Effect of sintering atmosphere on the pore-structure stability of cerium-doped nanostructured alumina

Abstract

Pore-structure stability of pure and Ce-doped alumina in air and argon atmospheres was studied using DTA, TGA, N 2 ads./des, and XRD with a view to understand the importance of the ionic size of the dopant cation on the porestructure stability of alumina. The ionic size effect was studied by heat treating the Ce-alumina system in both oxidizing and reducing atmospheres to have Ce 4÷ (87 pm) and Ce 3÷ (106 pm) respectively. No compound formation between Ce and alumina was observed. In the case of pure alumina there is a drastic reduction in porosity during the transformation to e-alumina. Ce-doped alumina has a higher DSC transformation temperature corresponding to the (~-alumina transformation compared to pure alumina. Ce-doped alumina showed higher pore-structure stability compared with pure alumina and the stability was relatively higher in reducing atmosphere (higher Ce3÷/Ce 4÷ ratio, higher effective ionic size) compared with oxidizing conditions (lower Ce3+/Ce 4÷ ratio, lower effective ionic size).

Pore Structure Evolution of Lanthana–Alumina Systems Prepared through Coprecipitation

“…Barium and rare-earth cations are the most studied dopants, and their behaviors at high temperature are different. [1][2][3][4][5][6][7][8][9][10] Machida et al 4 have reported the excellent stability of barium hexaaluminate (BaO⅐6Al 2 O 3 ) in maintaining a large surface area. They have reported a value of 11 m 2 ⅐g Ϫ1 for powder samples after heating at 1600°C for 6 h, and such a high value has not been reported for any other oxides so far.…”

Phase transformations in t-ZrO2-Al2O3 nanoprecursors and formation of the microstructure of related ceramic materials

“…According to Kumar et al [13], this phase can be considered a metastable solid solution based on the monoclinic aluminum oxide stabilized by Ce 4+ ions. The content of the θ-Al 2 O 3 phase is proportional to the content of the α-Al 2 O 3 phase and also depends on the sequence of precipitation of the components.…”

Effect of Monovalent Cation Additives on the γ‐Al2O3‐to‐α‐Al2O3 Phase Transition

“…T HE phase transition from ␥-Al 2 O 3 to ␣-Al 2 O 3 via ␦-Al 2 O 3 and -Al 2 O 3 has been investigated by many workers, [1][2][3][4][5][6][7][8][9][10][11][12][13][14] because this knowledge is very important for creating porous Al 2 O 3 ceramics used as high-temperature catalyst supports, [1][2][3][4][5][6][7][8][9][10][11][12] as well as for the low-temperature sintering and microstructural control of dense Al 2 O 3 ceramics. 13 Because this phase transition is from metastable to stable, the transition temperature is influenced by various factors, such as purity, 1 crystallinity 2 and the agglomerated state 3 of the ␥-Al 2 O 3 cation 4 -9 and anion 10 additives, and heating atmosphere.…”

“…13 Because this phase transition is from metastable to stable, the transition temperature is influenced by various factors, such as purity, 1 crystallinity 2 and the agglomerated state 3 of the ␥-Al 2 O 3 cation 4 -9 and anion 10 additives, and heating atmosphere. 11 Among these various factors, cation additives can accelerate or retard the transition, depending on the type, 4 -9 amount, 6,7 state, 12 and size 11,14 of the cation. For example, the addition of amorphous SiO 2 has been remarkably effective in retarding phase transition, but crystalline forms of SiO 2 , such as quartz and cristobalite, are reported to accelerate the transition.…”

“…11 Because the smaller cations are incorporated into the ␥-Al 2 O 3 before its transformation into ␣-Al 2 O 3 , whereas the larger cations form aluminate phases of magnetoplumbite structure, the aluminate phases that form on the surface of the ␥-Al 2 O 3 particles appear to determine the effect of the cations on the transition. However, incorporating divalent cations into ␥-Al 2 O 3 decreases the cation vacancy concentration, effectively suppressing diffusion reactions and, thus, retarding the phase transition.…”