Influence of a Ceramic Substrate on Aqueous Precipitation and Structural Evolution of Alumina Nano-Crystalline Coatings
Either boehmite (-AlOOH) or gibbsite (-Al(OH)3) nanocrystalline thin films (h≈100 nm) can be precipitated from AlCl3 solution at fixed pH and temperature onto different substrates. It depends on the nature of the substrate (mica flakes, SiO2 flakes, or α-Al2O3 flakes), on their crystallographic properties (crystalline or amorphous), and on some experimental parameters (agitation rate, addition rate). According to the surface charge of the substrates, different alumina species are involved in the precipitation process. When negative charges are present on the substrate, the [Al3O(OH)3(OH2)9]4+ polycation is promoted, leading to the formation of the (Al4) tetramer ([Al4O(OH)10(OH2)5]o) and then to the precipitation of bohemite. When positive charges are present, a ligand bridge containing complex ([Al3O(OH)3(O2H3)3(OH2)9]+) is likely favored, giving rise to hexagonal ring structures or amorphous solids that lead to the formation of gibbsite. Besides the surface effects, crystalline substrates can act as a template during precipitation of aluminum species as shown for the formation of gibbsite on muscovite. Finally, calcination at 850°C of boehmite samples leads to porous -Al2O3 layers, while calcination of gibbsite leads to -Al2O3 layers.
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