Fabrication of submicron alumina ceramics by pulse electric current sintering using Mg(2+)doped transition alumina powders

Dense submicron-grained alumina ceramics were fabricated by pulse electric current sintering (PECS) using Mg(2+)-doped transition alumina powders at 1200-1350 deg C under a uniaxial pressure of 40 or 80 MPa. The Mg(2+)-doped transition alumina powders (0-0.50 mass% MgO base) were prepared through a new sol-gel route using high-purity polyhydroxoaluminum (PHA) and MgCl2 solutions as starting materials. The composite gels obtained were calcined at 900 deg C and ground by planetary ball-milling. Upon heating, the composite gels transformed into a single-phase gamma-alumina or mixed phase of y- and x-aluminas, depending on the MgO content. The resultant transition alumina powders were solid solutions, in which Mg(2+) cations were substituted into the crystal lattice. The powders were re-calcined to increase the content of alpha-aLumina particles, which act as seeding for low-temperature densification. Densification depended on the MgO content and loading pressure. The critical Mg(2+)-doping for suppressing grain growth was found to be 0.10 mass% MgO. Higher loading pressures led to full densification at lower temperatures, resulting in a more uniform and finer microstructure. Thus, dense alumina ceramics (relative density > or = 99.6%) with a uniform microstructure composed of fine grains with an average size of 0.47mum could be obtained by PECS at 1250 C under 80 MPa.