Thermal stability and its improvement of the alumina membrane top-layers prepared by sol-gel methods

The thermal stability of unsupported alumina membrane top-layers was studied by determining the pore structure (mainly pore size) change of alumina gels, prepared by sol-gel methods, after sintering at different temperatures from 450 to 1200 C. The average pore size of the pure alumina membranes and PVA-added membranes increased sharply after sintering above 1000 C. Addition of 3% lanthanum, either by mixing lanthanum nitrate in the alumina sol or impregnating lanthanum nitrate into calcined alumina gel, followed by a second heat treatment, can considerably stabilise the pore structure of the alumina membrane top-layers. The pore diameter for the lanthanum-doped membranes was stabilised within 25 nm after sintering at 1200 C for 30 h, about one-sixth of that for the pure alumina membranes after sintering at 1200 C for 30 h. The substantial increase in the pore size for the pure alumina membranes at the sintering temperature of 1000 to 1200 C was accompanied by the phase transformation from gamma to alpha-alumina. The addition of lanthanum can raise this phase transformation temperature by about 200 C. 20 refs.