​Cubic Boron Nitride Nanopowders: The New Diamond, Properties, and Applications

Boron nitride is a chemical compound consisting of nitrogen and boron, having the chemical formula BN. It has various forms but the most common one is the cubic boron nitride form. It is actually a thermal compound that is resistant to heat. It is a lot more identical to the artificial diamond. This compound is manufactured by a process which is known as the sintering process or the frottage process. 

The main and basic property that is exhibited by cubic boron nitride is its hardness. It is also because of this property of cubic boron nitride that it is so useful and approachable in the market.Though it has more properties as well which include low melting and boiling point, thickness, and conductivity but its hardness is one such property that subsides all the rest of the properties. Because of this, it has a wide range of applications as well which makes it more sustainable in the usage and market.

Introduction

Boron nitride has BN as its chemical formula. It is nitrogen and boron’s chemically and thermally resistant refractory compound. Boron nitride exists in numerous crystalline forms that are isoelectronic to the carbon lattice of the same structure. Among BN polymorphs, the softest and most stable is the hexagonal form corresponding to graphite, and that’s why it is utilized as an additive to the products of cosmetics and as a lubricant. C-BN is the cubic (sphalerite structure) variety analogous to diamond. C-BN has superior chemical and thermal stability, despite being softer as compared to diamond. As compared to the cubic form, the rare wurtzite BN modification is softer but the same as the lonsdaleite.

Thermal and Chemical Stability

Traditionally, boron nitride ceramics are utilized as high-temperature equipment parts due to their remarkable chemical and thermal stability. Potential usage of boron nitride is seen in nanotechnology. BN's nanotubes of a similar structure to that of the carbon nanotubes, i.e. sheets of graphene rolled on themselves, can be formed but their characteristics are dissimilar.

Structure of Cubic Boron Nitride

As compared to diamond’s structure, cubic boron nitride has an analogous crystal structure. Graphite is consistently more stable as compared to diamond. Despite the hexagonal form being more stable than the cubic form, the rate of conversion at room temperature is negligible between the two, just like for diamond. Like the diamond, sphalerite crystal structure is possessed by the cubic form too, also known as c-BN or β-BN.

Polycrystalline Cubic Boron Nitride

In order to cut the ferrous and carbide=forming hard substances, polycrystalline cubic boron nitride is the best suitable material as compared to diamond because high chemical stability and extreme hardness are possessed by polycrystalline cubic boron nitride. The size of the grains of commercial polycrystalline CBNs is in the micrometer scale and they have a 33-45 GPa Vickers hardness (HV), which is way lower as compared to diamond. (100 GPa). CBN applications are less.

For a long time, different methods are used by scientists to improve the polycrystalline CBN's hardness meanwhile holding its remarkable chemical stability. With the help of the Hall-Petch effect, the hardness of the material increases when the size of the grain decreases. This effect has been observed in a broad range. The most broadly utilized approach is lessening the size of the grain in order to improve the hardness of a diamond or polycrystalline CBN.

Several conditions

For synthesis, high temperature and high-pressure conditions (HTHP) are needed as the rate of nucleation is enhanced by the temperature, and the growth of grain is suppressed by high pressure. The suppression is done by lessening the atomic diffusion, which is accountable for crystallization. Having 10-30 nm grain size, the nano grained (ng) diamonds have been experimentally synthesized. There are reports on High Knoop hardnesses of 110–140 GPa.

The size of the grain despite the efforts doesn't lessen to below 10 nm because of the intergranular fractures in these bulk ng-diamonds. The best attempts made so far for cBN have formed samples of ng-cBN at 20 GPa pressures with 14 nm minimum size of the grain and 85 GPa of hardness. As alloys and metals demonstrate, the size of the grain is near to the value at which the breakdown of the Hall-Petch relation occurs. Synthesizing polycrystalline CBNs with smaller grain sizes and excellent grain boundaries is a technical difficulty because large driving forces are generated by various high-energy grain boundaries in a polycrystalline material for grain growth. Within the twinning-capable materials, fine twin domains can be stabilized.

Coherence with the twin boundaries

The excess energy of almost one order of magnitude is possessed by coherent twin boundaries, thus making the nano-twinned structures more stable and energetic as compared to their nano-grained counterparts, therefore permitting domain size’s finer control. Experimentally, twin boundaries are assured to have a hardening effect. Against gliding dislocations, the twin boundaries function as barriers.

Preparation of Cubic BN

Like the diamond, the same methods are used in synthesizing c-BN. If hexagonal boron nitride is treated at high temperature and pressure, cubic boron nitride is formed, much as the synthetic diamond is synthesized from the graphite. At temperatures in between 1730-3230 C, and pressures in between 5 GPa and 18 GPa, the hexagonal boron nitride’s direct conversion to the cubic form is observed. The parameters for direct conversion of graphite to diamond are the same as the parameters of the direct conversion of hexagonal boron nitride to the cubic form. The temperature and pressure can be lessened to 1500 C and 4-7 GPa when boron oxide is added in a small amount. For instance, a catalyst (magnesium, potassium, lithium, hydrazine, water with ammonium compounds, their fluoronitrides, or their nitrides) is added for reducing the conversion temperatures and pressures in the synthesis of the diamond.

Crystal growth used in a temperature gradient and explosive shockwave are the other two industrial synthesis methods. Nitrogen, carbon, and boron’s super-hard compound, heterodiamond, is produced by utilizing the shock wave method. Low-pressure deposition of cubic boron nitride’s thin films is possible. The main problem in growing the diamond is suppressing the hexagonal phase’s growth (graphite or h-BN), meanwhile, this problem is solved by adding boron trifluoride for c-BN, and for diamond growth, hydrogen gas should be added. Other methods that are utilized are reactive sputtering, pulsed laser deposition, plasma-enhanced chemical vapor deposition, ion beam deposition, and other methods of physical vapor deposition.

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Properties of BN

In h-BN, the BN layer’s partly ionic structure lessens the electrical conductivity and covalency, however, when the interlayer interaction increases, it results in h-BN’s higher hardness, they are harder as compared to graphite. A large bandgap and color's absence indicates the reduced electron-delocalization in hexagonal-BN. There are basal planes, in which atoms of nitrogen and boron are bonded, covalently). Within the basal planes, the bonding is very different and strongly covalent whereas, between the basal planes, the bonding is weak, resulting in high anisotropy of h-BN's most characteristics.

For instance, the thermal and electrical conductivity, and hardness are way more within the planes as compared to perpendicular to the planes. In comparison, the characteristics of w-BN and c-BN are more isotropic and homogeneous.

The Hardness of the Bulk

As compared to the hardness of diamond, the bulk c-BN is a little less hard and w-BN is way harder. These materials are very hard. It is reported that polycrystalline c-BN, which have 10 nm size of their grains have Vickers hardness equal to or more than the hardness of diamond. In mechanical applications like machining steel, diamond is surpassed by c-BN due to c-BN's better stability to transition metals and heat.In front of all the electric insulators, the BN’s thermal conductivity is the highest.

The Doping Effect

Beryllium can dope boron nitride as p-type meanwhile, silicon, sulfur, and boron can dope boron nitride as n-type. If nitrogen and carbon co-dopes the boron nitride, it will be n-type. With a band-gap energy parallel to the ultraviolet region, both cubic and hexagonal BN are wide-gap semiconductors. C-BN and h-BN can be utilized potentially as lasers or light-emitting diodes (LEDs) because they emit UV light in the 215-250 nm range if the voltage is applied to both of them. No-one knows much about boron nitride’s melting behavior. At increased pressure, it melts, but at normal pressure and 2973 C, it sublimates, releasing boron and nitrogen gas.

CBN's thermal stability is summarized here. Further oxidation to 1300 C is prevented by the protective layer of B2O3 in oxygen or air, at 1400 C, the conversion to the hexagonal form is not possible. The conversion to h-BN is possible in nitrogen after 12 hours at 1525 °C. The conversion in a vacuum(10−5 Pa) to h-BN is possible at 1550–1600 °C.

Chemical Stability

In usual acids, boron nitride is not soluble, but in nitrides (Li3BN2, Ba3N2, Sr3N2, Mg3N2, Li3N, NaNO3, NaOH-Na2CO3, KOH, or LiOH, which are utilized for etching BN) and alkaline molten salts, it is soluble.

Thermal conductivity

Hexagonal boron nitride nanoribbon’s (BNNRs) theoretical thermal conductivity can reach 1700–2000 W/(mK), having a magnitude of the same order as the graphene's experimental measured value, and can be compared to graphene nanoribbon’s theoretical calculations. Also, the thermal transport is anisotropic in the BNNRs. As compared to the thermal conductivity of the armchair-edged nanoribbons, the zigzag-edged BNNRs thermal conductivity is about 20% larger at room temperature.

Applications

Cubic boron nitride (c-BN or CBN) has many usages. It is utilized broadly as an abrasive. It is very useful as at high temperatures, it is insoluble in nickel, iron, and other alloys, meanwhile in these metals, diamond is soluble. Diamond abrasives are utilized for stone, ceramics, and aluminum alloys, meanwhile, for machining steel, polycrystalline c-BN (PCBN) abrasives are utilized. Boron oxide’s passivation layer is formed by BN when BN is in contact at high temperatures with oxygen. Because of the production of interlayers of nitrides or metal borides, boron nitride binds great with metals. Most often, the materials that have cubic boron nitride crystals are utilized in the cutting tool’s tool bits. For applications of grinding, softer binders like soft metals, porous ceramics, and resin are utilized. The usage of ceramic binders is also well. Russian vendors know commercial products by the name ‘Cubonite’ and ‘Elbor', meanwhile, Diamond innovations know them as 'Borazon'.

In Comparison to Diamond

Large c-BN pellets are capable of being formed in a simple process (sintering) of annealing the CBN powders in the flow of nitrogen at temperatures, lesser than the temperature of BN decomposition. The fusing capability of h-BN and c-BN powders enables the affordable formation of large BN parts.

Like the diamond, c-BN also has the combination of the highest electrical resistivity and thermal conductivity, making them suitable and appropriate for heat spreaders. One of the famous materials for the x-ray membranes is cubic boron nitride as it is extremely mechanically and chemically robust and consists of light atoms. Small X-ray absorption results because of low mass. Due to good mechanical characteristics, thin membranes are utilized, resulting in a further reduction in the absorption.

Applications of BN

Boron nitride has many important applications. Some of them are the following:

• As lubricants of high-temperature
• Starting material for hot-pressed BN parts
• Composite ceramics
• Thermally conductive filler for polymers
• Mold release for injection molding and die casting
• The catalyst for high pressure, high-temperature treatment
• To be utilized in heat shielding material in the aerospace industry
• As a resistor material and special high-temperature electrolysis state
• Utilized as high voltage, high temperature, insulation, and cooling components when pressed into different shapes
• As a metal drawing and metal forming lubricant release agent
• In tools of high-speed, oil drilling, and subsurface investigation
• As plastic resin sealing the desiccant polymer additives in transistors
• In forming high-temperature crucible and other products, mold pressing boron nitride is capable of being utilized
• For components of the rocket engine, radar's pass box, radar antenna's mediums, atom reactor's structure materials, semi-conducting solid-phase admixtures, high-frequency induction furnace materials, plasma arc's insulators, high-voltage high-frequency electricity, and welding high-temperature coating automatically.
• To prevent the packing materials of neutron radiation

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Mechanical Applications

As compared to traditional abrasive materials, for instance, boron carbide, SiC, and Al2O3, c-BN is way harder. Therefore, the output performance of grinding with c-BN wheels is increased over traditional abrasive material wheels during the grinding of the chilled mill grade’s cast irons and hardened high-speed steels. Generally, because of diamond’s nature of reactivity with ferrous metals, the tools that are made out of diamond are not capable of being utilized in the above cases, meanwhile, in the existence of ferrous metals up to 1500-1600 K, c-BN is against to the chemical attack.

For the Cutting of Tools

Sintered c-BN is used to cut tools. In the thermodynamic stability region, the process of sintering of c-BN should be done while preventing its retransformation at higher temperatures into h-BN. Because of c-BN's strong covalence bonding nature and its stability at higher pressures and temperatures, c-BN is a material that is hard-to-sinter, just like a diamond. Two-step sintering and the instantaneous sintering of c-BN while h-BN’s conversion are the two possible processes. Previously, the initial step was h-BN’s transformation to c-BN (by using the method of pressure flux formation and a traditional higher temperature) exemplified by c-BN’s isolation and purification in the form of powder. The next step contains sintering during rest or movable during high-pressure conditions.

The Research Work

Following is the work concluded by Hirano et al. regarding the one-step process. By adding A1N, the CBN's concurrent sintering and transformation from h-BN. The temperature and pressure in this transformation operation are the same as the operating conditions in the formation of c-BN. According to observations, additives were used by Fukunaga for improving sintering and transformation which is 2 mol. % lesser as compared to that of magnesium nitride (Mg3N2). For 1 hour, a conversion or a total change was observed by him during 5.0 GPa and 1770 K. Semi-transparent c-BN polycrystals were formed by Fukunaga and co-workers when they utilized Mg3BN3 as the sintering agent at a temperature of 1770 K and pressure of 6 GPa. The c-BN polycrystals that were produced were with 99% calculated density.

Shock compaction about c-BN powder’s sintering was studied by Akashi and Sawaoka. Shock compression is utilized to directly handle the powders without any kind of additives. The starting powders should be rough. The starting powder's size strongly determines the microhardness and density of the formed c-BN compacts as they are dependent in nature. Compacts having 51.3 GPa microhardness and theoretical density of 98% were obtained. The mechanical characteristics associated with two sintered c-BN materials and the micro-structural metallographic studies were compared by Shintani et al. Sintered material having super hard nature like c-BN, w-BN, and the diamond was reviewed by Singh. He examined them via energy dispersive X-ray analysis, scanning electron microscopy (SEM), and X-ray diffraction.

Tungsten Carbide or Ceramic Substrates

Mostly, the compacted c-BN powder is cemented on ceramic substrates or tungsten carbide. Machining of ferrous metals, hardened steels, and chilled cast irons is allowed by the sintered cutting tools. The cutting speed is augmented and the tool’s life is influentially improved by 7-50 value (as compared to tungsten carbide tools). In many cases, the surface finish of high quality dispenses with further polishing and grinding actions.

Applications of BN in Electronics

c-BN is an extremely good insulator due to its broad band gap. It is an III-V semiconductor compound. In addition, for microwave devices, semiconductor lasers, etc. c-BN can be utilized as a heat sink because it has the highest thermal conductivity. The c-BN ceramic’s surface is coated via the CVD process with an aluminum (or aluminum alloy) or metal of group VIII in these applications. As Tanji and Kawasaki innovated, it is possible to metalize with nickel by a chemical vapor deposition process or with aluminum or gold by process of sputtering.

Mixture with Silicon

For getting n-type and p-type semiconductors respectively, c-BN is capable of being mixed with beryllium and silicon. Mishima et al. patented a growing method of semiconductive c-BN crystals.

For instance, when the LiCaBN2-Si mixture is utilized as a flux antecedent to the c-BN-” and h-BN transformation at 2070 K temperature and 5.5 GPa for a time period of 18 hours, n-type c-BN crystals of 1.2mm size are acquired by the researchers. These c-BN doped crystals are capable of producing such p-n junction diodes which operate even at high temperatures because of the higher thermal stability of c-BN. In the ultraviolet, Injection scintillation was observed at high pressure from a c-BN made a p-n junction.

Based on various conditions, the light emission occurs particularly at the junction, near a certain region. The production of p-n junctions takes place from c-BN semiconductors by various methods at high temperatures and high pressures. c-BN thin films were recently studied by Ahmad and Lichtman for UV sensor applications.

Development of the electronic applications of c-BN has just begun and in the recent future, their usage will improve significantly. Due to electronics having the requirement of their components being of small size, thin films or compounds with the higher value of electrically insulating and thermal conductivity characteristics are needed.

Conclusion

Cubic boron nitride is a common and basic form of a chemical compound of boron and nitride widely being used for various purposes but most commonly for industrial abrasion. This is due to the reason that it is hard or almost impossible for cubic boron nitride to dissolve with iron, zinc, nickel, and other such metals. Diamond is hard to find while cubic boron nitride is easily accessible so a better use is being made in the industry of this matter as it is highly impressive in all its forms and functions. Its authentic nature and functionality make it impressive for the consumers to consume it and bring it to more useful terms in both the fields of technology and industry.

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References

https://www.intechopen.com/books/recent-advances-in-boron-containing-materials/significance-of-boron-nitride-in-composites-and-its-applications

https://www.researchgate.net/publication/281769221_Synthesis_of_Cubic_Boron_Nitride_Nanoparticles_from_Boron_Oxide_Melamine_and_NH3_by_Non-Transferred_Ar-N2_Thermal_Plasma

https://www.azonano.com/article.aspx?ArticleID=3326#:~:text=The%20key%20applications%20of%20boron,resin%20sealing%20desiccant%20polymer%20additives

https://cutt.ly/CJSrriN