Densification and thermal conductivity of low-sintering-temperature AlN materials

Abstract The aim of the study was the sintering of aluminium nitride at rather low temperatures (≤1650°C). Various sintering aids were studied, in particular AlSi- and Ca-based oxides. CaCO 3 and 3CaO-3SiO 2 -Al 2 O 3 are the most efficient additives, because quantities as low as 0·5 wt% are enough to yield sintered AlN substrates with no open porosity. TEM observations showed that low concentrations of CaCO 3 or CaSiO 3 lead to location of the secondary phases at the triple points, whereas high concentrations lead to wetting of the grains. Thermal diffusivity of sintered materials was determined by a laser flash method. The highest conductivity (70 W m −1 K −1 ) is obtained with CaCO 3 additions. These additions promote liquid formation, which cleans the surface of the AlN grains from oxygen and thereby prevents oxygen from entering the AlN lattice. The CaAl 2 O 4 compound exhibits a lower thermal conductivity than the CaAl 4 O 7 and Ca 3 Al 10 O 18 compounds. The Ca 2 SiAl 2 O 7 compound and the 27R AlN polytype have a strong negative influence on the thermal conductivity of sintered materials.