Papers by Keyword: BN

Abstract: A high-temperature poly (m-phenyleneisophthalamide) (PMIA) dielectric composite was successfully manufactured with functionalized BN (fBN) fillers. Due to effective modification by KH-550, fBN particles evenly dispersed in PMIA matrix. The dielectric property, breakdown strength and thermal conductivity of PMIA/fBN dielectric composite were researched. The consequences indicate that fBN fillers can not only decrease the dielectric loss but also enhance the breakdown strength of PMIA/fBN dielectric composites. Furthermore, owing to the generated heat transfer pathways by fBN particles, the thermal conductivities improved from 0.23 W·m^-1·K^-1 of fBN-0 to 0.86 W·m^-1·K^-1 of fBN-30, demonstrating a significant improvement. These results present a novel method for fabricating high-temperature PMIA/fBN dielectric composites with improved breakdown strength and thermal conductivity.

Abstract: Processes and products of combustion of systems of "sodium azide – aluminum and boron halides " are studied. Halide salts AlF₃, Na₃AlF₆, KBF₄ and NH₄BF₄ are used as precursors of Al and B elements. The measured combustion temperature is in the range from 900 to 1800°C. It is established that the end product of combustion synthesis consists of several phases, not only the target phases (AlN and BN), but also the by-products (Na₃AlF₆, К₂NaAlF₆, NaF, KF), part of which is removed by water washing. The composite powder of AlN-BN is formed only in the case of AlF₃ as precursor of Al; in doing so, it is formed with side, impurity product К₂NaAlF₆ or Na₃AlF₆. In the case of precursor Na₃AlF₆, the washed product of SHS may only be a by-product К₂NaAlF₆ or Na₃AlF₆ or along with the by-product, it may contain AlN or BN, but not their composition. That is explained by different combustion temperature.The structure formation of the composite powder is investigated. The obtained agglomerated nitride nano- and micropowders have an irregular and predominantly acicular shape with an average particle size of 50-170 nm in the agglomerates.

Abstract: The composite powder of AlN-BN is of interest from the point of view of the use for sintering the AlN-BN composite ceramics. The ceramics compared with AlN ceramics would have better thermal shock resistance, less britlleness, better machinability, good heat conductivity and tribological properties. Possibility of obtaining a composite powder of AlN-BN with the application of process of self-propagating high-temperature synthesis (SHS) from powder mixtures of NaN₃ and precursors of aluminum and boron was investigated. Halide salts AlF₃, Na₃AlF₆, KBF₄ and NH₄BF₄ were used as the precursors. It was shown that these mixtures are capable of burning and obtaining agglomerated composite powders from nanosized (50-170 nm) particles.

Abstract: 2.5D SiC/SiC composites with and without BN interphase coating were fabricated by VSI process from SiC/C preforms that were produced after 3, 4 and 5 PIP C cycles, respectively. The effect of the BN interphase coating and PIP C cycles on the mechanical properties and microstructure of the composites was investigated. The results indicated that the composites with or without BN interphase coating exhibited catastrophic fracture behavior. Both the excess residual carbon and BN interphase coating played a positive role in protecting the SiC fibers from silication by vapor silicon, which accounted for the less obvious fibers pull-out phenomenon that occurred in the composites. The PIP cycles had an influence on the residual porosity and the content of the newly developed SiC in the composites, and therefore the mechanical properties of the composites were influenced. The composites fabricated from preforms after 4 PIP C cycles and containing BN interphase coating have a flexural strength of 122.5 MPa and a modulus of 86.5 GPa.

Abstract: This paper presents the results of a study on the preparation of nanostructured nitride composition TiN-BN in the boron halide - sodium azide - titanium halide system by the azide SHS technology. SHS-Az technology makes it possible to produce powders having the desired properties, only in one step, without blending separately prepared powders of titanium nitride and boron nitride, and to obtain the final product in the form of micro-and nanosized powder nitride compositions TiN-BN by using complex halides of titanium and boron. Thus, the composite powder TiN and BN obtained from the KBF₄-NaN₃-Na₂TiF₆» and «NH₄BF₄-NaN₃-Na₂TiF₆ systems can be classified as a nanostructured powder (nanopowder).

Abstract: Aiming at the optimal decision problem of multi factors, model is proposed of establishing BN with cause-and-effect relationships, constructing evaluation function with multi factors by decision objectives, finally comparing and selecting the optimal solution of evaluation function from all possible decisions. The model inherits the ability of numerical reasoning and can analyze multi factors. Example illuminates procedures of the model in the paper.

Abstract: To solve the problems caused by the complex failure mechanisms in hydraulic brake failure systems and the uncertain linkages between fault type and fault symptoms, a method of Bayesian network (BN) is offered for failure diagnosis. A statistical strategy is adopted there in this algorithm on the rule base provided by many experts to discard the weak causal relationship rules while keep the relatively strong ones during the BN structure learning, and a BN-based model of layered architecture is established for fault diagnosis of hydraulic braking failure systems. Experimental data analysis shows that the Bayesian network fault diagnosis model has higher accuracy than fuzzy logic diagnosis method, effectively solving the uncertainties in fault diagnosis.

Abstract: Polysulfone modified epoxy resin was chosen as the matrix of composite materials. The epoxy/glass fiber/boron nitride composite materials were prepared by heat press molding. The effect of different size and content of BN on the thermal, mechanical, electrical properties and thermal stability of the composite was investigated. The results showed that thermal conductivity and thermal stability of the composite increased with the content of the BN particles, the thermal conductivity of the composite reached 0.672W/m·K when filled with 1μm BN at 20 wt.%. Impact strength and flexure strength of the composite increased initially and then decreased with the increasing content of BN. The electrical insulating property of the composite remained with the content of 20 wt.% BN.

Abstract: The thermodynamic properties of (B1-xCx)(N1-xCx) (x=0, 0.25, 0.5) are calculated by the ultra-soft pseud-potentials within local density approximation in frame of density functional theory with virtual crystal approximation and using the quasi-harmonic Debye model. The normalized relative volume V/V0, bulk modulus, thermal expansion coefficient α and heat capacity were computed with different temperature and pressure. Meanwhile the lower limit of the thermal conductivity κmin is also examined in details.

Abstract: The structural properties and elastic constants of (B1-xCx)(N1-xCx) (x=0.0, 0.2, 0.4 and 0.6) are calculated by the ultra-soft pseudo-potentials within the generalized gradient approximation and local density approximation in frame of density functional theory with virtual crystal approximation. The elastic constants, the aggregate elastic modulus, poisson’s ratio, energy gap and hardness are computed too. The energy band structure, DOS, and Hellmann-Feynman stresses are also examined in details.