144 research outputs found
Thermodynamics of the Mg-B system: Implications for the deposition of MgB2 thin films
We have studied thermodynamics of the Mg-B system with the modeling technique
CALPHAD using a computerized optimization procedure. Temperature-composition,
pressure-composition, and pressure-temperature phase diagrams under different
conditions are obtained. The results provide helpful insights into appropriate
processing conditions for thin films of the superconducting phase, MgB2,
including the identification of the pressure/temperature region for
adsorption-controlled growth. Due to the high volatility of Mg, MgB2 is
thermodynamically stable only under fairly high Mg overpressures for likely
growth temperatures. This constraint places severe temperature constraints on
deposition techniques employing high vacuum conditions
Zentropy Theory for Positive and Negative Thermal Expansions
It has been observed in both natural and man-made materials that volume
sometimes decreases with increasing temperature. Though mechanistic
understanding has been gained for some individual materials, a general answer
to the question "Why does volume sometimes decrease with the increase of
temperature?" remains lacking. Based on the thermodynamic relation that the
derivative of volume with respect to temperature, i.e., thermal expansion, is
equal to the negative derivative of entropy with respect to pressure, we
developed a general theory in terms of multiscale entropy to understand and
predict the change of volume as a function of temperature, which is termed as
zentropy theory in the present work. It is shown that a phase at high
temperatures is a statistical representation of the ground-state stable and
multiple nonground-state metastable configurations. It is demonstrated that
when the volumes of the major nonground-state configurations are smaller than
that of the ground-state configuration, the volume of the phase may decrease
with the increase of temperature in certain ranges of temperature-pressure
combinations, depicting the negative divergency of thermal expansion at the
critical point. As examples, positive and negative divergencies of thermal
expansion are predicted at the critical points of Ce and Fe3Pt, respectively,
along with the temperature and pressure ranges for abnormally positive and
negative thermal expansion. The authors believe that the zentropy theory is
applicable to predict anomalies of other physical properties of phases because
the change of entropy drives the responses of a system to external stimuli
Parameter-free prediction of phase transition in PbTiO3 through combination of quantum mechanics and statistical mechanics
Thermodynamics of ferroelectric materials and their ferroelectric to
paraelectric (FE-PE) transitions including those in PbTiO3 is commonly
described by the phenomenological Landau theory and more recently by effective
Hamiltonian and various potentials, all with model parameters fitted to
experimental or theoretical data. Here we show that the zentropy theory, which
considers the total entropy of a system as a weighted sum of entropies of
configurations that the system may experience and the statistical entropy among
the configurations, can predict the FE-PE transition without fitting
parameters. For PbTiO3, the configurations are identified as the FE
configurations with 90- or 180-degree domain walls in addition to the ground
state of the FE configuration without domain wall. With the domain wall
energies predicted from first-principles calculations based on the density
functional theory in the literature as the only inputs, the FE-PE transition
for PbTiO3 is predicted showing remarkable agreement with experiments,
unveiling the microscopic fundamentals of the transition
Predicting hydrogen embrittlement of stainless steels using physics-based machine learning
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First-principles Investigation of Thermodynamic Properties of CrNbO4 and CrTaO4
In the present study, the DFT+U method was employed to predict the
thermodynamic properties of Cr2O3, Nb2O5, and Ta2O5. Results were benchmarked
with experimental data showing high accuracy, except for the negative thermal
expansion (NTE) of Nb2O5, which is attributed to its polymorphic complexity.
Additionally, we extended our analysis to rutile-type oxides CrNbO4 and CrTaO4,
examining their entropy and heat capacity at finite temperatures. CrNbO4
displayed slightly higher entropy and heat capacity at high temperatures. The
mean linear thermal expansion coefficients for CrNbO4 and CrTaO4 from 500 K to
2000 K were predicted to be 6.00*10-6/K and 13.49*10-6/K, respectively,
corroborating with DFT predictions and experimental evidence. Our research
highlights the precision of the DFT+U and phonon methods in predicting the
thermodynamic properties of oxide materials, offering insights into the design
of corrosion-resistant materials
Treatment of nonunions of humeral fractures with interlocking intramedullary nailing
ObjectiveTo introduce the experience of treating nonunions of humeral fractures with interlocking intramedullarynailing.MethodsTwelve patients with humeral nonunions were treated with interlocking intramedullary nailing. The time interval between trauma and surgery was 10.5 months on average. Open reduction with anterograde approach was performed. Axial compression was specially applied to the fracture site with humeral nail holder after insertion of distal locked screws. Iliac bone grafting was added.ResultsThe average follow-up period was 21 months (ranging 9-51 months). All patients achieved osseous union 5.8 months after treatment on average. Eleven patients had good functions of the shoulder joints and the upper extremities. No patient experienced any permanent neurological deficit. Refracture of the original ununited region occurred in one patient after removal of the internal fixator one year later, but union was achieved after closed re-intramedullarynailing fixation.ConclusionHumeral interlocking intramedullarynailing is an effective alternative treatment for humeral nonunion
Thermodynamic modeling of the Pd-Zn system with uncertainty quantification and its implication to tailor catalysts
Pd-Zn intermetallic catalysts show encouraging combinations of activity and
selectivity on well-defined active site ensembles. Thermodynamic description of
the Pd-Zn system, delineating phase boundaries, and enumerating site
occupancies within intermediate alloy phases, are essential to determining the
ensembles of Pd-Zn atoms as a function of composition and temperature.
Combining the present extensive first-principles calculations based on density
functional theory (DFT) and available experimental data, the Pd-Zn system was
remodeled using the CALculation of PHAse Diagrams (CALPHAD) approach. High
throughput modeling tools with uncertainty quantification, i.e., ESPEI and
PyCalphad, were incorporated in the phase analysis. The site occupancies across
the {\gamma}-phase composition region were given special attention. A
four-sublattice model was used for the {\gamma}-phase owing to its four Wyckoff
positions, i.e., the outer tetrahedral (OT) site 8c, the inner tetrahedral (IT)
site 8c, the octahedral (OH) 12e, and the cuboctahedral (CO) site 24g. The site
fractions of Pd and Zn calculated from the present thermodynamic model show the
occupancy preference of Pd in the OT and OH sublattices in agreement with
experimental observations. The force constants obtained from DFT-based phonon
calculations further supports the tendency of Pd occupying the OH sublattice
compared with the IT and CO sublattice. The catalytic assembles changing from
Pd monomers (Pd1) to trimers (Pd3) on the surface of the {\gamma}-phase is
attributed to the increase of Pd occupancy in the OH sublattice.Comment: 12 figure
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