Open Ceramics

The thermal stability, elemental/phase composition evolution and crystallization behaviour of Si–B–C–N ceramics obtained by the pyrolysis of a series of poly (vinylmethyl-co-methyl)silazanes displaying various boron contents were investigated through annealing in the temperature range 1000–1800 ​°C under nitrogen atmosphere. The increase of the boron content in the early stage of the process involved the nucleation of β-SiC, inhibited the crystallization of α-Si3N4 and modified the activity of the sp2-hybridised carbon phase in the derived ceramics obtained at 1000 and 1400 ​°C. At 1800 °C, low boron content Si–B–C–N ceramics gradually evolved toward a major SiC phase mainly formed via the carbothermal reaction of amorphous Si3N4 whereas high boron content Si–B–C–N ceramics led to highly stable materials with a complex microstructure made of SiC, Si3N4 and a BN-rich B(C)N phase that inhibited the activity of sp2-hybridised carbon toward the carbothermal reaction of amorphous Si3N4 and significantly reduced the SiC crystallization process

Mousses syntactiques : materiaux composites pour grandes profondeurs

SIGLECNRS TD 15166 MIC / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

New BN-based ceramics from organometallic precursors for plasma confinement : application to Hall-effect thruster

L’objectif de cette thèse est de développer une nouvelle génération de matériaux céramiques de confinement plasma pour les moteurs à effet Hall en mettant en oeuvre la voie PDCs pour polymer-derived ceramics ou voie des polymères précéramiques.Un état de l’art des différents matériaux de confinement et un bilan des travaux précédemment menés sur ce sujet ont permis de déterminer les paramètres essentiels de tels matériaux et de s’orienter vers l’utilisation des polymères précéramiques commeprécurseurs des matériaux envisagés. Après avoir détaillé les différents protocoles permettant de modifier chimiquement un polymère commercial avec le bore, de mettre en forme les composés obtenus puis de réaliser la pyrolyse pour générer la céramique, des pièces denses céramiques Si-B-(C)-N ont été réalisées avec une teneur variable en bore. Une étude complète allant de la structure chimique des polymères jusqu’aux propriétés des céramiques résultantes a permis de sélectionner la formulation optimale du polymère comme précurseur de céramique. Des composites c-BN/Si-B-(C)-N ont ensuite été préparés parajout de charges, puis mis en forme et caractérisés avant de procéder à un changement d’échelle visant à préparer des bagues céramiques de taille moteur. La dernière partie consiste en une ouverture sur l’utilisation des polymères précéramiques pour la réalisation de pièces denses Si-Al-(C)-N de composition contrôlée avec en particulier une étude sur l’impact de l’aluminium sur les propriétés des polymères et des céramiques.The main objective is here to develop a new generation of ceramic materials used for plasma confinement in Hall-effect thrusters using the PDCs (polymer-derived ceramics) route. A state of the art of the different confinement materials and a review of the previous work done on this topic allowed to determine the key parameters of such materials and to move towards the use of preceramic polymers as precursors of the materials envisaged. After having detailed the various protocols used to chemically modify a commercial polymer with boron, to shape the compounds obtained and then to convert the polymers into ceramics bypyrolysis, Si-B-(C)-N dense ceramic pieces have been produced with various boron content. A complete study from the chemical structure of the polymers to the properties of the resulting ceramics allowed selecting the optimal formulation of the polymer as a ceramic precursor. c-BN/Si-B-(C)-N composites were then prepared by filler addition, then shaped and characterized before a scale-up to prepare engine-size ceramic rings. The last part consists of an opening on the use of the PDCs route for the realization of dense Si-Al-(C)-N pieces of controlled composition, with a study of the impact of aluminum on the polymers andceramics properties

Mousses syntactiques : materiaux composites pour grandes profondeurs

SIGLECNRS TD 15166 MIC / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Dense SilicoBoron CarboNitride pieces: from Chemistry and Processing of Boron-Modified Polycarbosilazanes to preparation and characterization of final materials

International audienceSilicon-based non-oxide ceramics (SiC, Si3N4) have attracted much attention, primarily due to their good mechanical and chemical properties, and also their reliability at room and elevated temperatures. They have great potential for many industrialuses as engineering components. The addition of a second ceramic (nano)phase to SiC or Si3N4leads to materials which promise applications in many fields and offer solutions forthe market demands. However, the preparation of these materials is still a challenging task according to the fact that conventional processes unavoidably lead to size and structure inhomogeneities of the different phases and presence of impurities which affect the properties. Here, we propose an alternative strategy using a “ceramicthrough chemistry” concept. The Polymer-Derived Ceramics (PDCs) route is an attractive means for the design of advanced ceramics; in particular in non-oxide systems. Preceramic polymers are of great interest as they allow obtaining multi-element ceramics with controlled chemical composition and depending on the composition, they can provide high temperature resistant materials.In this study, boron is added to polycarbosilazanes to obtain silicoboroncarbonitride (Si/B/(C)/N) ceramics after pyrolysis.In this presentation, we will investigate through FTIRandsolid-state NMR, the chemistry of boron-modified polycarbosilazanes,as well as their pyrolysis behavior combining TG experiments and solid-state NMR. By controlling the boron content in the polymer at molecular scale, we can deliver after pyrolysis dense Si/B/(C)/N materials with tailored properties. The high temperature behavior is investigated by thermogravimetric analysis, XRD, elementalanalysis and Raman spectroscopy and we show how the boron content as well as the nature of the atmosphere affect the structural evolution of the Si/B/(C)/N phase at high temperature

From Chemistry to Processing of Boron-Modified Polycarbosilazanes: Toward the Preparation of SilicoBoron CarboNitride Ceramics

National audienceSilicon-based non-oxide ceramics (SiC, Si3N4) have attracted much attention, primarily due to their good mechanical and chemical properties, and also their reliability at room and elevated temperatures. They have great potential for many industrial uses as engineering components. The addition of a second ceramic (nano)phase to SiC or Si3N4leads to materials which promise applications in many fields and offer solutions for most of the market demands. However, the preparation of these materials is still a challenging task according to the fact that the conventional processes unavoidably lead to size and structure inhomogeneities of the different phases and presence of impurities (because of the use of sintering additives to consolidate the materials) which affect the properties. Here, we propose an alternative strategy using a “ceramic through chemistry” concept. The Polymer-Derived Ceramics (PDCs) route is an attractive means for the design of advanced ceramics; in particular in non-oxide systems. Preceramic polymers are of great interest as they allowed obtaining multi-element ceramics with controlled chemical composition and depending on the composition, they can provide high temperature resistant materials. The principle consists to incorporate one or more elements in the precursor (molecular or polymeric) at molecular scale to ensure compositional homogeneity. For example, boron can be added by specific reactions to polycarbosilazanes to be found after pyrolysis in the final ceramic, i.e., silicoboron carbonitride (Si/B/C/N), in the desired proportion. The type of backbone and functional side chains of precursor molecules influences the ceramic yield, the chemical composition, and the microstructure of Si/B/C/N materials.This is the objective we have fixed in our study. In particular, we demonstrate through FTIR, NMR that we can control the addition of boron in a particular way to deliver after pyrolysis Si/B/C/N materials with tailored structural properties. Furthermore, the boron content plays a key role in the processing of boron-modified polycarbosilazanes. This is demonstrated in the present paper

Design, Processing and Application of Polymer-Derived Single- and Multi-Phase Ceramics in the Si-X-C-N (X = Al, B and/or transition metals) System

International audienc

Design, Processing and Application of Polymer-Derived Multi-Element Ceramics in the Nitride, CarboNitride and Carbide Systems

International audienc

Multi-Component Ceramics via the Polymer-Derived Ceramics Route: From Design to Application

International audienc

Crosslinking chemistry of poly(vinylmethyl-<i>co</i>-methyl)silazanes toward low-temperature formable preceramic polymers as precursors of functional aluminium-modified Si–C–N ceramics

International audienceCrosslinking chemistry of a liquid poly(vinylmethyl-co-methyl)silazane with an alane hydride-based complex according to Si : Al ratios varying from 5 to 2.5 has been investigated in detail through the characterization of the as-obtained polymers using solid-state NMR, FT-IR and elemental analyses. This reaction allows tailoring the chemical and physical properties of the neat liquid polysilazane while extending its processability to lead to a series of low-temperature formable aluminium-modified polysilazanes. Structural models have been established based on solid-state NMR spectroscopy. Then, pyrolysis under nitrogen occurring the conversion of polymers into ceramics has been studied by coupling TG experiments with FTIR of pyrolysis intermediates. Pyrolysis at 1000 °C leads to X-ray amorphous Al-modified silicon carbonitride materials with higher ceramic yields compared to the materials obtained from the neat polysilazane. However, the increase of the ceramic yield is minimized with the decrease of the Si : Al ratio from 5 to 2.5 in the as-obtained polymers. This is due to the introduction of –NR3 (R = CH3 and C2H5) units as side groups during the polymer synthesis which are released in the low temperature regime of the pyrolysis. The structural evolution of the amorphous network of ceramics has been studied by annealing up to 1800 °C though X-ray diffraction and Raman spectroscopy. Such studies point out that samples remain amorphous even after annealing at 1400 °C (low Si : Al ratio) and 1600 °C (high Si : Al ratio) before forming Si3N4/SiC/AlN and AlN/SiC/C composites after annealing at 1800 °C depending on the Si : Al ratio fixed in the early stage of the process. Dense pieces could be prepared from these low-temperature formable polymers. The latter, especially those containing a certain portion of –NR3 (R = CH3 and C2H5) units acting as plasticizing groups during the process, display appropriate requirements for pressing at low temperature forming dense pieces with hardness and Young's modulus as high as 21.7 GPa and 192.7 GPa, respectively