5,723 research outputs found
Relationships Between Atomic Diffusion Mechanisms and Ensemble Transport Coefficients in Crystalline Polymorphs
Ionic transport in conventional ionic solids is generally considered to
proceed via independent diffusion events or "hops''. This assumption leads to
well-known Arrhenius expressions for transport coefficients, and is equivalent
to assuming diffusion is a Poisson process. Using molecular dynamics
simulations of the low-temperature B1, B3, and B4 AgI polymorphs, we have
compared rates of ion-hopping with corresponding Poisson distributions to test
the assumption of independent hopping in these common structure-types. In all
cases diffusion is a non-Poisson process, and hopping is strongly correlated in
time. In B1 the diffusion coefficient can be approximated by an Arrhenius
expression, though the physical significance of the parameters differs from
that commonly assumed. In low temperature B3 and B4 diffusion is characterised
by concerted motion of multiple ions in short closed loops. Diffusion
coefficients can not be expressed in a simple Arrhenius form dependent on
single-ion free-energies, and intrinsic diffusion must be considered a
many-body process
Molecular Dynamics Simulation of Coherent Interfaces in Fluorite Heterostructures
The standard model of enhanced ionic conductivities in solid electrolyte
heterostructures follows from a continuum mean-field description of defect
distributions that makes no reference to crystalline structure. To examine
ionic transport and defect distributions while explicitly accounting for
ion-ion correlations and lattice effects, we have performed molecular dynamics
simulations of a model coherent fluorite heterostructure without any extrinsic
defects, with a difference in standard chemical potentials of mobile fluoride
ions between phases induced by an external potential. Increasing the offset in
fluoride ion standard chemical potentials across the internal interfaces
decreases the activation energies for ionic conductivity and diffusion and
strongly enhances fluoride ion mobilities and defect concentrations near the
heterostructure interfaces. Non-charge-neutral "space-charge" regions, however,
extend only a few atomic spacings from the interface, suggesting a continuum
model may be inappropriate. Defect distributions are qualitatively inconsistent
with the predictions of the continuum mean-field model, and indicate strong
lattice-mediated defect-defect interactions. We identify an atomic-scale
"Frenkel polarisation" mechanism for the interfacial enhancement in ionic
mobility, where preferentially oriented associated Frenkel pairs form at the
interface and promote local ion mobility via concerted diffusion processes
A New Proof Rule for Almost-Sure Termination
An important question for a probabilistic program is whether the probability
mass of all its diverging runs is zero, that is that it terminates "almost
surely". Proving that can be hard, and this paper presents a new method for
doing so; it is expressed in a program logic, and so applies directly to source
code. The programs may contain both probabilistic- and demonic choice, and the
probabilistic choices may depend on the current state.
As do other researchers, we use variant functions (a.k.a.
"super-martingales") that are real-valued and probabilistically might decrease
on each loop iteration; but our key innovation is that the amount as well as
the probability of the decrease are parametric.
We prove the soundness of the new rule, indicate where its applicability goes
beyond existing rules, and explain its connection to classical results on
denumerable (non-demonic) Markov chains.Comment: V1 to appear in PoPL18. This version collects some existing text into
new example subsection 5.5 and adds a new example 5.6 and makes further
remarks about uncountable branching. The new example 5.6 relates to work on
lexicographic termination methods, also to appear in PoPL18 [Agrawal et al,
2018
Density Functional Theory screening of gas-treatment strategies for stabilization of high energy-density lithium metal anodes
To explore the potential of molecular gas treatment of freshly cut lithium
foils in non-electrolyte based passivation of high energy-density Li anodes,
density functional theory (DFT) has been used to study the decomposition of
molecular gases on metallic lithium surfaces. By combining DFT geometry
optimization and Molecular Dynamics, the effects of atmospheric (N2, O2, CO2)
and hazardous (F2, SO2) gas decomposition on Li(bcc) (100), (110), and (111)
surfaces on relative surface energies, work functions, and emerging electronic
and elastic properties are investigated. The simulations suggest that exposure
to different molecular gases can be used to induce and control reconstructions
of the metal Li surface and substantial changes (up to over 1 eV) in the work
function of the passivated system. Contrary to the other considered gases,
which form metallic adlayers, SO2 treatment emerges as the most effective in
creating an insulating passivation layer for dosages <= 1 mono-layer. The
substantial Li->adsorbate charge transfer and adlayer relaxation produce marked
elastic stiffening of the interface, with the smallest change shown by
nitrogen-treated adlayers
DEVELOPMENT AND EVALUATION OF ENVELOPE, SPECTRAL AND TIME ENHANCEMENT ALGORITHMS FOR AUDITORY NEUROPATHY
Auditory neuropathy (AN) is a hearing disorder that reduces the ability to detect temporal cues in speech, thus leading to deprived speech perception. Traditional amplification and frequency shifting techniques used in modern hearing aids are not suitable to assist individuals with AN due to the unique symptoms that result from the disorder. This study proposes a method for combining both speech envelope enhancement and time scaling to combine the proven benefits of each algorithm. In addition, spectral enhancement is cascaded with envelope and time enhancement to address the poor frequency discrimination in AN. The proposed speech enhancement strategy was evaluated using an AN simulator with normal hearing listeners under varying degrees of AN severity. The results showed a significant increase in word recognition scores for time scaling and envelope enhancement over envelope enhancement alone. Furthermore, the addition of spectral enhancement resulted in further increase in word recognition at profound AN severity
An Evaluation of Vegetated Roofing Technology: Application at Air Force Plant Four, Building 15
The United States Air Force maintains thousands of facilities around the world. Many of these facilities have asphalt built up roofs or some other less than sustainable roofing system. In an effort to find roofing systems suitable for Air Force facilities that are both economically and environmentally friendly, this thesis investigated vegetated roofing as a possible alternative to conventional roofing systems. While vegetated roofs are a relatively new roofing system, they exhibit performance qualities that seem to be in line with Air Force needs. An investigation into the feasibility of vegetated roofing technology revealed that this roofing system has many positive economic and environmental characteristics that could benefit the United States Air Force and the Department of Defense. The potential use of this technology was researched specifically for application to building 15 at Air Force Plant 4 (AFP4) in Ft. Worth Texas. A combination of case studies, site visits, and a life cycle economic evaluation was used to compare vegetated roofing with conventional asphalt built up roofing that is typically used at AFP4. The research revealed multiple environmental benefits and few disadvantages. The life cycle costs combined with the environmental benefits of vegetated roofing show that this roofing system is indeed a feasible alternative for building 15
Five Things I Learned from Working with the Student-Consultant for Teaching and Learning Program
Analysis of Antarctic Sea Ice Thickness: A Newly Created Database for 2000-2009
Observations of Antarctic sea ice thickness are sporadic in space and time, hindering knowledge of its variability. A proxy based on stage of development data from the National Ice Center (NIC) weekly operational charts is used to create a high-resolution time series of sea ice concentration, thickness and volume for 2000-2009.
Record-length mean thickness and volume of Antarctic sea ice are 66.7 cm and 7.7 x10^3 km^3. The mean growth and decay seasons in the Southern Ocean and in the Ross sector are 210 days and 155 days, but at least at least one week shorter (growth) and longer (decay) in the Amundsen/Bellingshausen sector. Over 90% of the Antarctic continental shelf is covered with sea ice for 3-5 months, and for 2 to 4 months longer periods in the Amundsen/Bellingshausen and Ross sectors.
Yearly mean sea ice area (extent) in the Southern Ocean increased at a rate of 0.71 x 10^6 km^2/decade (0.70 x 10^6 km^2/decade), equivalent to a 7.7 %/decade (6.3 %/decade) rise. A comparable trend of 9.1 %/decade (8.5 %/decade) is estimated in the Ross sector, at 0.21 x 10^6 km2/decade (0.23 x 10^6 km2/decade). The opposite trend is found in the Amundsen/Bellingshausen sector: a -0.15 x 10^6 km^2/decade (-0.17 x 10^6 km^2/decade) decline, or -14.6 %/decade (-13.4 %/decade).
The estimated annual increase of Antarctic sea ice thickness is 22.6 cm/decade (49.2 %/decade) and of volume is 3.78 x 10^3 km^3/decade (68.3 %/decade). The Ross sector showed similar trends for thickness, at 23.8 cm/decade (47.0 %/decade), and volume, at 1.11 x 10^3 km^3/decade (75.8 %/decade). Thickness has increased in the Amundsen/Bellingshausen sector, 20.7 cm/decade (44.8 %/decade), but with a less pronounced volume rise of 0.17 x10^3 km^3/decade (26.0 %/decade).
Monthly sea ice thickness anomalies show a weak response to the El Nino Southern Oscillation (ENSO) index. A strong positive response is observed in 2008 when a negative a negative ENSO index compounded to a positive Southern Annular Mode (SAM) index. Therefore the estimated increase of sea ice thickness in the Southern Ocean could be attributed to the prevailing atmospheric conditions with a positive SAM phase over the past decade
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