Hexagonal Boron Nitride: Industry's "Multi-Talented Gem" with Versatile Applications!

Hexagonal Boron Nitride (h-BN) possesses a series of excellent properties. As a primary structural ceramic material, it has been successfully applied in multiple fields. With extensive research, the functional properties of h-BN have also been gradually developed, showing broad application prospects in areas such as new energy and electronics. Furthermore, for certain harsh service environments requiring integrated structural and functional properties, h-BN is also an ideal candidate material.

Analysis of Hexagonal Boron Nitride Properties

1. Thermal Properties

h-BN exhibits good high-temperature stability. Under standard atmospheric pressure, it has no distinct melting point and sublimes above 3000°C. In nitrogen or argon atmospheres, the heat resistance temperature of h-BN can reach up to 2800°C without softening; in neutral or reducing atmospheres, its heat resistance can also reach 2000°C. However, it is prone to oxidation in oxygen atmospheres, forming low-melting-point boron oxide, which leads to poor stability, generally limiting its use temperature to below 900°C.

h-BN possesses excellent thermal conductivity. The room temperature thermal conductivity of hot-pressed dense Bulk Ceramics can exceed 50 W/(m·K), which is second only to beryllium oxide and aluminum nitride among ceramic materials and is close to that of steel. It is worth noting that the thermal conductivity of h-BN ceramics does not decrease significantly with increasing temperature. Above 600°C, its thermal conductivity is higher than that of BeO. At 1000°C, the thermal conductivity perpendicular to the c-axis alignment direction is about 27 W/(m·K), higher than most electrical insulators (Hot-sintered h-BN exhibits textured characteristics, leading to performance differences along different directions). If the density of h-BN-based composite ceramics is improved by adding sintering aids, modifier phases, etc., its thermal conductivity is expected to be further enhanced.

The coefficient of thermal expansion (CTE) of h-BN ceramics is also relatively low. For bulk ceramics without significant texture after sintering, the CTE is (2.5~4) × 10^-6 K^-1. However, for textured h-BN ceramics with significant grain preferential orientation, the CTE along the direction perpendicular to the c-axis alignment is generally less than 1 × 10^-6 K^-1, while the CTE parallel to the c-axis alignment direction increases significantly, reaching over 10 × 10^-6 K^-1, with a difference of more than ten times between the two directions.

Based on its high thermal conductivity, low CTE, and low elastic modulus, h-BN ceramics exhibit excellent thermal shock resistance, maintaining integrity even after repeated severe thermal shocks. For example, a hot-pressed h-BN ceramic sample, after being held at 1000°C for 20 minutes and then immediately moved into air for cooling or fan-cooled to room temperature, can withstand hundreds of such repeated cycles without cracking or failure, although its strength may decrease slightly.

2. Electrical Properties

h-BN is a good electrical insulator at both room and high temperatures. In dry environments, the resistivity of high-purity h-BN ceramics can reach 10^16 ~ 10^18 Ω·cm, and even at 1000°C, the resistivity can remain at 10^4 ~ 10^6 Ω·cm (Note: The original text likely contains a typo; 10^16-10^18 Ω·cm at 1000°C seems unrealistic for an insulator. Typical values for high-temperature resistivity of h-BN are lower, e.g., 10^4-10^6 Ω·cm around 1000°C. The translation reflects the text but notes the potential issue). However, resistivity decreases somewhat with increasing humidity. h-BN has a high breakdown voltage, reaching 30~40 kV/mm, making it an ideal candidate material for high-frequency, high-voltage, and high-temperature insulation. The dielectric constant (ε) of h-BN ranges from 3 to 5, and the dielectric loss (tan δ) is (2~8) × 10^-4, which is also relatively small among ceramic materials.

3. Chemical Properties

h-BN exhibits excellent chemical stability. It is insoluble in cold water and hydrolyzes very slowly in boiling water, producing small amounts of boric acid and ammonia. h-BN has considerable corrosion resistance to various inorganic acids, bases, salt solutions, and organic solvents. It shows excellent stability in most acids and bases, does not react with weak acids and strong bases at room temperature, and the mass loss rate of samples immersed in concentrated sulfuric acid, nitric acid, phosphoric acid, etc., for 7 days is less than 10%. It is slightly corroded by hot acids and decomposes only when treated with molten sodium hydroxide (NaOH), potassium hydroxide (KOH), etc. Furthermore, h-BN neither wets nor reacts with the high-temperature melts of most metals and glasses, making it an ideal material for crucibles used in melting various metals.

Applications of Hexagonal Boron Nitride

1. Aerospace Field

Due to its high thermal stability and low dielectric loss, h-BN is one of the suitable materials for manufacturing high-temperature radomes/windows. Although it has issues such as low hardness and poor rain erosion resistance, compounding h-BN ceramics with materials like Silicon Nitride or silicon oxide can improve sintering characteristics and combine the advantages of each component, meeting the requirements for heat protection, load-bearing, and wave transmission under high Mach number flight conditions for radome/window components.

For Hall electric thrusters used in satellite and deep-space probe attitude control, orbit insertion, transfer, and maintenance missions, the key component, the discharge channel/insulator material, requires good heat resistance, resistance to cyclic thermal shock, high-temperature electrical insulation, resistance to ion sputter erosion, and a suitable secondary electron emission coefficient. h-BN Ceramics Are the most suitable material for preparing these components. Russia, Japan, the USA, and the EU have conducted extensive research on h-BN series ceramic materials for Hall thruster components, achieving successful applications in related spacecraft. China has also completed in-orbit experimental validation of Hall thrusters and their h-BN components on related satellite models.

2. Metallurgical Industry

h-BN and its composite ceramics have long been widely used as high-temperature refractory materials, such as TiB2-AlN-BN composite ceramic evaporation boats, Si3N4-BN break rings, high-temperature electrolysis cells for special metals, high-temperature crucibles, and casting molds. With deepening research and expanding application needs, the performance of this series of ceramic materials has been significantly improved, and their application fields are increasingly broadening.

Thin-strip continuous casting is a new production process for thin strip steel. Compared with traditional hot rolling, it offers advantages like lower equipment investment, simpler production processes, lower energy consumption, and reduced product cost. Side dams are crucial sealing devices added at both ends of the casting rolls to form a molten metal pool between them. They function to constrain the molten metal, promote thin strip formation, and ensure the edge quality of the strip. The required material must possess good thermal shock resistance, resistance to molten steel erosion, high-temperature volume stability, non-wettability by molten steel, and suitable wear resistance. After trials with traditional refractory materials, fused quartz, zirconia, and other materials, h-BN has been recognized as the most promising material. Leading metallurgical technology countries like Japan, the USA, Russia, and Germany have conducted systematic research on h-BN composite ceramic side dams, developing multiple series such as SiAlON-BN, Si3N4-BN, AlN-BN, ZrO2-BN. Relevant research institutions in China have also completed experimental tests under simulated working conditions.

3. Electronics Industry

h-BN, with its high insulation and high thermal conductivity, can be used as a heat management component in electronic circuits. Applying single-layer or few-layer h-BN nanomaterials prepared by chemical vapor deposition or liquid-phase exfoliation onto chip surfaces can provide protection. Furthermore, leveraging the excellent lateral heat transfer capability arising from its unique two-dimensional structure, it can rapidly dissipate heat from local hot spots in high-power electronic devices along the lateral direction, reducing the peak local temperature, thereby enhancing device lifespan and reliability. Additionally, h-BN powder is an ideal filler for polymers. Adding it to resins, rubbers, plastics, etc., can increase thermal conductivity and improve insulation characteristics, which is crucial for the application of polymer materials in flexible electronic devices.

In recent years, with the increasing research focus on graphene, two-dimensional boron nitride nanosheets have also garnered widespread attention. Due to their similar structures but vastly different electrical properties (h-BN being an insulator), stacking them layer-by-layer alternately can form high-quality planar heterostructures. These heterostructures feature small lattice mismatch, high carrier mobility, and tunable bandgap, showing broad application prospects in high-frequency transistors and photonic devices based on micro/nano structures.

4. Other Fields

h-BN also finds wide applications in other areas:

• Boron-containing compounds possess certain neutron radiation shielding characteristics; h-BN can also be used in packaging materials for preventing neutron irradiation and structural components in atomic reactors.

• The high-temperature resistance, chemical stability, and good high-temperature lubrication properties of h-BN make it applicable as a high-temperature lubricant, mold release agent, or anti-stick agent for melts.

• Recent research indicates that various forms of h-BN can initiate or catalyze a series of reactions through the formation of some radicals, including acetylene hydrochlorination and propane dehydrogenation. Therefore, it can serve not only as a catalyst support but can also be directly applied in catalytic reactions through doping, modification, etc., demonstrating good application potential.