# Boron Nitride

Boron nitride is a thermally and chemically resistant refractory compound of boron and nitrogen with the chemical formula BN . It exists in various crystalline forms that are isoelectronic to the similarly structured carbon lattice . The graphite corresponding to the hexagonal form is the most stable and softest among BN polymorphs, and is therefore used as a lubricant and an additive to cosmetic products. Diamond- like Cube ( Sphalerite Structure ) Variety is called c-bn; It is softer than diamond, but has better thermal and chemical stability. The rare wurtzite Bn modification is similar to lonsdaleite but slightly softer than the cubic form.

Because of its excellent thermal and chemical stability, boron nitride ceramics are traditionally used as parts of high-temperature equipment. Boron nitride has potential uses in nanotechnology. Nanotubes of BN can be produced that have a structure similar to that of carbon nanotubes, that is, graphene (or BN) sheets rolled on themselves, but the properties are very different.

## Structure

Boron nitride exists in several forms that differ in the arrangement of the boron and nitrogen atoms, giving rise to different bulk properties of the material.

### Amorphous form (a-bn)

The amorphous form of boron nitride (a-BN) is non-crystalline, lacking long-range regularity in the arrangement of its atoms. It is similar to amorphous carbon.

All other forms of boron nitride are crystalline.

### Hexagonal Form (h-BN)

The most stable crystalline form is hexagonal, also known as h-BN, α-BN, g-BN, and graphitic boron nitride . Hexagonal boron nitride (point group = D 6H ; space group = P 6 3 / mmc) has a layered structure similar to graphite. Within each layer, the boron and nitrogen atoms are bound by strong covalent bonds, while the layers are held together by weak van der Waals forces. However, the interlayer “registry” of these sheets differs from the pattern observed for graphite, as the atoms are assumed, in which the boron atoms are on and above the nitrogen atoms. This registry shows the polarity of the B–N bond. Nevertheless, h-BN and graphite are very close neighbors and even BC6N hybrids have also been synthesized where carbon substitutes for some B and N atoms. [3]

### Cubic form (c-bn)

Cubic boron nitride has a crystal structure similar to that of diamond. As diamond is less stable than graphite, the cubic form is less stable than the hexagonal form, but the conversion rate between the two is negligible at room temperature, as it is for diamond. Sphalerite in cubic form has a crystal structure similar to that of diamond, and is also referred to as β-BN or c-BN.

### Wurtzite Form (W-BN)

Wurtzite has the same structure as the form of boron nitride (c; point group = c w-b n 6v ; space group = p 6 3 mc) as lonsdaleite, a rare hexagonal polymorph of carbon. In cubic form, the boron and nitrogen atoms are divided into tetrahedra. [4]As in wurtzite, the boron and nitrogen atoms are grouped into 6-membered rings; In cubic form all the rings are in chair configuration, in W-BN the rings between ‘layers’ are in boat configuration. Previously optimistic reports predicted the wurtzite form to be very strong, and was estimated by a simulation to potentially have a strength 18% stronger than that of diamond, but because only a small amount of the mineral is present in nature. , it has not yet been experimentally verified. [5] Recent studies measured w-BN hardness at 46 GPa to be slightly harder than commercial borides, but softer than the cubic form of boron nitride. [2]

## Virtue

### Body

Sources: amorphous BN, [7] [8] [9] crystalline BN, [10] [11] graphite, [12] diamond . [11 1]

The partially ionic structure of the BN layers in h-BN reduces the covalentness and electrical conductivity, while the interlayer interactions are increased resulting in the higher stiffness of h-BN relative to graphite. The low electron-delocalization in hexagonal-BN is also indicated by its absence of color and a large band gap. Very different bonds—strong covalent within the basal planes (the plane where boron and nitrogen atoms are covalently bonded) and weak between them—cause the high anisotropy of most properties of h-BN.

For example, rigidity, electrical and thermal conductivity within planes are much higher than those perpendicular to them. In contrast, the properties of c-BN and w-BN are more homogeneous and isotropic.

Those materials are extremely hard, the hardness of bulk c-BN is slightly smaller and even higher than that of w-BN diamond. [13] Polycrystalline c-BN with grain sizes on the order of 10 nm is also reported to have a Vickers hardness comparable or higher than that of diamond. [14] Because of its superior stability to heat and transition metals, c-BN overtakes diamond in mechanical applications such as machining steel. [15] The thermal conductivity of BN is the highest among all electrical insulators (see table).

Boron nitride can be co-doped with beryllium as p-type and n-type with boron, sulfur, silicon, or with carbon and nitrogen. [10] Both hexagonal and cubic BN are wide-gap semiconductors with band-gap energies corresponding to the UV region. If voltage is applied to h-BN [16] [17] or c-BN, [18] it emits UV light in the range of 215–250 nm and is therefore potentially a light emitting diode (L). ed) or can be used as a laser. ,

Little is known about the melting behavior of boron nitride. It sublimes at 2973 °C under normal pressure liberating nitrogen gas and boron, but melts at elevated pressure.

Little is known about the melting behavior of boron nitride. It sublimes at 2973 °C under normal pressure liberating nitrogen gas and boron, but melts at elevated pressure. [19] [20]

### Thermal stability

Hexagonal and cubic (and probably W-BN) BNs show remarkable chemical and thermal stability. For example, h-BN is stable to decomposition at temperatures up to 1000 °C in air, 1400 °C in vacuum, and 2800 °C in an inert atmosphere. The reactivity of h-BN and c-BN is relatively similar, and the data for c-BN are summarized in the table below.

The thermal stability of si-BN can be summarized as follows: [10]

• In air or oxygen: B2O3 protective layer prevents further oxidation to ~1300 °C; No conversion to hexagonal form at 1400 °C.
• In nitrogen: some conversion to h-BN after 12 h at 1525 °C.
• in a vacuum (10 −5  Pa ): conversion to h-BN at 1550–1600 °C.

### Chemical stability

Boron nitride is insoluble in normal acids, but soluble in alkaline molten salts and nitrides, such as LiOH , KOH , NaOH – Na 2 CO 3 , NaNO 3 , Li 3 N , Mg 3 N 2 , Sr 3 N 2 , Ba 3n 2 or Li 3bn 2 , which are therefore used to dig up the bn [10]

### Thermal conductivity

The theoretical thermal conductivity of hexagonal boron nitride nanoribbons (BNNRs) can approach 1700–2000 W/(m K), which is the same order of magnitude as the experimentally measured value for graphene, and to perform theoretical calculations for graphene may be comparable to nanoribbons. [21] [22] Furthermore, thermal transport in BNNRs is anisotropic. The thermal conductivity of the zigzag-edge BNNRs is about 20% higher than that of the armchair-edge nanoribbons at room temperature. [23]

## Natural phenomena

In 2009, a naturally occurring boron nitride mineral in cubic form (c-Bn) was reported in Tibet, and the name kingsongite was proposed. The substance was found in scattered micron-sized inclusions in chromium-rich rocks. In 2013, the International Mineralogical Association ratified the mineral and the name.

## Synthesis

### Hexagonal bn. preparation and reactivity

Boron nitride is produced synthetically. Hexagonal boron nitride is obtained by reactive boron trioxide (B 2 O 3 ) or boric acid (H 3 BO 3 ) with ammonia (NH 3 ) or urea (CO (NH 2 ) 2 ) in a nitrogen atmosphere: [28]

2 O 3 + 2 NH 3 → 2 BN + 3 H 2 O (t = 900 °C)

B (OH) 3 + NH 3 → BN + 3H 2 O (t = 900 °C)

2 O 3 + CO (NH 2 ) 2 → 2 Bn + CO 2 + 2H 2 O (t> 1000 ° C)

2 O 3 + 3 Cab 6 + 10 N 2 → 20 Bn + 3 CaO (t > 1500 °C)

The resulting disordered (amorphous) boron nitride contains 92–95% BN and 5–8 % B2O3 The remaining B 2 O 3 can be evaporated in the second step at a temperature > 1500 °C to obtain a BN concentration > 98%. Such annealing also crystallizes BN, with the size of the crystallite increasing with the annealing temperature. [15] [29]

H-BN parts can be cheaply fabricated by hot-pressing with subsequent machining. The parts are made from boron nitride powder by adding boron oxide for better compressibility. Thin films of boron nitride can be obtained by chemical vapor deposition from boron trichloride and nitrogen precursors. [30] Combustion of boron powder in nitrogen plasma at 5500 °C yields ultrafine boron nitrides used for lubricants and toners. [31]

Boron nitride reacts with iodine fluoride in trichlorofluoromethane at -30 °C to produce a highly sensitive contact explosive, Ni 3 , in low yield. [32] Boron nitride reacts with nitrides of lithium, alkaline earth metals and lanthanides to form nitrideborate compounds. [33] For example:

Li 3 n + bn → Li 3 bn 2

### Interrelation of hexagonal bn

Similar to graphite, various molecules, such as NH [34] or alkali metals, [35] can be added to the hexagonal boron nitride, which is inserted between its layers. Both experiment and theory suggest that intercalation is more difficult for BN than for graphite. [36]

### Cube bn. the preparation of

The synthesis of c-BN uses methods similar to those of diamond: cubic boron nitride is produced by treating hexagonal boron nitride at high pressure and temperature, much as synthetic diamond is produced from graphite. Direct conversion of hexagonal boron nitride to cubic form has been observed at pressures between 5 and 18 GPa and temperatures between 1730 and 3230 °C, which is the same parameter as the direct graphite–diamond conversion. [37] The addition of small amounts of boron oxide can reduce the required pressure to 4–7 GPa and the temperature to 1500 °C. As in diamond synthesis, to further lower the conversion pressure and temperature, a catalyst is added, such as lithium, potassium, or magnesium, their nitrides, their fluoronitrides, water with ammonium compounds, or hydrazine. [38] [39]Other industrial synthesis methods, again borrowed from diamond growth, use crystal growth in a temperature gradient, or explosive shock wave. The shock wave method is used to produce a material called heterodiamond, a superhard compound of boron, carbon and nitrogen. [40]

Low pressure deposition of thin films of cubic boron nitride is possible. As with diamond growth, the major problem is to suppress the growth of hexagonal phases (h-BN or graphite, respectively). Whereas in diamond growth this is achieved by adding hydrogen gas, boron trifluoride is used for si-BN. Ion beam deposition, plasma-enhanced chemical vapor deposition, pulsed laser deposition, reactive sputtering, and other physical vapor deposition methods are also used. [30]

### Wurtzite BN. the preparation of

Wurtzite BN can be obtained through static high pressure or dynamic shock methods. [41] The limits of its stability are not well defined. Both c-BN and w-BN are formed by compressing h-BN, but w-BN is formed at much lower temperatures, close to 1700 °C. [38]

### Production statistics

While data on production and consumption of the raw materials used for BN synthesis, namely boric acid and boron trioxide, are well known (see boron), the corresponding numbers for boron nitride are not listed in statistical reports. World production estimates for 1999 range from 300 to 350 metric tons. Major producers and consumers of BN are located in the United States, Japan, China, and Germany. In 2000, prices varied from about $75/kg to$120/kg for standard industrial-quality h-BN and to about $200–$400/kg for higher purity BN grades.

## Application

### Hexagonal bn

Hexagonal BN (h-BN) is the most widely used polymorphism. It is a good lubricant at both low and high temperatures (up to 900 °C, even in oxidizing environments). The h-BN lubricant is particularly useful when the electrical conductivity or chemical reactivity of graphite (alternative lubricant) is problematic. Another advantage of h-BN over graphite is that it does not require water or gas molecules trapped between the layers for its lubrication. Therefore, h-BN lubricants can also be used in vacuum, for example in space applications. The lubricating properties of fine-grained h-BN are used in cosmetics, paints, dental cement and pencil lead. [42]

Hexagonal BN was first used in cosmetics in Japan around 1940. However, due to its high price, h-BN was soon abandoned for this application. Its use was revived in the late 1990s with optimized h-BN production processes, and currently h-BN is used as a cosmetic product for foundation, makeup, eye shadow, blushers, kohl pencils, lipsticks and others. Almost all are used by the major producers. skin care products. [15]

Due to its excellent thermal and chemical stability, boron nitride ceramics are traditionally used as parts of high-temperature equipment. h-BN can be incorporated into ceramics, alloys, resins, plastics, rubbers and other materials, giving them self-lubricating properties. Such materials are suitable for the manufacture of bearings and steelmaking. [15] BN-filled plastics have low thermal expansion as well as high thermal conductivity and electrical resistivity. Due to its excellent dielectric and thermal properties, BN is used in electronics such as a substrate for semiconductors, microwave-transparent windows and as a structural material for seals. [43] It can also be used as a dielectric in resistive random access memory.[44] [45]

Hexagonal BN is used as a charge leakage barrier layer of photo drums in xerographic process and laser printers. [46] In the automotive industry, h-BN mixed with binder (boron oxide) is used to seal oxygen sensors, which provide feedback to adjust fuel flow. The binder utilizes the unique temperature stability and insulating properties of h-BN. [15]

Parts can be made by hot pressing from four commercial grades of h-BN. Grade hBN contains boron oxide binder; It is usable in oxidizing environments up to 550–850 °C and in vacuum up to 1600 °C, but is sensitive to water due to its boron oxide content. Grade HBr uses calcium borate binder and is usable at 1600 °C. Grades HBC and HBT contain no binder and can be used up to 3000 °C. [47]

Boron nitride nanosheets (h-BN) can be deposited by catalytic decomposition of borazine at a temperature of ~1100 °C in a chemical vapor deposition setup, in areas up to about 10 cm. Due to their hexagonal atomic structure, small lattice mismatch with graphene (~2%), and high homogeneity they are used as substrates for graphene-based devices. [48] ​​BN nanosheets are also excellent proton conductors. Their high proton transport rate, coupled with high electrical resistance, may lead to applications in fuel cells and water electrolysis. [49]

H-BN has been used as a bullet and bore lubricant in precision target rifle applications since the mid-2000s as an alternative to the molybdenum disulfide coating, commonly referred to as “moly”. It is claimed to increase effective barrel life, increase the interval between bore cleans, and reduce deviation in the point of impact between clean bore first shots and subsequent shots. [50]

### Cubic bn

Cubic boron nitride (CBN or si-BN) is widely used as an abrasive. [51]Its usefulness stems from its insolubility in iron, nickel and related alloys at high temperatures, whereas diamond is soluble in these metals. Therefore polycrystalline si-BN (PCBN) abrasives are used for machining steel, whereas diamond abrasives are preferred for aluminum alloys, ceramics and stone. When exposed to oxygen at high temperatures, BN forms a passive layer of boron oxide. Boron nitride binds well with metals due to the formation of interlayers of metal borides or nitrides. Materials containing cubic boron nitride crystals are often used in tool bits of cutting tools. For grinding applications, soft binders, such as resin, porous ceramics and soft metals, are used. Ceramic binders can also be used. commercial products called “Borazone” (by Diamond Innovations),[42]

Unlike diamond, large c-BN pellets can be produced in a simple process (called sintering) of annealing the c-BN powder in a nitrogen flow at temperatures slightly below the BN decomposition temperature. This ability to fuse si-BN and h-BN powders allows cheap production of larger BN parts. [42]

Similar to diamond, the combination of highest thermal conductivity and electrical resistivity in c-BN is ideal for heat dissipators.

Because cubic boron nitride has lighter atoms and is very strong chemically and mechanically, it is one of the popular materials for X-ray membranes: the low mass results in small X-ray absorption, and good mechanical properties for thin membranes. Allows the use of ions, thus further reducing absorption. [52]

### Amorphous bn

Layers of amorphous boron nitride (a-BN) are used in some semiconductor devices, such as MOSFETs. They can be prepared by chemical decomposition of trichloroborazine with cesium or by thermal chemical vapor deposition methods. Thermal CVD can also be used for the deposition of h-BN layers, or high temperature, c-BN. [53]

## Other Forms of Boron Nitride

### Atomically dilute boron nitride

Hexagonal boron nitride can be exfoliated on sheets of mono or few atomic layers. Because of its similar structure to graphene, atomically thin boron nitride is sometimes referred to as “white graphene”. [54]

Mechanical characteristics. Atomically dilute boron nitride is one of the strongest electrically insulating materials. Monolayer boron nitride has an average Young’s modulus of 0.865TPa and fracture strength 70.5GPa, and unlike graphene, whose strength decreases dramatically with increased thickness, few-layer boron nitride sheets have the same strength as monolayer boron nitride . [55]

thermal conductivity. Atomically thin boron nitride has one of the highest thermal conductivity coefficients (751 W/mK at room temperature) between a semiconductor and an electrical insulator, and its thermal conductivity increases with decreasing thickness due to low inter-layer coupling. . [56]

thermal stability. The air stability of graphene shows obvious thickness dependence: monolayer graphene is reactive to oxygen at 250 °C, strongly doped at 300 °C, and etched at 450 °C; In contrast, bulk graphite does not oxidize until 800 °C. [57] Atomically thinner boron nitride has better oxidation resistance than graphene. Monolayer boron nitride is not oxidized up to 700 °C and can persist in air up to 850 °C; The bilayer and trilayer boron nitride nanosheets have slightly higher oxidation initiation temperatures. [58]Excellent thermal stability, high impermeability to gas and liquid, and electrical insulation make atomically thin boron nitride potential coating materials to prevent surface oxidation and corrosion of metals [59 [60] and other two-dimensional (2 d) Materials, such as black phosphorus. [61]

Better surface adsorption. Atomically thin boron nitride has been found to have better surface adsorption capacity than bulk hexagonal boron nitride. [62] According to theoretical and experimental studies, surface adsorption of atomically thin boron nitride molecules as an adsorbent, experiences structural changes upon increase in adsorption energy and efficiency. The synergistic effect of atomic thickness, high ductility, strong surface adsorption capacity, electrical insulation, impermeability, high thermal and chemical stability of BN nanosheets can increase the Raman sensitivity up to two orders of magnitude, and meanwhile achieve long-term stability and exceptional reactivity. Applicability cannot be achieved by other materials. [63] [64]

Dielectric properties. Atomically dilute hexagonal boron nitride is an excellent dielectric substrate for graphene, molybdenum disulfide (MoS ), and many other 2D materials-based electronic and photonic devices. As shown by electric force microscopy (EFM) studies, screening of the electric field in atomically thin boron nitride shows a weak dependence on thickness, which is revealed by the first principles of the smooth decay of the electric field inside the few-layer boron nitride. is in line with. Calculation. [57]

Raman characteristics. Raman spectroscopy has been a useful tool to study a variety of 2D materials, and the Raman signature of high-quality atomically thin boron nitride was first reported by Gorbachev et al. in 2011. [65] and Lee et al. [58] However, both reported that the Raman results of monolayer boron nitride did not agree with each other. Therefore, Cai and others conducted systematic experimental and theoretical studies to reveal the intrinsic Raman spectrum of atomically thin boron nitride. [66]It turns out that atomically thin boron nitride without interaction with a substrate has the same G band frequency as bulk hexagonal boron nitride, but the strain induced by the substrate can cause a Raman shift. Nevertheless, the Raman intensity of the G band of atomically thin boron nitride can be used to estimate the layer thickness and sample quality.

### Boron Nitride Nanomesh

Boron nitride nanomesh is a nanostructured two-dimensional material. It consists of a BN layer, which by self-aggregation forms a highly regular mesh after a clean high temperature exposure of rhodium [68] or ruthenium [69] to surface borazine under ultra-high vacuum. The nanomesh looks like an assembly of hexagonal pores. The distance between the two pore centers is 3.2 nm and the pore diameter is ~2 nm. Other terms for this material are boronitrin or white graphene. [70]

Boron nitride nanomesh is stable to decomposition not only under vacuum, [68] air [71] and some liquids, [72] [73] but also up to temperatures of 800 °C. [68] In addition, it shows an exceptional ability to trap molecules [72] and metal clusters [69] of the same size as the nanomesh pores, forming a well-ordered array. These characteristics promise interesting applications of Nanomesh in areas such as catalysis, surface functionalisation, spintronics, quantum computing and data storage media like hard drives.

### Boron nitride nanotube

Boron nitride tubules were first created in 1989 by Shor and Dolan; this work was patented in 1989 and published in the 1989 thesis (Dolan) and then in 1993 Science. The 1989 work was also the first preparation of amorphous BN by b-trichloroborazine and cesium metal.

Boron nitride nanotubes were predicted in 1994 [76] and experimentally discovered in 1995. [77] They can be imagined as a rolled sheet of H-boron nitride. Structurally, it is a close analog of a carbon nanotube, i.e. a long cylinder with a diameter of several to a hundred nanometers and a length of several micrometers except for carbon atoms that are alternately replaced by nitrogen and boron atoms. However, the properties of BN nanotubes are very different: while carbon nanotubes can be metallic or semiconducting depending on the rolling direction and radius, BN nanotubes are an electrical insulator with a bandgap of ~5.5 eV, which is basically the tube. independent of chirality and morphology. [78]Furthermore, a layered BN structure is much more thermally and chemically stable than a graphitic carbon structure. [79] [80]

### Boron Nitride Airgel

Boron nitride aerogel is an airgel composed of highly porous BN. It usually consists of a mixture of deformed BN nanotubes and nanosheets. It can have a density as low as 0.6 mg/cm3 and a specific surface area as high as 1050 m2 / g, and therefore has potential applications as an absorber, catalytic support, and gas storage medium. BN aerogels are highly hydrophobic and can absorb up to 160 times their own weight in oil. They are resistant to oxidation in air at temperatures up to 1200 °C, and therefore the absorbed oil can be reused after burning with a flame. BN aerogels can be prepared by template-assisted chemical vapor deposition using borazine as the feed gas. [67]

### containing composites bn

The addition of boron nitride to silicon nitride ceramics improves the thermal shock resistance of the resulting material. For the same purpose, BN is also added to silicon nitride-alumina and titanium nitride-alumina ceramics. Other materials that can be reinforced with BN include alumina and zirconia, borosilicate glass, glass ceramics, enamel, and titanium boride-boron nitride, titanium boride-aluminum nitride-boron nitride, and silicon carbide-composite ceramics with boron nitride composition. . [81]

## Health issues

Boron nitride (along with Si 3 N 4 , NbN, and BNC) show weak fibrogenic activity, and causes pneumoconiosis when inhaled as particulates. The recommended maximum concentration for nitrides of nonmetals is 10 mg/m3 for Bn and 4 for Aln or ZrN.