Heat conductivity of diamond

Diamond, like glass or quartz, is a perfect electrical insulator. It may hence come as a surprise to hear that diamond – in complete contrast to glass and quartz – is a marvelous conductor of heat. Diamond has in fact the highest heat conductivity (thermal conductivity) of all materials, far better than that of metals, which are otherwise known as materials with very high heat conductivity:

                                               Heat 
                                               conductivity
                                   Material    (W/mK)
                                   ________    ____________ 

                                   Aluminum     220
                                   Copper       390
                                   Silver       420
                                   Diamond     2000

Stiff bonds

The reason for this surprisingly high thermal conductivity is the extremely strong sp3 chemical bond between the carbon atoms in a diamond crystal. While the thermal conductivity of metals depends on “freely mobile” valence electrons, which also give the metals their high electrical conductivity, the sp3 bond of diamond binds the electrons strongly to their respective nuclei. The thermal vibrations in a diamond crystal are instead propagated along the extremely “stiff” sp3 bonds. This gives the phonons (= quanta of thermal vibration) a path of very low resistance, while the unavailability of “freely mobile” electrons makes transport of electricity almost impossible.

Diamonds are not forever

It is interesting to note that while the sp3 bond of diamond is extremely strong, physically speaking, it is not the most energetically stable chemical bond that can occur between carbon atoms. The carbon-carbon bond with the most stable energy is instead the graphite bond. Hence all diamond crystals will in time convert to graphite, to the crystal structure with the optimal energy conditions. This transformation is very slow (on the order of millions of years), but the phrase “diamonds are forever” is clearly unjustified – if you wait long enough, you will find that your sparkling diamond in your engagement ring has turned into a dull slab of gray graphite. If you are not patient enough, then you can speed up the process by subjecting your ring to elevated temperature (around 1000 C), which will quickly transform your gem into graphite.

Heat sinks

The high component density in electronic integrated circuits necessitates heat sinks to conduct away the heat that the ICs generate. Up to now heat sinks out of metals like copper and aluminum, with or without cooling flanges, have been used.

For the next generation of microprocessors, with their extremely high component density, metals are no longer sufficiently good heat conductors. Instead, attention has turned to diamond.

Clearly, cutting down the Kohinoor diamond to suitable size to fit an IC would be prohibitively expensive. An alternative is to grow diamond films by Chemical Vapor Deposition (CVD). This gives well-conducting diamond sheets, several millimeters thick, but it is also relatively expensive.

Low-cost diamond composites

The presently preferred technique is to make heat sinks out of diamond composites. Here densely packed small diamond grains are bound together with some binder material. The price of diamonds falls abruptly as the grain size decreases, so small diamond grains can be bought for less than a dollar per gram. The presence of binder reduces the heat conductivity of the material, but well-designed composites still have much higher heat conductivity than that of any metal.