Abstract: In the present work the densification behavior and reaction sequences of the three types of AlN-polytypoids 15R, 12H and 21R were
studied. The results show that it is necessary to put small amount of additives for achieving fully dense of AlN-polytypoids. Among
these additives explored, Sm₂O₃·Al₂O₃ is the most effective. With 2.5wt% SmAlO₃, all these three polytypoids can reach near to
fully dense at 1600℃. 15R and 12H have similar reaction sequences. Below 1550℃ and 1600℃, 15R and 12H increase with increasing temperature respectively and reach to the top value around
95wt% and then both decrease with 12H and 21R occurring in 15R and 12H respectively. The formation of 21R is different from 15R and 12H. At lower
temperature, 12H forms firstly and with increasing temperature 12H decreases and 21R increases. Above 1750℃, pure 21R can be obtained. The
reaction sequences of these AlN-polytypoids were discussed in the paper.

Key words: AlN, AlN-polytypoids，rare-earth oxides，densification，reaction sequences