Secondary phases, microstructures and properties of AlN ceramics sintered by adding nitrate sintering additives

AlN ceramics were sintered at a temperature range from 1650 to 1800°C through adding the Ca and Y nitrate sintering additives. Secondary phases, microstructures and properties of the AlN ceramics were studied. When the AlN ceramics are sintered at 1650 or 1675°C, CaO and Y2O3 from the sintering additives react with Al2O3 in the AlN powder to generate CaAl4O7 and Y3Al5O12. Part of Y3Al5O12 reacts with CaO and Al2O3 to form CaYAl3O7 at 1700°C. At 1800°C, CaYAl3O7 decomposes into CaAl4O7 and Y3Al5O12. Finally, CaAl4O7 volatilises and only Y3Al5O12 remains. As the sintering temperature increases, the AlN grains grow continuously and the bending strength and thermal conductivity of the AlN ceramics increase first and then decrease. The AlN ceramics sintered at 1700°C are fully dense and have the highest bending strength and thermal conductivity of 373·7 MPa and 136·7 W m−1 K−1 in this work.