The NMR theory for a two-dimensional (2D) system of spins coupled by dipolar interaction and undergoing diffusive motion is presented. The correlation functions (c.f.) of the dipolar coefficients are split in two parts: a "slow" one and a "fast'" one. The c.r. are also written in a form which allows easy prediction of the angular dependence of the relaxation rates. It is demonstrated that, owing to the diffusion process, the slow part decays at long times as t -I and the fast one as t -2. Measurements of T 2 , T I , Tip on the protons of nitric acid intercalated in graphite have been performed in the temperature range -153 'C, + 18 'c. The prediction of the model was checked both in the solid phases (where WoTD> 1 with TD = diffusional correlation time) and in the "liquid" phase (above -20'C). The measurements at room temperature prove, over several decades of frequency, the logarithmic divergence of the slow part-spectral density for w-iO. Also in the solid phases, the 2D-diffusive model interprets the measurements. Using the experimental results, we can illustrate the angular dependence of the relaxation rates that are predicted by the theory for planar systems in different dynamical situations. The information given by NMR on graphite nitrate are also discussed.
The static and dynamic properties which characterize the structural phase transitions in NaNbO~have been widely investigated by transient NMR measurements of the Na" and Nb~' nuclei in a powdered sample (no 1arg~ough single crystal was available). The temperature range investigated (100-1100'K) includes both the purely structural transitions associated with the tilting of the NbO~octahedra and the ferroelectric and antiferroelectric transitions associated with the off-center motion of the Nb atoms. The static efFects have been investigated through the changes in the free-precession decay of the central hne due to secondwrder quadrupole broadening. The dephasing time of the free-precession decay has been related to the quadrupole coupling constant. The temperature dependence of the rotational displacement of the NbO~octahedra and of the off~ter displacement of the Nb ions is then obtained. The variation of the tilt angle P of the oxygen octahedra near the transition at 641'C is very. rapid and it is not possible to decide whether 4 goes to zero continuously or with a sm~8 discentinuity. The maximum value of iit). in the tetragonal phase, as deduced from a crude estimate of the electric field gradient in a point-charge model, is about 7 . No off-center displacement of the Nb atom is observed, on cooling, before the 480'C transition. The numerical values for the Nb displacements in the R and P phase (c 0.11 A at 373 C and c 0.15 A at 315'C) are in good agreement with previous indications of x-ray difFraction measurements. The dynamic effects have been investigated through nuclear spin-lattice quadrupole relaxation. A theory is developed which relates the relaxation rate to the critical pmuneters of the central peak in the dynamic cubic-tetragonal phase transition at 641'C, a value v 0.6 for the critical index of the correlation length can be estimated; the rotational fluctuations appear to have a quasiphmar correlation. The experimental results seem to indicate that for (T -T, )~4'C the slowing down reaches an angular frequency of the order of 300 MHz. Finally, an estimate for the root-mean-square local fluctuating angle near T, of about 1.6 is obtained.
Aspects of formation and characterization of a radioactive colloidal fraction released by the waste form or produced by association with microcolloids naturally existing in ground water or produced either by corrosion of container material or by degradation of backfill material are discussed. A filtration model has been developed in order to describe colloidal transport under field conditions. Comparison between data obtained with laboratory column experiments and theoretical evaluations is presented.
Small scale infiltration experiments were conducted with granite cored samples of various degrees of alteration. Hydrodynamic characteristics of the granite columns were determined with nonsorbing tracers. The transport behavior of Np was investigated in the presence of natural organic matter. This was found to decrease the retardation factor of Np. Results from dynamic experiments were compared with static sorption tests. A preferential association of Pu with fracture-filling material was observed.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.