1,383 research outputs found
From metallic glasses to nanocrystals: Molecular dynamics simulations on the crossover from glass-like to grain-boundary-mediated deformation behaviour
Nanocrystalline metals contain a large fraction of high-energy grain
boundaries, which may be considered as glassy phases. Consequently, with
decreasing grain size, a crossover in the deformation behaviour of nanocrystals
to that of metallic glasses has been proposed. Here, we study this crossover
using molecular dynamics simulations on bulk glasses, glass-crystal
nanocomposites, and nanocrystals of Cu64Zr36 with varying crystalline volume
fractions induced by long-time thermal annealing. We find that the grain
boundary phase behaves like a metallic glass under constraint from the abutting
crystallites. The transition from glass-like to grain-boundary-mediated
plasticity can be classified into three regimes: (1) For low crystalline volume
fractions, the system resembles a glass-crystal composite and plastic flow is
localised in the amorphous phase; (2) with increasing crystalline volume
fraction, clusters of crystallites become jammed and the mechanical response
depends critically on the relaxation state of the glassy grain boundaries; (3)
at grain sizes 10 nm, the system is jammed completely, prohibiting pure
grain-boundary plasticity and instead leading to co-deformation. We observe an
inverse Hall-Petch effect only in the second regime when the grain boundary is
not deeply relaxed. Experimental results with different grain boundary states
are therefore not directly comparable in this regime.Comment: 19 pages, 17 figure
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
Formation and switching of defect dipoles in acceptor doped lead titanate: A kinetic model based on first-principles calculations
The formation and field-induced switching of defect dipoles in acceptor doped
lead titanate is described by a kinetic model representing an extension of the
well established Arlt-Neumann model [Ferroelectrics {\bf 76}, 303 (1987)].
Energy barriers for defect association and reorientation of oxygen
vacancy-dopant (Cu and Fe) complexes are obtained from first-principles
calculations and serve as input data for the kinetic coefficients in the rate
equation model. The numerical solution of the model describes the time
evolution of the oxygen vacancy distribution at different temperatures and
dopant concentrations in the presence or absence of an alternating external
field. We predict the characteristic time scale for the alignment of all defect
dipoles with the spontanenous polarization of the surrounding matrix. In this
state the defect dipoles act as obstacles for domain wall motion and contribute
to the experimentally observed aging. Under cycling conditions the fully
aligned configuration is perturbed and a dynamic equilibrium is established
with defect dipoles in parallel and anti-parallel orientation relative to the
spontaneous polarization. This process can be related to the deaging behavior
of piezoelectric ceramics.Comment: 10 pages, 7 figure
Solid-state amorphization of Cu nanolayers embedded in a Cu64Zr36 glass
Solid-state amorphization of crystalline copper nanolayers embedded in a
Cu64Zr36 metallic glass is studied by molecular dynamics simulations for
different orientations of the crystalline layer. We show that solid-state
amorphization is driven by a reduction of interface energy, which compensates
the bulk excess energy of the amorphous nanolayer with respect to the
crystalline phase up to a critical layer thickness. A simple thermodynamic
model is derived, which describes the simulation results in terms of
orientation-dependent interface energies. Detailed analysis reveals the
structure of the amorphous nanolayer and allows a comparison to a quenched
copper melt, providing further insights into the origin of excess and interface
energy.Comment: 16 pages, 18 figure
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 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
Viral Encephalitis: phenotyping leukocyte infiltration into the central nervous system as a result of Rift Valley Fever Virus infection
Rift Valley Fever Virus (RVFV) is a vector-borne infection endemic to the Horne of Africa; However, outbreaks in the Arabian Peninsula in 2001 demonstrate its expanding range. Humans can develop encephalitis as a result of RVFV infection. Our lab uses an immunocompetent Lewis rat model to study neuropathogenesis of RVFV. With this model, Lewis rats uniformly develop lethal encephalitis within 7 – 8 days after aerosol exposure. In contrast, Lewis rats infected subcutaneously do not develop apparent disease except at very high doses. The public health significance of RVFV is that it has the potential to natural spread throughout the world and it could be used as a potential bioweapon. The goal of this study is to characterize the phenotypes, timing, and extent of immune cell infiltrate into the CNS of RVFV-infected Lewis rats. Lewis rats were infected with RVFV ZH501 by aerosol or subcutaneous routes, and subsequently serially-sacrificed between 1 and 7 days post infection. Rat immune cells were isolated from brains via percoll gradients, stained with the appropriate antibodies, run on a BD LSRII flow cytometer, and analyzed with FlowJo 7.6.5.
In aerosol-infected rats, the leukocyte infiltrate 5 days post infection and later consisted of primarily neutrophils, corresponding to the clinical window. Despite no apparent disease, rats infected subcutaneously had detectable infiltrate primarily consisting of phenotypic CD4+ T cells. These findings suggest that aerosol-infected rats mainly exhibit innate leukocyte infiltration to the brain while subcutaneous have limited leukocyte infiltration and display a stronger adaptive immune response. The relative contributions of viral cytopathology versus immunopathology remain to be determined, but this study represent the first attempt to characterize the leukocytic component of RVFV neurological disease
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
Ultrastructural analysis of platelets and fibrin networks in stroke patients
Ischaemic stroke represent more than 80% of the total stroke instances. The location of the occlusion and the amount of brain tissue involved determines the effect of the stroke. Stroke can result in paralysis, memory loss, speech impairment and even a “lock-in” state. The amount of neuronal damage will determine whether these symptoms will be temporary or permanent. Stroke is deemed the second leading cause of death for individuals over the age of 60. According to the World Stroke Organization (WSO) every six seconds stroke claims a life, regardless of age or gender. Stroke is a global burden and the medical costs and disability related to stroke in America for 2010 was projected at almost $73.7 billion. The morphology of platelets, fibrin networks and erythrocytes as well as the differential white blood cell counts of 20 thrombo-embolic ischaemic stroke patients were investigated. Internal and external alterations were revealed in the platelets of stroke patients when compared to healthy controls. The decreased numbers of alpha granules in the platelets of the stroke patients indicated these platelets to be activated. Substances released by activated platelets promote fibrin network structure, specifically the formation of fibrin strands and accumulation of additional platelets. The fibrin network of healthy individuals consists of major, thick fibers with minor, thin fibers distributed between them. The fibrin network of stroke patients exhibited an abnormally layered and matted ultrastructure comprising of mainly thin, minor fibrin fibers packed closely together. An uncharacteristic circular morphology was also observed. These alterations in the fibrin network indicate the activated platelets to be actively involved in the thrombotic event. Neuronal damage related to stroke is also advanced by the vasoactive substances released by activated platelets. It can therefore be deduced that the morphology of the fibrin network is altered long before the concrete thrombotic event transpire. Large numbers of abnormal erythrocytes were distinguished in the blood of stroke patients. Among these abnormal forms of erythrocytes specifically codocytes, knizocytes, stomatocytes and echinocytes were identified. Abnormal erythrocyte forms were significantly increased in hypertensive patients and females independently. Alterations in the ultrastructure of erythrocytes disturb blood flow in the microcirculation and could possibly augment the ischaemic event. Inflammation is closely related to ischaemic stroke. An increased monocyte count and a reduced number of neutrophils were a significant feature among all the stroke patients of this study. Patients with hypertension as well as patients consuming aspirin on a daily basis showed the greatest influence on the observed differential white blood cell counts. These morphological alterations observed in the platelets, fibrin network and erythrocytes as well as the differential white blood cell count could be incorporated in an analysis regime that could probably indicate an impending thrombotic event. Therefore treatment could be initiated before the ischaemic event to possibly prevent the stroke. For future studies a larger study population, a more refined patient enrolment as well as the analysis of follow-up blood samples from patients could substantiate the above-mentioned findings and provide additional information concerning the thrombotic event and the effectiveness of treatment procedures.Dissertation (MSc)--University of Pretoria, 2010.AnatomyUnrestricte
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