TY - JOUR

T1 - NMR study on reaction processes from aluminum chloride hydroxides to alpha alumina powders

AU - Nýblová, Daniela

AU - Senna, Mamoru

AU - Düvel, Andre

AU - Heitjans, Paul

AU - Billik, Peter

AU - Filo, Juraj

AU - Šepelák, Vladimír

N1 - Funding Information: This work was supported by the Scientific Grant Agency VEGA of The Ministry of Education, Science, Research and Sport of the Slovak Republic [grant numbers 2/ 0092/14, 1/0712/18]; and the Deutsche Forschungsgemeinschaft [grant number SE 1407/4‐1]. P. H. is grateful to the State of Lower Saxony (Germany) for a Niedersachsen Professorship, from which also D. N. was supported.

PY - 2019/3/2

Y1 - 2019/3/2

N2 - 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.

AB - 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.

UR - http://www.scopus.com/inward/record.url?scp=85054590515&partnerID=8YFLogxK

U2 - 10.15488/4943

DO - 10.15488/4943

M3 - Article

AN - SCOPUS:85054590515

VL - 102

SP - 2871

EP - 2881

JO - Journal of the American Ceramic Society

JF - Journal of the American Ceramic Society

SN - 0002-7820

IS - 5

ER -