52 research outputs found
Elastic and thermodynamic properties of the shape-memory alloy AuZn
The current work reports on the elastic shear moduli, internal friction, and
the specific heat of the B2 cubic ordered alloy AuZn as a function of
temperature. Measurements were made on single-crystal and polycrystalline
samples using Resonant Ultrasound Spectroscopy (RUS), semi-adiabatic
calorimetry and stress-strain measurements. Our results confirm that this alloy
exhibits the shape-memory effect and a phase transition at 64.75 K that appears
to be continuous (second-order) from the specific heat data. It is argued that
the combination of equiatomic composition and a low transformation temperature
constrain the chemical potential and its derivatives to exhibit behavior that
lies at the borderline between that of a first-order (discontinuous) and a
continuous phase transition. The acoustic dissipation does not peak at the
transtion temperature as expected, but shows a maximum well into the
low-temperature phase. The Debye temeprature value of 219 K, obtained from the
low-temperature specific heat data is in favorable agreement with that
determined from the acoustic data (207 K) above the transition.Comment: 25 pages, 6 figures, submitted to Phys. Rev.
Phase Progression of γ‑Al<sub>2</sub>O<sub>3</sub> Nanoparticles Synthesized in a Solvent-Deficient Environment
Our
simple and uniquely cost-effective solvent-deficient synthetic method
produces 3–5 nm Al<sub>2</sub>O<sub>3</sub> nanoparticles which
show promise as improved industrial catalyst–supports. While
catalytic applications are sensitive to the details of the atomic
structure, a diffraction analysis of alumina nanoparticles is challenging
because of extreme size/microstrain-related peak broadening and the
similarity of the diffraction patterns of various transitional Al<sub>2</sub>O<sub>3</sub> phases. Here, we employ a combination of X-ray
pair-distribution function (PDF) and Rietveld methods, together with
solid-state NMR and thermogravimetry/differential thermal analysis-mass
spectrometry (TG/DTA-MS), to characterize the alumina phase-progression
in our nanoparticles as a function of calcination temperature between
300 and 1200 °C. In the solvent-deficient synthetic environment,
a boehmite precursor phase forms which transitions to γ-Al<sub>2</sub>O<sub>3</sub> at an extraordinarily low temperature (below
300 °C), but this γ-Al<sub>2</sub>O<sub>3</sub> is initially
riddled with boehmite-like stacking-fault defects that steadily disappear
during calcination in the range from 300 to 950 °C. The healing
of these defects accounts for many of the most interesting and widely
reported properties of the γ-phase
- …