The properties of the material itself, in fact, determine its application to a large extent. Like most ceramic materials, the heat transfer performance is worse than that of metal materials, so the application in the field of heat conduction is not as much as that of metal materials.

But even so, because ceramic materials have high melting point, high hardness, high wear resistance, oxidation resistance, corrosion resistance, and excellent characteristics in sound, light, electricity, heat, and magnetism, therefore, in certain occasions, ceramics with better thermal conductivity and insulation can replace metals to play a role, generally oxides, nitrides, carbides, borides, etc., such as AlN､BeO, Si3N4, SiC, BN. In recent years, the application of these high thermal conductivity ceramics has become very extensive, and the most well-known of them is ceramic substrates.

The heat transfer principle of thermally conductive ceramics

The heat conduction process is the energy transmission process inside the material, but the energy transmission does not follow a straight line from one end to the other end of the object, but in the form of diffusion, which will deviate from the straight line direction due to collision during the propagation process. The carriers of thermal energy are electrons, phonons, photons and magnetic excitation.

Since ceramics are covalent compounds, the internal electrons are bound and cannot move freely. Therefore, the heat conduction is achieved through the mutual restriction or harmonious vibration of the crystal structure primitives (atoms, ions, or molecules). When the lattice is intact and defect-free, the larger the mean free path of phonons, the higher the thermal conductivity. Generally, non-metallic materials with high thermal conductivity have the characteristics of simple crystal structure, lattice defects, less impurities and voids, and high Debye temperature.

Influencing factors of thermal conductivity of ceramic materials

Although the characteristics of high thermal conductivity ceramics are explained above, in actual industrial applications, crystals are more or less defective, and the distribution of structural elements will be different. Therefore, in addition to the inherent phonon-phonon scattering that reduces the thermal conductivity of aluminum nitride ceramics, various defects in the ceramic are the main factors affecting the thermal conductivity. Let's take AlN ceramics as an example to list the annoying defects that affect its thermal conductivity.

1. Oxygen impurities

Like all solid media, the lattice impurities of aluminum nitride will adversely affect its thermal conductivity, and the main impurity is lattice oxygen. Based on its single crystal research, Slack proposed that oxygen atoms will be solid-dissolved into the aluminum nitride lattice. Since oxygen atoms and nitrogen atoms are non-equivalently replaced, according to the defect equation, one aluminum will be vacant with three oxygen atoms. As shown in the following defect equation:

This results in the generation of a large number of aluminum lattice sites and aluminum vacancies, which makes the aluminum nitride crystal lattice present anharmonicity, which affects the phonon scattering, so that the thermal conductivity of aluminum nitride ceramics is sharply reduced. Bachelard et al. study showed that when the oxygen content in aluminum nitride is 0.12wt%, its thermal conductivity drops to 185W/(m·K), and when the oxygen content rises to 0.31wt%, its thermal conductivity is only 130W / (m · k).

2. Density

According to the thermal conductivity of aluminum nitride, the large number of pores in low-density samples will affect the scattering of phonons, reduce their mean free path, and thereby reduce the thermal conductivity of aluminum nitride ceramics. At the same time, the mechanical properties of low-density samples may not meet relevant application requirements. Therefore, high density is a prerequisite for aluminum nitride ceramics to have high thermal conductivity.

3. Micro structure

Some additives are often added in the sintering process of aluminum nitride ceramics to reduce the sintering temperature of aluminum nitride ceramics. However, the second phase produced at the same time may exist in the aluminum nitride crystal lattice, and it will also be scattered during the heat conduction process of the aluminum nitride, thereby affecting the thermal conductivity of the aluminum nitride ceramic. The distribution of the second phase in the aluminum nitride lattice mainly has two forms, one is distributed at the grain boundary, which is a continuous phase distribution, and the other is distributed at the grain boundary triangle, which is an isolated phase distribution.

It can be seen from the figure that, compared to the aluminum nitride ceramics with the second phase distributed at the grain boundary, the aluminum nitride ceramics with the second phase distributed at the grain boundary triangle have better thermal conductivity, because the latter produces less coherent scattering in the heat conduction process of aluminum nitride. The above picture is a microscopic model of the different distribution of the second phase in aluminum nitride ceramics, which vividly illustrates that the second phase located at the grain boundary triangle has less influence on the heat conduction of aluminum nitride than at the grain boundary.

Conclusion

On the whole, in order to improve the thermal conductivity of ceramic materials, the following methods can be used: try to increase the purity of ceramic materials, and try not to add or add as little additives as possible. However, in order to increase the density of the material and control the grain size, it is necessary to add a certain amount of additives; proper control of the raw material particle size can significantly increase the thermal conductivity; increase the density of ceramic materials, reduce pores and glass phase, make it as close to the theoretical density as possible, and so on.