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Microstuctural Characterization of Water-Rich Boehmite (AlO(OH)): TEM Correlation of Apparently Divergent XRD and TGA Results

Published online by Cambridge University Press:  02 July 2020

L.M. Anovitz ,
L.F. Allard ,
W. D. Porter ,
D. W. Coffey ,
P. Benezeth ,
D.A. Palmer  and
D.J. Wesolowski
L.M. Anovitz
Affiliation:
Chemical and Analytical Sciences Div., Oak Ridge National Laboratory, Oak Ridge, TN37831
L.F. Allard
Affiliation:
High Temperature Materials Laboratory, Oak Ridge National Laboratory, Oak Ridge, TN37831
W. D. Porter
Affiliation:
High Temperature Materials Laboratory, Oak Ridge National Laboratory, Oak Ridge, TN37831
D. W. Coffey
Affiliation:
High Temperature Materials Laboratory, Oak Ridge National Laboratory, Oak Ridge, TN37831
P. Benezeth
Affiliation:
Chemical and Analytical Sciences Div., Oak Ridge National Laboratory, Oak Ridge, TN37831
D.A. Palmer
Affiliation:
Chemical and Analytical Sciences Div., Oak Ridge National Laboratory, Oak Ridge, TN37831
D.J. Wesolowski
Affiliation:
Chemical and Analytical Sciences Div., Oak Ridge National Laboratory, Oak Ridge, TN37831
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Extract

An understanding of the solid-phase thermodynamics and aqueous speciation of aluminum is critical to our ability to understand and predict processes in a wide variety of geologic and industrial settings. Boehmite (AlO(OH)) is an important phase in the system Al2O3-H2O that has been the subject of a number of structural and thermodynamic studies since its initial synthesis [1] and discovery in nature [2]. Unfortunately, it has long been recognized that thermogravimetric analysis (TGA) of both synthetic and natural boehmite samples (that appear well crystallized by powder XRD methods) yields significant excess water - typically losing 16-16.5 wt. % on heating as compared with a nominal expected weight loss of 15.0 wt. % [3,4]. The boehmite used in our experiments was synthesized hydrothermally from acid-washed gibbsite (Al(OH)3) at 200°C. Powder XRD and SEM examination showed no evidence of the presence a contaminant phase. The TGA patterns do not suggest that this is due to adsorbed water, so a structural source is likely. We therefore undertook to examine this material by TEM to clarify this phenomenon.

Boehmite is orthorhombic (a = 0.0285nm, b = 0.1224nm and c = 0.0365nm, Amam). The crystals were tabular with major surfaces normal to <010>. Simple powder dispersals onto holey carbon films typically resulted in b-axis orientations parallel to the electron beam. To view other orientations, specimens were mulled in M-Bond epoxy, pressed between plates of single-crystal Si (aligning the boehmite tablets parallel to the Si plates) while the epoxy cured. Electron transparent thin foils normal to the silicon plates were produced by argon ion milling techniques. Sample stability in the electron beam was dramatically improved by cooling to −140°C using an 2 cold stage.

Type
Geology/Mineralogy
Information
Microscopy and Microanalysis , Volume 5 , Issue S2: Proceedings: Microscopy & Microanalysis '99, Microscopy Society of America 57th Annual Meeting, Microbeam Analysis Society 33rd Annual Meeting, Portland, Oregon August 1-5, 1999 , August 1999 , pp. 540 - 541
Copyright
Copyright © Microscopy Society of America

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References

1.Böhm, J. and Niclassen, H., Z. f. Anorg. Chemie, 132, 5 (1924).Google Scholar
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6. Research sponsored by the Office of Basic Energy Sciences and the Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy, under contract DE-AC05-96OR22464, Oak Ridge National Laboratory, managed by Lockheed Martin Energy Research Corporation.Google Scholar