High‐Thermal‐Conductivity Aluminum Nitride Ceramics: The Effect of Thermodynamic, Kinetic, and Microstructural Factors

TLDR

Aluminum nitride’s thermal conductivity can be enhanced by additives with high thermodynamic affinity to alumina, as shown in the AlN‑Y₂O₃ system, and a contiguous AlN phase yields rapid heat conduction even at short sintering times. Rapid heat conduction arises from a contiguous AlN phase that forms even during short sintering times. Samarium oxide gives the highest, lutetium oxide the lowest thermal conductivity in AlN composites, and the sintering aid and dopant level outweigh microstructure, with extended 1850 °C annealing enhancing conductivity via grain purification and increased AlN contiguity.

Abstract

Improvement in the thermal conductivity of aluminum nitride (AlN) can be realized by additives that have a high thermodynamic affinity toward alumina (Al 2 O 3 ), as is clearly demonstrated in the aluminum nitride‐yttria (AlN‐Y 2 O 3 ) system. A wide variety of lanthanide dopants are compared at equimolar lanthanide oxide:alumina (Ln 2 O 3 : Al 2 O 3 , where Ln is a lanthanide element) ratios, with samaria (Sm 2 O 3 ) and lutetia (Lu 2 O 3 ) being the dopants that give the highest‐ and lowest‐thermal‐conductivity AlN composites, respectively. The choice of the sintering aid and the dopant level is much more important than the microstructure that evolves during sintering. A contiguous AlN phase provides rapid heat conduction paths, even at short sintering times. AlN contiguity decreases slightly as the annealing times increase in the range of 1–1000 min at 1850°C. However, a substantial increase in thermal conductivity results, because of purification of AlN grains by dissolution‐reprecipitation and bulk diffusion. Removal of grain‐boundary phases, with a concurrent increase in AlN contiguity, occurs at high annealing temperatures or at long times and is a natural consequence of high dihedral angles (poor wetting) in liquidphase‐sintered AlN ceramics.