The role of P 3s2 lone pair (E) in structure, properties and phase transitions of black phosphorus. Stereochemistry and ab initio topology analyses

An approach merging crystal chemistry and density functional theory (DFT) electron localization function (ELF) taking P 3s2 lone pair (E) into account induces a full renewal of stereochemistry of black phosphorus, its crystal network evolutions and phase transitions under increasing pressures from atmospheric up to 32 GPa. Orthorhombic (Cmce) black P at ambient pressure, shows a packing of puckered [P]n layers - orthogonal to [010] - separated by a large free interspace (3.071 Å), which actually is partially filled by lone pairs (E) (P-E ~ 0.8 Å). Each P exhibits its lone pair pointing outside the [P]n layer, sandwiching it between two [E]n layers into a new stacking sequence … [EP2E]n … denoted O-[PE]n. The free interspace between [EP2E]n layers is much smaller 1.858 Å but allows sliding along [001]. The pressure evolving up to 2.66 GPa, all structural details have been followed and reported, including the layer thickness reduction along [010] and the sliding along [001] of consecutive layers. A mechanism for the phase transition occurring around 5.5 GPa is proposed. Depicted in the trigonal system the new layered phase R-[PE]n involves a bond rearrangement through E-E layer in zigzag phosphorus layers and P-E rotation and alignment with the A axis. Now, the phosphorene layers have P-E patterns oriented towards each other in their interspace. A very particular phenomenon occurs around ~11 GPa the lone pair centroid Ec (P-Ec = 0.73 Å) splits into three partially occupied sites Ed around the A axis which explains observed variations in properties at this critical pressure. So, we claim that there are two trigonal phases, R1-[PE] up to 11 GPa followed by a second form R2-[PE] directly caused by lone pair displacement from Ec to Ed and its influence on layer stacking. A further layer sliding brings the phosphorus atomic layers close enough to each other to establish new P-P bonds and then to cause an ultimate transition to cubic system, with a new structure, isostructural to Po. The mechanisms of the transitions are detailed

La croissance des pyrocarbones

Cet article fait la synthèse des recherches récentes dans le domaine des pyrocarbones. Ce sont les formes solides de carbone qui se déposent sur une surface chaude par craquage d'hydrocarbures liquides ou gazeux au-delà de 900 °C. Les applications touchent les matériaux composites, les biomatériaux ou les applications nucléaires. Très récemment, une étape importante a été franchie dans la connaissance de ces carbones grâce à une démarche pluridisciplinaire. Il en résulte une classification basée sur les mesures des défauts de réseau et d'anisotropie par spectroscopie Raman. Elle permet de relier de façon satisfaisante les mécanismes de croissance, les structures et les propriétés des pyrocarbones de basse température

High-flux sublimation of a 3D carbon/carbon composite: surface roughness patterns

3D carbon-fibre reinforced carbon composites (3D ) are used as thermal protection systems for atmospheric re-entry, where they are exposed to strong ablation. Particularly, sublimation of the carbonaceous material plays an important role during the re entry. To study this, an arc image furnace under controlled Argon flow is used, with heat fluxes of 8 MWm−2 to 10 MWm−2. The furnace and the sample thermal response have been numerically simulated prior to the experiments and match in-situ temperature measurements. Scanning electron microscopy and 3D profilometry with digital optical microscopy were used in order to characterise the epi-macro-structural and the epi-micro-structural roughness of the composite surface, evidencing a faster recession of the fibres as compared to the matrix. Carbon nanotextures have been assessed by using High-Resolution Transmission Electron Microscopy and Polarised Light Optical Microscopy, showing that the matrix is more organised than the fibre. This can explain the “inverse” behaviour under sublimation as compared to oxidation. The results have been qualitatively interpreted using numerical simulation of differential surface recession

Low Temperature Pyrocarbon : a review

International audienceThis article is a synthetic survey of the recent researches in the field of pyrocarbons. Pyrocarbon is the form of carbon which deposits on hot surfaces above 900°C by cracking of hydrocarbons. Applications are in the fields of composite materials, nuclear reactors or biomaterials. Very recently, an important step was reached concerning the understanding of the growth processes. A classification of the low temperature pyrocarbons is introduced. It is based on the measure of carbon defects and anisotropy, using Raman spectroscopy. It provides a comprehensive relationship between growth mechanisms, structure and properties of pyrocarbons

Rippled nanocarbons from periodic arrangements of reordered bivacancies in graphene or SWCNTs

We report on various nanocarbons formed from a unique structural pattern containing two pentagons, three hexagons and two heptagons, resulting from local rearrange- ments around a divacancy in pristine graphene or nanotubes. This defect can be inserted in sheets or tubes either individually or as extended defect lines. Sheets or tubes containing only this defect as a pattern can also be obtained. These fully defective sheets, and most of the tubes, present a very pronounced rippled (wavy) structure and their energies are lower than other structures based on pentagons and heptagons published so far. Another particularity of these rippled carbon sheets is their ability to fold themselves into a two-dimensional porous network of inter- connected tubes upon heat treatment as shown by hybrid Monte Carlo simulations. Finally, contrary to the common belief that pentagon/heptagon based structures are metallic, this work shows that this defect pattern should give rise to semi-metallic conduction

Composites à matrice céramique industriels et commerciaux pour le chemisage d’alvéole de stockage de déchets radioactifs

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A flash characterisation method for thin cylindrical multilayered composites based on the combined front and rear faces thermograms

The flash method, known as the most used experimental technique to measure the thermal diffusivity of solids, has been adapted to cylindrical highly diffusive and heterogeneous multilayered samples. In order to overcome the heterogeneities issue and give a more accurate estimation of the sample thermal properties, the front face and rear face thermal responses have been simultaneously recorded using an infrared camera. A homogeneous monolithic SiC cylinder has been tested to validate this experimental method which has finally been applied to 2D bi-layered SiC$_f$/SiC cylinders used for nuclear fuel cladding

Optimal thickness of porous micro-electrodes operating a single reduction reaction

In the recent decades, porous electrodes with high specific surface, which are of major interest for the design of miniaturized electro-devices such as bio-batteries, have received a lot of attention both from modeling and experimental points of view. Such electrodes may provide much higher electrical current than classical flat electrodes of the same size [1]. Despite considerable progress in porous electrode manufacturing, effective tool for predicting the optimal thickness of such electrodes have not been developed in the literature. To fill this gap, this work aims at developing a thorough approach to estimate the optimal thickness of a porous electrode operating a single reduction reaction. This is achieved by making use of a macroscopic model formally derived from the microscopic coupled diffusion and electrochemical reaction model operating at the pore scale using an upscaling procedure [2] relying on the volume averaging method [3]. The solution of this macroscopic model was successfully compared to the solution of the microscale original model obtained from direct numerical simulations on the one hand and to experimental data on the other hand, validating our theoretical macroscale model. The macroscopic model is subsequently used in the steady-state regime (a situation which is of wide practical interest) to derive an analytical solution for the concentration profile of the dilute species allowing to express the current intensity available at the electrode. On this basis, an optimization procedure is finally proposed to estimate the effective electrode thickness which is defined as the crossover value of two asymptotic regimes characterizing the volume current density dependence upon the electrode thickness. This yields an analytical expression of the optimal thickness that is general for a cylindrical porous electrode, regardless the type of microstructure of the electrode material. An illustration is provided for the type of electrode used in the experiments reported in this work