Synthesis of Aluminum Nitride/Boron Nitride Composite Materials

Aluminum nitride/boron nitride composite was synthesized by using boric acid, urea, and aluminum chloride (or aluminum lactate) as the starting compounds. The starting materials were dissolved in water and mixed homogeneously. Ammonolysis of this aqueous solution resulted in the formation of a precomposite gel, which converted into the aluminum nitride/boron nitride composite on further heat treatment. Characterization of both the precomposite and the composite powders included powder X-ray diffraction, Fourier transform infrared spectroscopy, and scanning electron microscopy. Analysis of the composite revealed that the aluminum nitride phase had a hexagonal structure, and the boron nitride phase a turbostratic structures.     

Synthesis of fine boron nitride powders by combining direct boron nitridation with carbothermic boron oxide reduction

Fine boron nitride powders were synthesized by combining direct nitridation of boron with carbothermic reduction of boron oxide. It has been established that the heat released in direct nitridation of boron can be utilized to carry out carbothermic reduction of boron oxide in a mode of self-sustained combustion. Intermediate gaseous products were found to play a key role the SHS reactions under study. The article is published in the original.     

Polyethylene/boron nitride composites for space radiation shielding

Composites made with boron might be absorbers of low energy neutrons, and could be used for structural materials for spacecraft. Polyethylene/boron nitride composites were fabricated using conventional polymer processing techniques, and were evaluated for mechanical and radiation shielding properties. The boron nitride powder surfaces were also functionalized to improve interfacial adhesion. Addition of neat boron nitride to an injection molding grade HDPE increased the tensile modulus from 588 to 735 MPa with 15 vol % filler. The bonding of a trifunctional alkoxysilane to the powder surface prior to processing increases the composite modulus to 856 MPa at the same loading. Scanning electron microscopy of fracture surfaces verified that the silane‐treated powders had improved adhesion at the filler/polymer interface. Radiation shielding measurements of a 2 wt % boron nitride composite were improved over those of the neat polyethylene. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

The influence of polyelectrolyte on the synthesis of B4C/BN nanocomposite powders via sol-gel method

In the present study, B₄C-BN nanocomposite powders were synthesized by using the sol-gel method. To investigate the effects of polyelectrolyte on phase content, particle size, and final morphology of synthesized powders different amounts of ammonium polycarboxylate were used as a gel dispersing agent and a nitrogen source. Highly crystalline, sub-micron/micron-sized boron carbide particles with varying morphologies including polyhedral-equiaxed, belt-like, needle-like, and complex-shaped hierarchical structures were produced from the polymeric gel containing glycerine, tartaric acid, and citric acid as carbon sources, and boric acid as boron source. With the addition of ammonium polycarboxylate as a polymeric gel network modifier, nanocomposite powders containing micron-sized polyhedral-equiaxed boron carbide particles and boron nitride nanoflakes were obtained. The results indicated that the particles dimensions, crystallinity, and B₄C to BN phase ratio of the synthesized powders are directly related to the preliminary formation of borate-ammonium and/or amine complexes in the polymeric gel. The SEM inspections revealed that the size of boron carbide particles tends to increase from 2 μm up to 40 μm as a function of ammonium polycarboxylate content. It was also observed that the average size and thickness of boron nitride nanoflakes within the range of 80 nm to 3 μm and 10–150 nm, respectively. B₄C/BN nanocomposite powders were synthesized with up to 32% BN content using a 43 wt% ammonium polycarboxylate additive.     

Machinable α-SiAlON/BN Composites

Dense machinable α-SiAlON/BN composites were fabricated by hot-pressing using turbostratic boron nitride (tBN) obtained from nitridation of melamine diborate. The tBN was added to the starting powders, or introduced as a coating that formed in situ on α-Si₃N₄ carrier powders during nitridation, and was subsequently converted to hexagonal boron nitride (hBN) during hot pressing by solution reprecipitation. These composites maintain high strength at 1000°C and their strength/hardness are much higher than similar composites prepared using commercial hBN powder, which yielded a coarser microstructure. Good machinability was achieved despite a flat R curve.     

Study of fragmentation in cBN powders under ultra-high pressure

Studying the fragmentation law and refinement of cubic boron nitride powder under ultra-high pressure is crucial to producing a high-strength, high-density polycrystalline cubic boron nitride. In this paper, brown and black cBN crystalline powders with different micron sizes were selected as initial raw materials for an ultra-high-pressure simulation experiment. Single and mixed particles were extruded under 80MPa low pressure and 5.5GPa ultra-high pressure at ambient temperature for 1 min. The crushing behavior and particle size distribution of cBN powders with different particle sizes and ratios were investigated using a laser particle size analyzer and scanning electron microscopy. Results revealed no particle breakage or deformation at low pressure, and the compaction density was low. However, under ultra-high pressure, the cBN particles showed cracks, plastic deformation, and fragmentation, resulting in crushed fine particles filling in the voids of coarse particles, which led to a higher pressing density. Small-sized or mixed cBN particles with high density ratios were not easily crushed; the coarser the particle size, the more severe the ultra-high-pressure extrusion and crushing. The pressing density also declined, and brown cBN crystal particles with higher impact toughness demonstrated a lower particle breakage rate. The ultra-high-pressure crushing law should be considered and appropriate binders should be selected to improve the sintering performance of PcBN materials; ultra-high-pressure crushing of cBN powder contributes to cBN-cBN and cBN-M-cBN bonds under high temperatures and ultra-high pressure.     

A new processing aid for the extrusion of polyolefins

The influence of a new processing additive (a composition of fine particles of boron nitride) on the rheology and processability of molten polymers is studied. The equipment used includes both an Instron capillary rheometer with special annular dies (Nokia Maillefer wire coating crosshead) attached to the rheometer and a parallelplate rheometer. Two metallocene polyethylenes with several types of boron nitride powders varying in particle size distribution are tested at various concentration levels. The powder having the finest particle size was found to have the greatest influence in the processing of polyolefins using crosshead dies and tips. As a result, its use eliminates surface melt fracture and postpones the critical shear rate for the onset of gross melt fracture to significantly higher values depending on the additive concentration. The performance of the boron nitride in eliminating melt fracture is compared with that of a typical fluoroelastomer. It is shown that boron nitride is a superior processing aid.     

On some alkali- and alkaline-earth-metal boron nitrides, unsaturated with boron

Three new lithium, magnesium and calcium boron nitrides, namely Ca₆BN₅(L), Mg₆BN₅(H) and Li₃Mg₃B₂N₅(L), were synthesized using a metal nitride–boron nitride sintering technique at low (L) or high (H) pressures. All these new compounds are typical metal boron nitride powders and, like previously known Mg₃BN₃, seem to have the N³⁻ anion in their structures.     

New functional compositions of mixture for superhard composites based on bn blende

Results of a study of promising new compositions for cutting ceramics based on boron nitride blende are presented. The initial abrasive powders and powders of titanium-bearing binder are modified; the steel-cutting properties of the latter are shown to be affected by additional factors.     

Synthesis and Characterization of TiN‐coated Cubic Boron Nitride Powders

Titanium nitride‐coated cubic boron nitride (TiN/cBN) composite powders were prepared by nitridizing TiO₂/cBN powders in a NH₃ flow at 950°C. The TiO₂/cBN powders were synthesized via a sol‐gel process using tetra‐butyl titanate and concentrated‐HNO₃‐treated BN powders as starting materials. The techniques of XRD, SEM, TEM, FT‐IR, and TG‐DTA were used to characterize the products and their intermediates. The cBN powders were uniformly coated with TiN nanoparticles. During the nitridization, the morphology of the TiO₂/cBN powders is unchanged. The TiN/cBN powders can be used as starting materials to prepare polycrystalline cBN compacts, or as reinforcements to strengthen metal‐matrix composites.     

High pressure synthesis of new heterodiamond phase

New heterodiamond phase (structure type of cubic boron nitride) with boron and nitrogen atoms partially substituted by carbon has been synthesized by using high pressure-high temperature treatment of the mixture of boron and C₃N₄ carbon nitride powders. This phase consisted of up to 5 micron-sized individual crystals. The composition of new phase was established with the help of microanalysis and structure refinement.     

Reaction bonded aluminum oxide composites containing cubic boron nitride

The properties of alumina can be improved by incorporating second phases like zirconia or carbides. It has also been reported that reaction bonded aluminum oxide (RBAO) process can be used as a host matrix for large scale second phase reinforcement particles without causing harmful residual stresses normally encountered with shrinking matrix materials. The potential of using cubic boron nitride in reinforcing alumina has not yet been reported. This work reports some improvements in mechanical properties of alumina achieved by incorporating cubic boron nitride particles in a reaction bonded aluminum matrix. Attrition milled aluminum, alumina and cubic boron nitride powders were heat treated to 800 °C in air followed by sintering to 1300 °C in argon. Sintered samples were found to have better density, hardness and fracture toughness compared to conventionally sintered samples of same composition.     

Dynamic Compaction of Cubic Boron Nitride Powders

Compacts of cubic boron nitride with 94% of theoretical density and a Vickers microhardness of 30.3 GPa were produced from coarse c-BN powder by a shock compaction technique. The density and microhardness of these compacts depend strongly on the grain size of the starting powders.     

Conical Boron Nitride Nanorods Synthesized Via the Ball-Milling and Annealing Method

A boron nitride (BN) nanostructure, conical BN nanorod, has been synthesized in a large quantity on Si substrates for the first time via the ball-milling and annealing method. Nitridation of milled boron carbide (B₄C) powders was performed in nitrogen gas at 1300°C on the surface of the substrates to form the BN nanorods. The highly crystallized nanorods consist of conical BN basal layers stacked along the nanorod axis. Ball milling of the B₄C powders can significantly enhance the nitridation of the powders and thus facilitate the formation of nanorods during the annealing process.     

Large-scale synthesis and growth mechanism of boron nitride nanocomposite assembled by nanosheets and nanotubes

Boron nitride nanocomposites assembled by nanosheets and nanotubes can exert multi-dimensional synergistic toughening and strengthening effects. This material is expected to be a high-efficiency reinforcement additive in advanced structural ceramics. In this study, we designed a universal method for synthesizing gram-scale boron nitride nanocomposites by annealing the precursor containing catalyst in chemical vapor deposition equipment under flowing ammonia, and a combined growth mechanism of surface-diffusion and solid-liquid-solid is proposed. The boron nitride nanosheets were initially formed by a surface-diffusion reaction between boron trioxide and ammonia at 1300°C. At elevated temperatures (1400°C-1500°C), the boron nitride nanotubes grew in-situ from the nanosheets in the presence of catalysts through a solid-liquid-solid mechanism, forming the desired boron nitride nanocomposite.     

Combining boron nitride with a fluoroelastomer: An enhanced polymer processing additive

The effect of a new processing additive (boron nitride powder in combination with a fluoroelastomer) on the rheology and processability of molten polymers is studied. The equipment used includes an Instron capillary rheometer equipped with a special annular die (Nokia Maillefer wire coating cross‐head) and a parallel‐plate rheometer. Metallocene polyethylenes with and without boron nitride (BN) and fluoroelastomer are tested in extrusion. First, it is demonstrated that BN is a superior processing aid compared to conventional fluoropolymer ones. Second, it is found that the combination of BN powders with a small amount of a fluorelastomer improves even further the processability of molten polymers (melt fracture performance).     

Synthesis of nanocrystalline boron nitride by combustion process

Nanocrystalline boron nitride powders were synthesized by combustion process using urea as a fuel. Experiments were carried out by heating boric acid and urea in an N₂ atmosphere at 850°C. Boric acid was used as a source of boron while urea, as a source of nitrogen. The reactions were carried out in an autoclave with provisions for purging with nitrogen gas. The samples were characterized by powder X-ray diffraction, Fourier transform IR spectroscopy (FT-IR), FT-Raman spectroscopy, UV-VIS spectroscopy, and SEM. The article is published in the original.     

Study on Tribological Properties of Carbon/Aramid Fabric Coatings Modified by Boron Nitride of Single Layer

Carbon/aramid fabric composite coatings modified with boron nitride of single layer were fabricated through a dip-coating method. The composite coatings were cured with successive heating processes in an oven. The friction and wear properties of those as-prepared coatings were studied on a block-on-ring tester. The obtained results showed that the wear life of the coatings increased obviously after inclusion of boron nitride of single layer; however, the values of friction coefficients of the coatings almost remained constant. The optimal loadings of boron nitride of single layer in our experiments was 5 wt.%, and the wear life of the modified coating increased by ca. 360% compared with that of pristine fabric composite coating. The worn morphology of the sliding surface for both pristine fabric coating and the composite coatings filled with boron nitride of single layer was discussed, and the wear mechanisms were illuminated.     

Combustion synthesis of hexagonal boron nitride nanoplates with high aspect ratio

High crystalline hexagonal boron nitride nanoplates with high aspect ratio of ~400 have been synthesized by combustion synthesis method through magnesiothermic reduction reaction between B₂O₃ and Mg in N₂ pressure. The synthesized hexagonal boron nitride nanoplates were about 50 nm in thickness and larger than 20 μm in lateral size. The six-fold symmetric spots electron diffraction pattern of transmission electron microscopy shows that the nanoplate is well crystallized. Hexagonal boron nitride nanoplates grow via an Oswald ripening process and have larger lateral size when it was prepared with larger magnesium particles. High temperature liquid magnesium provides an important environment for the growth and crystallization of boron nitride. This work provides an effective way to achieve low-cost and large-scale preparation of high-quality boron nitride nanoplates.     

Excellent electrical conductivity of the exfoliated and fluorinated hexagonal boron nitride nanosheets

The insulator characteristic of hexagonal boron nitride limits its applications in microelectronics. In this paper, the fluorinated hexagonal boron nitride nanosheets were prepared by doping fluorine into the boron nitride nanosheets exfoliated from the bulk boron nitride in isopropanol via a facile chemical solution method with fluoboric acid; interestingly, these boron nitride nanosheets demonstrate a typical semiconductor characteristic which were studied on a new scanning tunneling microscope-transmission electron microscope holder. Since this property changes from an insulator to a semiconductor of the boron nitride, these nanosheets will be able to extend their applications in designing and fabricating electronic nanodevices.