A. Le Goff

Supercapacitor/biofuel cell hybrids based on wired enzymes on carbon nanotube matrices: autonomous reloading after high power pulses in neutral buffered glucose solutions

We report an original setup using carbon nanotube matrices as supercapacitors where redox enzymes serve for continuous charging of the capacitors. High currents can be delivered under short pulse discharges. This supercapacitor/biofuel cell hybrid system remains stable for at least 40 000 pulses of 2 mW.

Freestanding HRP–GOx redox buckypaper as an oxygen-reducing biocathode for biofuel cell applications

Horseradish peroxidase (HRP) was immobilized on redox buckypapers followed by electropolymerization of pyrrole-modified concanavalin A enabling the subsequent additional immobilization of the glycoprotein glucose oxidase (GOx). Biocatalytic buckypapers were formed using pyrene-modified 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) or bis-Pyr-ABTS, a redox mediator, as a cross-linker. ABTS-functionalized buckypaper enhances the electron transfer of the bioelectrocatalytic reduction of H2O2 by HRP. Since H2O2 is produced during glucose oxidation by GOx in the presence of oxygen, the bienzymatic GOx–HRP biocathode achieves the complete reduction of oxygen into water. A clearly improved performance of the biocathode was obtained by using an improved biocompatible immobilization strategy, enabling the prevention of enzyme loss while ensuring both diffusion of glucose and O2 and the local production of H2O2. These freestanding flexible oxygen-reducing biocathodes can operate under physiological conditions and show a high onset potential at 0.60 (±0.01) V. In the presence of glucose (5 mM), such biocathodes exhibit a stable current density output of 1.1 (±0.1) mA cm−2 at 0.1 V under continuous one-hour discharge. Furthermore, a marked increase in lifetime was observed, the biocathode displaying 64% of its initial electrocatalytic activity after 15 days.

Energy absorption in a bamboo foam

We discuss the ability of soap films to absorb the energy of an impacting projectile. If the impact velocity is large enough, solid objects or water drops pass through the films without breaking them. However, during each impact a small part of the original kinetic energy is extracted, which is revealed by considering collections of parallel films (so-called bamboo foams). Then, the impacting object is observed to stop after crossing numerous films. The total energy absorbed by the foam is found to be the total energy of distortion of the films, integrated over the multiple crossings. The case of inclined films is also considered, and found to affect the trajectory of the projectile.

Glucose fuel cell based on carbon nanotube-supported pyrene–metalloporphyrin catalysts

It is important to design a functionalization scheme for carbon nanotubes that preserves their outstanding proprieties, while adding new proprieties, thereby enabling their integration in fuel cell applications. In the present work, we describe the production of a non-covalently attached network of porphyrins to multi-walled carbon nanotubes (MWCNT) sidewalls. The approach is based on π–π stacking interactions of pyrene-modified metalloporphyrins onto MWCNT sidewalls. Two configurations of MWCNT–porphyrin hybrid electrodes were both electrochemically characterized and tested under alkaline conditions. Pyrene-functionalized rhodium deuteroporphyrin (Rh(DP)pyr2), was used as an anode in the electrocatalytic oxidation of glucose and pyrene-functionalized tetracarboxyphenyl cobalt porphyrin (Co(TCPP)pyr4) was itself used as a cathode in the electrocatalytic reduction of oxygen. Both electrodes were integrated into a glucose fuel cell system leading to a maximum power output of 0.9(±0.10) mW cm−2. Compared to alternative system approaches, pyrene-modified porphyrin hybrid electrodes and their corresponding fuel cell devices exhibited higher activity, power output, and long term stability.

Joint perpendicular anisotropy and strong interlayer exchange coupling in systems with thin vanadium spacers

We study the influence of the insertion of a vanadium spacer layer between an FeCoB layer and a [Co/Ni] multilayer in an MgO substrate-based system mimicking the reference system of a perpendicular anisotropy magnetic tunnel junction. The anisotropy of the [Co/Ni] multilayer gradually improves with the vanadium thicknesses t, up to an optimized state for t = 8 A, with little influence of the thermal annealing. The interlayer exchange coupling is ferromagnetic and very strong for t≤6 A. It can be adjusted by thermal treatment at t = 8 A from no coupling in the as-grown state to more than 2 mJ/m2 after 250 °C annealing. For this spacer thickness, the magnetic properties are consistent with the occurrence of a bcc (001) to an fcc (111) crystalline structure transition at the vanadium spacer. The remaining interlayer exchange coupling at t = 8 A is still substantially higher than the one formerly obtained with a Tantalum spacer, which holds promise for further optimization of the reference layers of tunnel ju...

Study of luminous spots observed on metallic surfaces subjected to high RF fields

The performance of high gradient superconducting RF cavities for electron accelerators is mainly limited by field emission. Major improvements have been recently obtained using different surface conditioning techniques confirming the involvement of metallic particles in field emission enhancement. In this paper we present the results obtained with an optical apparatus attached to an RF copper cavity equipped with a removable sample which is subjected to high RF fields (E/sub pk/>40 MV/m). Stable light spots are observed on the sample surface and their intensities and optical spectra are measured as a function of the surface electric field. The total emitted current is simultaneously measured by an isolated hollow electrode facing the sample. Particles of different types were deliberately sprinkled over the sample surface and the luminous features are studied. Light intensity, spectral power density and evolution of the luminous sites provide useful information for understanding the field emission phenomena and the conditioning effects.

Spin-wave thermal population as temperature probe in magnetic tunnel junctions

We study whether a direct measurement of the absolute temperature of a Magnetic Tunnel Junction (MTJ) can be performed using the high frequency electrical noise that it delivers under a finite voltage bias. Our method includes quasi-static hysteresis loop measurements of the MTJ, together with the field-dependence of its spin wave noise spectra. We rely on an analytical modeling of the spectra by assuming independent fluctuations of the different sub-systems of the tunnel junction that are described as macrospin fluctuators. We illustrate our method on perpendicularly magnetized MgO-based MTJs patterned in 50 × 100 nm2 nanopillars. We apply hard axis (in-plane) fields to let the magnetic thermal fluctuations yield finite conductance fluctuations of the MTJ. Instead of the free layer fluctuations that are observed to be affected by both spin-torque and temperature, we use the magnetization fluctuations of the sole reference layers. Their much stronger anisotropy and their much heavier damping render them e...

A comparison of enhanced field emission from broad surfaces in direct-current and radiofrequency regimes

The purpose of the present work was to compare enhanced field emission from broad-area surfaces in DC and RF regimes. Geometrical defects, known to be field emitters, were created artificially by solid contact on copper and niobium samples. Each sample was tested successively in a re-entrant RF cavity, then in a DC diode housed in a specially equipped scanning electron microscope (SEM). When the sample was out-gassed carefully and exposed to a limited field and a limited current to avoid irreversible modifications of the emitter geometry, the Fowler - Nordheim parameters extracted from the experiment were the same in DC and RF regimes.

Study of luminous phenomena observed on contaminated metallic surfaces submitted to high RF fields

Abstract The primary limitation to the increase of the attainable accelerating gradient in superconducting cavities is the strong field emission (FE) of electrons from the RF surface. Many studies devoted to the DC field emission have shown that the electron emission on a broad metallic surface stems from a few very localized sources, called emitters. Superficial dust particles (metallic or dielectric) or semiconducting impurities embedded in the surface can originate the emission. Different experimental results have led to several theoretical models explaining the physical mechanisms of the field emission. We have investigated the RF field emission from a sample subjected to high RF fields ( E peak > 40 MV/m) in a copper cavity. Our study is focused on the luminous emissions occurring on the RF surface simultaneously with the electron emission. The optical apparatus attached to the cavity permits one to observe the evolution of the emitters and the direct effect of the surface conditioning. Also, the parameters of the emitted radiation (intensity, glowing duration, spectral distribution) may provide additional information on the field emission phenomena. This paper relates some results concerning samples intentionally contaminated with particles (metallic or dielectric).