276 research outputs found
Role of Internal Motions and Molecular Geometry on the NMR Relaxation of Hydrocarbons
The role of internal motions and molecular geometry on H NMR relaxation
times in hydrocarbons is investigated using MD (molecular dynamics)
simulations of the autocorrelation functions for in{\it tra}molecular
and in{\it ter}molecular H-H dipole-dipole interactions
arising from rotational () and translational () diffusion, respectively.
We show that molecules with increased molecular symmetry such as neopentane,
benzene, and isooctane show better agreement with traditional hard-sphere
models than their corresponding straight-chain -alkane, and furthermore that
spherically-symmetric neopentane agrees well with the Stokes-Einstein theory.
The influence of internal motions on the dynamics and relaxation of
-alkanes are investigated by simulating rigid -alkanes and comparing with
flexible (i.e. non-rigid) -alkanes. Internal motions cause the rotational
and translational correlation-times to get significantly shorter
and the relaxation times to get significantly longer, especially for
longer-chain -alkanes. Site-by-site simulations of H's along the chains
indicate significant variations in and across the chain,
especially for longer-chain -alkanes. The extent of the stretched (i.e.
multi-exponential) decay in the autocorrelation functions are
quantified using inverse Laplace transforms, for both rigid and flexible
molecules, and on a site-by-site bases. Comparison of measurements
with the site-by-site simulations indicate that cross-relaxation (partially)
averages-out the variations in and across the chain of
long-chain -alkanes. This work also has implications on the role of
nano-pore confinement on the NMR relaxation of fluids in the organic-matter
pores of kerogen and bitumen
NMR Spin-Rotation Relaxation and Diffusion of Methane
The translational-diffusion coefficient and the spin-rotation
contribution to the H NMR relaxation time for methane (CH) are
investigated using MD (molecular dynamics) simulations, over a wide range of
densities and temperatures , spanning the liquid, supercritical, and
gas phases. The simulated agree well with measurements, without any
adjustable parameters in the interpretation of the simulations. A minimization
technique is developed to compute the angular-velocity for non-rigid spherical
molecules, which is used to simulate the autocorrelation function
for spin-rotation interactions. With increasing (i.e. decreasing ),
shows increasing deviations from the single-exponential decay
predicted by the Langevin theory for hard spheres, and the deviations are
quantified using inverse Laplace transforms of . is
derived from using the kinetic model "km" for gases
(), and the diffusion model "dm" for liquids ().
shows better agreement with measurements at higher ,
while shows better agreement with measurements at lower
. is shown to dominate over the MD simulated H-H
dipole-dipole relaxation at high , while the opposite is found
at low . At high , the simulated spin-rotation correlation-time
agrees with the kinetic collision time for gases, from which
a new relation is inferred, without any adjustable
parameters
Lunar lander conceptual design
This paper is a first look at the problems of building a lunar lander to support a small lunar surface base. A series of trade studies was performed to define the lander. The initial trades concerned choosing number of stages, payload mass, parking orbit altitude, and propellant type. Other important trades and issues included plane change capability, propellant loading and maintenance location, and reusability considerations. Given a rough baseline, the systems were then reviewed. A conceptual design was then produced. The process was carried through only one iteration. Many more iterations are needed. A transportation system using reusable, aerobraked orbital transfer vehicles (OTV's) is assumed. These OTV's are assumed to be based and maintained at a low Earth orbit (LEO) space station, optimized for transportation functions. Single- and two-stage OTV stacks are considered. The OTV's make the translunar injection (TLI), lunar orbit insertion (LOI), and trans-Earth injection (TEI) burns, as well as midcourse and perigee raise maneuvers
Abnormal left ventricular wall rotation in patients with complete left bundle branch block: analysis using magnetic resonance imaging tagging images
The ideal vaccine is a simple and stable formulation which can be conveniently administered and provides life-long immunity against a given pathogen. The development of such a vaccine, which should trigger broad and strong B-cell and T-cell responses against antigens of the pathogen in question, is highly dependent on tailored vaccine delivery approaches. This review addresses vaccine delivery in its broadest scope. We discuss the needs and challenges in the area of vaccine delivery, including restrictions posed by specific target populations, potentials of dedicated stable formulations and devices, and the use of adjuvants. Moreover, we address the current status and perspectives of vaccine delivery via several routes of administration, including non-or minimally invasive routes. Finally we suggest possible directions for future vaccine delivery research and development. (C) 2012 Elsevier B.V. All rights reserved
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Integrated, Multi-Scale Characterization of Imbibition and Wettability Phenomena Using Magnetic Resonance and Wide-Band Dielectric Measurements
The petrophysical properties of rocks, particularly their relative permeability and wettability, strongly influence the efficiency and the time-scale of all hydrocarbon recovery processes. However, the quantitative relationships needed to account for the influence of wettability and pore structure on multi-phase flow are not yet available, largely due to the complexity of the phenomena controlling wettability and the difficulty of characterizing rock properties at the relevant length scales. This project brings together several advanced technologies to characterize pore structure and wettability. Grain-scale models are developed that help to better interpret the electric and dielectric response of rocks. These studies allow the computation of realistic configurations of two immiscible fluids as a function of wettability and geologic characteristics. These fluid configurations form a basis for predicting and explaining macroscopic behavior, including the relationship between relative permeability, wettability and laboratory and wireline log measurements of NMR and dielectric response. Dielectric and NMR measurements have been made show that the response of the rocks depends on the wetting and flow properties of the rock. The theoretical models can be used for a better interpretation and inversion of standard well logs to obtain accurate and reliable estimates of fluid saturation and of their producibility. The ultimate benefit of this combined theoretical/empirical approach for reservoir characterization is that rather than reproducing the behavior of any particular sample or set of samples, it can explain and predict trends in behavior that can be applied at a range of length scales, including correlation with wireline logs, seismic, and geologic units and strata. This approach can substantially enhance wireline log interpretation for reservoir characterization and provide better descriptions, at several scales, of crucial reservoir flow properties that govern oil recovery
Spectroscopic Study of 41,45Sc Nuclei Through (d,n) Reactions on 40,44Ca at 25 MeV
開始ページ、終了ページ: 冊子体のページ付
Gamow-Teller Transition in the (p,n) Reaction on 42Ca and 44Ca
開始ページ、終了ページ: 冊子体のページ付
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