Aluminum-containing ytterbium nitrogen woehlerite solid solutions. Synthesis, structure, and some properties

A complete solid solution between Yb4Si2O7N2 (ytterbium nitrogen woehlerite) and Yb4Al2O9 with general formula Yb4Si2-xAlxO7+xN2-x has been established by pressureless sintering at 1670-1700 degrees C. X-ray powder diffraction patterns of intermediate members reveal a pronounced tetragonal pseudosymmetry. Unit cell parameters increase in a non-linear way with increasing alumina content. Weight loss of the solid solutions increases with increasing Al/Si ratio on hearing in the temperature range 1950-2000 degrees C under 1 bar nitrogen pressure which implies partial decomposition of the formed aluminates, although the temperature at which the thermal decomposition begins, rises at the same rime. Coarsening of the microstructure was observed with the increase of the amount of alumina dissolved. Oxidation resistance of the solid solutions improves considerably with higher alumina content. Thus, alumina-containing nitrogen woehlerite solid solutions may be considered as favourable secondary grain boundary phases in Si3N4-based ceramics. (C) 1998 Published by Elsevier Science Limited All rights reserved.