Why does C-C have a higher bond energy than B-N in hBN and graphene?

[james weaver]

Hi everyone, this is my first post in the chemistry section. I am doing a project for my engineering class on "cutting edge" materials and the one I chose is hexagonal boron nitride (specifically in the form of nanotubes). In comparing hBN to graphene, I need to explain to the class why graphene has a much higher thermal conductivity. From what I've read, non-metals transfer heat via vibrations in their lattice structures. As I understand it, atoms with higher bond energies have a stiffer spring-like effect which enables them to transfer heat quicker (makes sense because B-N has lower bond energy than C-C). My question is this:

Why does C-C have a higher bond energy than B-N? I thought that non-polar bonds in general have lower bond energies, and this case C-C has the less polar bond but higher bond energy.

 

[DrClaude]

The carbon-carbon bonds in graphene are double bonds and the electrons are delocalized over many carbon atoms (see https://en.wikipedia.org/wiki/Conjugated_system#Conjugated_cyclic_compounds). That makes for very strong bonds.

 

[Borek]

The carbon-carbon bonds in graphene are double bonds and the electrons are delocalized over many carbon atoms

hBN seems to be isoelectronic with graphene, which suggests bonds of a comparable strength, doesn't it?

 

[DrClaude]

hBN seems to be isoelectronic with graphene, which suggests bonds of a comparable strength, doesn't it?

Good point. But the bonds are highly polar.

 

[TeethWhitener]

The bond strength in hBN is similar to that of graphene. The thermal conductivity difference is almost entirely because of the electrical conductivity difference.

Thermal conductivity in general is determined by propagation of energy through the lattice, in the form of either phonons or electrons. hBN and graphene have similar phonon spectra, but hBN is an electrical insulator, whereas graphene is an electrical conductor, so electrons contribute substantially to the thermal conductivity in graphene but not hBN.