KINETIC INSTABILITY OF NANOCRYSTALLINE ALUMINUM PREPARED BY CHEMICAL SYNTHESIS - FACILE ROOM-TEMPERATURE GRAIN-GROWTH

Nanocrystalline aluminum (nano-Al) is synthesized by two chemical meth ods. Method A consists of the following: reaction of LiAlH4 and AlCl3 at 164 degrees C in 1,3,5-trimethylbenzene produces nano-Al with an av erage mean coherence length (crystallite size) of 160 +/- 50 nm. The b yproduct LiCl is removed by washing with MeOH at -25 or 0 degrees C. M ethod B consists of the following: nano-Al is produced by decompositio n of H3Al(NMe2Et) under reflux in 1,3,5-trimethylbenzene (ca. 100-164 degrees 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 less than or equal to 0.25 wt % each of C, O, and Cl and is greater than or equa l to 99 wt % Al. nano-Al produced by both methods has been consolidate d by uniaxial pressing at 350 MPa for 1 h at 25, 100, or 300 degrees C Rapid grain growth is observed at each pressing temperature in method -B powders. The mean grain size doubles at 25 degrees C within 1 h and continues to increase beyond the nanometer-size regime over longer pe riods. The low-temperature grain-size instability of the chemically sy nthesized (method-B) nano-Al contrasts markedly with the high-temperat ure stability (greater than or equal to 300 degrees C) of nano-Al prep ared by gas condensation or mechanical attrition. Facile grain growth in the chemically synthesized (method-B) nano-Al is attributed to high er grain-boundary purity and to aggregate structures that minimize adv entitious oxidation. The results establish that barriers to grain grow th in pure nano-Al are intrinsically low.