Hexagonal Boron Nitride Composites Research Articles

Insight into the tribological properties of cylinder liner-piston ring under the nano BNNs/Cu composite additive lubrication

To improve the tribological properties of the cylinder liner-piston ring, a two-dimensional hexagonal boron nitride/copper composite lubricant additive was prepared and characterized in detail. The tribological properties and lubrication mechanism of nano hexagonal boron nitride composites with different concentrations were studied through the reciprocating friction test on the Rtec friction and wear tester. The results show that copper is successfully reduced and attached to the surface of h-BN nanosheets through the self-polymerization of dopamine, and the spherical structure promotes the interlayer slip of the nanosheets during the reciprocating friction process. The appropriate concentration of nano composite additives has excellent anti-friction and anti-wear properties. At 1 Hz and 100 N, the friction coefficient and wear quality of the nano composite additive with a concentration of 2 wt% were reduced by 29.07% and 76%, respectively. The surface Sq value and Sz value of the cylinder liner sample decreased by 68.06% and 74.47%. At the same time, under the condition of high speed and heavy load, the average wear depth of the cylinder liner sample is reduced by 61.3%. The nano composite material additive forms an excellent friction protective film on the wear surface of the cylinder liner, which can better enter the wear surface of the cylinder liner and produce a filling and repairing effect. The research results provide a method for the use of nano hexagonal boron nitride composite additives to inhibit the wear of cylinder liner-piston ring of Marine diesel engines.

Enhanced surface flashover performance of oriented hexagonal boron nitride composites via anisotropic charge transportation

AbstractSurface flashover is a crucial issue for the miniaturisation of electronic facilities in military, industrial, and aerospace engineering. The oriented hexagonal boron nitride (hBN) composites, due to excellent thermal and electrical insulating properties, show a potential application in high‐voltage power equipment, while the surface flashover performance of hBN composites dependent on oriented hBN texture is rarely reported. The effects of hBN orientation and contents on the surface flashover performances of oriented hBN composites are investigated. The isothermal surface potential decay of the oriented hBN composites was also studied. It is found that the charge transportation could be adjusted by the hBN orientation, thus regulating surface flashover strength. The DC flashover voltage of the in‐plane oriented hBN composites with a thickness of 15 μm reached the maximum of 27.6 kV at the hBN loading of 20 wt%, 14.5% higher than that of the pure resin. The carrier mobility of out‐of‐plane oriented hBN composites is about three times greater than that of the in‐plane oriented composites, indicating that the charges are easily transported along the hBN basal plane. The larger carrier mobility causes charge dissipation in composites near the electrode at the hBN basal plane parallel to the axis of electrodes and inhibits the distortion of the surface electric field on the composites, thus enhancing the surface flashover. Consequently, developing oriented insulators for high‐voltage applications and enabling an optimum insulation design would be beneficial because of the compactness and high reliability of power apparatus for use in power grids.

Thermal conductivity of polycaprolactone/three-dimensional hexagonal boron nitride composites and multi-orientation model investigation

Three-dimensional boron nitride (3D-BN) scaffolds were fabricated by the ice-templating method, and polycaprolactone/3D-BN (PCL/3D-BN) composites were prepared by vacuum impregnation of caprolactone monomers into 3D-BN scaffolds and then polymerization via the microwave-assisted technique. The results reveal that hexagonal boron nitrides (hBNs) in the PCL/3D-BN composites present three orientation patterns, that is, parallel (z-direction) and vertical (x-direction) alignment in the direction of ice-crystal and random orientation. Moreover, the orientation of hBNs plays an essential role in improving the thermal conductivity of the composite. The maximum thermal conductivities of the PCL/3D-BN composite are 1.42 and 1.01 W m−1 K−1 in the z-direction and x-direction, which are 7.10 and 5.05 times higher than that of the pure PCL, respectively. However, the existing models cannot be used to analyze the orientation of hBNs in the composites quantitatively. In this work, based on two-phase models, a multi-orientation model was developed to explore the volume fractions of hBNs oriented in each pattern. The results indicate that the hBN ratio of random orientation (fr/f) enhances with the increase of hBN content in the PCL/3D-BN composites, and especially while increasing the hBN content from 13.41 vol% to 15.48 vol%, the value of fr/f has soared from 33.0% to 63.7%, leading to the decrease of thermal conductivities. The predictions from the multi-orientation model are in good agreement with the experimental data for the other composites, implying that the model could be used to analyze the orientation of fillers quantitatively and guide the design of thermally conductive scaffolds.

Temperature Effects on the Dielectric Properties and Breakdown Performance of h-BN/Epoxy Composites.

Epoxy–boron nitride composites are promising insulating materials, and it is highly important to understand their insulating performances at different temperatures with different nano-doping amounts. In this study, we investigated the effects of different mass fractions of epoxy–micron hexagonal boron nitride composites on their thermal conductivity, as well as the effects of temperature and mass fraction on their insulating performances. The results demonstrated that the thermal conductivity of epoxy–micron hexagonal boron nitride composites was superior to that of neat epoxy. The thermal conductivity of epoxy–micron hexagonal boron nitride composites increased with the mass fraction of hexagonal boron nitride, and their dielectric constant and dielectric loss increased with temperature. The dielectric constant of epoxy–micron hexagonal boron nitride composites decreased as the mass fraction of hexagonal boron nitride increased, while their dielectric losses decreased and then increased as the mass fraction of hexagonal boron nitride increased. Due to internal heat accumulation, the alternating current breakdown strength of epoxy–micron hexagonal boron nitride composites increased and then decreased as the mass fraction of hexagonal boron nitride increased. Additionally, as the temperature increased, the composites transitioned from the glassy state to the rubbery or viscous state, and the breakdown strength significantly degraded.

Insight into the Directional Thermal Transport of Hexagonal Boron Nitride Composites.

Ideal dielectric materials for microelectronic devices should have high directionally tailored thermoconductivity with low dielectric constant and loss. Hexagonal boron nitride (hBN) with excellent thermal and dielectric properties shows a promise for the fabrication of thermoconductive dielectric polymer composites. Herein, a simple method for the fabrication of lightweight polymer/hBN composites with high directionally tailored thermoconductivity and excellent dielectric properties is presented. The solid polymer/hBN composites are manufactured by melt-compounding and injection molding. The porous composites are successfully manufactured in an injection molding process through supercritical fluid (SCF) foaming. X-ray tomography provides direct visualization of the internal microstructure and hBN orientation, leading to an in-depth understanding of the directionally dependent thermoconductivity of the polymer/hBN composite. Shear-induced orientation of hBN platelets in the solid HDPE/hBN composites leads to a significant anisotropic thermal conductivity. The solid HDPE/23.2 vol % hBN composites show an in-plane thermoconductivity as high as 10.1 W m-1 K-1, whereas the through-plane thermoconductivity is limited to 0.28 W m-1 K-1. However, the generation of a porous structure via SCF foaming imparts in situ exfoliation, random orientation, and interconnectivity of hBN platelets within the polymer matrix. This results in highly isotropic thermoconductivity with higher bulk thermal conductivity in the lightweight porous composites as compared to their solid counterparts. Furthermore, the electrically insulating composites developed in this study exhibit low dielectric constant and ultralow dielectric loss. Thus, this study presents a simple fabrication method to develop lightweight dielectric materials with tailored thermal conductivity for modern electronics.

Polycarbonate–hexagonal boron nitride composites with better dielectric properties for electronic applications

Hexagonal boron nitride (h-BN) reinforced polycarbonate (PC) composites were fabricated using solution method followed by hot pressing. The content of h-BN was varied from 0 to 60 wt% (~ 44.4 vol%). The morphology, electrical volume resistivity, microhardness, and dielectric properties of the composites were characterized. Scanning electron microscopy showed uniform dispersion of h-BN particles in the PC matrix. X-ray diffraction and Fourier transforms infrared spectroscopy showed no change in morphology of PC present in the composites. The dielectric constant of the composites containing 60 wt% h-BN increased to 3.9. The dissipation factor of the composites reduced to less than 0.007 and the dispersion in dielectric constant of the composites became stable over the frequency from 1 kHz to 10 MHz compared to that of pure PC. The electrical volume resistivity of the composites remained close to that of pure PC. The microhardness increased approximately 25% at 50 wt% h-BN content and thereafter, a slight decrease in microhardness was observed.

Recyclable thermoplastic hexagonal boron nitride composites with high thermal conductivity

Polymers have been widely adapted as matrix incorporating relatively heavy but functional fillers to produce more practically useful materials with lighter weights in composite industries. In an engineering perspective, processability, ease of mass production, good interfacial interaction with the filler, and recyclability are crucial aspects of the polymeric matrix. To these ends, we herein describe a recyclable, processable, and also readily mass-producible polymer for including functional model hexagonal boron nitride (hBN) filler. The final composite possessing highly aligned hBN was obtained via simple melt pressing, attributing to a good interfacial interaction between the matrix and the hBN along with shear force generated upon manufacturing the composite. As a consequence, the composite exhibited exceptionally high thermal conductivity of 14 W/mK. To further highlight the versatility of the matrix, the incorporated filler was recovered from the composite by depolymerizing the matrix. The recovered hBN displayed equivalent quality as the stock, thus readily reusable for the future.

A detailed investigation on thermal and micro-structural properties of hexagonal boron nitride composites

This work reports on the processing and characterization of Boron Nitride filled polyester composites. It emphasizes the details of the test procedures and results in regard to the thermal and micro-structural characteristics of the polyester composites filled with micro-sized boron nitride (BN) with different concentrations. The estimation of effective thermal conductivity of the composites using the proposed theoretical models is done and the results are validated by corresponding experimental results. The effects of inclusion of BN on the effective thermal conductivity (keff), glass transition temperature (Tg), coefficient of thermal expansion (CTE) of composites are also studied. With the addition of BN, the thermal conductivity enhances. Again, the embedment of BN fillers results in lowering the CTE of the composites whereas the Tg of the composites is improved manifold. The other theoretical models as well as the proposed theoretical correlation can serve as very good empirical models for particulate fillers to estimate keff for composites within the percolation limit. Again, the polyester composites serve to be a better option than the epoxy composites as polyester tends to impart better mechanical and thermal properties. By enhancement of conduction capability the light weight and micro-sized BN filled polyester composites can be used for applications such as space flight and aviation industry,electronic encapsulations,engine parts etc. Similarly a hybrid composite is fabricated with another organic filler which showed even more distinctive properties suitable for particular applications

Mechanical properties of hydroxyapatite hexagonal boron nitride composites

Mechanical properties of hydroxyapatite hexagonal boron nitride composites

Consolidation of cubic and hexagonal boron nitride composites

Consolidating cubic boron nitride (cBN) typically requires either a matrix of metal bearing materials that are undesirable for certain applications, or very high pressures within the cBN phase stability field that are prohibitive to manufacturing size and cost. We present new methodology for consolidating high stiffness cBN composites within a hexagonal boron nitride (hBN) matrix (15–25vol%) with the aid of a binder phase (0–6vol%) at moderate pressures (0.5–1.0GPa) and temperatures (900–1300°C). The composites are demonstrated to be highly tailorable with a range of compositions and resulting physical/mechanical properties. Ultrasonic measurements indicate that in some cases these composites have elastic mechanical properties that exceed those of the highest strength steel alloys. Two methods were identified to prevent phase transformation of the metastable cBN phase into hBN during consolidation: 1. removal of hydrocarbons, and 2. increased cBN particle size. Lithium tetraborate worked better as a binder than boron oxide, aiding consolidation without enhancing cBN to hBN phase transformation kinetics. These powder mixtures consolidated within error of their full theoretical mass densities at 1GPa, and had only slightly lower densities at 0.5GPa. This shows potential for consolidation of these composites into larger parts, in a variety of shapes, at even lower pressures using more conventional manufacturing methods, such as hot-pressing.

Phase Relations and Transformations in Advanced Ceramic Materials

This article reviews the phase diagrams contributing to the understanding of compound formations, phase transformations, phase separation and microstructural development in advanced ceramic systems. Phase equilibrium relations in the Al 2 O 3 -AlN and AlN-B 2 O 3 systems are presented for the fabrication of translucent aluminum oxynitride spinel ceramics and in-situ hexagonal boron nitride composites. In oxide systems, especially in glass-forming systems, crystallized phases vary with not only composition and temperature but also time of heat-treatment. Considering this aspect, time-temperature-transformation (TTT) diagrams in the solidification processes of cordierite, high-T c superconducting oxide and fluorphlogopite melts and the phase transformations of ammonium aluminum carbonate hydroxide to α-alumina are introduced and specific phase transformations in these systems are also discussed. The effect of treatment atmosphere on microstructure in phase transformations is described in the surface-treatment of AlN ceramics and the sintering of transparent silica glass.