An insulator that moves heat
Metals carry heat mainly with free electrons, which is why a copper pan spreads heat quickly. Diamond has no free electrons to speak of, so it is an electrical insulator. Even so, a high-quality diamond conducts heat better than copper at room temperature, by a wide margin. Pure diamond is often described as having a thermal conductivity several times higher than copper, although the exact figure depends on the sample and is best taken from measurements on the specific material.
The explanation lies in how heat moves through a non-metallic crystal: not by electrons, but by vibrations of the lattice.
Heat as vibration
Atoms in a solid are never still. They vibrate about their positions, and because they are bonded to each other, a vibration in one place passes to its neighbors in the form of a wave. In a crystal these waves are quantized, and each quantum is called a phonon. Heat flow in a non-metal is, to a good approximation, the movement of phonons from the hot end to the cold end.
How well a crystal conducts heat then depends on two things: how fast the phonons move, and how far they travel before something scatters them. Diamond is excellent on both counts.
Why diamond’s phonons travel fast
Sound-like waves move faster in a stiff material made of light atoms. Diamond has both features. Carbon is a light element, and the bonds between the atoms are strong and short, making the lattice very stiff. The result is that vibrational waves move through diamond at very high speed, and the range of vibration energies available is wide. High stiffness is the same reason diamond is so hard, which is why the thermal behavior and the mechanical behavior are connected facets of one structure.
Why they travel far
Even a fast phonon would carry little heat if it was scattered every few atoms. Scattering happens when phonons collide with defects, with impurity atoms, with grain boundaries, with surfaces, or with other phonons. In a pure, well-ordered diamond lattice, the first of these are few, and the strong, regular bonding also limits phonon-phonon collisions at ordinary temperatures. Phonons can therefore travel a long distance before being deflected, which is the same as saying the heat flow is efficient.
What reduces conductivity in real material
Diamond’s top-level performance applies only to excellent material. Several things lower it:
- Impurities. Nitrogen, boron and other atoms disturb the lattice and scatter phonons. High-purity grades conduct best,, which is why impurity levels matter so much.
- Isotopes. Natural carbon is mostly carbon-12 with a small share of carbon-13. The slight mass difference acts as a weak disorder and scatters phonons. Diamond enriched in carbon-12 has been reported to conduct heat even better than natural-composition diamond, which shows how sensitive the property is.
- Grain boundaries. In polycrystalline diamond, boundaries between grains scatter phonons, so conductivity is typically lower than in a single crystal, particularly with very small grains.
- Non-diamond carbon and voids. Graphitic regions and pores in films or composites reduce conduction.
- Thickness and surfaces. In thin films, surfaces and boundaries limit how far phonons can travel, so a thin film can conduct noticeably worse than the bulk.
- Temperature. Conductivity changes with temperature. For diamond it is generally highest at low temperatures, well below room temperature, and decreases as temperature rises and phonon collisions become more frequent. It stays high at working temperatures, though.
Making use of it
This combination of high heat conduction and electrical insulation is rare and valuable. Applications include laser heat spreaders and heat sinks for high-power electronics, where hot spots must be spread out quickly. Diamond is also combined with metals in diamond composites for heat spreader materials, which try to capture much of the thermal benefit at lower cost and with better matched expansion. Anything that sits between a hot device and a heat sink also introduces an interface, and the thermal interface materials used there can limit overall performance more than the diamond itself.
Diamond also expands very little when heated, which keeps parts dimensionally stable, though it must still be joined to materials that expand much more. How that mismatch is handled is a design issue linked to thermal expansion in assemblies.
How it is measured
Thermal conductivity is usually derived from other measurements, such as thermal diffusivity from a laser-flash method, combined with density and heat capacity. The measurements are sensitive to sample thickness and surface preparation, which is one reason published values vary. For design work, data on the exact grade should be preferred over a single headline number.
Summary
Diamond conducts heat so well because its light, strongly bonded carbon atoms form a stiff lattice that carries vibrations quickly, and because a pure crystal scatters those vibrations rarely. Defects, impurities, isotopes, grain boundaries and thin geometry all cut into that performance, so the headline figure applies to the best material and not to every piece of diamond.