EFFECT OF SILICA ON THE THERMAL-CONDUCTIVITY OF ALUMINUM NITRIDE

The effect of SiO2 on the thermal conductivity of aluminium nitride pr essureless sintered with 3 wt% Y2O3 as a sintering aid was investigate d. SiO2 additions greatly decreased the thermal conductivity of the si ntered parts from values of around 160 W/m.K on undoped samples to abo ut 25 W/m.K with 5 wt% SiO2 added to the green body composition. Micro structural studies, combined with the temperature dependence of the th ermal conductivity and lattice parameter measurements, indicated that defect phonon scattering was the mechanism responsible for the decreas e in thermal conductivity. SiO2 can be incorporated in limited solid s olution into the AlN lattice, generating Al vacancies for charge compe nsation in a process not unlike the solution of oxygen in AlN. The mas s difference introduced by the vacancies is the main phonon scattering defect. Beyond a concentration threshold of 2%, the SiO2-induced defe cts cluster to form SiAlON polytypoids derived from the basic 2H AlN s tructure with stacking sequences that depend on the SiO2 levels in the sample.