How does diamond conduct heat?

With the continuous development of technology, more and more high-power electrical appliances and microelectronic components are gradually emerging, and people's demand for lightweight and high-performance electronic products is increasing. The power density of semiconductor components is constantly improving, and the heat flux will also increase. Conventional heat dissipation materials can no longer solve the heat dissipation problem well, and how to dissipate heat and cool down materials has become the primary challenge.

 

• So how to choose materials in the field of thermal conductivity and heat dissipation?

At present, the popular heat dissipation solutions mainly include graphite sheets, graphene, thermal interface materials, heat pipes and homogenization plates, and semi-solid die castings. However, natural graphite heat dissipation film products are thicker and have low thermal conductivity, which makes it difficult to meet the heat dissipation needs of future high-power and high-density integrated devices. At the same time, they also do not meet people's high-performance requirements such as ultra-thin and long battery life. Therefore, the search for new materials with super thermal conductivity is of great significance. This requires such materials to have extremely low thermal expansion rate, ultra-high thermal conductivity, and lightweight volume. Carbon materials such as diamond and graphene meet the requirements perfectly. They have high thermal conductivity, and their composite materials are a class of highly potential thermal conductivity and heat dissipation materials, which have become the focus of people's attention.

 

• The excellent thermal conductivity and heat dissipation of diamond

Faced with various limitations of traditional packaging materials, various new heat dissipation materials have been developed, which have low thermal expansion and light weight. Diamond, as a representative of these materials, is the substance with the highest thermal conductivity in nature. People often say that the thermal conductivity of diamond is five times that of copper. In fact, there are various types of diamonds, such as Type Ia, Ib, IIa, IIb, etc. For Type I and II diamonds, they are distinguished by the difference in their ultraviolet and infrared absorption spectra, while Type A and B diamonds are distinguished by the difference in their electron paramagnetic resonance absorption. Different types of diamond also have different thermal conductivity, which means that the thermal conductivity of the same type of diamond may not be the same. The thermal conductivity of diamond is related to the integrity of its internal structure and the types and contents of impurities it contains. The thermal conductivity of the same type of diamond also varies at different temperatures, as shown in the table below:

The thermal conductivity of diamond is not fixed and has a range of variation. The main materials used as diamond heat sinks are Type IIa single crystal diamond and polycrystalline diamond with required thermal conductivity, with a thermal expansion coefficient of about 0.8 × 10-6/K and insulation at room temperature.

 

• Principle of Diamond Thermal Conductivity

Diamond has a cubic crystal structure, where each carbon atom forms a covalent bond with four other carbon atoms in SP3 hybridized orbitals, forming a regular tetrahedron. Because all valence electrons are confined to the covalent bond region and there are no free electrons, diamond is non-conductive. High thermal conductivity is associated with high electrical conductivity. Unlike metals that rely on peripheral electrons for heat transfer, the thermal conductivity of diamond is mainly derived from the propagation of carbon atomic vibrations (i.e. phonons).

 

The mean free path of phonons is determined by the collisions between phonons and the scattering of phonons by defects in solids. Impurities, dislocations, cracks, and other crystal defects in diamond, as well as residual metal catalysts and lattice orientations, can collide with phonons and scatter them, thereby limiting the mean free path of phonons and reducing thermal conductivity.

 

When the composition of a substance is simpler, the structure is simpler, and there are fewer impurities, the phonon motion is faster, and the heat transfer rate is also faster. This is because the introduction of the second component and impurities can cause lattice distortion, distortion, and dislocations, disrupting the integrity of the crystal and increasing the scattering probability of phonons or electrons. The composition of diamond is only a single element of carbon, and its structure is also very simple. Among the four types of diamonds Ia, Ib, IIa, and IIb, IIa is the purest and has the least impurities, thus having the highest heat transfer rate.

 

In the past, when buying diamonds, some people would lick them with the tip of their tongue. If they felt the tip of their tongue cool, it was a real diamond; If it's warm, it's just glass. This process is actually using the tip of the tongue as a probe to conduct a comparative experiment on the thermal conductivity of a gemstone. Because the thermal conductivity of glass is very low, while the heat transfer rate of real diamond is over a thousand times that of glass, the sensitive tip of the tongue can easily distinguish the difference between the two.

 

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