BN Flakes Research Articles

Flexible hexagonal boron nitride@liquid metal/polydimethylsiloxane composites with excellent thermal conductivity and superior mechanical properties

AbstractHexagonal boron nitride (h‐BN) has a layered lattice structure, high intrinsic thermal conductivity, and good chemical stability, and it has a promising applications in thermal management materials. However, poor interfacial compatibility and dispersion of BN flakes in the polymer matrix cause thermal contact resistance and gaps, directly impairing the performance of the composites and limiting their thermal conduction applications. Herein, the P‐BN@LM/PDMS thermally conductive composites with superior mechanical properties were fabricated by incorporating dopamine and liquid metal (LM) into BN flakes. The dopamine self‐polymerizes to form polydopamine (PDA) on the BN surface, denoted as P‐BN, which effectively improves interface compatibility and dispersion. The LM attaches to the functionalized P‐BN surface by mechanical grinding, acting as a bridge between adjacent BN flakes to enhance the integrity of the thermal conductive network within the composites. The coordination between P‐BN and LM increases interface strength, effectively improving mechanical performance. Hence, the P‐BN@LM/PDMS composites exhibit excellent thermal conductivity (4.0 W/mK) and an enhancement factor of 1373%, which is 2.22 times that of BN/PDMS composites with the same 30 wt% loading. Additionally, the composite shows superior tensile strength (2.11 MPa) and elongation at break (121%), representing 441% and 203% improvements compared with pure polydimethylsiloxane (PDMS). The P‐BN@LM/PDMS composites, with significant advantages in thermal conductivity and mechanical properties, are promising for future applications as flexible thermal interface materials in thermal management.Highlights The P‐BN@LM/PDMS composites exhibit excellent thermal conductivity (4.0 W/mK) and an enhancement factor of 1373%, making them promising for future applications as flexible thermal interface materials in thermal management. The P‐BN@LM/PDMS composites present a superior tensile strength (2.11 MPa) and elongation at break (121%). PDA formed by the self‐polymerization of dopamine on the BN surface effectively enhances interfacial compatibility and dispersion. LM attaches to the functionalized BN surface through mechanical grinding, acting as a bridge between adjacent BN flakes, thus enhancing the integrity of the thermal conductive network within the composites.

Magnetic-induced dynamically enhanced in-plane or out-of-plane thermal conductivity of BN/Ag NWs@Ni/epoxy composites

The thermal conductivity of polymer composites could be improved through the orientation of fillers in specific directions. Here, a novel strategy to fabricate BN/Ag nanowires@Ni/epoxy composites (BN/Ag NWs@Ni/EP) to dynamically improve the in-plane or out-of-plane thermal conductivity of the composites in real-time by changing the direction of the magnetic field was investigated. The thermal conduction path was constructed by Ag NWs bridging between BN flakes. Ag NWs@Ni would be oriented in the in-plane or out-of-plane direction by changing the direction of the magnetic field. The out-of-plane or in-plane thermal conductivities of the composites were 0.824 and 0.723 W m−1K−1 at 40 wt% BN/Ag NWs@Ni content, and the corresponding TCEs were 429% and 359% respectively when Ag NWs@Ni was oriented in the out-of-plane or in-plane direction. Meanwhile, BN/Ag NWs@Ni/EP exhibited good mechanical and dielectric properties, which were beneficial for its industrial application in electronic packaging. This strategy provides the possibility for the applications that required the adjustment of the heat conduction direction in real-time.

Titanium nitrides as novel reactants for in-situ synthesis of TiB2-based composites with improved properties

A novel in-situ reactive approach based on the reactions among TiN, aluminum and boron has been developed to synthesize TiB 2 -based composites including TiB 2 - h BN (TB) and TiB 2 - h BN-AlN (TBA). Fully dense ceramics with fine-grained microstructure were successfully obtained via spark plasma sintering at 1850 °C/60 MPa/5 min. Microstructure analysis suggests h BN flakes were homogenously distributed in the TiB 2 matrix. For AlN, however, they were elongated into plate-like grains during sintering, in which lots of defects in terms of stacking faults and twinning structures were observed. The mechanical and thermophysical properties of as-sintered ceramics were comprehensively investigated and compared. Incorporating AlN significantly improved the flexure strength, hardness, fracture toughness and thermal conductivity of TiB 2 - h BN ceramics. The electrical conductivity of TB (3.06 × 10 6 S/m) is larger than that of TBA (2.35 × 10 6 S/m) at room temperature, but the value (6 × 10 5 S/m) was lower than that of TBA (6.9 × 10 5 S/m) at 1173 K. Based on the measurement of electrical and thermal conductivity, electron and phonon contributions to thermal conductivities of TB and TBA were calculated and their temperature dependences were illustrated.

Thermal and Electrical Properties of Additively Manufactured Polymer-Boron Nitride Composite.

The efficiency of electronic microchip-based devices increases with advancements in technology, while their size decreases. This miniaturization leads to significant overheating of various electronic components, such as power transistors, processors, and power diodes, leading to a reduction in their lifespan and reliability. To address this issue, researchers are exploring the use of materials that offer efficient heat dissipation. One promising material is a polymer-boron nitride composite. This paper focuses on 3D printing using digital light processing of a model of a composite radiator with different boron nitride fillings. The measured absolute values of the thermal conductivity of such a composite in the temperature range of 3-300 K strongly depend on the concentration of boron nitride. Filling the photopolymer with boron nitride leads to a change in the behavior of the volt-current curves, which may be associated with the occurrence of percolation currents during the deposition of boron nitride. The ab initio calculations show the behavior and spatial orientation of BN flakes under the influence of an external electric field at the atomic level. These results demonstrate the potential use of photopolymer-based composite materials filled with boron nitride, which are manufactured using additive techniques, in modern electronics.

Excitons and trions in WSSe monolayers

The possibility of almost linear tuning of the band gap and of the electrical and optical properties in monolayers (MLs) of semiconducting transition metal dichalcogenide (S-TMD) alloys opens up the way to fabricate materials with on-demand characteristics. By making use of photoluminescence spectroscopy, we investigate optical properties of WSSe MLs with a S/Se ratio of 57/43 deposited on SiO2/Si substrate and encapsulated in hexagonal BN flakes. Similarly to the ‘parent’ WS2 and WSe2 MLs, we assign the WSSe MLs to the ML family with the dark ground exciton state. We find that, in addition to the neutral bright A exciton line, three observed emission lines are associated with negatively charged excitons. The application of in-plane and out-of-plane magnetic fields allows us to assign undeniably the bright and dark (spin- and momentum-forbidden) negative trions as well as the phonon replica of the dark spin-forbidden complex. Furthermore, the existence of the single photon emitters in the WSSe ML is also demonstrated, thus prompting the opportunity to enlarge the wavelength range for potential future quantum applications of S-TMDs.

Facile preparation of BN coatings on graphite substrates and their binding strength

To decrease the curing temperature and achieve excellent binding strength of BN coatings on graphite substrates, a proper Al(H2PO4)3 binder has been carefully designed where MgAl2O4 and poly vinyl alcohol (PVA) were used as the curing agent and the dispersant, respectively. XRD, SEM, TEM and DSC showed that the desired binding phase AlPO4 had been successfully obtained, where MgAl2O4 and PVA worked synergistically to lower the curing temperature down to 70–100 °C and promote the formation of AlPO4. Such optimized binder might have formed hydrogen bond with –NH2 and –OH groups on the BN flakes. It might also form hydrogen bond with-OH and –COOH groups on graphite substrates to ensure an optimal binding strength of 5.5 N (∼22 MPa) as suggested by the scratch tests. This work affords a better understanding on the curing chemistry of Al(H2PO4)3 and the binding mechanism between BN flakes, AlPO4 binding phases and graphite substrates.

Temperature induced modulation of resonant Raman scattering in bilayer 2H-MoS2

The temperature evolution of the resonant Raman scattering from high-quality bilayer 2H-MoS_{2} encapsulated in hexagonal BN flakes is presented. The observed resonant Raman scattering spectrum as initiated by the laser energy of 1.96 eV, close to the A excitonic resonance, shows rich and distinct vibrational features that are otherwise not observed in non-resonant scattering. The appearance of 1st and 2nd order phonon modes is unambiguously observed in a broad range of temperatures from 5 to 320 K. The spectrum includes the Raman-active modes, i.e. E_{text {1g}}^{2}(Gamma) and A_{text {1g}}(Gamma) along with their Davydov-split counterparts, i.e. E_{text {1u}}(Gamma) and B_{text {1u}}(Gamma). The temperature evolution of the Raman scattering spectrum brings forward key observations, as the integrated intensity profiles of different phonon modes show diverse trends. The Raman-active A_{text {1g}}(Gamma) mode, which dominates the Raman scattering spectrum at T = 5 K quenches with increasing temperature. Surprisingly, at room temperature the B_{text {1u}}(Gamma) mode, which is infrared-active in the bilayer, is substantially stronger than its nominally Raman-active A_{text {1g}}(Gamma) counterpart.

Highly Thermally Conductive Epoxy Composites with AlN/BN Hybrid Filler as Underfill Encapsulation Material for Electronic Packaging.

In this study, the effects of a hybrid filler composed of zero-dimensional spherical AlN particles and two-dimensional BN flakes on the thermal conductivity of epoxy resin were studied. The thermal conductivity (TC) of the pristine epoxy matrix (EP) was 0.22 W/(m K), while the composite showed the TC of 10.18 W/(m K) at the 75 wt% AlN–BN hybrid filler loading, which is approximately a 46-fold increase. Moreover, various essential application properties were examined, such as the viscosity, cooling rate, coefficient of thermal expansion (CTE), morphology, and electrical properties. In particular, the AlN–BN/EP composite showed higher thermal stability and lower CTE (22.56 ppm/°C) than pure epoxy. Overall, the demonstrated outstanding thermal performance is appropriate for the production of electronic packaging materials, including next-generation flip-chip underfills.

(Digital Presentation) H-BN Enhanced Gel Polymer Electrolyte for Solid State Li-Ion Batteries

In order to address safety concerns of conventional carbonate liquid electrolytes, porous gel polymer electrolytes (GPE) can effectively encapsulate the solution while providing good electrolyte-electrode contact. In this work, a GPE is incorporated with exfoliated 2D hexagonal boron nitride nanosheets (BNNS) as an effective and safe polymer electrolyte for Li-ion batteries. With a facile Dr. Blade approach combined with phase inversion, a high porosity and electrolyte uptake can be maintained while still acting as a stable film. Utilising a 15 wt.% binary polymer mixture of polyvinylidene fluoride (PVDF) and polyethylene oxide (PEO) doped with exfoliated BN flakes, the final GPE is not only more thermal stable but able to effectively suppress dendrite growth through multiple cycles with a high ionic conductivity of 3.03 x10-3 S/cm at ambient conditions. Cell performance with this GPE includes strong cycling performance while sustaining a high coulombic efficiency. Figure 1

The effect of dielectric environment on the brightening of neutral and charged dark excitons in WSe2 monolayer

The dielectric environment of atomically thin monolayer (ML) of semiconducting transition metal dichalcogenides affects both the electronic bandgap and the excitonic binding energy in the ML. We investigate the effect of the environment on the in-plane magnetic field brightening of neutral and charged dark exciton emissions in the WSe2 ML. The monolayers placed in three dielectric environments are studied, in particular, the ML encapsulated in hexagonal BN (hBN) flakes, the ML deposited on a hBN layer, and the ML embedded between the hBN flake and SiO2/Si substrate. We observe that the brightening rates of the neutral and charged dark excitons depend on the dielectric environment, which may be related to the variation of the level of carrier concentration in the ML. Moreover, the surrounding media, characterized by different dielectric constants, weakly influence the relative energies of the neutral and charged dark excitons in reference to the bright ones.

Oriented BN/Silicone rubber composite thermal interface materials with high out-of-plane thermal conductivity and flexibility

Polymer composites with high out-of-plane thermal conductivity, flexibility, and low modulus have always been one of the ideal thermal interface materials. Using softer polymer composites can reduce thermal contact resistance and stress caused by the mismatch of the coefficient of thermal expansion, as well as heat can be quickly transferred due to its high out-of-plane thermal conductivity. Herein, we report a silicone rubber-based composites with high out-of-plane thermal conductivity and softness prepared by combining shear orientation and layer-by-layer stacking methods. The composites exhibit an out-of-plane thermal conductivity of 7.62 Wm-1K−1, while maintain the flexibility and high elastic recovery of silicone rubber. According to finite element simulation analysis, the increase in thermal conductivity comes from the orientation of the BN flakes and the formation of the thermal network. This work provides a simple method for the preparation of flexible thermal interface materials with high out-of-plane thermal conductivity for many potential applications.

Insight into BN Impurity Formation during Boron Nitride Nanotube Synthesis by High-Temperature Plasma.

The high-temperature plasma process has demonstrated great potential in growing high-quality boron nitride nanotubes (BNNTs) with small diameters (∼5 nm) and few walls (3–4 walls) and led to successful commercialization with a high production rate approaching 20 g/h. However, the process is still accompanied by the production of BN impurities (e.g., a-BN, BN shell, BN flakes) whose physicochemical properties are similar to those of BNNTs. This renders the post-purification process very challenging and thus hampers the development of their practical applications. In this study, we have employed both experimental and numerical approaches for a mechanistic understanding of BN impurity formation in the high-temperature plasma process. This study suggests that the flow structure of the plasma jet (e.g., laminar or turbulent) plays a key role in the formation of BN impurities by dictating the transport phenomena of BNNT seeds (e.g., B droplets), which play an important role in BNNT nucleation. We discussed that the turbulence enhances the radial diffusion of B droplets as well as their interparticle coagulation, which leads to a significant reduction in the population of effective BNNT seeds in the BNNT growth zone (T < 4000 K). This results in the generation of unreacted BN precursors (e.g., B-N-H species) in the BNNT growth zone that eventually self-assemble into BN impurities. Our numerical simulation also suggests that a higher thermal energy input makes the flow more turbulent in the BNNT growth zone due to the elevated velocity difference between the plasma jet and ambient cold gas. This finding provides critical insight into the process design that can suppress the BN impurity formation in the high-temperature plasma process.

Formation of thermal conductive network in boron nitride/polyvinyl alcohol by ice-templated self-assembly

Ceramic/resin composite is an important thermal management material for electronic packaging. The formation of a thermally conductive network of ceramic fillers in the composite is the key to improving thermal conduction performance. We design a method of ice-templated self-assembly followed by hot pressing to prepare oriented boron nitride/polyvinyl alcohol (BN/PVA) composites. First, the BN/PVA aerogel is made by freezing and then freeze-drying. BN flakes are squeezed by ice crystals to form a 3D network at the ice interfaces. After further removing the gaps between the BN flakes by hot pressing, a dense BN/PVA composite material is obtained. The resulting polymer composite has a thermal conductivity up to 10.04 W m−1 K−1 at h-BN content of 68 wt%. Theoretical analysis shows a low interfacial thermal resistance between BN flakes. This approach can potentially be used in advanced electronic packaging technology, especially in the design of thermal interface materials.

Thermal conductivity and mechanical properties of polyimide composites with mixed fillers of BN flakes and SiC@SiO2 whiskers

AbstractPolyimide (PI) composites with mixed fillers of BN flakes and SiC whiskers exhibit enhanced thermal conductivity and mechanical properties. In order to improve dispersion and interaction of these mixed fillers within the PI matrix, BN flakes were modified by a titanate coupling agent while SiC whiskers were oxidized at 750°C for 60 minutes to produce SiC@SiO2 followed by silane coupling agent modification. PI composites reached a maximum thermal conductivity of 0.95 W/m K at volume fraction of mixed fillers of 27.6 vol% when the weight ratio of BN flakes to SiC@SiO2 whiskers was 1:4. The enhanced thermal conductivity is likely attributed to the formation of heat conductive networks constructed by BN flakes and SiC@SiO2 whiskers and the improved interfacial affinity between fillers and matrix. The optimized Nielsen‐mold confirms the distribution and morphology of fillers affect the thermal conductivity of PI composites. In addition, SiC whiskers enhanced the mechanical property of PI composites and the influence of fillers on the mechanical property was further elaborated.

BN@PPS core-shell structure particles and their 3D segregated architecture composites with high thermal conductivities

Hexagonal boron nitride (h-BN) presents an excellent thermal conductivity and the outstanding insulating property for electronic packaging. However, a traditional method can enhance the thermal conductivity of the composites by melt mixing fillers and polymers usually leading to composites with limited thermal conductivity. Here, BN@PPS (Polyphenylene sulfide) core-shell structure particles and their composites with the 3D segregated architecture are presented. The composite achieves a high thermal conductivity of 4.15 W/mK containing 40 vol% BN, which is 16 times higher than that of PPS resin of ∼0.25 W/mK and 1.69 times higher than that PPS/BN blend composite with at the same BN loading of 2.45 W/mK. The outstanding thermally conductive property of segregated architecture PPS/BN composite is attributed to the formation of BN flakes networks in the PPS matrices, which can provide effective thermal conductive pathway. This segregated architecture composite is an optimal material for insulating electronic packaging.

Si-B-C-N monoliths with LaB6-induced well-developed BN(C) flakes

Si-B-C-N ceramic monoliths with LaB6 additive were prepared by mechanical alloying followed by spark plasma sintering. The microstructures and mechanical properties were investigated compared to LaB6-free Si-B-C-N counterparts. After sintered at 1900°C/50MPa, the La/Si-B-C-N monoliths provide well-developed BN(C) flakes (≤100nm in thickness) and improved fracture toughness, while the BN(C) ones show no lamellar microstructures in Si-B-C-N systems. The solid-solution of La in BN(C) grains promotes ordered arrangement of BN(C) along the (0002) plane causing in situ growth of BN(C) thus leading to an increase in toughness by 40% (from 5.6 to 7.8MPam−1/2).

DFT and TD-DFT studies on the electronic and optical properties of explosive molecules adsorbed on boron nitride and graphene nano flakes

The binding affinity of explosive molecules with 2D BN flakes is higher than G flakes due to more charge transfer in the BN-explosive complexes.

Synthesis, microstructure and mechanical properties of reactively sintered ZrB 2 –SiC–ZrN composites

ZrB 2 –SiC–ZrN composites were fabricated by reactive hot pressing using Zr, Si 3 N 4 , and B 4 C powders as starting materials. Sintering was conducted at temperatures of 1800 to 2000 °C under a load of 20 MPa in Ar atmosphere. The composite was densified at 2000 °C. The in situ formed BN flakes which distributed uniformly at the grain boundaries were identified by X-ray diffraction and scanning electron microscopy. The formation of h- BN phase and its effect on the mechanical properties of the composite are discussed.

Effect of the Edge Type and Strain on the Structural, Electronic and Magnetic Properties of the BNRs

We present the effect of edge structures on the edge energy and stress of BN nanoribbons. Ab initio density functional calculations show that the armchair edge is lower in energy than the zigzag edge by 0.43 eV/angstrom. Both types of the edges are under the compressive stress. The zigzag edges are mechanically more stable than the armchair edges. Based on the calculated edge energies, the equilibrium shape of the BN flakes are found to be regular hexagonal, and dominated by the armchair edges. The zigzag ribbons are found to be half-metallic, whereas the armchair ribbons are semiconducting.

Machinability of silicon nitride-boron nitride composite materials : E.Sentoku et al. (Kanazawa Inst. of Technology, Ishikawa, Japan.) J.Japan Soc Powder/Powder Metall., Vol 48, No 7, 2001, 655–659. In Japanese

Hexagonal graphitic boron nitride (h-BN) composites show excellent corrosion and thermal shock resistance, good mechanical tolerance and machinability, especially Si3N4–BN and Sialon–BN composites; they have already been used as break rings for horizontal continuous casting of steel. However, the strength of the conventionally processed BN composites were remarkably degraded by the addition of BN due to the poor densification behavior and the existence of large BN flakes or agglomerates of BN flakes that acted as fracture flaws. This means that BN dispersoids with fine particle size and homogeneous distribution are the key factors to obtain high strength composites. By in situ process, such microstructural features can be realized. In this work, by using the proposed in situ reactions, synthesis, microstructures and properties of various in situ nonoxide-boron nitride (Nobn) composites including SiC–BN, Si3N4–BN, AlN–BN, Sialon–BN and Alon–BN composites were investigated. For some Nobn composite systems, due to the large volume expansion during the reaction processes, near-net shape sintering can be realized. For example, the sintering shrinkage of AlN-30 vol.% BN was 3.1% and that of Alon-21 vol.% BN was 4.2%. This will be an advantage for the fabrication of large and complicated products.