Fabrication of High-Thermal-Conductivity Polycrystalline Aluminum Nitride: Thermodynamic and Kinetic Aspects of Oxygen Removal

Abstract

It is now well recognized that the principal factor that lowers the thermal conductivity of polycrystalline AlN well below its theoretical value of ∼320W/mK at room temperature is the presence of dissolved lattice oxygen. Covalently bonded AlN is difficult to fabricate by pressureless sintering due to very low diffusion coefficients. It is also now well-established that certain additives, predominantly oxides, facilitate densification of AlN by liquid phase sintering, and in the process lead to the purification of AlN lattice—a prerequisite to attaining high thermal conductivity. The additive must have the ability to form a liquid phase, which facilitates sintering at the processing temperature. The additive must also have a strong enough affinity for aluminum oxide (Al2O3), which is the form in which oxygen is dissolved in AlN, so that various aluminates can be formed as secondary phases—effectively scavenging Al2O3 and purifying the lattice. The thermodynamic considerations relate to the affinity between Al2O3 and the additive to form aluminates, which can be described in terms of the standard free energy of formation, ΔG o , of the respective aluminate. The greater the |ΔG o |, with δG o < 0, the greater is the ability of the additive to scavenge Al2O3. The kinetics relate to various processes, such as the sintering kinetics and the kinetics of the removal of dissolved oxygen from within the grains to grain boundaries. In addition, other kinetic processes involve changes at the microstructural level, such as changing the wetting characteristics of secondary grain boundary phases and the occurrence of grain growth. This chapter presents a brief review of the role of dissolved oxygen on the thermal conductivity of AlN and a brief review of the role of sintering and processing procedures used in the fabrication of high-thermal-conductivity AlN ceramics. The focus will be on using the lattice model of thermal conductivity of AlN, the role of thermodynamics and phase equilibria in the purification of AlN lattice, and the kinetics of various processes central to the fabrication of high-thermal-conductivity AlN ceramics.