Hexagonal Boron Nitride

• Hexagonal Boron Nitride
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  • Hexagonal boron nitride is a white slippery solid with a layered structure, physically similar to graphite in this respect.
    • Like layers of graphite or graphene, it is a 2D planar giant covalent network.
    • Because of its colour, it sometimes, confusingly, called 'white graphite'!
  • It is a very good insulator (thermal and electrical?) and chemically very inert i.e. great chemical stability - very unreactive!
  • It melts under pressure at ~3000^oC testament to its great thermal stability.
  • In the hexagonal form of boron nitride, alternate boron and nitrogen atoms are linked to form interlocking hexagonal rings, just like the carbon atoms in graphite do.
  • Therefore in each hexagonal ring there are 3 boron atoms and 3 nitrogen atoms and all the bond lengths are 0.145 nm, so it isn't an alternate single-double bond system but the above diagram is just a simple valence-bond representation.
  • The B-N-B or N-B-N bond angle is 120^o, i.e. that expected for perfect hexagonal ring bond network e.g. as found in graphite.
  • sp² hybridisation is quoted for the boron atom bonds.
  • The B-N bonding in the 2D layers is very strong giving boron nitride great thermal stability, i.e. very melting point.
  • However, the layers are held together by weak intermolecular forces (Van der Waal forces, instantaneous dipole - induced dipole forces) and the layers are 0.334 nm apart.
    • This distance is similar to the inter-layer gap in graphite, not surprising, bearing in carbon lies between boron and nitrogen in period 2 of the periodic table.
  • As in graphite and graphene, there is pi bonding BUT the energy levels are too high to allow good electrical conduction you find in graphite.
  • Hexagonal boron nitride (HBN) is used as a lubricant (weakly held layers can slide over each other), and can have semiconductor properties (after doping?).
    • Because of its 'soft' and 'slippery' crystalline nature, HBN is used in lubricants and cosmetic preparations.
  • Hexagonal boron nitride can be made in single layers and can also be formed into nanotubes.
  • Bundles of boron nanotubes are used for wire sleeving.
  • Boron nanotubes are used as a catalyst support, as in the case of carbon nanotubes.
  • Boron nitride is NOT an electron deficient compound like semi-conductors.
  • Hexagonal boron nitride can be incorporated in ceramics, alloys, resins, plastics, rubbers to give them self-lubricating properties.
    • Plastics filled with HBN have decreased thermal expansion, increased thermal conductivity, increased electrical insulation and cause reduced wear to adjacent parts.
• Because of their excellent thermal stability, thermal shock stability and chemical stability, boron nitride ceramics are often used as parts of high-temperature equipment ( a typical melting range is 2700-3000^oC). They are stable in air to ~1000^oC whereas carbon-graphite based materials would have long since ignited!