Quasielastic neutron scattering study of hydrogen motion in NbC0.71H0.28
In order to study the mechanism and parameters of H jump motion in the nonstoichiometric Nb carbides, we have performed quasielastic neutron scattering (QENS) measurements for NbC0.71H0.28 over the temperature range 11– 475 K. Our results indicate that about 30% of H atoms in this system participate in a fast diffusive motion. The temperature dependence of the corresponding H jump rate in the range 298–475 K follows the Arrhenius law with an activation energy of 328 ± 9 meV. The Q dependence of the QENS data suggests that the observed jump motion corresponds to long-range diffusion of H atoms along chains of the off-centre sites in carbon vacancies.
Crystal structures and thermal properties of titanium carbo-deuterides as prepared by combustion synthesis
Titanium carbo-deuterides of various compositions have been prepared by thermal radiation and cold syntheses, and by self-propagating ¯ high-temperature synthesis. While the former methods favored the formation of a cubic structure of composition TiC D (Fm3m, 0.48(1) 0.60(2) ¯ ˚ ˚ a54.30963(3) A) the latter yielded a trigonal structure of composition TiC D (P3m1, a53.08208(2), c55.04052(6) A). 0.50(2) 0.707(4) Differential thermal analysis and thermogravimetry data suggest the cubic structure to be thermally less stable (desorption onset temperature 823 K) than the trigonal structure (1093 K). Joint Rietveld structure refinements based on neutron and X-ray powder diffraction data show that carbon occupies octahedral interstices in the f.c.c. and h.c.p. metal atom networks of the cubic and trigonal phase, respectively, while deuterium occupies both octahedral and tetrahedral interstices in the cubic but tetrahedral interstices only in the trigonal phase. The different thermal stability of the compounds is attributed to these structural differences.  2003 Elsevier B.V. All rights reserved. able. In this paper we present the first structure refinements of, and thermal stability data for the Ti-C-H system. The Hydrogen reacts with transition metal carbides to form samples investigated were obtained by combustion syn- so-called carbo-hydrides. Representative examples occur in thesis (11-13). This method is of particular interest in the systems based on titanium (1-6), zirconium (3-5,7-10), context of hydrogen storage materials because it is capable hafnium (2,3,5), vanadium (9,10), niobium (5) and others. of yielding large quantities of materials at a relatively low The compounds crystallise mostly with face-centred cubic cost. Consequently, the structural and thermal properties ( f.c.c.) and / or hexagonal close-packed (h.c.p.) metal atom reported in this paper refer to relatively extreme conditions arrangements in which carbon and hydrogen atoms occupy of synthesis.
Microstructure of the solder-casting zone in bridges of dental gold alloys.
The composition and structure were studied in the cast and in the solder of bridges which had failed clinically. Both materials were gold alloys but with different contents of Pt, Ag and Zn. The metallographic investigation revealed defects mostly in the solder, situated in the subsurface layer. SEM studied of the fracture surface revealed large porosites and a structure of dense parallel lines--striations--, indicating that the material had failed from fatigue. The materials in the fractured bridges were identified by micro-probe measurements. It was stated that cast material and solder material used together, were of different composition.