Philippe Miele

Boron Nitride Fibers Prepared from Symmetric and Asymmetric Alkylaminoborazines

The thermolysis of 2,4,6-[(CH3)2N]3B3N3H3 (1), 2,4-[(CH3)2N]2-6-(CH3HN)B3N3H3 (2), and 2-[(CH3)2N]-4,6-(CH3HN)2B3-N3H3 (3) led to polyborazines 4, 5, and 6 respectively. The polymers display direct B–N bonds between borazinic B3N3 rings and, in addition, a proportion of –N(CH3)– bridges for 5 and 6, as clearly underlined by 13C NMR spectroscopy. Melt-spinning of these three polymeric precursors exemplified that their ease of processing increases in the order 4 < 5 < 6. Nevertheless, polyborazine filaments could be prepared from each of them and a subsequent thermal treatment up to 1800u2009°C resulted in the formation of crystalline hexagonal boron nitride fibers, which were characterized by X-ray diffraction analysis, Fourier transform infrared (FTIR) spectroscopy, and Raman spectroscopy. Scanning electron microscopy (SEM) images showed that the ceramic fibers are circular and dense without major defects. The mechanical properties for 4-derived fibers could not be measured because of their brittleness, whereas measurements on 5- and 6-derived fibers gave tensile strength σR = 0.51 GPa, Young’s modulus E = 67 GPa, and σR = 0.69 GPa, E = 170 GPa, respectively. The improvement in mechanical properties for ceramic fibers prepared respectively from 4, 5, and 6 could be explained to a large extent by the improvement of the processing properties of the preceramic polymers. This evolution could be related to the increased ratio of bridging –N(CH3)– groups between the B3N3 rings within the polymers 4, 5, and 6 and therefore to the functionalities of the starting monomers 1, 2, and 3.

Recent Developments in Polymer‐Derived Ceramic Fibers (PDCFs): Preparation, Properties and Applications – A Review

This review states recent developments in the preparation of ceramic fibers using a chemical procedure denoted as Pyrolysis of Preceramic Polymers. Synthesis strategies which are used to prepare Polymer‐Derived Ceramics (PDCs) fibers from molecular and polymeric precursors are presented by refering to the crystalline boron nitride (BN) fibers and amorphous silicoboron carbonitride (SiBCN) fibers as models. The influence of the structure of the molecular and polymeric precursors, in which specific functional groups and structural motifs are incorporated on the melt‐spinnability, polymer‐to‐ceramic conversion and fiber properties is discussed. A minimum cross‐linking degree combined with the presence of flexible structural patterns and plasticizing functional groups yield controlled melt‐viscoelastic properties and appropriate thermal stability at low temperature to produce high quality fine‐diameter endless green fibers in a stable melt‐spinning process. These structural motifs and functional groups are also capable to undergo cross‐linking reactions and provide fiber curability allowing the polymer backbone to be interlocked, the fiber integrity to be thereby maintained. As‐cured fibers are then pyrolyzed in a controlled atmosphere with retention of the shape to produce the ceramic fibers in the desired composition and structure. †Member of IUF (Institut Universitaire de France)

Controlling the chemistry, morphology and structure of boron nitride-based ceramic fibers through a comprehensive mechanistic study of the reactivity of spinnable polymers with ammonia

The present paper describes an access to polycrystalline boron nitride fibers from poly[B-(methylamino)borazine]. Solid-state NMR and IR spectroscopies, thermo-analytical experiments, SEM and XRD investigations were applied to provide a comprehensive mechanistic study of the fiber transformation and understand the role played by ammonia during the polymer-to-ceramic conversion. It was shown that a typical melt-spinnable poly[B-(methylamino)borazine] (Tsynthesis = 180 °C) is composed of borazine rings connected via a majority of NCH3 bridges and a small proportion of NB3-containing motifs forming a cross-linked network. In addition, a low proportion of peripheral N(H)CH3 groups, which are present in the starting molecular precursor, B-tri(methylamino)borazine, is identified. The polymer is capable of melting without decomposition in flowing nitrogen to produce high quality green fibers at moderate temperature. A curing process of green fibers in flowing ammonia at 400 °C through transamination and condensation forming cross-linked NB3 motifs in the polymer network is seen as the most appropriate way to retain the fiber integrity during the polymer-to-ceramic conversion. The use of ammonia during the subsequent pyrolysis from 400 to 1000 °C allows the basal unit of the “naphthalenic-type structure” of boron nitride to be established at 1000 °C through important structural rearrangements and the crystallization tendency to be improved during further heating from 1000 to 1800 °C. Finally, incorporation of nitrogen using ammonia allows the production of polycristalline fibers in which the stoichiometry approaches that of BN.

High-performance boron nitride fibers obtained from asymmetric alkylaminoborazine

In order to prepare boron nitride fibers displaying improved mechanical properties, different polymers have been prepared from 2-[(CH3)2N]-4,6-(CH3HN)2B3N3H3xa0(1) by varying thermolysis conditions. Analytical data showed that the resulting polyborazines 2, 3, 4 and 5 present polymerisation degrees included between 0.7 and 1.1, and confirmed that they are composed of borazinic rings connected mainly through direct B–N inter-ring bonds and to a lesser extent through –N(CH3)– bridges. Spinning properties of these polymers and ceramisation conditions of the derived crude fibers have been studied. While the spinnability of the higher cross-linked polymer 5 was poor, all of the others displayed very good processing properties as well as no reactivity in spinning conditions allowing the fabrication of high performance boron nitride fibers. Their tensile strengths are over 0.80 GPa and often reach 1.10 GPa. The three main characteristics of the polymeric precursors of fibers, namely polymerisation degree, glass-transition and spinning temperature are shown to be closely related. We observed that when a polymer displays good processing properties, the higher the polycondensation degree is, and the higher the mechanical properties are. A final pyrolysis temperature of 1600 °C is sufficient to ensure the crystallization of the BN fibers obtained.

Direct synthesis of amorphous silicon dioxide nanowires and helical self-assembled nanostructures derived therefrom

Bulk quantities of amorphous silica nanowires and novel braided helical silica nanostructures have been synthesized by a simple and cheap route. Actually, direct thermal treatment of a commercial silicon powder in the presence of graphite yields pure amorphous silica nanowires with lengths up to 500 µm for diameters in the range 10–300 nm. Electron Energy-Loss Spectroscopy (EELS) analysis indicates that the nanowires consist of Si and O elements in atomic ratio 1 ∶ 2, corresponding to SiO2. The formation of silicon dioxide nanowires can be related to the in-situ formation and subsequent decomposition of silicon oxide SiO (g). The nanowires are gathered to form bundles and braid-like nanostructures have been observed in some cases. The formation of these helical nanoobjects results from the self-assemblage of silica nanowires, may be due to the gas flowing during the process.

Direct synthesis of β-SiC and h-BN coated β-SiC nanowires

Abstract β-Silicon carbide (β-SiC) nanowires (NWs) have been grown by thermal treatment of commercial silicon particles disposed in a graphite crucible under nitrogen atmosphere. By the same way, treatment under argon of a mixture of a boron nitride (BN) based powder and silicon particles led to h-BN coated β-SiC nanowires. The structures of both nanoobjects have been investigated by HRTEM, EDX and EELS.

Synthesis, characterization and optical power limiting behaviour of phenylazo- and 4-nitrophenylazo-tetrahydroxytetrathiacalix[4]arene

p-Tetrakis(4-nitrophenylazo)tetrahydroxytetrathiacalix[4]arene and p-tetrakis(phenylazo)tetrahydroxytetrathiacalix[4]arene were prepared and fully characterized using 1H and 13C NMR, mass spectroscopy, thermogravimetric analysis and differential scanning calorimetry. The solid-state structure of p-tetrakis(4-nitrophenylazo)tetrahydroxytetrathiacalix[4]arene was investigated by single crystal X-ray diffraction. It crystallized in the triclinic system (space group: P). z-Scan experiments were performed on the p-tetrakis(4-nitrophenylazo)tetrahydroxytetrathiacalix[4]arene showing non linear absorption due to two photon absorption with a TPA cross-section of about 50xa0×xa010−50xa0cm4xa0s per photon. Optical power limiting measurements on the p-tetrakis(phenylazo)tetrahydroxytetrathiacalix[4]arene nat 532xa0nm (48% linear transmission) was found to limit the energy to ∼9xa0µJ.

Texture, structure and chemistry of a boron nitride fibre studied by high resolution and analytical TEM

Abstract The present work is devoted to a TEM (transmission electron microscopy) study of the texture, structure and chemistry of a boron nitride (BN) fibre. The general structure of the fibre consists of two concentric parts; the near surface region is finely nano-crystallised, while the bulk of the fibre exhibits larger crystallites, although still of nanometric sizes. All grains appear to be randomly oriented with respect to the fibre axis, which makes the mechanical properties of this material remain modest. From a crystallographic point of view, both hexagonal and rhombohedral BN forms have clearly been identified by high resolution TEM (HRTEM). From a chemical point of view, EELS (electron energy loss spectroscopy) analysis shows that the N/B atomic ratio remains close to one, although it tends to decrease slightly from the outer surface to the ‘core’. No significant amount of impurities (e.g. carbon and oxygen) has been detected. The study of the B–K and N–K edges reveals a great similarity between the hexagonal and the rhombohedral forms.

Si3N4BN composites obtained from aminoboranes as BN precursors and sintering aids

Abstract The Si 3 N 4 -BN composite ceramic has been elaborated by combining Si 3 N 4 fine powder with boron nitride provided by the thermolysis of a molecular precursor. The properties of this new type of sample has been compared with those of composite ceramics obtained by the classical hot-pressing method using Si 3 N 4 -BN powders and sintering aids (Y 2 O 3 , Al 2 O 3 ). Giving the best ceramic yield, tris (methylamino) borane (TMB) has been used as BN precursor and sintering aid. Boron nitride formed from TMB thermolysis was poorly crystallized when the hot-pressing was run up to a temperature lower than 1800 °C. The density of the composite ceramic samples obtained from molecular precursors was higher than expected, this could be related to the sintering activation properties of the precursor. Moreover the problems due to the BN platelets orientations were removed. The Vickers hardness was clearly improved for samples without BN platelets, however the bending strength was not increased. The precursor improved some properties of the composite and could be considered as a sintering activator avoiding oxide addition as sintering aid.

Rheological Behavior of Poly[(B‐alkylamino)borazine] in a Fiber Spinning Process

Polymer‐derived ceramics (PDCs) are innovative materials with a wide range of novel applications (micro fibers, protective coatings, porous materials, MEMS). Among high‐performance, non‐oxide ceramics, hexagonal boron nitride offers interesting potentialities as fibrous reinforcing agent for ceramic composites. Boron nitride can easily be produced from preceramic polymers and previous studies have shown the great potential of poly[(B‐methylamino)borazines], called polyMAB as melt‐spinnable precursors for the preparation of this type of ceramics. The goal of the present study is to provide a comprehensive structural and rheological characterization of polyMAB‐type polymers using a combination of thermal, structural, and chemical experiments, as well as rheological investigations and constitutive modeling, in order to predict and control polymer spinnability from the early stage of material formation. The results from dynamic shear rheology are consistent with the rheological behavior observed in spinning. The experimental measurements of the fiber diameter during steady spinning put in evidence the difference in extensional rheological behavior and spinnability between samples. Numerical simulations of 1D model of fiber spinning emphasis the importance of heat transfer at the exit from the die and relevant differences for the polymers under investigation in the evolution of spin extensional viscosity along the fiber. The present work evidences for the first time the rheological behavior of polyMAB samples and the link to their chemistry, especially in relation to the fiber spinning process.