The heat transfer of AIN belongs to phonon heat conduction. When the lattice is complete and defect-free, the mean free path of phonons is large, and the thermal conductivity is high. The room temperature thermal conductivity of pure AIN crystal is 319W/(m·K) (Slack, 1987). Some studies have shown that for AIN ceramics with 100~220 W/(m·K), the mean free path of phonons at room temperature is only 10~30nm, which is much smaller than the AIN grain size (1~40μm). Therefore, the crystal size factor has little effect on thermal conductivity. The impurities (carbon, oxygen, silicon), especially the defects caused by oxygen impurities, have the greatest impact on thermal conductivity.
The surface of AlN powder is often covered with a layer of Al2O3 film due to oxidation, which can be regarded as the existence form of oxygen. Since AIN has a very strong affinity for oxygen, oxygen impurities diffuse into the AIN lattice during the sintering process, replacing the position of nitrogen and generating aluminum vacancies.
The scattering of phonons by aluminum vacancies will lead to a decrease in thermal conductivity. Slack believed that the independent existence of aluminum vacancies causes unit cell contraction, which is evenly distributed in AIN and scatters phonons. As oxygen dissolves into the AIN lattice, accompanied by the formation of structural defects such as aluminum vacancies, the phonon scattering area increases, the phonon means free path decreases, the thermal resistance of AIN increases, and the thermal conductivity decreases.
Harris et al. (1990) believed that defects in the AIN lattice are closely related to the oxygen concentration. When the oxygen concentration is different, the form of defects is also different, and there is a critical molar fraction x”(O)~0.75% (atomic fraction). When x(O)<r”(O), oxygen is evenly distributed in the AIN lattice, occupying the position of nitrogen, and accompanied by the generation of aluminum vacancies. Due to the generation of aluminum vacancies, the volume of the adjacent area shrinks, and the aluminum vacancy VAl and the three adjacent nitrogen vacancies ON form a composite scatterer. When x(O)>x”(O), isolated defects will agglomerate, and aluminum atoms and oxygen atoms will form a more stable coordination octahedron centered on aluminum. Each coordination octahedron will annihilate two VAl, which is more stable from the perspective of crystal chemistry. When x(O) is further increased, the aluminum-oxygen octahedron extension defects, such as oxygen-containing stacking faults and inversion domain boundary core polymorphs, will produce strong phonon-defect scattering, reducing the thermal conductivity of AlN.
Therefore, the main scattering mechanism in AIN ceramics is still oxygen defect scattering. Therefore, suitable additives are selected to have a strong binding ability with Al2O3, to remove oxygen impurities and purify the AlN lattice, thereby reducing the phonon scattering cross section, increasing the phonon mean free path, and improving thermal conductivity. On the other hand, since the thermal conductivity of the second phase (such as the grain boundary phase) is much lower than that of the AIN crystal phase, from the perspective of microstructure, if the second phase is an isolated phase or completely eliminated, it will help improve the thermal conductivity of the material.
Edgetech can provide aluminum nitride ceramic products with thermal conductivity ranging from 170-250W/mk.