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Dynamics of Formation of Self‐Organized Mesoporous AlO(OH)·αH 2 O Structure in Al‐Metal Surface Hydrolysis in Humid Air
Author(s) -
Ram S.
Publication year - 2003
Publication title -
journal of the american ceramic society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.9
H-Index - 196
eISSN - 1551-2916
pISSN - 0002-7820
DOI - 10.1111/j.1151-2916.2003.tb03605.x
Subject(s) - exothermic reaction , mesoporous material , amorphous solid , chemistry , catalysis , chemical engineering , materials science , crystallography , biochemistry , organic chemistry , engineering
In humid air, a nascent Al‐metal surface (S) with a surface Hg 2+ catalyst hydrolyzes in divided reaction centers (micelles) with a vigorous exothermic reaction, {Al + 2OH − }+αH 2 O → AlO(OH)·αH 2 O + 3e − + H + . It yields amorphous AlO(OH)·αH 2 O with a huge ∼90% porosity with α= 0.25. The primary driving forces of the reaction are the chemical potential μ e between the reaction species, the mechanical stress ς induced in expansion of S, and the flow of the reaction species. They drive it in a common direction perpendicular to S. The heat released in it flows primarily along S. It disrupts and stops the directional hydrolysis if the local temperature in the micelle reaches a critical value T c (hot spot). The hot spot cools to the operating value T 0 , and the reaction restarts and runs over to T c in a periodic manner, at a time scale of Δ t i ∼ 5 s, per the dynamics of hot spots, forming a self‐organized mesoporous structure of 15–50‐nm diameter ellipsoidal shaped particles (halo) separated through 3–5‐nm pores. A pore, in continuous formation of the sample, forms in disrupted reaction during the hot spot as it cools from T c to T 0 . The result is modeled in terms of the microstructure and dynamics of the hot spots.