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The development of materials science promotes social progress and new types of materials keep emerging. Inorganic graphite boron nitride, due to its similarity to graphite and its unique chemical composition, has excellent performance and great potential for application. And it has become a research hotspot in materials science.

Inorganic graphite name

Why boron nitride is called inorganic graphite?

Boron nitride is called inorganic graphite because it has a similar appearance to graphite, both being layered and having a slippery touch. The hexagonal crystal structure of boron nitride is also similar to that of graphite, and it has good thermal stability. However, graphite is composed of carbon elements, while boron nitride is a nitrogen-boron compound.

Structure of inorganic graphite

Inorganic graphite formula

Inorganic graphite – as known as boron nitride, whose chemical formula is BN. In boron nitride, boron (B) and nitrogen (N) are chemically bonded together in a 1:1 stoichiometric ratio through covalent bonds. The outer electron configuration of boron atoms is 2s²2p¹, and that of nitrogen atoms is 2s²2p³. During the formation of boron nitride, boron atoms provide 3 valence electrons, and nitrogen atoms also provide 3 electrons to participate in bonding. The interaction of these electrons forms stable covalent bonds, which constitute the basic structural unit of boron nitride.

Detailed analysis of crystal structure

Boron nitride mainly exists in three crystal structures: hexagonal boron nitride (h-BN), cubic boron nitride (c-BN), and rhombohedral boron nitride (r-BN). H-BN has a layered structure similar to graphite, with each layer formed by alternating boron and nitrogen atoms in a hexagonal planar network. Layers interact with each other through relatively weak van der Waals forces. And this layered structure endows hexagonal boron nitride with good lubrication properties and certain peelability. The crystal structure of c-BN is similar to that of diamond, with boron and nitrogen atoms connected in a tetrahedral form to form a three-dimensional close-packed structure. This structure endows it with extremely high hardness, second only to diamond. The structure of r-BN lies between hexagonal and cubic boron nitride. And its crystal structure is relatively more complex, with relatively less research and application.

The arrangement of atoms

Hexagonal boron nitride is layered, and the boron and nitrogen atoms in the same layer are connected by covalent bonds. Each boron atom is surrounded by three nitrogen atoms, forming B-N bonds with a bond length of 0.145 nm. These bonds form a hexagonal network that extends infinitely in the plane. The atoms within each layer are arranged closely and orderly, while the layers are held together by van der Waals forces. The interlayer spacing is approximately 0.333 nm. In cubic boron nitride, the boron and nitrogen atoms form tetrahedral structures through covalent bonds. Each of them connects to four opposite atoms, making it hard and stable.

Inorganic graphite hybridization

In boron nitride, the boron and nitrogen atoms in hexagonal and cubic boron nitride are mainly sp²hybridized. (Cubic boron nitride has a small amount of sp³hybridization). Taking hexagonal boron nitride as an example, the sp²hybridized orbitals of boron and nitrogen atoms overlap to formσbonds, forming a hexagonal plane. The remaining unhybridized p orbitals are perpendicular to the plane and overlap shoulder-to-shoulder to form delocalizedπbonds, similar to the hybridization of graphite. This is the key to its similar electrical and thermal properties.

Comparison with the similarities and differences between graphite structure

The similarities between hexagonal boron nitride and graphite are that they both have layered structures, with covalent bonds within the layers and van der Waals forces between the layers. The atoms form delocalizedπbonds through sp²hybridization, and have certain electrical conductivity and thermal conductivity. The differences lie in that the interlayer forces in graphite are weaker, making it easier to slide and lubricate. And graphite is composed of 탄소 원자, while boron nitride is composed of boron and nitrogen atoms. The atomic electronegativities are different, and their chemical and some physical properties are also different.

Properties of inorganic graphite

물리적 속성

Inorganic graphite (taking hexagonal boron nitride as an example) has good lubricity due to the weak interlayer interactions in its layered structure. Its density is approximately 2.27 g/cm³, and it has advantages in aerospace and other fields where weight is a critical factor. Cubic boron nitride has extremely high hardness, with a Mohs hardness of 9.5 – 10. And you can often use it to manufacture wear-resistant materials such as cutting tools.

화학적 특성

Boron nitride has good chemical stability, so it doesn’t react with water or common acids and bases at room temperature. And it is relatively stable at high temperatures and strong acids and bases. It only undergoes slow oxidation in the presence of high temperatures, strong oxidants, etc.. This enables it to be widely used in industrial production with harsh chemical environments.

열 속성

Inorganic graphite has excellent thermal properties. The thermal conductivity of hexagonal boron nitride is up to 300 – 400 W/(m·K), which is conducive to heat dissipation of electronic devices. Its melting point is approximately 3000°C, and its structure and properties remain stable at high temperatures. This makes it suitable as a thermal protection material in aerospace and other fields.

전기적 특성

Hexagonal boron nitride is a wide-bandgap semiconductor material with a bandgap width of approximately 6.4 eV. It has a unique prospect in the semiconductor field. Due to the delocalized large π bonds between layers, it has certain conductivity, but it is weaker than metals.

Preparation method of inorganic graphite

High temperature and high pressure synthesis method

This method operates under the conditions of 1000 – 2000℃high temperature and 5 – 10 GPa high pressure. Boron powder, borate, and other boron sources, as well as ammonia and nitrogen gas, etc., are used as raw materials. To promote the reaction of boron and nitrogen atoms to form boron nitride crystals. This method produces cubic and hexagonal boron nitride with high crystallinity and purity, which is suitable for the production of high-end cutting tools. However, the equipment is expensive, energy consumption is high, and the output is low.

Chemical vapor deposition

It uses borane and other gaseous boron sources, ammonia and other nitrogen sources, etc.. To transport them to the reaction chamber under the joint action of high temperature and catalysts. It reacts on the substrate surface to form a boron nitride film. It can precisely control the thickness and quality of the film and is often used in semiconductor device manufacturing. Such as the preparation of boron nitride-based field-effect transistor insulating layers. However, the equipment is complex, the cost is high, and the growth speed is slow.

Sol-gel method

This method is a mild preparation method. First, dissolve borate esters and other boron sources, organic amines and other nitrogen sources in organic solvents to form a uniform solution. After hydrolysis and condensation, a sol is formed. Then, it is aged, dried, and transformed into a gel. Finally, it undergoes high-temperature heat treatment to decompose the organic components and generate boron nitride. This method is simple to operate, has low cost, and is easy to produce on a large scale. It can produce high-purity boron nitride powder, but the crystallinity is poor and needs to be optimized.

Application field of inorganic graphite

전자 분야

Semiconductor  

It is a wide bandgap semiconductor. The high-temperature performance of boron nitride-based field-effect transistors is superior to that of traditional silicon-based products. LEDs fabricated with boron nitride can emit short-wave light and you can use it for ultraviolet communication and disinfection.

Heat dissipation of electronic devices  

In computer chips, mobile phone processors and other devices, you can use it as heat sinks or coatings. It can quickly dissipate heat and improve performance and extend lifespan.

에너지 분야

Battery electrode materials  

It has a high theoretical specific capacity and stable cycling performance, is studied for use in lithium-ion, sodium-ion batteries, etc. The 전극 with 탄소 소재 composites can improve the battery’s rate and cycle life.

Hydrogen storage material  

Due to its special structure and electronic properties, it can adsorb and store hydrogen. After modification treatment, the hydrogen storage capacity and stability can be improved.

항공우주 분야

Thermal protection materials  

It has a high melting point, good thermal stability, and low thermal conductivity. When an aircraft is flying at high speed, boron nitride-based thermal protection materials can prevent heat from entering and protect the internal structure and equipment.

Aircraft parts  

It has a low density and high strength. Its base composite materials are used to manufacture aircraft wings, fuselage structural components, etc.. It can reduce weight and improve structural strength and reliability.

Machinery field

High temperature lubricant  

The layered structure of hexagonal boron nitride makes it have good lubricity and high-temperature stability. So you can use it as a lubricant in high-temperature production processes  to reduce friction, reduce wear, and improve efficiency.

Wear-resisting material

Cubic boron nitride has high hardness. The tools and grinding tools made from it have excellent wear resistance and cutting performance during cutting and grinding. And it can improve processing accuracy and extend tool life.

결론

Inorganic graphite (boron nitride) has a unique structure, excellent performance, and diverse preparation methods. Although it faces challenges such as cost and mass production. With the development of research and technology, it is expected to make breakthroughs in more fields.