6. Cubic and hexagonal boron nitride structure uses applications BN nanotubes gcse igcse gce A Level AS A2 chemistry revision notes

Part 6. Boron nitrides BN

Doc Brown's Chemistry Revision Notes

NANOCHEMISTRY Nanoscience Nanotechnology Nanostructures

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Part 6. Cubic and hexagonal boron nitride

Boron Nitride
- What is boron nitride? What is boron nitride's molecular structure? What might we use boron nitride for?
- Boron nitride has the formula (BN)_n, (n is a very large number, but the empirical formula is BN)
- It forms cubic and hexagonal structures which correspond (analogous) to carbon in the form of diamond and graphite respectively.
Cubic Boron Nitride
- In the cubic form of boron nitride, alternately linked boron and nitrogen atoms form a tetrahedral bond network, exactly like carbon atoms do in diamond. So it is a 3D giant covalent lattice.
  - In fact it is isoelectronic with the giant covalent lattices of carbon.
- The B-N-B or N-B-N bond angle is 109^o i.e. that expected for tetrahedral bond network e.g. as found in the 3D C-C bond network in diamond.
- Nitrogen's lone pair of electrons can accepted by boron to give the tetrahedral bond network shown in the diagram (sp3 hybridisation?).
  - A situation similar to adduct compounds formed e.g.
  - ammonia + boron trifluoride ==> F₃B<=:NH₃
  - This gives a tetrahedral arrangement of bounds around both the boron and nitrogen atoms.
- Cubic boron nitride is extremely hard and will even scratch diamond. It is the second hardest material known, second only to diamond.
- Cubic boron nitride has very high thermal conductivity, excellent wear resistance and good chemical inertness, all very useful properties for a material subjected to extreme conditions.
- Because of its hardness, chemical inertness, high melting temperature (2973^oC) cubic boron nitride is used as an abrasive and wear-resistant coating.
- Cubic boron nitride (CBN) is used for cutting tools and abrasive components for shaping/polishing with low carbon ferrous metals. BN based tools behave in a similar way to diamond tools but can be used on iron and low carbon alloys without risk of reaction because CBN is chemically inert. CBN doesn't lose its cutting properties until 1100-1200^oC.
- Boron nitride is stable in air, BUT is slowly hydrolysed by water ...
  - to give ammonia + boric acid ?
  - BN + 3H₂O ==> H₃BO₃ + NH₃ ?
Hexagonal Boron Nitride
- 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!