Materials fall under two camps: electrical conductors (metals) can conduct heat (phonon), but not emit heat (IR), and electrical insulators (ceramics) are just the opposite. However, amorphous diamond is both the thermal conductor and thermal emitter. Amorphous diamond is not only thermionic; it is also the perfect black body. Typically a metal has a low emissivity (e.g. 2%), but an insulator has a poor thermal conductance (e.g. 10 W/mK), so both of them cannot sustain the emission far infrared from a warm surface. However, amorphous diamond has a thermal conductivity (about 500 W/mK) that is even higher than the best metal (420 W/mK for silver), and its emissivity is nearly 100%. It was measured that at a temperature as low as 70°C, the sustained heat emission was 0.088 W/cm2, this equals to what predicted by Stefan-Boltzmann's equation (5.67 x 10-8 T[K]4/m2) for black body. This implies that
amorphous diamond has an emissivity of about 100% and the emission is not limited by its thermal conductivity. The exceptional ability to emit heat makes amorphous diamond an excellent thermal radiator for cooling high-powered LED (Figs. 9.8 and 9.9).
As the black body is reversible, amorphous diamond can be used as the heat absorber for applications related to advanced thermal imaging of infrared source or mundane water heating by absorbing sunlight.
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