In this article, the different synthesis techniques of boron nitride are discussed, comparing their methodologies and benefits.
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Introduction to Boron Nitride
Boron nitride (BN) is a chemically and thermally resistant compound composed of equal parts nitrogen and boron. It comes as a powder or a solid and shares structural and electronic similarities with carbon. While it can dissolve in molten sodium hydroxide, potassium hydroxide and lithium hydroxide, BN remains insoluble in water and most solvents.
Key properties of BN include high thermal conductivity, thermal shock resistance, low thermal expansion, electrical resistance, microwave transparency, non-toxicity and chemical inertness.
Applications of BN expand to various industries. For instance, it is used for coil forms, substrates and heat sinks in electronics, with specific use in silicon semiconductor processing. In aerospace and ceramics, BN finds use in furnace fixtures, insulators, seals, tubes and brazing fixtures.
BN exists in three main crystalline forms: hexagonal boron nitride (h-BN), cubic boron nitride (c-BN) and wurtzite boron nitride (w-BN). h-BN has a hexagonal lattice structure and is typically a white powder. 2D h-BN usually finds applications in coatings, nanoelectronics, and dielectrics for high-temperature resistive layers.
c-BN possesses a cubic structure and is widely used as an abrasive and insulator. Its notable thermal conductivity makes it ideal for heat sinks in microwave devices and semiconductor lasers. However, c-BN must be sintered in a stable temperature range to prevent conversion to h-BN.
w-BN has an iconic wurtzite crystal structure, with alternating layers of boron and nitrogen atoms. Its plate-shaped particles enable the sintering of coarse grit polycrystalline powders, making it suitable for steel polishing applications like dies and molds.
Common Boron Nitride Synthesis Techniques
CVD
CVD is performed by vaporizing the nitrogen and boron precursors using a hot filament/plasma for boron nitride synthesis. The vaporized species then form boron nitride by reacting with the substrate.
CVD is a versatile synthesis technique that can synthesize high-purity boron nitride films with various properties, such as crystallinity, roughness, and thickness, and deposit the films on different substrates. However, the method is expensive compared to other synthesis methods, not easily scalable and can synthesize films with defects.
In a study published in the journal Nano Letters, researchers synthesized monolayer h-BN on a copper substrate using the CVD method with heating zones under low pressure. Ammonia borane was used as a BN precursor due to its higher stability under ambient conditions and easy accessibility.
Surface-mediated growth of monolayer h-BN was observed on the copper foil, which was similar to the graphene growth on copper under low pressure. The copper surface morphology affected the density and location of the h-BN nucleation.
Physical Vapor Deposition (PVD)
PVD is performed using the evaporation/sputtering process for boron nitride synthesis. In the sputtering process, ion bombardment on a boron nitride target leads to the ejection/sputtering of atoms from the target surface. These ejected atoms are then deposited on the substrate, such as a silicon wafer.
In the evaporation process, the boron nitride is heated to high temperatures until the material is vaporized. Subsequently, the vaporized material is deposited on a substrate to form a thin film.
Although PVD is easily scalable, can synthesize high-purity boron nitride films, and is a less expensive boron nitride synthesis technique compared to CVD, the method is not as versatile as CVD. Moreover, controlling the deposited boron nitride film properties using this method is extremely difficult.
In a study published in the journal Surface and Coatings Technology, researchers investigated the growth of boron nitride films at the transition region between h-BN and c-BN phases using PVD methods. The findings demonstrated that sp2 boron nitride films with three g/cm3 density and a level of stress up to 13 GPa can be obtained under ion bombardment.
Advanced Boron Nitride Synthesis Techniques
Sol-Gel Method
In this method, a solution containing organic/inorganic nitrogen and boron precursors is hydrolyzed to obtain a sol, a colloidal particle suspension in a liquid. Subsequently, a crosslinking agent, such as citric acid, is added to the sol for crosslinking the boron nitride particles in the sol to obtain the gel.
Eventually, the solvent from the gel is removed by evaporation to complete the gelation process. Thin boron nitride films can be obtained using the gel through different methods, including spray coating, spin coating, and dip coating.
The method is simple, inexpensive, and can synthesize high-purity uniform boron nitride films. However, the sol-gel technique is not as versatile as PVD/CVD, extremely time-consuming, and is not easily scalable.
In a study published in the journal Optik, researchers deposited BN thin films using the sol-gel dip coating method. Boron nitrate and boric acid were used as precursors. The deposited films possessed the preferred orientation along (002) plane with a direct relationship between crystallite size and precursor molarity. The film thickness increased with an increase in boric acid molarity and decreased with an increase in boron nitrate molarity.
Hydrothermal Synthesis
In hydrothermal synthesis, a solution of nitrogen and boron compounds is heated and pressurized in a hydrothermal reaction vessel to obtain boron nitride precipitates. Although the method is simple and inexpensive, hydrothermal synthesis has several disadvantages, including the challenges of controlling the synthesized boron nitride properties and the possibility of explosion due to the high pressure and temperature generated in the vessel.
In a study published in the Journal of The Electrochemical Society, researchers successfully synthesized h-BN nanosheets using a low-temperature hydrothermal method. The spectroscopic and structural characterizations of the synthesized nanosheets demonstrated the incorporation of maximum induced strain and formation of a few layers h-BN nanosheets.
Conclusion
Among the four BN synthesis methods discussed in this article, CVD was the most suitable. However, more research is required to overcome the limitations of this method so that BN synthesis can be carried out at scale without defects.
See More: Classifying the Crystalline Compounds of Boron Nitride
References and Further Reading
Wurtzite Boron Nitride. [Online]. Available at: https://borates.today/wurtzite-boron-nitride/
Ross, L.K. 5 Boron Nitride Powder Production Methods. [Online]. (Accessed on 03 September 2023)
Tabbakh, T. A., et al. (2022). Boron Nitride Fabrication Techniques and Physical Properties. IntechOpen. doi.org/10.5772/intechopen.106675
Vasin, A. V., et al. (2004). Deposition of boron nitride films by PVD methods: Transition from h-BN to c-BN. Surface and Coatings Technology, 242, pp. 174-177. doi.org/10.1016/j.surfcoat.2003.10.156.
Liu, H., et al. (2021). Synthesis of hexagonal boron nitrides by chemical vapor deposition and their use as single photon emitters. Nano Materials Science, 3(3), pp. 291-312. doi.org/10.1016/j.nanoms.2021.03.002
Boron Nitride. [Online] Available at https://www.sciencedirect.com/topics/chemical-engineering/boron-nitride
Rathinasabapathy. S., et al. (2020). Significance of Boron Nitride in Composites and Its Applications. IntechOpen. dx.doi.org/10.5772/intechopen.81557
Boron Nitride. [Online]. Available at: https://www.bn.saint-gobain.com/
Ertug, B. (2012). Powder Preparation, Properties and Industrial Applications of Hexagonal Boron Nitride. IntechOpen. doi.org/10.5772/53325
Kim, K. K., et al. (2012). Synthesis of Monolayer Hexagonal Boron Nitride on Cu Foil Using Chemical Vapor Deposition. Nano Letters, 12, 1, pp. 161–166. doi.org/10.1021/nl203249a
Kayani, Z. N., et al. (2021). Sol-gel synthesized boron nitride (BN) thin films for antibacterial and magnetic applications. Optik, 243, p. 167502. doi.org/10.1016/j.ijleo.2021.167502
Sharma, K. & Puri, N. K. (2021). Enhanced Electrochemical Performance of Hydrothermally Exfoliated Hexagonal Boron Nitride Nanosheets for Applications in Electrochemistry. Journal of The Electrochemical Society. doi.org/10.1149/1945-7111/abfe41.
