CRYSTALLIZATION OF YTTRIA UNDER HYDROTHERMAL CONDITIONS

The effect of precursor type and hydrothermal crystallization conditio ns on the morphology of yttria grains was studied. Yttrium nitrate and yttrium chloride were used as yttrium salts soluble in water. Yttrium hydroxide obtained by the reaction of the chosen salt with ammonia wa s subjected to hydrothermal conditions (200-270 degrees C, 0-120 min) in an auto-clave. The shape of the yttria grains was strongly dependen t on the crystallization temperature and the type of precursor used. W ith yttrium chloride as a precursor and at crystallization temperature s up to 498K, yttria grains are isometric in shape. Above this tempera ture yttria grains get elongated with a needle-like shape. The size of the needles depends on the crystallization time. For the case of the yttrium nitrate precursor, the critical temperature of crystallization , above which extended yttria grains appear, is about 25 degrees C hig her. From the results a model mechanism for the hydrothermal crystalli zation of yttria is proposed. (C) 1996 Elsevier Science Limited.