Pyrolysis behavior of poly[(n-propylamino/methylamino)borazine]

A processable poly[(n-propylamino/methylamino)borazine] (PPAB) has been pyrolyzed in Ar to study its thermal decomposition behavior. The structural evolution and chemical composition change during pyrolysis were characterized by chemical analysis, thermal gravimetry–mass spectrometry (TG–MS), Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The results indicated that the polymer-to-ceramic transition of PPAB involved two steps. Below 400 °C, the gas species were mainly methane and methylamine, while from 400 to 900 °C those were methane and n-propylamine. The PPAB displayed a ceramic yield of 52 wt% at 1000 °C and the pyrolyzed product was amorphous boron nitride (BN) with a small quantity of carbon impurity, in presence as CC and CN bonds. Moreover, for the pyrolyzed product, further heat treatment resulted in the occurrence of a transformation from amorphous to turbostratic.     

BN analogues of graphene: On the formation mechanism of boronitrene layers — solids with extreme structural anisotropy

Boron nitride is an extremely useful material for applications in material sciences and appears in a manifold of crystalline modifications, with hexagonal and cubic boron nitride as most prominent substances. In hexagonal boron nitride, threefold coordinated boron and nitrogen atoms form two-dimensional layers, which resemble the boron nitride analogue of graphene layers, and we refer to this two-dimensional network as boronitrene layers. So far, there is little knowledge about the elementary growth reactions of these boronitrene layers.Here we show that it is possible to regenerate a previously prepared 14 × 14 BN/13 × 13 Rh(111) superstructure [obtained by CVD of borazine (B₃N₃H₆)] after an oxidation step from an unordered monolayer of a boron–oxygen compound by chemical reactions induced by ammonia as nitrification agent on the surface. The individual steps of the experiment were controlled by a LEED- and XPS- analysis and indicate that the original BN superstructure can be recovered without traceable amounts of oxygen in the film. The presented results indicate that highly mobile species from the B–N–O–H system can be considered as surfactants in the elementary formation and degradation reactions of highly ordered boronitrene layers. An understanding of these reactions is a fundamental issue in tuning the crystal growth and quality of the BN phases.     

Chemical vapor deposition of pyrolytic boron nitride ceramics from single source precursor

Pyrolytic boron nitride ceramics were prepared on graphite substrates from borazine as the single source precursor by hot-wall chemical vapor deposition in deposition temperature range from 1300 °C to 1600 °C with a total pressure of 200 Pa. The chemical composition and the effect of deposition temperature on the morphology, phase, and structure of the pyrolytic boron nitride were investigated. A high purity product with stoichiometric B/N ratio is obtained. The deposition surface of the product exhibited a pebble-like structure, and the fracture surface showed an apparent laminar structure having a preferential (002) orientation parallel to the surface of the substrate at temperatures above 1400 °C. The product contained some turbostratic and amorphous boron nitride as evidenced from XRD and FTIR examinations. With the increase of deposition temperature, the crystallinity of the pyrolytic boron nitride increased with the turbostratic and amorphous boron nitride turned into hexagonal structure, and the crystallinity of the product became higher.     

Structural and thermal properties of boron nitride nanoparticles

In the present study, our main motivation was to investigate the structural and thermal stability of BN nanoparticles (B_1.0N_0.9-NPs) produced by spray-pyrolysis (SP) of borazine at 1400 °C by thermogravimetric experiments and X-ray diffraction. We observed that B_1.0N_0.9-NPs are relatively stable in air below 850 °C in which only oxidation of the NP surface proceeded. Above 850 °C, the powders started to strongly react with air due to bulk oxidation. Under nitrogen, they appeared to be less stable than plate-like BN synthesized from borazine at 1400 °C through conventional pyrolysis. This is related to the low degree of crystallization of B_1.0N_0.9-NPs that clearly affects their stability. Using a post-pyrolysis treatment at 1400 °C, B_1.0N_0.9-NPs remained stable up to 1600 °C similarly to plate-like BN. However, above 1600 °C, a relatively fast weight loss occurred for B_1.0N_0.9-NPs, whereas plate-like BN remained stable up to 1800 °C. This indicated that their lower size also affects their high temperature thermal behavior.     

Characterisation of BN-supported palladium oxide catalyst used for hydrocarbon oxidation

Hexagonal boron nitride (BN), with a graphite-type structure and with surface area of 184 m²/g was used as a support for palladium oxide (PdO/BN). About 1 wt% of palladium was deposited on BN by incipient wetness method by using palladium nitrate as precursor. The support and the catalyst were characterized by BET, TEM, XRD, XPS, ICP, TG, TPD, in situ ac electrical conductivity and by ammonia adsorption microcalorimetry. Oxidation of propylene and methane were used as model reactions to study the catalytic properties of the PdO/BN catalyst. The BN support was practically inactive in propylene oxidation up to 400 °C, while the onset of the oxidation was detected around 200 °C on PdO/BN, which points out the role of the palladium in adsorption of the reactive hydrocarbon species. At the same time, this temperature is coincident with the increase of the electronic conductivity on both BN and PdO/BN samples, which is important for oxygen adsorption/activation as electrophilic species. The catalyst was inactive in methane oxidation below 400 °C. Only about 2% CH₄ conversion was observed at 400 °C, increasing sharply up to 87% at 550 °C with methane transformation only to CO₂ and water.     

Thermal stability of mesoporous boron nitride templated with a cationic surfactant

The preparation of mesoporous boron nitride by using tris(monomethylamino)borazine (MAB) as boron nitride source and cetyl-trimethylammonium bromide (CTAB) as structurating agent is reported. The X-ray diffraction, TEM and pore size analysis show that highly porous boron nitride (specific surface area of 800 m²/g and mesoporous volume of 0.5 cm³/g) is synthesized with mesopores of 6 nm in diameter. Moreover, the mesoporosity is conserved up to 1600 °C under an inert atmosphere.     

Complete characterisation of BN fibres obtained from a new polyborylborazine

A new polymer was prepared at room temperature from a di-chloroborazine and a reactive aminoborane. It displays borazine rings unambiguously linked through three atoms N–B–N bridges. This connecting mode was evidence by ¹⁵N solid state NMR. This polyborazine was processed into a continuous polymer fibre of about 21 μm diameter, which was subsequently heat-treated under NH₃/N₂ up to 1800 °C for conversion into BN fibres. The achievement of hexagonal boron nitride was confirmed by X-ray diffraction and Raman spectroscopy. Tensile tests were carried out on the ceramic fibres. The average tensile strength is about 1000 MPa and the Young's modulus is close to 200 GPa. Structural characterisation of the BN fibres was undertaken by polarised light and transmission electronic microscopies.     

Atomic structures of bamboo-type boron nitride nanotubes with cup-stacked structures

Bamboo-type boron nitride (BN) nanotubes with cup-stacked structures were produced by annealing of Fe₄N and boron particles at 1000 °C for 5 h in nitrogen atmosphere. The iron nitride particles were reduced to α-Fe. Atomic structure models and the formation mechanism were proposed from the results of high-resolution electron microscopy (HREM), image simulations and molecular mechanics calculations. The nanotube structures would be stabilized by stacking of BN cup-layers.     

Reactive sintering cBN-Ti-Al composites by spark plasma sintering

When synthesizing polycrystalline cubic boron nitride (PcBN) at normal pressure, cBN had a trend of hexagonal transformation, which reduces the hardness and strength of PcBN. The cBN-Ti-Al composite was prepared by spark plasma sintering with introducing Ti and Al to absorb hexagonal boron nitride (hBN) transformed from cBN. By the results of X-ray diffraction (XRD), Ti and Al reacted with BN and forming TiN, TiB₂, and AlN, which combined cBN as the binder by chemical bonding. The mechanical properties of the prepared composite increased as the increment of sintering temperature. The threshold temperature for preparing composite without hBN phase was at 1400 °C. The composite with optimal mechanical properties was prepared at 1400 °C, and the relative density, the bending strength, hardness, and fracture toughness were 98.9 ± 0.1%, 390.7 ± 4.4 MPa, 14.1 ± 0.5 GPa, and 7.6 ± 0.1 MPa·m^0.5, respectively.     

Synthesis and ceramic conversion of a novel processible polyboronsilazane precursor to SiBCN ceramic

Polyboronsilazane (PBSZ) precursors for SiBCN ceramics were prepared by using 9-borabicyclo-[1,3,3] nonane (9-BBN) and copolysilazanes (CPSZ) as starting materials, involving the hydroboration reaction between vinyl groups of PSZ and BH groups of 9-BBN under mild conditions. The as-synthesized PBSZ was obtained as a soluble liquid, which was characterized by FT IR and NMR. The polymer-to-ceramic conversion of PBSZ at a ceramic yield of 62.2–79.9% was investigated by means of FT IR and TGA. The crystallization behavior and microstructures of PBSZ-derived SiBCN ceramics were studied by XRD, SEM and HRTEM. The SiBCN ceramic began to crystallize at 1600 °C. Further heating at 1800 °C induced partial crystallization to give mixed XRD patterns for SiC, Si₃N₄, and BN(C). It is observed that the introduction of boron improves the thermal stability of SiBCN ceramics, especially under high temperatures of 1600–1800 °C. In addition, the introduction of boron significantly improves the ceramic density while inhibits the SiC crystallization.     

The equilibrium phase boundary between hexagonal and cubic boron nitride

The equilibrium phase boundary between hexagonal and cubic boron nitride was determined over the pressure range 3–6 GPa and the temperature range 1200–2200°C. Absolute pressure and temperature were estimated based on the minimum P–T point of diamond formation in the conventional metal catalyst system using the Kennedy–Kennedy equilibrium line between graphite and diamond. Above 3.8 GPa, we determined the phase boundary by detecting the formation of cubic BN in the catalyst-containing system and by the reverse transformation from cubic BN to hexagonal BN. Below 3.8 GPa, we determined the boundary by careful observation of the transformation behavior of cubic BN powder. The rate of phase transformation from cubic BN to hexagonal BN was evaluated from the ratio of X-ray diffraction peak intensities of both phases. We found that the amount of cubic BN phase clearly changed at the phase boundary. Two sets of the results could be plotted with a uniquely determined boundary line expressed by the equation P(GPa)=T(°C)/465+0.79, which is located about 300°C higher at 5 GPa than the line determined by Bundy and Wentorf in 1963.     

Room and high temperature flexural failure of spark plasma sintered boron carbide

Dense (95–98.6%) bulk boron carbide prepared by Spark Plasma Sintering (SPS) in Ar or N₂ atmospheres were subject to three-point flexural tests at room and at 1600 °C. Eight different consolidation conditions were used via SPS of commercially available B₄C powder. Resulting specimens had similar grain size not exceeding 4 µm and room-temperature bending strength (σ_25 °C) of 300–600 MPa, suggesting that difference in σ_25 °C is due to development of secondary phases in monolithic boron carbide ceramics during SPS processing. To explain such difference the composition of boron carbide and secondary phases observed by XRD and Raman spectroscopy. The variation in intensity of the Raman peak at 490 cm⁻¹ of boron carbide suggests modification of the boron carbide composition and a higher intensity correlates with a higher room-temperature bending strength (σ_25 °C) and Vickers hardness (HV). Secondary phases can modify the level of mechanical characteristics within some general trends that are not dependent on additives (with some exceptions) or technologies. Namely, HV increases, σ_25 °C decreases, and the ratio σ_1600 °C/σ_25 °C (σ_1600 °C – bending strength at 1600 °C) is lower when fracture toughness (K_IC) is higher. The ratio σ_1600 °C/σ_25 °C shows two regions of low and high K_IC delimited by K_IC=4.1 MPa m^0.5: in the low K_IC region, boron carbide specimens are produced in nitrogen.     

Room temperature synthesis of boron nitride thin films by dual-ion beam sputtering deposition

Boron nitride (BN) films are prepared by dual-ion beam sputtering deposition at room temperature (~25 °C). An assisting argon/nitrogen ion beam (ion energy E_i=0–300 eV) directly bombards the substrate surface to modify the properties of the BN films. The effects of assisting ion beam energy on the characteristics of BN films were investigated by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, Raman spectra, atomic force microscopy, and optical transmittance. The density of the B–N bond in the film increased with the increase in assisting ion beam energy. The highest transmittance of more than 95% in the visible region was obtained under the assisting ion beam energy of 300 eV. The band gap of BN films increased from 5.54 eV to 6.13 eV when the assisted ion-beam energy increased from 0 eV to 300 eV.     

Synthesis of boron nitride nanotubes by catalytic pyrolysis of organic-inorganic hybrid precursor

Large quantities of hexagonal boron nitride (h-BN) nanotubes (BNNTs) with high purity have been successfully synthesized under ammonia gas flow at 1200 °C via catalytic pyrolysis of organic-inorganic hybrid precursor which was pre-prepared through a wet chemistry method at 95 °C. Several characterizations, such as SEM, TEM, XRD, FTIR, EDS, XPS and SAED measurements, were used to confirm the morphology, composition and crystalline structure of the as-synthesized powders. It was observed that the obtained product was a kind of nanotubes (NTs) in hexagonal BN phase with a curved shape and smooth surface. The diameter of BNNTs was distributed in a range of 60–200 nm while its length was about tens of microns. The possible growth mechanism of BNNTs was also proposed in this paper.     

Highly-dispersible boron nitride nanoparticles by spray drying and pyrolysis

A spray drying and pyrolysis synthesis route was developed and it successfully prepared boron nitride (BN) nanoparticles with high dispersivity. During the experiment, the extremely rapid drying of the boric acid/urea solution during the spray-drying process resulted in the formation of homogeneous precursors, which was the key for the final pyrolysis synthesis of BN nanoparticles with high dispersibility and uniform diameters (~20 nm). Compared with boron nitride synthesized without using spray drying, the as-prepared BN nanoparticles possess higher specific surface area (145.01 m² g⁻¹) and larger pore volume (0.41 cm³ g⁻¹). Especially, we used the BN nanoparticles as lubricant and incorporated it into the liquid paraffin (LP). The experiment results show that the LP presents outstanding antifriction properties for a BN content of 1.5 wt%. These results suggest that the h-BN nanoparticles have significant potential applications in the field of tribology.     

B4C/metal boride composites derived from B4C/metal oxide mixtures

This study examines the reactions occurring from room temperature to 2180 °C during the heating under argon of mixtures of B₄C and metal oxides, as well as the properties of the ceramic composites prepared by these reactions. The cations of the oxides investigated, belonged to the transition metal and to the lanthanide groups. The mixtures underwent solid-state reactions in the range between 1100 and 1900 °C. These reactions resulted in composites in which the metal borides and B₄C are the majority phases. The boron carbide/metal boride(s) mixtures resulted from these reactions exhibited a sintering aptitude significantly higher than that of pure boron carbide. The improvement in the sintering aptitude was proportional to the oxide content present in the initial mixture, up to an upper limit. B₄C/boride(s)-type composites, exhibiting bulk densities ≥97% TD, could be prepared for certain compositions by pressureless heating at 2180 °C. The ceramic parts prepared under these conditions are characterized by strength and hardness values similar to those determined for pure boron carbide.     

Shaping potentialities of aluminum nitride polymeric precursors: Preparation of thin coatings and 1D nanostructures in liquid phase

The present paper is focused on the synthesis of a series of poly[N-(alkylimino)alanes] of the type [HAlNR]_n as preceramic polymers for the preparation of aluminum nitride (AlN). Polymers were characterized by means of Fourier transform infrared spectra (FT-IR), liquid-state ¹H and ¹³C NMR spectrometry and elemental analyses. The polymers were prepared in different physical states going from viscous liquid to solid (soluble and/or fusible) compounds with the decrease of the carbon content in the polymer chain. Such properties offer potentialities in the preparation of complex forms of ceramics including thin coatings and 1D nanostructures. AlN thin coatings and 1D nanostructures were obtained from a solution of poly[N-(isopropylimino)alane] in toluene followed by heat-treatment in flowing ammonia up to 1000 °C resulting in a ceramic yield of 50.6%. Subsequent heat-treatment to 1800 °C in flowing nitrogen allowed the production of crystalline AlN coatings and nanorods identified by Raman spectrometry and X-ray diffraction.     

Electrical and mechanical properties of electrically conductive adhesives from epoxy,micro-silver flakes,and nano-hexagonal boron nitride particles after humid and thermal aging

In this study, we incorporated micro-silver flakes and nano-hexagonal boron nitride (BN) particles into a matrix resin to prepare electrically conductive adhesives (ECAs). The humid and thermal aging results under a constant relative humidity level of 85% at 85 °C revealed that the aged ECAs containing 3 wt% of nano-hexagonal BN particles had high reliability. The contact resistance was low and the shear strength high. Nano-hexagonal BN particles have a good effect on the reliability of ECAs that can be used to improve the properties of ECAs.     

Fabrication and properties of borazine derived boron nitride bonded porous silicon aluminum oxynitride wave-transparent composite

Boron nitride bonded porous silicon aluminum oxynitride composite was fabricated by gel-casting, precursor infiltration and pyrolysis process, and the composition, microstructure, mechanical and dialectical properties of the composite were characterized. The results show that the composite is comprised of β-SiAlON (z = 3) synthesized from mixed ceramic powders, and continuous h-BN pyrolyzed from borazine, with a relatively high porosity of 24.21%. The flexural strength, elastic modulus and fracture toughness of the composite are 178.58 MPa, 75.51 GPa and 4.54 MPa m^1/2, respectively. The sintering shrinkage of SiAlON ceramics can be greatly decreased by the borazine infiltration and pyrolysis process. The existence of h-BN phase and the high porosity reduce the values of dielectric constant and loss tangent of the composite, which are 3.51–3.69 and 0.9–3.1 × 10⁻³ at the frequency from 7 to 18 GHz with the elevating temperature from 25 to 1200 °C.     

Mechanical properties and microstructure of porous BN–SiO2–Si3N4 composite ceramics

Porous silicon nitride (Si₃N₄) ceramics incorporated with hexagonal boron nitride (h-BN) and silica (SiO₂) nanoparticles were fabricated by pressureless-sintering at relatively low temperature, in which stearic acid was used as pore-making agent. Bending strength at room and high temperatures, thermal shock resistance, fracture toughness, elastic modulus, porosity and microstructure were investigated in detail. The mechanical properties and thermal shock resistance behavior of porous Si₃N₄ ceramics were greatly influenced by incorporation of BN and SiO₂ nanoparticles. Porous BN–SiO₂–Si₃N₄ composites were successfully obtained with good critical thermal shock temperature of 800 °C, high bending strength (130 MPa at room temperature and 60 MPa at 1000 °C) and high porosity.