Understanding AlN sintering through computational thermodynamics combined with experimental investigation

Aluminum nitride (AlN) has attracted large interest recently as a suitable material for hybrid integrated circuit substrates because of its high thermal conductivity, high flexural strength, good dielectric properties, thermal expansion coefficient matches that of Si and non-toxic nature. Yttria (Y2O3) is the best additive for AlN sintering. Binary diagrams of Al2O3–Y2O3, AlN–Al2O3, and AlN–Y2O3 were thermodynamically modelled. The obtained Gibbs free energies of components, stoichiometric phases and solution parameters were used to build a thermodynamic database of AlN–Al2O3–Y2O3 system. Liquid-phase sintering of AlN with Y2O3 as an additive has been studied and compared with the thermodynamic results. Sintering was performed in the temperature range of 1750–1950 °C for upto 4 h under a N2 atmosphere to optimise the sintering conditions for each composition. The relative density exceeded 95% for all the compositions sintered at 1850 °C and full densification was achieved for all four compositions sintered at 1900 °C. The microstructure and assemblage of the secondary phase have a significant effect on the final thermal conductivity of the sintered AlN. Thermodynamic modelling of AlN–Al2O3–Y2O3 system provided an important basis for understanding the sintering behaviour and interpreting the experimental results.