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NMR study on reaction processes from aluminum chloride hydroxides to alpha alumina powders
Author(s) -
Nýblová Daniela,
Senna Mamoru,
Düvel Andre,
Heitjans Paul,
Billik Peter,
Filo Juraj,
Šepelák Vladimír
Publication year - 2019
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/jace.16108
Subject(s) - anhydrous , aqueous solution , nucleation , hydroxide , hydrolysis , amorphous solid , chemistry , solid state nuclear magnetic resonance , chloride , inorganic chemistry , nuclear chemistry , chemical engineering , crystallography , organic chemistry , nuclear magnetic resonance , physics , engineering
Starting from gelatinous aluminum chloride hydroxide, the transformation process toward α‐Al 2 O 3 was examined using 27 Al NMR , both in the liquid and solid states, as a main analytical tool. By increasing the hydrolysis ratio ( h , defined as [ OH − ]/[Al 3+ ]) of the starting aqueous precursor up to h = 2.5, the transition temperature to the final product, α‐Al 2 O 3 , decreased to as low as 500°C. In this case, the structural change from amorphous alumina to α‐Al 2 O 3 took place without intermediate transition Al 2 O 3 phases. Examining the process of networking during the transition from aqueous sol–through the state of xerogel–to final anhydrous oxide by nuclear magnetic resonance ( NMR ) revealed the presence of highly polymeric species mainly ascribed to δ‐[Al 2 O 8 Al 28 ( OH ) 56 (H 2 O) 24 ] 18+ (δ‐Al 30 ). δ‐Al 30 species were found in the solution phase and became predominant after drying. We conclude that the lower temperature synthesis of α‐Al 2 O 3 became possible due to preformation of polymerized AlO 6 construction units in the precursor, reducing the energy barrier for the nucleation of the final α‐Al 2 O 3 phase.