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Structure of Boron nitride (Inorganic Graphite):

Diborane react with excess NH3 at temperature to form boron nitride (BN) x.while when diborane and NH3 react in 1:2 ratios at low temperature give Borazole.

Boron Nitrides exist two forms just like allotropic forms of carbon (Graphite and Diamond) and both have formula (BN)x.

(i) Boron Nitride (Inorganic Graphite):

(1) Boron nitride is a hexagonal 2D planar giant covalent network , slippery  and a white solid with a layered structure like graphite. Doe to similar structure with graphite it know as “Inorganic graphite” and due to white colour  it is also called “white graphite”

(2) The thermodynamically stable phase of boron nitride, BN, consists of planar sheets of atoms like those in graphite The planar sheets of alternating B and N atoms consist of edge shared hexagons and, as in graphite, the B-N distance within the sheet (145 pm) is much shorter than the distance between the sheets (333 pm,). The difference between the structures of graphite and boron nitride, however, lies in the register of the atoms of neighboring sheets:

(3) The B-N-B or N-B-N bond angle is 120oc . It may be expected for perfect hexagonal ring bond network just like graphite. And boron and nitrogen atoms are sp2 hybridized.

(4) Boron nitride (Inorganic graphite ) is a very good insulator (thermal and electrical) and chemically very inert , chemically posses great stability due to the  very strong B-N bonding  in the 2D layers structure. It melts under pressure at  3000oc  so it is great thermal stability.

(5) In (BN)x the hexagonal rings are stacked directly over each other, with B and N atoms alternating in successive layers; in graphite, the hexagons are staggered. Molecular orbital calculations suggest that the stacking

(6) In (BN)x stems from a partial positive charge on B and a partial negative charge on N. This charge distribution is consistent with the electronegativity difference of the two elements.

 (7)In  Boron nitride(BN)x the Vander Waals forces holding the sheet in line with each other are stronger, so boron nitride is not as good a good lubricant as graphite. However , the use of boron nitride as a lubricant is noted as high temperature due to its chemical stability.

(8) As with impure graphite, layered boron nitride is a slippery material that is used as a lubricant. Unlike graphite, however, it is a colorless electrical insulator, as there is a large energy gap between the filled and vacant π bands

Uses:

(1) Boron nitride ceramics us in high temperature (range 2700-3000oc)  equipment due to excellent thermal stability, thermal shock stability and chemical stability.

(2) Boron nitride based ceramics are stable in air at 1000oc while carbon-graphite based materials ignited at that temperature.

(3) Hexagonal boron nitride can be made in single layers and can also be formed into nanotubes. And that nanotubes are used for wire sieving and a catalyst support.

(4) Hexagonal boron nitride can be incorporated in ceramics, alloys, resins, plastics, rubbers to give them self-lubricating properties. And plastics based hexagonal boron nitride  decrease thermal expansion, increased thermal conductivity, increased electrical insulation. 

Structure of "Borazon" (Cubic boron nitride):

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