The structural, spectroscopic and electronic properties of Na and K birnessites were investigated from ambient conditions (birA) to complete dehydration, and the involved mechanisms were scrutinized. Density Functional Theory (DFT) simulations were employed to derive structural models for lamellar A0.33MnO2xH2O (A = Na+ or K+, x = 0 or 0.66), subsequently compared with the experimental results obtained for Na0.30MnO20.75H2O and K0.22MnO20.77H2O materials. Thermal analysis (TGA-DSC), X-ray diffraction (XRD), Fourier Transform Infrared (FTIR) spectroscopy, and Near Ambient Pressure X-ray Photoemission Spectroscopy (NAP-XPS) measurements were conducted for both birnessites. Dehydration under vacuum, annealing, or controlled relative humidity were considered. Results indicated that complete birnessite dehydration was a two-stage process. In the first stage, water removal from the interlayer of fully hydrated birnessite (birA) down to a molar H2O/A ratio of similar to 2 (birB) led to the progressive shrinkage of the interlayer distance (3% for Na birnessite, 1% for K birnessite). In the second stage, water-free (birC) domains with a shorter interlayer distance (20% for Na birnessite, 10% for K birnessite) appeared and coexisted with birB domains. Then, birB was essentially transformed into birC when complete dehydration was achieved. The vibrational properties of birA were consistent with strong intermolecular interactions among water molecules, whereas partially dehydrated birnessite (birB) showed a distinct feature, with 3 (for Na-bir) and 2 (for K-bir) vibrations that were reproduced by DFT calculations for organized water into the interlayer (x = 0.66). The study also demonstrated that the electronic structure of Na birnessite depends on the interlayer water content. The external Na+ electronic level (Na 2p) was slightly destabilized (+0.3 eV binding energy) under near ambient conditions (birA) compared to drier conditions (birB and birC). The structural, spectroscopic and electronic properties of Na and K birnessites were investigated from ambient conditions to complete dehydration, and the involved mechanisms were scrutinized.

In the field of energy conversion, enzyme-based biofuel cells (EBFCs) are considered as energy-harvesting devices. These devices involve the use of redox enzymes for the oxidation of specific fuels at the anode and the reduction of oxidants at the cathode. The enzymes are immobilized on the electrode surface to prevent their denaturation and improve their stability. Owing to potential difference between the two electrodes, electron transport leads to the generation of electric power. EBFCs offer several advantages over traditional fuel cells working from metal catalysts, including reaction selectivity, use of nontoxic components, diversity of fuel, and ability to operate at mild conditions. However, these devices suffer from severe kinetic limitations caused by enzyme loading, enzyme instability, and slow electron transfer between the catalysts and the electrode surface. Many works have been devoted to the development of electrode materials with characteristics including high electric conductivity, surface area, porosity, and mechanical strength. To meet these demands, nanostructured materials with three-dimensional architectures are relevant for optimizing both surface area of enzyme-based electrodes and fluid delivery. This article presents briefly the different strategies developed to prepare enzyme electrodes from nanostructured materials for the efficient development of enzyme biofuel cells.

New nanostructured electrodes, promising for the production of clean and renewable energy in biofuel cells, were developed with success. For this purpose, carbon nanofibers were produced by the electrospinning of polyacrylonitrile solution followed by convenient thermal treatments (stabilization followed by carbonization at 1000, 1200 and 1400° C), and carbon nanotubes were adsorbed on the surfaces of the fibers by a dipping method. The morphology of the developed electrodes was characterized by several techniques (SEM, Raman spectroscopy, electrical conductivity measurement). The electrochemical properties were evaluated through cyclic voltammetry, where the influence of the carbonization temperature of the fibers and the beneficial contribution of the carbon nanotubes were observed through the reversibility and size of the redox peaks of K3Fe(CN)6 versus Ag/AgCl. Subsequently, redox enzymes were immobilized on the electrodes and the electroreduction of oxygen to water was realized as a test of their efficiency as biocathodes. Due to the fibrous and porous structure of these new electrodes, and to the fact that carbon nanotubes may have the ability to promote electron transfer reactions of redox biomolecules, the new electrodes developed were capable of producing higher current densities than an electrode composed only of electrospun carbon fibers.

A series of experimental boron nitride (BN) fibers were prepared from a Balkylaminoborazine-based polymer (=poly[(methylamino)borazine]) according to the PolymerDerived Ceramic (PDC) route. The polymer was melt-spun in N2 into monoand multifilaments, prior the curing of the resulting green fibers in NH3 at 400°C and the subsequent pyrolysis of the ascured fibers in NH3 (1000°C), then N2 up to 1800°C to generate BN fibers. It was established that the melt-spinning operation could deliver BN fibers with different microtexture/microstructure, and therefore with different mechanical behavior depending on the spinning parameters. The relationship between mechanical properties and fiber microtexture/microstructure could be studied in the present paper. Melt-spinning operation into multifilaments provided either low-modulus fibers with a featureless cross-sectional microtexture as glassy-like materials or fibers with a microtextural skincore heterogeneity decreasing the fiber strength. The disordered microstructure of low-modulus BN fibers consisted of disoriented nanosized grains mixed in an amorphous matrix. In contrast, the meltspinning operation into monofilament produced high-modulus fibers with a coarse-grained microtexture. In such samples, the extended grains were ordered along the fiber-axis increasing the crack propagation along the cleavage basal planes, and therefore decreasing the failure strain of fibers. INTRODUCTION Hexagonal boron nitride (h-BN) represents an advanced ceramic material with a layer anisotropic structure and attractive properties such as high stiffness and toughness along the basal layers. It also exhibits a good oxidative resistance up to T 1000°C, a good thermal stability up to T ~ 2500°C in an inert atmosphere and a low coefficient of thermal expansion (CTE) along the basal layers. Based on these properties, h-BN should be promising for preparing continuous fiberreinforced ceramic-matrix composites (CFCCs) intended for high temperature applications. For CFCCs fabrication, high-modulus and strength oxidation resistant fibers with small diameter are required. Additionally, reinforcing fibers must be capable of retaining the structure, stiffness and strength under processing (matrix deposition) and service conditions. Keeping these in view, the preparation of a new generation of BN fibers-reinforced BN composites appears to be an excellent opportunity to replace the traditional carbon/carbon (C/C) composites which are very sensitive to oxidative and hydrolytic environment above 400°C. In addition, BN/BN composites could be used in radiation-transparent structures (low dielectric constant of h-BN) as well as in aerospace applications requiring, among others, ultra-light weight in accordance with its low density (2.27) compared with that of SiC or oxide-based materials, for example. Above all, the layered structure of h-BN as Advances in Ceramic Matrix Composites XI 3 matrix could protect the fibers against the notch effect arising from matrix microcracking, deflect the microcracks parallel to the fiber surface and allow fiber sliding. In that case, the deposition of BN interphases to weaken the fiber-matrix bonding will be not necessary. The first objective for preparing such composite materials concerns the development of BN fibers with controllable mechanical properties. Preceramic polymers are ideally suited for the preparation of ceramic fibers using spinning and pyrolysis procedures.* In our lab, we have prepared BN fibers from B-alkylaminoborazine-derived polymers (=poly[(alkylamino)borazines])." In particular, the poly[(methylamino)borazine] is seen as a melt-spinnable polymer for providing BN fibers with high mechanical properties, fine diameters and a low density. For example, in a previous paper, we showed that this polymer could deliver BN fibers with a tensile strength of 2 GPa, a modulus of 440 GPa, a density of 1.85 and a wellordered and oriented microstructure. Permanent advances in polyMAB-derived BN fibers are important in the aim of improving the performances of the CFCCs. With this aim in mind, our work has been mostly dedicated to the study of the precursor synthesis, and to the numerous reactions occurring during the polymer-to-ceramic conversion. These investigations allowed to introduce significant improvements in the preparation of BN fibers. By varying the experimental conditions of melt-spinning, the present paper also shows that different types of fibers can be developed. In this context, the present study aims at the effect of the as-obtained microtextures/microstructures on the mechanical properties of BN fibers. EXPERIMENTAL SECTION General comments All synthesis reactions were carried out in Ar, whereas N2 and NH3 with electronic purity were used during the fiber preparation. Tensile tests and diameter measurements were achieved from 50 filaments with a gauge length of 10 mm. Diameters were measured by laser interferometry and mechanical properties were determined using a standard tensile tester (Adamel Lhomargy DY 22). Modulus and strains were averaged from the 50 tests and the strength distribution was described by Weibull statistics. Strength were averaged for a failure probability P=0.632. XRD was performed using a Philips apparatus (CuKa radiation; X = 1.5406 À at 40 kV and 30 m A). Fibers were crushed, prior characterization. SEM (Hitachi S800) was used to observe the cross-sectional microtexture of fibers. An Au/Pd film was deposed on fibers, prior observation. TEM was investigated using a Topcon EMB-002B microscope. Samples were embedded in a resin and cut into thin foils with an ultramicrotome. Foils were then set on microgrids to observe the longitudinal microtexture. Fiber preparation A same lot of polyMAB was used in the present paper. Its synthesis and characterization was previously described. Green fibers were prepared in a glove-box in N2 by the melt-extrusion of the polymer followed by the stretching of the resulting monoor multifilament by a spool. The as-spun fiber wound on the spool was transferred into a silica furnace to achieve the curing and pyrolysis processes in NH3 (25°C-1000°C, 0.8°C.min~, dwell time of lh). After such heating, the fiber was transferred into a graphite furnace to undergo heat-treatment (lOX.min) in N2 up to 1800°C (dwell time of lh). As-pyrolyzed BN fibers were white colored and their typical elemental composition (N2.8B3) showed that pure boron nitride was produced. The level in oxygen was extremely low (<2wt%). 4 Advances in Ceramic Matrix Composites XI RESULTS AND DISCUSSION Melt-spinning of polyMAB-based fibers Figure 1 details the melt-spinning as well as the process variables which are controlled to provide the different microtextures in the final BN fibers. Typically, the polyMAB is molten into an extruder at 180°C, then compacted, before pushing it through a filter for removing unexpected impurity. After passing through the filter, the melt is driven through a spinneret with one or seven capillaries, 200 |im in diameter for each. As the melt exits, it cools and emerges as an endless monoCone capillary) or multi(seven capillaries) filament which is finally continuously wound on a spool. It should be noted that the spinning temperature is closely related to the polymerization degree of the polymer. Piston, piston rate (m.min)

Si–Zr–C–N membranes were studied for an application of hydrogen separation at high temperature. The synthesis of single-source molecular precursor incorporating four elements Si, Zr, C and N in the same molecule has been developed leading to a volatile compound that can be deposited in a gaseous way by plasma enhanced chemical vapor deposition. The obtained thin films were characterized by scanning electron microscopy, infrared spectroscopy and ellipsometry. Gas permeation tests were also performed to determine their performance in terms of permeance and selectivity. A helium permeance of about 4.5.10−8molm−2s−1Pa−1 and an ideal selectivity α* He/N2 around 60 have been obtained at 150°C with a transmembrane pressure of 105Pa.

We have developed hybrid devices based on n type silicon nanowires (SiNWs) dispersed in a p type poly(3-hexylthiophene): P3HT, polymer film. The strong photoluminescence at 860 nm exhibited by SiNWs can be assigned to the electronic confinement effect for nanowire diameters smaller than 10 nm. An optimum dissociation of the electron-hole pairs photogenerated in P3HT by a 10 wt.% concentration of SiNWs in the blend is indicated by the extinction of the P3HT photoluminescence which is however incomplete. The current/voltage characteristics under simulated sun-light shows an open circuit voltage reaching 0.5 V. a filling factor of the order of 0.35 and a short circuit of some mA/cm(2) depending on the surface treatment of SiNWs. Improved results are obtained for silicon surfaces grafted with polystyrene chains leading to surface state passivation and increased dispersion of the SiNWs in the polymer layer coming from an improved compatibility between organic and derivatized inorganic phases. (C) 2011 Elsevier B.V. All rights reserved.

The stabilized aqueous solution of sodium borohydride NaBH4 is a promising hydrogen fuel but the stored hydrogen has to be released with the help of a catalyst through hydrolysis. In the present study, we developed Co- and clay-based supported catalysts. Three raw clays were taken from soil in Lebanon. Once purified and annealed, they were used as supports. Two of them, mainly composed of kaolinite and illite respectively, showed to be promising owing to their attractive specific surface areas (58.0 and 67.1m2g−1) as well as the high reactivity of the corresponding 15wt.% Co catalysts (i.e. NaBH4 conversions of 100% and hydrogen generation rates up to ∼31L(H2)min−1g−1(Co)). A kinetic study was also carried out. The main results are reported and discussed herein.

Back-gated field effect transistors (FETs) based on catalyst-free grown 3C-SiC nanowires (NWs) were fabricated and electrical characterization is presented. Silvaco simulation was used to fit the I-V characteristics and to extract information about the carrier (electrons) concentration and the oxide/NW interface quality. The high trap density and fixed charges at the nanowire/oxide interface, Dit~5x1011 cm-2eV-1 and Qf ~3x1013cm-2, and the high electron concentration (~3x1019 cm-3) originating from unintentional doping severely affect the electrical conduction through the nanowires which has as a result low values of mobility and transconductance, 0.11 cm2/Vs and 7x10-10 A/V, respectively.

Shaped catalysts are crucial for technological applications. In this context, we have developed Co–αAl2O3 catalyst films deposited over Cu plates to be used in hydrogen generation by hydrolysis of sodium borohydride NaBH4 in alkaline solution. The Co–αAl2O3 films were prepared by electrophoretic deposition according to six different routes. While five of them failed in fabricating adhering films, the sixth route, consisting of electrodepositing Co-impregnated αAl2O3, showed promising results. The as-obtained shaped catalysts were stable when hydrogen vigorously bubbled and catalyzed the NaBH4 hydrolysis with attractive hydrogen generation rates. These results open an alternative route for preparing shaped catalysts in this reaction.

Polyurethane-based nanocomposite films were prepared by incorporating carbon-coated SiC nanowires (SiC@C) into the polymer matrix. Electric field-induced strain measurements revealed that a loading of 0.5 wt% SiC@C increased the strain level by a factor of 1.7 at a moderate field strength (6.5V mu m(-1)). Current-electric field characteristics and the film thickness dependence of strain demonstrated that the improvement of the electromechanical response was linked to a more pronounced space charge effect in the nanocomposite than in the polymer host. DSC measurements revealed that the level of phase mixing in the PU matrix remained unchanged after SiC@C filling; hence, the nano-objects themselves acted as charge traps.

We report here field-emission (FE) studies of individual single-crystal SiC nanowires that showed several distinct I/V regimes including strong saturation resulting in highly nonlinear Fowler-Nordheim plots. The saturation is due to the formation of a depletion layer near the nanowire ends as predicted for FE from semiconductors and appears after in situ control of the surface cleanliness. This work opens the door to improving the uniformity, stability, and photon control of mass-produced planar nanowire FE cathodes and shows how FE can be used for transport measurements on individual semiconducting nanowires.

BN-based coatings on metallic (titanium, aluminium and copper) substrates are prepared by infrared irradiation of a dip-coated polyborazylene. This innovative strategy creates an access towards the molecular design of hexagonal-boron nitride coatings as protective materials for low melting point metals.

The formation of dots by CVD in the hetero-system SiC-Si was studied in the two possible ways : Si dots on SiC substrate and SiC dots on Si substrate. The substrates underwent special surface treatment to reveal a network of parallel steps before deposition of the dots. In the Si on SiC case, the dots density on the 8°off 4H-SiC substrate varied in the range 107 – 7x108 cm-2 and mainly depends on the SiH4 flux and the deposition time. The Si dots are in majority aligned along the step edges of the substrate. In the other hetero-system, only propane was introduced in the reactor to performed a localised carbonisation of the Si(111) 1.5°off substrate. The SiC dots obtained at 1200°C have similar density the Si ones but with smaller size.

Hybrid organic-inorganic bis-tetrabutylammonium-7,7'-(iminundecahydro-closo-dodecaborate)-9,9'-(dihexyl)-2,2'-bifluorene (5) was prepared, and the influence of the closo-dodecaborate cluster on the two-photon absorption properties of bi-fluorene derivatives was demonstrated.

The thermal behaviour of a series of poly[B-(methylamino)borazine] prepared at various temperatures ranging from 140 to 200°C is studied in the present paper as potential BN fiber precursors. It was shown that the softening capability of poly[B-(methylamino)borazine] can be tailored by controlling the temperature at which polymers were prepared to achieve melt-spinning and produce high quality green fibers. Thus as-spun fibers could be next converted into boron nitride fibers using ammonia (25–1000°C) and nitrogen (25–1800°C) atmospheres. The quality of boron nitride fibers was shown to depend on the first part of the pyrolysis step (25 and 1000°C; ammonia atmosphere) in which the great majority of the weight loss necessary for boron nitride production occurs. Ideal poly[B-(methylamino)borazine] as BN fiber precursors are those prepared between 170 and 180°C. They display appropriate melt-spinnability and ceramic conversion capability.

A gradual downshift and broadening of the Si optical phonon peak was observed by Raman scattering measurements on individual undoped and p-doped silicon nanowire (SiNW) when heated by a laser beam. This dual effect can be interpreted by an induced compressive stress resulting from structural defects. The p-doped SiNW was shown to be the most sensitive to heating and its structure was clearly modified with a large diffusion of gold atoms forming numerous gold nanoparticles.

BN tubular micro-/nanostructures were prepared in large scale from a simple unique chemical process onto the surface of a graphite substrate, using polypropylene and boron oxide as solid sources, and under an ammonia flow. The as-grown product is featured by a unique hierarchical tubular structure with BN microtubes covered by BN multiwall nanotubes a few nanometers in diameters. Elegant BN flowers were grown on the same substrate when omitting carbon precursors. These BN micro-/nanotubes open new perspectives in (nano)composite materials with enhanced matrice-to-fibers bonding.

We present here comparative measurements by scanning electron microscopy (SEM) and field emission (FE) of the mechanical resonances of singly clamped, batch-fabricated SiC nanowires as well as an extensive theoretical description. The mechanical resonances of six nanowires, which were glued to the ends of tungsten support tips, were electrostatically excited and detected visually in the SEM configuration and then by FE microscopy image processing. The large tensions generated by electric field pulling in FE that tune the resonance frequencies and the complex boundary conditions at both the free and clamped nanowire ends complicate the interpretation of the resonance frequencies necessary for extracting intrinsic mechanical parameters. Our model fully takes into account these effects and results in an excellent agreement with the measured resonance modes in both configurations. Analytical solutions with their validity conditions are given for the low and high tension ranges and semianalytical solutions for the intermediary range. Viable estimates of Young's modulus are thus achieved for the ultra high vacuum (UHV) environment of FE. Progressive in situ cleaning was performed in the FE-UHV configuration in the range of 600-1350 K, which increased the Q factor of the first mechanical resonance by up to x100 and did not alter the value of the Young's modulus measured previously in the SEM configuration. The agreement between the SEM and FE techniques means that we can now profit from their different strengths for better understanding the mechanics of nanowires and nanotubes.