HIGH-THERMAL-CONDUCTIVITY ALUMINUM NITRIDE CERAMICS - THE EFFECT OF THERMODYNAMIC, KINETIC, AND MICROSTRUCTURAL FACTORS

Improvement in the thermal conductivity of aluminum nitride (AIN) can be realized by additives that have a high thermodynamic affinity towar d alumina (Al2O3), as is clearly demonstrated in the aluminum nitride- yttria (AlN-Y2O3) system, A wide variety of lanthanide dopants are com pared at equimolar lanthanide oxide:alumina (Ln(2)O(3): Al2O3, where L n is a lanthanide element) ratios, with samaria (Sm2O3) and lutetia (L u2O3) being the dopants that give the highest- and lowest-thermal-cond uctivity AIN composites, respectively, The choice of the sintering aid and the dopant level is much more important than the microstructure t hat evolves during sintering, A contiguous AIN phase provides rapid he at conduction paths, even at short sintering times, AlN contiguity dec reases slightly as the annealing times increase in the range of 1-1000 min at 1850 degrees C. However, a substantial increase in thermal con ductivity results, because of purification of AIN grains by dissolutio n-reprecipitation and bulk diffusion, Removal of grain-boundary phases , with a concurrent increase in AIN contiguity, occurs at high anneali ng temperatures or at long times and is a natural consequence of high dihedral angles (poor wetting) in liquid-phase-sintered AIN ceramics.