Hydrogen in the C15 Laves-phase material TaV2Hx has been studied by
means of resonant ultrasound spectroscopy over the temperature range of
15-345 K for a series of hydrogen concentrations (x = 0.00-0.53). Ultrasonic
loss peaks and frequency shifts (dispersion) associated with the hydrogen
motion were observed, yielding parameters for the hydrogen motion. Hydrogen in
these materials is known to occupy the tetrahedral g sites which form a
series of interlinked hexagons. The ultrasonic results were associated with H
hopping between g-site hexagons. The relaxation rates for x⩽0.18 were
best described as a sum of two Arrhenius processes. For x = 0.34 and 0.53 only
a single Arrhenius process was needed to fit the results, although the
presence of a second Arrhenius mechanism could not be over-ruled. A single
relaxation rate was sufficient to fit the data; a distribution of rates was
not required. The magnitudes of the attenuation and dispersion depended
linearly on the hydrogen concentration implying that it is the relaxation of
isolated H atoms (the Snoek effect) that is responsible for the mechanical
damping. The faster local motion of H reported from nuclear magnetic resonance
measurements for motion within g-site hexagons was not observed in the
present study. This suggests that the H hopping rate for the local motion
remains above the ultrasonic frequencies over the temperature range of study,
or perhaps that too few H atoms participate in the local motion.
Nanoparticles composed of iron and carbon have been produced by chemical vapor synthesis. A detailed structural, electronic, and magnetic characterization has been performed by several methods. The atomic arrangement in the as-prepared particles is strongly affected and stabilized by excess carbon. Small clusters of different ferrous phases are the building blocks of the particles. Due to the in situ formation of a carbonaceous shell the particles are stable against oxidation at ambient conditions. The magnetic properties are influenced by the exceptionally small particle size. The particles exhibit superparamagnetic behavior with a blocking temperature of 30K and the temperature dependence of the magnetization is governed by the finite size of the system.
Resonant ultrasound spectroscopy has been used for the measurement of the elastic moduli of a Ti-based quasicrystal, one of the major classes of quasicrystals. Both the bulk and shear elastic moduli of a Ti-Zr-Ni multigrained icosahedral quasicrystal and a polycrystalline 1/1 approximant were measured over the temperature range of 15-300 K. The elastic moduli of these two materials are very nearly the same, a result in agreement with the belief that the structures of these two materials are closely related. The temperature dependence of the moduli resembles that of ordinary metals. The results have been used to predict the acoustic contribution to the low-temperature specific heat. This contribution is much larger in these Ti-based materials than for the Al-based quasicrystals reported by others. Besides their general interest, the elastic moduli are useful for constructing atomic potentials in these unusual materials. ͓S0163-1829͑99͒09517-X͔
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.