193 research outputs found
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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Lorenz Number and Electronic Thermoelectric Figure of Merit: Thermodynamics and Direct DFT Calculations
The Lorenz number (L) contained in the Wiedemann-Franz law represents the
ratio of two kinetic parameters of electronic charge carriers: the electronic
contribution to the thermal conductivity (K_el) and the electrical conductivity
(sigma), , and can be expressed as LT=K_el/sigma where T is temperature. We
demonstrate that the Lorenz number simply equals to the ratio of two
thermodynamic quantities: the electronic heat capacity (c_el) and the
electrochemical capacitance (c_N) through LT=c_el/c_N , a purely thermodynamic
quantity, and thus it can be calculated solely based on the electron density of
states of a material. It is shown that our thermodynamic formulation for the
Lorenz number leads to: i) the well-known Sommerfeld value L=pi^2/3(k_B/e)^2 at
the low temperature limit, ii) the Drude value L=3/2(k_B/e)^2 at the high
temperature limit with the free electron gas model, and iii) possible higher
values than the Sommerfeld limit for semiconductors. It is also demonstrated
that the purely electronic contribution to the thermoelectric figure-of-merit
can be directly computed using high-throughput DFT calculations without
resorting to the computationally more expensive Boltzmann transport theory to
the electronic thermal conductivity and electrical conductivity
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Suitability of binary oxides for molecular-beam epitaxy source materials: A comprehensive thermodynamic analysis
We have conducted a comprehensive thermodynamic analysis of the volatility of 128 binary oxides to evaluate their suitability as source materials for oxide molecular-beam epitaxy (MBE). 16 solid or liquid oxides are identified that evaporate nearly congruently from stable oxide sources to gas species: As2O3, B2O3, BaO, MoO3, OsO4, P2O5, PbO, PuO2, Rb2O, Re2O7, Sb2O3, SeO2, SnO, ThO2, Tl2O, and WO3. An additional 24 oxides could provide molecular beams with dominant gas species of CeO, Cs2O, DyO, ErO, Ga2O, GdO, GeO, HfO, HoO, In2O, LaO, LuO, NdO, PmO, PrO, PuO, ScO, SiO, SmO, TbO, Te2O2, U2O6, VO2, and YO2. The present findings are in close accord with available experimental results in the literature. For example, As2O3, B2O3, BaO, MoO3, PbO, Sb2O3, and WO3 are the only oxides in the ideal category that have been used in MBE. The remaining oxides deemed ideal for MBE awaiting experimental verification. We also consider two-phase mixtures as a route to achieve the desired congruent evaporation characteristic of an ideal MBE source. These include (Ga2O3 + Ga) to produce a molecular beam of Ga2O(g), (GeO2 + Ge) to produce GeO(g), (SiO2 + Si) to produce SiO(g), (SnO2 + Sn) to produce SnO(g), etc.; these suboxide sources enable suboxide MBE. Our analysis provides the vapor pressures of the gas species over the condensed phases of 128 binary oxides, which may be either solid or liquid depending on the melting temperature. © 2020 Author(s)
DFTTK: Density Functional Theory Tool Kit for High-throughput Calculations of Thermodynamic Properties at Finite Temperatures
In this work, we present a software package in Python for high-throughput
first-principles calculations of thermodynamic properties at finite
temperatures, which we refer to as DFTTK (Density Functional Theory Tool Kit).
DFTTK is based on the atomate package and integrates our experiences in the
last decades on the development of theoretical methods and computational
software. It includes task submissions on all major operating systems and task
execution on high-performance computing environments. The distribution of the
DFTTK package comes with examples of calculations of phonon density of states,
heat capacity, entropy, enthalpy, and free energy under the quasi-harmonic
phonon scheme for the stoichiometric phases of Al, Ni, Al3Ni, AlNi, AlNi3,
Al3Ni4, and Al3Ni5, and the fcc solution phases treated using the special
quasirandom structures at the compositions of Al3Ni, AlNi, and AlNi3.Comment: 49 pages, 18 figure
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