floculation de produit intermkdiaire. En ce qui concerne l'hydrolyse, la quantitk relative d'eau Coprecipitation of yttrium and aluminium hydroxide ddtermine le degrb d'agglom~risation. L'influence des for the preparation of pure yttrium aluminium garnet liquides de dispersion diffkrentes est Otudike ce qui a ( YAG) powder with small grain size is the subject of conduit aux conditions optimalisbes pour la this study. Starting materials are sulphates and floculation. chlorides of yttrium and aluminium. To obtain pure YAG (Y3AlsOI2), the pH duringflocculation of the 1 Introduction precursor must be chosen carefully. The presence of water increases the degree of agglomeration. To Polycrystalline yttrium aluminium garnet (YAG, minimize agglomeration, the influence of dispersion Y3AlsO12) sintered to full density, has interesting liquids has been studied, leading to optimized optical I and mechanical 2 properties. As it has a conditions for precipitation, cubic crystallographic structure there are no birefringence effects and thermal expansion is isotropic. Moreover YAG has a good resistance to corrosion Das Thema dieser Arbeit ist die Kopriizipitation yon from alkali metals. 3 Sintering behaviour is largely Yttrium und Aluminium zur Darstellung yon reinem determined by precursor powder characteristics. Yttrium-Aluminium-Granat-Pulver mit kleiner Korngr6fle. Ausgangsmaterialien sinddie Sulfate und The latter depend on the conversion steps in the Chloride yon Yttrium und Aluminium. Um reines powder preparation process. In a wet chemical YAG (Y3Als012) zu erhalten muss der pH-Wert synthesis precipitation, calcination and conversion wiihrend der Fiillung des Zwischenproduktes sorginto garnet will contribute to the properties of the faltig gewiihlt werden. Die Anwesenheit yon Wasser resulting powder. Agglomerates, formed during hat einen nachteiligen Effekt auf den Grad der precipitation and remaining in the powder compact Agglomeration. Um diesen nachteiligen Effekt beseitiafter calcination, are disastrous for sintering to full gen zu kOnnen wird der Einflufl verschiedener density. 4'5 To optimize process conditions the Dispersionsfliissigkeite untersucht. Damit ist es influence of the dispersion medium on the agglom-m6glich geworden die optimale Bedingungen fiir die eration of precursor particles during precipitation F~illung festzustellen, has been studied. Furthermore, to obtain a stoichiometric precipitate in the quasi-binary system it is important to take into account the dependence on On ddcrit la coprkcipitation d'yttrium et d'aluminium pH for precipitation of Y and A1 hydroxide. The pour obtenir unepoudred'yttrium aluminate trOsfine present paper describes the circumstances for et de haute puretd. Comme materiaux de d~part sont obtaining an ultrafine precursor powder of the employbs des sulphates et des chlorures d'yttrium et proper stoichiometry. Conversion into YAG and d'aluminium. La prOparation de YAG (Y3Als012) sintering behaviour will be discussed in a following pur demande un choix prdcis du pH pendant la paper. 47