My daughter is a AS level student. She need the answer of following question.
Why carbon in the form of diamond is an insulator? but in the form of graphite is a reasonablely good conductors?
Thanks for help
best wishes
Ali

 

The difference between carbon in graphite and diamond form is the bonding.

In graphite form carbon is sp3 hybridized (look this up in any chemistry textbook).
In diamond form carbon is sp2 hybridized.

Now what you want to analyze is the resulting energy band structure. I believe (correct me if I'm wrong), diamond is an insulator, while graphite is a semi-metal.

The bandgap is the energy difference between the valence and conduction bands in a material. Only electrons in the conduction band and holes in the valence band are available to conduct. However the valence band is usually filled with electrons and the conduction band is usually filled with holes. In order for the electrons and holes to conduct they have to jump the bandgap.

In metals there is no bandgap, so any electron can contribute to conduction.
In semiconductors there is a small bandgap. At high termperatures semiconductors behave as good conductors while at low temperatures they behave as insulators.
In insulators there is a large bandgap, it is very difficult for electrons or holes to jump the bandgap (lots of energy (temperature) is required), so they are poor conductors.

In semi-metals the bands overlap... I don't remember exactly how this works out in explaining why a semi-metal is a better conductor than an insulator.

But anyway,

Just from looking up numbers on yahoo, I find that the bandgap of diamond is around 6eV, while that of carbon is -0.05eV. Assuming the -0.04eV means that the bands are overlapping (ie graphite is a semi-metal), that should explain why graphite is a better conductor than diamond.

hope this helps.

 

In semi-metals the bands overlap... I don't remember exactly how this works out in explaining why a semi-metal is a better conductor than an insulator.

As far as I'm aware semi-metals beahve in the same way as semi-conductors, as you have explained before. The situations where the band-gaps overlap critically occurs in the Field Effect Transistors (particularly the MOSFET if I remember correctly), where large fields change the doping type of the material at the gates creating a essential depletion region. I can't full remember so I will have to dig out the old first year notes!!