Multifunctional Boron Nitride

  Boron nitrideis a crystal composed of nitrogen and boron atoms. In addition to the commonly known hexagonal boron nitride (white graphite), there are also cubic boron nitride (CBN), rhombohedral boron nitride (RBN), and wurtzite boron nitride (WBN) variants, and scientists have even discovered two-dimensional boron nitride crystals with properties similar to graphene.Different variants of boron nitride exhibit distinct characteristics and applications. Taking hexagonal white graphite as an example, the hexagonal lattice structure formed by nitrogen and boron atoms is remarkably similar to the hexagonal lattice structure of carbon atoms in graphite, thus possessing similar properties in certain aspects, such as heat resistance, Wear Resistance, and lubricity. However, white graphite also has some unique properties; for instance, while graphite can conduct both heat and electricity, white graphite can conduct heat but not electricity.

The mobile phone has overheated; how about using some boron nitride?

  Boron nitride exhibits strong thermal conductivity, a low thermal expansion coefficient, and excellent insulation properties, while also being corrosion-resistant and able to withstand high temperatures. The thermal conductivity of hexagonal boron nitride is 56.94 watts per meter·degree Celsius, while that of cubic boron nitride is 79.54 watts per meter·degree Celsius, second only to diamond. A foreign study indicates that the thermal conductivity of monolayer hexagonal boron nitride can reach up to 751 watts per meter·degree Celsius at room temperature, making it a promising candidate for the thermal management of next-generation flexible electronic devices.Effective thermal management is an urgent issue for high-density and high-power electronic products. For example, with the popularization of LED technology, 'agricultural factories' have emerged. To compensate for the lack of sunlight, using LED plant growth lights as a replacement for sunlight has become a mature solution.

  Although LED lights have a high energy conversion efficiency compared to other lighting equipment, the theoretical total electro-optical conversion efficiency is still only 54%. This means that LED plant illuminators will still have a lot of heat energy released. Especially when the temperature of LED chips exceeds 140°C, the shortening of their lifespan becomes a problem that cannot be ignored. How to cool down LED lights, hexagonal boron nitride has once again entered the field of vision of scientists. Using hexagonal boron nitride as a filler to make thermally conductive plastics with excellent electrical insulation and chemical stability can improve their thermal conductivity.

Equip the 'electric rocket' with a 'ceramic heart'

  With the launch and orbiting of China's space station 'Tianhe' core module, the 'ceramic heart' of the Hall thruster has become a focal point of interest. This 'ceramic heart' is made from white graphite composite material.In the quest for space, humanity has traditionally relied on chemical propulsion, which generates thrust by burning chemical propellants. After a spacecraft is launched into orbit, additional propulsion is needed to support orbital and attitude adjustments, necessitating the carrying of chemical fuel or in-orbit refueling. However, carrying chemical fuel not only increases launch costs but also to some extent impacts the spacecraft's operational capability in space missions. Against this backdrop, electric propulsion technology has gradually stepped into the limelight. China's research on space electric propulsion technology began in the 1960s, and after decades of technological efforts, multiple breakthroughs have been achieved. In January 2020, China's first 20 kW high-power Hall thruster successfully completed its ignition test and reached an internationally advanced level.

  The four Hall effect thrusters configured in the "Tianhe" core module utilize the electrical energy generated by the solar panels of the core module to provide power support for maintaining the orbit and safe flight of the space station. Hall effect thrusters are a type of plasma thruster that operate on the principle of using a strong electric field to accelerate ions and expel them, allowing for attitude control or orbital adjustments through their reaction force. Hall effect thrusters feature low thrust and high specific impulse. Specific impulse is a technical parameter that evaluates the performance of rocket propellants; a higher specific impulse indicates that, under certain conditions, the propellant creates a greater increase in velocity.While the space station is in orbit, the effects of microgravity and the thin atmosphere at near-Earth space inevitably lead to orbital decay. However, maintaining orbit does not require a large amount of thrust. Although electric propulsion provides low thrust, it can be precisely controlled, enhancing mission execution capabilities. High specific impulse significantly reduces the amount of chemical fuel the spacecraft must carry, thereby expanding the scope of space missions.

  In Hall-effect thrusters, the ionization and acceleration of plasma must occur within the discharge chamber. Hall-effect thrusters require a strong 'heart' to generate precisely adjustable thrust. To create this robust 'heart', it is essential to meet the conditions of high temperature resistance, thermal shock resistance, resistance to ion sputtering, and good insulation properties to withstand the harsh operating conditions of the discharge chamber. The boron nitride ceramic-based composites developed by the Shenyang National Laboratory for Materials Science of the Chinese Academy of Sciences perfectly meet the special requirements for discharge chamber materials in electric thrusters.

Become as hard as diamonds

  Cubic boron nitride, known for its smoothness, can also be hardened. In the 1950s, scientists synthesized single crystals of cubic boron nitride by altering the structure of white graphite. It is a superhard material that emerged after the invention of synthetic diamonds, exhibiting a hardness slightly lower than that of diamonds, but with far superior high-temperature resistance, especially demonstrating excellent chemical stability with iron-based metallic elements. In the 1970s, polycrystalline cubic boron nitride (PCBN) was introduced. PCBN has a very high hardness, second only to diamonds; its bending strength and fracture toughness lie between those of carbide and ceramics; its thermal stability surpasses that of synthetic diamonds, allowing cutting operations to be performed at 1300°C; at high temperatures ranging from 1200 to 1300°C, it is less susceptible to chemical reactions with iron-based materials.Cubic boron nitride, renowned for its hardness, serves two primary purposes: the manufacture of grinding tools such as grinding wheels and oil stones, and the production of cutting tools like drill bits, turning tools, twist drills, and milling cutters. It is particularly advantageous when processing hard materials such as hardened steel, wear-resistant cast iron, and titanium alloys, and is exceptionally suitable for CNC machine tool operations.

Special skills are particularly used, and the prospects for boron nitride are promising

  The silicon-based semiconductor industry has profoundly highlighted the appeal of modern electronic products. However, electronic devices made from silicon semiconductors struggle to meet the challenges posed by extreme conditions, such as high temperatures. In this context, the properties of white graphite—including its wide bandgap, high thermal conductivity, high resistivity, and high mobility—have drawn the attention of scientists.In particular, the derivative product of white graphite, cubic boron nitride, is expected to become a third-generation semiconductor material. Research institutions have developed high-temperature semiconductor PN junction devices using boron nitride materials, which can operate normally at temperatures up to 650°C. This expands the perspective for manufacturing electronic devices that can withstand extreme conditions, bringing new hope to the semiconductor industry.

  The preparation of electronic devices that can withstand extreme conditions such as high temperature, high frequency, high power, and high radiation using boron nitride materials has the potential to solve many application challenges in specific scenarios. In recent years, the fabrication of boron nitride thin films has become a research hotspot in semiconductor materials. Due to the high hardness and thermal resistance of boron nitride films, coupled with their high transmittance across the entire spectrum from ultraviolet to far-infrared, they are suitable for use as window materials in high-power lasers and detectors.The synthesis of boron nitride nanotubes further creates opportunities for advanced applications of boron nitride materials. It is reported that boron nitride nanotubes possess heat insulation, oxidation resistance, and strong elasticity and toughness, showing promise for applications in special industries such as aviation and aerospace. Research institutions plan to apply boron nitride nanotubes in lithium-sulfur batteries to enhance their performance and facilitate commercialization.