In crystallography, the diamond structure is also called the diamond cubic crystal structure, which is formed by the covalent bonding of carbon atoms. Many of the extreme properties of diamond are the direct result of the sp³ covalent bond strength that forms a rigid structure and a small number of carbon atoms. Metal conducts heat through free electrons, and its high thermal conductivity is associated with high electrical conductivity. In contrast, heat conduction in diamond is only accomplished by lattice vibrations (ie, phonons). The extremely strong covalent bonds between diamond atoms make the rigid crystal lattice have a high vibration frequency, so its Debye characteristic temperature is as high as 2,220 K.
Since most applications are much lower than the Debye temperature, the phonon scattering is small, so the heat conduction resistance with the phonon as the medium is extremely small. But any lattice defect will produce phonon scattering, thereby reducing thermal conductivity, which is an inherent characteristic of all crystal materials. Defects in diamond usually include point defects such as heavier ˡ³C isotopes, nitrogen impurities and vacancies, extended defects such as stacking faults and dislocations, and 2D defects such as grain boundaries.
The diamond crystal has a regular tetrahedral structure, in which all 4 lone pairs of carbon atoms can form covalent bonds, so there are no free electrons, so diamond cannot conduct electricity.
In addition, the carbon atoms in diamond are linked by four-valent bonds. Because the C-C bond in diamond is very strong, all valence electrons participate in the formation of covalent bonds, forming a pyramid-shaped crystal structure, so the hardness of diamond is very high and the melting point is high. And this structure of diamond also makes it absorb very few light bands, most of the light irradiated on the diamond is reflected out, so although it is very hard, it looks transparent.
At present, the more popular heat dissipation materials are mainly members of the nano-carbon material family, including nanodiamond, nano-graphene, graphene flakes, flake-shaped nano-graphite powder, and carbon nanotubes. However, natural graphite heat dissipation film products are thicker and have low thermal conductivity, which is difficult to meet the heat dissipation requirements of future high-power, high-integration-density devices. At the same time, it does not meet people’s high-performance requirements for ultra-light and thin, long battery life. Therefore, it is extremely important to find new super-thermal conductive materials. This requires such materials to have extremely low thermal expansion rate, ultra-high thermal conductivity, and lightness. Carbon materials such as diamond and graphene just meet the requirements. They have high thermal conductivity. Their composite materials are a kind of heat conduction and heat dissipation materials with great application potential, and they have become the focus of attention.
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