Aluminum nitride (AlN) ceramics have excellent thermal conductivity, good electrical insulation properties, thermal expansion coefficient matching with silicon materials, and high mechanical strength. AlN ceramics show a wide application potential in high-tech fields such as electronic packaging, circuit substrates, and heat dissipation components. Especially with the rapid development of microelectronic technology to high power and high density integration, AlN ceramics have attracted great attention as an ideal heat dissipation and packaging material. However, AlN as a covalent bond compound, its inherent strong covalent bond, low diffusivity and high melting point make it difficult to achieve densification under conventional sintering conditions, thus restricting the optimization of its microstructure and the improvement of its performance, as well as its application in a wider range of fields. To overcome this bottleneck, the introduction of sintering aids becomes one of the most feasible and effective ways to achieve high density and high performance AlN ceramics. In recent years, researchers have extensively explored different types of sintering aid systems, including oxides, fluorides, composite additives, and other types of sintering aids. Their effects on the sintering behavior, microstructure and properties of AlN ceramics are investigated, obtaining the remarkable results.

In this review, the sintering aids are divided into oxide sintering aids, fluoride sintering aids, composite sintering aids and other types of sintering aids according to the chemical composition. According to the different types of sintering aids, the performance optimization mechanism of AlN ceramics is analyzed. Rare-earth oxides (such as Y₂O₃, CeO₂, etc.) are the most widely used sintering aids. They can react with the inherent Al₂O₃ oxide layer on the surface of AlN particles to form rare-earth aluminate liquid phase. The liquid phase has a low viscosity and a good wettability at high temperatures, which can promote the rearrangement of AlN particles. The oxygen impurities in the AlN lattice are effectively fixed at the grain boundary in the form of grain boundary phase. The degree of oxygen solid solution in the AlN lattice is significantly reduced, and the phonon scattering in the lattice is reduced. The unique advantage of fluoride sintering aids (such as CaF₂, YF₃, etc.) is that they can form eutectic liquid phase or react with Al₂O₃ on the surface of AlN to form volatile fluorine-containing substances during sintering, thus promoting the rearrangement of AlN particles and filling the pores to achieve densification. There is no second phase residue at the grain boundary of AlN after sintering. The composite sintering additives can often reduce the lattice oxygen content through the synergistic effect of each component, i.e., one of which focuses on removing oxygen impurities and the another focuses on forming an effective liquid phase at a lower temperature to promote densification. AlN ceramics can form a liquid phase, which is more conducive to sintering at a lower temperature, and achieve more fine regulation of grain microstructure. Other types of sintering aids are mainly represented by carbides or rare-earth hydrides, which also promote densification via forming a low melting point liquid phase, and remove oxygen impurities from the AlN lattice and fix them to the grain boundaries through chemical reactions. The mechanical and thermal properties of AlN ceramics are improved to achieve the purpose of obtaining AlN ceramics with a high strength and a high thermal conductivity.