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

Un modèle dynamique simple pour la croissance de polytypes périodiques et désordonnés de SiC en CVD/CVI

The generation of periodic and chaotic polytypes of SiC is described by means of a discrete dynamical model based on the repetition of a function linking the deformations of the surface Si-C bilayer on one hand, and the deformations and relative orientation of the growing bilayer on the other hand. A simplified function acting on only one variable has been determined from results of former studies. The combinatorial study of this model reveals that it can produce any polytypic sequence, either periodic or disordered, by a variation of two parameters. The "routes to chaos" recognized in the bifurcation diagram are, despite their diversity, related to one central combinatorial theorem. The physical bases of the model have been studied using a "cluster" method. The influence of the deformations of the surface bilayer on the orientation choice and the deformations of the new bilayer has been confirmed.La génération de polytypes périodiques et chaotiques du SiC est décrite par un modèle dynamique discret basé sur la répétition d'une fonction reliant les déformations de la bicouche Si-C de surface d'une part et les déformations et l'orientation relative de la bicouche en formation d'autre part. Une fonction simplifiée agissant sur une seule variable a pu être déterminée d'après les résultats d'études antérieures. L'étude combinatoire de ce modèle révèle qu'il peut produire n'importe quelle séquence polytypique, périodique ou désordonnée, grâce à la variation de deux paramètres. Les "routes vers le chaos" qui ont été mises en évidence dans le diagramme de bifurcations sont liées, malgré leur diversité apparente, à un théorème combinatoire unique. Par ailleurs, la nature physique des interactions entre bicouches a été étudiée par une méthode de "clusters". L'influence des déformations de la bicouche de surface sur le choix d'orientation et les déformations de la bicouche nouvelle est confirmée

Un modèle dynamique simple pour la croissance de polytypes périodiques et désordonnés de SiC en CVD/CVI

The generation of periodic and chaotic polytypes of SiC is described by means of a discrete dynamical model based on the repetition of a function linking the deformations of the surface Si-C bilayer on one hand, and the deformations and relative orientation of the growing bilayer on the other hand. A simplified function acting on only one variable has been determined from results of former studies. The combinatorial study of this model reveals that it can produce any polytypic sequence, either periodic or disordered, by a variation of two parameters. The "routes to chaos" recognized in the bifurcation diagram are, despite their diversity, related to one central combinatorial theorem. The physical bases of the model have been studied using a "cluster" method. The influence of the deformations of the surface bilayer on the orientation choice and the deformations of the new bilayer has been confirmed.La génération de polytypes périodiques et chaotiques du SiC est décrite par un modèle dynamique discret basé sur la répétition d'une fonction reliant les déformations de la bicouche Si-C de surface d'une part et les déformations et l'orientation relative de la bicouche en formation d'autre part. Une fonction simplifiée agissant sur une seule variable a pu être déterminée d'après les résultats d'études antérieures. L'étude combinatoire de ce modèle révèle qu'il peut produire n'importe quelle séquence polytypique, périodique ou désordonnée, grâce à la variation de deux paramètres. Les "routes vers le chaos" qui ont été mises en évidence dans le diagramme de bifurcations sont liées, malgré leur diversité apparente, à un théorème combinatoire unique. Par ailleurs, la nature physique des interactions entre bicouches a été étudiée par une méthode de "clusters". L'influence des déformations de la bicouche de surface sur le choix d'orientation et les déformations de la bicouche nouvelle est confirmée

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

Numerical study of effective heat conductivities of foams by coupled conduction and radiation

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Modèle d'écoulement dans les CMCs

On propose un modèle asymptotique pour l'écoulement incompress- ible de l'oxyde produit dans les fissures de compositesà matrice céramique auto- cicatrisants. Le modèle est validé par de simulations d'étalement de gouttesWe propose an asymptotic model for the incompressible flow of the oxide pruduced in cracks of self-healing ceramic matrix composites. The model is validated on simulations of drop spreading

Analytical and Numerical studyof the Densification of Carbon/Carbon Composites by a Film-Boiling Chemical Vapor Infiltration Process

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