77 research outputs found
Elastic effects on relaxation volume tensor calculations
Relaxation volume tensors quantify the effect of stress on diffusion of
crystal defects. Continuum linear elasticity predicts that calculations of
these parameters using periodic boundary conditions do not suffer from
systematic deviations due to elastic image effects and should be independent of
supercell size or symmetry. In practice, however, calculations of formation
volume tensors of the interstitial in Stillinger-Weber silicon
demonstrate that changes in bonding at the defect affect the elastic moduli and
result in system-size dependent relaxation volumes. These vary with the inverse
of the system size. Knowing the rate of convergence permits accurate estimates
of these quantities from modestly sized calculations. Furthermore, within the
continuum linear elasticity assumptions the average stress can be used to
estimate the relaxation volume tensor from constant volume calculations.Comment: 31 pages, 6 figures, submitted to Phys. Rev.
The "MINOTOR" H2020 project for ECR thruster development
Electric propulsion has been identified by European actors as a strategic technology for improving competitiveness in different space areas such as in-space operations and transportation. The European Commission has set up the "In-space Electrical Propulsion and Station-Keeping" Strategic Research Cluster (SRC) in the "Horizon 2020" funding framework with the goal of enabling major advances in Electric Propulsion for in-space operations and transportation. In this framework, the MINOTOR project was funded to mature a potentially disruptive cathodeless electric propulsion technology, the Electron Cyclotron Resonance (ECR) thruster. In recent years, the consortium leader ONERA has built up a large experience on ECR technology for electric propulsion, and the MINOTOR project will bring the expertise from three industrial partners (TMI, TAS-B and SAFRAN) and two university partners (UC3M and JLU) to take the next step.This work was made in the framework of project MINOTOR that has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 730028
Meteors: A Delivery Mechanism of Organic Matter to the Early Earth
All potential exogenous pre-biotic matter arrived to Earth by ways of our atmosphere, where much material was ablated during a luminous phase called "meteors" in rarefied flows of high (up to 270) Mach number. The recent Leonid showers offered a first glimpse into the clusive physical conditions of the ablation process and atmospheric chemistry associated with high-speed meteors. Molecular emissions were detected that trace a meteor's brilliant light to a 4,300 K warm wake rather than to the meteor's head. A new theoretical approach using the direct simulation by Monte Carlo technique identified the source-region and demonstrated that the ablation process is critical in the heating of the meteor's wake. In the head of the meteor, organic carbon appears to survive flash heating and rapid cooling. The temperatures in the wake of the meteor are just right for dissociation of CO and the formation of more complex organic compounds. The resulting materials could account for the bulk of pre-biotic organic carbon on the early Earth at the time of the origin of life.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43257/1/11038_2004_Article_310535.pd
Tuning the conductance of single-walled carbon nanotubes by ion irradiation in the Anderson localization regime
Carbon nanotubes are a good realization of one-dimensional crystals where
basic science and potential nanodevice applications merge. Defects are known to
modify the electrical resistance of carbon nanotubes. They can be present in
as-grown carbon nanotubes, but controlling externally their density opens a
path towards the tuning of the nanotube electronic characteristics. In this
work consecutive Ar+ irradiation doses are applied to single-walled nanotubes
(SWNTs) producing a uniform density of defects. After each dose, the room
temperature resistance versus SWNT-length [R(L)] along the nanotube is
measured. Our data show an exponential dependence of R(L) indicating that the
system is within the strong Anderson localization regime. Theoretical
simulations demonstrate that mainly di-vacancies contribute to the resistance
increase induced by irradiation and that just a 0.03% of di-vacancies produces
an increase of three orders of magnitude in the resistance of a 400 nm SWNT
length.Comment: 16 pages, 4 figure
X-ray Nanodiffraction on a Single SiGe Quantum Dot inside a Functioning Field-Effect Transistor
For advanced electronic, optoelectronic, or mechanical nanoscale devices a detailed understanding of their structural properties and in particular the strain state within their active region is of utmost importance. We demonstrate that X-ray nanodiffraction represents an excellent tool to investigate the internal structure of such devices in a nondestructive way by using a focused synchotron X-ray beam with a diameter of 400 nm. We show results on the strain fields in and around a single SiGe island, which serves as stressor for the Si-channel in a fully functioning Si-metal-oxide semiconductor field-effect transistor
Mach 3 shock-wave unsteadiness alleviation using a negative corona discharge
International audienc
Optimization of a coaxial electron cyclotron resonance plasma thruster with an analytical model
International audienceA new cathodeless plasma thruster currently under development at Onera is presented and characterized experimentally and analytically. The coaxial thruster consists of a microwave antenna immersed in a magnetic field, which allows electron heating via cyclotron resonance. The magnetic field diverges at the thruster exit and forms a nozzle that accelerates the quasi-neutral plasma to generate a thrust. Different thruster configurations are tested, and in particular, the influence of the source diameter on the thruster performance is investigated. At microwave powers of about 30W and a xenon flow rate of 0.1mg/s (1 SCCM), a mass utilization of 60% and a thrust of 1mN are estimated based on angular electrostatic probe measurements performed downstream of the thruster in the exhaust plume. Results are found to be in fair agreement with a recent analytical helicon thruster model that has been adapted for the coaxial geometry used here
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