Structural Properties and Sintering Behavior

For composition near the extremes of the formation range of layered-alumina phases, samples with typical px (Ba poor composition) and pri (Ba-rich composition) structure were obtained. For the intermediate composition, BaAl12Oi9, 

These data indicate that sintering resistance is related not only to the formation of a layered-alumina structure. It appears that a critical Ba content is required for the effective suppression of crystal growth along the c axis of crystallites. Indeed, in BaAli4022, sintering proceeded even after the formation of Ba-P-Al203. 

Further characterization of BaAl O based on FTIR of adsorbed probe molecules (C02 and pyridine) and measurements of isoelectric point29 showed that the surface was basic. Because of the low bulk Ba content of this material such basic character can hardly be explained assuming a statistical exposure of the different crystallographic planes. This suggests that the Ba-containing mirror planes are preferentially exposed. 

Arai and co-workers investigated the effects of cation composition in the mirror plane of Mn-substituted hexaaluminates.5,15 Investigation of a series of rare-earth-based hexaaluminates (AMnAluOi9, A=La, Pr, Nd, Sm) prepared via hydrolysis of alkoxides and calcined at 1200 °C for 5 h showed that surface area increases with the ionic radius of A. The La-substituted sample, the largest cation of the series, showed a surface area of 15 m2/g, which compared with 5 m2/g for the Sm-substituted one, i.e., the smallest cation of the series. These data are consistent with the mechanism of sintering resistance reported above. Indeed, it is expected that the larger the cation, the more difficult its diffusion along the c axis. 

Partial replacement of cation in the mirror plane was also studied for AA MniAluOi9 samples (A=Ba, Sr, Ca, La, K A = Sr, Ca, La, K). It was reported that substitution of cations in the mirror plane resulted in enhanced 

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