SUN Wei ^{1, 2}, FAN Binbin ¹, WANG Chang’an ¹, XIE Zhipeng ¹

(1. School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China;

2. Wuxi Haigood New Technology Co., Ltd., Wuxi 214000, Jiangsu, China)

Extended abstract:

[Significance] Aluminum nitride (AlN) ceramics have emerged as one of the most strategically important thermal management materials for advanced electronic and energy-related systems, owing to their unique high intrinsic thermal conductivity, excellent electrical insulation, low dielectric constant and thermal expansion compatibility with silicon-based devices. With the rapid development of high-power electronics, high-frequency high-speed communication technologies and highly integrated semiconductor equipment, the dissipation of localized heat flux has become a critical bottleneck limiting device reliability and performance. In this context, AlN ceramics are regarded as indispensable functional materials for next-generation electronic packaging and thermal management components. Despite the theoretically high thermal conductivity of AlN crystals, practical AlN ceramics often exhibit significantly reduced thermal transport performance. This discrepancy primarily originates from microstructural imperfections, including residual porosity, secondary grain-boundary phases and oxygen-related lattice defects, all of which intensify phonon scattering. Therefore, achieving AlN ceramics with high thermal conductivity fundamentally relies on the precise control of sintering processes, particularly through the rational design of sintering additives and the selection of appropriate sintering technologies to promote densification, while minimizing thermal resistance at grain boundaries.

[Progress] Extensive efforts have been devoted to tailoring sintering additive systems to enhance the densification behavior and increase thermal conductivity of AlN ceramics. Alkaline-earth metal additives, such as Ca-based compounds, have been demonstrated to effectively react with surface alumina layers on AlN particles, forming low-melting liquid phases that facilitate particle rearrangement and densification. However, their limited oxygen-gettering capability and volatility at elevated temperatures restrict the achievable thermal conductivity. Rare-earth oxide additives, especially Y₂O₃, have been more widely adopted, due to their strong oxygen-scavenging ability and chemical stability. During sintering, Y₂O₃ reacts with surface Al₂O₃ to form yttrium–aluminate secondary phases, while simultaneously driving dissolved oxygen from the AlN lattice toward grain boundaries. The phase constitution and distribution of these secondary phases are highly sensitive to additive content and sintering temperature, exerting a decisive influence on purity and phonon transport of the grain boundaries. An optimal additive range is therefore required to balance densification rate and grain-boundary continuity. To further reduce sintering temperature and energy consumption, increasing attention has been paid to multicomponent and nontraditional additive systems, including fluoride-, carbide- and lithium-containing compounds, as well as synergistic combinations of rare-earth and alkaline-earth additives. These composite systems enable low-temperature liquid-phase sintering through eutectic reactions and in some cases facilitate the removal of oxygen impurities via volatilization or reduction mechanisms. Nevertheless, issues related to phase stability, volatilization control and environmental compatibility remain challenges for large-scale application. In parallel, advanced sintering technologies have been extensively explored to optimize microstructural evolution. Conventional pressureless sintering remains industrially attractive but typically requires high temperatures and long dwell times. Hot pressing significantly enhances densification and grain-boundary purification through the combined effects of temperature and external pressure, yielding AlN ceramics with superior thermal conductivity, albeit at the expense of shape complexity and production cost. Building upon conventional hot pressing, oscillatory hot pressing involves a periodically modulated pressure field during sintering, which effectively reduces interparticle friction, promotes particle rearrangement and facilitates liquid-phase redistribution along grain boundaries, thereby enabling high densification and high thermal conductivity at relatively low temperatures and pressures. Spark plasma sintering enables rapid densification at reduced temperatures by utilizing pulsed electric current and Joule heating, effectively suppressing abnormal grain growth while promoting high density. Microwave sintering, characterized by volumetric heating and accelerated mass transport, has also demonstrated potential for producing dense AlN ceramics with refined microstructures and improved thermal properties. Each technique presents distinct advantages and limitations in terms of efficiency, microstructural control and scalability. Driven by these advances, high-thermal-conductivity AlN ceramics have found increasing application in power electronic packaging, semiconductor manufacturing equipment, high-frequency high-speed devices and thermal management structural components, where stringent requirements on thermal stability, electrical insulation and chemical inertness must be simultaneously satisfied.

[Conclusions and prospects] Recent progress in high-thermal-conductivity AlN ceramics has been systematically summarized, with particular emphasis on the roles of sintering additives and sintering technologies in governing densification behavior, grain-boundary chemistry and thermal conductivity. The interplay between liquid-phase formation, oxygen removal and grain-boundary phase evolution is identified as the key factor determining the attainable thermal conductivity of AlN ceramics. Future research should be focused on the precise regulation of grain-boundary structure through optimized multicomponent additive systems and advanced sintering strategies, aiming to reconcile low-temperature densification with high thermal conductivity. In addition, the coupling between sintering process design and application-specific performance requirements deserves greater attention, especially for emerging high-frequency high-speed electronic systems and complex thermal management structures. Continued progress along these directions is expected to further unlock the potential of AlN ceramics as a core material platform for next-generation electronic and thermal management technologies.

Key words: high thermal conductivity; aluminum nitride; thermal conduction mechanism; sintering additives; sintering technology