SYNTHESIS OF THE BI3ZN2SB3O14 PYROCHLORE PHASE BY DIRECT OXIDATION OFA PRECURSORY ALLOY (DOPA)

ZnO varistors are multi-constituted ceramics and their composition alw ays includes small quantities of antimony and bismuth. The reason is t hat an intermediate compound, the pyrochlore phase Bi3Zn2Sb3O14, takes a prominent part in the sintering reaction. The aim of the present wo rk is to elaborate the pure ternary oxide by Direct Oxidation of the P recursory Alloy. The DOPA method consists of preparing beforehand an a lloy powder of accurate composition by atomization. Then, the precurso r powder is oxidized in order to form the mixed oxide to be used. The oxidation of the powder is studied by thermogravimetric analysis and X -ray powder diffraction. The reaction mechanism takes place in four st eps, corresponding to the formation of Zn (II), Sb (III), Bi (III) and Sb (V). The oxidation path is described by the quaternary system Zn-S b-Bi-O-2. This phase diagram shows the successive formation of the ZnO , ZnSb2O4, Bi2O3 and Bi3Zn2Sb3O14 phases. The X-ray pattern of the pur e pyrochlore phase is thus established for the first time and all the lines are indexed.