stretched exponential below 72 K with varying stretched exponent. Low-temperature T1 behavior is interpreted in terms of methyl groups undergoing quantum rotational tunneling. 相似文献
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Dambaru Bhatta Mallikarjunaiah S. Muddamallappa Daniel N. Riahi 《Nonlinear Analysis: Real World Applications》2010,11(5):4010-4020
The problem of weakly nonlinear convective flow in a mushy layer, with a permeable mush–liquid interface and constant permeability, is studied under operating conditions for an experiment. A Landau type nonlinear evolution equation for the amplitude of the secondary solutions, which is based on the Landau theory and formulation for the Rayleigh, , number close to its critical value, , is developed. Using numerical and analytical methods, the solutions to the evolution equation are calculated for both supercritical and subcritical conditions. We found, in particular, that for , the amplitude of the secondary solutions decays with time. For , the tendency for chimney formation in the mushy layer increases with time. In addition, in such a supercritical regime, the basic flow is linearly unstable and we see the presence of steady flow for large values of time. These results suggest a possible slow transition to turbulence in such a flow system. 相似文献
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Mallikarjunaiah KJ Jugeshwar Singh K Ramesh KP Damle R 《Magnetic resonance in chemistry : MRC》2008,46(2):110-114
(CH3)4NGeCl3 is prepared, characterized and studied using 1H NMR spin lattice relaxation time and second moment to understand the internal motions and quantum rotational tunneling. Proton second moment is measured at 7 MHz as function of temperature in the range 300-77 K and spin lattice relaxation time (T1) is measured at two Larmor frequencies, as a function of temperature in the range 270-17 K employing a homemade wide-line/pulsed NMR spectrometers. T1 data are analyzed in two temperature regions using relevant theoretical models. The relaxation in the higher temperatures (270-115 K) is attributed to the hindered reorientations of symmetric groups (CH3 and (CH3)4N). Broad asymmetric T1 minima observed below 115 K down to 17 K are attributed to quantum rotational tunneling of the inequivalent methyl groups. 相似文献
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Mallikarjunaiah S. Muddamallappa Dambaru Bhatta Daniel N. Riahi 《Transport in Porous Media》2009,79(2):301-317
In this article, an investigation is conducted to analyze the marginal stability with and without magnetic field in a mushy
layer. During alloy solidification, such mushy layer, which is adjacent to the solidification front and composed of solid
dendrites and liquid, is known to produce vertical chimneys. Here, we carry out numerical investigation for particular range
of parameter values, which cover those of available experimental studies, to determine the convective flow at the onset of
motion. The governing coupled non-linear partial differential equations are non-dimensionalised and solved to get the steady
basic-state solution. The thickness of the mushy layer is determined as a part of the solution. Using multiple shooting technique,
we determine the steady-state solutions in a range of critical Rayleigh number. We analyse the effect of several parameters,
Chandrasekhar number Q, and Robert’s number τ on the problem. It was found that an increase in Q has a stabilizing effect on solidification and the critical Rayleigh number increases on increasing Q. It was also found that for moderate or small values of Robert’s number τ the critical Rayleigh number is mostly insensitive. 相似文献
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(CH(3))(4)NPF(6) is studied by NMR measurements to understand the internal motions and cross relaxation mechanism between the heterogeneous nuclei. The spin lattice relaxation times (T(1)) are measured for (1)H and (19)F nuclei, at three (11.4, 16.1 and 21.34 MHz) Larmor frequencies in the temperature range 350-50K and (1)H NMR second moment measurements at 7 MHz in the temperature range 300-100K employing home made pulsed and wide-line NMR spectrometers. (1)H NMR results are attributed to the simultaneous reorientations of both methyl and tetramethylammonium groups and motional parameters are evaluated. (19)F NMR results are attributed to cross relaxation between proton and fluorine and motional parameters for the PF(6) group reorientation are evaluated. 相似文献
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Dambaru Bhatta Mallikarjunaiah S. Muddamallappa Daniel N. Riahi 《Transport in Porous Media》2010,82(2):385-399
This present study considers the problem of steady magneto-convection in a horizontal mushy layer with variable permeability
and an impermeable mush–liquid interface during directional solidification of binary alloys. We model the flow by introducing
a uniform magnetic field in the mushy layer which is considered as a porous medium where Darcy’s law holds and the permeability
is a function of the local solid volume fraction. Basic-state solutions are obtained analytically using the no-flow condition.
With the help of multiple shooting techniques, we obtain numerical solutions to the linear perturbation system for non-magnetic
and magnetic cases. Numerical results are presented showing the effects of the magnetic field and the permeability of the
layer. These results demonstrate that the application of an external magnetic field has stabilizing effects on the convection
and can reduce the tendency for chimney formation in the mushy layer. In addition, variable permeability, which corresponds
to an active mushy layer, indicates more stable and realizable flow system as compared to the case of constant permeability. 相似文献
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