Aluminum hydroxide microstructural units in gelled media aged, or nonaged, with alcohol and water

Effect of vacuum and of strong adsorbed water films on micropore formation in aluminum hydroxide xerogel powders

“…3B). As it possible to observe, in good agreement with our previous results [5], the average pore size of this xerogel sample is about 9.5 nm. Fig.…”

“…Since the S BET values of any powders are related to the corresponding average crystallite dimensions [27], our data suggest that the alcoholic xerogel crystallites are finer than the aqueous ones. These results are well known since the pioneering work of Haberko [10], and they have been recently confirmed also by us for the same aluminum hydroxide systems aged or not aged in water and ethanol [5].…”

“…A further experimental observation that can be drawn from the data of Table 1 deals with the different average dimensions of the individual crystallites of the alcoholic and the aqueous xerogels. As shown in previous work [5], the ethanolic individual crystallites are finer than the aqueous ones. The present results suggest that also methanol and isopropanol could have a comparable effect.…”

“…As soon as the sample reached the selected temperature, drying continued at constant temperature under the same vacuum conditions for a total time of 11 h. It was observed that within this time period the samples reached constant weight at a thermobalance sensitivity of 10 −5 mg/s. Isotherms of N 2 adsorption at 78 K [5] were taken on both xerogels produced either in air or under vacuum to determine their specific surface area, microporosity, mesoporosity and pore size distribution. It was used as a reference t-curve [23] the one obtained experimentally from N 2 adsorption at 78 K on aluminum hydroxide powders with negligible porosity, specific surface area of 44 m 2 /g and comparable values of the c BET constant [5].…”

“…This is clear evidence that a critical level of stress has been reached inside the xerogel matrix. Mosquera et al [4] and Beruto et al [5] pointed out that, when the vaporization rate is kept low, crack-free xerogel matrix can be obtained, and individual crystallites and cluster dimensions can be conserved. Taken for granted what we have learned in our previous study [5], it is interesting to inquire what it happens when the vaporization rate increases due to two combined factors: vacuum and vaporization temperature.…”