Fabrication of Submicron Alumina Ceramics by Pulse Electric Current Sintering Using Mg²⁺-Doped Transition Alumina Powders

Abstract

Dense submicron-grained alumina ceramics were fabricated by pulse electric current sintering (PECS) using Mg²⁺-doped transition alumina powders at 1200-1350°C under a uniaxial pressure of 40 or 80 MPa. The Mg²⁺-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°C and ground by planetary ball-milling. Upon heating, the composite gels transformed into a single-phase γ-alumina or mixed phase of γ- and χ-aluminas, depending on the MgO content. The resultant transition alumina powders were solid solutions, in which Mg²⁺ cations were substituted into the crystal lattice. The powders were re-calcined to increase the content of α-alumina particles, which act as seeding for low-temperature densification. Densification depended on the MgO content and loading pressure. The critical Mg²⁺-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≥99.6%) with a uniform microstructure composed of fine grains with an average size of 0.47 μm could be obtained by PECS at 1250°C under 80 MPa.