Densification of Ca-doped alumina nanopowders prepared by a new sol–gel route with seeding

Abstract

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.

Densification of rare-earth (Lu, Gd, Nd)-doped alumina nanopowders obtained by a sol–gel route under seeding

“…The order of Pm seems to correspond to the order of ionic size of the RE dopants: a larger dopant cation tends to retard the - transformation, showing a similar trend reported by Kumar et al [17]. The shift of Pm to higher temperatures due to doping seems to suggest that dopants might be substituted into the defect spinel lattice of -alumina, when the dopant amount was as low as 0.10 mol%, as was the case in the Ca 2+ doping system [8]. Intensive studies on the variation of lattice constant with increasing dopants couldn't reveal the clear evidence of the increase in lattice constant.…”

“…A high-purity PHA solution was prepared by dissolving Al metal in HCl solution with paying special attention to the impurities [8,19]. The Al2O3 concentration and OH/Al ratio of the final PHA solution thus obtained were determined to be 23.7 mass% and 2.50, respectively, based on the chemical analysis of Al and Cl.…”

“…The disk-shaped samples obtained by slip casting were sintered at 1200-1500°C for 1 h in air. For comparison, the same temperature-time schedule to that of Ca-doped alumina nanopowders [8] were employed: a heating rate of 50°C/min (below 1300°C) and 12°C/min (above 1300°C) was employed, paying special attention to preventing overshoot at the predetermined temperature. A cooling rate of 17°C/min was employed down to 300°C.…”

“…We have shown that the use of M 2+ -doped -aluminas [3,7,8] synthesized through a polyhydroxoaluminum (PHA) sol-gel process [9][10][11] yields a fine grain size upon sintering due to the intimate mixing of dopants at an atomic level. Among the divalent cation dopants reported in our previous study, Ca 2+ is by far the best dopant for suppressing the grain growth of -alumina upon heating [7].…”

“…The addition of -alumina seeds provides multiple low-energy sites for nucleation [8,12], which allows early impingement of the growing -alumina colonies, and hence, full densification at lower temperatures. In fact, the use of Ca-doping along with -alumina seeding was demonstrated to be very effective in producing low-temperature densifiable nanopowders [8].…”