Degree Type

Dissertation

Date of Award

1988

Degree Name

Doctor of Philosophy

Department

Physics and Astronomy

First Advisor

R. G. Barnes

Abstract

Proton spin relaxation rates have been measured across the random alloy system Nb[subscript] yV[subscript] 1 - yH[subscript] x at temperatures from 10 to 800 K and various frequencies from 4.45 to 127.5 MHz. Enhanced spin-lattice relaxation rates found below 100 K in all samples with Nb, including NbH[subscript]0.2, at frequencies of 40 MHz and below have been explained by a process of dipolar cross relaxation between proton Zeeman energy levels and the combined Zeeman-quadrupole levels of [superscript]93Nb. A semi-quantitative model, applicable in metal-hydrogen as well as other multi-spin systems, has been developed to describe the cross relaxation mechanism. Unusual features found experimentally and interpreted via this model include: (1) cross relaxation in unrotated polycrystalline samples, (2) cross relaxation to spins with pure quadrupole frequencies substantially less than the proton Larmor frequency, and (3) both temperature-dependent and temperature-independent contributions which can vary with frequency. Effects of Zeeman-quadrupole splittings on motional relaxation rates at higher temperatures were also estimated;Phase boundaries between solid solutions and ordered hydrides were found by analyzing non-experimental magnetization recovery curves. The temperature at which hydrides precipitated was lower in all the alloys as compared to pure Nb-H and V-H. Alloys with Nb concentrations of 50 at.% and above could retain substantial amounts of H in solid solution without hydride precipitation, at least down to 10 K;Motional relaxation rates of H in solid solution could in most cases be modeled by hopping rates governed by asymmetric Gaussian distributions of activation energies. The mean activation energies varied from 160 to 280 meV, depending on both V and H composition. A different analysis combined the present NMR results with internal friction data to derive activation energies somewhat smaller than those above, but in closer agreement with previous determinations. The dependence of the hopping rates on H concentration suggests that H atoms are attracted to tetrahedral sites with several V nearest neighbors. The relaxation rates could not be modeled by a Gaussian distribution of site energies with constant saddle point energy.

DOI

https://doi.org/10.31274/rtd-180813-11597

Publisher

Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/

Copyright Owner

Lyle Raymond Lichty

Language

en

Proquest ID

AAI8909167

File Format

application/pdf

File Size

188 pages

Share

COinS