American Chemical Society
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Kinetic Instability of Nanocrystalline Aluminum Prepared by Chemical Synthesis; Facile Room-Temperature Grain Growth

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journal contribution
posted on 1998-10-13, 00:00 authored by Joel A. Haber, William E. Buhro
Nanocrystalline aluminum (nano-Al) is synthesized by two chemical methods. Method A consists of the following:  reaction of LiAlH4 and AlCl3 at 164 °C in 1,3,5-trimethylbenzene produces nano-Al with an average mean coherence length (crystallite size) of 160 ± 50 nm. The byproduct LiCl is removed by washing with MeOH at −25 or 0 °C. Method B consists of the following:  nano-Al is produced by decomposition of H3Al(NMe2Et) under reflux in 1,3,5-trimethylbenzene (ca. 100−164 °C), with or without added decomposition catalyst Ti(O-i-Pr)4. Here the mean particle size (40−180 nm) and degree of aggregation of the nano-Al depend on the mole percent of decomposition catalyst used (0−1%). The nano-Al produced by method A contains 3−4 wt % each of C, O, and Cl; whereas that produced by method B contains only ≤0.25 wt % each of C, O, and Cl and is ≥99 wt % Al. nano-Al produced by both methods has been consolidated by uniaxial pressing at 350 MPa for 1 h at 25, 100, or 300 °C. Rapid grain growth is observed at each pressing temperature in method-B powders. The mean grain size doubles at 25 °C within 1 h and continues to increase beyond the nanometer-size regime over longer periods. The low-temperature grain-size instability of the chemically synthesized (method-B) nano-Al contrasts markedly with the high-temperature stability (≥300 °C) of nano-Al prepared by gas condensation or mechanical attrition. Facile grain growth in the chemically synthesized (method-B) nano-Al is attributed to higher grain-boundary purity and to aggregate structures that minimize adventitious oxidation. The results establish that barriers to grain growth in pure nano-Al are intrinsically low.