Hexagonal boron nitride is also called white graphite. Similar to the hexagonal carbon mesh in graphite, nitrogen and boron in hexagonal boron nitride also form hexagonal mesh layers, which overlap each other to form crystals. Its crystal is similar to graphite, with diamagnetism and high anisotropy, and the crystal parameters are also quite similar. Due to its excellent properties, boron nitride is mainly used for refractory materials, semiconductor solid-phase doping sources, structural materials for atomic stacks, packaging materials for preventing neutron radiation, rocket engine constituent materials, high-temperature lubricants, and mold release agents.
Advantages of Hexagonal Boron Nitride
Good thermal stability
Hexagonal boron nitride is stable at temperatures between 800 - 900°C in air and 1400°C in vacuum (better than graphene). It has a very high melting point of around 3000°C. This is due to the strong covalent bonds between boron and nitrogen within the hexagonal boron nitride layers. The partially ionic character of the covalent bonds adds an electrostatic attraction to the bonds. This increases the total energy required to break the bonds.
High thermal conductivity
At room temperature, h-BN has a thermal conductivity of 300-2000 W m 1 K 1, which is higher than most metals and ceramic materials, but not as good as graphene. This means that it dissipates heat efficiently, reducing the risk of hot spots that could cause material degradation. Strong covalent bonds are good at transmitting thermal vibrations, which can easily propagate in a plane due to the tight hexagonal lattice.
Wide bandgap
The direct bandgap of a single layer of hexagonal boron nitride (h-BN) is around 6 eV. This means that there is no change in momentum when an electron jumps from the valence band to the conduction band. However, as the h-BN layers are added to the bulk form, the band gap becomes indirect - around 5.95 eV. This is similar to the changes seen in materials such as molybdenum disulfide, where the band energy shifts to different locations in the Brillouin zone.
Good flexibility
The atomic thickness of a single layer of h-BN enables it to bend. Its strong intralayer bonds allow it to withstand this bending. The covalent bonds remain deformed, allowing the overall planar structure to bend.
Low friction
The layers of h-BN are held together by weak van der Waals forces. These interactions are easily broken and the layers can slide over each other almost frictionlessly. As the layers of h-BN slide over each other, small flakes or plates of the material can transfer to the sliding surface, forming a lubricating layer. This transferred film acts to separate the sliding surfaces, reducing direct contact and minimizing wear.
Chemical resistance
Ultra-thin h-BN films are considered chemically inert. The chemical resistance of h-BN starts at the atomic level, where the strong covalent bonds between boron and nitrogen form a very stable lattice. Its atoms neither readily donate electrons nor readily accept electrons from other substances, thereby minimizing unwanted chemical reactions.
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The Characteristics of Hexagonal Boron Nitride as a Chemical Additive Are
Hexagonal boron nitride (white graphite) is a loose, lubricating, moisture-absorbing white powder with a true density of 2.27/cm3, a Mohs hardness of 2, and low mechanical strength, but higher than graphite. It has no obvious melting point and sublimates at 3000℃ in 0.1Mpa nitrogen. Its stability in an oxygen atmosphere is poor, and its use temperature is below 1000°C.
Hexagonal boron nitride has a low expansion coefficient and high thermal conductivity, so it has excellent thermal shock resistance, and it will not be damaged even after hundreds of cycles at 1200-20°C. The expansion coefficient of BN is equivalent to that of quartz, but the thermal conductivity is 10 times that of quartz.
Hexagonal boron nitride is characterized by a good heat conductor and a typical electrical insulator. Its room temperature resistivity can reach 1016~1018. Even at 1000℃, the resistivity is still 104~106Ω.cm. The dielectric constant of BN is 3~5, the dielectric loss is (2~8) *10-4, and the breakdown strength is twice that of Al2O3, reaching 30-40 Kv/mm. Hexagonal boron nitride has good lubricity, oxidation resistance, corrosion resistance, insulation, thermal conductivity, and chemical stability. It can be used to manufacture TiB2/BN composite ceramics, high-grade refractory materials and super hard materials, horizontal continuous rolling steel separation rings, high-temperature resistant lubricants, and high-temperature coatings, and also a raw material for the synthesis of cubic boron nitride.
It is characterized by excellent chemical stability, neither wetting nor acting on most metal melts, such as steel, stainless steel, Al, Fe, Ge, Bi, Si, Cu, Sb, Sn, In, Cd, Ni, Zn, etc. Therefore, it can be used as a high-temperature galvanic couple protection cover, melting metal crucible, utensils, pipes for conveying liquid metal, pump parts, cast steel abrasive tools, and high-temperature electrical insulation materials. Due to the heat and corrosion resistance of BN, it can be used to manufacture high-temperature components, rocket combustion chamber linings, heat shields for spacecraft, and corrosion-resistant parts for magnetic flow generators.
Hexagonal Boron Nitride vs. Graphene
Although HBN and graphene are both two-dimensional materials with similar honeycomb atomic structures, they have a variety of different properties.
--Electrical properties:
Graphene is renowned for its exceptional electrical conductivity. It is a zero-bandgap material, meaning it has excellent electrical conductivity and behaves like a semimetal or a zero-gap semiconductor, so it is ideal for use in electronic devices, sensors, and conductive coatings. Graphene's electrical properties have the potential to revolutionize the electronics industry, enabling faster and more efficient devices.
HBN, on the other hand, is an excellent electrical insulator. Its wide bandgap and low electron mobility make it an ideal material for electrical insulation and isolation. Boron nitride's electrical properties are particularly advantageous in high-voltage applications, where electrical insulation is crucial to prevent short circuits and ensure safety.
--Thermal features:
When it comes to thermal properties, both boron nitride and graphene excel in their own ways. Graphene possesses high thermal conductivity, allowing it to efficiently conduct heat. This property renders it suitable for thermal management in electronic devices, heat sinks, and other applications where efficient heat dissipation is crucial.
HBN, on the other hand, exhibits even higher thermal conductivity than graphene. Its ability to transfer heat efficiently makes it an excellent choice for thermal management in high-temperature environments, such as aerospace and power electronics. Boron nitride's thermal stability at extreme temperatures further enhances its suitability for applications requiring effective heat dissipation.
--Other characteristics:
L mechanical properties: Graphene is the strongest material known to date with exceptional mechanical properties, including high tensile strength, stiffness, and flexibility. While HBN is also mechanically strong but not as strong as graphene. Yet, it has good resistance to thermal and mechanical stresses.
L optical properties: Graphene has unique optical properties and exhibits remarkable transparency and absorption characteristics over a wide electromagnetic spectrum, from ultraviolet to infrared. While HBN is transparent and has a wide bandgap. It is an excellent insulator for optics and photonics applications.
Materials and Methods for Hexagonal Boron Nitride
Hbns synthesis
Boric acid, colemanite, and boron trioxide, as boron and ammonia as nitrogen source were used. The synthesis was performed based on a method previously reported by our group (emanet et al., 2017). First, 2 g of boric acid or colemanite or boron trioxide were suspended in 3 ml of 13.38 m ammonia solution. This mixture was transferred onto a silicon carbide boat and dried on a hot plate adjusted to 100°c for approximately 20 min. Then, this silicon carbide boat was placed in a protherm furnace ptf 14/50/450 and heated until 1,300°c with a heating rate of 10°c/min under ammonia gas flow for 2 h. Following the heating, the silicon carbide boat was removed from furnace at around 550°c and hbns were scratched from the surface of the silicon carbide boat with the help of spatula and stored under room conditions.
Sem and tem imaging
The morphology and size of the synthesized hbns were characterized using sem and tem. Sem (helios nano-lab 600i fib/sem, fei) imaging was carried out on samples previously gold-sputtered for 25 s at 60 na, obtaining a 3-nm thick conductive layer over the hbns. Tem images were acquired with a jeol-2100 hrtem microscopy system at 200 kv (equipped with lab6 filament and an oxford instruments 6498 eds system).
Uv-vis, xrd, ft-ir, and raman spectroscopy
The hbns were dispersed in double distilled water (ddh2o) by sonication for 2 min at before the analysis (bandelin sonopuls hd 3100). A perkin elmer lambda 25 uv-vis spectrometer was used to obtain absorption spectra. Ir spectra were acquired with a thermo nicolet is50 spectrometer. Xrd analysis was performed using a shimadzu xrd-6000 with a icdd pdf 4 software. The scanning area was in continuous mode with a scanning range of 2.000–69.980° and a scanning speed of 2.0000°/min. The sampling pitch was set to 0.0200°, and the preset time was set to 0.60 s. Raman spectra of the hbns were recorded using a renishaw in via reflex raman microscopy system (renishaw plc., new mills, wotton-under-edge, uk) equipped with a 514 nm argon ion laser. A minimum of 16 spectra was acquired from a 16-μm2 hbns sample area. All measurements were performed at least three times.
Thermogravimetric analysis (tga)
Tga analyses were performed using a mettler toledo tga/sdta 851 instrument. The samples were analyzed under 20-ml/min n2 gas flow. The temperature was increased up to 700°c and was set at 10°c/min.
Dynamic light scattering (dls) and zeta potential measurements
The colloidal stability analysis of the hbns was carried out by monitoring the size distribution and the z-potential using a malvern zetasizer nano zs. The hbns were sonicated for optimal dispersion in ddh2o at for 2 min. The concentration of the hbns was fixed as 1 mg/ml. Then, the sonicated samples were analyzed using dls at different times (0 and 60 min) to assess their time-dependent colloidal stability. Furthermore, zeta potential measurements were performed and all experiments were repeated at least three times.
Biodegradation studies
Biodegradation of hbns was performed under two different conditions: In lysosome mimicking solution (lms) for oxidative degradation, and in phosphate-buffered saline (pbs) for hydrolytic degradation. To observe the oxidative degradation of hbns in lysosome mimicking condition, a lms was prepared according to table 1 (russier et al., 2011). The ph of the solution was brought to 4.5. Then, 25 mg of hbns were dispersed in 25 ml of lms, which contains physiological concentration of hydrogen peroxide (h2o2, 1 mm). Then, they were sonicated for 1 h. To maintain the physiological oxidizing environment of lysosomes, 1 mm of h2o2 was weekly added. For hydrolytic degradation of hbns in pbs, 25 mg hbns were dispersed in 25 ml pbs and sonicated for 1 h. After the sonication, all samples for oxidative and hydrolytic degradation were placed in dark and incubated at 37°c while shaking at 180 rpm up to 30 days. In both cases, 1 ml sample was taken from the lms or pbs suspensions at 0, 1, 3, 7, 14, and 30 days of the incubation, and stored at 4°c in the dark until characterization.
Application of Hexagonal Boron Nitride
HBN in electronics
One of the most attractive applications of HBN is in van der waals heterostructures, formed by stacking different layers together. Graphene devices built on HBN substrates show that the heterostructures have mobility almost as good as those on sio2, and carrier inhomogeneity almost as good as those on sio2.
Using HBN as a protective barrier
Due to its good impermeability and its inactivity to most chemicals, HBN is an effective barrier to protect metals from corrosion and oxidation. More specifically, since HBN is electrically insulating, it does not produce electrochemical corrosion. For example, researchers found that stainless steel had improved corrosion resistance after being coated with HBN/polyvinyl alcohol in a simulated marine environment.
HBN in thermal applications
The low density, high strength, and large elastic modulus of HBN are beneficial for enhancing the mechanical properties of polymer composites. At room temperature, HBN has a high thermal conductivity of 400 w/mk, making it a potential contender for thermal applications.
HBN in batteries
Conventional polymer separators act as a physical barrier that prevents electrical connection between electrodes, and lithium-ion batteries often face safety issues when they overheat. Researchers have studied an electrolyte composition based on ionic liquids and HBN that enables lithium-ion batteries to operate efficiently over a wide temperature range of up to 150°c. Overall, the presence of HBN in the electrolyte improves oxidation resistance.
Applications of HBN in biomedicine
Due to its biocompatibility, HBN is used in a variety of exciting applications such as drug delivery and additives in tissue scaffolds to improve thermal and mechanical properties of biocomposites.
Commercial applications of HBN
Due to its commercial viability, HBN has been widely used in various industries. For example, HBN fillers for optimizing thermal conductivity of polymers are one of the most common applications of HBN.
HBN is widely used in electrical insulation in consumer electronics such as lithium-ion batteries, supercapacitors, and other high-performance components to improve heat dissipation.
FAQ
Q: What is hexagonal boron nitride used for?
Q: What is the difference between cubic and hexagonal boron nitride?
Q: What is the difference between graphite and hexagonal boron nitride?
Q: What is hexagonal boron nitride in cosmetics?
Q: Is CBN harder than diamond?
Q: Why is hexagonal boron nitride an lubricant?
Q: Is hexagonal boron nitride toxic?
Q: Can hexagonal boron nitride conduct electricity?
Q: Is hexagonal boron nitride a ceramic?
Q: Who manufactures hexagonal boron nitride?
Q: How do you make hexagonal boron nitride?
Q: Is hexagonal boron nitride hydrophobic?
Q: What is the application of hexagonal boron nitride?
Q: Is hexagonal boron nitride a 2D material?
Q: Is hexagonal boron nitride an insulator?
Hexagonal boron nitride is widely considered to be the most promising insulator for FETs based on 2D materials.
Q: Why is CBN so expensive?
Q: Which is stronger CBN or CBG?
Q: Can CBN cut carbide?
Q: What is the hardness of hexagonal boron nitride?
Q: When was hexagonal boron nitride discovered?
