The alloy-design strategy of combining multiple elements in near-equimolar ratios has shown great potential for producing exceptional engineering materials, often known as 'high-entropy alloys'. ...Understanding the elemental distribution, and, thus, the evolution of the configurational entropy during solidification, is undertaken in the present study using the Al₁.₃CoCrCuFeNi model alloy. Here we show that, even when the material undergoes elemental segregation, precipitation, chemical ordering and spinodal decomposition, a significant amount of disorder remains, due to the distributions of multiple elements in the major phases. The results suggest that the high-entropy alloy-design strategy may be applied to a wide range of complex materials, and should not be limited to the goal of creating single-phase solid solutions.
Abstract
Structure and thermodynamics of pure cubic ZrO
2
and HfO
2
were studied computationally and experimentally from their tetragonal to cubic transition temperatures (2311 and 2530 °C) to their ...melting points (2710 and 2800 °C). Computations were performed using automated
ab initio
molecular dynamics techniques. High temperature synchrotron X-ray diffraction on laser heated aerodynamically levitated samples provided experimental data on volume change during tetragonal-to-cubic phase transformation (0.55 ± 0.09% for ZrO
2
and 0.87 ± 0.08% for HfO
2
), density and thermal expansion. Fusion enthalpies were measured using drop and catch calorimetry on laser heated levitated samples as 55 ± 7 kJ/mol for ZrO
2
and 61 ± 10 kJ/mol for HfO
2
, compared with 54 ± 2 and 52 ± 2 kJ/mol from computation. Volumetric thermal expansion for cubic ZrO
2
and HfO
2
are similar and reach (4 ± 1)·10
−5
/K from experiment and (5 ± 1)·10
−5
/K from computation. An agreement with experiment renders confidence in values obtained exclusively from computation: namely heat capacity of cubic HfO
2
and ZrO
2
, volume change on melting, and thermal expansion of the liquid to 3127 °C. Computed oxygen diffusion coefficients indicate that above 2400 °C pure ZrO
2
is an excellent oxygen conductor, perhaps even better than YSZ.
The previously unknown experimental HfO2–Ta2O5‐temperature phase diagram has been elucidated up to 3000°C using a quadrupole lamp furnace and conical nozzle levitator system equipped with a CO2 ...laser, in conjunction with synchrotron X‐ray diffraction. These in‐situ techniques allowed the determination of the following: (a) liquidus, solidus, and invariant transformation temperatures as a function of composition from thermal arrest experiments, (b) determination of equilibrium phases through testing of reversibility via in‐situ X‐ray diffraction, and (c) molar volume measurements as a function of temperature for equilibrium phases. From these, an experimental HfO2–Ta2O5‐temperature phase diagram has been constructed which is consistent with the Gibbs Phase Rule.
HfO2–Ta2O5‐Temperature phase space built based on the observed equilibrium phases from in situ X‐ray powder diffraction and the Gibbs Phase Rule.
Here, we report the structural characterization of several amorphous drugs made using the method of quenching molten droplets suspended in an acoustic levitator. 13C NMR, X-ray, and neutron ...diffraction results are discussed for glassy cinnarizine, carbamazepine, miconazole nitrate, probucol, and clotrimazole. The 13C NMR results did not find any change in chemical bonding induced by the amorphization process. High-energy X-ray diffraction results were used to characterize the ratio of crystalline to amorphous material present in the glasses over a period of 8 months. All the glasses were stable for at least 6 months except carbamazepine, which has a strong tendency to crystallize within a few months. Neutron and X-ray pair distribution function analyses were applied to the glassy materials, and the results were compared with their crystalline counterparts. The two diffraction techniques yielded similar results in most cases and identified distinct intramolecular and intermolecular correlations. The intramolecular scattering was calculated based on the crystal structure and fit to the measured X-ray structure factor. The resulting intermolecular pair distribution functions revealed broad- nearest and next-nearest neighbor molecule-molecule correlations.
Low cation coordination in oxide melts Skinner, L B; Benmore, C J; Weber, J K R ...
Physical review letters,
2014-Apr-18, Volume:
112, Issue:
15
Journal Article
Peer reviewed
The complete set of partial pair distribution functions for a rare earth oxide liquid are measured by combining aerodynamic levitation, neutron and x-ray diffraction on Y2O3, and Ho2O3 melts at ...2870 K. The average Y-O (or Ho-O) coordination of these isomorphic melts is measured to be 5.5(2), which is significantly less than the octahedral coordination of crystalline Y2O3 (or Ho2O3). Investigation of La2O3, ZrO2, and Al2O3 melts by x-ray diffraction and molecular dynamics simulations also show lower-than-crystal cation-oxygen coordination. These measurements suggest a general trend towards lower coordination compared to their crystalline counterparts. It is found that the coordination drop is larger for lower field strength, larger radius cations and is negligible for high field strength (network forming) cations, such as SiO2. These findings have broad implications for predicting the local structure and related physical properties of metal-oxide melts and oxide glasses.
The structure of liquid alumina at a temperature ~2400 K near its melting point was measured using neutron and high-energy x-ray diffraction by employing containerless aerodynamic-levitation and ...laser-heating techniques. The measured diffraction patterns were compared to those calculated from molecular dynamics simulations using a variety of pair potentials, and the model found to be in best agreement with experiments was refined using the reverse Monte Carlo method. The resultant model shows that the melt is composed predominantly of AlO sub(4) and AlO sub(5) units, in the approximate ratio of 2:1, with only minor fractions of AlO sub(3) and AlO sub(6) units. The majority of Al-O-Al connections involve corner-sharing polyhedra (83%), although a significant minority involve edge-sharing polyhedra (16%), predominantly between AlO sub(5) and either AlO sub(5) or AlO sub(4) units. Most of the oxygen atoms (81%) are shared among three or more polyhedra, and the majority of these oxygen atoms are triply shared among one or two AlO sub(4) units and two or one AlO sub(5) units, consistent with the abundance of these polyhedra in the melt and their fairly uniform spatial distribution.
Knowledge of thermal expansion and high temperature phase transformations is essential for prediction and interpretation of materials behavior under the extreme conditions of high temperature and ...intense radiation encountered in nuclear reactors. Structure and thermal expansion of Lu2O3 and Yb2O3 were studied in oxygen and argon atmospheres up to their melting temperatures using synchrotron X-ray diffraction on laser heated levitated samples. Both oxides retained the cubic bixbyite C-type structure in oxygen and argon to melting. In contrast to fluorite-type structures, the increase in the unit cell parameter of Yb2O3 and Lu2O3 with temperature is linear within experimental error from room temperature to the melting point, with mean thermal expansion coefficients (8.5 ± 0.6) · 10−6 K−1 and (7.7 ± 0.6) · 10−6 K−1, respectively. There is no indication of a superionic (Bredig) transition in the C-type structure or of a previously suggested Yb2O3 phase transformation to hexagonal phase prior to melting.
Display omitted
•Lu2O3 and Yb2O3 retain bixbyite-type structure in oxygen and argon to melting.•Thermal expansion is close to linear from room temperature to melting.•There is no indication of Bredig transition or transformation to hexagonal phase.
Borate melts containing <20 mol% Na2O have been studied using high‐energy synchrotron X‐ray diffraction. Temperature dependencies of the mean B–O bond lengths are shown to vary strongly with soda ...content, by comparison to previous measurements on liquid B2O3 and Na2B4O7. Whereas in liquid B2O3 linear thermal expansion of the BØ3 units is observed, with coefficient αBO = 3.7(2) × 10−6 K−1, this expansion is apparently slightly suppressed in melts containing <20 mol% Na2O, and is dramatically reversed at the diborate composition. These effects are interpreted in terms of changes in the mean B–O coordination number, where the reaction BØ4− + BØ3 ⇌ BØ3 + BØ2O− shifts to the right with increasing temperature. The empirical bond‐valence relationship is used to convert measured bond lengths, rBO, to coordination numbers, nBO, including a correction for the expected thermal expansion. This method is more accurate and precise than direct determination of nBO from peak areas in the radial distribution functions. Gradients of ΔnBO/ΔT = −3.4(3) × 10−4 K−1 close to the diborate composition, and ΔnBO/ΔT = −0.3(1) × 10−4 K−1 for a 13(3) mol% Na2O melt are observed, in reasonable agreement with Raman spectroscopic observations and thermodynamic modeling, with some quantitative differences. These observations go toward explaining isothermal viscosity maxima and changes in fragility across the sodium borate system.
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