Toward a wire medium endoscope for MRI imaging

This paper shows, numerically and experimentally, that imaging is realizable with a strongly curved wire medium. In a first part, simulation results obtained for various curvatures are presented and sub-wavelength imaging on a wide bandwidth is demonstrated. It is also shown that the collimation phenomenon remains very effective for structures having a cross-section corresponding to about a hundredth of the wavelength. In a second part, experimental results are shown for transmission of a medical image through a 63° curved wire medium in which the receiving antenna has been inserted.     

Analysis of arabic gum: Study of degradation and water desorption processes

Arabic gum is very much used in the agroalimentary industry, mainly as an emulsifier. It is a natural origin polysaccharide and therefore subject to more or less important variabilities of its properties. This work deals with two similar polysaccharides exuded by Acacia senegal of different history and an Acacia seyal sample. To characterize these samples, wide angle X-ray scattering (WAXS) and thermogravimetric analysis in dynamic and isothermal mode are used.The initial structure checked by X-ray diffraction allows to affirm that samples are equivalent in their crystalline structure. The activation energy used to characterize the main degradation process is the same for A. senegal samples, but higher for the A. seyal gum sample. Differences are furthermore observed for the kinetics of water desorption. We conclude that the samples from the same type of gum (A. senegal) are very similar and that A. seyal exudates can be detected by this technique. The water desorption of the two exudates is different.     

Deposit formation from lubricant degradation: a uniform layer deposition model

Reported is a new model describing deposit formation and ageing along the so‐called universal mass deposition curve. The penn state micro‐oxidation (PSMO) apparatus produced model deposits at varied test durations in order to study the ageing mechanism of thin film deposits from mineral base oil. Along the ‘universal’ mass deposition curve, mass deposition rates were correlated to film chemistry. Formed by concerted oxidation‐polymerisation reactions, chemical group and elemental content analyses were performed by Fourier transformed infrared spectroscopy‐attenuated total reflectance and energy dispersive spectroscopy, respectively. The results showed that deposit formation initiates as a film — uniform in composition. After an induction period, the film growth accelerates then slows and eventually falls off, with a decrease in mass often observed. These stages reflect underlying physical and chemical changes as the deposit progresses from an initial polymeric, lacquer‐type deposit, towards a final dark, carbonaceous deposit, along the universal mass deposition curve. Carbonyl, hydroxyl and sp³ hybridised carbon were identified as the main functional groups found across all deposit types albeit at different levels. Correlative behaviour was observed between changes in functional group infrared intensities and O‐atom and C‐atom content. These trends are interpreted as reflecting concerted deoxygenation and dehydrogenation processes, throughout the deposit, over the course of ageing, thus providing a foundation for the uniform layer model. Copyright © 2013 John Wiley & Sons, Ltd.     

Electroactive superelongation of carbon nanotube aggregates in liquid crystal medium

We report an effect of superelongation of carbon nanotube (CNT) aggregates driven by the electric field in a liquid crystal (LC) medium. The CNT aggregates started to elongate above a certain threshold field and sustained the elongation up to nearly 400% in the linear region with a large electroactive constant of 70 (V/microm)-1. The original morphology of the CNT aggregates was restored upon removal of the field. The elongation was fully reversible below a certain breakdown field, irrespective of the nematic or isotropic phase of the LC medium. The overall process involved (i) the alignment of CNT aggregates to increase the dipole energy of aggregates in the presence of the bias voltage, (ii) stretching of the CNTs by sliding-out from the bundles, and (iii) the entropic elasticity that restores the randomly entangled CNT network of the original aggregates after suppression of the bias voltage.     

Comparison of IGBT short-circuit failure “ohmic mode”: Epoxy molded package versus silicone gel module for new fail-safe and interruptible power converters

An epoxy molded package is compared with a silicone gel module with IGBTs chips in short-circuit failure modes with respect to critical energy, I²T_melting and explosion energy capabilities. Special importance was attached to “ohmic mode” assessment and ageing of the failed chips. The molded technology yields a very low and stable R_sc (<10 mΩ) as a “residual ohmic value” of the dies in low energy short-circuit failure, which is analysed through a complete reverse. Continuous thermal cycling tests over a medium time duration (>1000 h) also exhibit an acceptable drift of the R_sc property (<20%). The silicone gel module clearly exhibits an unstable R_sc value due to damage of the “free moving” wire-bonding on the chips. The authors show that the paralleled wires connections and the multiple parallel melting pits allow a sort of active redundancy and a possible on-state operation. All these results are used for the design of new and original failsafe converters. These topologies use only one paralleled safety leg that is spontaneously and directly connected in series with the failed devices, through the low R_sc value of the failed chips, without any additional complexity or extra cost.     

Study of the high throughput flexographic process for silicon solar cell metallisation

A commercially available silver paste was modified to match the flexographic process requirements. Rotational and oscillatory rheological tests were carried out to assess the printability and spreading behaviour of the resulting inks. Then, a multifactorial approach was used on a laboratory‐scale printing press to adapt the flexographic process for the front side metallisation of Cz‐Si solar cells, especially for the seed layer deposit of two layer contacts. To quickly identify the significant process parameters, a fractional design of experiment based on a screening approach at two levels was performed. Afterwards, two full factorial designs of experiments were implemented. While the first one allows a better understanding of the effect of the main factors and interactions, the second allows a fine tuning and a confirmation of the first results. Additionally, this methodology allows corroborating the influence of the ink rheological properties on the printing results. Following the process study and optimisation, a seed layer with an average width of 25 µm was printed at a high 0.3 m/s throughput. Additional results suggest that the line width and the throughput can be further improved, which underlines the potential of flexography for photovoltaic applications. Finally, the light‐induced process was used to thicken the seed layer after a standard firing‐through step, leading to an encouraging 17.9% efficiency on Cz‐Si solar cells. Copyright © 2015 John Wiley & Sons, Ltd.     

The Detector System for the Stratospheric Kinetic Inductance Polarimeter (Skip)

The stratospheric kinetic inductance polarimeter is a proposed balloon-borne experiment designed to study the cosmic microwave background, the cosmic infrared background and Galactic dust emission by observing 1,133 deg \(^2\) of sky in the Northern Hemisphere with launches from Kiruna, Sweden. The instrument contains 2,317 single-polarization, horn-coupled, aluminum lumped-element kinetic inductance detectors (Lekids). The Lekids will be maintained at 100 mK with an adiabatic demagnetization refrigerator. The polarimeter operates in two configurations, one sensitive to a spectral band centered on 150 GHz and the other sensitive to 260 and 350 GHz bands. The detector readout system is based on the ROACH-1 board, and the detectors will be biased below 300 MHz. The detector array is fed by an F/2.4 crossed-Dragone telescope with a 500 mm aperture yielding a 15 arcmin FWHM beam at 150 GHz. To minimize detector loading and maximize sensitivity, the entire optical system will be cooled to 1 K. Linearly polarized sky signals will be modulated with a metal-mesh half-wave plate that is mounted at the telescope aperture and rotated by a superconducting magnetic bearing. The observation program consists of at least two, 5-day flights beginning with the 150 GHz observations.     

Origin of an intermediate peak in DMTA analysis of multilayer ABS/PC samples

Three layers film shaped by thermocompression of acrylonitrile butadiene styrene copolymer and polycarbonate have been analyzed in dynamic mechanical thermal analysis in tensile mode. They present two peaks as the film is loaded perpendicularly to the layers and three peaks as the film is loaded in parallel to the layers. Numerical computations confirm that the origin of this peak is not related to a mechanical issue such as the transmission of the imposed deformation from one layer to the other. Using this method, it is demonstrated that this third peak can only be obtained assuming a material transition with its own behavior between layers. Tan δ measurements provide a simple and useful experimental tool to understand more about the interfacial zone in polymer blends.     

Nanosized ammonia borane for solid-state hydrogen storage: Outcomes,limitations, challenges and opportunities

Ammonia borane NH₃BH₃ (AB), a material for solid-state hydrogen storage, can be nanosized by confinement into the porosity of a scaffold like mesoporous silica, carbon cryogel, graphene oxide, ZIF-8 as a metal organic framework, poly (methyl acrylate), boron nitride and manganese oxide. In doing so, nanosized AB is destabilized and shows better dehydrogenation properties than bulk AB in terms of temperature, activation energy, enthalpy and kinetics. Such improvements are due to the confinement-driven nanosizing effect, but not only. A catalytic effect may also have a contribution and, in some cases, it even overpasses the nanosizing effect. These effects are explained in detail herein. The present review aims at reporting the outcomes of the AB confinement strategy to help understand the advantages and to identify the limitations which are still not adequately defined. Based on this analysis, the challenges ahead are listed and discussed, and it appears that there are new opportunities to explore. Though nanosized AB is not mature enough for implementation, it has the potential to be developed further. Avenues worth exploring are given.