Synthetic Routes to Obtain Hexagonal Boron Nitride Ceramic

Hexagonal boron nitride (h-BN), also known as white graphite, is chemically and thermally stable and is a wide-band-gap semiconductor. With its unique combination of properties such as low friction, high thermal conductivity, excellent electrical insulation, and resistance to oxidation, h-BN has gained considerable interest for various applications in the fields of electronics, aerospace, and other advanced industries. In this article, we will explore the synthetic routes to obtain hexagonal boron nitride ceramic powder, which serves as the raw material for boron nitride ceramic, a major product of QS Advanced Materials Inc.

 

Nitridation of Boron Oxide (B2O3)

Nitridation of boron oxide is another common method to produce h-BN powder. In this process, boron oxide is mixed with a nitrogen-containing compound, such as urea, melamine, or ammonium chloride, and heated to high temperatures in an inert atmosphere. The nitrogen source reacts with the boron oxide to form h-BN and by-products such as water, ammonia, or hydrogen chloride.

The overall nitridation reaction using urea (CO(NH2)2) can be represented as:

 2B2O3 + 6CO(NH2)2 → 4BN + 6CO2 + 6H2O

 This method is relatively simple and cost-effective, but it often results in impure h-BN powder due to the presence of residual boron oxide or other by-products. The purity of the h-BN powder can be improved by optimizing the reaction conditions, such as the temperature and the ratio of the reactants, or by employing additional purification steps.

Molten Salt Synthesis

Molten salt synthesis is an alternative approach to produce h-BN powder. In this method, a mixture of boron oxide and a nitrogen source (e.g., urea, melamine, or ammonium salts) is heated in a molten salt medium, such as alkali metal halides (e.g., NaCl, KCl) or alkaline earth metal halides (e.g., CaCl2). The molten salt serves as both a solvent and a heat transfer medium, promoting the formation of h-BN at a lower temperature than in the nitridation process.

 The overall molten salt synthesis reaction using urea can be represented as:

 2B2O3 + 6CO(NH2)2 → 4BN + 6CO2 + 6H2O

The advantages of molten salt synthesis include lower reaction temperatures, shorter reaction times, and higher yields compared to the nitridation process. However, the method may introduce impurities from the molten salt medium, and the removal of the salt after the reaction can be challenging.

 

Polymer-Derived Ceramics (PDCs)

Polymer derived ceramics (PDCs) has been greatly developed over the past 40 years, due to its advantages of simple operation, low sintering temperature and strong design ability.Polymer-derived ceramics are synthesized by pyrolysis of preceramic polymers containing boron and nitrogen elements. Typically, borazine (B3N3H6) or its derivatives are used as the starting materials. The polymers are heated to high temperatures in an inert atmosphere, during which volatile by-products are released, and the remaining solid transforms into h-BN.

 The advantages of the PDC route include a high degree of control over the composition and structure of the h-BN product, as well as the ability to fabricate complex shapes and structures. However, this method is limited by the availability and cost of suitable preceramic polymers. On the other hand, it usually obtains porous material with lower density, which limits its application in structural ceramics.

 

Chemical Vapor Deposition (CVD)

Chemical vapor deposition is a popular method to synthesize h-BN. With a graphite mold, it could directly obtain the desired shape of the BN component. This type of boron nitride ceramic is also called pyrolytic boron nitride (PBN). In this process, volatile precursors, typically boron and nitrogen-containing gases, are introduced into a reaction chamber. The gases react at high temperatures and deposit h-BN layers on a substrate. Products obtained by this method The quality of the h-BN layers can be controlled by adjusting the substrate temperature, gas flow rates, and chamber pressure.

 

The most common precursors used in the CVD process are ammonia borane (NH3BH3), boron trichloride (BCl3), and ammonia (NH3). The precursors decompose at high temperatures, releasing boron and nitrogen species that react on the substrate surface to form h-BN. The overall reaction can be represented as:

BCl3+ NH3→BN+HCl

CVD-based h-BN synthesis is advantageous due to its high purity, scalability, and tunable thickness. However, it is also associated with high equipment costs, complex processing parameters, and the potential for environmental hazards due to the use of toxic precursors.

 

Conclusion

Multiple synthetic routes can be employed to obtain hexagonal boron nitride ceramic powder, each with its advantages and limitations. The choice of method depends on factors such as the desired purity, particle size, and morphology of the h-BN powder, the scalability of the process, and the cost and availabilityof the starting materials. Chemical vapor deposition offers high purity and tunable thickness, but it is associated with high equipment costs and the use of toxic precursors. Nitridation of boron oxide is a relatively simple and cost-effective method but may result in impure h-BN powder. Molten salt synthesis lowers the reaction temperature and time but may introduce impurities from the salt medium. Lastly, polymer-derived ceramics offer a high degree of control over the product's composition and structure but are limited by the availability and cost of suitable preceramic polymers.

As the demand for h-BN and its derivatives continues to grow in various advanced industries, the development of more efficient, scalable, and environmentally friendly synthesis methods will be crucial. Researchers and companies like QS Advanced Materials Inc. are actively exploring novel synthesis routes and optimizing existing methods to meet the needs of the market.