With the continuous development of science and technology, more and more high-power electrical appliances and high-power microelectronic components gradually appear, as well as people's demand for lightweight and efficient performance of electronic products is getting higher and higher, the power density of semiconductor components is constantly improving, and the heat flux will become larger and larger. Ordinary heat dissipation materials can no longer solve the heat dissipation problem well, so how to cool the heat dissipation of materials has become the primary problem.

So in the field of heat conduction and heat dissipation, how to choose the material?

At present, the more popular heat dissipation solutions mainly include graphite sheets, graphene, thermal interface materials, heat pipes and heat transfer plates, and semi-solid die castings. The natural graphite thermal film products are thick, and the thermal conductivity is not high, it is difficult to meet the future high-power, high-integrated density device heat dissipation needs, but also does not meet people's ultra-thin, long life and other high-performance requirements. Therefore, it is of great significance to find new superthermal conductive materials. This requires such materials to have low thermal expansion, high thermal conductivity, and thin volume. Diamond, graphene and other carbon materials just meet the requirements, they have a high coefficient of thermal conductivity, the composite material is a kind of thermal conductivity material with application potential, has become the focus of attention.

Superior Properties of Heat Conduction and Heat Dissipation of Diamond

In the face of various limitations of traditional packaging materials, a variety of new heat dissipation materials have been developed, which have low thermal expansion rate and very light quality. Diamond, as a representative of the above materials, is a material with high thermal conductivity in nature. It is often said that the thermal conductivity of diamond is five times that of copper. In fact, there are various types of diamonds, such as Ia, Ib, IIa, IIb, etc. For type I and II diamonds, they are distinguished by the difference of ultraviolet and infrared absorption spectra of diamonds, while type A and B are distinguished by the difference of electron paramagnetic resonance absorption. Different types of diamonds have different thermal conductivity, that is, the thermal conductivity of the same diamond is not necessarily the same. The thermal conductivity of diamond is related to the integrity of its internal structure and the type and content of impurities, and the thermal conductivity of the same type of diamond at different temperatures is also different, as shown in the following table:

diamond single crystal, fine grain diamond, synthetic diamond, diamond

The thermal conductivity of diamond is not fixed, there is a range of variation, as the diamond heat sink is mainly IIa type of single crystal diamond and thermal conductivity meet the requirements of polycrystalline diamond, thermal expansion coefficient of about 0.8 × 10-6/K, and insulation at room temperature.

Principle of Diamond Heat Conduction

Diamond is a cubic crystal structure, each carbon atom is a SP3 hybrid orbital and the other four carbon atoms to form a covalent bond, constitute a regular tetrahedron, because all the valence electrons are confined to the covalent bond region, there is no free electron, so the diamond is not conductive. High thermal conductivity is associated with high electrical conductivity, and unlike metals, which rely on peripheral electrons for heat transfer, the thermal conductivity of diamond results essentially from the propagation of carbon atom vibrations (I. e., phonons).

The phonon mean free path is determined by the mutual collision between phonons and the scattering of phonons by defects in solids. Impurity elements in diamond, crystal defects such as dislocations and cracks, residual metal catalysts and lattice positions will collide with phonons to scatter them, thus limiting the mean free path of phonons and reducing thermal conductivity.

When the composition of the substance is simpler, the structure is simpler, and the impurities are less, the phonon motion is faster and the heat transfer rate is faster. This is because the introduction of the second component and impurities will cause lattice distortion, distortion and dislocation, destroy the integrity of the crystal, and increase the scattering probability of phonons or electrons. The composition of diamond is only a single element carbon, the structure is also very simple, Ia, Ib, IIa and IIb 4 kinds of diamonds, IIa pure, less impurities, so it has a high heat transfer rate.

In the past, when buying diamonds, some people would lick them with the tip of the tongue. If the tip of the tongue feels cool, it is a real diamond. If it is warm, it is only glass. This process is actually using the tip of the tongue as a probe to do a thermal conductivity comparison experiment on the gem. Because the thermal conductivity of glass is very small, and the heat transfer rate of real drill is more than a thousand times that of glass, so the sensitive tip of the tongue is really easy to distinguish the difference between the two.

In addition, diamond also has the characteristics of high resistivity and high breakdown field strength, low dielectric constant, low thermal expansion and so on. It has obvious advantages in the heat dissipation of high power optoelectronic devices, which also shows that diamond has great application potential in the field of heat dissipation.