Boron nitride

Boron nitride
IUPAC name

Boron nitride
Identifiers
CAS number 10043-11-5 Y
PubChem 66227
ChemSpider 59612 Y
EC number 233-136-6
MeSH Elbor
ChEBI CHEBI:50883 Y
RTECS number ED7800000
Gmelin Reference 216
Jmol-3D images Image 1
Properties
Molecular formula BN
Molar mass 24.82 g mol−1
Exact mass 25.012379471 g mol−1
Appearance Colorless crystals
Density 2.1 (hBN); 3.45 (cBN) g cm−3
Melting point

2973 °C, 3246 K, 5383 °F (sublimes (cBN))
Electron mobility 200 cm2/(V·s) (cBN)
Refractive index (nD) 1.8 (hBN); 2.1 (cBN)
Structure
Crystal structure hexagonal, sphalerite, wurtzite
Thermochemistry
Std enthalpy of
formation ΔfHo298		 -250.91 kJ mol−1
Standard molar
entropy So298		 14.77 J K−1 mol−1
Hazards
EU classification Xi
R-phrases R36/37
S-phrases S26, S36
NFPA 704
0
0
0
Related compounds
Related compounds Boron arsenide

Boron carbide
Boron phosphide
Boron trioxide
 Y (verify) (what is: Y/N?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Boron nitride is a chemical compound with chemical formula BN, consisting of equal numbers of boron and nitrogen atoms. BN is isoelectronic to a similarly structured carbon lattice and thus exists in various crystalline forms. The hexagonal form corresponding to graphite is the most stable and softest among BN polymorphs, and is therefore used as a lubricant and an additive to cosmetic products. The cubic (sphalerite structure) variety analogous to diamond is called c-BN. Its hardness is inferior only to diamond, but its thermal and chemical stability is superior. The rare wurtzite BN modification is similar to lonsdaleite and may even be harder than the cubic form.

Boron nitride is not found in nature and is therefore produced synthetically from boric acid or boron trioxide. The initial product is amorphous BN powder, which is converted to crystalline h-BN by heating in nitrogen flow at temperatures above 1500 °C. c-BN is made by annealing h-BN powder at higher temperatures, under pressures above 5 GPa. Contrary to diamond, larger c-BN pellets can be produced by fusing (sintering) relatively cheap c-BN powders. As a result, c-BN is widely used in mechanical applications.

Because of excellent thermal and chemical stability, boron nitride ceramics are traditionally used as parts of high-temperature equipment. Boron nitride has a great potential in nanotechnology. Nanotubes of BN can be produced that have a structure similar to that of carbon nanotubes, i.e. graphene (or BN) sheets rolled on themselves, however the properties 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 wide bandgap of ~5.5 eV (same as in diamond), which is almost independent of tube chirality and morphology. Similar to other BN forms, BN nanotubes are more thermally and chemically stable than carbon nanotubes which favors them for some applications.

Contents

Structure

Boron nitride has been produced in an amorphous (a-BN) and crystalline forms. The most stable crystalline form is the hexagonal one, also called h-BN, α-BN, or g-BN (graphitic BN). It 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 polarity of the B-N bonds. Still, h-BN and graphite are very close neighbors and even the BC6N hybrids have been synthesized where carbon substitutes for some B and N atoms.[1]

As diamond is less stable than graphite, cubic BN is less stable than h-BN, but the conversion rate between those forms is negligible at room temperature. The cubic form has the sphalerite crystal structure, same as diamond structure, and is also called β-BN or c-BN. The wurtzite BN form (w-BN) has similar structure as lonsdaleite, rare hexagonal polymorph of carbon. In both c-BN and w-BN boron and nitrogen atoms are grouped into tetrahedra, but the angles between neighboring tetrahedra are different.[2]

α-BN, hexagonal
β-BN, sphalerite structure
BN, wurtzite structure

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 a-BN h-BN c-BN w-BN graphite diamond
Density (g/cm3) 2.28 ~2.1 3.45 3.49 ~2.1 3.515
Mohs hardness 1–2 ~10 ~10 1–2 10
Knoop hardness (GPa) 10 45 34 100
Bulk modulus (GPa) 100 36.5 400 400 34 440
Thermal conductivity (W/cm K) 0.03 6║; 0.3 ┴ 7.4 2–20║; 0.02–0.8┴ 6–20
Thermal expansion (10−6/°C) −2.7║; 38┴ 1.2 2.7 −1.5║; 25┴ 0.8
Bandgap (eV) 5.05 5.2 6.4 4.5–5.5 0 5.5
Refractive index 1.7 1.8 2.1 2.05 2.4
Magnetic susceptibility (µemu/g)[3] −0.48║; -17.3┴ −0.2..-2.7║; -20..-28┴ −1.6

Sources: amorphous BN,[4][5][6] crystalline BN,[7][8] graphite,[9] diamond.[8]

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.

Those materials are extremely hard, with the hardness of c-BN being slightly smaller and w-BN even higher than that of diamond.[10] Because of much better stability to heat and metals, c-BN surpasses diamond in mechanical applications.[11] The thermal conductivity of BN is among the highest of all electric insulators (see table).

Boron nitride can be doped p-type with Be and n-type with boron, sulfur, silicon or if co-doped with carbon and nitrogen.[7] 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 [12][13] or c-BN,[14] 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.[15][16]

Thermal stability

Hexagonal and cubic (and probably w-BN) BN show remarkable chemical and thermal stabilities. For example, h-BN is stable to decomposition in 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[7]
Solid Ambient Action Threshold T (°C)
Mo 10−2 Pa vacuum reaction 1360
Ni 10−2 Pa vacuum wetting[17] 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:[7]

Chemical stability

Boron nitride is insoluble in 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.[7]

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.[18][19] 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.[20]

Synthesis

Boron nitride has not been found in nature and therefore is produced synthetically. The most common raw materials for BN synthesis, boric acid and boron trioxide are produced on industrial scales by treating minerals borax and colemanite with sulfuric acid or hydrochloric acid:

Na2B4O7·10H2O (borax) + H2SO4 → 4 H3BO3 (boric acid) + Na2SO4 + 5 H2O

Boron trioxide is obtained by heating boric acid.[21]

Preparation and reactivity of hexagonal BN

Hexagonal boron nitride is obtained by the reacting boron trioxide (B2O3) or boric acid (B(OH)3) with ammonia (NH3) or urea (CO(NH2)2) in nitrogen atmosphere:[22]

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.[11][23]

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 chemical vapor deposition from boron trichloride and nitrogen precursors.[24] Combustion of boron powder in nitrogen plasma at 5500 °C yields ultrafine boron nitride used for lubricants and toners.[25]

Boron nitride reacts with iodine fluoride in trichlorofluoromethane at −30 °C to produce an extremely sensitive contact explosive, NI3, in low yield.[26]

Intercalation of hexagonal BN

See also: Graphite intercalation compound

Similar to graphite, various molecules, such as NH3[27] or alkali metals,[28] 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.[29]

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.[16] 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.[30][31] 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.[32]

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.[24]

Preparation of wurtzite BN

Wurtzite BN can be obtained via static high-pressure or dynamic shock methods.[33] 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.[30]

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 boron), the corresponding numbers for the boron nitride are not listed in statistical reports. An estimate for the 1999 world production is 300 to 350 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/kg to $120/kg for standard industrial-quality h-BN and were about up to $200–$400/kg for high purity BN grades.[22]

Applications

Hexagonal BN

Hexagonal 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. 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 even in vacuum, e.g. in space applications. The lubricating properties of fine-grained h-BN are used in cosmetics, paints, dental cements, and pencil leads.[34]

Hexagonal BN was first used in cosmetics around 1940 in Japan. However, because of its high price, h-BN was soon abandoned for this application. Its use was revitalized in the late 1990s with the optimization h-BN production processes, and currently h-BN is used by nearly all leading producers of cosmetic products for foundations, make-up, eye shadows, blushers, kohl pencils, lipsticks and other skincare products.[11]

Because of its excellent thermal and chemical stability, boron nitride ceramics are traditionally used as parts of 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.[11] 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 e.g. as a substrate for semiconductors, microwave-transparent windows, and as a structural material for seals.[35]

Hexagonal BN is used in xerographic process and laser printers as a charge leakage barrier layer of the photo drum.[36] 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.[11]

Parts can be made of h-BN by hot pressing. Union Carbide Corporation produces three grades of BN. HBN, with boron oxide binder, usable 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. HBR uses calcium borate binder and is usable at 1600 °C. HBC grade uses no binder and can be used to 3000 °C.[37]

Cubic boron nitride

Cubic boron nitride (CBN or c-BN) is widely used as an abrasive.[38] Its usefulness arises from its insolubility in iron, nickel, and related alloys at high temperatures, whereas diamond is soluble in these metals to give carbides. Polycrystalline c-BN (PCBN) abrasives are therefore used for machining steel, whereas diamond abrasives are preferred for aluminum alloys, ceramics, and stone. When in contact with oxygen at high temperatures, BN forms a passivation layer of boron oxide. Boron nitride binds well with metals, due to formation of interlayers of metal borides or nitrides. Materials with cubic boron nitride crystals are often used in the tool bits of cutting tools. For grinding applications, softer binders, e.g. resin, porous ceramics, and soft metals, are used. Ceramic binders can be used as well. Commercial products are known under names "Borazon" (by Diamond Innovations), and "Elbor" or "Cubonite" (by Russian vendors). Similar to diamond, the combination in c-BN of highest thermal conductivity and electrical resistivity is ideal for heat spreaders. 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.[34]

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, thus further reducing the absorption.[39]

Amorphous boron nitride

Layers of amorphous boron nitride (a-BN) are used in some semiconductor devices, e.g. MISFETs. 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.[40]

Other BN forms

Boron nitride fibers

Hexagonal BN can be prepared in the form of fibers, structurally similar to carbon fibers, by thermal decomposition of extruded borazine (B3N3H6) fibers with addition of boron oxide in a nitrogen atmosphere at 1800 °C. An alternative method is thermal decomposition of cellulose fibers impregnated with boric acid or ammonium tetraborate in an atmosphere of ammonia and nitrogen above 1000 °C. Boron nitride fibers are used as reinforcement in composite materials, with the matrix materials ranging from organic resins to ceramics to metals (see Metal matrix composites).[41]

Boron nitride nanomesh

Main article: Nanomesh

Boron nitride nanomesh is an inorganic 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[42] or ruthenium[43] surface to borazine under ultra-high vacuum. The nanomesh looks like an assembly of hexagonal pores. The distance between 2 pore centers is 3.2 nm and the pore diameter is ~2 nm.

The boron nitride nanomesh is not only stable to decomposition under vacuum,[42] air[44] and some liquids,[45][46] but also up to temperatures of 800 °C.[42] In addition, it shows the extraordinary ability to trap molecules[45] and metallic clusters[43] which have similar sizes to the nanomesh pores, forming a well-ordered array. These characteristics promise interesting applications of the nanomesh in areas like nanocatalysis, surface functionalisation, spintronics, quantum computing and data storage media like hard drives.[47]

Boron nitride nanotubes

See also: Carbon nanotube

Boron nitride nanotubes were theoretically predicted in 1994[48] and experimentally discovered in 1995.[49] They can be imagined as a rolled up sheet of 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.[50] In addition, a layered BN structure is much more thermally and chemically stable than a graphitic carbon structure.[51][52]

All well-established techniques of carbon nanotube growth, such as arc-discharge,[49][53] laser ablation[54][55] and chemical vapor deposition,[56] are used to synthesize BN nanotubes. BN nanotubes can also be produced by ball milling of amorphous boron, mixed with a catalyst: iron powder, under NH3 atmosphere. Subsequent annealing at ~1100 °C in nitrogen flow transforms most of the product into BN.[57][58]

Electrical and field emission properties of the thus prepared nanotubes can be tuned by doping with gold atoms via sputtering of gold on the nanotubes.[57][59] Doping rare-earth atoms of europium turns a BN nanotube into a phosphor material emitting visible light under electron excitation.[58]

Like BN fibers, boron nitride nanotubes show promise for aerospace applications where integration of boron and in particular the light isotope of boron (10B) into structural materials improves their radiation-shielding properties; the improvement is due to strong neutron absorption by 10B. Such 10BN materials are of particular theoretical value as composite structural materials in future manned interplanetary spacecraft, where absorption-shielding from cosmic ray spallation neutrons is expected to be a particular asset in light construction materials.[60]

Composites containing BN

Addition of boron nitride to silicon nitride ceramics improves the thermal shock resistance of the resulting material. For the same purpose, BN is added also to silicon nitride-alumina and titanium nitride-alumina ceramics. Other materials being reinforced with BN are, e.g., alumina and zirconia, borosilicate glasses, glass ceramics, enamels, and composite ceramics with titanium boride-boron nitride and titanium boride-aluminium nitride-boron nitride and silicon carbide-boron nitride composition.[61]

Health issues

Boron nitride (along with Si3N4, NbN, and BNC) is reported to show weak fibrogenic activity and cause pneumoconiosis. The maximum concentration recommended for nitrides of nonmetals is 10 mg/m3 for BN and 4 for AlN or ZrN.[7]

See also

Notes and references

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