In-situ comprehensive calibration of a tri-port nano-electro-mechanical device

We report on experiments performed in vacuum and at cryogenic temperatures on a tri-port nano-electro-mechanical (NEMS) device. One port is a very nonlinear capacitive actuation, while the two others implement the magnetomotive scheme with a linear input force port and a (quasi-linear) output velocity port. We present an experimental method enabling a full characterization of the nanomechanical device harmonic response: the nonlinear capacitance function C(x) is derived, and the normal parameters k and m (spring constant and mass) of the mode under study are measured through a careful definition of the motion (in meters) and of the applied forces (in Newtons). These results are obtained with a series of purely electric measurements performed without disconnecting/reconnecting the device, and rely only on known dc properties of the circuit, making use of a thermometric property of the oscillator itself: we use the Young modulus of the coating metal as a thermometer, and the resistivity for Joule heating. The setup requires only three connecting lines without any particular matching, enabling the preservation of a high impedance NEMS environment even at MHz frequencies. The experimental data are fit to a detailed electrical and thermal model of the NEMS device, demonstrating a complete understanding of its dynamics. These methods are quite general and can be adapted (as a whole, or in parts) to a large variety of electromechanical devices.     

Long term ageing of polyamide 6 and polyamide 6 reinforced with 30% of glass fibers: physicochemical,mechanical and morphological characterization

Hygrothermal ageing of polyamide 6 (PA6) and polyamide 6 reinforced with 30 wt% of glass fibers (PA6GF30) was undertaken. Immersion was conducted in distilled water at 90 °C and 100% relative humidity (RH) for up to 80 days (1920 h). Results revealed a noteworthy decrease either in glass transition temperature T_g or in tensile properties, at early stage of ageing, for both studied materials. This decline was mainly caused by the plasticization effect of water and the weakness of the interfacial interactions leading as a consequence to a loss of adhesion between fiber and matrix. Afterwards, physical and mechanical properties decrease monotonically testifying the occurrence of exhaustive damages and chemical reaction phenomena. Such phenomena were yellowing and crazing formation which were observed for both materials after 1920 h of conditioning. The former is caused by the thermo- oxidation whereas the latter results from the release of internal stresses induced by water sorption. These chemical reactions were monitored by infrared spectroscopy. Thus, an increase of the free N-H stretch and the carbonyl groups (imides) was noted. Accordingly, it seems that long term immersion in distilled water at high temperature induces chemical reactions which indicate the severity of the damage.     

The interaction between molten gallium and the hydrocarbon medium induced by ultrasonic energy—can gallium carbide be formed?

Ultrasonic irradiation of molten gallium in organic liquids (decane, dodecane, etc.) results in dispersion of the gallium into nanometric spheres. These were examined by several analytical methods XRD, DSC, Raman and IR spectroscopy) as well as electron microscopy (SEM, TEM) and found to be composed of Ga and C. The DSC analysis indicates that the Ga has possibly reacted with carbon, while the Raman spectrum of the product demonstrates a strong additional peak that could not be identified. This work explores the possibility that the product is gallium carbide or another gallium‐carbon complex. To investigate the nature of the product, we performed detailed extended X‐ray absorption fine structure (EXAFS) and X‐ray absorption near‐edge structure (XANES) analyses. On the basis of DSC, IR, and Raman it appear to be formation of GaC, whereas the analysis by EXAFS and XANES demonstrated that the gallium is found to be in a higher reduced state (almost metallic), supported by carbon. The question that remains open in addition to the one related to the formation of galium carbide is whether a complex structure, including oxygen contamination is involved in the layers surrounding the Ga as indicated by the EXAFS results.     

Synthesis of AlN nanopowder coated with a thin layer of C,using a thermal plasma reactor

High-purity nanocrystalline aluminum nitride powders were synthesized by using a 12?kW non-transferred arc plasma. The synthesis was conducted in a versatile, new designed, one-chamber thermal plasma reactor (TPR). The novel experimental assembly incorporated better working conditions like: high temperature gradient between the crucible and reactor's wall, and high super-saturation of the system by nitrogen and carbon. Thermodynamic modelling of the synthesis was conducted in order to achieve the best conditions for AlN formation. In this study, aluminum discs of Al 1100 were used as precursor material and pure nitrogen was the only gas used as reagent and plasmogenic gas.Nanopowders collected from reactor's wall were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-Ray diffraction (XRD). Synthesized h-AlN nano-powders were found to be free of oxides and aluminum metal. A thin carbon-layer around the particles was detected. TEM results indicated that the carbon-layer was around 5 and 10?nm. This outcome could make a significant difference with other synthesis reported in the literature since the occurrence of the carbon-layer, could delay AlN oxidation, prevent hydration, and could avoid the agglomeration of the particles.     

Thermal Resonance at the Microscale in AC Scanning Thermal Microscopy with a Thermal-Resistive Probe

Investigation of the thermal-resistive probe response in an AC scanning thermal microscope (SThM) as a function of the distance probe–material surface under ambient conditions and the current excitation frequency is presented. The analysis of temperature experimental results points out a phenomenon which can be interpreted as a thermal wave resonance. The modeling of the thermal response with the finite element method considering the surrounding medium as a thermally conducting medium tends to confirm this. This phenomenon is independent of the sample nature and is related to the thermal diffusion length of the air medium between the probe and the sample. An equivalent resonance factor is defined: it shows a linear dependence with the characteristic distance for which the thermal resonance phenomenon is an extremum. The system—probe/sample surface—behaves as a resonant cavity at the microscale. This configuration is not specific to a SThM and can occur within electronic devices.     

Some investigations into the optimal dimensional synthesis of parallel robots

In this paper, we will perform a comparison between two approaches of dimensional synthesis of parallel robots. The first one concerns the single-objective optimization approach; in this case, the dimensional synthesis is expressed by taking into account only one performance criterion but enables to get a final solution if it exists. The second one concerns the multi-objective optimization approach; it enables to simultaneously take into account several performance criteria. However, this approach appears to provide a set of solutions instead of a single expected final solution which should directly enable to carry out the structural synthesis. In fact, the search of a single final solution is postponed to a further step where the designers have to impose and/or restrict certain parameters. And we will establish if it is really necessary to make a multi-objective optimization approach or if a single-objective is sufficient to reach the objectives set in the specifications (user requirements). A discussion is proposed concerning the arising questions related to each approach and leading to the optimal dimensional synthesis. The PAR2 robot with two degree-of-freedom is used to exemplify the analysis and the comparison of the two approaches. The proposed comparison can be applied to any classes of parallel robots.     

The generalized amalgam method for modal reduction

In this paper, we propose an extension of the amalgam method to reduce advection diffusion problem with time dependent parameters. From a full general basis, this technique consists to select rapidly the most influent modes, and to add to them the remaining modes balanced by a coefficient. The originality of this paper lies in the fast obtention of a full degraded model, used as a reference in a minimization process of the reduced model’s error. Transcribed to the physical space, this error approaches the error between the reduced model and the real physical finite elements model. The treated application is a disk rotating at a variable velocity with time dependent thermal inputs. Comparison between the reduced and the finite elements model gives a gain in term of computational time of 36 for a mean error on the entire domain equal to 0.42%.     

Theoretical considerations on the modelling of transport in a three-phase electrode and application to a proton conducting solid oxide electrolysis cell

This paper presents a numerical model for fuel cells and electrolysis cells that use cermets as electrodes. The mass and charge continuity equations were demonstrated inside the electrodes and in the divergence term, surface ratios were used instead of more usual volumetric ratios. The Butler–Volmer equation for electrokinetics was used with concentration coefficients in order to predict correctly the concentration effects on the value of the transfer current density in the cermets. In addition, it was considered that the reaction takes place inside all of cermet's volume inste1 ad of a thin layer near the electrolyte. The model was tested for the proton-conducting SOEC technology on a generic cell design and the calculations were performed with COMSOL Multiphysics 4.1™. A parametric analysis was carried out on a proton-conducting SOEC in galvanostatic mode in order to evaluate the influence of parameters on oxygen production across the electrode in the anodic compartment. This analysis showed that the structural parameters of the cermets such as grain radius or volumetric ratios of the conductors play a major role in the distribution of reaction kinetics.     

Strictly periodic scheduling in IMA-based architectures

The avionic industry has recently adopted the Integrated Modular Avionics (IMA). Such architectures allow the execution of avionic functions on a shared computing platform while avoiding any interference between them. This is done through hard memory and temporal segregation constraints. Although IMA reduces the weight and the power consumption and shortens the design-cycle times, it gives rise to a complex multiprocessor scheduling problem. One of the key difficulties of this problem is related to the strict periodicity of tasks, which means that the time separating two successive executions of the same task is strictly equal to the associated period. In order to help the system designer in producing a proper schedule, an exact formulation based on Integer Linear Programming and a heuristic inspired from Game Theory are proposed. To enhance the solution quality of the heuristic, a?multi-start method, which gives some probabilistic guarantees on the optimality of the solutions, is also introduced.