The Role of Cerium Orthosilicate in the Densification of Si 3 N 4

The existence of compounds between Si 3 N 4 ‐CeO 2 and Si 3 N 4 ‐Ce 2 O 3 was investigated for firing temperatures of 1600° to 1700°C. The two new monoclinic compounds found were Ce 2 O 3 ·2Si 3 N 4 with lattice parameters a = 16.288, b = 4.848, and c =7.853 Å and β=91.54° and Ce 4 Si 2 O 7 N 2 with lattice parameters a = 10.360, b = 10.865, and c =3.974 Å and β=90.33°. Cerium orthosilicate (Ce 4.67 (SiO 4 ) 3 O) is present during firing as a glassy intermediate phase which promotes sintering and densification and then reacts with silicon nitride to form cerium silicon oxynitrde (CeSiO 2 N).