Sonochemically Generated Air-Stable Bimetallic Nanopowders of Group 4 Transition Metals with Aluminum

Abstract

Air-stable reactive metal nanopowders (RMNPs) of group 4 transition metals and aluminum were produced via a sonochemically agitated reaction of MCl4 (M = Ti, Zr, Hf) with 4 equiv of lithium aluminum hydride (LiAlH4, LAH) under an atmosphere of N2 in Et2O, generating in situ the tetrahydroaluminate of the respective group 4 metal. The subsequent decomposition of the tetrahydroaluminate complex via elimination of H2 produced black powders that were further dried and subsequently annealed in vacuo to temperatures ranging from 620 to 860 °C, simultaneously subliming LiCl and Al byproducts for ease of purification, and producing air-stable RMNPs, which were then analyzed by elemental analysis, X-ray diffraction (XRD), scanning electron microscopy/energy-dispersive spectroscopy (SEM/EDS), magic-angle spinning (MAS) NMR, thermogravimetric analysis/differential scanning calorimetry (TGA/DSC), and oxygen bomb calorimetry. Elemental analysis of the materials showed a significant presence of H, C, and N in the product materials. For each of the metal combinations the XRD analysis showed the presence of two main phases that have the same lattice parameters as the cubic metal carbide phase and the tetragonal Al3M phase of each of the group 4 transition metals. Each of the phases exhibited nanocrystallites, with the more refractory carbide phase producing approximately 5 nm crystallites, and the aluminide phase producing 20 nm crystallites based on Scherrer analysis. SEM confirmed that the materials consist of small individual crystallites that are agglomerated into larger structures. The observed 27Al MAS NMR resonances can be assigned to Al0, Al3M alloys, and Aln+ ionic species. The TGA data showed the superior stability of these materials exhibiting reactivity with O2 only at temperatures approaching 400 °C. The combustion energy of these RMNPs was then measured using oxygen bomb calorimetry, which confirmed that the air-exposed RMNPs are capable of producing at least 26 kJ/g when combusted in O2 (as in the case of the titanium aluminum material), and therefore may have potential to be useful fuels.