Spherical Boron Research Articles

Enhancing thermal conductivity of spherical boron nitride/silicone rubber composites by matching interfacial modulus

ABSTRACT Lightweight, electrically insulating and high thermally conductive materials are highly desirable in electronic and power systems because they can effectively transfer heat and avoid temperature rise of the equipment and devices, thus guaranteeing normal operation. In this work, silica-coated spherical boron nitride (SiO2@s-BN) was successfully fabricated and prepared as a thermally conductive composite by blending with silicone rubber (SR). Due to the modulus matching, SiO2@BN could decrease the thermal resistance between the SR matrices and the thermally conductive fillers, thus achieving higher thermal conductivity compared with the original BN at the same filler loading. When the SR composite is filled with 40 wt% SiO2@BN, the thermal conductivity can reach 1.2 W m−1 K−1, which is 16.5% higher than that of the 40 wt% s-BN/SR composite. Simultaneously, the 40 wt% SiO2@BN/SR composite material not only exhibits excellent dielectric, thermal, and mechanical properties but also demonstrates outstanding processability, maintaining a low precursor viscosity of only 14.5 Pa·s. Overall, this work provides a novel strategy for developing high thermally conductive SR composites with good processability.

Enhanced Thermal Conductivity of High-Density Polyethylene Composites with Hybrid Fillers of Flaky and Spherical Boron Nitride Particles.

The synergistic effect between different fillers plays a crucial role in determining the performance of composites. In this work, spherical boron nitride (BN) and flaky BN are used as hybrid fillers to improve the thermal conductivity (TC) of high-density polyethylene (HDPE) composites. A series of HDPE composites were prepared by adjusting the mass ratio (1:0, 4:1, 2:1, 1:1, 1:2, 1:4, and 0:1) of spherical BN and flaky BN. The SEM results indicate that the spherical BN (with a particle size of 3 μm) effectively filled the gaps between the flaky BN (with a particle size of 30 μm), leading to the formation of more continuous heat conduction paths with the composite. Remarkably, when the mass ratio of spherical BN to flaky BN was set to 1:4 (with a total BN filling amount of 30 wt%), the TC of the composite could reach up to 1.648 Wm-1K-1, which is obviously higher than that of the composite containing a single filler, realizing the synergistic effect of the hybrid fillers. In addition, the synergistic effect of fillers also affects the thermal stability and crystallization behavior of composites. This work is of great significance for optimizing the application of hybrid BN fillers in the field of thermal management.

Solid-state excimer emission of o-carborane derivatives and applications for stimuli-responsive luminescent materials.

o-Carborane is an icosahedral cluster containing boron and has been paid attention as a versatile platform for constructing stimuli-responsive materials with solid-state luminescence properties by suppressing aggregation-caused quenching (ACQ), which is the emission annihilation process observed in condensed states, such as concentrated solutions, solids and films. Non-specific intermolecular interactions in the condensed states resulting in ACQ are disturbed by the spherical boron cluster. More recently, it has been demonstrated from several reports that o-carborane is capable of assisting solid-state excimer emission in aryl-modified o-carboranes. Moreover, some of these materials provide unique stimuli-responsiveness. In the commodity design for solid-state luminescent materials, o-carboranes are responsible for disturbing intermolecular interactions and subsequently presenting solid-state luminescence. On the other hand, excimer emission, which is inducible from the intermolecular photophysical process, has been discovered in the solid state based on o-carborane materials. In this manuscript, recent progress in stimuli-responsive luminescent materials via excimer formation will be described. Moreover, as the further potential of o-carborane materials, recent findings on the photo-salient effect which can be induced through excimer formation followed by photoreaction will also be explained.

Theoretical study on thermal conductivity of epoxy composites doped with boron nitride with multiple dimensions

Epoxy is widely used in electrical equipment owing to its exceptional physical, chemical, and electrical insulation properties. However, its low conductivity limits its applications. The incorporation of fillers with high thermal conductivities into epoxy matrices is one of the most effective and common methods for enhancing the thermal conductivities of epoxy composites. In this study, Digimat software was used to construct the models, and a numerical simulation based on representative volume elements (RVEs) was employed to investigate the relationship between the thermal conductivity and microstructure of the epoxy composites. The epoxy composites exhibited different thermal conductivities in various directions owing to the pronounced anisotropy of the boron nitride (BN) fibers. In composites filled with both spherical boron nitride (S-BN) and BN fibers, BN fibers play a dominant role in enhancing the effective thermal conductivity (ETC). However, at the same filling fraction, the composites achieved their maximum ETC when the S-BN-to-BN fiber ratio was 2:8. Furthermore, the ETC increased linearly with the filling fraction of the filler. When the volume fraction of the fillers was kept constant, the smaller S-BN particles increased the ETC of the composites. Similarly, when the number of fillers and volume fraction were kept constant, an increased aspect ratio of the BN fibers resulted in a higher ETC of the composites. An investigation of the electrothermal coupling between pure epoxy resin and epoxy-resin composite materials revealed that the stable temperature of the composites was considerably lower than that of pure epoxy. The heat transfer to the bottom of the composites was faster, which indicated a substantial improvement in the ETC.

Preparation of isotropic submicron spherical boron nitride particles by restricted template method

AbstractHexagonal boron nitride (h‐BN) has received considerable attention, due to its high thermal conductivity and electrical insulation. However, the intrinsic platelike structure with the strong anisotropic property restricts its applications, and it is necessary to synthesize isotropic spherical h‐BN particles (SP‐BNs) with submicron size. Till now, methods to prepare (SP‐BN) still exist problems, such as high oxygen impurities and pollution, generated by the ammonia and pyrolysis of precursors. Here, a relatively green reaction between the restricted template of carbon nanospheres and boron trioxide (B2O3) under elevated temperature is conducted, and the SP‐BNs with an average diameter of 200–300 nm (named Nano‐BN‐s) have been successfully synthesized. Comprehensive scanning electron microscopy, transmission electron microscopy, and X‐ray diffraction characterizations confirm the obtained products are spherical boron nitride. With the analysis of X‐ray photoelectron spectroscopy and Fourier transform infrared, the reaction mechanism is briefly discussed. These results indicate the reaction occurs on the restricted template of carbon nanospheres, and the C atoms are substituted by B and N atoms as the reaction progress, forming the Nano‐BN‐s. What is more, the restricted template method plays a key role in the design and improves h‐BN‐based materials in the future and may also be extended to form other novel materials.

Controllable preparation of boron nitride microspheres and their epoxy-based thermoconductive composites

How to prepare spherical boron nitride (BN) particle with different size is an extremely challenging work. In this paper, the controllable preparation of spherical BN particle from nanospheres to microsphere was realized by changing the synthesis temperature of trimethyl borate (B(OMe)3) and ammonia. The spherical precursor (SP) with high oxygen content was obtained first, and then it was heated under flowing ammonia atmosphere to form stable boron nitride microspheres (BNMS). The BNMS exhibits onion-like cavitation structure with a diameter of 0.8–3.4 μm. The effects of the lower reaction temperature (700–825 °C) and gas flow rate on the spherical precursor are discussed. A possible mechanism is proposed to explain the formation of precursors and the appearance of onion-like structure. It is believed that the formation of microsphere is due to the deposition and growth of BO species during the flow process of nanosphere. In addition, the effect of the addition of BNMS on the thermal conductivity of epoxy resin (EP) composites was investigated.

Studies on drug carrier potential of spherical boron nitride nanoparticles in cancer therapy

Studies on drug carrier potential of spherical boron nitride nanoparticles in cancer therapy

Polydopamine-assisted silver-coated spherical boron nitride as dual functional filler for thermal management and electromagnetic interference shielding

Polydopamine-assisted silver-coated spherical boron nitride as dual functional filler for thermal management and electromagnetic interference shielding

Proportional Optimization Model of Multiscale Spherical BN for Enhancing Thermal Conductivity

The power electronics tend to become miniaturized and multifunctionalized, such as thermal rotators, circuit breakers, and microchips, which have necessitated creating instruments for investigating thermal mechanisms and enhancing thermal conductivity. In this paper, we construct the heat flow network of spherical boron nitride (BN) and used multiscale spherical BN to improve the thermal conductivity of the composite synergistically. The multiscale spherical filler ratio optimization model based on the Dinger–Funk particle stacking theory is established, which obtained the optimal volume ratio of 0.224:0.374:0.402 with D50 of 20, 70, and 160 μm. Meanwhile, the effects of multiscale filler ratio, morphology, filler content, and temperature are investigated. The thermal conductivity of composites can reach up to 1.84 W/(m·K) at 20 vol %. Significantly, the thermal conductivity of composites is 4.82 W/(m·K) at 30 vol %, which is achieved by optimizing the multiscale filler and particle size distribution.

Spherical boron nitride/silicone rubber composite with high isotropic thermal conductivity via pre‐constructing thermally conductive networks

AbstractThermal interface material (TIM) is crucial for electronic devices to dissipate heat, but the high interface thermal resistance between polymer matrix and filler is a major problem affecting its thermal conductivity (TC). In this study, we prebuilt excellent thermally conductive pathway of isotropic spherical boron nitride (sBN) using a facile method, that is, pressing fillers, to decrease the interface thermal resistance before filling matrix silicon rubber (SR), and prepared a TIM with high isotropic TC. At 50 wt% filler content, the through‐plane and in‐plane TCs of sBN/SR composite reached 9.36 and 7.82 W/(m·K), respectively, which are higher than the highest value of previous research on bulk BN/SR composites (not including film), 4.13 and 6.56 W/(m·K), respectively. Meanwhile, the thermal decomposition temperature of TIM is 47.2°C higher than that of pure SR. In addition, the TIM has a low hardness (shore A hardness <70) and can be bent and folded, so can be used to substitute traditional thermally conductive silicone pad.

Spherical boron nitride/pitch‐based carbon fiber/silicone rubber composites for high thermal conductivity and excellent electromagnetic interference shielding performance

AbstractComposites with high thermal conductivity and excellent electromagnetic interference (EMI) shielding performance are very important for the long‐term stable operation of electronic equipment. In this work, we report a simple and effective method to construct thermally and electrically conductive network pathways by hybridizing pitch‐based carbon fiber (PCF) and spherical boron nitride (s‐BN), in which PCF plays the role in building the framework, and s‐BN acts as a bridge to connect the PCF framework. This can simultaneously enhance the thermal conductivity and EMI shielding efficiency (SE) of silicone rubber (SR) composites. The s‐BN10/PCF20/SR composite could achieve 5.81 W m−1 K−1 of bulk thermal conductivity. The total EMI SE reached 51 dB at the X‐band of 8.2–12.4 GHz. In addition, the s‐BN10/PCF20/SR would have a good heat resistance index (HRI). The tensile strength and elongation at break of the s‐BN10/PCF20/SR were 1.08 MPa and 21.98%, respectively, and its density was only 1.391 g/cm3. In brief, this work provides a simple and effective method to fabricate SR composites with light weight, good thermal stability and suitable mechanical properties to be applied in thermal management and EMI shielding of current electronic equipment.

Effect of particle morphology on reactivity, ignition and combustion of boron powders

Spherical boron powders with 2–20 µm sized particles were prepared by ball milling commercial boron in the presence of emulsion with hexane and acetonitrile serving as the continuous and droplet phases, respectively. A small amount of Fluorel®, a fluorocarbon serving as binder was dissolved in the acetonitrile. For comparison, an irregularly-shaped boron powder was prepared by milling commercial boron with the same acetonitrile/Fluorel® solution. Reactivity of the spherical boron powders was compared to those of milled and as received commercial powders with irregularly-shaped particles. Thermo-gravimetric measurements in Ar/O2 gas flow showed that milling boron with or without Fluorel® shifts the onset of boron oxidation to lower temperatures. The rate of initial oxidation was greater for the finer spherical powders compared to coarser spheres and to irregularly shaped milled powder. The milling did not appreciably change the activation energy of oxidation of boron and the observed accelerated oxidation was attributed to its increased specific surface area. Two of the finer prepared spherical powders could be ignited when coated on an electrically heated filament in the temperature range of ca. 600 – 1000 °C. Commercial powders, irregularly-shaped milled powder, and coarser spherical powders did not ignite. All boron powders aerosolized and ignited in an enclosed vessel in air generated similar pressures. In another experiment, blended boron and potassium nitrate powders were heated to ignition in room air using a laser beam. The ignition delays for the blends made with spherical powders were substantially shorter vs. similar blends with commercial powder. Combustion of the blends with spherical powders was more vigorous than with commercial boron. Particles were ejected more rapidly, and had shorter apparent burn times.

Characteristic and Synthesis of High-Temperature Resistant Liquid Crystal Epoxy Resin Containing Boron Nitride Composite.

Five liquid crystal epoxy resins and composites containing flat boron nitride (f-BN) and spherical boron nitride (s-BN) were successfully synthesized. The chemical structures, crystal diffraction, and thermal conductivity of the liquid crystal (LC) epoxy composites were measured using Nuclear Magnetic Resonance (NMR), Differential Scanning Calorimetry (DSC), X-ray, and Discovery Xenon Flash. In this study, the molecular arrangement of five LC epoxy resins and the thermal conductivity of their composites were carefully discussed. Several different amounts of flat boron nitride and spherical boron nitride were added to the five LC epoxy resins. The influence of nano-scale ceramic materials, f-BN, and s-BN, on the thermal conductivity of the LC epoxy resins, was studied. It is worth noting that the thermal conductivity of the spherical boron nitride composite demonstrated a better result than that of the flat boron nitride composite. In simpler terms, the thermal conductivity of the composites is closely related to the molecular arrangement of the LC resin and the amount of BN added. The results demonstrate that the SBPDAE/s-BN (60%) composite shows the highest thermal conductivity of 9.36 W/mK in the vertical direction. These data prove that the LC alignment of the matrix will greatly enhance the thermal conductivity of the composites.

Enhancing thermal conductivity of silicone rubber via constructing hybrid spherical boron nitride thermal network

AbstractIn this work, a series of spherical boron nitride/silicone rubber (sBN/SR) composites were prepared, and the influence of different shapes and sizes of BN on the thermal conductivities of the composites was investigated. Additionally, hybrid spherical boron nitride/silicon rubber (hybrid sBN/SR) composites were fabricated by synchronously incorporating two kinds of sBN particles into SR to perfect the thermal conduction paths, and a synergistic effect of the fillers was achieved. The thermal conductivity of the composite could reach 1.70 W/(m·K) at the 35 wt% filler content, which is clearly superior to the composite containing single type of filler and stands at a very high level at such a filler fraction according to the reported literature. Also, the experimental results were supported by the Agari's equation. In addition, the composites show a low hardness (shore A hardness < 60) due to the low filling content. The prepared materials hold great promise in thermal interface materials for thermal management of state‐of‐the‐art electronic equipment.

Study on a High-Boron-Content Stainless Steel Composite for Nuclear Radiation.

In this research, a high-boron-content composite material with both neutron and γ rays shielding properties was developed by an optimized design and manufacture. It consists of 304 stainless steel as the matrix and spherical boron carbide (B4C) particles as the functional particles. The content of B4C is 24.68 wt%, and the particles’ radius is 1.53 mm. The density of the newly designed material is 5.17 g·cm−3, about 68.02% of that of traditional borated stainless steel containing 1.7 wt% boron, while its neutrons shielding performance is much better. Firstly, focusing on shielding properties and material density, the content and the size of B4C were optimized by the Genetic Algorithm (GA) program combined with the MCNP program. Then, some samples of the material were manufactured by the infiltration casting technique according to the optimized results. The actual density of the samples was 5.21 g cm−3. In addition, the neutron and γ rays shielding performance of the samples and borated stainless steel containing 1.7 wt% boron was tested by using an 241Am–Be neutron source and 60Co and 137Cs γ rays sources, respectively, and the results were compared. It can be concluded that the new designed material could be used as a material for nuclear power plants or spent-fuel storage and transportation containers with high requirements for mobility.

Electrical Treeing Breakdown Characteristics of Epoxy/Spherical Boron Nitride with Card‐House Structure Composites

A spherical boron nitride with a card‐house structure (sBNch) was developed to enhance the filler's thermal property. To develop a thermal conductive insulating composite material with higher thermal conductive and acceptable electrical breakdown strength, an epoxy (EP)/an sBNch composite was produced. The treeing breakdown voltage of the composite using a needle‐rod electrode and the thermal conductivity were measured. The treeing breakdown voltage of EP/sBNch composite with the sBNch content of 50 wt% was higher than that of EP without the sBNch. The protrusion on the surface of the sBNch increases the effective distance between the electrodes. The thermal conductivity of EP/sBNch composite with the sBNch content of 50 wt% was higher than that of EP. From these results, the sBNch is considered a strong candidate filler of a thermally conductive composite. © 2021 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC.

Spherical boron powders prepared by mechanical milling in immiscible liquids

Spherical boron powders with sizes ranging from 4 to 100 μm were prepared by ball milling a commercial boron powder. A planetary mill was used with a process control agent consisting of two immiscible fluids: hexane and acetonitrile. The spherical powders formed when the volume of the loaded boron was close to that of acetonitrile in presence of excess hexane. Particle sizes were measured using low-angle laser light scattering, they were examined using scanning electron microscopy, and their specific surface area was measured using nitrogen adsorption. The size distributions of the prepared spheres were much narrower than is common for solids processed by high-energy milling. Spherical particles were fragile; their mechanical stability was improved when a small amount of Fluorel®, a fluorocarbon binder, was preliminarily dissolved in acetonitrile. Experiments showed that the prepared spherical powders exhibited a higher bulk density and much better flowability than starting boron powder. Process-property relations were explored by systematically varying milling parameters and analyzing their effect on the size distribution of the formed spheres. The size of spherical particles was found to decrease with the decrease in the amount of acetonitrile used; it was also decreased with an increase in milling time, number of milling balls, and speed of milling.

Improved thermal conductivity of fluids and composites using boron nitride (BN) nanoparticles through hydrogen bonding

A new method, which utilized hydrogen bonding to enhance the effective thermal conductivity of materials is presented and discussed. Spherical boron nitride (BN) nano particles were mixed and exfoliated to form hydrogen bonds between the particles, fluids, and polymer solutions containing glycerol, water, krytox oil, polyurethane solutions, and other similar substances. The resulting long chain carbon fibers were found to significantly enhance the thermal conductivity of these substances, due to increases in the contact and interfacial surface area. The experimental results indicated that a 35 wt% BN loading in polyurethane could increase the effective thermal conductivity of the base polyurethane composite to a value of 1.78 W/mK, which is a 390 % increase over the base polyurethane composite. In addition, a 20 wt% BN loading in polyurethane with a 5 wt% carbon fiber loading, was found to increase the effective thermal conductivity to a value of 2.10 W/mK, or a 480 % increase over the base polyurethane composite. These results are among the highest values ever reported and can in part, be attributed to the three roll-milling technique utilized to enhance the contact. This work may open the way to further explore enhanced thermal materials with numerous and significant potential commercial applications.

Hypergravity-Induced Accumulation: A New, Efficient, and Simple Strategy to Improve the Thermal Conductivity of Boron Nitride Filled Polymer Composites.

Thermal conductive polymer composites (filled type) consisting of thermal conductive fillers and a polymer matrix have been widely used in a range of areas. More than 10 strategies have been developed to improve the thermal conductivity of polymer composites. Here we report a new “hypergravity accumulation” strategy. Raw material mixtures of boron nitride/silicone rubber composites were treated in hypergravity fields (800–20,000 g, relative gravity acceleration) before heat-curing. A series of comparison studies were made. It was found that hypergravity treatments could efficiently improve the microstructures and thermal conductivity of the composites. When the hypergravity was about 20,000 g (relative gravity acceleration), the obtained spherical boron nitride/silicone rubber composites had highly compacted microstructures and high and isotropic thermal conductivity. The highest thermal conductivity reached 4.0 W/mK. Thermal interface application study showed that the composites could help to decrease the temperature on a light-emitting diode (LED) chip by 5 °C. The mechanism of the improved microstructure increased thermal conductivity, and the high viscosity problem in the preparation of boron nitride/silicone rubber composites, and the advantages and disadvantages of the hypergravity accumulation strategy, were discussed. Overall, this work has provided a new, efficient, and simple strategy to improve the thermal conductivity of boron nitride/silicone rubber and other polymer composites (filled type).

Improvement of thermal conductivities and simulation model for glass fabrics reinforced epoxy laminated composites via introducing hetero-structured BNN-30@BNNS fillers

Hetero-structured thermally conductive spherical boron nitride and boron nitride nanosheets (BNN-30@BNNS) fillers were prepared via electrostatic self-assembly method. And the corresponding thermally conductive & electrically insulating BNN-30@BNNS/Si-GFs/E-44 laminated composites were then fabricated via hot compression. BNN-30@BNNS-III (fBNN-30/fBNNS, 1/2, wt/wt) fillers presented the optimal synergistic improvement effects on the thermal conductivities of epoxy composites. When the mass fraction of BNN-30@BNNS-III was 15 wt%, λ value of the BNN-30@BNNS-III/E-44 composites was up to 0.61 W m−1K−1, increased by 2.8 times compared with pure E-44 (λ = 0.22 W m−1K−1), also higher than that of the 15 wt% BNN-30/E-44 (0.56 W m−1K−1), 15 wt% BNNS/E-44 (0.42 W m−1K−1), and 15 wt% (BNN-30/BNNS)/E-44 (direct blending BNN-30/BNNS hybrid fillers, 1/2, wt/wt, 0.49 W m−1K−1) composites. The λ in-plane (λ//) and λ cross-plane (λ┴) of 15 wt% BNN-30@BNNS-III/Si-GFs/E-44 laminated composites significantly reached 2.75 W m−1K−1 and 1.32 W m−1K−1, 186.5 % and 187.0 % higher than those of Si-GFs/E-44 laminated composites (λ// = 0.96 W m−1K−1 and λ┴ = 0.46 W m−1K−1). Established models can well simulate heat transfer efficiency in the BNN-30@BNNS-III/Si-GFs/E-44 laminated composites. Under the condition of point heat source, the introduction of BNN-30@BNNS-III fillers were conducive to accelerating heat flow transfer. BNN-30@BNNS-III/Si-GFs/E-44 laminated composites also demonstrated outstanding electrical insulating properties (cross-plane withstanding voltage, breakdown strength, surface & volume resistivity of 51.3 kV, 23.8 kV mm−1, 3.7 × 1014 Ω & 3.4 × 1014 Ω·cm, favorable mechanical properties (flexural strength of 401.0 MPa and ILSS of 22.3 MPa), excellent dielectric properties (ε of 4.92 and tanδ of 0.008) and terrific thermal properties (Tg of 167.3 °C and THRI of 199.2 °C).