Question: 4. Diamond is a good thermal conductor. But it is an excellent electrical insulator. How?

Nathan Langford answered on 27 Jun 2012:

Hi again sizzle1999,

Another good question.

You’re right that materials that are good conductors of electricity are generally also good thermal conductors, because there a lots of free electrons moving around which can carry energy from place to place, taking it from one part of a crystal to another.

However, diamond is a good thermal conductor in another way. The reason why diamond is a good electrical insulator is because of its special structure and the special element it is made up from – carbon. Diamond is one of the most amazing natural materials in the world. It has in some ways the most beautifully symmetrical structure. Every carbon atom in pure diamond is bound to exactly 4 other diamond atoms in exactly the same overall shape – called a tetrahedron, which is a special type of 4 sided pyramid, where every side is a perfect equilateral triangle. Now it turns out that 4 really is the perfect number, because carbon has exactly four electrons which are available to form bonds with other atoms (these are called “valence electrons”). But not only that, once it has made these 4 bonds to other carbon atoms, it turns out that its number of bonds are full. There are no more “free spots” for new bonds to form. If you think again about the bookshelves I described to you earlier (http://ias.im/87.357), then the next free bookshelf is a long way up, so that you can’t reach it without using a lot of energy (like bringing a long ladder or something).

So once each carbon atom has made these 4 bonds to other carbon atoms, there are no electrons left that are free to move around and create a current flow when a voltage is applied to them.

BUT, and here’s the thing about its thermal properties, because the crystal structure of diamond is so beautiful and symmetric, and because all of those bonds are so strong and stiff, it is very easy for vibrations to travel through a diamond crystal from one place to another. And this is what heat normally comes from.

Heat is normally the result of atoms moving around quickly. In gas form, a hot gas is one where the atoms or molecules are whizzing around really quickly, occasionally changing direction as they collide with each other or the walls of the container, but always moving really fast. For example, under normal room conditions of temperature and pressure, nitrogen molecules are moving around at around 500 metres per second or *1100 miles per hour* – now that’s fast! But in solids, the movement of molecules happens in the form of vibrations.

So in diamond, because it’s so hard and because all the carbon bonds are so stiff, the vibrations move around really easily… and because its structure is so symmetrical in all directions (which we call “isotropic”), these vibrations travel easily throughout the entire solid. And if diamond can carry vibrations easily throughout the crystal, then it can also carry heat really easily.

Hope this helps!
Cheers,
Nathan.