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.