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.     

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.     

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.     

Heat propagation in thermally conductive polymers of PA6 and hexagonal boron nitride

Thermally conductive polymers offer new possibilities for the heat dissipation in electric and electronic components, for example, by a three‐dimensional shaping of the heat sinks. To face safety regulations, improved fire performance of those components is required. In contrast to unfilled polymers, those materials exhibit an entirely different thermal behavior. To investigate the flammability, a phosphorus flame retardant was incorporated into thermally conductive composites of polyamide 6 and hexagonal boron nitride. The flame retardant decreased the thermal conductivity only slightly. However, the burning behavior changed significantly, due to a different heat propagation, which was investigated using a thermographic camera. An optimum content of hexagonal boron nitride for a sufficient thermal conductivity and fire performance was found between 20 and 30 vol%. The improvement of the fire performance was due to a faster heat release out of the pyrolysis zone and an earlier decomposition of the flame retardant. For higher contents of hexagonal boron nitride, the heat was spread faster within the part, promoting an earlier ignition and increasing the decomposition rate of the flame retardant.     

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 (15–25 vol%) with the aid of a binder phase (0–6 vol%) at moderate pressures (0.5–1.0 GPa) 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 1 GPa, and had only slightly lower densities at 0.5 GPa. 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.     

Lightweight and thermally conductive thermoplastic polyurethane/hollow glass bead/boron nitride composites with flame retardancy

Polymeric composites with low density and high thermal conductivity (TC) are greatly demanded in some specific applications such as aeronautics, astronautics, and deep-sea exploration. It is a great challenge to obtain lightweight and thermally conductive polymer composites because the heat fillers have high density (>2 g/cm³) Herein, lightweight and thermally conductive thermoplastic polyurethane/hollow glass bead/boron nitride composites (TPU/HGB/BN) were prepared with the construction of a 3D BN network under the assistance of ultralightweight HGB by a solution-mixing and hot-pressing method. A 3D BN heat network has been constructed in the TPU matrix due to the alignment of the BN platelets along with the HGB microspheres during hot-pressing, which leads to a higher TC (5.34 W/mK) of the TPU/HGB/BN composites with a low density of 1.23 g/cm³, which is close to the density of pure TPU (1.20 g/cm³). In addition, the TPU/HGB/BN composites show good thermal stability with TC losses of 4.24% and 2.22%, respectively, even after treated for 50 hot-cold cycles and heated at 80 °C for 50 h. Moreover, the limiting oxygen index (LOI) of the TPU/HGB/BN composites is 51%, and they can extinguish in 8 s after ignition and exhibit enhanced flame retardancy. This work presents a simple method to design and prepare lightweight, flame retardant and thermally conductive composite materials, which can be used as lightweight thermal management materials.     

Development of thermally conductive polymer matrix composites by foaming‐assisted networking of micron‐ and submicron‐scale hexagonal boron nitride

Thermally conductive polymer matrix composite (PMC) foams with effective thermal conductivities (k_eff) higher than their solid counterparts have been developed for the first time. Using a material system consists of low density polyethylene and micron‐scale or submicron‐scale hexagon boron nitride platelets as a case example, this article demonstrates that foaming‐assisted filler networking is a feasible processing strategy to enhance PMC's k_eff, especially at a low hBN loading. Parametric studies were conducted to identify the structure‐to‐property relationships between foam morphology (e.g., cell population density, cell size, and foam expansion) and the PMC foam's k_eff. In particular, there exists an optimal cell size to maximize the PMC foam's k_eff for foams with up to 50% volume expansion. However, an optimal cell size is absent for PMC foams with higher volume expansion. X‐ray diffraction (XRD) analyses reveal that both the presence of hBN platelets and foam expansion promoted the crystallization of LDPE phase. Moreover, the XRD spectra also provide evidence for the effect of foam expansion on the orientation of hBN platelets. Overall, the findings provide new directions to design and fabricate thermally conductive PMC foams with low filler contents for heat management applications. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 42910.     

基于变径-变压塔的甲醇/氯仿二元共沸物精馏分离过程热集成与动态控制

研究了基于变径-变压塔(VDC-PSD)联合处理二元共沸物甲醇-氯仿分离过程的热集成,基于Aspen dynamics对动态控制方案进行了仿真验证,并通过改进控制结构实现了对进料流量和进料组成在±20%扰动下的自动控制;在没有增加控制难度的情况下,带有热集成的VDC-PSD可以实现热量的综合利用,提高PSD的经济性。     

Interfacial modification of boron nitride nanoplatelets for epoxy composites with improved thermal properties

Interface is a critical factor in determining the properties of polymer composites. Generally, the physicochemical properties of the interface are closely associated with the surface chemistry of fillers. In this study, we report a simple method to fabricate boron nitride (BN) nanoplatelets using a sonication-centrifugation technique and investigate the effects of functionalization BN nanoplatelets on thermal properties of epoxy composites. Two methods have been used for functionalizing BN nanoplatelets: non-covalent functionalization by octadecylamine (ODA) and covalent functionalization by hyperbranched aromatic polyamide (HBP). The functionalized BN nanoplatelets were characterized by Fourier-transform infrared (FT-IR), nuclear magnetic resonance (¹H NMR), thermogravimetric analyzer (TGA), and transmission electron microscopy (TEM). Epoxy composites were fabricated by incorporating three kinds of fillers: BN nanoplatelets, BN nanoplatelets functionalized by ODA (BN-ODA), and BN nanoplatelets functionalized by HBP (BN-HBP). Our results show that the BN-HBP results in a strong interface and thus the composites exhibit significantly increased glass transition temperature, thermal decomposition temperature, thermal conductivity and dynamic thermal mechanical modulus. BN-ODA produced intermediate interface interaction, resulting in a moderate improvement of thermal properties. The composites with BN nanoplatelets show the least improvements of thermal properties.     

Nucleation and growth of single crystal graphene on hexagonal boron nitride

Direct graphene growth was demonstrated on exfoliated hexagonal boron nitride (h-BN) single crystal flakes by low pressure CVD. The size of the hexagonal single crystal graphene domain increases with deposition time, with maximum size of ～270 nm. Most domains were found to nucleate at screw dislocation sites, and a step-flow growth mechanism was observed at atomic steps on the h-BN surface. Understanding the nucleation and growth mechanisms is an important step towards the synthesis of large single crystal graphene on h-BN substrates.     

Production and characterization of spark plasma sintered (Ti,Nb)B2 solid solutions with graphene nanoplatelets and hexagonal boron nitride

For this study, (Ti,Nb)B₂ solid solutions were consolidated by spark plasma sintering. In addition, (Ti,Nb)B₂ with graphene nanoplatelets (GNPs) and hexagonal boron nitride (h-BN) were produced to evaluate the potential of the new structural materials. The phase formation, microstructure, mechanical properties, oxidation resistance and room temperature reflectance, and absorbance features of (Ti,Nb)B₂ were investigated. X-ray diffraction and Transmission electron microscopy observations showed that a complete solid solution phase was formed when the samples were sintered at 1850 °C for 5 min under 50 MPa. Ti_0.75Nb_0.25B₂ exhibited a relative density of ～98.6%, a hardness of ～20.5 GPa, and an indentation fracture toughness of ～3.4 MPa·m^1/2. It was found that the presence of 1 vol% h-BN as an additive enhanced the hardness (～10%) and fracture toughness (～30%) of Ti_0.75Nb_0.25B₂ by activating toughening mechanisms. The GNP added Ti_0.75Nb_0.25B₂ proved to have better oxidation resistance and optical absorbance than the other materials used in the study.     

Preparation and properties of thermally conductive photosensitive polyimide/boron nitride nanocomposites

A new thermally conductive photoresist was developed. It was based on a dispersion of boron nitride (BN) nanoflakes in a negative‐tone photosensitive polyimide (PSPI) precursor. 3‐Mercaptopropionic acid was used as the surfactant to modify the BN nanoflake surface for the dispersion of BN nanoflakes in the polymer. The thermal conductivity of the composite films increased with increasing BN fraction. The thermal conductivity of the PSPI/BN nanocomposite was up to 0.47 W m⁻¹ K⁻¹ for a mixture containing 30 wt % nanosized BN filler in the polyimide matrix. Patterns with a resolution of 30 μm were obtained from the PSPI/BN nanocomposites. The PSPI/BN nanocomposites had excellent thermal properties. Their glass‐transition temperatures were above 360°C, and the thermal decomposition temperatures were over 460°C. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Synergistic enhancement of thermal conductivity by addition of graphene nanoplatelets to three-dimensional boron nitride scaffolds for polyamide 6 composites

Three-dimensional boron nitride/graphene nanoplatelets (3D-BN-GNP) scaffolds were fabricated using an ice-templating method and polyamide 6 (PA6)-based composites were prepared by vacuum impregnation of caprolactam monomers into the scaffolds, followed by polymerization. The BN sheets in the PA6/3D-BN and PA6/3D-BN-GNP composites display a predominant parallel alignment along the ice-crystal formation constructing thermally conductive paths. The addition of few GNPs assists the dispersion of BN sheets in the PA6/3D-BN-GNP composites and repair the broken thermal paths caused by local agglomeration of the BN sheets. Consequently, GNPs play a morphology-promoted synergistic role in the enhancement of the thermal conductivity of the PA6/3D-BN-GNP composites. The PA6/3D-BN-GNP composite prepared with 23.40 wt% BN sheets and 2.60 wt% GNPs exhibits the highest thermal conductivity of 2.80 W m⁻¹ K⁻¹, which is 833% and 33% higher than the values recorded for the pure PA6 and the PA6/3D-BN composite at BN loading of 26.18 wt%, respectively. Infrared imaging analysis revealed that the surface of the PA6/3D-BN-GNP composite has a fast response to heating and cooling, suggesting the potential of the composites in thermal management applications.     

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.     

Van der Waals epitaxy and characterization of hexagonal boron nitride nanosheets on graphene

Graphene is highly sensitive to environmental influences, and thus, it is worthwhile to deposit protective layers on graphene without impairing its excellent properties. Hexagonal boron nitride (h-BN), a well-known dielectric material, may afford the necessary protection. In this research, we demonstrated the van der Waals epitaxy of h-BN nanosheets on mechanically exfoliated graphene by chemical vapor deposition, using borazine as the precursor to h-BN. The h-BN nanosheets had a triangular morphology on a narrow graphene belt but a polygonal morphology on a larger graphene film. The h-BN nanosheets on graphene were highly crystalline, except for various in-plane lattice orientations. Interestingly, the h-BN nanosheets preferred to grow on graphene than on SiO₂/Si under the chosen experimental conditions, and this selective growth spoke of potential promise for application to the preparation of graphene/h-BN superlattice structures fabricated on SiO₂/Si.     

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.     

Modification of hexagonal boron nitride nanoparticles with fluorosilane

Surfaces of hexagonal boron nitride (hBN) nanoparticles were modified with perfluorooctyl-triethoxysilane (FTS). Experiments were performed for 40–120 min in 70–150 °C range with FTS/hBN weight ratio in the range of 0.5–1.5. The products were analyzed by FT-IR, TGA, FESEM, HRTEM and EDX. Results of FT-IR analyses indicated that modification takes place in 80 min at 150 °C under reflux with a FTS/hBN ratio of 1.5. Presence of FTS on hBN nanoparticles was confirmed by the weight losses in TGA, and by TEM, TEM-EDX analyses.     

Mechanical and thermal properties of wood‐plastic composites reinforced with hexagonal boron nitride

Mechanical, thermal, and morphological properties of injection molded wood‐plastic composites (WPCs) prepared from poplar wood flour (50 wt%), thermoplastics (high density polyethlyne or polypropylene) with coupling agent (3 wt%), and hexagonal boron nitride (h‐BN) (2, 4, or 6 wt%) nanopowder were investigated. The flexural and tensile properties of WPCs significantly improved with increasing content of the h‐BN. Unlike the tensile and flexural properties, the notched izod impact strength of WPCs decreased with increasing content of h‐BN but it was higher than that of WPCs without the h‐BN. The WPCs containing h‐BN were stiffer than those without h‐BN. The tensile elongation at break values of WPCs increased with the addition of h‐BN. The differential scanning calorimetry (DSC) analysis showed that the crystallinity, melting enthalpy, and crystallization enthalpy of the WPCs increased with increasing content of the h‐BN. The increase in the crystallization peak temperature of WPCs indicated that h‐BN was the efficient nucleating agent for the thermoplastic composites to increase the crystallization rate. POLYM. COMPOS., 35:194–200, 2014. © 2013 Society of Plastics Engineers     

Enhanced mechanical and anisotropic thermal conductive properties of polyimide nanocomposite films reinforced with hexagonal boron nitride nanosheets

To attain thermally conductive but electrically insulating polymer films, in this study, polyimide (PI) nanocomposite films with 1–30 wt% functionalized hexagonal boron nitride nanosheets (BNNSs) were fabricated via solution casting and following imidization. The microstructures, mechanical and thermal conductive properties of PI/BNNS nanocomposite films were examined by taking account of the relative content, anisotropic orientation, and interfacial interaction of BNNS and PI matrix. The scanning electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and X-ray diffractometry data revealed that BNNSs with hydroxy and amino functional groups have specific molecular interactions with PI matrix and they form stacked aggregates in the nanocomposite films with high BNNS loadings of 10–30 wt%. The tensile mechanical strength/modulus, thermal degradation temperatures, and thermal conductivity of the nanocomposite films were found to be significantly enhanced with increasing the BNNS loadings. For the nanocomposite films with 1–30 wt% BNNS loadings, the in-plane thermal conductivity was measured to be 1.82–2.38 W/mK, which were much higher than the out-of-plane values of 0.35–1.14 W/mK. The significant anisotropic thermal conductivity of the nanocomposite films was found to be owing to the synergistic anisotropic orientation effects of both BNNS and PI matrix. It is noticeable that the in-plane and out-of-plane thermal conductivity values of the nanocomposite film with 30 wt% BNNS were ~1.31 and ~3.35 times higher than those of neat PI film, respectively.     

Matching micro- and nano-boron nitride hybrid fillers for high-thermal conductive composites

Incorporating hybrid fillers into polymer has been considered as one of the effective ways to obtain composites with high-thermal conductivities (TCs). Herein, we fabricated polytetrafluoroethylene (PTFE) composites by using micro-boron nitride nanosheets (mBNNs) and nano-BNNs (nBNNs) as fillers, and studied the optimum ratios of mBNNs to nBNNs (i.e., mBNNs:nBNNs) for obtaining high-thermal conductive composites at different filler's contents. The results indicated that for the composites with total BNNs contents of 10, 20, and 30 wt%, the optimum mBNNs:nBNNs for obtaining the highest TC were 9:1, 9:1, and 5:5, respectively. The highest TC of the composites with 30 wt% BNNs could reach 1.46 W·m⁻¹·K⁻¹, which was 356% higher than that of PTFE. The reasons for optimum mBNNs:nBNNs values were interpreted by observing the composite's microstructures. Moreover, the fabricated composites also exhibited excellent electrical insulation properties. This study has important implications for obtaining high-thermal conductive composites using hybrid fillers.