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 a similarly structured carbon lattice. The hexagonal form corresponding to graphite is the most stable and soft among BN polymorphs, and is therefore used as a lubricant and an additive to cosmetic products. The cubic (zincblende aka sphalerite structure) variety analogous to diamond is called c-BN; it is softer than diamond, but its thermal and chemical stability are superior.
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.
History Boron nitride was discovered by chemistry teacher of the Liverpool Institute, in 1842 via reduction of boric acid with charcoal in the presence of potassium cyanide. The earliest mention of a patent for the production of boron nitride exists in a 1913 paper on the synthesis of nitrides and ammonia. This patent involved heating charcoal impregnated with boric acid in the presence of nitrogen gas at "bright red heat and elevated temperature," resulting in meagre yields. In 1890, Stähler and Elbert succeeded in obtaining yields of 82-85.5 % at 1600 °C and . Nitrides of boron, alongside titanium, aluminium, and silicon, were considered as candidates for nitrogen fixation, prior to the large-scale implementation of the Haber-Bosch Process, but it was not until the 1950s that the electrically insulative properties of BN were detailed and the graphite-like hexagonal form (h-BN) was produced on an industrial level by Union Carbide and Carborundum Universal.
In 1957, the cubic form of boron nitride (c-BN) was synthesised by Robert H. Wentworf, Jr., a physical chemist working for General Electric. The polymorph was officially trademarked by the company in 1969 as Borazon, and applied as a more thermally stable abrasive than its structural analogue, diamond.
In 1994, boron nitride nanotubes (BNNTs) were theorised by Marvin Cohen and synthesised by Alex Zettl the following year. Boron nitride has applications in mechanical insulation and pharmaceuticals, while its electronic, thermal, and optical properties make it a candidate for high-power, deep-ultraviolet, and two-dimensional electronics.
It was previously known as æthogenOED
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, graphitic boron nitride and "white graphite". Hexagonal boron nitride (point group = D3h; 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. 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.
Studies into the optical properties of h-BN at the few- and mono-layer level have been conducted using specialized techniques like deep ultraviolet (DUV) hyperspectral imaging due to its very wide bandgap. Research at low temperatures revealed that monolayer h-BN exhibits photoluminescence consistent with a direct bandgap semiconductor, with emission observed around 6.1 eV. In contrast to observations in other 2D materials like TMDs, the photoluminescence intensity remains remarkably high in few-layer h-BN, which has been attributed to a high radiative efficiency despite the indirect bandgap nature of bulk h-BN. Complementary AFM investigations, particularly in tapping and Kelvin probe modes, have provided nanoscale insight into surface morphology and potential distribution across mono- and few-layer regions. These AFM-based techniques not only assist in confirming flake thickness and uniformity but also support optoelectronic analyses by correlating topographical and electrical variations with luminescence behavior.
Functionalization and heterostructures Modifications of hexagonal boron nitride have led to novel materials like hBNCF, a vertical heterostructure involving graphene and h-BN functionalized with fluorine. This material, synthesized from h-BN via graphitization and fluorination, was found to exhibit room-temperature ferromagnetism. Crucially, magnetic force microscopy (MFM), a specialized mode of atomic force microscopy (AFM), was employed to investigate the magnetic properties at the nanoscale. These MFM studies confirmed the ferromagnetic nature of the hBNCF powder. Furthermore, the MFM analysis provided evidence that the observed magnetism was intrinsic to the hBNCF structure, helping to exclude extrinsic metallic magnetic impurities as the origin. The material was also characterized as a wide band gap semiconductor (~3.89 eV) with potential applications as an MRI contrast agent.
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 = F3m), 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 wurtzite form of boron nitride (w-BN; point group = C6v; space group = P63mc) has the same structure as lonsdaleite, a rare hexagonal polymorph of carbon. As in the cubic form, the boron and nitrogen atoms are grouped into tetrahedra. In the wurtzite form, the boron and nitrogen atoms are grouped into 6-membered rings. In the cubic form all rings are in the chair configuration, whereas in w-BN the rings between 'layers' are in 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. Its hardness is 46 GPa, slightly harder than commercial borides but softer than the cubic form of boron nitride.
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 Diamond a- h- c- 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 10.1-10.7 4.5–5.5 0 5.5 Refractive index 1.7 1.8 2.1 2.05 2.4 Magnetic susceptibility (µemu/g) −0.48 ∥, −17.3 ⟂ −0.2 – −2.7 ∥, −20 – −28 ⟂ −1.6
The partly ionic structure of BN layers in h-BN reduces covalency and electrical conductivity, whereas the interlayer interaction increases resulting in higher hardness of h-BN relative to graphite. The reduced electron-delocalization in hexagonal-BN is also indicated by its absence of color and a large band gap. Very different bonding – strong covalent within the 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. Polycrystalline c-BN with grain sizes on the order of 10 nm is also reported to have Vickers hardness comparable or higher than diamond. Because of much better stability to heat and transition metals, c-BN surpasses diamond in mechanical applications, such as machining steel. 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. 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 or c-BN, 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 of the melting behavior of boron nitride. It degrades at 2973 °C, but melts at elevated pressure.
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 Solid Ambient Action Threshold temperature (°C) Mo vacuum Reaction 1360 Ni vacuum Wetting 1360 Fe, Ni, Co Argon React 1400–1500 Al vacuum Wetting and reaction 1050 Si vacuum Wetting 1500 Cu, Ag, Au, Ga, In, Ge, Sn vacuum No wetting 1100 B No wetting 2200 vacuum No reaction 1360
Thermal stability of c-BN can be summarized as follows: In air or oxygen: 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 (): conversion to h-BN at 1550–1600 °C.
Chemical stability Boron nitride is not attacked by the usual acids, but it is soluble in alkaline molten salts and nitrides, such as LiOH, KOH, NaOH-, , , , , or , which are therefore used to etch BN.
Thermal conductivity The theoretical thermal conductivity of hexagonal boron nitride nanoribbons (BNNRs) can approach 1700–2000 W/(m⋅K), which has the same order of magnitude as the experimental measured value for graphene, and can be comparable to the theoretical calculations for graphene nanoribbons. Moreover, the thermal transport in the BNNRs is anisotropic. The thermal conductivity of zigzag-edged BNNRs is about 20% larger than that of armchair-edged nanoribbons at room temperature.
Mechanical properties BN nanosheets consist of hexagonal boron nitride (h-BN). They are stable up to 800 °C in air. The structure of monolayer BN is similar to that of graphene, which has exceptional strength, a high-temperature lubricant, and a substrate in electronic devices.