200 research outputs found
Thermodynamics of mono and di-vacancies in barium titanate
The thermodynamic and kinetic properties of mono and di-vacancy defects in
cubic (para-electric) barium titanate are studied by means of
density-functional theory calculations. It is determined which vacancy types
prevail for given thermodynamic boundary conditions. The calculations confirm
the established picture that vacancies occur in their nominal charge states
almost over the entire band gap. For the dominating range of the band gap the
di-vacancy binding energies are constant and negative. The system, therefore,
strives to achieve a state in which under metal-rich (oxygen-rich) conditions
all metal (oxygen) vacancies are bound in di-vacancy clusters. The migration
barriers are calculated for mono-vacancies in different charge states. Since
oxygen vacancies are found to readily migrate at typical growth temperatures,
di-vacancies can be formed at ease. The key results of the present study with
respect to the thermodynamic behavior of mono and di-vacancies influence the
initial defect distribution in the ferroelectric phases and therefore the
conditions for aging.Comment: 9 pages, 4 figures, 4 table
Influence of elastic strain on the thermodynamics and kinetics of lithium vacancy in bulk LiCoO2
The influence of elastic strain on the lithium vacancy formation and
migration in bulk LiCoO2 is evaluated by means of first-principles calculations
within density functional theory (DFT). Strain dependent energies are
determined directly from defective cells and also within linear elasticity
theory from the elastic dipole tensor (Gij) for ground state and saddle point
configurations. We analyze finite size-effects in the calculation of Gij,
compare the predictions of the linear elastic model with those obtained from
direct calculations of defective cells under strain and discuss the
differences. Based on our data, we calculate the variations in vacancy
concentration and mobility due to the presence of external strain in bulk
LiCoO2 cathodes. Our results reveal that elastic in-plane and out-of-plane
strains can significantly change the ionic conductivity of bulk LiCoO2 by an
order of magnitude and thus strongly affect the performance of Li-secondary
batteries
Influence of Br/S site-exchange on Li diffusion mechanism in LiPSBr -- a computational study
We investigate the influence of Br/S site-exchange on lithium
diffusion in the agyrodite-type solid electrolyte LiPSBr by ab-initio
molecular dynamics simulations. Based on the calculated trajectories a new
mechanism for the internal lithium reorganization within the Li-cages around
the sites is identified. This reorganization mechanism is highly concerted
and cannot be described by one single rotation axis only. Simulations with
Br/S defects reveal that Li interstitials are the dominant
mobile charge carriers, which originate from Frenkel pairs. These are formed
because Br defects on the sites cause the transfer of one or
even two Li to the neighboring 12 cages. The lithium interstitials then
carry out intercage jumps via interstitial and interstitialcy mechanisms. With
that, one single Br defect enables Li diffusion over an extended
spatial area explaining why low degrees of site-exchange are sufficient to
trigger superionic conduction. The vacant sites of the Frenkel pairs, namely
V, are mostly immobile and bound to the Br defect. To
a lesser degree also S defects induce disturbances in the lithium
distribution and act as sinks for lithium interstitials restricting the
Li motion to the vicinity of the S defect
Influence of Crystalline Nanoprecipitates on Shear-Band Propagation in Cu-Zr Based Metallic Glasses
The interaction of shear bands with crystalline nanoprecipitates in
Cu-Zr-based metallic glasses is investigated by a combination of
high-resolution TEM imaging and molecular-dynamics computer simulations. Our
results reveal different interaction mechanisms: Shear bands can dissolve
precipitates, can wrap around crystalline obstacles, or can be blocked
depending on size and density of the precipitates. If the crystalline phase has
a low yield strength, we also observe slip transfer through the precipitate.
Based on the computational results and experimental findings, a qualitative
mechanism map is proposed that categorizes the various processes as a function
of the critical stress for dislocation nucleation, precipitate size, and
distance.Comment: 16 pages, 15 figure
Computersimulationen zu Struktur und Wachstum von Bornitrid
Dünne Filme aus kubischem Bornitrid (c-BN) sind aufgrund ihrer herausragenden thermomechanischen, chemischen und elektronischen Eigenschaften von besonderem Interesse für die Materialforschung. Bornitrid in der kubischen Phase ist das nach Diamant härteste bekannte Material. Anders als Kohlenstoff verhält es sich gegenüber ferrithaltigen Metallen chemisch inert und zeigt auch bei hohen Temperaturen eine vergleichsweise geringere Oxidationsneigung. Wegen seiner hohen thermischen Stabilität und der Möglichkeit, dünne Schichten bei niedrigen Temperaturen zu synthetisieren, ist Bornitrid als Hard-Coating-Material für Werkzeuge besonders geeignet. Mögliche Anwendungen für mikroelelektronische Hochleistungsbauelemente ergeben sich aus der hohen Wärmeleitfähigkeit und der weiten Bandlücke (Eg ~ 6 eV). Bornitrid kann mit Beryllium und Silizium n- bzw. p-Typ dotiert werden und läßt sich mit einer Oxidschicht passivieren. Die optische Transparenz im sichtbaren und Infrarotbereich macht das Material zudem für die Oberflächenvergütung optischer Bauelemente geeignet.
Interface-controlled creep in metallic glass composites
In this work we present molecular dynamics simulations on the creep behavior
of metallic glass composites. Surprisingly, all composites
exhibit much higher creep rates than the homogeneous glass. The glass-crystal
interface can be viewed as a weak interphase, where the activation barrier of
shear transformation zones is lower than in the surrounding glass. We observe
that the creep behavior of the composites does not only depend on the interface
area but also on the orientation of the interface with respect to the loading
axis. We propose an explanation in terms of different mean Schmid factors of
the interfaces, with the amorphous interface regions acting as preferential
slip sites.Comment: 11 pages, 13 figure
Low temperature heat capacity of severely deformed metallic glass
The low temperature heat capacity of amorphous materials reveals a
low-frequency enhancement (boson peak) of the vibrational density of states, as
compared with the Debye law. By measuring the low-temperature heat capacity of
a Zr-based bulk metallic glass relative to a crystalline reference state, we
show that the heat capacity of the glass is strongly enhanced after severe
plastic deformation by high-pressure torsion, while subsequent thermal
annealing at elevated temperatures leads to a significant reduction. The
detailed analysis of corresponding molecular dynamics simulations of an
amorphous Zr-Cu glass shows that the change in heat capacity is primarily due
to enhanced low-frequency modes within the shear band region.Comment: 5 pages, 2 figure
Determination of optimal reversed field with maximal electrocaloric cooling by a direct entropy analysis
Application of a negative field on a positively poled ferroelectric sample
can enhance the electrocaloric cooling and appears as a promising method to
optimize the electrocaloric cycle. Experimental measurements show that the
maximal cooling does not appear at the zero-polarization point, but around the
shoulder of the P-E loop. This phenomenon cannot be explained by the theory
based on the constant total entropy assumption under adiabatic condition. In
fact, adiabatic condition does not imply constant total entropy when
irreversibility is involved. A direct entropy analysis approach based on work
loss is proposed in this work, which takes the entropy contribution of the
irreversible process into account. The optimal reversed field determined by
this approach agrees with the experimental observations. This study signifies
the importance of considering the irreversible process in the electrocaloric
cycles
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