Water-Vapor Sorption Processes in Nanoporous MgO-Al2O3 Ceramics: the PAL Spectroscopy Study

Abstract

The water-vapor sorption processes in nanoporous MgO-Al2O3 ceramics are studied with positron annihilation lifetime (PAL) spectroscopy employing positron trapping and positronium (Ps)-decaying modes. It is demonstrated that the longest-lived components in the four-term reconstructed PAL spectra with characteristic lifetimes near 2 and 60–70 ns can be, respectively, attributed to ortho-positronium (o-Ps) traps in nanopores with 0.3- and 1.5–1.8-nm radii. The first o-Ps decaying process includes “pick-off” annihilation in the “bubbles” of liquid water, while the second is based on o-Ps interaction with physisorbed water molecules at the walls of the pores. In addition, the water vapor modifies structural defects located at the grain boundaries in a vicinity of pores, this process being accompanied by void fragmentation during water adsorption and agglomeration during water desorption after drying.

Water-Sorption Effects near Grain Boundaries in Modified MgO-Al2O3 Ceramics Tested with Positron-Positronium Trapping Algorithm

“…Obtained effects are described in detail in [17]. The PAL spectra were decomposed in four components, but in this work we shall focuse mainly on transformations in the second (τ 2 , I 2 ) component.…”

“…The PAL measurements were performed with the OR-TEC instrument ( 22 Na source) [17]. The first experiment were performed at 22 • C and relative humidity RH = 35% after immersion of ceramic samples into distillated water for 12 h. Then, the PAL measurements were repeated once more with water-immersed ceramics at the same conditions.…”

“…The first experiment were performed at 22 • C and relative humidity RH = 35% after immersion of ceramic samples into distillated water for 12 h. Then, the PAL measurements were repeated once more with water-immersed ceramics at the same conditions. In the second case PAL measurement for ceramic samples was carried out at 22 • C and relative humidity RH= 35% after 1 day of water exposure (water wetting in distiller at RH = 100%) and drying in a vacuum at 120 • C during 4 h [17].…”

“…Further processing of PAL spectra was performed with LT 9.0 program [20] considering the three-or four-component treatment [7,17,21]. In spinel ceramics the lifetime τ 2 is related to the size of free-volume defects (or voids) near grain boundaries, and I 2 intensity reflects their amount [17]. The third and fourth components (τ 3 , I 3 ) and (τ 4 , I 4 ) originate from annihilation of o-Ps atoms in intrinsic nanopores of MgO-Al 2 O 3 ceramics.…”

“…Early we used PAL method to study influence of sintering conditions in the MgO-Al 2 O 3 ceramics in general (to characterization of MgO-Al 2 O 3 ceramics sintered at 1100-1400 • C) and water-sorption processes in nanopores of these materials (at analysis of third and fourth com-ponents of PAL spectra) [10,17]. In this work, to clarify this feature, we shall analyze physical water-sorption process near grain boundaries in the modified MgO-Al 2 O 3 ceramics using positron-positronium trapping algorithm proposed in [18] (at interpretation of three-component treatment of PAL spectra) in comparison with chemical water-sorption process at decomposition of PAL spectra into four components.…”