Boron nitrides BN
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Buckminsterfullerenes-bucky balls *
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Part 6. Cubic and hexagonal 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
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.
The B-N-B or N-B-N bond
angle is 109o i.e. that expected for tetrahedral bond network
e.g. as found in the 3D C-C bond network in diamond.
lone pair of
electrons can accepted by boron to give the tetrahedral bond network shown
in the diagram (sp3 hybridisation?).
similar to adduct compounds formed e.g.
boron trifluoride ==> F3B<=:NH3
This gives a
tetrahedral arrangement of bounds around both the boron and nitrogen
Cubic boron nitride is
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
Because of its hardness,
chemical inertness, high melting temperature (2973oC) 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 1100-1200oC.
Boron nitride is stable in
air, BUT is slowly hydrolysed by water ...
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
~3000oC testament to its great thermal stability.
In the hexagonal form of boron
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 120o, 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.
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?).
Hexagonal boron nitride can be
made in single layers and can also be formed into nanotubes.
Bundles of boron nanotubes are
used for wire
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
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 2700-3000oC). They are stable in air to
~1000oC whereas carbon-graphite based materials would have long
NANOSCIENCE - NANOCHEMISTRY INDEX
Introduction to nanoscience,
nanoparticles, commonly used terms explained
Part 2. Nanochemistry - introduction,
uses & potential
Uses of Nanoparticles of titanium(IV) oxide (e.g. sun
cream), fat (e.g. cosmetics), silver (e.g. medical applications)
From fullerenes & bucky balls to carbon nanotubes -
structure, properties, uses
graphene oxide and
fluorographene - structure, properties, uses
Cubic and hexagonal boron nitride BN
Problems, issues and
implications associated with
see also INDEX
Smart materials pages
A general survey of materials - natural & synthetic,
their properties & uses
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