258 research outputs found
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Mechanistic Study Of Plasma Damage Of Low k Dielectric Surfaces
Plasma damage to low k dielectric materials was investigated from a mechanistic point of view. Low k dielectric films were treated by plasma Ar, O-2, N-2/H-2, N-2 and H-2 in a standard RIE chamber and the damage was characterized by Angle Resolved X-ray Photoelectron Spectroscopy (ARXPS), X-Ray Reflectivity (XRR), Fourier Transform Infrared Spectroscopy (FTIR) and Contact Angle measurements. Both carbon depletion and surface densification were observed on the top surface of damaged low k materials while the bulk remained largely unaffected. Plasma damage was found to be a complicated phenomenon involving both chemical and physical effects, depending on chemical reactivity and the energy and mass of the plasma species. A downstream hybrid plasma source with separate ions and atomic radicals was employed to study their respective roles in the plasma damage process. Ions were found to play a more important role in the plasma damage process. The dielectric constant of low k materials can increase up to 20% due to plasma damage and we attributed this to the removal of the methyl group making the low k surface hydrophilic. Annealing was generally effective in mitigating moisture uptake to restore the k value but the recovery was less complete for higher energy plasmas. Quantum chemistry calculation confirmed that physisorbed water in low k materials induces the largest increase of dipole moments in comparison with changes of surface bonding configurations, and is primarily responsible for the dielectric constant increase.Microelectronics Research Cente
Simulation and analysis of in vitro DNA evolution
We study theoretically the in vitro evolution of a DNA sequence by binding to
a transcription factor. Using a simple model of protein-DNA binding and
available binding constants for the Mnt protein, we perform large-scale,
realistic simulations of evolution starting from a single DNA sequence. We
identify different parameter regimes characterized by distinct evolutionary
behaviors. For each regime we find analytical estimates which agree well with
simulation results. For small population sizes, the DNA evolutional path is a
random walk on a smooth landscape. While for large population sizes, the
evolution dynamics can be well described by a mean-field theory. We also study
how the details of the DNA-protein interaction affect the evolution.Comment: 11 pages, 11 figures. Submitted to PNA
Dime como educas y te diré como es tu ambiente
La crisis ambiental es un problema de conocimiento, por ello involucra a la educación, siendo el desafío de ella el de pasar de una cultura economicista, que refuerza y que es reforzada por la globalización, a una cultura de pertenencia, de compromiso, de resistencia, de solidaridad. Debemos focalizar los esfuerzos por construir desde la complejidad ambiental una ciencia para la sustentabilidad que hunda sus raíces en la justicia ambiental y que destierre las ideas simplificadoras y reduccionistas de la sociedad y comience a fundar el nuevo paradigma basado en la interdisciplinaridad y la transdisciplinariedad. Este proyecto didáctico se elabora para aplicar en dos cursos "paralelos" de diferentes escuelas de la ciudad de Rafaela, provincia de Santa Fe. Se formarán equipos de trabajo que interactuarán en esta propuesta: docentes (biología, historia, derecho, lengua, geografía, economía), directivos y los alumnos, con las adaptaciones a sus problemáticas, su universo y territorialidad, implementando ILS-indicadores locales de sustentabilidad.Trabajos del área Ciencias NaturalesDepartamento de Ciencias Exactas y Naturale
The role of aerosol in altering North Atlantic atmospheric circulation in winter and its impact on air quality
Numerical model scenarios of future climate depict a global increase in temperatures and changing precipitation patterns, primarily driven by increasing greenhouse gas (GHG) concentrations. Aerosol particles also play an important role by altering the Earth's radiation budget and consequently surface temperature. Here, we use the general circulation aerosol model ECHAM5-HAM, coupled to a mixed layer ocean model, to investigate the impacts of future air pollution mitigation strategies in Europe on winter atmospheric circulation over the North Atlantic. We analyse the extreme case of a maximum feasible end-of-pipe reduction of aerosols in the near future (2030), in combination with increasing GHG concentrations. Our results show a more positive North Atlantic Oscillation (NAO) mean state by 2030, together with a significant eastward shift of the southern centre of action of sea-level pressure (SLP). Moreover, we show a significantly increased blocking frequency over the western Mediterranean. By separating the impacts of aerosols and GHGs, our study suggests that future aerosol abatement may be the primary driver of both the eastward shift in the southern SLP centre of action and the increased blocking frequency over the western Mediterranean. These concomitant modifications of the atmospheric circulation over the Euro-Atlantic sector lead to more stagnant weather conditions that favour air pollutant accumulation, especially in the western Mediterranean sector. Changes in atmospheric circulation should therefore be included in future air pollution mitigation assessments. The indicator-based evaluation of atmospheric circulation changes presented in this work will allow an objective first-order assessment of the role of changes in wintertime circulation on future air quality in other climate model simulations
Burst avalanches in solvable models of fibrous materials
We review limiting models for fracture in bundles of fibers, with
statistically distributed thresholds for breakdown of individual fibers. During
the breakdown process, avalanches consisting of simultaneous rupture of several
fibers occur, and the distribution of the magnitude of
such avalanches is the central characteristics in our analysis. For a bundle of
parallel fibers two limiting models of load sharing are studied and contrasted:
the global model in which the load carried by a bursting fiber is equally
distributed among the surviving members, and the local model in which the
nearest surviving neighbors take up the load. For the global model we
investigate in particular the conditions on the threshold distribution which
would lead to anomalous behavior, i.e. deviations from the asymptotics
, known to be the generic behavior. For the local
model no universal power-law asymptotics exists, but we show for a particular
threshold distribution how the avalanche distribution can nevertheless be
explicitly calculated in the large-bundle limit.Comment: 28 pages, RevTeX, 3 Postscript figure
Scaling in the time-dependent failure of a fiber bundle with local load sharing
We study the scaling behaviors of a time-dependent fiber-bundle model with
local load sharing. Upon approaching the complete failure of the bundle, the
breaking rate of fibers diverges according to ,
where is the lifetime of the bundle, and is a quite
universal scaling exponent. The average lifetime of the bundle scales
with the system size as , where depends on the
distribution of individual fiber as well as the breakdown rule.Comment: 5 pages, 4 eps figures; to appear in Phys. Rev.
Gravastar Solutions with Continuous Pressures and Equation of State
We study the gravitational vacuum star (gravastar) configuration as proposed
by other authors in a model where the interior de Sitter spacetime segment is
continuously extended to the exterior Schwarzschild spacetime. The multilayered
structure in previous papers is replaced by a continuous stress-energy tensor
at the price of introducing anisotropy in the (fluid) model of the gravastar.
Either with an ansatz for the equation of state connecting the radial and
tangential pressure or with a calculated equation of state with
non-homogeneous energy/fluid density, solutions are obtained which in all
aspects satisfy the conditions expected for an anisotropic gravastar. Certain
energy conditions have been shown to be obeyed and a polytropic equation of
state has been derived. Stability of the solution with respect to possible
axial perturbation is shown to hold.Comment: 19 pages, 9 figures. Latest version contains new and updated
references along with some clarifying remarks in the stability analysi
Fracture of disordered solids in compression as a critical phenomenon: I. Statistical mechanics formalism
This is the first of a series of three articles that treats fracture
localization as a critical phenomenon. This first article establishes a
statistical mechanics based on ensemble averages when fluctuations through time
play no role in defining the ensemble. Ensembles are obtained by dividing a
huge rock sample into many mesoscopic volumes. Because rocks are a disordered
collection of grains in cohesive contact, we expect that once shear strain is
applied and cracks begin to arrive in the system, the mesoscopic volumes will
have a wide distribution of different crack states. These mesoscopic volumes
are the members of our ensembles. We determine the probability of observing a
mesoscopic volume to be in a given crack state by maximizing Shannon's measure
of the emergent crack disorder subject to constraints coming from the
energy-balance of brittle fracture. The laws of thermodynamics, the partition
function, and the quantification of temperature are obtained for such cracking
systems.Comment: 11 pages, 2 figure
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