Contrasting Electrical Properties of Boron Nitride and Graphite: An Examination of Structure and Bonding
While graphite is a well-known conductor of electricity, BN stands out as an insulator. Understanding the reasons behind this stark contrast can shed light on the intriguing nature of these two materials.
The contrasting electrical properties of graphite and boron nitride can be elucidated through analysis of their crystal symmetries. Graphite possesses sublattice symmetry with two identical carbon atoms per unit cell, resulting in degeneracy at the Dirac points that protects gapless conductivity. Conversely, the symmetry is broken in boron nitride, which has alternating boron and nitrogen atoms per unit cell. This lifts the degeneracy and opens a band gap, conferring insulation.
Additionally, the ionic nature of B-N bonds localizes electrons, inhibiting conductivity. The strong polarity arises from differences in electronegativity between boron and nitrogen. This gives boron nitride a stable layered structure as free electrons preferentially bond with nitrogen atoms.
In graphite, the trigonal planar carbon atoms exhibit sp2 hybridization with delocalized pi orbitals oriented perpendicular to the sheets. The symmetry of the resonating pi-pi bonds between carbon atoms allows the free movement of electrons, enabling metallic conductivity along the sheets.
In summary, while the symmetric carbon-carbon bonding in graphite permits conduction, the asymmetric and ionic boron-nitrogen bonds in BN localize electrons. This explains the insulating behavior of boron nitride compared to the high conductivity of graphite.
Furthermore, the difference in electronegativity between boron and nitrogen in BN leads to a more ionic character in the bonds compared to the purely covalent bonding in graphite. This ionic character further restricts the mobility of electrons in BN, contributing to its insulating behavior.
The unique electrical properties of BN and graphite make them suitable for different applications. Graphite's conductivity makes it ideal for use in electrical circuits, electrodes, and other electronic components. Its ability to carry electrical current with low resistance and its thermal conductivity make it a valuable material in various industries.
In contrast, BN's insulating properties are advantageous in applications that require electrical insulation, such as insulating layers for electronic devices, high-voltage equipment, and thermal management systems. BN's high thermal stability, excellent thermal conductivity in the plane of the layered structure, and resistance to chemical attack make it a preferred choice in high-temperature environments where electrical insulation is needed.