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 exist in various crystalline forms. The hexagonal form corresponding to graphite is the most stable and softest among BN polymorphs, and is therefore used as 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 applications.
Structure
Boron nitride has been produced in an amorphous (a-BN) and several 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 BC 6 N hybrids have been synthesized where carbon substitutes for some B and N atoms.
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.
Properties
Physical
Sources: amorphous BN, crystalline BN, graphite, diamond .
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. Because of much better stability to heat and metals, c-BN surpasses diamond in mechanical applications. 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. 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 on melting behavior of boron nitride. It sublimates at 2973 °C at normal pressure releasing nitrogen gas and boron, but melts at elevated pressure.
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.
Thermal stability of c-BN can be summarized as follows:
- In air or oxygen: B 2 O 3 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 usual acids, but is 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 3 N 2 or Li 3 BN 2 , which are therefore used to etch BN.
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:
Boron trioxide is obtained by heating boric acid.
Preparation and reactivity of hexagonal BN
Hexagonal boron nitride is obtained by the reacting boron trioxide (B 2 O 3 ) or boric acid (B(OH) 3 ) with ammonia (NH 3 ) or urea (CO(NH 2 ) 2 ) in nitrogen atmosphere:
The resulting disordered (amorphous) boron nitride contains 92-95% BN and 5-8% B 2 O 3 . The remaining B 2 O 3 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.
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. Combustion of boron powder in nitrogen plasma at 5500 °C yields ultrafine boron nitride used for lubricants and toners.
Boron nitride reacts with iodine fluoride in trichlorofluoromethane at -30 °C to produce an extremely sensitive contact explosive, NI 3 , in low yield.
Intercalation of hexagonal BN
See also: Graphite intercalation compoundSimilar to graphite, various molecules, such as NH 3 or alkali metals, 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.
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. 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. 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.
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-
BBN Technologies Network Research
The Network Research group at BBN Technologies, conducts leading-edge research to develop networking technologies of the future, with both commercial and government customers.
BBN
Traditional Christian radio in Spanish, Portuguese and English. Network of over 200 radio stations in North and South America.
Parliament < Information & Knowledge Technologies < Technology ...
To learn more about BBN's technology development, call 617-873-8000 or email us at technology@bbn.com.
Boomerang from BBN Technologies
Over 4,000 systems deployed. Easily Integrated with other systems: Remote Weapons Systems; Situational Awareness Systems; Command & Control Systems
BBN Technologies - Wikipedia, the free encyclopedia
BBN Technologies (originally Bolt, Beranek and Newman) is a high-technology company which provides research and development services. BBN is based next to Fresh Pond in Cambridge ...
AI and Machine Learning Group of BBN Technologies
The Artificial Intelligence (AI) and Machine Learning (ML) group develops and uses models of tasks, task-related knowledge, and human ...
Asio Tool Suite < Information & Knowledge Technologies < Technology ...
To learn more about BBN's technology development, call 617-873-8000 or email us at technology@bbn.com.
Internships < Careers | BBN Technologies
BBN Technologies, Internships, The BBN Internship Program strives to provide our interns with hands-on experience and real-work challenges in their field. Work at BBN during the ...
Home | BBN Technologies
BBN Technologies, a wholly owned subsidiary of Raytheon Company, is a legendary R&D organization with expertise spanning information security, speech and language processing ...
BBN Distributed Systems
The Distributed Systems Technology Group of BBN Technologies conducts research and development in distributed systems and the infrastructure needed to support them.