247 research outputs found
Urban mobility demand profiles: Time series for cars and bike-sharing use as a resource for transport and energy modeling
The transport sector is currently facing a significant transition, with strong drivers including decarbonization and digitalization trends, especially in urban passenger transport. The availability of monitoring data is at the basis of the development of optimization models supporting an enhanced urban mobility, with multiple benefits including lower pollutants and CO2 emissions, lower energy consumption, better transport management and land space use. This paper presents two datasets that represent time series with a high temporal resolution (five-minute time step) both for vehicles and bike sharing use in the city of Turin, located in Northern Italy. These high-resolution profiles have been obtained by the collection and elaboration of available online resources providing live information on traffic monitoring and bike sharing docking stations. The data are provided for the entire year 2018, and they represent an interesting basis for the evaluation of seasonal and daily variability patterns in urban mobility. These data may be used for different applications, ranging from the chronological distribution of mobility demand, to the estimation of passenger transport flows for the development of transport models in urban contexts. Moreover, traffic profiles are at the basis for the modeling of electric vehicles charging strategies and their interaction with the power grid
Suppression of Giant Magnetoresistance by a superconducting contact
We predict that current perpendicular to the plane (CPP) giant
magnetoresistance (GMR) in a phase-coherent magnetic multilayer is suppressed
when one of the contacts is superconducting. This is a consequence of a
superconductivity-induced magneto-resistive (SMR) effect, whereby the
conductance of the ferromagnetically aligned state is drastically reduced by
superconductivity. To demonstrate this effect, we compute the GMR ratio of
clean (Cu/Co)_nCu and (Cu/Co)_nPb multilayers, described by an ab-initio spd
tight binding Hamiltonian. By analyzing a simpler model with two orbitals per
site, we also show that the suppression survives in the presence of elastic
scattering by impurities.Comment: 5 pages, 4 figures. Submitted to PR
When are battery electric vehicles economically convenient? A sensitivity analysis based on multi-carrier residential energy system renovation modelling
Processing of UHTCMCs
There is an increasing demand for advanced materials with temperature capability in highly corrosive environments for aerospace. Rocket nozzles of solid/hybrid rocket motors must survive harsh thermochemical and mechanical environments produced by high performance solid propellants (2700-3500°C). Thermal protection systems (TPS) for space vehicles flying at Mach 7 must withstand projected service temperatures up to 2500°C associated to convective heat fluxes up to 15 MWm-2 and intense mechanical vibrations at launch and re-entry into Earth’s atmosphere. The combination of extremely hot temperatures, chemically aggressive environments and rapid heating/cooling is beyond the capabilities of current materials.
As indicated by the previous talk, the main purpose of C3HARME is to design, develop, manufacture, test and validate a new class of out-performing, reliable, cost-effective and scalable Ultra High Temperature Ceramic Matrix Composites (UHTCMCs) based on C fibre preforms enriched with ultra-high temperature ceramics (UHTCs) and capable of in-situ repairing damage induced during operation in severe aerospace environments. Two main applications are envisaged: near-ZERO erosion rocket nozzles that must maintain dimensional stability during firing in combustion chambers, and near-ZERO ablation thermal protection systems enabling hypersonic space vehicles to maintain flight performance.
This talk aims at providing an indication of progress to date within Work Package 2, which is focused on the processing of Cf-ZrB2 UHTCMCs. Four primary routes are being investigated, these include: green forming of fibre reinforced UHT ceramics followed by spark plasma sintering; radio-frequency enhanced chemical vapour infiltration of UHTCMCs; reactive melt infiltration of UHTCMCs and polymer infiltration and pyrolysis of UHTCMCs. All four approaches will be outlined and conclusions drawn, plus there will be a brief mention of ongoing work into atomistic modelling of processes at materials interfaces and nanoparticle dispersion with a view to imparting self-healing properties.
Acknowledgements: This work has received funding from the European Union’s Horizon 2020 “Research and innovation programme” under grant agreement N°685594 (C3HARME
AFLOW-CHULL: Cloud-Oriented Platform for Autonomous Phase Stability Analysis
A priori prediction of phase stability of materials is a challenging practice, requiring knowledge of all energetically-competing structures at formation conditions. Large materials repositories - housing properties of both experimental and hypothetical compounds - offer a path to prediction through the construction of informatics-based, ab-initio phase diagrams. However, limited access to relevant data and software infrastructure has rendered thermodynamic characterizations largely peripheral, despite their continued success in dictating synthesizability. Herein, a new module is presented for autonomous thermodynamic stability analysis implemented within the open-source, ab-initio framework AFLOW. Powered by the AFLUX Search-API, AFLOW-CHULL leverages data of more than 1.8 million compounds currently characterized in the AFLOW.org repository and can be employed locally from any UNIX-like computer. The module integrates a range of functionality: the identification of stable phases and equivalent structures, phase coexistence, measures for robust stability, and determination of decomposition reactions. As a proof-of-concept, thorough thermodynamic characterizations have been performed for more than 1,300 binary and ternary systems, enabling the identification of several candidate phases for synthesis based on their relative stability criterion - including 18 promising C15b-type structures and two half-Heuslers. In addition to a full report included herein, an interactive, online web application has been developed showcasing the results of the analysis, and is located at aflow.org/aflow-chull
Ferromagnetism in Mn doped GaAs due to substitutional-interstitial complexes
While most calculations on the properties of the ferromagnetic semiconductor
GaAs:Mn have focussed on isolated Mn substituting the Ga site (Mn), we
investigate here whether alternate lattice sites are favored and what the
magnetic consequences of this might be. Under As-rich (Ga-poor) conditions
prevalent at growth, we find that the formation energies are lower for
Mn over interstitial Mn (Mn).As the Fermi energy is shifted towards
the valence band maximum via external -doping, the formation energy of
Mn is reduced relative to Mn. Furthermore, under epitaxial growth
conditions, the solubility of both substitutional and interstitial Mn are
strongly enhanced over what is possible under bulk growth conditions. The high
concentration of Mn attained under epitaxial growth of p-type material opens
the possibility of Mn atoms forming small clusters. We consider various types
of clusters, including the Coulomb-stabilized clusters involving two Mn
and one Mn. While isolated Mn are hole killers (donors), and therefore
destroy ferromagnetism,complexes such as Mn-Mn-Mn) are found
to be more stable than complexes involving Mn-Mn-Mn. The
former complexes exhibit partial or total quenching of holes, yet Mn in
these complexes provide a channel for a ferromagnetic arrangement of the spins
on the two Mn within the complex. This suggests that ferromagnetism in
Mn doped GaAs arises both from holes due to isolated Mn as well as from
strongly Coulomb stabilized Mn-Mn-Mn clusters.Comment: 7 figure
First principles study of the origin and nature of ferromagnetism in (Ga,Mn)As
The properties of diluted GaMnAs are calculated for a wide range
of Mn concentrations within the local spin density approximation of density
functional theory. M\"ulliken population analyses and orbital-resolved
densities of states show that the configuration of Mn in GaAs is compatible
with either 3d or 3d, however the occupation is not integer due to the
large - hybridization between the Mn states and the valence band of
GaAs. The spin splitting of the conduction band of GaAs has a mean field-like
linear variation with the Mn concentration and indicates ferromagnetic coupling
with the Mn ions. In contrast the valence band is antiferromagnetically coupled
with the Mn impurities and the spin splitting is not linearly dependent on the
Mn concentration. This suggests that the mean field approximation breaks down
in the case of Mn-doped GaAs and corrections due to multiple scattering must be
considered. We calculate these corrections within a simple free electron model
and find good agreement with our {\it ab initio} results if a large exchange
constant (eV) is assumed.Comment: 15 pages, 14 figure
Annealing-Dependent Magnetic Depth Profile in Ga[1-x]Mn[x]As
We have studied the depth-dependent magnetic and structural properties of
as-grown and optimally annealed Ga[1-x]Mn[x]As films using polarized neutron
reflectometry. In addition to increasing total magnetization, the annealing
process was observed to produce a significantly more homogeneous distribution
of the magnetization. This difference in the films is attributed to the
redistribution of Mn at interstitial sites during the annealing process. Also,
we have seen evidence of significant magnetization depletion at the surface of
both as-grown and annealed films.Comment: 5 pages, 3 figure
Backward diode composed of a metallic and semiconducting nanotube
The conditions necessary for a nanotube junction connecting a metallic and
semiconducting nanotube to rectify the current are theoretically investigated.
A tight binding model is used for the analysis, which includes the Hartree-Fock
approximation and the Green's function method.
It is found that the junction has a behavior similar to the backward diode if
the gate electrode is located nearby and the Fermi level of the semiconducting
tube is near the gap.
Such a junction would be advantageous since the required length for the
rectification could be reduced.Comment: 4 pages, RevTeX, uses epsf.st
Tunneling through molecules and quantum dots: master-equation approaches
An important class of approaches to the description of electronic transport
through molecules and quantum dots is based on the master equation. We discuss
various formalisms for deriving a master equation and their interrelations. It
is shown that the master equations derived by Wangsness, Bloch, and Redfield
and by Koenig et al. are equivalent. The roles of the large-reservoir and
Markov approximations are clarified. The Markov approximation is traced back to
nonzero bias voltage and temperature, whereas interactions and the
corresponding rapid relaxation in the leads are shown to be irrelevant for the
transport under certain conditions. It is explained why the T-matrix formalism
gives incomplete results except for diagonal density operators and to second
order in the tunneling amplitudes. The time-convolutionless master equation is
adapted to tunneling problems and a diagrammatic scheme for generating
arbitrary orders in the tunneling amplitudes is developed.Comment: 13 pages including 5 figure
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