Solvent-free fabrication of thermally conductive insulating epoxy composites with boron nitride nanoplatelets as fillers

A solvent-free method for the fabrication of thermally conductive epoxy-boron nitride (BN) nanoplatelet composite material is developed in this study. By this method, polymer composites with nearly any filler fractions can be easily fabricated. The maximum thermal conductivity reaches 5.24 W/mK, which is 1,600% improvement in comparison with that of pristine epoxy material. In addition, the as-fabricated samples exhibit excellent overall performances with great mechanical property and thermal stability well preserved.     

Enhanced out-of-plane thermal conductivity of polyimide composite films by adding hollow cube-like boron nitride

Boron nitride nanosheet (BNNS) is widely used in electronic thermal management due to its excellent planar thermal conductivity and insulating properties. However, it is challenging to improve the out-of-plane thermal conductivity of BNNS-doped composites due to the anisotropy of the thermal conductivity of BNNS. Therefore, the BNNS in the matrix must be oriented to obtain composites with high out-of-plane thermal conductivity. In this study, BNNS powders with directional structures were synthesized directly using sodium chloride templates. The as-obtained BNNS powders have a unique hollow cube-like structure with an ultra-low density of 2.67 × 10⁻² g/cm³ and nearly 8 times the volume of the same mass of two-dimensional (2D) BNNS, making it easy to form the out-of-plane thermal conductivity paths in the polymer matrix. In addition, the high out-of-plane thermal conductivity of 4.93 W m⁻¹ K⁻¹ at 23.3 wt% loadings was obtained by doping it into a polyimide (PI) matrix. This value is 9.7 times higher than that of 2D BNNS-doped PI at the same loadings, 17.6 times higher than pure PI, and 6.1 times higher than the thermally conductive PI film sold by DuPont. Therefore, the prepared composite film has great potential for application in electronic thermal management.     

Mechanical,dielectric and thermal properties of porous boron nitride/silicon oxynitride ceramic composites prepared by pressureless sintering

Porous boron nitride/silicon oxynitride (BN/Si₂N₂O) composites were fabricated by pressureless sintering at 1650 °C with Li₂O as sintering aid. The influence of Li₂O and hexagonal boron nitride (h-BN) contents on phase, microstructure, mechanical, dielectric and thermal properties of the resulting porous BN/Si₂N₂O composites was investigated. Increasing Li₂O content facilitated densification and decomposition of Si₂N₂O into Si₃N₄. The apparent porosity of the composites increases with the h-BN content increases and Si₂N₂O grain growth was restrained by the dispersed h-BN particles. The dielectric properties and thermal conductivities (TC) were affected mainly by porosity. Porous BN/Si₂N₂O ceramic composites with 4 mol% Li₂O and 25 mol% BN exhibit both low dielectric constant (3.83) and dielectric loss tangent (0.008) with good mechanical and thermal performance, suggesting possible use as high-temperature structural/functional materials.     

Highly thermally conductive hexagonal boron nitride/alumina composite made from commercial hexagonal boron nitride

Hexagonal BN is an unusual material in that it is both highly thermally conductive as well as an electrical insulator. Additionally, hBN is also thermally stable in air. This unusual combination of properties makes hBN of significant interest for thermal management. Unfortunately, hBN is not easily consolidated into substrates without the addition of second phases which generally result in poorer thermal performance. This research investigates the potential to utilize this material to dissipate heat from high‐voltage, high‐power electrical devices. Specifically, a process to coat individual platelets of commercial hexagonal BN powder with a layer of amorphous aluminum oxide was developed. The coated hexagonal BN was then hot‐pressed to form a highly thermally conductive substrate. The process to coat hexagonal BN platelets with aluminum oxide was accomplished by mixing hexagonal BN with AlCl₃ containing some water, then evaporation of excess AlCl₃ to form a Al, Cl, and O layer on hexagonal BN. This product was then heated in air to convert the surface layer into aluminum oxide. Following hot pressing to 1950°C and 10 ksi, the consolidated composite has through‐plane and in‐plane thermal conductivity of 14 and 157 W·(m·K)^?1, respectively, at room temperature.     

Synergistic effect between hollow glass beads and aluminium hydroxide in flame retardant EVA composites

Abstract

Flame retardant ethylene vinyl acetate (EVA) composites were prepared based on different contents of hollow glass beads (HGBs) and aluminium hydroxide (ATH) in this paper, and the flame retardant properties were studied using limiting oxygen index, UL-94 test and cone calorimeter test respectively. The results showed that EVA-4 with 1·0 wt-%HGBs has the lowest heat release rate, total heat release and smoke production rate among all samples, and HGBs could promote to form a compact char layer on the surface of the sample. It indicates that there is an obvious synergistic flame retardant effect between HGBs and ATH in this composite.     

High thermal conductive shape-stabilized phase change materials based on water-borne polyurethane/boron nitride aerogel

Phase change materials (PCMs) applied in energy storage and temperature control system are important for energy conservation and environmental protection. In this work, structure-adjustable water-borne polyurethane (WPU)/boron nitride (BN) aerogels were synthesized via directional freeze-drying method, and used as supporting scaffolds to confine paraffin wax (PW) and obtain composite phase change materials. The three-dimensional (3D) porous thermal conductivity network of BN was derived by the in-situ ice crystal mound in aerogel, which endows the PW/WPU/BN composite PCM-2.5 with high thermal conductivity (0.96 W m^?1 K^?1) and high energy storage density (140.04 J/g). Shape-stabilized PCMs with high thermal conductivity and excellent electrical insulation prepared by the simple method have great potential for the thermal management of electronic products.     

Preparation and properties of boron nitride nanosheets/cellulose nanofiber shear-oriented films with high thermal conductivity

A highly thermally conductive boron nitride nanosheets/cellulose nanofiber (BNNS/CNF) oriented film was prepared by doctor blading method via mechanical shear-induced orientation. The SEM images for cross-sectional parts showed that BNNS were well aligned within the film, forming a good layered structure. The XRD results further confirmed the high orientation effect of BNNS. Due to the excellent thermal stability of BNNS and its good physical barrier effect on the matrix after the orientation treatment, the thermal stability of shear-oriented films was largely improved. Resulting from the shear-induced orientation, BNNS were closely contacted with each other, forming a good thermally conductive pathway within the CNF matrix. Thus the influence of the interfacial thermal resistance was dramatically reduced, and the thermal conductivity of shear-oriented films increased in proportion to filler loading. With 50 wt% BNNS, the thermal conductivity of the shear-oriented film reached 24.66 W/(m·K), which exhibited 1106% enhancement compared to the pure CNF.     

Mechanical,thermal and rheological properties of hollow glass microsphere filled thermoplastic polyurethane composites blended by normal vane extruder

In this study, different proportions of hollow glass microsphere (HGM) filled thermoplastic polyurethane (TPU) elastomer composites were prepared by normal vane extruder. The phase morphology, rheological properties, mechanical properties and thermal stability of TPU/HGM composites were investigated in detail. It was observed that the incorporation of HGM increased the thermal stability of TPU. With the increase in HGM content from 5 to 20?wt-%, the glass transition temperature, storage modulus and complex viscosity values increased continuously, while the density of the composite monotonously decreased. Moreover, the tensile modulus of the composites increased from 43.5 to 81.3?MPa. The results showed that, although raw HGM was used without any modification, the HGM played an important role in improving the properties of TPU.     

Mechanical and dielectric properties of functionalized boron nitride nanosheets/silicon nitride composites

Uniformly dispersed boron nitride nanosheets (BNNSs) reinforced silicon nitride (Si₃N₄) composites were prepared by surface modification assisted flocculation combined with SPS sintering. In order to improve the dispersibility of the BNNSs in the composites, the liquid phase stripped BNNSs are surface functionalized by a two-step covalently modification. The amino-modified BNNSs (NH₂-BNNSs) and Si₃N₄ powders have opposite surface potential, mixed evenly by electrostatic interaction during flocculation. The results showed that mechanical properties of Si₃N₄ composites were obviously enhanced by adding NH₂-BNNSs. The fracture toughness and bending strength of Si₃N₄ composites added 0.75 wt% NH₂-BNNSs were increased by 34% and 28%, respectively, compared with monolithic Si₃N₄. Toughening mechanisms are synergistic action of the torn, pull-out or bridging of BNNSs and crack deflection mechanisms with microstructural analyzes. The dielectric properties of the Si₃N₄ ceramics are also improved after the addition of NH₂-BNNSs.     

环氧树脂/玻纤/BN导热绝缘复合材料制备研究

采用高温模压成型法制备环氧树脂/玻纤/BN导热复合材料,探讨了BN用量对复合材料力学性能、导热性能和电性能的影响,结果表明.当BN用量为10%时,复合材料的冲击强度和弯曲强度较佳;导热性能随BN用量的增加而提高,当BN用量为20%耐.热导率为0.7438 W/mk,此时复合材料仍保持较好的绝缘性能.     

耐高温高导热环氧树脂／玻纤／BN复合材料的制备

以4,4-二氨基二苯砜（DDS）和内亚甲基四氢邻苯二甲酸酐（NA）为复配固化剂,采用高温模压成型法制备耐高温高导热环氧树脂／玻纤／氮化硼（BN）复合材料。探讨了BN用量和偶联剂处理对复合材料冲击强度、导热性能和电阻率的影响。结果表明：当nDDS：nNA=3：1时,复合材料的耐热性能最佳。当BN质量分数为8％时,复合材料的冲击强度最高;导热性能随BN用量的增加而增加,当BN用量为15％时,热导率为0．7560W／（mk）,此时复合材料仍保持较高的体积、表面电阻率;当BN填充量为一定值时,偶联剂处理使冲击强度和导热性能得到进一步提高。     

Design of carbon fiber reinforced boron nitride matrix composites by vacuum-assisted polyborazylene transfer molding and pyrolysis

In the present study, the vacuum-assisted polyborazylene transfer molding (VAPTM) followed by pyrolysis at 1450 °C under nitrogen was successfully applied to prepare C/BN composites with a relative density of 94.7%, and an open porosity of 5.1 vol%. Rheology measurements combined with TG experiments and measurement of the weight gain of the composites after each polymer impregnation pyrolysis (PIP) cycles allowed fixing elaboration parameters (10 PIP cycles using two types of polyborazylene) to generate C/BN composites as high temperature vacuum compatible materials which demonstrate potentialities for vacuum technology and space applications. Scanning electron micrographs of composites demonstrated that the BN matrix appropriately filled the space between the fibers with a low fiber-matrix bonding which affects the compressive modulus. The thermal diffusivity and electrical conductivity of the BN matrix have been measured and showed that carbon fibers have poor effects on the intrinsic properties of BN.     

Fabrication of thermally conductive composite with surface modified boron nitride by epoxy wetting method

Boron nitride (BN) particles fabricated with different surface treatments were used to prepare thermally conductive polymer composites by epoxy wetting. The polar functionality present on the BN particles allowed the permeation of the epoxy resin because of a secondary interaction, which allowed the fabrication of a composite containing high filler concentration. The different cohesive energy densities of the synthesized material due to a functional-group-induced surface treatment effect on surface free energy and wettability determined the thermal and mechanical properties of the polymer. The results indicate that surface-curing agents interrupt the interaction between the filler and matrix, and do not always enhance thermal conductivity. Moreover, the composites showed maximum thermal conductivity at 30 wt% epoxy loading when the fixed-pore volume fraction reached in the filtrated BN film. The measured storage modulus was also enhanced by surface treatment because of the sufficient interface produced and interaction between the large amount of the filler and epoxy.     

氮化硼填充甲基乙烯基硅橡胶导热复合材料的性能

用0.3,6.0,20.0μm 3种粒径的氮化硼(BN)(质量比为1:1:3)混合填充甲基乙烯基硅橡胶(MVQ),研究了BN用量对MVQ导热系数、热失重、热膨胀系数、硫化特性的影响.结果表明,随着BN用量的增加,MVQ的导热系数和热分解温度升高,热失重量和热膨胀系数明显降低,但对MVQ的硫化反应影响不大;当BN填充量为150份时,MVQ的综合性能较佳.     

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.     

Thermal conductivity and mechanical properties of thermally conductive composites based on multifunctional epoxyorganosiloxanes and hexagonal boron nitride

As electronics become portable and compact with concomitant thermal issues, the demand for high-performance thermal interface materials has increased. However, the low thermal conductivity of polymers and the poor dispersion of fillers impede the realization of high filler loading composites, and this in turn limits the increase in thermal conductivity. To overcome this, multifunctional epoxyorganosiloxanes (MEOSs) were synthesized and used to fabricate thermally conductive composites in this study. In the first part of this study, the effect of the molecular weights of MEOSs on the curing behaviors of the MEOSs/trimethylolpropane tris(3-mercaptopropioante)/1-methyl imidazole systems was investigated by a DSC analysis. Both the nonisothermal and isothermal curing of the epoxy compositions (ECs) verified that the reaction rate of EC-1 containing MEOS-1 with lower molecular weight was faster than that of EC-2. In addition, mechanical properties of the cured EC-1 were superior to those of its counterpart because of a higher density in crosslinking. In the second part, EC-1 was admixed with h-BN to fabricate thermally conductive (TC) composites. Owing to the low viscosity (1.6 Pa s at 0.1 Hz) of EC-1, a TC-3 composite containing 45 wt% h-BN fillers was obtained, and the in-plane and through-plane thermal conductivity of the cured TC-3 composite reached 3.55 ± 0.29 Wm^?1K^?1 and 1.08 ± 0.08 Wm^?1K^?1, respectively. Furthermore, the tensile modulus of the cured TC-3 was measured as 76.3 ± 6.1 MPa, which was 9.1 times higher than that of the cured EC-1. Both the high thermal conductivity and good mechanical properties of the cured TC-3 composite were ascribed to the percolation of h-BN networks stemming from the high filler loading.     

Ablation behavior and mechanism of boron nitride - magnesium aluminum silicate ceramic composites in an oxyacetylene combustion flame

In the present study, ablation behavior and properties of BN-MAS (magnesium aluminum silicate) composites impinged with an oxyacetylene flame at temperatures up to 3100 °C were investigated. As ablation time ranged from 5 to 30 s, the mass and linear ablation rates increased from 0.0027 g/s and 0.001 mm/s to 0.0254 g/s and 0.087 mm/s, respectively. A SiO₂-rich protective oxide layer formed during the ablation process, which contributed to the oxidation resistance of the composites. Ablation products mainly consisted of magnesium-aluminum borosilicate glass, mullite, spinel and indialite. The thermal oxidation of h-BN during flame ablation and scouring of MAS by high-speed gas flow were the main ablation mechanisms.     

Highly electrically and thermally conductive silicon carbide-graphene composites with yttria and scandia additives

Dense silicon carbide/graphene nanoplatelets (GNPs) and silicon carbide/graphene oxide (GO) composites with 1 vol.% equimolar Y₂O₃–Sc₂O₃ sintering additives were sintered at 2000 °C in nitrogen atmosphere by rapid hot-pressing technique. The sintered composites were further annealed in gas pressure sintering (GPS) furnace at 1800 °C for 6 h in overpressure of nitrogen (3 MPa). The effects of types and amount of graphene, orientation of graphene sheets, as well as the influence of annealing on microstructure and functional properties of prepared composites were investigated. SiC-graphene composite materials exhibit anisotropic electrical as well as thermal conductivity due to the alignment of graphene platelets as a consequence of applied high uniaxial pressure (50 MPa) during sintering. The electrical conductivity of annealed sample with 10 wt.% of GNPs oriented parallel to the measuring direction increased significantly up to 118 S·cm⁻¹. Similarly, the thermal conductivity of composites was very sensitive to the orientation of GNPs. In direction perpendicular to the GNPs the thermal conductivity decreased with increasing amount of graphene from 180 W·m⁻¹ K⁻¹ to 70 W·m⁻¹ K⁻¹, mainly due to the scattering of phonons on the graphene – SiC interface. In parallel direction to GNPs the thermal conductivity varied from 130 W·m⁻¹ K⁻¹ up to 238 W·m⁻¹ K⁻¹ for composites with 1 wt.% of GO and 5 wt.% of GNPs after annealing. In this case both the microstructure and composition of SiC matrix and the good thermal conductivity of GNPs improved the thermal conductivity of composites.     

Alkylated modified boron nitride nanosheets/polyimide composite films with advanced thermal conductivity and low dielectric constant

Owing to the rapid development of the latest micro-electronic devices, polymer composite materials that combine high thermal conductivity and low permittivity have aroused the interest of researchers. However, it is a huge challenge to balance the above parameters. In this work, hexagonal boron nitride (h-BN) powder was ultrasonically exfoliated to obtain alkylated boron nitride nanosheets (Alkyl-BNNS). Then, a series of polyimide (PI) composites were synthesized with different amounts of Alkyl-BNNS. Attributed to more robust interfacial non-covalent interactions between Alkyl-BNNS and polymer chains to inhibit interfacial polarization, Alkyl-BNNS can be scattered well in PI substrate. Thus, the obtained PI composite behaved a high thermal conductivity of 6.21 W/(mK) and a low dielectric constant (3.23) under the load of 20 wt%. Besides, Alkyl-BNNS/PI composites have efficient thermal management capability, low water absorption, favorable electrical resistance, and prominent tensile strength. Importantly, these composite films are expected to be excellent candidates in the field of microelectronics.