CRYSTALLIZATION OF Y3AL5O12 GARNET FROM DEEP UNDERCOOLED MELT EFFECT OF THE AL-GA SUBSTITUTION

Melts of garnet Y3Al5O12 and compositions lying on both sides of this compound were rapidly cooled in contactless conditions by using an aer odynamic levitation device coupled with a CO2 laser. The crystallized phases were analyzed at room temperature by XRD and Al-17 MAS NMR. As the temperature reached by the supercooled melt decreases. the solidif ication was shown to lead successively to a mixture of YAlO3 and Al2O3 , a monophasic garnet type structure and a glass. The formation of Y3A l5O12 starts at about 920 degrees C under the melting temperature and only 50 degrees C above the glass transition temperature. The solidifi cation behaviour of liquid Y3Al5-xGaxO12 (x = 0.5, l) was similar. Res ults clearly show the difficulties of garnet to nucleate. They are con sistent with a rapid increase of the melt viscosity in the temperature range just above the formation of glass. (C) 1997 Elsevier Science B. V.