On the structural integrity of the spinel block in the β"-alumina structure.
The β''-alumina structure is examined in detail and an analysis is presented of the three-dimensional integrity of the lattice. The layer structure that is responsible for the very high sodium conduction rate is the specific focus. Rigid layers that are derived from the cubic spinel structure are interleaved by more open honeycomb pathways where rapid ion diffusion takes place. The three-dimensional rigidity of the spinel block in this structure makes it possible to accurately quantify the conduction layer thickness based only on the hexagonal unit-cell dimensions, as suggested originally by Harbach [(1983), J. Mater. Sci. 18, 2437-2452]. His calculation is tested rigorously against the many single-crystal structure determinations that have been made on the β''-alumina family compounds and excellent correlation is found.
Conduction plane and structure of Li-stabilized Na+ β″-alumina: A powder neutron diffraction study
The structure of Li-stabilized Na+ β″-alumina has been refined by Rietveld analysis of high resolution time-of-flight powder diffraction data. The sample was a sintered tube of the type used for battery separators. The Li was found to reside in the Al(2) site in the spinel block. Fourier mapping and Rietveld analysis revealed that Na is located in both 6c and 18h sites (hexagonal setting of space group R3m) separated 1.03(3) A in the conduction slab.
Neutron Residual Stress Measurements on Rail Sections for Different Production Conditions
Rail sectioning with subsequent neutron diffraction experiments has been used to assess residual stresses in the rails. In this study we present the results of neutron stress measurements performed at the NIST Center for Neutron Research (NCNR) on rail sections from rails that were produced under various conditions. Specifically, these are air-cooled, air-cooled and roller-straightened, head-hardened and head-hardened and roller-straightened. More significantly, a head-hardened and roller-straightened rail was also studied after service to elucidate evolution of the service-induced residual stresses. In the latter case both a transverse-cut slice and the central region of a 0.53 m long piece were studied. Measurements on this piece are the first in which triaxial stresses have been determined for an intact rail. Neutron strain measurements with 3×3×3 mm3 spatial resolution were successfully employed for transversally cut slices to verify the difference in the stress state depending on the production process. Although examination of slices allows determination of only two-dimensional stresses in the plane of the slice, additional measurements on obliquely-cut slices, which were also carried out, and utilization of FEM gives the possibility of reconstructing the full triaxial stress distribution. Together, these approaches provide a better understanding of rail fabrication and the possibility of improving the durability and safety of rails in the future.Copyright © 2004 by ASME