The rapid development of technologies such as artificial intelligence, 5G communications, and new energy vehicles has led to continuously increasing power density and integration levels in electronic devices, making "heat dissipation" a key factor limiting device performance. Compared with traditional materials such as Al₂O₃, BeO, and SiC, AlN (aluminum nitride), with its excellent thermal conductivity, high mechanical strength, good thermal expansion matching, chemical stability, non-toxicity, low dielectric constant, and high electrical resistivity, has become a critical material for electronic packaging and thermal management. However, AlN powder is prone to hydrolysis in practical applications. When exposed to humid air, its surface undergoes an irreversible reaction with water, producing aluminum hydroxide [Al(OH)₃], AlOOH, and other products. This leads to nitrogen vacancies, increased porosity and oxygen content in subsequently sintered AlN ceramics, a significant drop in thermal conductivity, and even the release of ammonia gas during use, which can affect surrounding components (e.g., poisoning platinum catalysts). Therefore, finding suitable anti-hydrolysis methods is crucial for the development of AlN.
Hydrolysis Behavior of Aluminum Nitride
The hydrolysis behavior of AlN can be roughly divided into three characteristic stages:
(1) Induction period: The pre-existing amorphous hydroxide compound (AHC) on the AlN surface dissolves slowly, and the pH remains largely stable (lasting 17–180 min at room temperature; the induction period disappears at 90 °C).
(2) Rapid reaction period: After the AHC dissolves, the AlN surface is exposed and undergoes rapid hydrolysis to form amorphous AlOOH, which subsequently transforms into crystalline boehmite (AlOOH). The pH rises sharply to 9–10.
(3) Product stabilization period: At low temperatures (22–50 °C), boehmite further dissolves and recrystallizes into bayerite (α-Al(OH)₃). At high temperatures (80–90 °C), boehmite remains stable and no bayerite forms.
Key Factors Influencing the Hydrolysis Process
1. Environmental factors: The environmental medium is a key factor determining the duration of each stage and the nature of the hydrolysis products.
(1) Temperature: Fukumoto et al. found that when the temperature is below 77.85 °C, the final hydrolysis product is Al(OH)₃; above 77.85 °C, the main product is AlOOH. Strong acids (e.g., HCl) or strong bases (e.g., NaOH) accelerate hydrolysis, whereas moderately strong acids (e.g., H₃PO₄) can inhibit hydrolysis by forming a protective phosphate layer.
(2) pH: Kocjan found that the initial pH does not change the overall hydrolysis rate, but at pH = 10 the induction period is eliminated (amorphous AHC cannot exist stably under alkaline conditions). In acidic environments of pH = 1–3, H⁺ ions can combine with OH⁻ on the AlN surface, delaying hydrolysis.
(3) Atmosphere: Hou et al. studied high-temperature environments (1000–1150 °C) and found that because H₂O promotes the inward diffusion of H⁺, the hydrolysis/oxidation tendency of AlN is highest in an Ar–20 vol% H₂O atmosphere, with a mass increase of 21.1% after 15 h. In an air–20 vol% H₂O atmosphere, O₂ competes with H₂O, resulting in a mass increase of only 12.0%, indicating that oxygen suppresses high-temperature hydrolysis.
2. Particle size: Studies have shown that the hydrolysis rate of nano-sized AlN is 5–10 times faster than that of submicron-sized AlN. Hydrolysis initiates at surface defects such as steps, demonstrating the significant influence of particle size and surface morphology on hydrolysis kinetics.
3. Preparation process: Currently, the industrial production of AlN powder is dominated by carbothermal reduction and direct nitridation methods. Li et al. found that AlN prepared by carbothermal reduction has a stable γ-Al₂O₃ layer on its surface, resulting in the longest induction period (>24 h), whereas the induction period for AlN prepared by direct nitridation is only 6 h.