This paper demonstrates that the Ca-doped alumina nanopowders prepared by a new sol-gel route using polyhydoxoaluminum (PHA) and CaCl 2 solutions under α-alumina seeding represent a viable option for producing fine-grained ceramics. Appropriate conditions for producing Ca-doped alumina nanopowders suitable for low-temperature finer-grained densification were 0.10 mol% Ca-doping, 5 mass% α-alumina seeding and a calcination temperature of 900˚C. These conditions led to the formation of new nano-sized alumina powders, which consisted of ~80 nm α-alumina particles and γ-alumina nanoparticles. Using these Ca-doped nanopowders, fully densified alumina ceramics with a uniform microstructure composed of fine grains with an average grain size of 0.66 μm could be obtained at 1375˚C.Clearly, the Ca-doped nanopowders obtained here through the proposed sol-gel route are very suitable for fabricating dense, fine-grained alumina ceramics because an undoped sample with 5 mass% seeds led to a microstructure with an average grain size of 1.39 μm at 1375˚C.

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Rare-earth (RE: Lu, Gd, Nd, 0.10 mol%)-doped alumina nanopowders were prepared by a new sol-gel route using polyhydroxoaluminum (PHA) and RECl3 solutions under -alumina (~75 nm) seeding. Among the rare-earth dopants studied, Lu yields the most suitable nanopowders for low-temperature densification. The 0.10 mol% Lu-doped nanopowders, which were obtained at a calcination temperature of 900°C under 5 mass% -alumina seeding, consisted of ~80-nm -alumina particles and -alumina nanoparticles. Using these Lu-doped alumina nanopowders, fully densified alumina ceramics with a uniform microstructure composed of fine grains with an average size of 0.61 μm could be obtained at 1400°C by pressureless sintering. Clearly, the Lu-doped nanopowders obtained here represent a viable option for fabricating dense, finer-grained alumina ceramics because an undoped sample with 5 mass% seeds gave a microstructure with an average grain size of 1.78 μm at 1400°C.

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Dense submicron-grained alumina ceramics were fabricated by pulse electric current sintering (PECS) using M 2+ (M: Mg, Ca, Ni)-doped alumina nanopowders at 1250°C under a uniaxial pressure of 80 MPa. The M 2+ -doped alumina nanopowders (0-0.10 mass%) were prepared through a new sol-gel route using high-purity polyhydroxoaluminum (PHA) and MCl2 solutions as starting materials. The composite gels obtained were calcined at 900°C and ground by planetary ball-milling. The powders were re-calcined at 900°C to increase the content of α-alumina particles, which act as seeding for low-temperature densification. Densification and microstructural development depend on the M 2+ dopant species. Dense alumina ceramics (relative density ≥ 99.0%) thus obtained had a uniform microstructure composed of fine grains, where the average grain size developed for non-doped, Ni-doped, Mg-doped and Ca-doped samples was 0.67, 0.67, 0.47 and 0.30 m, respectively, showing that Ca-doping is the most promising method for tailoring of nanocrystalline alumina ceramics.Keywords: A. Sintering; A. Sol-gel processes; B. Grain size; D. Al2O3 3 IntroductionAs the grain size of fully dense alumina is reduced, significant benefits are observed in terms of improved mechanical properties [1,2], superior wear resistance [3] and optical transparency [4]. To obtain dense submicron-grained or nanocrystalline alumina ceramics (NCAs), studies on the densification of alumina have focused on the use of nanosized transition alumina powders as a starting powder [5,6]. In these studies, transition aluminas were densified by hot-pressing [5] or sinter-forging [6], and alumina ceramics with an -alumina grain size of 200 nm or less were obtained. We have also focused on the use of transition alumina powders and attempted densification by pulse electric current sintering (PECS) using polyhydroxoaluminum (PHA) gel-derived γ-alumina powders [7][8][9]. As a result, we obtained fully densified alumina ceramics with a relatively high bending strength of ~860 MPa under optimized conditions [9]. However, these alumina ceramics showed an inhomogeneous microstructure consisting of submicron-sized grains and a large number of elongated large grains due to the presence of impurities such as CaO and SiO2.It is well known that additives such as , CaO [14,15] and NiO [11] affect the sintering behavior of -alumina. MgO plays a particularly beneficial role in inhibiting grain growth of -alumina, which can be explained in terms of a solute drag (pinning) model [11,12]. While the effect of MO (M: Mg, Ca, Ni) addition to -alumina powders on grain growth inhibition has been studied extensively, there are few reports concerning the effects of M 2+ -doping on densification of γ-aluminas except for Mg 2+ -doping [16]. In particular, systematic studies on the densification behavior induced by PECS for various M 2+ -doped γ-alumina powders, in which M 2+ cations are substituted into the crystal lattice of γ-alumina, have been lacking.Thus, the present study extends the potentia...