SOLID-SOLID INTERACTION IN THE PURE AND LI2O-DOPED MOO3-AL2O3 SYSTEM

The effects of Li2O doping (1.5 and 6 mol%) on solid-solid interaction s and the phase transformation process in the MoO3-Al2O3 system were i nvestigated using thermogravimetry, differential thermal analysis and X-ray diffraction (TG, DTA and XRD) techniques. The proportions of mol ybdena expressed as wt% MoO3 were 22 and 41.4. The results obtained re vealed that MoO3 interacted with Al2O3 at temperatures starting from 5 00 degrees C, forming orthorhombic Al-2(MoO4)(3); doping with lithium oxide was found to promote the reaction. The complete transformation o f molybdenum trioxide into aluminium molybdate required heating of the mixed oxides at 700 degrees C, The molybdate produced decomposed at t emperatures above 800 degrees C yielding alpha-Al2O3 and MoO3. The Li2 O doping enhanced the crystallization of cc-alumina and retarded the t hermal decomposition of Al-2(MoO4)(3). The MoO3 produced partly sublim ed and the remaining portion dissolved in the alumina matrix forming a n MoO3-Al2O3 solid solution. Li2O doping increased the amount of MoO3 sublimed at 900-1100 degrees C to an extent proportional to the concen tration of the dopant. In other words, Li2O treatment decreased the so lubility of MoO3 in Al2O3. The promoting effect of Li2O on Al-2(MoO4)( 3) formation at 500 degrees C was attributed to dissolution of a small portion of Li2O in the MoO3 lattice, with subsequent increase in the mobility of Mo6+ ions. The possible increase in the mobility of these ions might also account for the observed decrease in the solubility of MoO3 in Al2O3.