Boron nitride

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Boron nitride
Magnified sample of crystalline hexagonal boron nitride
Names
IUPAC name
Boron nitride
Identifiers
3D model (
JSmol
)
ChEBI
ChemSpider
ECHA InfoCard
100.030.111 Edit this at Wikidata
EC Number
  • 233-136-6
216
MeSH Elbor
RTECS number
  • ED7800000
UNII
  • InChI=1S/BN/c1-2 checkY
    Key: PZNSFCLAULLKQX-UHFFFAOYSA-N checkY
  • InChI=1S/B2N2/c1-3-2-4-1
    Key: AMPXHBZZESCUCE-UHFFFAOYSA-N
  • InChI=1S/B3N3/c1-4-2-6-3-5-1
    Key: WHDCVGLBMWOYDC-UHFFFAOYSA-N
  • InChI=1/BN/c1-2
    Key: PZNSFCLAULLKQX-UHFFFAOYAL
  • [B].[N]
Properties
BN
Molar mass 24.82 g/mol
Appearance Colorless crystals
Density 2.1 g/cm3 (h-BN); 3.45 g/cm3 (c-BN)
Melting point 2,973 °C (5,383 °F; 3,246 K) sublimates (c-BN)
Insoluble
Electron mobility 200 cm2/(V·s) (c-BN)
1.8 (h-BN); 2.1 (c-BN)
Structure
wurtzite
Thermochemistry
19.7 J/(K·mol)[1][2]
14.8 J/K mol[3][2]
Std enthalpy of
formation
fH298)
−254.4 kJ/mol[4][2]
−228.4 kJ/mol[5][2]
Hazards
GHS labelling:
GHS07: Exclamation mark
Warning
H319, H335, H413
P261, P264, P271, P273, P280, P304+P340, P305+P351+P338, P312, P337+P313, P403+P233, P405, P501
NFPA 704 (fire diamond)
NFPA 704 four-colored diamondHealth 0: Exposure under fire conditions would offer no hazard beyond that of ordinary combustible material. E.g. sodium chlorideFlammability 0: Will not burn. E.g. waterInstability 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no code
0
0
0
Related compounds
Related compounds
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
☒N verify (what is checkY☒N ?)

Boron nitride is a thermally and chemically resistant

wurtzite BN modification is similar to lonsdaleite but slightly softer than the cubic form.[6]

Because of excellent thermal and chemical stability, boron nitride ceramics are used in high-temperature equipment and

metal casting
. Boron nitride has potential use in nanotechnology.

Structure

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

Amorphous form (a-BN)

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

All other forms of boron nitride are crystalline.

Hexagonal form (h-BN)

The most stable crystalline form is the hexagonal one, also called h-BN, α-BN, g-BN, and graphitic boron nitride. Hexagonal boron nitride (point group = D6h; space group = P63/mmc) has a layered structure similar to graphite. Within each layer, boron and nitrogen atoms are bound by strong covalent bonds, whereas the layers are held together by weak van der Waals forces. The interlayer "registry" of these sheets differs, however, from the pattern seen for graphite, because the atoms are eclipsed, with boron atoms lying over and above nitrogen atoms. This registry reflects the local polarity of the B–N bonds, as well as interlayer N-donor/B-acceptor characteristics. Likewise, many metastable forms consisting of differently stacked polytypes exist. Therefore, h-BN and graphite are very close neighbors, and the material can accommodate carbon as a substituent element to form BNCs. BC6N hybrids have been synthesized, where carbon substitutes for some B and N atoms.[7] Hexagonal boron nitride monolayer is analogous to graphene, having a honeycomb lattice structure of nearly the same dimensions. Unlike graphene, which is black and an electrical conductor, h-BN monolayer is white and an insulator. It has been proposed for use as an atomic flat insulating substrate or a tunneling dielectric barrier in 2D electronics. .[8]

Cubic form (c-BN)

Cubic boron nitride has a crystal structure analogous to that of diamond. Consistent with diamond being 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. The cubic form has the sphalerite crystal structure (space group = F43m), the same as that of diamond (with ordered B and N atoms), and is also called β-BN or c-BN.

Wurtzite form (w-BN)

The

boat configuration. Earlier optimistic reports predicted that the wurtzite form was very strong, and was estimated by a simulation as potentially having a strength 18% stronger than that of diamond. Since only small amounts of the mineral exist in nature, this has not yet been experimentally verified.[10] Its hardness is 46 GPa, slightly harder than commercial borides but softer than the cubic form of boron nitride.[6]

Properties

Physical

Properties of amorphous and crystalline BN, graphite and diamond.
Some properties of h-BN and graphite differ within the basal planes (∥) and perpendicular to them (⟂)
Material Boron nitride (BN) Graphite[11] Diamond[12]
a-[13][14][15] h- c-[16][12] w-
Density (g/cm3) 2.28 ~2.1 3.45 3.49 ~2.1 3.515
Knoop hardness (GPa) 10 45 34 100
Bulk modulus (GPa) 100 36.5 400 400 34 440
Thermal conductivity

(W/m·K)
3 600 ∥,
30 ⟂
740 200–2000 ∥,
2–800 ⟂
600–2000
Thermal expansion (10−6/K) −2.7 ∥, 38 ⟂ 1.2 2.7 −1.5 ∥, 25 ⟂ 0.8
Band gap (eV) 5.05 5.9–6.4 [17] 10.1-10.7 [18] 4.5–5.5 0 5.5
Refractive index 1.7 1.8 2.1 2.05 2.4
Magnetic susceptibility
(µemu/g)[19]
−0.48 ∥,
−17.3 ⟂
−0.2 – −2.7 ∥,
−20 – −28 ⟂
−1.6

The partly

basal planes (planes where boron and nitrogen atoms are covalently bonded) and weak between them – causes high anisotropy
of most properties of h-BN.

For example, the hardness, electrical and thermal conductivity are much higher within the planes than perpendicular to them. On the contrary, the properties of c-BN and w-BN are more homogeneous and isotropic.

Those materials are extremely hard, with the hardness of bulk c-BN being slightly smaller and w-BN even higher than that of diamond.

Vickers hardness comparable or higher than diamond.[21] Because of much better stability to heat and transition metals, c-BN surpasses diamond in mechanical applications, such as machining steel.[22]
The thermal conductivity of BN is among the highest of all electric insulators (see table).

Boron nitride can be doped p-type with beryllium and n-type with boron, sulfur, silicon or if co-doped with carbon and nitrogen.[16] Both hexagonal and cubic BN are wide-gap semiconductors with a band-gap energy corresponding to the UV region. If voltage is applied to h-BN[23][24] or c-BN,[25] then it emits UV light in the range 215–250 nm and therefore can potentially be used as light-emitting diodes (LEDs) or lasers.

Little is known on melting behavior of boron nitride. It sublimates at 2973 °C at normal pressure releasing nitrogen gas and boron, but melts at elevated pressure.[26][27]

Thermal stability

Hexagonal and cubic BN (and probably w-BN) show remarkable chemical and thermal stabilities. 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.

Reactivity of c-BN with solids[16]
Solid Ambient Action Threshold temperature (°C)
Mo 10−2 Pa vacuum Reaction 1360
Ni 10−2 Pa vacuum Wetting[a] 1360
Fe, Ni, Co Argon React 1400–1500
Al 10−2 Pa vacuum Wetting and reaction 1050
Si 10−3 Pa vacuum Wetting 1500
Cu, Ag, Au, Ga, In, Ge, Sn 10−3 Pa vacuum No wetting 1100
B No wetting 2200
Al2O3 + B2O3 10−2 Pa vacuum No reaction 1360

Thermal stability of c-BN can be summarized as follows:[16]

  • 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 at 1525 °C after 12 h.
  • In vacuum (10−5 Pa): conversion to h-BN at 1550–1600 °C.

Chemical stability

Boron nitride is insoluble in the usual acids, but is soluble in alkaline molten salts and nitrides, such as LiOH, KOH, NaOH-Na2CO3, NaNO3, Li3N, Mg3N2, Sr3N2, Ba3N2 or Li3BN2, which are therefore used to etch BN.[16]

Thermal conductivity

The theoretical thermal conductivity of hexagonal boron nitride nanoribbons (BNNRs) can approach 1700–2000 

anisotropic. The thermal conductivity of zigzag-edged BNNRs is about 20% larger than that of armchair-edged nanoribbons at room temperature.[30]

Natural occurrence

In 2009, a naturally occurring boron nitride mineral in the cubic form (c-BN) was reported in

micron-sized inclusions in chromium-rich rocks. In 2013, the International Mineralogical Association affirmed the mineral and the name.[31][32][33][34]

Synthesis

Preparation and reactivity of hexagonal BN

Boron nitride is produced synthetically. Hexagonal boron nitride is obtained by the reacting boron trioxide (B2O3) or boric acid (H3BO3) with ammonia (NH3) or urea (CO(NH2)2) in a nitrogen atmosphere:[35]

B2O3 + 2 NH3 → 2 BN + 3 H2O (T = 900 °C)
B(OH)3 + NH3 → BN + 3 H2O (T = 900 °C)
B2O3 + CO(NH2)2 → 2 BN + CO2 + 2 H2O (T > 1000 °C)
B2O3 + 3 CaB6 + 10 N2 → 20 BN + 3 CaO (T > 1500 °C)

The resulting disordered (amorphous) boron nitride contains 92–95% BN and 5–8% B2O3. The remaining B2O3 can be evaporated in a second step at temperatures > 1500 °C in order to achieve BN concentration >98%. Such annealing also crystallizes BN, the size of the crystallites increasing with the annealing temperature.[22][36]

h-BN parts can be fabricated inexpensively by hot-pressing with subsequent machining. The parts are made from boron nitride powders adding boron oxide for better compressibility. Thin films of boron nitride can be obtained by

ultrafine boron nitride used for lubricants and toners.[38]

Boron nitride reacts with iodine fluoride in trichlorofluoromethane at −30 °C to produce an extremely sensitive contact explosive, NI3, in low yield.[39] Boron nitride reacts with nitrides of lithium, alkaline earth metals and lanthanides to form nitridoborate compounds.[40] For example:

Li3N + BN → Li3BN2

Intercalation of hexagonal BN

Structure of hexagonal boron nitride intercalated with potassium (B4N4K)

Similar to graphite, various molecules, such as NH3[41] or alkali metals,[42] can be intercalated into hexagonal boron nitride, that is inserted between its layers. Both experiment and theory suggest the intercalation is much more difficult for BN than for graphite.[43]

Preparation of cubic BN

Synthesis of c-BN uses same methods as that 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 the cubic form has been observed at pressures between 5 and 18 GPa and temperatures between 1730 and 3230 °C, that is similar parameters as for direct graphite-diamond conversion.[44] The addition of a small amount of boron oxide can lower the required pressure to 4–7 GPa and temperature to 1500 °C. As in diamond synthesis, to further reduce the conversion pressures and temperatures, a catalyst is added, such as lithium, potassium, or magnesium, their nitrides, their fluoronitrides, water with ammonium compounds, or hydrazine.[45][46] 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 material called heterodiamond, a superhard compound of boron, carbon, and nitrogen.[47]

Low-pressure deposition of thin films of cubic boron nitride is possible. As in 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 c-BN. Ion beam deposition, plasma-enhanced chemical vapor deposition, pulsed laser deposition, reactive sputtering, and other physical vapor deposition methods are used as well.[37]

Preparation of wurtzite BN

Wurtzite BN can be obtained via static high-pressure or dynamic shock methods.[48] The limits of its stability are not well defined. Both c-BN and w-BN are formed by compressing h-BN, but formation of w-BN occurs at much lower temperatures close to 1700 °C.[45]

Production statistics

Whereas the production and consumption figures for the raw materials used for BN synthesis, namely boric acid and boron trioxide, are well known (see

metric tons. The major producers and consumers of BN are located in the United States, Japan, China and Germany. In 2000, prices varied from about $75–120/kg for standard industrial-quality h-BN and were about up to $200–400/kg for high purity BN grades.[35]

Applications

Hexagonal BN

Ceramic BN crucible

Hexagonal BN (h-BN) is the most widely used polymorph. It is a good lubricant at both low and high temperatures (up to 900 °C, even in an oxidizing atmosphere). h-BN lubricant is particularly useful when the electrical conductivity or chemical reactivity of graphite (alternative lubricant) would be problematic. In internal combustion engines, where graphite could be oxidized and turn into carbon sludge, h-BN with its superior thermal stability can be added to engine lubricants. As with all nano-particle suspensions, Brownian-motion settlement is a problem. Settlement can clog engine oil filters, which limits solid lubricant applications in a combustion engine to automotive racing, where engine re-building is common. Since carbon has appreciable solubility in certain alloys (such as steels), which may lead to degradation of properties, BN is often superior for high temperature and/or high pressure applications. Another advantage of h-BN over graphite is that its lubricity does not require water or gas molecules trapped between the layers. Therefore, h-BN lubricants can be used in vacuum, such as space applications. The lubricating properties of fine-grained h-BN are used in cosmetics, paints, dental cements, and pencil leads.[49]

Hexagonal BN was first used in cosmetics around 1940 in

make-up, eye shadows, blushers, kohl pencils, lipsticks and other skincare products.[22]

Because of its excellent thermal and chemical stability, boron nitride ceramics and coatings are used high-temperature equipment. h-BN can be included in ceramics, alloys, resins, plastics, rubbers, and other materials, giving them self-lubricating properties. Such materials are suitable for construction of e.g. bearings and in steelmaking.[22] Recently, h-BN coatings have been used in 3d printing applications. Plastics filled with BN have less thermal expansion as well as higher thermal conductivity and electrical resistivity. Due to its excellent dielectric and thermal properties, BN is used in electronics as a substrate for semiconductors, microwave-transparent windows, as a heat conductive yet electrically insulating filler in thermal pastes and as a structural material for seals.[50] Many quantum devices use multilayer h-BN as a substrate material. It can also be used as a dielectric in resistive random access memories.[51][52]

Hexagonal BN is used in

laser printers as a charge leakage barrier layer of the photo drum.[53] In the automotive industry, h-BN mixed with a binder (boron oxide) is used for sealing oxygen sensors, which provide feedback for adjusting fuel flow. The binder utilizes the unique temperature stability and insulating properties of h-BN.[22]

Parts can be made by hot pressing from four commercial grades of h-BN. Grade HBN contains a boron oxide binder; it is usable up to 550–850 °C in oxidizing atmosphere and up to 1600 °C in vacuum, but due to the boron oxide content is sensitive to water. Grade HBR uses a calcium borate binder and is usable at 1600 °C. Grades HBC and HBT contain no binder and can be used up to 3000 °C.[54]

water electrolysis.[56]

h-BN has been used since the mid-2000s as a bullet and bore lubricant in precision target rifle applications as an alternative to molybdenum disulfide coating, commonly referred to as "moly". It is claimed to increase effective barrel life, increase intervals between bore cleaning and decrease the deviation in point of impact between clean bore first shots and subsequent shots.[57]

h-BN is used as a release agent in molten metal and glass applications. For example, ZYP Coatings developed and currently produces a line of paintable h-BN coatings that are used by manufacturers of molten aluminium, non-ferrous metal, and glass.[58] Because h-BN is nonwetting and lubricious to these molten materials, the coated surface (i.e. mold or crucible) does not stick to the material.[59][60][61][62]

Cubic BN

Cubic boron nitride (CBN or c-BN) is widely used as an

cutting tools. For grinding applications, softer binders such as resin, porous ceramics and soft metals are used. Ceramic binders can be used as well. Commercial products are known under names "Borazon" (by Hyperion Materials & Technologies[64]), and "Elbor" or "Cubonite" (by Russian vendors).[49]

Contrary to diamond, large c-BN pellets can be produced in a simple process (called sintering) of annealing c-BN powders in nitrogen flow at temperatures slightly below the BN decomposition temperature. This ability of c-BN and h-BN powders to fuse allows cheap production of large BN parts.[49]

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

As cubic boron nitride consists of light atoms and is very robust chemically and mechanically, it is one of the popular materials for X-ray membranes: low mass results in small X-ray absorption, and good mechanical properties allow usage of thin membranes, further reducing the absorption.[65]

Amorphous BN

Layers of amorphous boron nitride (a-BN) are used in some semiconductor devices, e.g. MOSFETs. They can be prepared by chemical decomposition of trichloroborazine with caesium, or by thermal chemical vapor deposition methods. Thermal CVD can be also used for deposition of h-BN layers, or at high temperatures, c-BN.[66]

Other forms of boron nitride

Atomically thin boron nitride

Hexagonal boron nitride can be exfoliated to mono or few atomic layer sheets. Due to its analogous structure to that of graphene, atomically thin boron nitride is sometimes called white graphene.[67]

Mechanical properties

Atomically thin 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 of 70.5GPa, and in contrast to graphene, whose strength decreases dramatically with increased thickness, few-layer boron nitride sheets have a strength similar to that of monolayer boron nitride.[68]

Thermal conductivity

Atomically thin boron nitride has one of the highest thermal conductivity coefficients (751 W/mK at room temperature) among semiconductors and electrical insulators, and its thermal conductivity increases with reduced thickness due to less intra-layer coupling.[69]

Thermal stability

The air stability of graphene shows a clear 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 is not oxidized until 800 °C.

black phosphorus.[74]

Better surface adsorption

Atomically thin boron nitride has been found to have better surface adsorption capabilities than bulk hexagonal boron nitride.[75] According to theoretical and experimental studies, atomically thin boron nitride as an adsorbent experiences conformational changes upon surface adsorption of molecules, increasing adsorption energy and efficiency. The synergic effect of the atomic thickness, high flexibility, stronger surface adsorption capability, electrical insulation, impermeability, high thermal and chemical stability of BN nanosheets can increase the Raman sensitivity by up to two orders, and in the meantime attain long-term stability and reusability not readily achievable by other materials.[76][77]

Dielectric properties

Atomically thin hexagonal boron nitride is an excellent dielectric substrate for graphene, molybdenum disulfide (MoS2), and many other 2D material-based electronic and photonic devices. As shown by electric force microscopy (EFM) studies, the electric field screening in atomically thin boron nitride shows a weak dependence on thickness, which is in line with the smooth decay of electric field inside few-layer boron nitride revealed by the first-principles calculations.[70]

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.[78] and Li et al.[71] However, the two reported Raman results of monolayer boron nitride did not agree with each other. Cai et al., therefore, conducted systematic experimental and theoretical studies to reveal the intrinsic Raman spectrum of atomically thin boron nitride.[79] It reveals that atomically thin boron nitride without interaction with a substrate has a G band frequency similar to that of bulk hexagonal boron nitride, but strain induced by the substrate can cause Raman shifts. Nevertheless, the Raman intensity of G band of atomically thin boron nitride can be used to estimate layer thickness and sample quality.

BN nanomesh observed with a scanning tunneling microscope. The center of each ring corresponds to the center of the pores
Top: absorption of cyclohexane by BN aerogel. Cyclohexane is stained with Sudan II red dye and is floating on water. Bottom: reuse of the aerogel after burning in air.[80]

Boron nitride nanomesh

Boron nitride nanomesh is a nanostructured two-dimensional material. It consists of a single BN layer, which forms by self-assembly a highly regular mesh after high-temperature exposure of a clean rhodium[81] or ruthenium[82] surface to borazine under ultra-high vacuum. The nanomesh looks like an assembly of hexagonal pores. The distance between two pore centers is 3.2 nm and the pore diameter is ~2 nm. Other terms for this material are boronitrene or white graphene.[83]

The boron nitride nanomesh is air-stable[84] and compatible with some liquids.[85][86] up to temperatures of 800 °C.[81]

BN nanotubes are flame resistant, as shown in this comparative test of airplanes made of cellullose, carbon buckypaper and BN nanotube buckypaper.[87]

Boron nitride nanotubes

Boron nitride tubules were first made in 1989 by Shore and Dolan This work was patented in 1989 and published in 1989 thesis (Dolan) and then 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[88] and experimentally discovered in 1995.[89] They can be imagined as a rolled up sheet of h-boron nitride. Structurally, it is a close analog of the carbon nanotube, namely a long cylinder with diameter of several to hundred nanometers and length of many micrometers, except carbon atoms are alternately substituted by nitrogen and boron atoms. However, the properties of BN nanotubes are very different: whereas carbon nanotubes can be metallic or semiconducting depending on the rolling direction and radius, a BN nanotube is an electrical insulator with a bandgap of ~5.5 eV, basically independent of tube chirality and morphology.[90] In addition, a layered BN structure is much more thermally and chemically stable than a graphitic carbon structure.[91][92]

Boron nitride aerogel

Boron nitride aerogel is an

hydrophobic and can absorb up to 160 times their weight in oil. They are resistant to oxidation in air at temperatures up to 1200 °C, and hence can be reused after the absorbed oil is burned out by flame. BN aerogels can be prepared by template-assisted chemical vapor deposition using borazine as the feed gas.[80]

Composites containing BN

Addition of boron nitride to

titanium boride-boron nitride, titanium boride-aluminium nitride-boron nitride, and silicon carbide-boron nitride composition.[93]

Health issues

Boron nitride (along with Si3N4, NbN, and BNC) is generally considered to be non-toxic and does not exhibit chemical activity in biological systems.[94] Due to its excellent safety profile and lubricious properties, boron nitride finds widespread use in various applications, including cosmetics and food processing equipment.[95][96]

See also

Notes

  1. ^ Here wetting refers to the ability of a molten metal to keep contact with solid BN

References

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