Boron Nitride Matrix Research Articles

Formation of ε-FexN/BN magnetic nanocomposite and its thermodynamic and kinetic analyses

A nanocomposite of nanometer-sized magnetic granular e-FexN embedded in a nonmagnetic amorphous boron nitride matrix was prepared by ball milling mixture of α-Fe and hexagonal boron nitride in argon atmosphere. The grain size of the e-FexN alloy was about 10–20 nm. The nitrogen concentration in the e-FexN alloy increases with extending milling time. Both thermodynamic calculation and the present experiment show that iron and nitrogen atoms have higher alloying driving force than iron and boron atoms. Analyses of thermodynamics and kinetics about formation of the e-FexN alloy suggested that the formation of the e-FexN alloy is related to amorphization of the hexagonal boron nitride and refinement of the α-Fe. It was found from the present experiment that a critical grain size of the α-Fe reacting with nitrogen in the amorphous boron nitride is about 8 nm.

Viscoelastic sliding and diffusive relaxation along grain boundaries in polycrystalline boron nitride

Dense hexagonal boron nitride (BN) materials were prepared via two different processing routes: (i) hot-pressing with the addition of a Ca/B-containing glass and (ii) chemical vapor deposition (CVD). The resulting microstructure of both materials was studied by scanning and transmission electron microscopy. While the hot-pressed BN material shows, apart from large BN matrix grains, an inhomogeneous distribution of residual glass at room temperature, the CVD deposition yields a homogeneous fine grained microstructure with no amorphous residue detectable. Internal-friction experiments were performed to study the micromechanical response of the materials when exposed to high temperatures. The CVD material revealed no relaxation peak during testing up to 2300°C, while the glass-doped sample showed a pronounced relaxation peak at a peak-top temperature of about 600°C. This temperature corresponds to the softening temperature known for bulk Ca/B-glasses and it is, therefore, concluded that the glass homogeneously wets the BN grains at elevated temperatures. The results presented are seen as the first clear evidence that the internal friction peak monitored for various glass-containing ceramics is indeed related to a viscous sliding process along grain boundaries.

Characterization of fiber/matrix interfaces in composites with a boron nitride matrix

The fracture behavior of boron nitride (BN) composites reinforced with several types of carbon and ceramic fibers has been examined. Fiber properties and fiber/matrix interface characteristics were found to control the mechanical strength and toughness of the composites. Transmission electron microscopy demonstrated preferential orientation of the BN matrix parallel to the fiber surfaces. Auger electron spectroscopy was then used to examine the fiber/matrix interface and locate the path of interface debonding. Single-fiber push-out experiments were used to quantify the mechanical properties of the fiber/matrix interface. The bulk composite fracture properties were consistent with current understanding in that a lower interfacial toughness was found to enhance fiber pull-out and increase the composite toughness. Matrix abrasion during fiber sliding had a significant effect on the sliding behavior of the fibers.

ChemInform Abstract: Fabrication and Properties of Ceramic Composites with a Boron Nitride Matrix.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Fabrication and Properties of Ceramic Composites with a Boron Nitride Matrix

Boron nitride (BN) matrix composites reinforced by a number of different ceramic fibers have been prepared using a low‐viscosity, borazine oligomer which converts in very high yield to a stable BN matrix when heated to 1200°C. Fibers including Nicalon (SiC), FP (A12O3), Sumica and Nextel 440 (Al2O3‐SiO2) were evaluated. The Nicalon/BN and Sumica/BN composites displayed good flexural strengths of 380 and 420 MPa, respectively, and modulus values in both cases of 80 GPa. On the other hand, FP/BN and Nextel/BN composites exhibited very brittle behavior. Nicalon fiber with a carbon coating as a buffer barrier improved the strength by 30%, with a large amount of fiber pullout from the BN matrix. In all cases except for Nicalon, the composites showed low dielectric constant and loss.

Fabrication of oxidation-resistant carbon fiber/boron nitride matrix composites

A low-viscosity borazine oligomer has been prepared which readily wets carbon fiber layups and converts in very high yield to a stable boron nitride (BN) matrix when heated at 1200°C. Composites could be prepared after only one or two impregnations which displayed excellent oxidation resistance up to 850°C. In most cases, the composites showed outstanding mechanical properties after heating at 1200-1500°C with flexural strength and modulus values as high as 130 ksi and 15 Msi, respectively

The Properties of a Plasma Deposited Candidate Insulator for Future Multilevel Interconnects Technology.

ABSTRACTResults are reported from an investigation of the properties of boron nitride thin films. The BN films were deposited by the method of plasma enhanced chemical vapor deposition (PECVD) from mixtures of ammonia and diborane. The deposition variables were RF power, pressure, ammonia partial pressure, diborane partial pressure, diluent type (nitrogen or argon), and temperature. The films were characterized for their stoichiometry, hydrogen content, stress, refractive index, and especially for their dielectric properties. It was observed that the main parameters affecting the dielectric constant were the ratio of ammonia to diborane flow, and the deposition temperature. The dielectric constant decreased with increasing ammonia to diborane ratio, while it increased with decreasing deposition temperature. The lowest dielectric constant value achieved was 4.0 ·, 0.1. Nitrogen dilution allowed the deposition of boron nitride films with the lowest dielectric constant value at ammonia to diborane ratios smaller than those required under argon dilution, which suggests some incorporation of nitrogen into the boron nitride matrix from the diluent.

Boron Nitride Composites By Chemical Vapor Deposition

Composites of boron nitride ( BN) have been made by the chemical vapor deposition ( CVD) of a BN matrix on a BN felt fiber substrate. Reactant gases were boron trifluoride and ammonia. The composites have a relatively high density (1.70 g/cm3), a crystallite size LC = 150 A and an interlayer spacing d002 = 3.35 A. Measurements of elastic modulus and thermal conductivity and expansion showed some anisotropy as a result of the preferred fiber orientation of the substrate.