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

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.